Nanotribology Lab NC State

Micro-Electro-Mechanical Systems:

These Squeaky Wheels will get no Grease

Jacqueline Krim, Department of Physics

Iron on copperTemp. = 4 K

M.F. Crommie, C.P. Lutz, D.M. Eigler, E.J. Heller.,Surf. Rev. and Lett. 2 (1), 127-137 (1995)

U. Kunze and B. Klehn,Adv. Mat. 11, 1473 (1999)

R. Overney and E. Meyer,MRS Bulletin, May 1993, p. 26.

Polymer mask over SiO2 film on Si,etched in HF.

50 nm line widths

Flourocarbon / Hydrocarbon mixtures.

70 nm line widths

Molecular Machines: Future or Fancy?

Atomic Scale Engines:Cars and Wheels

M. Porto et al., PRL 84, 1608 (2000)

Every week

• Apply a few drops of engine oil to the spark and throttle cross-shaft brackets

• Apply sufficient amounts of engine oil to all brake clevised, oiler, and cross-shaft brackets, at least 12 locations

• Force a “grease gun full”(half cup) of grease into the universal joint

• Pack the ball joints of the steering mechanism with grease more…..

Every Day

• Check Oil in Engine, oil lubricated clutch, tranmission, and differential gear housing

• Turn grease cup caps on the 8 spring bolts, one turn

• Apply a few drops of engine oil to the tie rod clevises

• Turn the grease cup on the fan support

• …….more

1916 Maxwell Owner’s Manual

2000 miles

• Drain rear axle, flush with kerosene and refill.

• Drain crank case, flush with kerosene, and refill (several quarts)

• Jack up car by the frame, pry spring leaves apart, and insert graphite grease between the leaves.

Monthly

• Force a “grease gun full” of grease into the engine timing gear.

• Force a “grease gun full” of grease into the steering gear case.

• Apply a few drops of 3-in-1 oil to the magneto bearing.

• Turn the grease cup on the generator drive shaft, one turn.

• Turn the grease cup on the drive shaft bearing, one turn.

• ……more

1916 Maxwell Owner’s Manual

Regularly • Check engine valve action• Inspect ignition wiring.• Check battery fluid level and

color.• Inspect cooling system for

leaks.• Check fan belt tension.• Inspect steering parts.• Tighten body and fender

bolts.• Check effectiveness of

brakes.

• more…..

Biweekly

• Check engine compression.

• Listen for crankshaft bearing noises.

• Clean and regap spark plugs.

• Adjust carburetor mixture.

• Clean gasoline strainer.

• Drain water from carburetor bowl .

• Inspect springs.• More….

1916 Maxwell Owner’s Manual

MicroElectro Mechanical Systems

Advantages: Mass-fabrication, low-cost and IC integration

Application : Whole new line of applications, limited only by imagination

• MEMS Microsurgery devices

• Miniature valves, pumps

• MEMS accelerometer used in Airbags

An emerging cutting-edge technology which relies on microfabrication of small scale IC compatible mechanical components

Photo Courtesy M. Adrian Michalicek, University of Colorado at Boulder

MEMS Application

Science News, July 22, 2000

Surface effects that dominate bulk effectsHigh temperature processing conditionsLubricant deliveryLubricant replenishment

Tribological issues related:

• Stiction (Release and/or In-use)

• Friction and wear

Stiction: Unintentional adhesion of microstructure surfaces where the restoring forces are unable to overcome interfacial forces

MEMS Tribology Issues

Release related stiction

• Caused mainly by liquid capillary forces

Adhesion of micromachined structures to the underlying substrate after the final sacrificial layer etch

Photo Courtesy University of California at Berkley

Approaches to solving release related stiction

• Self-assembled monolayer (SAMs) OTS, FDTS, DDMS

- Capillary pull can be made into a push if the contact angle is made larger than 90 °

- Due to hydrophobicity of these coatings, capillary forces responsible for release-related stiction are eliminated

Photo Courtesy University of California at Berkley

• Supercritical carbon dioxide drying of Microstructures

• Freeze sublimation drying

- Avoidance of liquid-vapor interfaces through supercritical fluid

- Allows samples to be dried without any surface tension, thus reducing the likelihood of stiction

Alternate release methods

• developing stiction reducing chemical additives for final rinse stages

• development of vapor-phase lubricants for use in extreme MEMS operating environments

• A knowledge of nanotribology is required, as contact areas may include only tens of atoms!

