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

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-

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

(MPa m

1/2

)
Thermal
Conductivity

b(W/m K)
Coeff. of
Thermal
Expansion

b(

×

10

–6

/°C)
Melting
Point (°C)

(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

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.


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.)


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


+

-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


-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

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


at 40 µN (nm)
Wear Depth

cat 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

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


+

-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