A Brief Introduction to MEMS and NEMS

A Brief Introd

9.A Brief Introduction to MEMS and NEMS

Wendy C.Crone

The expanding and developingfields of

micro-electromechanical systems(MEMS)and

nano-electromechanical(NEMS)are highly in-

terdisciplinary and rely heavily on experimental mechanics for materials selection,process

validation,design development,and device char-acterization.These devices range from mechanical sensors and actuators,to microanalysis and chem-ical sensors,to micro-optical systems and bioMEMS for microscopic surgery.Their applications span the automotive industry,communications,de-fense systems,national security,health care,

information technology,avionics,and environ-mental monitoring.This chapter gives a general introduction to the fabrication processes and ma-terials commonly used in MEMS/NEMS,as well as

a discussion of the application of experimental

mechanics techniques to these devices.Mechanics issues that arise in selected example devices are also presented.

9.1Background (203)

9.2MEMS/NEMS Fabrication (206)

9.3Common MEMS/NEMS Materials

and Their Properties (206)

9.3.1Silicon-Based Materials (207)

9.3.2Other Hard Materials (208)

9.3.3Metals (208)

9.3.4Polymeric Materials (208)

9.3.5Active Materials (209)

9.3.6Nanomaterials (209)

9.3.7Micromachining (210)

9.3.8Hard Fabrication Techniques (211)

9.3.9Deposition (211)

9.3.10Lithography (211)

9.3.11Etching (212)

9.4Bulk Micromachining

versus Surface Micromachining (213)

9.5Wafer Bonding (214)

9.6Soft Fabrication Techniques (215)

9.6.1Other NEMS Fabrication Strategies..215

9.6.2Packaging (216)

9.7Experimental Mechanics

Applied to MEMS/NEMS (217)

9.8The Influence of Scale (217)

9.8.1Basic Device Characterization

Techniques (218)

9.8.2Residual Stresses in Films (219)

9.8.3Wafer Bond Integrity (220)

9.8.4Adhesion and Friction (220)

9.9Mechanics Issues in MEMS/NEMS (221)

9.9.1Devices (221)

9.10Conclusion (224)

References (225)

9.1Background

The acronym MEMS stands for micro-electromechanical system,but MEMS generally refers to microscale devices or miniature embedded systems involving one or more micromachined component that enables higher-level functionality.Similarly NEMS,nano-electromechanical system,refers to such nanoscale devices or nanodevices.MEMS and NEMS are fab-ricated microscale and nanoscale devices that are

often made in batch processes,usually convert be-

tween some physical parameter and a signal,and

may be incorporated with integrated circuit technol-

ogy.Thefield of MEMS/NEMS encompasses devices

created with micromachining technologies originally developed to produce integrated circuits,as well as

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204Part A Solid Mechanics Topics

Table 9.1Sample applications of MEMS /NEMS

Sensors Accelerometers,biochemical analyzers,environmental assay devices,gyroscopes,medical diagnostic sensors,pressure sensors

Actuators

Data storage,drug-delivery devices,drug synthesis,fluid regulators,ink-jet printing devices,micro fuel cells,micromirror devices,microphones,optoelectric devices,radiofrequency devices,surgical devices

Passive structures

Atomizers,fluid spray systems,fuel injection,medical inhalers

non-silicon-based devices created by the same micro-machining or other techniques.They can be classified as sensors,actuators,and passive structures (Ta-ble 9.1).

Sensors and actuators are transducers that convert one physical quantity to another,such as electro-magnetic,mechanical,chemical,biological,optical or thermal phenomena.MEMS sensors commonly meas-ure pressure,force,linear acceleration,rate of angular motion,torque,and flow.For instance,to sense pressure an intermediate conversion step,such as mechanical stress,can be used to produce a signal in the form of electrical energy.The sensing or actuation con-version can use a variety of methods.MEMS /NEMS sensing can employ change in electrical resistance,piezoresistive,piezoelectric,change in capacitance,and magnetoresistive methods (Table 9.2).MEMS /NEMS actuators provide the ability to manipulate physical parameters at the micro/nanoscale,and can employ electrostatic,thermal,magnetic,piezoelectric,piezore-sistive,and shape-memory transformation methods.Passive MEMS structures such as micronozzles are used in atomizers,medical inhalers,fluid spray systems,fuel injection,and ink-jet printers.Table 9.2Physical quantities used in MEMS /NEMS sensors and actuators (after Maluf [9.1])

Method Description

Physical and material Order of energy parameters

density (J /cm 3)Electrostatic Attractive force between two components Electric field,dielectric permittivity ≈0.1carrying opposite charge

Piezoelectric Certain materials that change shape Electric field,Young’s modulus,≈0.2under an electric field

piezoelectric constant

Thermal Thermal expansion or difference Coefficient of expansion,temperature ≈5in coefficient of thermal expansion change,Young’s modulus

Magnetic

Electric current in a component Magnetic field,magnetic permeability

≈4

surrounded by a magnetic field

gives rise to an electromagnetic force Shape memory Certain materials that undergo Transformation temperature ≈10

a solid–solid phase transformation producing a large shape change

MEMS have a characteristic length scale be-tween 1mm and 1μm,whereas NEMS devices have a characteristic length scale below 1μm (most strictly,1–100nm).For instance a digital micromirror de-vice has a characteristic length scale of 14μm,a quantum dot transistor has components measuring 300nm,and molecular gears fall into the 10–100nm range [9.2].Additionally,although an entire device may be mesoscale,if the functional components fall in the microscale or nanoscale regime it may be referred to as a MEMS or NEMS device,respectively.MEMS /NEMS inherently have a reduced size and weight for the function they carry out,but they can also carry ad-vantages such as low power consumption,improved speed,increased function in one package,and higher precision.

There is no distinct MEMS /NEMS market,in-stead there is a collection of niche markets where MEMS /NEMS become attractive by enabling a new function,bringing the advantage of reduced size,or lowering cost [9.1].

Despite this characteristic,the MEMS industry is al-ready valued in tens of billions of dollars and growing rapidly.The Small Times Tech Business Directory TM

Part A 9.1

lists more than700manufacturers/fabricators of mi-crosystems and nanotechnologies[9.3].High-volume production with lucrative sales have been achieved by several companies making devices such as accelero-meters for automobiles(Analog Devices,Motorola, Bosch),micromirrors for digital projection displays (Texas Instruments),and pressure sensors for the automotive and medical industries(NovaSensor).Cur-rently,the MEMS markets with the largest commer-cial value are ink-jet printer heads,optical MEMS (which includes the Digital Micromirror Device TM discussed below),and pressure sensors,followed by microfluidics,gyroscopes,and accelerometers[9.4]. The MEMS market was reported to be US$5.1bil-lion in2005and projected to reach US$9.7billion by2010[9.4].The NEMS industry,while still young, has been growing in value.The market researchfirm Report Buyer recently released a market report on Nanorobotics and NEMS indicating that the global mar-ket for NEMS increased from US$29.5million to US$34.2million between2004and2005and project-ing that the market will reach US$830.4million by 2011[9.5].

Microfluidic and nanofluidic devices also fall under the umbrella of MEMS/NEMS and are often classified as bioMEMS/bioNEMS devices when involving biolog-ical materials.These devices incorporate channels with at least one microscale or nanoscale dimension in which fluidflows.The small scale of these devices allow for smaller sample size,faster reactions,and higher sen-sitivity.Microfluidic devices commonly use both hard and soft fabrication techniques to produce channels and otherfluidic structures[9.6].The common feature of these devices is that they allow forflow of gas and/or liquid and use components such as pumps,valves,noz-zles,and mixers.Commercial and defense applications include automotive controls,pneumatics,environmen-tal testing,and medical devices.The advantages of the microscale in these applications include high spa-tial resolution,fast time response,smallfluid volumes required for analysis,low leakage,low power consump-tion,low cost,appropriate compatibility of surfaces,and the potential for integrate signal processing[9.7].At the microscale,pressure drop over a narrow channel is high andfluidflow generated by electricfields can be substantial.

The micrometer and nanometer length scales are particularly relevant to biological materials because they are comparable to the size of cells,molecules,dif-fusions length for molecules,and electrostatic screening lengths of ionic conductingfluids[9.8].Device exam-ples include biofluidic chips for biochemical analyses, biosensors for medical diagnostics,environmental as-

says for toxin identification,implantable pharmaceu-

tical drug delivery,DNA and genetic code analysis,

imaging,and surgery.NEMS is often associated with biotechnology because this size scale allows for in-

teraction with biological systems in a fundamental

way.BioNEMS may be used for drug delivery,drug synthesis,genome synthesis,nanosurgery,and artifi-

cial organs comprised of nanomaterials.The sensitivity

of such bioNEMS devices can be exquisite,selec-

tively binding and detecting a single biomolecule.More

complete background information on microfluidic de-

vices can be found in Beeby[9.7],Koch[9.9]and

Kovacs[9.10].

Semiconductor NEMS devices can offer microwave resonance frequencies,exceptionally high mechanical

quality factors,and extraordinarily small heat capaci-

ties[9.11,12].Examples of NEMS devices also include transducers,radiating energy devices,nanoscale inte-

grated circuits,and optoelectronic devices[9.13,14].