Permanent adhesion through acceleration or electrostatic forces and/or adhesive forces between surfaces causing permanent device failure

Approaches towards solving Stiction/Friction

In-use stiction and/or Friction

Nanotribology Lab NC State

Nanotribology and the Atomic-Scale Origins of Friction:

What Once Was Old Is New Again.

m

mg FN

v

Ff = FN

m

v

Amontons, 1699

F = ma

Newton, 1686

F

Ff

m

mg FN

v

1, 2 Guillaume Amontons, 1699 3 Charles-Augustin de Coulomb, 1785

Ff

Classical Laws of Friction:

1) Ff = FN

2) independent of apparent contact area3) independent of sliding speed

depends on whether object is at rest or moving - “static friction” vs. “kinetic friction”.

s k

Meanwhile, for solid-liquid interfaces, “viscous friction” applies,

where, vm

F

Leonardo da VinciCodex AtlanticusCodex Arundelca. 1500k = 0.25

Charles-Augustin de CoulombThéorie des Machines Simple1785

• QCM: unconfined geometry, “viscous friction”, no static friction

• SFA: Confined “planar” geometry, higher friction levels, “barrier to induce motion” always observed

• LFM: Confined “point” geometry, highest friction levels, static friction always observed.

J. Krim, Scientific American, Oct. 1996.

vm

F

SFA Measurement

Compared toluene on mica to C60/toluene solution on mica.Found that C60 formed 1-2 monolayers on the mica--and these adsorbed layers “possess unusually high fluidity and are easily disrupted.”Found that the viscous response of the fluid near the mica surface was completely different for the C60/toluene solution as compared to the toluene alone. The C60 toluene solution exhibited full-slip boundary conditions. Does this imply it will be a good additive to lubricants?

Toluene alone C60/Toluene Solution

S.E. Campbell, G. Luengo, V.I. Srdanov, F. Wudl, and J.N. Israelachvili, Nature, 382, 520-522 (1996).

-1.0 -0.5 0.0 0.5 1.0-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Fri

ctio

na

l Fo

rce

(vo

lts)

Normal Force (volts)

Toluene on Mica C

60/Toluene Solution on Mica

AFM Measurements

Scan speed: 500 nm/sec with silicon nitride cantilever10% C60/Toluene solution

AFM Measurements

mica under toluene, force error image

mica under toluene, lateral force image

mica under ~20% C60/toluene solution, force error image

60 angstroms

50 angstroms

Ag(111)

Ag(111)

Cu(111)

Ag(111) Control

0 HzWhen C60 molecules form a monolayer on Cu(111), the molecules lock in to a specific direction on the terraces and the free rotation is suppressed.T. Sakurai et al, Applied Surface Science 87/88 (1995) 405-413.

109 HzFor Ag(111), the C60 molecules in the second layer rotate at frequencies matching that of bulk C60. E.I. Altman

and R.J. Colton, Surface Science 295 (1993) 13-33

< 1 HzFor Ag(111), the C60 molecules in the first monolayer do rotate, but slowly. E.I. Altman and R.J. Colton, Surface Science 295 (1993) 13-33

C60 Rotation

AFM Results

The films were evaporated in UHV conditions onto freshly cleaved mica surfaces. They were then transferred to a liquid cell and completely submerged in methanol for the AFM measurements. The measurements were acquired under methanol in order to avoid capillary effects.

Quartz Crystal Microbalance

Single crystal quartz

Metal film electrode

A.

