Bhushan B. Nanotribology and Nanomechanics: An Introduction

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1264 Bharat Bhushan

Pinned

Free

Fig. 22.45. SEM micrograph

of micromachined array of

polysilicon cantilever beams

of increasing length. The

micrograph shows the onset

of pinning for beams longer

than 34 µm [179]

to separate unit areas of the two surfaces from contact is called the work of adhe-

sion. To measure the work of adhesion, electrostatic actuation is used to bring allthe

beams into contact with the substrate; see Fig. 22.45 [179,181]. Once the actuation

force is removed, the beams begin to peel themselves oﬀ the substrate, which can

be observed with an optical interference microscope (e.g., a Wyko surface proﬁler).

For beams shorterthan acharacteristic length, the so-calleddetachment length,their

stiﬀness is suﬃcient to free them completely from the substrate underneath. Beams

larger than the detachment length remain adhered. The beams at the transition re-

gion start to detach and remain attached to the substrate just at the tips. For this

case, by equating the elastic energy stored within the beam and the beam–substrate

interfacial energy, the work of adhesion, W

, can be calculated by the following

equation [179]

3Ed

8

, (22.5)

where E is the Young’s modulus of the beam, d is the spacing between the unde-

ﬂected beam and the substrate, t is the beam thickness, and P

is the detachment

length. The technique has been used to screen methods for adhesion reduction in

polysilicon microstructures.

22.6.3 Microtriboapparatus for Adhesion, Friction,

and Wear of Microcomponents

To measure adhesion, friction,and wear between two microcomponents,a microtri-

boapparatus has been used. Figure 22.46 shows a schematic of a microtriboappa-

ratus, capable of adopting MEMS components [184]. In this apparatus, an upper

specimen, mounted on a soft cantileverbeam, comes into contact with a lower spec-

imen mounted on a lower specimen holder. The apparatus consists of two piezos

(x-andz-piezos), and four ﬁber-optic sensors (x-andz-displacement sensors, and

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1265

z-displacement sensor

z-force sensor

Cantilever holder

Cantilever

Upper specimen

Lower specimen

Lower specimen holder

x-piezo

x-displacement sensor

Cantilever

Mirrors

z-piezo

x-force sensor

Fig. 22.46. Schematic of the microtriboapparatus including specially designed cantilever

(with two perpendicular mirrors attached on the end), lower specimen holder, two piezos (x-

and z-piezos), and four ﬁber-optic sensors (x-andz-displacement sensors and x-andz-force

sensors) [184]

x-andz-force sensors). For adhesion and friction studies, z-andx-piezos are used

to bring the upper specimen and lower specimen in contact and to apply a relative

motion in the lateral direction, respectively. The x-andz-displacement sensors are

used to measure the lateral position of the lower specimen and vertical position of

the upper specimen, respectively. The x-andz-force sensors are used to measure

the friction force and normal load/adhesive force between these two specimens,

respectively, by monitoring the deﬂection of the cantilever.

As most MEMS/NEMS devices are fabricated from silicon, study of silicon-

on-silicon contacts is important. This contact was simulated by a ﬂat single-crystal

Si(100) wafer (phosphorus-doped)specimen sliding against a single-crystal Si(100)

ball (1 mm in diameter, 5×10

atoms/cm

boron doped) mounted on a stainless-

steel cantilever [184,185]. Both of them have native oxide layer on their surfaces.

The other materials studied were 10-nm-thick DLC deposited by ﬁltered cathodic

arc deposition on Si(100), 2.3-nm-thick chemically bonded PFPE (Z-DOL, BW)

on Si(100), and hexadecane thiol (HDT) monolayer on evaporated Au(111) ﬁlm to

investigate their anti-adhesion performance.

It is well known that in computer rigid disk drives, the adhesive force increases

rapidly with increasing rest time between a magnetic head and a magnetic disk [34].

Considering that adhesion and friction are the major issues that lead to the fail-

ure of MEMS/NEMS devices, the eﬀect of rest time on the microscale on Si(100),

1266 Bharat Bhushan

Adhesive force (μN)

1,000

800

600

400

200

Rest time (s)

Meniscus force (μN)

15.7

15.6

15.5

15.4

15.3

15.2

Rest time (s)

22°C, RH 50%

Si(100)

DLC

PFPE

HDT

R= 50 μm

= 1nm

η= 0.25 Pa s

Fig. 22.47. (a) The inﬂuence

of rest time on the adhesive

force of Si(100), DLC, chem-

ically bonded PFPE, and

HDT, and (b) Single-asperity

contact modeling results of

the eﬀect of rest time on the

meniscus force for an asper-

ity of R in contact with a ﬂat

surface with a water ﬁlm of

thickness of h

and absolute

viscosity of η

[186]

DLC, PFPE, and HDT was studied, and the results are summarized in Fig. 22.47a.