NEMS manufacturing is being further enabled by the

drive towards nanometer feature sizes in the microelec-

tronics industry.Terascale computational ability will

require nanotransistors,nanodiodes,nanoswitces,and

nanologic gates[9.15].NEMS also opens the door

for fundamental science at the nanometer scale inves-

tigating phonon-mediated mechanical processes[9.16]

and quantum behavior of mesoscopic mechanical sys-

tems[9.17].

Although there is some discussion as to whether the

NEMS definition requires a characteristic length scale

below1000nm or100nm,there is no argument that the

field of NEMS is in its infancy.Existing commercial

devices are limited at this point,but research on NEMS

is extremely active and highly promising.Many chal-

lenges remain,including assembly of nanoscale devices

and mass production capabilities.

In the long term,a number of issues must be addressed in analysis,design,development,and fabri-

cation for high-performance MEMS/NEMS to become ubiquitous.Of most relevance to the focus of this hand-

book,advanced materials must be well characterized

and MEMS/NEMS testing must be further developed. Additionally for commercialization,MEMS/NEMS de-

sign must consider issues of market(need for prod-

uct,size of market),impact(enabling new systems,

paradigm shift for thefield),competition(other macro

and micro/nanoproducts existing),technology(avail-

able capability and tools),and manufacturing(manufac-

turability in volume at low cost)[9.18].

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Thefield of MEMS/NEMS is highly multidisci-plinary,often involving expertise from engineering, materials science,physics,chemistry,biology,and medicine.Because of the breadth of thefield and the range of activities that fall under the scope of MEMS/NEMS,a comprehensive review is not possi-ble in this chapter.After providing general background, the focus will be on mechanics and specifically exper-imental mechanics as it is applied to MEMS/NEMS.Mechanics is critical to the design,fabrication,and performance of MEMS/NEMS.A broad range of ex-perimental tools has been applied to MEMS/NEMS. This chapter will provide an overview of such work. Additional information on the application of mechan-ics to MEMS/NEMS can be found in the proceedings of the annual symposium held by the MEMS and Nanotechnology Technical Division of the Society for Experimental Mechanics(see,for example,[9.19]).

9.2MEMS/NEMS Fabrication

Traditionally MEMS/NEMS are thought of in the con-text of microelectronics fabrication techniques which utilize silicon.This approach to MEMS/NEMS brings with it the momentum of the integrated circuits in-dustry and has the advantage of ease of integration with semiconductor devices,but fabrication is expen-sive in both the infrastructure and equipment required and the time investment needed to create a work-ing prototype.An alternative approach that has seen significant success,especially in its application to mi-crofluidic devices,is the use of soft materials such as polydimethylsiloxane(PDMS).Soft MEMS/NEMS fabrication can often be conducted with bench-top tech-niques with no need for the clean-room facilities used in microelectronics fabrication.Additionally,polymers offer a range of properties not available in silicon-based materials such as mechanical shock tolerance, biocompatibility,and biodegradability.However,poly-mers can carry disadvantages for certain applications because of their viscoelastic behavior and low ther-mal stability.Ultimately a combination of function and economics decides the medium of choice for device construction.

Whether we talk about hard or soft MEMS/NEMS, the basic approach to device construction is similar. Material is deposited onto a substrate,a lithographic step is used to produce a pattern,and material removal is conducted to create a shape.For traditional micro-electronics fabrication,the substrate is often silicon, material deposition is achieved by vapor deposition or sputtering,lithography involves patterning of a chem-ically resistant polymer,and material is removed by a chemical etch.Alternatively,for soft MEMS/NEMS materials,fabrication often utilizes a glass or plas-tic substrate,material in the form of a monomer is flowed into a region,a lithographic mask allows expo-sure of a pattern to ultraviolet(UV)radiation triggering polymerization,and the unpolymerized monomer is re-moved with aflushing solution.For both hard and soft MEMS/NEMS fabrication there are a number of varia-tions on these basic steps which allow for a wide array of structures and devices to be constructed.

9.3Common MEMS/NEMS Materials and Their Properties

Materials used in MEMS/NEMS must simultaneously satisfy a range requirements for chemical,structural, mechanical,and electrical properties.For biomedical and bioassay devices,material biocompatibility and bioresistance must also be considered.

Most MEMS/NEMS devices are created on a sub-strate.Common substrate materials include single-crystal silicon,single-crystal quartz,fused quartz, gallium arsenide,glass,and plastics.Devices are made with a range of methods by machining into the sub-strate and/or depositing additional material on top of the substrate.The additional materials may be structural, sacrificial,or active.

Although traditionally MEMS in particular have re-lied on silicon,the materials used in MEMS/NEMS are becoming more heterogeneous.Selected properties are given in Table9.3for comparative purposes,but an extensive list of properties for the wide range of ma-terials used in MEMS/NEMS cannot be included here. It should be noted,however,that the constitutive behav-ior of materials used in MEMS/NEMS applications can be sensitive to fabrication method,processing parame-

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9.3A Brief Introduction to MEMS and NEMS9.3Common MEMS/NEMS Materials and Their Properties207

Table9.3Properties of selected materials(after Maluf[9.1]and Beeby[9.7])

Property Si SiO2Si3N4Quartz SiC Si(111)Stainless Al

steel

Young’s modulus(GPa)1607332310745019020070

Yield strength(GPa)78.414921730.17

Poisson’s ratio0.220.170.250.160.140.220.30.33

Density(g/cm3)2.42.33.12.653.22.382.7

Coefficient of thermal2.60.552.80.554.22.31624

expansion(10−6/◦C)

Thermal conductivity1.570.0140.190.013851.480.22.37

at300K(W/cm·K)

Melting temperature(◦C)1415170018001610283014141500660

ters,and thermal history due to the relative similarity between characteristic length scales and device dimen-sions.A good resource compiling characterization data from a number of sources is the material database at http://www.memsnet.org/material/[9.20].The follow-ing books,used as references for the discussion here, are valuable resources for more extensive information: Senturia[9.18],Maluf[9.1],and Beeby[9.7].

9.3.1Silicon-Based Materials

Silicon,Polysilicon,and Amorphous Silicon Silicon-based materials are the most common ma-terials currently used in MEMS/NEMS commercial production.MEMS/NEMS devices often exploit the mechanical properties of silicon rather than its electrical properties.Silicon can be used in a number of differ-ent forms:oriented single-crystal silicon,amorphous silicon,or polycrystal silicon(polysilicon).

Single-crystal silicon,which has cubic crystal struc-ture,exhibits anisotropic behavior which is evident in its mechanical properties such as Young’s modulus.

A high-purity ingot of single-crystal silicon is grown, sawn to the desired thickness,and polished to create a wafer.Single-crystal silicon used for MEMS/NEMS are usually the standard100mm(4inch diameter, 525μm thickness)or150mm(6inch diameter,650μm thickness)wafers.Although larger8-inch and12-inch wafers are available,they are not used as prevalently for MEMS fabrication.

The properties of the wafer depend on both the ori-entation of crystal growth and the dopants added to the silicon(Fig.9.1).Impurity doping has a significant impact on electrical properties but does not generally impact the mechanical properties if the concentration is approximately<1020cm−3.Silicon is a group IV semiconductor.To create a p-type material,dopants from group III(such as boron)create mobile charge

carriers that behave like positively charged species.To

create an n-type material,dopants from group V(such

as phosphorous,arsenic,and antimony)are used to cre-

ate mobile charge carriers that behave like negatively

charged electrons.Doping of the entire wafer can be ac-complished during crystal growth.Counter-doping can

be accomplished by adding dopants of the other type

to an already doped substrate using deposition followed

by ion implantation and annealing(to promote diffusion

and relieve residual stresses).For instance,p-type into

n-type creates a pn-junction.

Amorphous and polysiliconfilms are usually de-

posited with thicknesses of<5μm,although it is also

possible to create thick polysilicon[9.21].The residual

stress in deposited polysilicon and amorphous silicon

thinfilms can be large,but annealing can be used to

provide some relief.Polysilicon has the disadvantage

of a somewhat lower strength and lower piezoresis-

to identify crystallographic orientation and doping(after

Senturia[9.18])

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9.3208Part A Solid Mechanics Topics

modulus may vary significantly because the diameter of a single grain may comprise a large fraction of a com-ponent’s width[9.22].

Silicon,polysilicon,and amorphous silicon are also piezoresistive,meaning that the resistivity of the mater-ial changes with applied stress.The fractional change in resistivity,Δρ/ρ,is linearly dependent on the stress components parallel and perpendicular to the direc-tion of the resistor.The proportionality constants are the piezoresistive coefficients,which are dependent on the crystallographic orientation,and the dopant type/concentration in single-crystal silicon.This prop-erty can be used to create a strain gage.

Silicon Dioxide

The success of silicon is heavily based on its ability to form a stable oxide which can be predictably grown at elevated temperature.Dry oxidation produces a higher-quality oxide layer,but wet oxidation(in the presence of water)enhances the diffusion rate and is often used when making thicker oxides.Amorphous silicon diox-ide can be used as a mask against etchants.It should be noted that thesefilms can have large residual stresses.

Silicon Nitride

Silicon nitride can be deposited by chemical vapor de-position(CVD)as an amorphousfilm which can be used as a mask against etchants.It should be noted that these films can have large residual stresses.

Silicon Carbide

Silicon carbide is an attractive material because of its high hardness,good thermal properties,and resistance to harsh environments.Additionally,silicon carbide is piezoresistive.Although it can be produced as a bulk polycrystalline material it is generally grown or de-posited on a silicon substrate by epitaxial growth(single crystal)or by chemical vapor deposition(polycrystal).

