Handbook of Micro and Nano Tribology P16

Bhushan, B. “Micro/Nanotribology and Micro/Nanomechanics of MEMS...”
Handbook of Micro/Nanotribology.
Ed. Bharat Bhushan
Boca Raton: CRC Press LLC, 1999

© 1999 by CRC Press LLC

16

Micro/Nanotribology
and
Micro/Nanomechanics

of MEMS Devices

Bharat Bhushan

16.1 Introduction

Background • Tribological Issues

16.2 Experimental Techniques

Description of Apparatus and Test Procedures • Test Samples

16.3 Results and Discussion

Micro/Nanotribological Studies of Virgin, Coated, and Treated
Silicon Samples • Micro/Nanotribological Studies of Doped
and Undoped Polysilicon Films, SiC Films, and Their
Comparison to Single-Crystal Silicon • Macroscale

Tribological Studies of Virgin, Coated, and Treated Samples •
Boundary Lubrication Studies • Component Level Studies

16.4 Closure
References

16.1 Introduction

16.1.1 Background

The advances in silicon photolithographic process technology since 1960s have led to the development
of microcomponents or microdevices, known as microelectromechanical systems (MEMS). More
recently, lithographic processes have been developed to process nonsilicon materials. These lithographic
processes are being complemented with nonlithographic micromachining processes for fabrication of
milliscale components or devices. Using these fabrication processes, researchers have fabricated a wide
variety of miniaturized devices, such as acceleration, pressure and chemical sensors, linear and rotary
actuators, electric motors, gear trains, gas turbines, nozzles, pumps, fluid valves, switches, grippers,
tweezers, and optoelectronic devices with dimensions in the range of a couple to a few thousand microns
(for an early review, see Peterson, 1982; for recent reviews, see Muller et al., 1990; Madou, 1997; Trimmer,
1997; and Bhushan, 1998a). MEMS technology is still in its infancy and the emphasis to date has been
on the fabrication and laboratory demonstration of individual components. MEMS devices have begun

© 1999 by CRC Press LLC

to be commercially used, particularly in the automotive industry. Silicon-based high-

g

acceleration
sensors are used in airbag deployment (Bryzek et al., 1994). Acceleration sensor technology is slightly

less than a billion-dollar-a-year industry dominated by Lucas NovaSensor and Analog Devices. Texas
Instruments uses deformable mirror arrays on microflexures as part of airline-ticket laser printers and
high-resolution projection devices.
Potential applications of MEMS devices include silicon-based acceleration sensors for anti-skid braking
systems and four-wheel drives, silicon-based pressure sensors for monitoring pressure of cylinders in
automotive engines and of automotive tires, and various sensors, actuators, motors, pumps, and switches
in medical instrumentation, cockpit instrumentation, and many hydraulic, pneumatic, and other con-
sumer products (Fujimasa, 1996). MEMS devices are also being pursued in magnetic storage systems
(Bhushan, 1996a), where they are being developed for supercompact and ultrahigh-recording-density
magnetic disk drives. Horizontal thin-film heads with a single-crystal silicon substrate, referred to as
silicon planar head (SPH) sliders are mass-produced using integrated-circuit technology (Lazarri and
Deroux-Dauphin, 1989; Bhushan et al., 1992). Several integrated head/suspension microdevices have
been fabricated for contact recording applications (Hamilton, 1991; Ohwe et al., 1993). High-bandwidth
servo-controlled microactuators 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 (Miu and Tai, 1995; Fan et al., 1995b). Millimeter-sized wobble motors and actu-
ators for tip-based recording schemes have also been fabricated (Fan and Woodman, 1995a). 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 high reliability and 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 previously impractical fields.
The fabrication techniques for MEMS devices employ photolithography and fall into three basic
categories: bulk micromachining, surface micromachining, and LIGA a German acronym (Lithographie
Galvanoformung Abformung) for lithography, electroforming, and plastic molding. The first two
approaches, bulk and surface micromachining, use planar photolithographic fabrication processes devel-
oped for semiconductor devices in producing two-dimensional (2D) structures (Jaeger, 1988; Madou,
1997; Bhushan, 1998a). Bulk micromachining employs anisotropic etching to remove sections through
the thickness of a single-crystal silicon wafer, typically 250 to 500 µm thick. Bulk micromachining is a
proven high-volume production process and is routinely used to fabricate microstructures such as
acceleration and pressure sensors and magnetic head sliders. Surface micromachining is based on depos-