It is found that the adhesive force of Si(100) logarithmically increases with the rest

time up to a certain equilibrium time (t = 1000s), after which it remains constant.

Figure 22.47a also shows that the adhesive force of DLC, PFPE, and HDT does not

changewith rest time. Single-asperitycontactmodelingof the dependenceof menis-

cus force on the rest time has been carried out by Chilamakuri and Bhushan [186],

and the modeling results Fig. 22.47b verify experimental observations. Due to the

presence of thin-ﬁlm adsorbed water on Si(100), the meniscus forms around the

contacting asperities and grows with time until equilibrium occurs, which causes

the eﬀect of rest time on its adhesive force. The adhesive forces of DLC, PFPE, and

HDT do not change with rest time, which suggests that the water meniscus is not

present on their surfaces.

The measured adhesive forces of Si(100), DLC, PFPE, and HDT at a rest time

of 1 s are summarized in Fig. 22.48, which shows that the presence of solid ﬁlms

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1267

Adhesive force (μN)

1,000

800

600

400

200

Material

22°C, RH 50%

Si(100)

DLC PFPE HDT

Si(100)

DLC PFPE HDT

Fig. 22.48. Adhesive forces

of Si(100), DLC, chem-

ically bonded PFPE, and

HDT at ambient condition

and a schematic showing

the relative size of the water

meniscus on diﬀerent speci-

mens

of DLC, PFPE, and HDT greatly reduces the adhesive force of Si(100), whereas,

the HDT ﬁlm has the lowest adhesive force. It is well known that the native ox-

ide layer (SiO

) on the top of Si(100) wafers exhibits hydrophilic properties, and

water molecules, produced by capillary condensation of water vapor from the envi-

ronment, can easily be adsorbed onto this surface. The condensed water will form

a meniscus as the upper specimen approaches to the lower specimen surface. The

meniscus force is a major contributor to the adhesive force. In the case of DLC,

PFPE, and HDT, the ﬁlms are found to be hydrophobic based on contact angle

measurements and the amount of condensed water vapor is low compared to that

on Si(100). It should be noted that the measured adhesive force is generally higher

than that measured by AFM, because the larger radius of the Si(100) ball compared

to that of an AFM tip induces a larger meniscus and van der Waals forces.

To investigate the eﬀect of velocity on friction, the friction force as a function

of velocity was measured and is summarized in Fig. 22.49a. It indicates that, for

Si(100), the friction force initially decreases with increasing velocity until equilib-

rium occurs. Figure 22.49a also indicates that the velocity almost has no eﬀect on

the friction properties of DLC, PFPE, and HDT . This implies that the friction mech-

anisms on DLC, PFPE, and HDT do not change with the variation of velocity. For

Si(100), at high velocity, the meniscus is broken and does not have enough time

to rebuild. In addition, it is also believed that tribochemical reactions plays an im-

portant role. The high velocity leads to tribochemical reactions of Si(100) (which

has a native oxide SiO

) with water molecules to form a Si(OH)

ﬁlm. This ﬁlm is

removedand continuouslyreplenishedduring sliding. The Si(OH)

layer at the slid-

ing surface is knownto have low shear strength.The breaking of the water meniscus

and the formation of a Si(OH)

layer results in a decrease in the friction force of

1268 Bharat Bhushan

1,200

1,000

800

600

400

200

1,000

800

600

400

200

Effect of velocity

Effect of relative humidity Effect of temperature

Friction force (μN)

Sliding velocity (μm/s) Relative humidity (%) Temperature (°C)

b) c)

0 500 1,000 1,500 2,000 2,500 0 20 40 60 80 0 25 50 75 100 125 150

Adhesive force (μN)

22 °C, RH 50%, 2,000 μN

Si(100)

PFPE

HDT

Si(100)

PFPE

DLC

HDT

Si(100)

DLC

PFPE

HDT

Si(100)

DLC

PFPE

HDT

DLC

PFPE

HDT

Si(100)

DLC

22 °C, 2,000 μN, 720 μm/s

RH 50%22 °C

RH 50%, 2,000 μN, 720 μm/s

Fig. 22.49. The inﬂuence of (a) sliding velocity on the friction forces, (b) relative humidity

on the adhesive and friction forces, and (c) temperature on the adhesive and friction forces of

Si(100), DLC, chemically bonded PFPE, and HDT

Si(100). For DLC, PFPE, and HDT, their surfaces exhibit hydrophobic properties,

and can only adsorb a few water molecules in ambient conditions. The aforemen-

tioned meniscus breaking and tribochemical reaction mechanisms do not exist for

these ﬁlms. Therefore, their friction force does not change with velocity.