Silicon on Insulator(SOI)

Silicon on insulator(SOI)wafers are also used for MEMS sensors and actuators[9.23].Different SOI materials are distinguished by their properties.Buried oxide layers can be produced either through ion implan-tation or wafer bonding processes;these techniques are discussed further below[9.24].

9.3.2Other Hard Materials

Gallium Arsenide

Gallium arsenide(GaAs)is a III–V compound semi-conductor which is often used to create lasers,optical devices,and high-frequency components.

Quartz

Single-crystal quartz,which has a hexagonal crystal structure,can be used in natural or synthesized form. Like silicon,it can be etched selectively but the re-sults are less ideal than in silicon because of unwanted facets and poor edge definition.Single-crystal quartz can be used as substrate material in a range of cuts which have different temperature sensitivities for piezo-electric or mechanical properties.Detailed information about quartz cuts can be found in Ikeda[9.25].Quartz is also piezoelectric,meaning that there is a relationship between strain and voltage in the material.Fused quartz (silica)is a glassy noncrystalline material that is also occasionally used in MEMS/NEMS devices.

Glass

Glasses such as phosphosilicate and borosilicate (Pyrex)can be used as a substrate or in conjunction with silicon and other materials using wafer bonding (discussed below).

Diamond

Diamond is also attractive because of its high hard-ness,high fracture strength,low thermal expansion,low heat capacity,and resistance to harsh environments. Diamond is also piezoresistive and can be doped to produce semiconducting and metal-like behavior[9.26]. Because of its hardness,diamond is particularly attrac-tive for parts exposed to wear.The most promising synthetic forms are amorphous diamond-like carbon, nanocrystalline diamond,and ultra-nanocrystalline di-amondfilms created by pulsed laser deposition or chemical vapor deposition[9.27–31].

9.3.3Metals

Metals are usually deposited as a thinfilm by sputtering, evaporation or chemical vapor deposition(CVD).Gold, nickel and iron can also be electroplated.Aluminum is the most common metal used in MEMS/NEMS,and is often used for light reflection and electrical conduction. Gold is used for electrochemistry,infrared(IR)light re-flection,and electrical conduction.Chromium is often used as an adhesion layer.Alloys of Ni,such as NiTi and Permalloy TM,can be used for actuation and are discussed in more detail below.

9.3.4Polymeric Materials

Photoresists

Polymeric photoresist materials are generally used as a spin-castfilm as part of a photolithographic pro-cess.Thefilm is modified by exposure to radiation

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A Brief Introduction to MEMS and NEMS9.3Common MEMS/NEMS Materials and Their Properties209

such as visible light,ultraviolet light,x-rays or elec-trons.Exposure is usually conducted through a mask so that a pattern is created in the photoresist layer and subsequently on the substrate through an etching or de-position process.Resists are either positive or negative depending on whether the radiation exposure weakens or strengthens the polymer.In the developer step,chem-icals are used to remove the weaker material,leaving a patterned photoresist layer behind.Important pho-toresist properties include resolution and sensitivity, particularly as feature sizes decrease.

Polydimethylsiloxane

Polydimethylsiloxane(PDMS)is an elastomer used as both a structural component in MEMS devices and a stamping material for creating micro-and nanoscale features on surfaces.PDMS is a common silicone rub-ber and is used extensively because of its processibility, low curing temperature,stability,tunable modulus,op-tical transparency,biocompatibility,and adaptability by

a range functional groups that can be attached[9.32,33].

9.3.5Active Materials

There are several types of active materials that success-fully perform sensing and actuation functions at the microscale.Several examples of active materials are given below.

NiTi

Near-equiatomic nickel titanium alloy can be deposited as a thinfilm and used an as active material.This ma-terial is of particular interest to MEMS because the actuation work density of NiTi is more than an or-der of magnitude higher than the work densities of other actuation schemes.These shape-memory alloys (SMA s)undergo a reversible phase transformation that allows the material to display dramatic and recover-able stress-and temperature-induced transformations. The behavior of NiTi SMA is governed by a phase transformation between austenite and martensite crys-tal structures.Transformation between the austenite (B2)and martensite(B19)phases in NiTi can be produced by temperature cycling between the high-temperature austenite phase and the low-temperature martensite phase(shape-memory effect),or loading and unloading the material to favor either the high-strain martensite phase or the low-strain austenite phase(superelasticity).Thus both stress and tempera-ture produce the transformation between the austenite and martensite phases of the alloy.The transfor-mation occurs in a temperature window,which can

be adjusted from−100◦C to+160◦C by changing

the alloy composition and heat treatment process-

ing[9.34].

Permalloy TM

Permalloy TM,Ni x Fe y,displays magnetoresistance prop-

erties and is used for magnetic transducing.Mul-

tilayered nanostructures of this alloy give rise to

a giant-magnetoresistance(GMR)phenomenon which

can be used to detect magneticfields.It has been widely

applied to read the state of magnetic bits in data storage

media.

Lead Zirconate Titanate(PZT)

Lead zirconate titanate(PZT)is a ceramic solid solution

of lead zirconate(PbZrO3)and lead titanate(PbTiO3).

PZT is a piezoelectric material that can be deposited

in thinfilm form by sputtering or using a sol–gel pro-

cess.In addition to natural piezoelectric materials such

as quartz,other common synthetic piezoelectric ma-

terials include polyvinylidenefluoride(PVDF),zinc

oxide,and aluminum nitride.Actuation performed by piezoelectrics has the advantage of being capable of

achieving reasonable displacements with fast response,

but the material processing is complex.

Hydrogels

Hydrogels,such as poly(2-hydroxyethyl methacrylate (HEMA))gel,with volumetric shape-memory cap-

ability are now being employed as actuators,fluid

pumps,and valves in microfluidic devices.In an aque-

ous environment,hydrogels will undergo a reversible

phase transformation that results in dramatic volumetric

swelling and shrinking upon exposure and removal of

a stimulus.Hydrogels have been produced that actuate

when exposed to such stimuli as pH,salinity,electri-

cal current,temperature,and antigens.Since the rate of

swelling and shrinking in a hydrogel is diffusion lim-

ited,the temporal response of hydrogel structures can

be reduced to minutes or even seconds in microscale

devices.

9.3.6Nanomaterials

Nanostructuring of materials can produce unique me-chanical,electrical,magnetic,optical,and chemical properties.The materials themselves range from poly-

mers to metals to ceramics,it is their nanostructured

nature that gives them exciting new behaviors.In-

creased hardness with decreasing grain size allows for

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Table9.4Micromachining processes and their applications(after Kovacs[9.10])

Process Example applications

Lithography Photolithography,screen printing,electron-beam lithography,x-ray lithography

Thin-film deposition Chemical vapor deposition(CVD),plasma-enhanced chemical vapor deposition(PECVD),

physical vapor deposition(PVD)such as sputtering and evaporation,spin casting,sol–gel deposition Electroplating Blanket and template-delimited electroplating of metals

Directed deposition Electroplating,LIGA,stereolithography,laser-driven CVD,screen printing,microcontact printing,

dip-pen lithography

Etching Plasma etching,reactive-ion enhanced etching(RIE),deep reactive-ion etching(DRIE),

wet chemical etching,electrochemical etching

Machining Drilling,milling,electrical discharge machining(EDM),focused ion beam(FIB)milling,

diamond turning,sawing

Bonding Direct silicon-fusion bonding,fusion bonding,anodic bonding,adhesives

Surface modification Wet chemical modification,plasma modification,self assembled monolayer(SAM)deposition,

grinding,chemomechanical polishing

Annealing Thermal annealing,laser annealing

hard coatings and protective layers,lower percolation threshold impacts conductivity,and narrower bandgap with decreasing grain size enable unique optoelec-tronics[9.35].Hundreds of different synthesis routes have been created for manufacturing nanostructured materials.(See,for example,the proceedings of the International Conferences on Nanostructured Mater-ials[9.36].)A few examples of such materials are given below.

Carbon Nanotubes and Fullerenes

Carbon nanotubes(CNT s)and fullerenes(buckyballs, e.g.,C60)are self-assembled carbon nanostructures. CNT s are cylindrical graphene structures of single-or multiwall form which are extremely strong andflexi-ble.They possess metallic or semiconducting electronic behavior depending on the details of the structure(chi-rality).They can be created in an arc plasma furnace, laser ablation,or grown by chemical vapor deposition (CVD)on a substrate using catalyst particles[9.37].

Quantum Dots,Quantum Wires,

and Quantum Films

Quantum behavior occurs in semiconductor mater-ials(such as GaAs)when electrons are confined to nanoscale dimensions.The confined space forces elec-trons to have energy states that are clustered around specific peaks,producing fundamentally different elec-trical and optical properties than would be found in the same material in bulk form.The number of directions free of confinement is used to classify structures,thus two-dimensional(2-D)confinement leads to a quan-tumfilm,one-dimensional(1-D)confinement leads to a quantum wire,and zero-dimensional(0-D)confine-ment leads to a quantum dot.The dimension of the confined direction(s)is so small that the energy states are quantized in that direction[9.37].

