Bhushan B. Nanotribology and Nanomechanics: An Introduction

Подождите немного. Документ загружается.

1234 Bharat Bhushan

750

500

250

100

5.00

2.50

5.00

μm

750

500

250

100

5.00

2.50

5.00

μm

μN

750

500

250

100

5.00

2.50

5.00

μm

20 μN

750

500

250

100

5.00

2.50

5.00

μm

μN

5.00

200

–200

100 80 60 40 20

2.500 μm

5.00

200

–200

100 80 60 40 20

2.500 μm

μN

5.00

200

–200

80 60 40 20

2.500 μm

μN

200

–200

100 80 60 40 20

2.500 μm

μN

100

μN

Undoped Si(100)

Undoped polysilicon film

–type polysilicon film

SiC film

Fig. 22.19. AFM 3D maps and averaged 2D proﬁles of scratch marks on various sam-

ples [155]

results from microscale wear tests on the various ﬁlms. For all the materials, wear

depth increases almost linearly with increasing number of cycles. This suggests that

the material is removed layer by layer in all the materials. Here also, SiC ﬁlm

exhibits lower wear depths than the other samples. Doped polysilicon ﬁlm wears

less than the undoped ﬁlm. The higher fracture toughness and higher hardness of

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1235

SiC compared to Si(100) is responsible for its lower wear. Also the higher ther-

mal conductivity of SiC (see Table 22.2 compared to the other materials leads to

lower interface temperatures, which generally results in less degradation of the sur-

face [34,46,81]). Doping of the polysilicon does not aﬀect the scratch/wear resis-

tanceand hardness much.The measurementsmade on the dopedsample areaﬀected

by the presenceof grain boundaries.These studies indicate that SiCﬁlm exhibitsde-

sirable tribological properties for use in MEMS devices.

22.3 Lubrication Studies for MEMS/NEMS

Several studies of liquid perﬂuoropolyether (PFPE) lubricant ﬁlms, self-assembled

monolayers (SAMs), and hard diamond-like carbon (DLC) coatings have been car-

ried out for the purpose of minimizing adhesion, friction, and wear [46,47,82,102,

122–125,154,158–166]. Many variations of these ﬁlms are hydrophobic (low sur-

face tension and high contact angle)and have low shear strength, which providelow

adhesion, friction, and wear. Relevant details are presented here.

22.3.1 Perﬂuoropolyether Lubricants

The classical approach to lubrication uses freely supported multimolecular layers

of liquid lubricants [46,47,79,81]. The liquid lubricants are sometimes chemically

bonded to improve their wear resistance. Partially chemically bonded, molecularly

thick perﬂuoropolyether (PFPE) lubricants are widely used for lubrication of mag-

netic storage media because of their thermal stability and extremely low vapor pres-

sure [34], and are found to be suitable for MEMS/NEMS devices.

Adhesion, friction, and durability experiments have been performed on virgin

Si (100) surfaces and silicon surfaces lubricated with two commonly used PFPE lu-

bricants – Z-15 (with −CF

nonpolar end groups) and Z-DOL (with −OH polar end

groups) [46,47,158,160,161,165].Z-DOL ﬁlm was thermally bonded at 150

◦

Cfor

30minutes and the unbonded fraction was removed by a solvent (bonded washed,

BW) [34]. The thicknesses of the Z-15 and Z-DOL (BW) ﬁlms were 2.8nm and

2.3nm, respectively. Nanoscale measurements were made using an AFM. The ad-

hesive forces of Si(100), Z-15 and Z-DOL (BW) measured by plots of forcecalibra-

tion and friction force versus normal load are summarized in Fig. 22.20. The results

measured by these two methods are in good agreement. Figure 22.20 shows that

the presence of mobile Z-15 lubricant ﬁlm increases the adhesive force compared to

that of Si(100)by meniscus formation[79,81,167].In contrast,the presence of solid

phase Z-DOL (BW) ﬁlm reduces the adhesive force as compared that of Si(100) be-

cause of the absence of mobile liquid. The schematic (bottom) in Fig. 22.20 shows

the relative size and sources of meniscus. It is well known that the native oxide

layer (SiO

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

water molecules can be adsorbed on this surface. The condensed water will form

a meniscus as the tip approaches the sample surface. The larger adhesive force in

Z-15 is not only caused by the Z-15 meniscus, the nonpolarized Z-15 liquid does

1236 Bharat Bhushan

100

Z-DOL (BW)Z-15Si(100)

Z-DOL

Z-15

Z-15 Z-DOL (BW)Si(100)

22 °C, RH 45– 55%

Adhesive force (nN)

Force

calibration plot

Friction

force plot

Fig. 22.20. Summary of the adhesive forces of Si(100) and Z-15 and Z-DOL (BW) ﬁlms

measured by plots of force calibration and friction force versus normal load in ambient air.