iting and etching structural and sacrificial films to produce a free-standing structure. These films are
typically made of low-pressure chemical vapor deposition (LPCVD) polysilicon film with 2 to 20 µm
thickness. Surface micromachining is used to produce surprisingly complex micromechanical devices
such as motors, gears, and grippers. LIGA is used to produce high-aspect ratio (HAR) MEMS devices
that are up to 1 mm in height and only a few microns in width or length (Becker et al., 1986). The LIGA
process yields very sturdy 3D structures due to their increased thickness. The LIGA process is based on
the combined use of X-ray photolithography, electroforming, and molding processes. One of the limi-
tations of silicon microfabrication processes originally used for fabrication of MEMS devices is lack of
suitable materials which can be processed. With LIGA, a variety of nonsilicon materials such as metals,
ceramics and polymers can be processed. Nonlithographic micromachining processes, primarily in
Europe and Japan, are also being used for fabrication of millimeter-scale devices using direct material
microcutting or micromechanical machining (such as micromilling, microdrilling, microturning) or
removal by energy beams (such as microspark erosion, focused ion beam, laser ablation, and machining,
and laser polymerization) (Friedrich and Warrington, 1998; Madou, 1998). Hybrid technologies including
LIGA and high-precision micromachining techniques have been used to produce miniaturized motors,
gears, actuators, and connectors (Lehr et al., 1996, 1997; Michel and Ehrfeld, 1998). These millimeter-
scale devices may find more immediate applications.

© 1999 by CRC Press LLC

Silicon-based MEMS devices lack high-temperature capabilities with respect to both mechanical and
electrical properties. Recently, researchers have been pursuing SiC as a material for high-temperature
microsensor and microactuator applications (Tong et al., 1992; Shor et al., 1993). SiC is a likely candidate
for such applications since it has long been used in high-temperature electronics, high-frequency and
high-power devices, such as SiC metal–semiconductor field effect transistors (MESFETS) (Spencer et al.,
1994) and inversion-mode metal-oxide-semiconductor field effect transistors (MOSFETS). Many other
SiC devices have also been fabricated including ultraviolet detectors, SiC memories, and SiC/Si solar cells.
SiC has also been used in microstructures such as speaker diaphragms and X-ray masks. For a summary
of SiC devices and applications, see Harris (1995). Table 16.1 compares selected bulk properties of SiC
and Si(100). Because of the large band gap of SiC, almost all devices fabricated from SiC have good high-

temperature properties. This high-temperature capability of SiC combined with its excellent mechanical
properties, thermal dissipative characteristics, chemical inertness, and optical transparency makes SiC
an ideal choice for complementing polysilicon (polysilicon melts at 1400°C) in MEMS devices. Since
MEMS devices need to be of low cost to be viable in most applications, researchers have found low-cost
techniques of producing single-crystal 3C-SiC (cubic or

β

-SiC) films via epitaxial growth on large area
silicon substrates (Zorman et al., 1995). This technique allows high-volume batch processing and has the
advantage of having silicon as the substrate, an inexpensive material for which microfabrication and
micromachining technologies are well established. It is believed that these films will be well suited for
MEMS devices.

16.1.2 Tribological Issues

In MEMS devices, various forces associated with the device scale down with the size. When the length
of the machine decreases from 1 mm to 1 µm, the area decreases by a factor of a million and the volume
decreases by a factor of a billion. The resistive forces such as friction, viscous drag, and surface tension
that are proportional to the area, increase a thousand times more than the forces proportional to the
volume, such as inertial and electromagnetic forces. The increase in resistive forces leads to tribological
concerns, which become critical because friction/stiction (static friction), wear and surface contamination
affect device performance and in some cases, can even prevent devices from working.
Examples of two micromotors using polysilicon as the structural material in surface micro-
machining — a variable capacitance side drive and a wobble (harmonic) side drive — are shown in
Figures 16.1 and 16.2, which can rotate up to 100,000 rpm. Microfabricated variable-capacitance side-
drive micromotor with 12 stators and a 4-pole rotor shown in Figure 16.1 is produced using a three-
layer polysilicon process and the rotor diameter is 120 µm and the air gap between the rotor and stator
is 2 µm (Tai et al., 1989). It is driven electrostatically to continuous rotation (by electrostatic attraction
between positively and negatively charged surfaces). The intermittent contact at the rotor–stator interface

and physical contact at the rotor–hub flange interface result in wear issues, and high stiction between
the contacting surfaces limits the repeatability of operation or may even prevent the operation altogether.
Figure 16.2 shows the SEM micrograph of a microfabricated harmonic side-drive (wobble) micromotor