Nanowires

A variety of methods have been developed for mak-ing nanowires of a wide range of metals,ceramics, and polymers.Examples include gold nanowires made by a solution method[9.38],palladium nanowires cre-ated by electroplating on a stepped surface[9.39],and zinc oxide nanowires created by a vapor/liquid/solid method[9.40].In one popular technique,electroplating is conducted inside a nanoporous template of alumina or polycarbonate to direct the growth of nanowires[9.41, 42].The template can be chemically removed,leaving the nanowires behind.In another application,litho-graphically patterned metal is used as a catalyst for silicon nanowire growth,creating predefined regions of nanowires on a surface[9.43,44].Using various com-binations of metal catalysts and gases,a wide range of nanowire compositions can be created from chemical vapor deposition methods.

9.3.7Micromachining

Micromachining is a set of material removal and forming techniques for creating microscale movable features and complex structures,often from silicon. The micromachining processes listed in Table9.4can be applied to other materials such as glasses,ce-

Part A

9.3A Brief Introduction to MEMS and NEMS9.3Common MEMS/NEMS Materials and Their Properties211

ramics,polymers,and metals,but silicon is favored because of its widespread use and the availability of design and processing techniques.Other advantages of silicon include the availability of relatively inexpen-sive pure single-crystal substrate wafers,its desirable electrical properties,its well-understood mechanical properties,and ease of integration into a circuit for control and signal processing[9.7].Although often per-formed in batch processes,micromachining for MEMS application may make large-aspect-ratio features and incorporation of novel or active materials a higher pri-ority than batch manufacturing.This opens the door for a wider range of fabrication techniques such as focused ion-beam milling,laser machining,and electron-beam writing[9.22,45,46].

A brief overview of micromachining is provided below.The following books,used as references for the discussion here,are valuable resources for more extensive information[9.1,2,10,18,22,33].Addi-tional information can be found in Taniguchi[9.47] and Evans[9.48]on microfabrication technology, Bustillo[9.49]on surface micromachining,and Gen-tili[9.50]on nanolithography.

9.3.8Hard Fabrication Techniques

Hard MEMS utilizes enabling technologies for fabri-cation and design from the microelectronics industry. The MEMS industry has modified advanced techniques, leveraging well beyond the capability to fabricate inte-grated circuits.

Micromachining involves three fundamental pro-cesses:deposition,lithography,and etching.Deposition may employ oxidation,chemical vapor deposition, physical vapor deposition,electroplating,diffusion,or ion implantation.Lithography methods include optical and electron-beam techniques.Etching methods include wet and dry chemical etches,which can be either isotropic(uniform etching in all directions,resulting in rounded features)or anisotropic(etching in one prefer-ential direction,resulting in well-defined features). 9.3.9Deposition

Physical Vapor Deposition(PVD)

Physical vapor deposition(PVD)includes evaporation and sputtering.The evaporation method is used to deposit metals on a surface from vaporized atoms re-moved from a target by heating with an electron beam. This technique is performed under high vacuum and produces very directional deposition and can create shadows.Sputtering of a metallic or nonmetallic mater-

ial is accomplished by knocking atoms off a target with

a plasma of an inert gas such as argon.Sputtering is less directional and allows for higher deposition rates.

Chemical Vapor Deposition(CVD)

In chemical vapor deposition(CVD),precursor material

is introduced into a heated furnace and a chemical reac-

tion takes place on the surface of the wafer.The CVD

process is generally performed under low-pressure

conditions and is sometimes explicitly referred to as

low-pressure CVD(LP CVD).A range of materials can

be deposited by CVD,includingfilms of silicon(formed

by decomposition of silane(SiH4)),silicon nitride

formed by reacting dichlorosilane(SiH2Cl2)with am-

monia(NH3)),and silicon oxide(formed by silane with

an oxidizing species).LPCVD can produce amorphous

inorganic dielectricfilms and polycrystalline polysili-

con and metalfilms.Epitaxy is a CVD process where temperature and growth rate are controlled to achieve

ordered crystalline growth in registration with the sub-

strate.PE CVD is a plasma-enhanced CVD process.

Electroplating

A variety of electroplating techniques are used to make

micro-and nanoscale components.A mold is created

into which metal is plated.Gold,copper,chromium,

nickel,and iron are common plating metals.

Spin Casting

Spin casting is used to createfilms from a solution.The

most common spin-cast material is polymeric photore-

sist.

Sol–Gel Deposition

A range of sol–gel processes can be used to makefilms

and particles.The general technique involves a colloidal suspension of solid particles in afluid that undergo

a reaction to generate a gelatinous network.After de-

position of the gel,the solvent can be removed to

transform the network into a solid phase which is sub-

sequently sintered.Piezoelectric materials such as PZT

can be deposited with this method.

9.3.10Lithography

Most of the micromachining techniques discussed be-

low utilize lithography,or pattern transfer,at some point

in the manufacturing process.Depending on the res-

olution required to produce the desired feature sizes

and the aspect ratio necessary,lithography is either per-

Part

A

9.3212Part A Solid Mechanics Topics

formed with ultraviolet light,an ion beam,x-rays,or an electron beam.X-ray lithography can produce features down to10nm and electron beams can be focused down to less than1nm[9.50].Optical lithography allows as-pect ratios of up to three whereas x-ray lithography can produce aspect ratios>100.This large depth of focus, lack of scattering effects,and insensitivity to organic dust make x-ray lithography very attractive for NEMS production.Electron-beam lithograph has the attractive feature that a pattern can be directly written onto a re-sist,as well as the fact that it produces lower defect densities with a large depth of focus,but the process must be performed in vacuum.

In most cases a mask that carries either a posi-tive or negative image of the features to be created mustfirst be produced.Masks are commonly made with a chromium layer on fused silica.Photoresist cover-ing the chromium is exposed with an optical pattern generated from a sequence of small rectangles used to draw out the pattern desired.Other mask production techniques include photographic emulsion on quartz, electron-beam lithography with electron-beam resist, and high-resolution ink-jet printing on acetate or mylar film.

Photolithographic fabrication techniques have a long history of use with ceramics,plastics,and glasses.In the case of silicon fabrication,the wafer is coated with a polymeric photoresist layer sensitive to ultraviolet light.Exposure of the photoresist layer is conducted through a mask.Depending on whether a positive or negative photoresist is used,the light either weakens the polymer or strengthens the polymer.In the developer step,chemicals are used to remove the weaker mater-ial,leaving a patterned photoresist layer behind.The photoresist acts as a protective layer when etching is conducted.

Contact lithography produces a1:1ratio of the mask size and feature size.Proximity lithography also gives a1:1ratio with slightly lower resolution because a gap is left between the mask and the substrate to minimize damage to the mask.A factor of5–10reduction is common for projection step-and-repeat lithography.Be-cause this technique allows for the production of feature sizes smaller then the mask,only a small region is ex-posed at one time and the mask must be stepped across the substrate.

9.3.11Etching

A number of wet and dry etchants have been developed for silicon.Important properties include orientation dependence,selectivity,and the geometric details of

the etched feature(Fig.9.2).A common isotropic wet

etchant for silicon is HNA(a combination of HF,

HNO3,and CH3COOH),while anisotropic wet etchants

include KOH,which etches{100}planes100times

faster than{111}planes,tetramethylammonium hy-

droxide(called TMAH or(CH3)4NOH),which etches {100}planes30–50times faster than{111}planes but leaves silicon dioxide and silicon nitride unetched,and

ethylenediamine pyrochatechol(EDP),which is very

hazardous but does not etch most metals.

Wet etchants such as HF for silicon dioxide,H3PO4

for silicon nitride,KCl for gold,and acetone for organic

layers,can be performed in batch processes with little

cost[9.51].An important feature of an etchant is its

selectivity;for example,the etch rate of an oxide by

HF is100nm/min compared to0.04nm/min for sili-

con nitride[9.51].The etching reaction can be either

reaction rate controlled or mass transfer limited.Be-

cause wet etchants act quickly,making it hard to control

depth of the etch,electrochemical etching is sometimes

employed using an electric potential to moderate the re-

action along with a precision thickness epitaxial layer

used for etch stop.

The challenge comes with drying after the wet etch-

ing process is complete.Capillary forces can easily

draw surfaces together,causing damage and stiction.

Supercritical drying,where the liquid is converted to

a gas,can be used to prevent this.Alternatively,ap-

plication of a hydrophobic passivation layer such as

afluorocarbon polymer can be used to prevent stiction.

Chemically reactive vapors and plasmas are highly

effective dry etchants.Xenon difluoride(XeF2)is

a commercially important highly selective vapor etchant

for silicon.Dry etchants such as CHF3+O2for sili-

con dioxide,SF6for silicon nitride,Cl2+SiCl4

for

Fig.9.2

cesses(after Maluf[9.1])

Part A

9.3

A Brief Introduction to MEMS and NEMS 9.4Bulk Micromachining versus Surface Micromachining 213

aluminum,and O 2for organic layers,are used as a plasma [9.51].The process is conducted in a specially designed system that generates a chemically reactive plasma species of ion neutrals and accelerates them to-wards a substrate with an electric or magnetic field.Plasma etching is the spontaneous reaction of neu-trals with the substrate materials,while reactive-ion etching involves a synergistic role between the ion bom-bardment and the chemical reaction.Deep reactive-ion etching (DRIE )allows for the creation of high-aspect-ratio features.DRIE involves periodic deposition of a protective layer to shield the sidewalls either through condensation of reactant gasses produced by cryogenic cooling of the substrate or interim deposition cycles to put down a thin polymer film.

