Bhushan B. Nanotribology and Nanomechanics: An Introduction

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21 Micro/Nanotribology and Micro/Nanomechanics of Magnetic Storage Devices 1193

0 50 75 10025 125

0.15

0.10

0.05

70 nN, 0.4 μm/s, 22°C, RH 45– 55 %

Friction force (nN)

Si(100)

Z-DOL(BW)

Z-15

Number of cycles

Coefficient of friction

Si(100)

Z-DOL(BW)

Z-15

Increasing scan number

Z-15

1194 Bharat Bhushan

in ﬁlm thickness. Carbon coatings deposited by cathodic arc and ECR-CVD pro-

cesses are superior in wear and mechanical properties followed by ion beam and

sputtering processes. AFM has been modiﬁed for nanoindentation hardness meas-

urements with depth of indentation as low as 5nm. Scratching and indentation on

nanoscales are the powerful ways of evaluation of the mechanical integrity of ultra-

thin ﬁlms.

AFM/FFM friction experiments show that lubricants with polar (reactive) end

groups dramatically increase the load or contact pressure that a liquid ﬁlm can sup-

port before solid-solid contact and thus exhibit long durability. The lubricants with

the absence of mobile liquid exhibit low friction and adhesion and don’t exhibit rest

time eﬀect.

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1196 Bharat Bhushan

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thin amorphous carbon coatings. J. Mater. Res., 14:2328–2337, 1999.

52. X. Li and B. Bhushan. Mechanical and tribological studies of ultra-thin hard carbon

overcoats for magnetic recording heads. Z. Metallk., 90:820–830, 1999.

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carbon coatings using atomic force/friction force microscopy. Wear, 225-229:678–689,

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21 Micro/Nanotribology and Micro/Nanomechanics of Magnetic Storage Devices 1197

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Nanotribology and Materials Characterization

of MEMS/NEMS and BioMEMS/BioNEMS

Materials and Devices

Bharat Bhushan

Summary. Micro/nanoelectromechanical systems (MEMS/NEMS) need to be designed

to perform their expected functions with short duration, typically in millisecond to pi-

cosecond timescales. Expected life of the devices for high-speed contacts can vary from

a few hundred thousand to many billions of cycles, e.g., over a hundred billion cycles

for digital micromirror devices (DMDs), which puts serious requirements on materials. For

BioMEMS/BioNEMS, adhesion between biological molecular layers and the substrate, and

friction and wear of biological layers may be important. There is a need for the devel-

opment of a fundamental understanding of adhesion, friction/stiction, wear, and the role

of surface contamination, and environment. Most mechanical properties are known to be

scale-dependent. Therefore, the properties of nanoscale structures need to be measured.

MEMS/NEMS materials need to exhibit good mechanical and tribological properties on the

micro/nanoscale. There is a need to develop lubricants and identify lubrication methods that

are suitable for MEMS/NEMS. Methods need to be developed to enhance adhesion between

biomolecules and the device substrate. Component-level studies are required to provide a bet-

ter understanding of tribological phenomena occurring in MEMS/NEMS. The emergence

of micro/nanotribology and techniques based on atomic force microscopy has provided re-

searchers with a viable approach to address these problems. This chapter presents a review

of micro/nanoscale adhesion, friction, and wear studies of materials and lubrication stud-

ies for MEMS/NEMS and BioMEMS/BioNEMS, and component-level studies of stiction

phenomena in MEMS/NEMS devices.

22.1 Introduction

Microelectromechanical systems (MEMS) refers to microscopic devices that have

a characteristic length of less than 1 mm but more than 100nm and combine elec-

trical and mechanical components. Nanoelectromechanical systems (NEMS) re-

fer to nanoscopic devices that have a characteristic length of less than 100 nm

and combine electrical and mechanical components. In mesoscale devices, if the

functional components are on the micro- or nanoscale, they may be referred to

as MEMS or NEMS, respectively. These are referred to as an intelligent minia-

turized system consisting of sensing, processing, and/or actuating functions and

1200 Bharat Bhushan

combining electrical and mechanical components. The acronym MEMS originated

in the U.S.A. The termcommonlyused in Europeis microsystemtechnology(MST)

and in Japan is micromachines. Another term generally used is micro/nanodevices.

The terms MEMS/NEMS are also now used in a broad sense and include elec-

trical, mechanical, ﬂuidic, optical, and/or biological functions. MEMS/NEMS for

optical applications are referred to as micro/nano-optoelectromechanical systems

(MOEMS/NOEMS). MEMS/NEMS for electronic applications are referred to as

radio-frequency MEMS/NEMS (RF-MEMS/RF-NEMS). MEMS/NEMS for bio-

logical applications are referred to as BioMEMS/BioNEMS.