B.

Figure 8

Measuring friction with a quartz crystal microbalance (QCM)

• Thin crystal disk oscillates in a shear mode

• Adsorbed material lowers the resonant frequency

• If the shear stress is below about 103 N/m2, it will “slip” enough to be detected by the QCM:

• The slip time is deduced from Q and f:fQ 4)( 1

)()( 11 AQ

(Krim and Widom, PRB, v. 38, n.17, 1988)

QCM Results

Toluene on Ag

Toluene on C60/Ag

QCM Results

Toluene on Ag

Toluene on Ag/C60

Here, we find that C60 is sticky, while toluene is slippery.

Pre

ssur

e, M

Pa

Velocity, mm/s0 50 100 150 200 250 800 1000

0

200

400

600

800

1000

AFM/IFM

SFA

micromachines

atomisticsimulation

microenginespeed record

STM-QCMP = 0 - 1000 GPaV = 100 - 3000 mm/sContact radius = 10 nm - 1 m

Proceedings of NIST Nanotribology WorkshopGaithersburg, MD March 13-15, 2000

“Existing molecular scale test methods do not duplicate the operating P-V space of micromachines” -- M.T. Dugger, Sandia Labs

They also do not duplicate the operatingP-V space of macroscopic machines….

Alternate approaches are required to study MEMS

lubricants.“Existing molecular scale test methods do not duplicate the operating P-V space of micromachines” -- M.T. Dugger, Sandia Labs

Pre

ssur

e, M

Pa

Velocity, mm/s0 50 100 150 200 250 800 1000

0

200

400

600

800

1000

AFM/IFM

SFA

micromachines

atomisticsimulation

microenginespeed record

STM-QCMP = 0 - 1000 GPaV = 100 - 3000 mm/sContact radius = 10 nm - 1 m

Proceedings of NIST Nanotribology WorkshopGaithersburg, MD March 13-15, 2000

Surface micromachined device to investigate friction & wear

Photo Courtesy University of California at Berkley

Comb-drive

Photo courtesy Sandia National Laboratories

MEMS Friction tester Diagram

05

1015202530354045

0 1 2 3 4 5 6

Time

Am

plit

ude

(um

)

Vacuum system used to release the vapor-phase lubricants

Why vapor-phase lubricants?

• semiconductor-like fabrication of MEMS devices

• small size

• monolithic nature of micromachines

Difficulties in lubricating MEMS devices because of

Vapor phase may ultimately prove to be an effective and perhaps exclusive means to deliver and/or replenish lubricants

Current focus on:

• Development of realistic laboratory test set-ups which are both well controlled and relevant to operating machinery

• Understanding the chemical and tribochemical reaction which occur in sliding contact

• Characterization of the microstructural and mechanical properties of the micromachined contact region

Quartz Crystal Microbalance

Scanning Tunneling Microscope

STM-QCM

quartz disk

metal electrodesfilm

metal tip

500 × 500 nm2

STM tip

TunnelingcurrentMetal electrode

Quartz

stationary vibrating

Desirable Properties of a MEMS lubricant

Low friction Low wear Effective as very thin film Uniform adhesion to substrate Durable and Replenishable Specificity Usable in extreme environments (temperature,

pressure)

TCP is known to exhibit many of these properties in macroscopic tests. (downside: possible corrosion)

QUESTION: Does it exhibit these same favorable properties in nanometer-scale tests using STM-QCM?

P

O

C

H

TCP in purified form

3D-TRICRESYL PHOSPHATE (TCP)

P

O

C

H

Atomic-scale studies of an anti-wear additive proven effective in extreme

environments of high temperature and pressure.