The inﬂuence of relative humidity was studied in an environmentally controlled

chamber. The adhesive force and friction force were measured by making measure-

ments at increasing relative humidity, and the results are summarizedin Fig. 22.49b,

which shows that, for Si(100), the adhesive force increases with relative humidity,

but the adhesive forces for DLC and PFPE only show a slight increase when the

humidityis higher than 45%, while the adhesive force of HDT does not change with

humidity. Figure 22.49b also shows that, for Si(100), the friction force increases

with relative humidity increases up to 45%, and then shows a slight decrease with

further increases in the relative humidity. For PFPE, there is an increase in the fric-

tion force when the humidity is higher than 45%. In the whole testing range, the

relative humidity does not have any apparent inﬂuence on the friction properties of

DLC and HDT . In the case of Si(100), the initial increase in relative humidity up

to 45% causes more adsorbed water molecules, and the formation of a larger water

meniscus,which leads to an increase of the friction force. However, at very high hu-

midity of 65%, large quantities of adsorbed water can form a continuouswater layer

that separates the tip and sample surfaces, and acts as a kind of lubricant, which

causes a decrease in the friction force. For PFPE, dewetting of the lubricant ﬁlm at

humidities larger than 45% results in an increase in adhesive and friction forces. For

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1269

DLC and HDT , their surfaces show hydrophobic properties, and increasing relative

humidity does not play a large role in their friction force.

The inﬂuence of temperature was studied using a heated stage. The adhesive

forceand frictionforce weremeasured bymaking measurementsat increasingtemp-

eratures of 22–125

◦

C. The results are presented in Fig. 22.49c, which shows that,

once the temperature is higher than 50

◦

C, increasing temperature causes a signiﬁ-

cant decrease of adhesive and friction forces of Si(100) and a slight decrease in the

case of DLC and PFPE. However, the adhesion and friction forces of HDT do not

show any apparent change with test temperature. At high temperature, desorption

of water, and the reduction of surface tension of water lead to the decrease of adhe-

sive and friction forces of Si(100), DLC, and PFPE. However, in the case of HDT

ﬁlm, as only a few water molecules are adsorbed on the surface, the aforementioned

mechanisms do not play a large role. Therefore, the adhesive and friction forces of

HDT do not showany apparentchange with temperature.Figure 22.49shows that in

the whole velocity, relative humidity, and temperature test range, the adhesive force

and friction force of DLC, PFPE, and HDT are always smaller than that of Si(100),

and that HDT has the smallest value.

To summarize, several methods can be used to reduce adhesion in microstruc-

tures. MEMS/NEMS surfaces can be coated with hydrophobic coatings such as

PFPEs, SAMs, and passivated DLC coatings. It should be noted that other meth-

ods to reduce adhesion include the formation of dimples on the contact surfaces to

reduce contact area [13, 79, 81, 177,181]. Furthermore, an increase in hydropho-

bicity of the solid surfaces (high contact angle approaching 180

◦

) can be achieved

by using surfaces with suitable roughness, in addition to lowering their surface en-

ergy [173,176].The hydrophobicityof surfaces is dependent upon a subtle interplay

between surface chemistry and mesoscopic topography. The self-cleaning mechan-

ism or so-called lotus eﬀect is closely related to the ultra-hydrophobicproperties of

the biologicalsurfaces, which usually show microsculptures of speciﬁc dimensions.