To put the dimensionsof MEMS and NEMSin perspective,see Fig. 22.1 andTa-

ble 22.1. Individual atoms are typically a fraction of a nanometer in diameter, de-

oxyribonucleicacid (DNA) molecules are about 2.5nm wide, biological cells are in

the range of thousands of nm in diameter, and human hair is about 75µmindiam-

eter. NEMS shown in the ﬁgure have a size of 15–300nm and MEMS have a scale

of 12,000nm. The mass of a micromachined silicon structure can be as low as 1nN

and NEMS can be built with mass as low as 10

−20

N with cross sections of about

10nm. In comparison, the mass of a drop of water is about 10 µNandthemassof

an eyelash is about 100nN.

Size (nm)

0.1 1 10 100 1000 10 000 100000

Human hair

50–100

μm

DMD 12 μm

Red blood cell

μm

Quantum-dot transistor

300 nm

Drain

Molecular gear

10 nm–100 nm

DNA 2.5nm

C atom 0.16 nm

Yoke

Landing

tip

MEMS: Characteristic length less than 1 mm,

larger than 100 nm

NEMS: Less than 100

SWCNT transistor

15 nm

Fig. 22.1. Dimensions of MEMS and NEMS in perspective. MEMS/NEMS examples shown

are of a vertical single-walled carbon nanotube (SWCNT) transistor (5 nm wide and 15 nm

high) [7], of molecular dynamic simulations of a carbon-nanotube-based gear [8], quantum-

dot transistor obtained from van der Wiel et al. [9], and DMD obtained from www.dlp.com

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1201

Table 22.1. Dimensions and masses in perspective

(a) Dimensions in perspective

NEMS characteristic length: < 100 nm

MEMS characteristic length: < 1mmand> 100 nm

Molecular gear ≈ 10 nm

Vertical SWCNT transistor ≈ 15 nm

Quantum-dot transistor 300 nm

Digital micromirror 12,000 nm

Individual atoms typically a fraction of a nm in diameter

DNA molecules ≈ 2.5nmwide

Biological cells in the range of thousands of nm in diameter

Human hair ≈ 75,000 nm in diameter

(b) Masses in perspective

NEMS built with cross sections of about 10 nm – as low as 10

−20

Micromachines silicon structure – as low as 1 nN

Water droplet ≈ 10 µN

Eyelash ≈ 100 nN

Micro/nanofabrication techniques include top-down methods, in which one

buildsdownfrom thelargeto the small, andbottom-upmethods,in whichone builds

up from the small to the large. Top-down methods include micro/nanomachining

methods and methods based on lithography as well as non-lithographic miniatur-

ization, mostly for the fabrication of MEMS and a few NEMS. In bottom-up meth-

ods, also referred to as nanochemistry, the devices and systems are assembled from

their elemental constituents for NEMS fabrication, much like the way nature uses

proteins and other macromolecules to construct complex biological systems. The

bottom-up approach has the potential to go far beyond the limits of top-down tech-

nologyby producingnanoscalefeatures throughsynthesisand subsequentassembly.

Furthermore,the bottom-up approach oﬀers the potential to produce structures with

enhanced and/or completely new functions. It allows combination of materials with

distinct chemical composition, structure, and morphology. For a brief overview of

fabrication techniques, see Sect. 22.A.

MEMS and emerging NEMS are expected to have a major impact on our lives,

comparable to that of semiconductor technology, information technology, or cel-

lular and molecular biology [1–4]. MEMS/NEMS are used in electromechani-

cal, electronics, information/communication,chemical,and biologicalapplications.

The MEMS industry in 2004 was worth about $4.5 billion, with a projected annual

growth rate of 17% [5]. Growth of Si-based MEMS may slow down and non-silicon

MEMS may pick up during this decade. The NEMS industry was worth about $10

billion in 2004, mostly in nanomaterials [6]. It is expected to expand in this decade,

in nanomaterials,biomedical applicationsas well as in nanoelectronicsor molecular

electronics. Due to the enabling nature of these systems and because of the signiﬁ-

cant impact they can have on both commercial and defense applications, industry as

1202 Bharat Bhushan

well as federal governmentshave taken special interest in seeing growth nurtured in

this ﬁeld. MEMS/NEMS are the next logical step in the silicon revolution.

22.1.1 Introduction to MEMS

Advancesin silicon photolithographic processtechnology since the 1960s led to the

development of MEMS in the early 1980s. More recently, lithographic processes

have also been developed to process non-silicon materials. The lithographic pro-

cesses are being complemented with non-lithographic processes for fabrication of

componentsor devices made from plastics or ceramics. Using these fabrication pro-

cesses, researchers have fabricated a wide variety of devices with dimensions in the

range of submicronto a fewthousandmicrons (see e.g., [10–20]). MEMSfor mech-

anical applications include acceleration, pressure, ﬂow, and gas sensors, linear and

rotary actuators, and othermicrostructuresor microcomponentssuch as electricmo-

tors, gear chains, gas turbine engines, ﬂuid pumps, ﬂuid valves, switches, grippers,

and tweezers. MEMS for chemical applications include chemical sensors and var-

ious analytical instruments. MOEMS are devices that include optical components,

such as micromirror arrays for displays, infrared image sensors, spectrometers, bar-

code readers, and optical switches. RF-MEMS include inductors, capacitors, an-

tennas, and RF switches. High-aspect-ratio MEMS (HARMEMS) have also been

introduced.