TABLE 16.1

Selected Bulk Properties

a

of 3C (

β

- or cubic) SiC and Si(100)

Sample
Density
(kg/m

3

)
Hardness
(GPa)
Elastic
Modulus
(GPa)
Fracture
Toughness

(MPa m

1/2

)
Thermal
Conductivity

b


(W/m K)
Coeff. of
Thermal
Expansion

b


(

×

10

–6

/°C)
Melting
Point (°C)

Band-Gap
(eV)

β

–SiC 3210 23.5–26.5 440 4.6 85–260 4.5–6 2830 2.3
Si(100) 2330 9–10 130 0.95 155 2–4.5 1410 1.1

a

Unless stated otherwise, data shown were obtained from Bhushan and Gupta (1997).

b

Obtained from Shackelford et al. (1994).

© 1999 by CRC Press LLC

(Mehregany et al., 1988). In this motor, the rotor wobbles around the center bearing post rather than
the outer stator. Again friction/stiction and wear of rotor-center bearing interface are of concern. There
is a need for development of bearing/bushing materials that are both compatible with MEMS fabrication
processes and which provide superior friction and wear performance. Monolayer lubricant films are also
of interest. Figure 16.3 shows the SEM micrograph of an air turbine with gear or blade rotors, 125 to
240 µm in diameter, fabricated using polysilicon as the structural material in surface micromachining.
The two flow channels on the top are connected to the two independent input ports and the two flow
channels at the bottom are connected to the output port. Wear at the contact of gear teeth is a concern.
In microvalves used for flow control, the mating valve surfaces should be smooth enough to seal while

FIGURE 16.1

(a) SEM micrograph, and (b) schematic cross-section of a variable capacitance side-drive micromotor
fabricated of polysilicon film. (From Tai et al., 1989,

Sensors Actuators

A21–23, 180–83. With permission.)

FIGURE 16.2

SEM micrograph of a harmonic side-drive (wobble) micromotor. (From Mehregany, M. et al., 1990,
in

Proc. IEEE Micro Electromechanical Systems,

pp. 1–8, IEEE, New York. With permission.)

© 1999 by CRC Press LLC

maintaining a minimum roughness to ensure low friction/stiction (Bhushan, 1996a, 1998b). Studies have
been conducted to measure the friction/stiction in micromotors (Tai and Muller, 1990), gear systems
(Gabriel et al., 1990) and polysilicon microstructures (Lim et al., 1990) to understand friction mechanisms.
Several studies have been conducted to develop solid and liquid lubricant and hard films to minimize
friction and wear (Bhushan et al., 1995b; Deng et al., 1995; Beerschwinger et al., 1995; Koinkar and
Bhushan, 1996a,b; Bhushan, 1996b; Henck, 1997).
In a silicon planar head slider for magnetic disk drives shown in Figure 16.4, wear and friction/stiction
are an issue because of the close proximity between the slider and disk surfaces during steady operation
and continuous contacts during start and stops (Lazzari and Deroux-Dauphin, 1989; Bhushan et al.,
1992). Hard diamondlike carbon (DLC) coatings are used as an overcoat for protection against corrosion
and wear. Two electrostatically driven rotary and linear microactuaters (surface-micromachined, poly-
silicon microstructure) for a magnetic disk drive shown in Figure 16.5, consist of a movable plate

connected only by springs to a substrate, on which there are two sets of mating interdigitated electrodes
which activate motion of the plate in opposing directions. Any unintended contacts may result in wear
and stiction.
Figure 16.6 shows an SEM micrograph of a micromechanical switch (Peterson, 1979). As the voltage
is applied between the deflection electrode and the p