22.6.4 Static Friction Force (Stiction) Measurements in MEMS

In MEMS devices involving parts in relative motion to each other, such as micro-

motors, large friction forces become the limiting factor for the successful operation

and reliability of the device. It is generally known that most micromotors cannot

be rotated as manufactured and require some form of lubrication. It is therefore

critical to determine the friction forces present in such MEMS devices. To meas-

ure in situ the static friction of a rotor–bearing interface in a micromotor, Tai and

Muller [187] measured the starting torque (voltage) and pausing position for dif-

ferent starting positions under a constant bias voltage. A friction-torque model was

used to obtain the coeﬃcient of static friction. To measure the in situ kinetic friction

of the turbine and gear structures, Gabriel et al. [188] used a laser-based measure-

ment system to monitor the steady-state spins and decelerations. Lim et al. [189]

designed and fabricated a polysilicon microstructure to measure in situ the static

friction of various ﬁlms. The microstructure consisted of a shuttle suspended above

the underlying electrode by a folded beam suspension. A known normal force was

1270 Bharat Bhushan

applied and the lateral force was measured to obtain the coeﬃcient of static friction.

Beerschwinger et al. [190] developed a cantilever-deﬂection rig to measure fric-

tion of LIGA-processed micromotors. (Also see [191].) These techniques employ

indirect methods to determine the friction forces or involve fabrication of complex

structures.

A novel technique to measure the static friction force (stiction) encountered in

surface-micromachinedpolysilicon micromotors using an AFM has been developed

by Sundararajanand Bhushan [178]. Continuousphysical contact occurs duringro-

tor movement (rotation) in the micromotors between the rotor and lower hub ﬂange.

In addition, contact occurs at other locations between the rotor and the hub surfaces

and between the rotor and the stator. Friction forces will be present at these contact

regions during motor operation. Although the actual distribution of these forces is

not known, they can be expectedto be concentrated near the hub where there is con-

tinuous contact. If we therefore represent the static friction force of the micromotor

as a single force F

acting at point P

(as shown in Fig. 22.50a), then the magnitude

of the frictional torque about the center of the motor (O) that must be overcome

before rotor movement can be initiated is

= F



, (22.6)

where 

is the distance OP

, which is assumed to be the average distance from

the center at which the friction force F

occurs. Now consider an AFM tip moving

against a rotor arm in a direction perpendicular to the long axis of the cantilever

Lateral deflection

100 –200 nm

Rotor

Fig. 22.50. (a) Schematic of the technique used to measure the force F

required to initiate

rotor movement using an AFM/FFM. (b) As the tip is pushed against the rotor, the lateral

deﬂection experienced by the rotor due to the twisting of the tip prior to rotor movement is

a measure of static friction force F

of the rotors. (c) Schematic of lateral deﬂection expected

from the aforementioned experiment. The peak V

is related to the state of the rotor [178]

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1271

beam (the rotor-arm edge closest to the tip is parallel to the long axis of the can-

tilever beam), as shown in Fig. 22.50a. When the tip encounters the rotor at point

, the tip will twist, generating a lateral force between the tip and the rotor (eventA

in Fig. 22.50b). This reaction force will generate a torque about the center of the

motor. Since the tip is trying to move further in the direction shown, the tip will

continue to twist to a maximum value at which the lateral force between the tip and

the rotor becomes high enough that the resultant torque T

about the center of the

motor equals the static friction torque T

. At this point, the rotor will begin to rotate

and the twist of the cantilever decreases sharply (event B in Fig. 22.50b). The twist

of the cantilever is measured in the AFM as a change in the lateral deﬂection signal

(in volts), which is the underlying concept of friction force microscopy (FFM). The

change in the lateral deﬂection signal corresponding to the aforementioned events

as the tip approaches the rotor is shown schematically in Fig. 22.50c. The value of

the peak V

is a measure of the force exerted on the rotor by the tip just before the

static friction torque is matched and the rotor begins to rotate.

Using this technique, the viability of PFPE lubricants for micromotors has been

investigated and the eﬀect of humidity on the friction forces of unlubricated and

lubricated devices was studied as well. Figure 22.51 shows static friction forces,

normalized overthe weight of the rotor, of unlubricated and lubricated micromotors

as a function of rest time and relative humidity. Rest time here is deﬁned as the

time elapsed between the ﬁrst experiment conducted on a given motor (solid sym-

bol at time zero) and subsequent experiments (open symbols). Each open-symbol

data point is an average of six measurements. It can be seen that, for the unlu-

bricated motor and the motor lubricated with a bonded layer of Z-DOL(BW), the

static friction force is highest for the ﬁrst experiment and then drops to an almost

constant level. In the case of the motor with an as-is mobile layer of Z-DOL, the

values remain very high up to 10days after lubrication. In all cases, there is neg-

ligible diﬀerence in the static friction force at 0% and 45% RH. At 70% RH, the

unlubricated motor exhibits a substantial increase in the static friction force, while

the motor with bonded Z-DOL shows no increase in static friction force due to the

hydrophobicity of the lubricant layer. The motor with an as-is mobile layer of the

lubricant shows consistently high values of static friction force that vary little with

humidity.