The schematic (bottom) showing the eﬀect of meniscus, formed between AFM tip and the

surface sample, on the adhesive and friction forces [158]

not have good wettability and strong bonding with Si(100). Consequently, in the

ambient environment,the condensed water molecules from the environmentperme-

ate through the liquid Z-15 lubricant ﬁlm and compete with the lubricant molecules

present on the substrate. The interaction of the liquid lubricant with the substrate is

weakened, and a boundary layer of the liquid lubricant forms puddles [160,161].

This dewetting allows water molecules to be adsorbed on the Si(100) surface as ag-

gregates along with Z-15 molecules. And both of them can form a meniscus while

the tip approaches the surface. Thus, the dewetting of the liquid Z-15 ﬁlm results

in a higher adhesive force and poorer lubrication performance. In addition, as the

Z-15 ﬁlm is fairly soft compared to the solid Si(100) surface, penetration of the tip

in the ﬁlm occurs while pushing the tip down. This leads to the large area of the tip

involved to form the meniscus at the tip–liquid (mixture of Z-15 and water) inter-

face. It should also be noted that Z-15 has a higher viscosity than water, therefore

Z-15 ﬁlm provides higher resistance to lateral motion and coeﬃcient of friction. In

the case of Z-DOL (BW) ﬁlm, the active groups of Z-DOL molecules are mostly

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1237

bonded on Si(100) substrate, thus the Z-DOL (BW) ﬁlm has low free surface en-

ergy and cannot be displaced readily by water molecules or readily adsorb water

molecules. Thus, the use of Z-DOL (BW) can reduce the adhesive force.

To study the eﬀect of relative humidity on friction and adhesion, the variation

of friction force, adhesive force, and coeﬃcient of friction of Si(100), Z-15, and Z-

DOL (BW) as a function of relative humidity are shown in Fig. 22.21. This shows

that, for Si(100) and Z-15 ﬁlm, the friction force increases with a relative humid-

ity increase up to 45%, and then shows a slight decrease with further increases in

200

175

150

125

100

0.15

0.10

0.05

20 8060400

Z-15

Z-DOL (BW)

Si(100)

Increasing relative humidity

Z-DOL

70 %

Z-15

70 nN, 2 μm/s, 22°C

Friction force (nN)

Si(100)

Z-DOL (BW)

Z-15

Relative humidity (%)

From friction force plot

Adhesive force (nN)

Si(100)

Z-DOL (BW)

Z-15

Coefficient of friction

Si(100)

Z-DOL (BW)

Z-15

Thermally treated Si(100)

Fig. 22.21. The inﬂuence of relative humidity of the friction force, adhesive force, and coef-

ﬁcient of friction of Si(100) and Z-15 and Z-DOL (BW) ﬁlms at 70 nN, 2 µm/s, and in 22

◦

air. Schematic (right) shows the change of meniscus while increasing the relativehumidity. In

this ﬁgure, the thermal treated Si(100) represents the Si(100) wafer that was baked at 150

◦

for 1h in an oven (in order to remove the adsorbed water) just before it was placed in the

0% RH chamber [158]

1238 Bharat Bhushan

the relative humidity. Z-DOL (BW) has a smaller friction force than Si(100) and

Z-15 over the whole testing range, and its friction force shows a relative appar-

ent increase when the relative humidity is higher than 45%. For Si(100), Z-15 and

Z-DOL (BW), their adhesive forces increase with relative humidity. And their co-

eﬃcients of friction increase with a relative humidity up to 45%, after which they

decrease with further increasing of the relative humidity. It is also observed that the

eﬀect of humidity on Si(100) really depends on the history of the Si(100) sample.

As the surface of Si(100) wafer readily adsorb water in air, without any pretreat-

ment the Si(100) used in our study almost reaches its saturated stage of adsorbed

water, and shows less eﬀect during increasing relative humidity. However, once the

Si(100) wafer was thermally treated by baking at 150

◦

Cfor1h,alargereﬀect was

observed.

The schematic (right) in Fig. 22.21 shows that Si(100), because of its high free

surface energy, can adsorb more water molecules with increasing relative humidity.