+

ground plane, the cantilever beam is deflected and
the switch closes, connecting the contact electrode and the fixed electrode; wear during contact is of
concern. Figure 16.7 shows an SEM micrograph of a pair of tongs (Mehregany et al., 1988). The jaws
open when the linearly sliding handle is pushed forward, demonstrating the linear slide and the linear-
to-rotary motion conversion; for this pair of tongs, the jaws open up to 400 µm in width. Wear at the
teeth is of concern.
As an example of nonsilicon components, Figure 16.8a shows a DC brushless permanent magnet
millimotor (diameter = 1.9 mm, length = 5.5 mm) with an integrated milligear box which is produced
with parts obtained by hybrid fabrication processes including the LIGA process, micromechanical
machining, and microspark erosion techniques (Lehr et al., 1996, 1997; Michel and Ehrfeld, 1998). The
motor can rotate up to 100,000 rpm and deliver a maximum torque of 7.5 µNm. The rotor, supported
on two ruby bearings, consists of a tiny steel shaft and a diametrically magnetized rare earth magnet.
The rotational speed of the motor can be converted by the use of a milligear box to increase the torque
for a specific application. Gears are made of metal (e.g., electroplated Ni–Fe) or injected polymer materials
(e.g., POM) using the LIGA process, Figures 16.8b and c. Optimum materials and liquid and solid
lubrication approaches for bearings and gears are needed.

FIGURE 16.3

SEM micrograph of a gear train with three meshed gears, in an air turbine. (From Mehregany, M.
et al., 1988,

IEEE Trans. Electron Devices

35, 719–723. With permission.)

© 1999 by CRC Press LLC

There are tribological issues in the fabrication processes as well. For example, in surface microma-
chining, the suspended structures can sometimes collapse and permanently adhere to the underlying
substrate, Figure 16.9 (Guckel and Burns, 1989). The mechanism of such adhesion phenomena needs to
be understood (Mastrangelo, 1997).
Friction/stiction and wear clearly limit the lifetimes and compromise the performance and reliability
of microdevices. Since microdevices are designed to small tolerances, environmental factors, surface
contamination, and environmental debris affect their reliability. There is a need for development of a
fundamental understanding of friction/stiction, wear, and the role of surface contamination and envi-
ronment in microdevices (Bhushan, 1998a). A few studies have been conducted on the tribology of bulk
silicon and polysilicon films used in microdevices (Bhushan and Venkatesan, 1993a,b; Gupta et al., 1993;
Venkatesan and Bhushan, 1993, 1994; Gupta and Bhushan, 1994; Bhushan and Koinkar, 1994; Bhushan,
1996b). Mechanical properties of polysilicon films are not well characterized (Mehregany et al., 1987;
Ericson and Schweitz, 1990; Schweitz, 1991; Guckel et al., 1992; Bhushan, 1995; Fang and Wickert, 1995).
The advent of atomic force/friction force microscopy (AFM/FFM) (Bhushan, 1995, 1997; Bhushan et al.,
1995a) has allowed the study of surface topography, adhesion, friction, wear, lubrication, and measure-
ment of mechanical properties, all on a micro- to nanometer scale. Recently, microtribological studies

FIGURE 16.4

Schematic (a) of a silicon planar head slider and (b) of cross section of the slider for magnetic disk
drive applications. (From Bhushan, B. et al., 1992,

IEEE Trans Magn.

28, 2874–2876. With permission.)

© 1999 by CRC Press LLC

have been conducted using the AFM/FFM on undoped and doped silicon and polysilicon films and SiC
films that are used in MEMS devices (Bhushan, 1996b, 1997, 1998; Bhushan et al., 1994, 1997a,b, 1998;
Li and Bhushan, 1998; Sundararajan and Bhushan, 1998).
This chapter presents a review of macro- and micro/nanotribological studies of single-crystal silicon
and polysilicon, oxidized and implanted silicon, doped and undoped polysilicon films and SiC films. A
summary of limited component-level tests is also presented.

FIGURE 16.5

Schematics of (a) a microactuator in place with magnetic head slider, and (b) top view of two
electrostatic, rotary and linear microactuators (electrode tree structure). (From Fan, L.S. et al., 1995;

IEEE Trans. Ind.
Electron.

42, 222–233. With permission.)