Ions can also be used to sputter away material.For example,argon plasma will remove material from all parts of the wafer.Ion milling refers to selective sputtering and can be done uniformly over a wafer or with focusing electrodes by focused ion-beam milling (FIB ).FIB is also becoming a more important technique for test sample production and the application of grat-ings used for interferometry [9.52].In addition to FIB ,techniques such as scanning probe microscope (SPM )lithography and molecular-beam epitaxy can also be used to create micro-and nanoscale gratings [9.53].Beyond the use of etching as part of the initial fab-rication of a device,some small adjustments may be required after the device is fabricated due to small vari-ations that occur in processing.Compensation can be performed by trimming resistors and altering mechan-ical dimensions via techniques such as laser ablation and FIB milling.Calibration can be performed electron-ically with correction coefficients.

9.4Bulk Micromachining versus Surface Micromachining

The processes for silicon micromachining fall into two general categories:bulk (subtraction of sub-strate material)and surface (addition of layers to the substrate).Other techniques used on a range of ma-terials include surface micromachining,wafer bonding,thin film screen printing,electroplating,lithography galvanoforming molding (LIGA ,from the German

Fig.9.3a–c (a )(b )and (c )LIGA .All views are shown from the side (after Bhushan [9.2]Chap.50)

electric-discharge machining (EDM ),and focused ion beam (FIB ).Figure 9.3provides a basic comparison of bulk micromachining,surface micromachining,and LIGA .

Bulk Micromachining

Removal of significant regions of substrate mater-Part A 9.4

214Part A Solid Mechanics Topics

anisotropic etching of a silicon single-crystal wafer.The fabrication process includes deposition,lithography,and etching.Bulk micromachining is commonly used for high-volume production of accelerometers,pressure sensors,and flow sensors.

Surface Micromachining

Alternating structural and sacrificial thin film layers are built up and patterned in sequence for surface mi-cromachining.The process used by Sandia National Laboratory uses up to five structural polysilicon and five sacrificial silicon dioxide layers,whereas Texas Instrument’s digital micromirror device (discussed be-low)is made from a stack of structural metallic layers and sacrificial polymer layers [9.1,54].De-position methods include oxidation,chemical vapor deposition (CVD ),and sputtering.Annealing must sometimes be used to relax the mechanical stresses that build up in the films.Lithography and etching

are used to produce a free-standing structure.Sur-face micromachining is attractive for integrating MEMS sensors with electronic circuits,and is commonly used for micromirror arrays,motors,gears,and grip-pers.

LIGA

Lithography galvanoforming molding (LIGA ,from the German Lithografie-Galvanik-Abformung )is a litho-graphy and electroplating method used to create very high-aspect-ratio structures (aspect ratios of more than 100are common).The use of x-rays in the lithography process takes advantage of the short wavelength to create a larger depth of focus compared to photolitho-graphy [9.14].Devices can be up to 1mm in height with another dimension being only a few microns and are commonly made of materials such as metals,ceram-ics,and polymers.See Guckel [9.55],Becker [9.56],and Bley [9.57]for additional details.

9.5Wafer Bonding

Although microelectronics fabrication processes allow stacking layers of films,structures are relatively two dimensional.Wafer bonding provides an opportunity for a more three-dimensional structure and is com-monly used to make pressure sensors,accelerometers,and microfluidic devices (Fig.9.4).Anodic and di-

bonding can also be achieved by using intermedi-ate layers such as polymers,solders,and thin-film metals.

Anodic (electrostatic)bonding can be used to bond silicon to a sodium-containing glass substrate (with a matched coefficient of thermal expansion)using an field.This is accomplished with the ap-of a large voltage at elevated temperature to Na +ions mobile.The positively charged held to the negatively charged glass by elec-attraction.

(silicon-fusion)bonding requires two flat,in intimate contact.Direct bonding of stack can be achieved by applying pres-wafer bonding allows joining of two silicon or silicon and silicon dioxide surfaces and is to create SOI wafers.After treatment to produce hydroxyl (OH)groups,inti-allows van der Waals forces to make the followed by an annealing step to create reaction at the interface.

and polishing is sometimes needed to wafer.Annealing must be performed af-to remove defects incurred during grinding.chemomechanical polishing can be used chemical etching with the mechanical action Part A 9.5

A Brief Introduction to MEMS and NEMS9.6Soft Fabrication Techniques215

9.6Soft Fabrication Techniques

Self-Assembly

Partly because of the high cost of nanolithography and the time-consuming nature of atom-by-atom place-ment using probe microscopy techniques,self-assembly is an important bottom-up approach to NEMS fab-rication[9.59].To offset the time it takes to build unit by unit to create a useful device,massive par-allelism and autonomy is required.The advantage of self-assembly is that it occurs at thermodynamic min-ima,relying on naturally occurring phenomena that govern at the nanoscale and create highly perfect as-semblies[9.58].The atoms,molecules,collections of molecules,or nanoparticles self-organize into function-ing entities using thermodynamic forces and kinetic control[9.60].Such self-organization at the nanoscale is observed naturally in liquid crystals,colloids,mi-celles,and self-assembled monolayers[9.61].Reviews of self-assembly can be found in[9.62–65].

At the nanoparticle level,a variety of methods have been used to promote self-assembly.Three basic re-quirements must be met:there must be some sort of bonding force present between particles or the particles and a substrate,the bonding must be selective,and the particles must be in random motion to facilitate chance interactions with a relatively high rate of occurrence. Additionally,for the technique to be practical,the par-ticles must be easily synthesized.Selectivity can be facilitated by micromachining the substrate including patterns with geometric designs that allow for only cer-tain orientations of the mating particle.

Particularly powerful are self-assembly methods using complementary pairs and molecular building blocks(analogous to DNA replication).Complemen-tary pairs can bind electrostatically or chemically(using functional groups with couple monomers).Molecular Table9.5Techniques for creating patterned SAM s[9.58]

Method Scale of features Microcontact printing100nm–some cm Micromachining100nm–someμm Microwriting with pen≈10–100μm Photolithography/lift-off>1μm

Photochemical patterning>1μm

Photo-oxidation>1μm

Focused ion-beam writing≈someμm

Electron-beam writing25–100nm

Scanning tunneling15–50nm

microscope writing building blocks can use a number of different bonds

and linkages(ionic bonds,hydrogen bonds,transition

metal complex bonds,amide linkages,and ester link-

ages)to create building blocks for three-dimensional

(3-D)nanostructures and nanocrystals such as quantum

dots.

Self-assembled monolayers(SAM s)can be pro-

duced in patterned form by several techniques that

produce features in a range of micro-and nanoscale

sizes(Table9.5).Combined with lithography,defined

areas of self-assembly on a surface can be created. Applications of SAM s include fundamental studies of

wetting and electrochemistry,control of adhesion,sur-

face passivation(to protect from corrosion,control oxidation,or use as resist),tribology,directed assembly,

optical systems,colloid fabrication,and biologically ac-

tive surfaces for biotechnology[9.58].

Soft Lithography

The term soft lithography encompasses a number

of techniques that can be used to fabricate micro-

and nanoscale structures using replica molding and

self-assembly.These techniques include microcon-

tact printing,replica molding,microtransfer molding, micromolding in capillaries,and solvent-assisted mi-cromolding[9.66].

As an example,microcontact printing uses a self-assembled monolayer as ink in a stamping operation

that transfers the SAM to a surface(Fig.9.5).The stamp

is fabricated from of an elastomeric material such as

PDMS by casting onto a master with surface features.

The master can be produced with a range of photolitho-

graphic techniques.The polymeric replica mold is used

as a stamp to enable physical pattern transfer.The ad-

vantages of microcontact printing are its simplicity,

conformal contact with a surface,the reusability of the

stamp,and the ability to produce multiple stamps from

one master.Although defect density and registration of

patterns over large scales can be issues,theflexibility of

the stamp can be use to make small features(≈100nm)

using compression or pattern transfer onto curved sur-

faces[9.58].The aspect ratio of features is a constraint

with PDMS however.Ratios between0.2and2must be

used to ensure defect-free stamps and molds[9.67].

9.6.1Other NEMS Fabrication Strategies

Nanoscale structures can be created from both top-

down and bottom-up approaches.Because of the push

Part

A

9.6216Part A Solid Mechanics Topics

to miniaturize commercial electronics,many top-down methods are refinements of micromachining techniques with the goal of achieving manufacturing accuracy on the nanometer scale.Bottom-up methods rely on additive atomic and molecular techniques,such as self-organization,self-assembly,and templating,using building blocks similar and size to those used in na-ture[9.68].A brief review of some additional examples is provided below.

Nanomachining

Scanning probe microscopes(SPM s)are a valuable set of tools for NEMS characterization,but these

tools from the side(after Wilbur[9.58])can also be used for NEMS manufacturing.These mi-croscopes share the common feature that they employ a nanometer-scale probe tip in the proximal vicinity of a surface.They are many times more powerful than scanning electron microscopes because their resolution is not determined by wavelength for the interaction with the surface under investigation.

The scanning tunneling microscope(STM)can be used to create a strong electricfield in the vicinity of the probe tip to manipulate individual atoms.Atoms can be induced to slide over a surface in order to move them into a desired arrangement by mechanosynthesis[9.69]. Resolution is effectively the size of a single atom but practically the process is exceptionally time consuming and the sample must be held at very low temperature to prevent movement of atoms out of place[9.70].With slightly less resolution but still less than100nm,an STM can also be used to write on a chemically ampli-fied negative electron-beam resist.