As discussed earlier, for the Z-15 ﬁlm in the humid environment, the condensed

water from the humid environment competes with the lubricant ﬁlm present on the

sample surface, and the interaction of the liquid lubricant ﬁlm with the silicon sub-

strate is weakened and a boundary layer of the liquid lubricant forms puddles. This

dewetting allows water molecules to be adsorbed on the Si(100) substrate mixed

with Z-15 molecules [160,161]. Obviously, more water molecules can be adsorbed

on theZ-15surfacewith increasingrelativehumidity.The largeramountof adsorbed

water in the case of Si(100), along with the lubricant molecules in the case of the

Z-15 ﬁlm, forms a larger water meniscus, which leads to an increase of the friction

force, adhesive force, and coeﬃcient of friction of Si(100) and Z-15 with humid-

ity. However, at a very high humidity of 70%, large quantities of adsorbed water

can form a continuous water layer that separates the tip and sample surface, acting

as a kind of lubricant, which causes a decrease in the friction force and coeﬃcient

of friction. For Z-DOL (BW) ﬁlm, because of their hydrophobic surface proper-

ties, water molecules can be adsorbed at humidity higher than 45%, and causes an

increase in the adhesive force and friction force.

To study the durability of lubricant ﬁlms at nanoscale, the friction of Si(100),

Z-15, and Z-DOL (BW) as a function of the number of scanning cycles are shown

in Fig. 22.22. As observed earlier, the friction force and coeﬃcient of friction of

Z-15 are higher than that of Si(100) with the lowest values for Z-DOL(BW). During

cycling,thefrictionforce andcoeﬃcientof frictionof Si(100)showa slightdecrease

during the initial few cycles then remain constant. This is related to the removal of

the top adsorbed layer. In the case ofZ-15ﬁlm,thefrictionforceandcoeﬃcient

of friction show an increase during the initial few cycles and then approach higher

stable values. This is believed to be caused by the attachment of the Z-15 molecules

onto the tip. The molecular interaction between these attached molecules to the

tip and molecules on the ﬁlm surface is responsible for an increase in the friction.

But after several scans, this molecular interaction reaches equilibrium and after that

the friction force and coeﬃcient of friction remain constant. In the case of Z-DOL

(BW) ﬁlm, the friction force and coeﬃcient of friction start out low and remain low

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1239

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

Fig. 22.22. Friction force and coeﬃcient of friction versus number of sliding cycles for

Si(100) and Z-15 and Z-DOL (BW) ﬁlms at 70 nN, 0.4 µm/s, and in ambient air. Schematic

(bottom) shows that some liquid Z-15 molecules can be attached onto the tip. The molecular

interaction between the attached molecules onto the tip with the Z-15 molecules in the ﬁlm

results in an increase of the friction force with multiple scanning [158]

1240 Bharat Bhushan

during the entire test for 100 cycles. This suggests that Z-DOL (BW) molecules do

not become attached or displaced as readily as Z-15.

22.3.2 Self-Assembled Monolayers (SAMs)

For lubrication of MEMS/NEMS, another eﬀective approach involves the deposi-

tion of organizedand dense molecularlayers of long-chainmolecules.Two common

methods to produce monolayers and thin ﬁlms are Langmuir–Blodgett(LB) deposi-

tion and self-assembled monolayers (SAMs) by chemical grafting of molecules.

LB ﬁlms are physically bonded to the substrate by weak van der Waals attrac-

tion, while SAMs are chemically bonded via covalent bonds to the substrate.

Because of the choice of chain length and terminal linking group that SAMs of-

fer, they hold great promise for boundary lubrication of MEMS/NEMS. A num-

ber of studies have been conducted to study the tribological properties of various

SAMs [122–125,159,162–164,166,168].

Bhushan and Liu [162] studied the eﬀect of ﬁlm compliance on adhesion

and friction. They used hexadecane thiol (HDT), 1,1,biphenyl-4-thiol (BPT), and

crosslinked BPT (BPTC) solvent deposited on an Au(111) substrate, Fig. 22.23a.

The average values and standard duration of the adhesive force and coeﬃcient of

friction are presented in Fig. 22.23b.Based on the data, the adhesiveforce and coef-

ﬁcient of friction of SAMs are lower than correspondingsubstrates. Among various

ﬁlms, HDT exhibits the lowest values. Based on stiﬀness measurements of various

SAMs, HDT wasthe most compliant,followedby BPT and BPTC. Based on friction

and stiﬀnessmeasurements,SAMs withhigh-compliancelongcarbon chainsexhibit

low friction; chain compliance is desirable for low friction. Friction mechanism of

SAMs is explained by a so-called molecular spring model Fig. 22.24. According

to this model, the chemically adsorbed self-assembled molecules on a substrate are

just like assembled molecular springs anchored to the substrate. An asperity sliding

on the surface of SAMs is like a tip sliding on the top of molecular springs or brush.

