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 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 bearing Micro-motor operation is based on the electrostatic-drive principles. It’s composed of three main components: Stator, Rotor and Hub (bearing). Energy E= Torque 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 –C n H 2n+1 COOH 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 Ground Plane 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 SiO 2 Si 3 N 4 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 –H 2 SO 4 :H 2 O 2  Covalent bonding to the surface –Cross-linking After Deng, et. al. (1995) CH 3 Si [ [ n O O CH 3 Si [ [ n O O O OH Cl CH 3 Si [ [ n CH 3 Si [ [ n OH Oxide Silicon

Under ideal conditions SAM used in Micro-motors 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 yU Y Fluid mechanics model 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 Governing Equation Moment of Inertia I= 0.5 M (R 2 2 -R 1 2 ) Comparing this result with the experimental curve, we can get an estimate of C d Theoretical solution

SAM used in Micro-motors 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.