© 1999 by CRC Press LLC

16.2 Experimental Techniques

16.2.1 Description of Apparatus and Test Procedures

16.2.1.1 Micro/Nanoscale Tests

A modified AFM/FFM (Nanoscope III, Digital Instruments, Santa Barbara, CA), was used for the

micro/nanotribological studies. Surface roughness and microscale friction measurements were simulta-
neously made over a scan size of 10

×

10 µm with an Si

3

N

4

tip (tip radius ~ 50 nm, cantilever stiffness
~ 0.6 N/m) sliding over the sample surface orthogonal to the long axis of the cantilever at 25 µm/s. A
coefficient of friction and conversion factors for converting the friction signal voltage to force units (nN)
were obtained through the methods developed previously by Bhushan and co-workers (Bhushan, 1995).
The normal loads used in the friction measurements varied between 50 to 300 nN. The reported values
are each an average of six separate measurements.

FIGURE 16.6

SEM micrograph of single-contact and double-con-
tact (with two orientations of the fixed electrodes) designs of micro-
mechanical switches (Peterson, 1979). (From Peterson, K.E., 1979,

IBM J. Res. Dev.

23, 376. With permission.)

FIGURE 16.7

SEM micrograph of a partially released pair of tongs. (From Mehregany, M. et al., 1988,

IEEE Trans.
Electron. Devices

35, 719–723. With permission.)

© 1999 by CRC Press LLC

FIGURE 16.8

Schematics of (a) permanent magnet millimotor with integrated milligear box, (b) of wolfrom-type
system made of Ni–Fe metal (Lehr et al., 1996), and (c) of multistage planetary gear system made with microinjected
POM plastic showing a single gear and the gear system. (From Thurigen, C. et al., 1998, in

Tribology Issues and
Opportunities in MEMS,

B. Bhushan, ed., Kluwer Academic, Dordrecht. With permission.)

© 1999 by CRC Press LLC

For the scratch and wear tests, specially fabricated diamond microtips were used (Bhushan et al., 1997a;
Sundararajan and Bhushan, 1998). These microtips consisted of single-crystal natural diamond, ground
to the shape of a three-sided pyramid, with an apex angle of 60° and tip radius of about 70 nm, mounted
on a platinum-coated stainless steel cantilever beam whose stiffness was 50 N/m. Samples were scanned
orthogonal to the long axis of the cantilever with loads ranging from 20 to 100 µN to generate scratch/wear
marks. Scratch tests consisted of generating scratches in a reciprocating mode at a given load for 10 cycles

over a scan length (stroke length) of 5 µm at 10 µm/s. Wear marks were generated over a scan area of
2

×

2 µm at 4 µm/s and the wear marks were observed by scanning a larger 4

×

4 µm area with the wear
mark at the center. Imaging scans of both scratch and wear tests were done at a low normal load of 0.5.
The reported scratch/wear depths are an average of three runs at separate instances. All measurements
were performed in an ambient environment (21 ± 1°C, 45 ± 5% RH).
Hardness and elastic modulus were calculated from load–displacement data obtained by nanoinden-
tation using a commercially available nanoindenter (Bhushan, 1995; Bhushan et al., 1997b; Li and Bhus-
han, 1998). The instrument monitored and recorded dynamic load and displacement of a three-sided
pyramidal diamond (Berkovich) indenter with a force resolution of about 75 nN and displacement
resolution of about 0.1 nm. Multiple loading and unloading were performed to examine reversibility of
the deformation and thereby ensuring that the regime was elastic.
The fracture toughness measurements were made using a microindentation technique. A Vickers
indenter (four-sided diamond pyramid) was used to indent samples in a microhardness tester at a normal
load of 0.5 N. The indentation impressions were examined in an optical microscope to measure the
length of median-radial cracks to calculate the fracture toughness (Li and Bhushan, 1998).