Figure 22.52 summarizes static friction force data for two motors, M1 and M2

along with schematics of the meniscus eﬀects for the unlubricated and lubricated

surfaces. Capillary condensation of water vapor from the environment results in

the formation of meniscus bridges between contacting and near-contacting asperi-

ties of the two surfaces in close proximity to each other, as shown in Fig. 22.52.

For unlubricated surfaces, more menisci are formed at higher humidity resulting in

a higher friction force between the surfaces. The formation of meniscus bridges is

supported by the fact that the static friction force for unlubricated motors increases

at high humidity Fig. 22.52. Solid bridging may occur near the rotor–hub interface

due to silica residues after the ﬁrst etching process. In addition the drying process

after the ﬁnal etch can result in liquid bridging, formed by the drying liquid due to

1272 Bharat Bhushan

Normalized static friction force

0255075

Relative humidity (%)

Rest time (days)

0 5 10 20 30

Normalized static friction force

Rest time =12h

Z-DOL, 2 nm (As is)

Z-DOL, 1 nm (BW)

Unlubricated

RH=45%

Fig. 22.51. Static friction

force values of unlubricated

motors and motors lubricated

using PFPE lubricants, nor-

malized over the rotor weight,

as a function of rest time and

relative humidity. Rest time

is deﬁned as the time elapsed

between a given experiment

and the ﬁrst experiment in

which motor movement was

recorded (time 0). The mo-

tors were allowed to sit at

a particular humidity for 12 h

prior to measurement [178]

meniscus force at these areas [79,81,179,180].The initial static friction force there-

fore will be quite high, as evidenced by the solid data points in Fig. 22.52. Once

the ﬁrst movement of the rotor permanently breaks these solid and liquid bridges,

the static friction force of the motors will drop (as seen in Fig. 22.52) to a value

dictated predominantly by the adhesive energies of the rotor and hub surfaces, the

real area of contact between these surfaces and meniscus forces due to water vapor

in the air, at which point, the eﬀect of lubricant ﬁlms can be observed. Lubrica-

tion with a mobile layer, even a thin one, results in very high static friction forces

due to meniscus eﬀects of the lubricant liquid itself at and near the contact regions.

It should be noted that a motor submerged in a liquid lubricant would result in

a fully ﬂooded lubrication regime. In this case there is no meniscus contribution

and only the viscous contribution to the friction forces would be relevant. However,

submerging the device in a lubricant may not be a practical method. A solid-like

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1273

Normalized static friction force

Motor

Z-DOL,

2 nm (As is)

Z-DOL,

1 nm (BW)

Unlubricated

M1 M2

Unlubricated

Hub

Rotor

Mobile liquid lubricant

Bonded liquid

lubricant

Rotor

Hub

Rotor

Fig. 22.52. Summary of the eﬀect of liquid and solid lubricants on static friction force of

micromotors. Despite the hydrophobicity of the lubricant used (Z-DOL), a mobile liquid lu-

bricant (Z-DOL as is) leads to very high static friction force due to increased meniscus forces

whereas a solid-like lubricant (bonded Z-DOL, BW) appears to provide some reduction in

static friction force

hydrophobic lubricant layer (such as bonded Z-DOL) results in favorable friction

characteristics of the motor. Thehydrophobic nature of the lubricant inhibits menis-

cus formation between the contact surfaces and maintains low friction even at high

humidity Fig. 22.52). This suggests that solid-like hydrophobic lubricants are ideal

for lubrication of MEMS while mobile lubricants result in increased values of the

static friction force.

22.6.5 Mechanisms Associated with Observed Stiction Phenomena

in Digital Micromirror Devices (DMD) and Nanomechanical Characterization

DMDs are used in digital projection displays, as described earlier. The DMD has

a layered structure, consisting of an aluminium alloy micromirror layer, yoke and

hinge layer, and metal layer on a CMOS memory array [27–29]. A blown-up

view of the DMD and the corresponding AFM surface-height images are presented

in Fig. 22.53 [192]. Single-layered aluminium alloy ﬁlms are used for the construc-