Nanolithography

Surface micromachining can be conducted at the nanoscale using electron-beam lithography to cre-ate free-standing or suspended mechanical objects. Although the general approach parallels standard litho-graphy(see above),the small-scale ability of this technique is enabled by the fact that an electron beam with energy in the keV range is not limited by diffrac-tion.The electron beam can be scanned to create a desired pattern in the resist[9.8].

Nanoscale resolution can also be obtained using alternative lithographic techniques such as dip-pen nanolithography(DPN)[9.71].This technique employs an atomic force microscope(AFM)probe tip to de-posit a layer of material onto a surface,much as a pen writes on paper.A pattern can be drawn on a surface using a wide range of inks such as thiols,silanes,met-als,sol–gel precursors,and biological macromolecules. Although the DPN process is inherently slower that standard mask lithographic techniques,it can be used for intricate functions such as mask repair and the ap-plication of macromolecules in biosensor fabrication,or it can be parallelized to increase speed[9.72].This and other nanofabrication techniques using AFM to modify and pattern surfaces are reviewed by Tang[9.73].

9.6.2Packaging

Packaging of a MEMS/NEMS device provides a protec-tive housing to prevent mechanical damage,minimize stresses and vibrations,guard against contamination,

Part A

9.6A Brief Introduction to MEMS and NEMS9.8The Influence of Scale217

protect from harsh environmental conditions,dissi-pate heat,and shield from electromagnetic interfer-ence[9.15].Packaging is critical because it enables the usefulness,safety,and reliability of the device. Hermetic packaging made of metal,ceramic,glass or silicon is used to prevent the infiltration of moisture, guard against corrosion,and eliminate contamination. The internal cavity is evacuated orfilled with an in-ert gas.For MEMS/NEMS the packaging may also be required to provide access to the environment through electrical and/orfluid interconnects and optically trans-parent windows.In these cases,the devices are left more vulnerable in order for them to interact with the envi-ronment to perform their function.Although there are well-established techniques for packaging of common microelectronics devices,packaging of MEMS/NEMS presents particular challenges and may account for 75–95%of the overall cost of the device[9.1].

Packaging design must be conducted in parallel with design of the MEMS/NEMS component.Design considerations include thickness of the device,wafer dicing(separation of the wafer into separate dice), sequence offinal release,cooling of heat-dissipating devices,power dissipation,mechanical stress isola-tion,thermal expansion matching,minimization of creep,protective coatings to mitigate damaging en-vironmental effects,and media isolation for extreme environments[9.1].

In the die-attach process,each individual die is mounted into a package,by bonding it to a metal,ce-ramic or plastic platform with a metal alloy solder or an adhesive.For silicon and glass,a thin metal layer must be placed over the surface prior to soldering to allow for wetting.Electrical interconnects can be produced with wire bonding(thermosonic gold bonding with ul-trasonic energy and elevated temperature)andflip-chip bonding(using solder bumps between the die and pack-age pads).Fluid interconnects are created by insertion of capillary tubes,mating of self-aligningfluid ports, and laminated layers of plastic[9.1].

9.7Experimental Mechanics Applied to MEMS/NEMS

With a basic understanding of the materials and pro-cesses used to make MEMS/NEMS devices,the role of mechanics in materials selection,process validation, design development,and device characterization can now be discussed.The remainder of this chapter will focus on the forces and phenomena dominant at the micrometer and nanometer scales,basic device charac-terization techniques,and mechanics issues that arise in MEMS/NEMS devices.

9.8The Influence of Scale

To gain perspective on the micrometer and nanometer size scales,consider that the diameter of human hair is 40–80μm and a DNA molecule is2–3nm wide.The weight of a MEMS structure can be about1nN and that of a NEMS components about10−20N.Compare this to the mass of a drop of water(10μN)or an eye-lash(100nN)[9.2].The minuscule size of forces that influence behavior at these small scales is hard to imag-ine.For instance,if you take a10cm length of your hair and hold it like a cantilever beam,the amount of force placed on the tip of the cantilever to deflect it by1cm is on the order of1pN.That piece of hair is 40–80μm in diameter,which is large compared to most MEMS/NEMS components.

In dealing with micro-and nanoscale devices,en-gineering intuition developed through experience with macroscale behavior is often misleading.It should be

noted that many macroscale techniques can be applied

at the micro-and nanoscales,but advantages come not

from miniaturization but rather working at the relevant

size scale using the uniqueness of the scale.The bal-

ance of forces at these scales differs dramatically from

the macroscale(Table9.6).Compared to a macroscale counterpart of the same aspect ratio,the structural stiff-

ness of a microscale cantilever increases relative to

inertially imposed loads.When the length scale changes

by a factor of a thousand,the area decreases by a fac-

tor of a million and the volume by a factor of a billion.

Surface forces,proportional to area,become a thou-

sand times large than forces that are proportional to

volume,thus inertial and electromagnet forces become negligible.At small scales,adhesion,friction,stiction

Part

A

9.8218Part A Solid Mechanics Topics

Table9.6Scaling laws and the relative importance of phenomena as they depend on linear dimension,i(after Madou[9.22])

Importance Phenomena Power of linear

at small scale dimension

Flow l4

Diminished Gravity l3

Inertial force l3

Magnetic force l2,l3or l4

Thermal emission l2or l4

Electrostatic force l2

Friction l2

Pressure l2

Piezoelectricity l2

Shape-memory l2

effect

Velocity l

Surface tension l

Increased Diffusion l1/2

van der Waal l1/4

force

(static friction),surface tension,meniscus forces,and viscous drag often govern.Acceleration of a small object becomes rapid.At the nanoscale,phenomena such as quantum effects,crystalline perfection,statis-tical time variation of properties,surface interactions, and interface interactions govern behavior and materials properties[9.35].

Additionally,the highly coupled nature of thermal transport properties at the microscale can be either an advantage or disadvantage depending on the device.En-hanced mass transport due to large surface-to-volume ratio can be a significant advantage for applications such as capillary electrophoresis and gas chromato-graphy.However,purging air bubbles in microfluidic systems can be extremely difficult due to capillary forces.The interfacial surface tension force will cause small bubble less than a few millimeters in diameter to adhere to channel surfaces because the mass of liquid in a capillary tube produces an insubstantial inertial force compared to the surface tension[9.10].

Some scaling effects favor particular micro-and nanoscale situations but others do not.For instance, large surface-to-volume ratio in MEMS devices can un-dermine device performance because of the retarding effects of adhesion and friction.However,electrostatic force is a good example of a phenomena that can have substantial engineering value at small scales.Transla-tional motion can be achieved in MEMS by electrostatic force because this scales as l2as compared to in-ertial force which scales as l3.Microactuation using electrostatic forces between parallel plates is used in comb drives,resonant microstructures,linear motors, rotary motors,and switches.In relation to MEMS test-ing,gripping of a tension sample can be achieved using an electrostatic force between a sample and the grip[9.74].

It is also important to note that,as the size scale decreases,breakdown in the predictions of continuum-based theories can occur at various length scales.In the case of electrostatics,electrical breakdown in the air gap between parallel plates separated by less than5μm does not occur at the predicted voltage[9.75].In optical devices,nanometer-scale gratings can produce an effec-tive refractive index different from the natural refractive index of the material because the grating features are smaller than the wavelength of light[9.76].For reso-nant structures continuum mechanics predictions break down when the structure’s dimensions are on the order of tens of lattice constants in cross section[9.11].De-tailed discussions of issues related to size scale can be found in Madou[9.22]and Trimmer[9.77].

9.8.1Basic Device Characterization

Techniques

A range of mechanical properties are needed to facili-tate design,predict allowable operating limits,and con-duct quality control inspection for MEMS.As with any macroscale device or component,structural integrity is critical to MEMS/NEMS.Concerns include fric-tion/stiction,wear,fracture,excessive deformation,and strength.Properties required for complete understand-ing of the mechanical performance of MEMS/NEMS materials include elastic modulus,strength,fracture toughness,fatigue strength,hardness,and surface to-pography.In MEMS devices the minimum feature size is on the order of1μm,which is also the natural length scale for microstructure(such as the grain size,disloca-tion length,or precipitate spacing)in most materials. Because of this,many of the mechanical properties of interest are size dependent,which precipitates the need for new testing methods given that knowledge of material properties is essential for predicting de-vice reliability and performance.A detailed discussion about micro-and nanoscale testing can be found in Part

B of this handbook as well as in references such as Sharpe[9.78],Srikar[9.79],Haque and Saif[9.80], Bhushan[9.2],and Yi[9.81].The following sections

Part A

9.8A Brief Introduction to MEMS and NEMS9.8The Influence of Scale219

provide a review of some mechanics issues that arise at the device level.The following sources,used as refer-ences for the discussion below,should be consulted for additional background on mechanics,metrology,and MEMS:Trimmer[9.77],Madou[9.22],Bhushan[9.2], and Gorecki[9.82].

9.8.2Residual Stresses in Films

Many MEMS/NEMS devices involve thinfilms of ma-terials.Properties of thin-film material often differ from their bulk counterparts due to the high surface-to-volume ratio of thinfilms and the influence of surface properties.Additionally,thesefilms must have good adhesion,low residual stress,low pinhole den-sity,good mechanical strength,and good chemical resistance[9.22].These properties often depend on de-position and processing details.