16.2.1.2 Macroscale Tests

Macroscale studies were conducted using either a ball-on-flat tribometer under reciprocating motion or
a magnetic rigid disk drive. In the ball-on-flat tribometer tests, a 5-mm diameter alumina ball (hardness
~ 21 GPa) was slid in a reciprocating mode (2 mm amplitude and 1 Hz frequency) under a normal load
of 1 N in the ambient environment (Gupta et al., 1993). The coefficient of friction was measured during

the tests using a strain gauge ring. Wear volume was measured by measuring the wear depth using a
stylus profiler.
In the magnetic disk drive tests, a modified disk drive was used. The silicon pins or magnetic head
slider specimens to be tested were slid in a unidirectional sliding mode against a magnetic thin-film disk
under a normal load of 0.15 N and the rotational speed of 200 rpm. The sliding speeds at track radii
ranging from 45 to 55 mm varied from 0.9 to 1.2 m/s (Bhushan and Venkatesan, 1993). At these speeds,
the pin or slider specimen remained in contact throughout the period of testing. The coefficient of friction
was measured during the tests using a strain gauge beam. Samples were examined using scanning electron
microscopy to detect any wear. Chemical analyses of the samples were also carried out to study failure
mechanisms.

FIGURE 16.9

Schematics of microstructures during fabrication using surface micromachining before and after
removal of sacrificial/spacer layer.

© 1999 by CRC Press LLC

16.2.2 Test Samples

Materials of most interest for planar fabrication processes using silicon as the structural material are
undoped and boron-doped (

p

+

-type) single-crystal silicon and phosphorus-doped (

n

+

-type) LPCVD
polysilicon films. For tribological reasons, silicon needs to be coated with a solid and/or liquid overcoat
or be surface treated, which exhibits low friction and wear.
Studies have been conducted on various types of virgin silicon samples: undoped (lightly doped)
single-crystal Si(100), Si(111), and Si(110) and the following types of treated/coated silicon samples:
PECVD-oxide-coated Si(111), dry-oxidized, wet-oxidized, and C

+

-implanted Si(111) (Bhushan and Ven-
katesan, 1993; Bhushan and Koinkar, 1994). Studies have also been conducted on heavily doped (

p

+

-type)
single-crystal Si(100), undoped polysilicon film, heavily doped (

n

+

-type) polysilicon film and 3C-SiC
(cubic or

β

-SiC) film (Bhushan et al., 1997a,b, 1998; Li and Bhushan, 1998; Sundararajan and Bhushan,
1998). A 10

×

10 mm coupon of each sample was ultrasonically cleaned in methanol for 20 min and
dried with a blast of dry air prior to measurements. The undoped Si(100) was a

p

-type material grown
by the CZ process. It had a boron concentration of 1.7

×

10

15

ions/cm

3

from intrinsic doping during the
manufacturing process. The doped wafer (

p

+

-type single-crystal silicon) was heavily doped with boron
ions (from a solid source of oxide of boron) with concentration of 7

×

10

19

ions/cm

3

down to a depth of
5.5 µm using thermal diffusion. The grain size of polysilicon wafer was about 5 mm. The polysilicon film
was produced as follows: (1) The substrate used was thermally oxidized Si(100) wafers with the oxide
layer grown using a standard wet oxidation recipe to a nominal thickness of about 100 nm; (2) the
polysilicon film was grown on the substrate using an LPCVD process (deposition temperature, 610°C;
silane flow rate, 285 sccm; deposition pressure, 230 mtorr), using the thermal decomposition of silane
vapor. The films were about 3 µm thick, with columnar grains and a grain size of about 750 nm. X-ray
diffraction and transmission electron microscope characterization showed the film to be highly oriented
(110). The

n

+

-doped polysilicon film was obtained by doping the polysilicon film with phosphorus ions
from a solid source of P

2

O

5

by thermal diffusion at 875°C for 90 min. The 3C-SiC films were grown
through an atmospheric pressure chemical vapor deposition (APCVD) process on an Si(100) substrate.
To grow the SiC film on the wafer by carbonizing its surface, the wafer is placed on an SiC-coated graphite
susceptor, which is induction-heated by an RF-generator to the growth temperature of 1360°C in the
presence of propane and silane at 1 atm. Prior to film growth, the wafer is heated to 1000°C in the
presence of hydrogen, which etches the native oxide from the wafer surface (Zorman et al., 1995). The
films obtained were about 2 µm thick. Both as-deposited and polished versions of the undoped polysilicon
and SiC films were studied. The polysilicon film was chemomechanically polished in a Struers Planopol-
3 polishing machine using 100 ml colloidal silica dispersion (Rippey Corporation, particle size of 30 to
100 nm) mixed in 2000 ml deionized water at a force of 210 N for 15 min, with the pads running at
150 rpm in the same direction. The SiC and doped polysilicon films were polished in a Buehler Ecomet-
3 polishing machine with diamond slurry (General Electric Company, particle size of 100 to 500 nm)
for 30 min for SiC and 12 min for doped polysilicon film at a load of 50 N, with the pads running at
10 rpm in the same direction. Doped polysilicon film was polished using Fuji film lapping tape, LT-2,
the main lapping agent being 37-µm-sized Cr

2

O

3

particles.