Experimental System

Quartz Crystal Microbalance

(QCM)

TCP vapor

Metal substrate

TCP film

0 10 20 30 40 50 60 700

10

20

30

40

50

60

-F

(Hz)

0 10 20 30 40 50 60 700.000.020.040.060.080.100.120.14

(1/

A)

(v-1)

10-7 *

(1

/Q) fi

lm

Time (min)

0

50

100

150

200

Mas

sD

epo

site

d (

ng

)

02468101214

TRICRESYL PHOSPHATE (TCP)/Fe

M. Abdelmaksoud, J. Bender and J. Krim, Trib. Lett. submitted

O

C

P

H

0.0 0.5 1.0 1.5 2.05E-10

1E-9

2E-9

3E-94E-95E-9

1E-8

2E-8

3E-84E-85E-8

S

lip T

ime

(s)

Coverage (monolayers)

TCP slip times are comparable to those of physisorbed monolayers!

O

C

P

H

0 2 4 6 8 10 12 14

0

200

400

600

TCP/Fe at 3000C

TCP/Cr at 3000C

O2 uptake

on TCP exposed surfaces

-F

(Hz)

0 2 4 6 8 10 12 14

0

20

40

60

80

10-7 *

(1

/Q) fil

m

Time (min)

TCP at high temperature: Polymeric materialFormation observed in combination with

oxygen gas uptake, but only for iron substrates.

How does this film respond to tribological contact?

STM-QCM of 10 Å TBPP film

200 × 200 nm2

QCM OFF

QCM ON

Room Temperature

Liquid TBPP cannot diffuse back into the rubbed region faster than the QCM vibrational speed. Therefore, an image can be obtained when the QCM is vibrating.

STM-QCM of 10 Å TBPP film

40 × 40 nm2

After Heating

F = + 0.6 Hz

After annealing, the polymeric surface is more conducting , and ‘nonrigid’.

STM-QCM of 10 Å TBPP film

40 × 40 nm2

QCM OFF

QCM ON

After annealing

Images from article by Peter Weiss in Science News, July 22, 2000

F = 0.0 Hz

F = + 2.3 Hz

After rubbing, the rubbed region exhibits lower friction, and evidence for a tribochemical reaction.

Frequency shift data for 10 Å film of TBPP

0 10 20 30 40 50 60 70 80 90100-80-60-40-20

020406080

100120140160

F (

Hz)

Normal Load (a.u.)

Lubricated contact Lubricated and annealed contact Unlubricated contact

Molecularly thin films dramatically change interfacial properties.

Films are tenacious, durable, yet very thin.

Best performance in extreme environments - high temperature and pressure

~100 GPa

Conclusions• A good MEMS lubricant exhibits high

flexibility, very low friction and high adhesion.

• Static friction (and the closely related phenomenon of stick-slip) is largely associated with confined (like a sandwich) geometries

• Static friction can be totally absent in an unconfined geometry at both microscopic and macroscopic scales, and for both solid-solid and solid-liquid interfaces.

• Sliding friction has been observed to increase by five orders of magnitude in going from an unconfined to confined geometry

Nanotribology Lab NC State

Micro-Electro-Mechanical Systems:

These Squeaky Wheels will get no Grease

Jacqueline Krim, Department of Physics

The end !! *

*On the QCM-STM test setup

Bucky Ball Background

C60 Trivia• an aromatic molecule

• molecular diameter: 0.71 nm• forms an FCC lattice with• nearest neighbor distance

of 1.003 nm• a blackish powder, dissolvesreadily in toluene and benzene

• sublimes readily at 450 C

Some Previous Work

B. Bhushan et al., Appl. Phys. Lett., 1993.ExperimentThey coated silicon with C60 in vacuum. They performed friction andwear tests with a ball on flat tribometer under reciprocating motion. Thefriction force was measured with strain gauges.ResultsThe C60 did reduce the coefficient of friction, but the films scratched offat high load.

T. Thundat et al., Appl. Phys. Lett., 1993.ExperimentThey sublimed C60 onto silicon wafers and cleaved mica. They acquired AFM images and LFM. ResultsThey found a higher coefficient of friction for C60 on mica (0.9) than for bare mica (0.1). They found that for high forces the tip pushed through the fullerene layers.