Transcript Friction Reduction in Micro-motors using Self

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
Micro-motor operation is based
on the electrostatic-drive
principles. It’s composed of three
main components: Stator, Rotor
and Hub (bearing).
Energy
E=
excitation
bearing
stator
1
CV 2
2
Torque T ( )  1 V 2 C ( )
2

rotor
Rotor rotation
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)

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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

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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
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Organosilicon Derivatives
– alkyloxysilanes, alkylaminosilanes
– Driving force is in situ formation of
polysiloxane
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Organosulfur Adsorbates on
Metal Surfaces
– alkanethiolets on Au (111)
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Multilayers of Diphosphates
Alkyl monolayers on Si
Synchronous Micro-motor Schematic -Top View
Stator
Rotor
Ground
Plane
Hub
After Fan, et. al. (1988)
Micro-motor Fabrication
Silicon/Poly-Si
SiO2
Si3N4
 Insulate
the Si substrate
Mask #1
– Thermal Oxide
– CVD Silicon Nitride - will also act as an etch stop
 Deposit
polysilicon and pattern grounding plate with
Mask #1
Micro-motor Fabrication
PSG
Mask #2
Mask #3
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Deposit and pattern phosphosilicate glass (PSG) using Mask #2
Deposit polysilicon and pattern rotors and stators using Mask #3
Micro-motor Fabrication
Mask #4
Mask #5
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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
Micro-motor Fabrication
Mask #6
SAM
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Deposit polysilicon to form hub, using Mask #6
Use BHF to remove PSG, and deposit SAMs from solution
SAM deposition of alkyl-siloxanes
OH
[
OH
n
Si
Cl

[
Silicon
Cl Cl
[
[ [
[
CH3
CH3
CH3
CH3
[ [
Oxide
n
n
Si
OH
OH OH
Si
Si
O
O
n
O
O
O
Oxidize surface
– Native, thermal
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Hydrate Surface, Hydrolysis of trichloro-silane
– H2SO4:H2O2
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Covalent bonding to the surface
– Cross-linking
After Deng, et. al. (1995)
Gear ratio is defined as the
ratio of the electrical
excitation frequency to the
rotor rotational frequency.
Under ideal conditions
n  n0 
rbe

Gear Ratio
SAM used in Micro-motors
80
70
60
50
40
30
20
10
ideal gear ratio
with OTS
without OTS
20
40
60
80
Excitation Voltage(V)
From the figure, we can see how
much the OTS monolayer
reduces the friction.
100
120
SAM used in Micro-motors
The torque due to the frictional forces
y
U
Td  cd gMRc
Y
Shear stress on the bottom of rotor
   (U / Y )
Fluid mechanics model
The torque due to the viscous forces
T
2 4
( R2  R14 )
4Y
SAM used in Micro-motors
Governing Equation
I
d (t )
 CV  (t )  cd gMRc  0
dt
 (t )  0
Moment of Inertia
R1
I= 0.5 M (R22-R12)
Theoretical solution
t
C   Cd gMRc
I
ln( V 0
)
CV
CV   Cd gMRc
Comparing this result with the
experimental curve, we can get an
estimate of Cd
Rc
R2
Geometric description
SAM used in Micro-motors
Microscratch Test:
Normal Load
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
Normal Load
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
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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.