The molecular spring assembly has compliant features and can experience orienta-

tion and compression under load. The orientation of the molecular springs or brush

under normal load reduces the shearing force at the interface, which in turn reduces

the friction force. The orientation is determined by the spring constant of a single

molecule as well as the interaction between the neighboring molecules, which can

be reﬂected by packing density or packing energy. It should be noted that the orien-

tation can lead to conformational defects along the molecular chains, which lead to

energy dissipation.

SAMs with high-compliancelong carbon chains also exhibitthe best wear resis-

tance [162,163]. In wear experiments, curves of wear depth as a function of normal

load show a critical normal load. A representative curve is shown in Fig. 22.25. Be-

low the critical normal load, SAMs undergo orientation, at the critical load SAMs

wear away from the substrate due to weak interface bond strengths, while above the

critical normal load severe wear takes place on the substrate.

Bhushan et al. [122], Kasai et al. [124], and Tambe and Bhushan [164] stud-

ied perﬂuorodecyltricholorosilane(PFTS), n-octyldimethyl (dimethylamino) silane

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1241

Au(111)

SSSS

Adhesive force (nN)

HDT BPT BPTC

Coefficient of friction

0.08

0.06

0.04

0.02

Materials

HDT BPT BPTC

Hexadecane thiol

(HDT)

1,1'–biphenyl–4–thiol

(BPT)

Cross-linked 1,1'–

biphenyl–4–thiol

(BPTC)

Biphenyl

–(C

)

–

Au(111)

Alkyl

–(CH

)

–

Au(111)

Fig. 22.23. (a) Schematics of

structures of hexadecane thiol

and biphenyl thiol SAMs on

Au(111) substrates, and (b)

adhesive force and coeﬃcient

of friction of Au(111) sub-

strate and various SAMs

(ODMS) (n = 7), and n-octadecylmethyl(dimethylamino)silane (ODDMS) (n = 17)

vapor-phase-deposited on a Si substrate, and octylphosphonate (OP) and octade-

cylphosphonate (ODP) on an Al substrate, Fig. 22.26a. Figure 22.26b presents the

contact angle, adhesive force, friction force, and coeﬃcient of friction of the two

substrates and with various SAMs. Based on the data, PFTS/Si exhibits a higher

1242 Bharat Bhushan

Substrate

Tip

Fig. 22.24. Molecular spring

model of SAMs. In this ﬁg-

ure, α

<α

, which is caused

by the further orientation un-

der the normal load applied

by an asperity tip [162]

Critical load

Decrease of surface height (nm)

Normal load (μN)

–1

123456

Fig. 22.25. Illustration of

the wear mechanism of

SAMs with increasing nor-

mal load [163]

SiCH

(CH

)

(CF

)

(CF

)

(CH

)

(CH

)

SiCH

(CH

)

n-Dimethyl(dimethylamino)silane Perfluorodecyltrichlorosilane n-Phosphonate

Octodecyl (ODDMS) Deca (PFTS) Octadecyl (ODP)Octa (ODMS) Octa (OP)

Fig. 22.26. (a) Schematics of structures of perﬂuoroalkylsilane and alkylsilane SAMs on Si

with native oxide substrates, and alkylphosphonate SAMs on Al with native oxide

contact angle and lower adhesive force compared to of ODMS/Si and ODDMS/Si.

The data for ODMS and ODDMS on the Si substrate are comparable to those for

OP and ODP on the Al substrate. Thus the substrate had little eﬀect. The coeﬃcient

of friction of various SAMs were comparable.

For wear performance studies, experiments were conducted on various ﬁlms.

Figure 22.27a shows the relationship between the decrease of surface height and the

22 Characterization of MEMS/NEMS and BioMEMS/BioNEMS 1243

ODDMSODMSPFTSSi Al OP ODP

Contact angle

(deg)

100

120

ODDMSODMSPFTSSi Al OP ODP

Adhesive force (nN)

ODDMSODMSPFTSSi Al OP ODP

Friction force (nN)

0.5

1.5

2.5

At 5 nN normal load

ODDMSODMSPFTSSi Al OP ODP

Coefficient of friction

0.01

0.02

0.04

0.05

0.06

0.03

Si substrate Al substrate

Fig. 22.26. (continued)

(b) contact angle, adhesive

force, friction force, and co-

eﬃcient of friction of Si with

native oxide and Al with na-

tive oxide substrates and with

various SAMs