The stress state of a thinfilm is a combination of external applied stress,thermal stress,and intrinsic residual stress that may arise due to factors such as doping(in silicon),grain boundaries,voids,gas entrap-ment,creep,and shrinkage with curing(in polymeric materials).Stresses that develop during deposition of thin-film material can be either tensile or compressive and may give rise to cracking,buckling,blistering,de-laminating,and void formation,all of which degrade device performance.Residual stresses can arise be-cause of coefficient of thermal expansion mismatch, lattice mismatch,growth processes,and nonuniform plastic deformation.Residual stresses that do not cause mechanical failure may still significantly affect de-vice performance by causing warping of released structures,changes in resonant frequency of resonant structures,and diminished electrical characteristics.In some instances,however,residual stresses can be used productively,such as in shape setting of shape-memory alloyfilms or stress-modulated growth and arrangement of quantum dots.

There are numerous techniques for measuring resid-ual stresses in thinfilms.Fundamental techniques rely on the fact that stresses within afilm will cause bending in its substrate(tension causing concavity,compression causing convexity).Simple displacement measurements can be conducted on a circular disk or a micromachined beam and stress calculated from the radius of curvature of the bent substrate or the deflection of a cantilever. Strain gages may also be made directly in thefilm and used to make local measurements.Freestanding por-tions of the thinfilm can be created by micromachining so that thefilms stresses can be explored by applied pressure,external probe,critical length for buckling,

or resonant frequency measurements.For instance,the

critical stress to cause buckling in a doubly supported

beam can be estimated from:

σCR=E

π2t2

KL2

,

where K is a constant determined by the boundary con-

ditions(3for a doubly supported beam),E is Young’s modulus,t is the beam thickness,and L is the shortest

length of beam displaying buckling[9.10].The stress or

strain gradient over a region of afilm can be found by measuring deflections in a simple cantilever.The up-

ward or downward deflection along the length of the

beam can be measured by optical methods and used

to estimate the internal bending moment M from the expression:

δ(x)

x

=K+(1−ν

2)

2EI

Mx,

whereδ(x)is the vertical deflection at a distance x from

the support,E is Young’s modulus,νis Poisson’s ratio,

I is the moment of the beam cross section about the

axis of bending,and K is a constant determined by the

boundary conditions at the support[9.83].

A number of techniques have been developed for determining residual stresses including an American

Society for Testing and Materials(ASTM)standard in-

volving optical interferometry[9.84].The bulge test is

a basic technique for measuring residual stress in a free-

standing thinfilm[9.85].The bulge test structure can

be easily created by micromachining with well-defined

boundary conditions.The M-test is an on-chip test that

uses bending of an integrated free-standing prismatic

beam[9.86].The principle of an electrostatic actuator is

used to conduct the test tofind the onset of instability in

the structure.The wafer curvature test is regularly used

for residual stress measurement in nonintegratedfilm structures,and can be used even when thefilm thick-

ness is much smaller than the substrate thickness[9.87].

Dynamic testing can be used to measure resonant fre-

quency and extract information about residual stress and modulus.Resonant frequency increases with tension

and decreases in compression[9.88,].Air damping

can significantly impact theses measurements,however,

so they must be conducted in vacuum[9.18].Other es-

tablished techniques that can be employed to measure

residual stresses infilms include passive strain sensors,

Raman spectroscopy,and nanoindentation[9.79].

More recently,nanoscale gratings created by fo-

cused ion-beam(FIB)milling have been used to

Part

A

9.8220Part A Solid Mechanics Topics

produce moiréinterference between the grating on the specimen surface and raster scan lines of a scanning electron microscope(SEM)image[9.90].This tech-nique can be used to provide details of residual strains in microscale structures as they evolve with etching of the underlying sacrificial layer[9.52].Digital image corre-lation(DIC)has also been applied to SEM and atomic force microscopy(AFM)images in combination with FIB.DIC is used to capture deformationfields while nearby FIB milling of the specimens releases residual stresses,allowing very local evaluation[9.91].

9.8.3Wafer Bond Integrity

Wafer bonding is often an essential device fabrica-tion step,particularly for microfluidic devices,mi-croengines,and microscale heat exchangers.Although direct bonding of silicon can achieve strengths com-parable to bulk silicon,the process is sensitive to bonding parameters such as temperature and pressure. The appearance of voids and bubbles at the interface is particularly undesirable for both strength and elec-trical conductivity[9.92].An important nondestructive technique for assessing the bond quality of bonded silicon wafers is infrared transmission.At IR wave-lengths of about1.1μm silicon is transparent[9.1]. Quantification of bond strength can be conducted with techniques such as the pressure burst test,tensile/shear test,knife-edge test,or four-point bend-delamination test[9.93].

Although a range of techniques and processes can be employed to bond both similar and dissimilar ma-terials,the stresses and deformation of the wafers that develop are consistent.The residual stress stored in the bonded wafers is important because it may provide the elastic strain energy to drive fracture.Details of the wafer geometry can impact thefinal shape of the bonded pair and the integrity of the bond interface[9.94].

9.8.4Adhesion and Friction

Adhesion is both essential and problematic for MEMS/NEMS.For multilayered devices,good adhe-sion between layers is critical for overall performance and reliability,where delamination under repetitive ap-plied mechanical stresses must be avoided.Adhesion between material layers can be enhanced by improved substrate cleanliness,increased substrate roughness,in-creased nucleation sites during deposition,and addition of a thin adhesion-promoting layer.Standard tests for film adhesion include:the scotch-tape test,abrasion,scratching,deceleration using ultrasonic and ultracen-trifuge techniques,bending,and pulling[9.95].In situ testing of adhesion can also be conducted by pres-surizing the underside of afilm until initiation of delamination.This method also allows the determina-tion of the average work of adhesion.

Adhesion can be problematic if distinct components or a component and the nearby substrate come into con-tact,causing the device to fail.For example,although the mass in an accelerometer device is intended to be free standing at all points of operation,adhesion can occur in the fabrication process.Commonly with free-standing portions of MEMS structures,the capillary forces present during the drying of a device after etch-ing to remove sacrificial material are large enough to cause collapse of the structure and failure due to adhe-sion[9.96].To avoid this problem,supercritical drying is used.

Contacting surfaces that must move relative to one another in MEMS/NEMS are minimized or eliminated altogether,the reason being that friction and adhesion at these scales can overwhelm the other forces at play. Because silicon readily oxidizes to form a hydrophilic surface,it is much more susceptible to adhesion and accumulation of static charge[9.97].When contacting surfaces are involved,lubricantfilms and hydrophobic coatings with low surface energy can be applied to mini-mize wear and stiction(the large lateral force required to initiate relative motion between two surfaces).For instance,Analog Devices uses a nonpolar silicone coat-ing in its accelerometers to resist charge buildup and stiction[9.98].

Processing plays a major role in surface prop-erties such as friction and adhesion.Polishing will dramatically affect roughness,as in the case of polysili-con where roughness can be reduced by an order of magnitude from the as-deposited state[9.2].The doping process can also lead to higher roughness.Or-ganic monolayerfilms show promise for lubrication of MEMS to reduce friction and prevent wear.The atomic force microscope and the surface force apparatus used to quantify friction and MEMS test structures such as those developed at Sandia National Laboratory are aiding the development of detailed mechanics models addressing friction[9.99–102].

Flow Visualization

Flow in the microscale domain occurs in a range of MEMS devices,particularly in bioMEMS,microchan-nel networks,ink-jet printer heads,and micropropulsion systems.The different balance of forces at micro-

Part A

9.8

A Brief Introduction to MEMS and NEMS9.9Mechanics Issues in MEMS/NEMS221

scopic length scales can influencefluidflow to produce counterintuitive behavior in microscopicflows.Addi-tionally,the breakdown in continuum laws forfluid flow begins to occur at the microscale.For in-stance,the no-slip condition no longer applies and the friction factor starts to decrease with channel reduc-tion.

Particle image velocimetry(PIV)is a technique commonly used at macroscopic length scales to meas-ure velocityfields through the use of particles seeded in thefluid.The technique has been adapted to measure flowfields in microfluidic devices,where micron-scale spatial resolution is critical[9.103].Microparticle im-age velocimetry(μPIV)has been used to characterize such things as microchannelflow[9.104]and micro-fabricated ink-jet printer headflow[9.105].For the high-velocity,small-length-scaleflows found in mi-crofluidics,high-speed lasers and cameras are used in conjunction with a microscope to image the particles seeded in theflow.WithμPIV techniques,theflow boundary topology can be measured to within tens of nanometers[9.106].

9.9Mechanics Issues in MEMS/NEMS

9.9.1Devices

A wide range of MEMS/NEMS devices is discussed in the literature,both as research and commercialized devices.These devices are commonly planar in na-ture and employ structures such as cantilever beams,fixed–fixed beams,and springs that are loaded in bend-ing and torsion.A range of mechanics calculations are needed for device characterization,including the effec-tive stiffness of composite beams,deflection analysis of beams,modal analysis of a resonant structures,buck-ling analysis of a compressively loaded beams,fracture and adhesion analysis of structures,and contact me-chanics calculations for friction and wear of surfaces.

A substantial literature is available on the application of mechanics to MEMS/NEMS devices.The selected MEMS/NEMS examples presented below were chosen for their illustrative nature.

