friction reduction in micro-motors using self-assembled monolayers

Friction Reduction in Micro-motors using

Self-Assembled Monolayers

ME 395 Project

Y. Zhu, J. Gregie & P. Prabhumirashi

5th June, 2000

SAM used in Micro-motors

Micro-motor is operating based on the electrostatic-drive principles. It’s composed of three main components: Stator, Rotor and Hub(bearing).

Friction becomes a serious problem compared with the usual macroscopic situation. So, contacting parts would have a limited lifetime due to wear.

SAM used in Micro-motors

stator

rotor

bearingMicro-motor operation is based on the electrostatic-drive principles. It’s composed of three main components: Stator, Rotor and Hub (bearing).

2

2

1CVEnergy E=

Torque

)(

2

1)( 2 C

VT Rotor rotation

excitation

Self-Assembled Monolayers (SAMs)

Surface Engineering - One of the major issues of concern.– Stiction - peeling

– Friction - vertical pull-off force

Modification of Surface– Topographic Modification

– Chemical Modification» Hydrogen Terminated Surfaces

» fluorocarbon films » Diamond-like Carbon Coatings

» SAMs

Self-Assembled Monolayers (SAMs)

Introduced in 1946 by Zisman Ordered molecular structures formed by adsorption on an active

surface Original application as building blocks for super-molecular structures Dense and Stable structures

– Applications in corrosion prevention, wear protection

Biocompatible nature– Applications in chemical and biochemical sensors

Used in semiconductor patterning Used in transducer technology Molecular level understanding of surface phenomena

Types of SAMs

Monolayers of Fatty acids– CnH2n+1COOH type acids– Driving force is the formation of a

surface salt between anion and cation Organosilicon Derivatives

– alkyloxysilanes, alkylaminosilanes– Driving force is in situ formation of

polysiloxane Organosulfur Adsorbates on Metal

Surfaces– alkanethiolets on Au (111)

Multilayers of Diphosphates Alkyl monolayers on Si

Synchronous Micro-motor Schematic -Top View

Stator

Rotor

Hub

GroundPlane

After Fan, et. al. (1988)

Micro-motor Fabrication

Insulate the Si substrate– Thermal Oxide– CVD Silicon Nitride - will also act as an etch stop

Deposit polysilicon and pattern grounding plate with Mask #1

Silicon/Poly-Si

SiO2

Si3N4

Mask #1

Micro-motor Fabrication

Deposit and pattern phosphosilicate glass (PSG) using Mask #2 Deposit polysilicon and pattern rotors and stators using Mask #3

Mask #2

Mask #3

PSG

Micro-motor Fabrication

Deposit an additional layer of PSG using Mask #4, to act as a spacer between the rotor and the hub.

Use PR and Mask #5 to etch PSG to form cavity for hub– RIE etch, followed by isotropic etch

Mask #4

Mask #5

Micro-motor Fabrication

Deposit polysilicon to form hub, using Mask #6 Use BHF to remove PSG, and deposit SAMs from solution

Mask #6

SAM

SAM deposition of alkyl-siloxanes

Oxidize surface– Native, thermal

Hydrate Surface, Hydrolysis of trichloro-silane – H2SO4:H2O2

Covalent bonding to the surface– Cross-linking

After Deng, et. al. (1995)

CH3

Si

[ [

n

OO

CH3

Si

[ [

n

O OOOH OH

Cl ClCl

CH3

Si

[ [

n

CH3

Si

[ [

n

OH OHOH

Oxide

Silicon

Under ideal conditions

SAM used in Micro-motors

1020304050607080

20 40 60 80 100 120Excitation Voltage(V)

Gea

r R

atio

ideal gear ratiowith OTSwithout OTS

ber

nn 0

Gear ratio is defined as the ratio of the electrical excitation frequency to the rotor rotational frequency.

From the figure, we can see how much the OTS monolayer reduces the friction.

SAM used in Micro-motors

y U

Y

Fluid mechanics model

cdd gMRcT

)/( YU

)(4

2 41

42 RR

YT

The torque due to the frictional forces

Shear stress on the bottom of rotor

The torque due to the viscous forces

SAM used in Micro-motors

R2

R1

Rc

Geometric description

0)(0)()(

tgMRctCdt

tdI cdV

Governing Equation

Moment of Inertia

I= 0.5 M (R22-R1

2)

Comparing this result with the experimental curve, we can get an estimate of Cd

Theoretical solution

)ln( 0

cdV

cdV

V gMRCC

gMRCC

C

It

SAM used in Micro-motorsNormal Load

Normal Load

Microscratch Test:

1) approaching the surface

2) indent into sample surface by loading the tip to 0.2mN

3) translating the sample at a constant load of 0.2mN

4) translating the sample in the opposite direction at ramping loads

5) unloading of the tip to 0.2mN

6) translating the sample at constant load of 0.2mN

7) final unloading of the tip

Conclusions

SAMs provide a means of reducing stiction and friction in micro-motors.

The size and chemistry of SAMs can be controlled and optimized from friction reduction

Deposition of SAMs on wear surfaces is an inexpensive and simple process.