Boundary lubrication studies have been conducted on silicon samples coated with perfluoropolyether
lubricants (Koinkar and Bhushan, 1996a,b) and Langmuir–Blodgett and chemically grafted self-assem-
bled monolayer films (Bhushan et al., 1995b).

16.3 Results and Discussion

Reviews of five studies are presented in this section. The first study compares micro/nanotribological
properties of various forms of virgin, coated, and treated silicon samples. The second study is composed
of similar studies conducted on SiC film and compares this material to other materials currently used
in MEMS devices. The third study compares the macroscale friction and wear data of virgin, coated, and

© 1999 by CRC Press LLC

treated silicon samples. The fourth study discusses various forms of boundary lubrication that may be
suitable for MEMS devices. Finally, the fifth study presents a review of component level studies.

16.3.1 Micro/nanotribological Studies of Virgin, Coated,
and Treated Silicon Samples

Table 16.2 summarizes the results of the studies conducted on various silicon samples (Bhushan and
Koinkar, 1994). Coefficient of microscale friction values of all the samples are about the same. Table 16.3
compares macroscale and microscale friction values for two of the samples. When measured for small
contact areas and very low loads used in microscale studies, indentation hardness and elastic modulus
are higher than that at the macroscale. This reduces wear. This, added to the effect of the small apparent
area of contact reducing the number of trapped particles on the interface, results in less plowing contri-
bution in the case of microscale friction measurements. Figure 16.10 and Table 16.2 show microscale
scratch data for the various silicon samples (Bhushan and Koinkar, 1994). These samples could be
scratched at 10 µN load. Scratch depth increased with normal load. Crystalline orientation of silicon has
little influence on scratch resistance. PECVD-oxide samples showed the best scratch resistance, followed
by dry-oxidized, wet-oxidized, and ion-implanted samples. Ion implantation does not appear to improve

scratch resistance. Microscratching experiments just described can be used to study failure mechanisms
on the microscale and to evaluate mechanical integrity (scratch resistance) of ultrathin films at low loads.
Wear data on the silicon samples are presented in Table 16.2 (Bhushan and Koinkar, 1994). PECVD-
oxide samples showed superior wear resistance followed by dry-oxidized, wet-oxidized, and ion-
implanted samples. This agrees with the trends seen in scratch resistance. In PECVD, ion bombardment

TABLE 16.2

rms, Microfriction, Microscratching/Microwear and Nanoindentation Hardness Data for Various

Virgin, Coated, and Treated Silicon Samples

Material
rms
Roughness

a


(nm)
Microscale
Coefficient
of Friction

b

Scratch Depth

c

at 40 µN (nm)
Wear Depth

c


at 40 µN (nm)
Nanohardness

c

at
100 µN (GPa)

Si(111) 0.11 0.03 20 27 11.7
Si(110) 0.09 0.04 20 — —
Si(100) 0.12 0.03 25 — —
Polysilicon 1.07 0.04 18 — —
Polysilicon (lapped) 0.16 0.05 18 25 12.5
PECVD-oxide coated Si(111) 1.50 0.01 8 5 18.0
Dry-oxidized Si(111) 0.11 0.04 16 14 17.0
Wet-oxidized Si(111) 0.25 0.04 17 18 14.4
C

+

-implanted Si(111) 0.33 0.02 20 23 18.6

a

Scan size of 500

×

500 nm.

b

Versus Si

3

N

4

ball, ball radius of 3 mm at a normal load of 0.1 N (0.3 GPa) at an average sliding speed of 0.8 mm/s.

c

Measured using an AFM with a diamond tip of radius of 100 nm.

