friction reduction in micro-motors using self-assembled monolayers
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Friction Reduction in Micro-motors using Self-Assembled Monolayers. ME 395 Project Y. Zhu, J. Gregie & P. Prabhumirashi 5 th June, 2000. SAM used in Micro-motors. - PowerPoint PPT PresentationTRANSCRIPT
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Friction Reduction in Micro-motors using
Self-Assembled Monolayers
ME 395 Project
Y. Zhu, J. Gregie & P. Prabhumirashi
5th June, 2000
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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.
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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
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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
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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
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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
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Synchronous Micro-motor Schematic -Top View
Stator
Rotor
Hub
GroundPlane
After Fan, et. al. (1988)
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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
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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
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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
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Micro-motor Fabrication
Deposit polysilicon to form hub, using Mask #6 Use BHF to remove PSG, and deposit SAMs from solution
Mask #6
SAM
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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
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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.
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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
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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
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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
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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.