A variety of MEMS devices have been produced and some are used commer-

cially [11,13–16,18–20]. A variety of sensors are used in industrial, consumer, de-

fense, and biomedical applications. The largest killer industrial applicationsinclude

accelerometers, pressure sensors, thermal and piezoelectric-type inkjet printheads,

and digital micromirror devices. Integrated capacitive-type silicon accelerometers

have been used in airbag deployment in automobiles since 1991 [21,22]. Some 90

million units were installed in vehicles in 2004. Accelerometer technology was over

a billion-dollar-a-year industry in 2004, dominated by Analog Devices followed by

Freescale Semiconductor (formerlyMotorola) and Bosch. These accelerometers are

being used for many other applications such as vehicle stability, rollover control,

and gyro sensors for automotive applications, and various consumer applications,

including handheld devices, e.g., laptops (2003), cellular phones (2004), and per-

sonal digital assistants (PDAs) Silicon-based piezoresistive pressure sensors were

launched in 1990 by GE NovaSensorfor manifold absolute pressure(MAP) sensing

for engines and for disposable blood-pressure sensors, and their annual sales were

more than 30 million units and more than 25 million units, respectively, in 2004.

MPA sensors measurethe pressurein theintakemanifold,whichis fed toa computer

that determines the optimum air/fuel mixture to maximize fuel economy. Most ve-

hicles havethese as part ofthe electronicengine-controlsystem. Capacitivepressure

sensors for tire-pressure measurements were launched by Freescale Semiconductor

(formerlyMotorola) in early 2000 and are also manufacturedby Inﬁneon/SensoNor

and GE Novasensor (2003).Piezoresistive-typesensors are also used, manufactured

by various companies such as EnTire Solutions (2003). The sensing module is lo-

cated inside the rim of the wheel and relays the information via radio-frequency

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1203

link to a central processing unit (CPU) in order to display it to the driver. In 2005,

about 9.2 million vehicles were equipped with sensors, which translates to about

37 million units. These sales are expected to grow rapidly as they become required

in automobiles. They will be required in the U.S. by 2008, which will aﬀect 17

million vehicles (one in each tire) sold every year. Thermal inkjet printers were de-

velopedindependentlyby HP and Cannon and commercializedin 1984 [23–26]and

today are made by Canon, Epson, HP, Lexmark, Xerox, and others. They typically

cost less initially than dry-toner laser printers and are the solution of choice for

low-volume print runs. Annual sales of thermal inkjet printheads with microscale

functional components were more than 500 million units in 2004.

Micromirror arrays are used for displays. Commercial digital light processing

(DLP) equipments, using digital micromirror devices (DMD), were launched in

1996 by Texas Instruments for digital projection displays in computer projectors,

high-deﬁnitiontelevision (HDTV)sets andmovie projectors(DLP cinema)[27–29].

Several million projectors had been sold up to 2004 (about $700 million rev-

enue for TI in 2004). Electrostatically actuated, membrane-type or cantilever-type

microswitches have been developed for direct current (DC), RF, and optical ap-

plications [30]. There exists two basic forms of RF microswitches: the metal-

to-metal contact microswitch (ohmic) and the capacitive microswitch. RF mi-

croswitches can be used in a variety of RF applications including cellular phones,

phase shifters, smart antennas, multiplexers for data acquisition, and more [31].

Optical microswitchesare ﬁnding applications in optical networking,telecommuni-

cations, and wireless technologies [30,32].

Other applications of MEMS devices include chemical/biological and gas sen-

sors [20,33], microresonators,infrared detectorsand focal-planearrays for earth ob-

servation, space science and missile defense applications, pico-satellites for space

applications, fuel cells, and many hydraulic, pneumatic, and other consumer prod-

ucts. MEMSdevicesare also being pursuedin magneticstorage systems [34],where

they are being developed for super-compact ultrahigh-recording-density magnetic

disk drives. Several integrated head/suspension microdevices have been fabricated

for contact recording applications [35]. High-bandwidth servo-controlled microac-

tuators have been fabricated for ultrahigh-track-density applications which serve as

the ﬁne-position control element of a two-stage, coarse/ﬁne servo system, coupled

with a conventional actuator [36,37].

Micro/nano-instruments and micro/nano-manipulators are used to move, posi-

tion, probe, pattern, and characterize nanoscale objects and nanoscale features [38].

Miniaturized analytical equipments include gas chromatography and mass spec-

trometry. Other instruments include micro-STM, where STM stands for scanning

tunneling microscope.

In some cases, MEMS devices are used primarily for their miniature size, while

in others, as in the case of the air bags, because of their low-cost manufacturing

techniques. This latter fact has been possible since semiconductor processing costs

have reduced drastically over the last decade, allowing the use of MEMS in many

ﬁelds.