Digital Micromirror Device

Optical MEMS devices range from bar-code readers to fiber-optic telecommunication,and use a range of wide-band-gap materials,nonlinear electro-optic polymers, and ceramics[9.107].(See Walker and Nagel[9.108] for more information on optical MEMS.)A well-established commercial example of an optical MEMS device is the Digital Micromirror Device TM(DMD) by Texas Instruments used for projection display (Fig.9.6)[9.109].These devices have superior resolu-tion,brightness,contrast,and convergence performance compared to conventional cathode ray tube technol-ogy[9.2].The DMD contains a surface micromachined array of half a million to two million independently controlled,reflective,hinged micromirrors that have a mechanical switching time of15μs[9.110].This de-vice steers a reflected beam of light with each individual

mirrored aluminum pixel.Pixel motion is driven by an electrostaticfield between the yoke and an underlying electrode.The yoke rotates and comes to rest on me-

chanical stops and its position is restored upon release

by torsional hinge springs[9.111].

Almost all commercial MEMS structures avoid any

contact between structural members in the operation of

the device,and sliding contact is avoided completely

because of stiction,friction,and wear.The DMD is

currently the only commercial device where structural components come in and out of contact,with contact

occurring between the mirror spring tips and the under-

lying mechanical stops,which act as landing sites.To

prevent adhesion problems in the DMD,a self healing

perfluorodecanoic acid coating is used on the structural

aluminum components[9.112].

Other challenges for the DMD include creep and fa-

tigue behavior in the hinge,shock and vibration,and

sensitivity to debris within the package[9.2].The pri-

mary failure mechanisms are surface contamination and

hinge memory due to creep in the metallic alloy result-

ing in a residual tilt angle[9.1].Heat transfer,which contributes to the creep problem,is also an issue for mi-cromirrors.When the reflection coefficient is less than

100%some of the optical power is absorbed as heat and

can cause changes in theflatness of the mirror,dam-

age to the reflective layer,and alterations in the dynamic

behavior of the system[9.113].

Micromirrors for projection display involve rotat-

ing structures and members in torsion.Such torsional

springs must be well characterized and their mechan-

ics well modeled.For production devices extensive

finite element models are developed to optimize perfor-

mance[9.114].For initial design calculations however,

Part

A

9.9

222Part A Solid Mechanics Topics

some closed-form solutions for mechanics analysis can be employed.For instance,an appropriate material can be chosen or the basic dimensional requirements can be found from calculation of the maximum shear stress τmax in a beam of elliptical cross section in torsion (with a and b the semi-axis lengths)using:

τmax =2G αa 2b

a 2

+b 2

,for a >b ,where G is the shear modulus and αin the angular twist [9.107].Mechanical integrity of the DMD relies on low stresses in the hinge,thus the tilt angle is limited to ±10◦[9.1].

Biomolecular Recognition Device

Biological molecules can be probed by external meth-ods using techniques such as optical tweezers [9.116],a)

b)

CMOS substrate

Mirror –10 deg

Mirror +10 deg

Hinge

Y oke

Landing tip

Fig.9.6(a )SEM image of yoke and hinges of one pixel with mirror removed (b )Schematic of two tilted pixels with

mirrors (shown as transparent)(reprinted with permission,Hornbeck [9.111],Bhushan [9.2])

atomic force microscopy [9.117,118],and magnetic beads [9.119],but these techniques have the disad-vantage of requiring external probes,labeling,and/or optical excitation.Alternatively,there are several meth-ods using molecular recognition and the small-scale forces created by events such as DNA hybridization and receptor–ligand binding to produce bending in cantilevers to create sensors with high selectivity and resolution [9.115,120].

Microcantilever sensors have been used for some time to detect changes in relative humidity,temperature,pressure,flow,viscosity,sound,natural gas,mercury vapor,and ultraviolet and infrared radiation.More re-

Oligonucleotide

100μm

Fig.9.7array and schematics illustrating functionalized cantilevers with selective sensing capability (reprinted with permis-sion,Fritz [9.115])

Part A 9.9

A Brief Introduction to MEMS and NEMS 9.9Mechanics Issues in MEMS /NEMS 223

cently micromachined cantilevers have been used to interact and probe material at the molecular level.De-vices employing these micromachined cantilevers can be dynamic,which are sensitive to mass changes down to 10–21g (the single molecule level),or static,which are sensitive to surface stress changes in the low mN /m range (changes in Gibbs free energy caused by binding site-analyte interactions)[9.121].In this case adhesion is required between the device and the material to be detected.

In a functionalized cantilever array device produced to measure biomechanical forces created by DNA hy-bridization or receptor–ligand binding,detection of the mass change is accomplished by measuring a shift in resonant frequency.The responsiveness of the device to a change in mass is given by the expression:Δm ≈2

M eff

ω0

Δωwhere M eff is the effective vibratory mass of the resonator,and ω0is the resonance frequency of the de-vice [9.11].The mass sensitivity of NEMS devices with micromachined cantilevers can be as small as a single small molecule (in the range of a single Dalton).

In a device such as that shown in Fig.9.7,a li-quid medium,which contains molecules that dock to a layer of receptor molecules attached to one side of the cantilever,is injected into the device.Sensi-tizing an array of cantilevers with different receptor allows docking of different substances in the same so-lution [9.115].Hybridization can be done with short strands of single-stranded DNA and proteins

known

to recognize antibodies.When docking occurs,the in-crease in the molecular packing density leads to surface stress,causing bending (10–20nm of deflection).This deflection can be measured by a laser beam reflected off of the end of the cantilever [9.115].Alternatively,sim-ple geometric interference by interdigitated cantilevers that act as diffraction gratings can be used to provide output of a binding event [9.120].

Thermomechanical Data Storage Device

Much of the drive to nanometer-scale devices orig-inates in the desire for higher density and faster computational devices.Magnetic data storage has been pushed into the nanoscale regime,but limitations have prompted the development of alternative methods for data storage such as the NEMS device known as the Millipede,developed by IBM.The Millipede,or scanning probe array memory device,is an array of individually addressable scanning probe tips (similar to atomic force microscope probe tips)that makes precisely positioned indentations in a polymer thin film.The Millipede is scanned to address a large area for data storage.The indentations are bits of digital information.A polymer thin film (50nm thick)of poly-methyl methacrylate (PMMA )is used for write,read,erase,and rewrite operations.Each individual bit is a nanoscale feature,which allows the Millipede to ex-tend storage density to the Tbit /in 2range with a bit size of 30–40nm [9.123].The device uses multiple cantilever probe tips equipped with integrated heaters which allow for data transfer rates of up to a few Mb /s [9.124].

Part A 9.9

224Part A Solid Mechanics Topics

Substrate Polymer Cantilever

Sensing current

Inscribed pins

Data stream 1

Output signal

0011111010001010100101110

Write current

Erasure current

Pit:

25nm deep 40nm wide (maximum)

The Millipede device is a massively parallel struc-ture with a large array of thousands of probe tips (100cantilevers /mm 2),each of which is able to address a region of the substrate where it produces indenta-tions for use as data storage bits (Fig.9.8)[9.125].As illustrated in Fig.9.9,the probes writes a bit by heating and a mechanical force applied between a can-

Fig.9.9Schematic of the writing,erasing,and reading op-

erations in the Millipede device.Data are mechanically stored in pits on a surface (reprinted with permission,Vet-tiger [9.127])

tilever tip and a polymer film.Erasure of a bit is also conducted with heating by placing a small pit just ad-jacent to the bit to be erased or using the spring back of the polymer when a hot tip is inserted into a pit.Reading is also enabled by heat transfer since the sens-ing relies on a thermomechanical sensor that exploits temperature-dependent resistance [9.126].The change in temperature of a continuously heated resistor is mon-itored while the tip is scanned over the film and relies on the change in resistance that occurs when a tip moves into a bit [9.123].

Scanning x ,y manipulation is conducted magnet-ically with the entire array at once.The data storage substrate is suspended above the cantilever array with leaf springs which enables the nanometer-scale scan-ning tolerances required.The cantilevers are precisely curved using residual stress control of a silicon nitride layer in order to minimize the distance between the heating platform of the cantilever and the polymer film while maximizing the distance between the cantilever array and the film substrate to ensure that only the tips come into contact [9.123].Fabrication details are given in Despont [9.125].

Thermal expansion is a major hurdle for this device since a shift of ≈30nm can cause misalignment of the data storage substrate and the cantilever array.A 10nm tip position accuracy of a 3mm ×3mm silicon area re-quires that temperature of the device be controlled to 1◦C using several sensors and heater elements [9.123].Tip wear due to contact between the tip and the under-lying silicon substrate is an issue for device reliability.Additionally,the PMMA is prone to charring at the temperatures necessary for device operation (around 350◦C)[9.128]so new polymeric formulations had to be developed to minimize this problem.The feasibility of using thin-film NiTi shape-memory alloy (SMA )for thermomechanical data storage as an alternative to the polymer thin film has also been shown [9.129].

9.10Conclusion

The sensors,actuators,and passive structures devel-oped as MEMS and NEMS devices require a highly interdisciplinary approach to their analysis,design,de-velopment,and fabrication.Experimental mechanics plays a critical role in design development,mater-ials selection,prediction of allowable operating lim-

Part A 9.10

its,device characterization,process validation,and quality control inspection.Commercial devices ex-ist,and research in the area of MEMS/NEMS is extremely active,but many challenges remain.Ad-vanced materials must be well characterized and MEMS/NEMS testing must be further developed.This chapter has provided a brief review of the fabrication processes and materials commonly used and exper-imental mechanics as it is applied to MEMS and NEMS.

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Part A

9