TABLE 16.3

Surface Roughness and Coefficients of Micro- and

Macroscale Friction of Selected Samples

Material

rms
Roughness
(nm)
Coefficient of
Microscale
Friction

a

Coefficient of
Macroscale
Friction

b

Si(111) 0.11 0.03 0.18
C

+

-implanted Si(111) 0.33 0.02 0.18

a

Versus Si

3

N

4

tip, tip radius of 50 nm in the load range of 10–150 nN
(2.5–6.1 GPa) at a scanning speed of 5 µm/s over a scan area of 1

×

1 µm.

b

Versus Si

3

N

4

ball, ball radius of 3 mm at a normal load of 0.1 N
(0.3 GPa) at an average sliding speed of 0.8 mm/s.

© 1999 by CRC Press LLC

during the deposition improves the coating properties such as suppression of columnar growth, freedom
from pinhole, decrease in crystalline size, and increase in density, hardness and substrate–coating adhe-
sion. These effects may help in improving mechanical integrity of the sample surface.
The wear resistance of ion-implanted silicon samples was further studied, Figure 16.11 (Bhushan and
Koinkar, 1994). For tests conducted at various loads on Si(111) and C

+

-implanted Si(111), it is noted
that wear resistance of implanted sample is slightly poorer than that of virgin silicon up to about 80 µN.
Above 80 µN, the wear resistance of implanted Si improves. As one continues to run tests at 40 µN for
a larger number of cycles, the implanted sample exhibits higher wear resistance than the unimplanted
sample. Damage from the implantation in the top layer results in poorer wear resistance; however, the
implanted zone at the subsurface is more wear resistant than the virgin silicon.
Nanoindentation hardness values of all samples are presented in Table 16.2. Coatings and treatments
improved nanohardness of silicon. Note that dry-oxidized and PECVD films are harder than wet-oxidized
films, as these films may be porous. High hardness of oxidized films may be responsible for measured
low wear on the microscale and macroscale (data to be presented later). Figure 16.12 shows the inden-
tation marks generated on virgin and C

+

-implanted Si(111) at a normal load of 70 µN with a depth of
indentation about 3 nm and hardness values of 15.8 and 19.5 GPa, respectively (Bhushan and Koinkar,
1994). Hardness values of virgin and C

+

-implanted Si(111) at various indentation depths (normal loads)
are presented in Figure 16.13 (Bhushan and Koinkar, 1994). Note that the hardness at a small indentation
depth of 2.5 nm is 16.6 GPa and it drops to a value of 11.7 GPa at a depth of 7 nm and a normal load
of 100 µN. Higher hardness values obtained in low-load indentation may arise from the observed pres-
sure-induced phase transformation during the nanoindentation (Pharr, 1991; Callahan and Morris,
1992). Additional increase in the hardness at an even lower indentation depth of 2.5 nm reported here
may arise from the contribution by complex chemical films (not from native oxide films) present on the
silicon surface. At small volumes there is a high probability that indentation would be made into a region

that was initially dislocation free. Furthermore, at small volumes, it is believed that there is an increase
in the stress necessary to operate dislocation sources (Gane and Cox, 1970; Sargent, 1986). These are
some of the plausible explanations for an increase in hardness at smaller volumes. If the silicon material
is to be used at very light loads such as in microsystems, the high hardness of surface films would protect
the surface until it is worn.
From Figure 16.13, hardness values of C

+

-implanted Si(111) at a normal load of 50 µN is 20.0 GPa
with an indentation depth of about 2 nm which is comparable to the hardness value of 19.5 GPa at 70 µN,
whereas measured hardness value for virgin silicon at an indentation depth of about 7 nm (normal load
of 100 µN) is only about 11.7 GPa. Thus, ion implantation results in an increase in hardness. Note that
the surface layer of the implanted zone is much harder compared with the subsurface, and may be brittle
leading to higher wear on the surface. The subsurface of the implanted zone is harder than the virgin
silicon, resulting in higher wear resistance, which will be shown later in macroscale tests conducted at
high loads.

FIGURE 16.10

Scratch depth as a function of normal force after 10 cycles for various silicon samples, virgin, treated,
and coated. (From Bhushan, B. and Koinkar, V.N., 1994,

J. Appl. Phys.

75, 5741–5746. With permission.)