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Tribology Concerns in MEMS Devices: The Materials and Fabrication Techniques Used to Reduce Them

Tribology Concerns in MEMS Devices: The Materials and Fabrication Techniques Used to Reduce Them. ME 381 – Final Project David Brass, Dan Fuller, Jim Lovsin December 6, 2004. Tribology on the Microscale. Surface Contact Surface Roughness Interfacial Forces Adhesion Friction Wear

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Tribology Concerns in MEMS Devices: The Materials and Fabrication Techniques Used to Reduce Them

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  1. Tribology Concerns in MEMS Devices: The Materials and Fabrication Techniques Used to Reduce Them ME 381 – Final Project David Brass, Dan Fuller, Jim Lovsin December 6, 2004

  2. Tribology on the Microscale • Surface Contact • Surface Roughness • Interfacial Forces • Adhesion • Friction • Wear • Models • Environmental Effects

  3. Surfaces Interfacial Forces Capillary Forces Electrostatic Forces Van der Waals Forces Surface Contact asperities • Adhesion • Friction Bhusan, B. Handbook of Micro/Nano Tribology

  4. Wear Wear debris identified as amorphous oxidized silicon with no polysilicon. Tanner, D. M.; Peterson, K. A.; Irwin, L. W.; Tangyunyong, P.; Miller, W. M.; Eaton, W. P.; Smith, N. F. Proceedings of SPIE1998, 3512, 215-226.

  5. Adhesive Low contact pressures Augmented asperities Abrasive High contact pressures Wear tracks Wear Mechanisms Tanner, D. M.; Peterson, K. A.; Irwin, L. W.; Tangyunyong, P.; Miller, W. M.; Eaton, W. P.; Smith, N. F. Proceedings of SPIE1998, 3512, 215-226.

  6. Environmental Effects on Wear • Humidity • Volume of wear debris • Morphology of wear debris Tanner, D. M.; Peterson, K. A.; Irwin, L. W.; Tangyunyong, P.; Miller, W. M.; Eaton, W. P.; Smith, N. F. Proceedings of SPIE1998, 3512, 215-226.

  7. Diamond Coatings • Diamond has a variety of useful properties compared to Silicon • Low wear, low coefficient of friction, thermally stable, isotropic hardness • Diamond cannot be simply made into smaller and smaller flakes, then deposited on MEMS devices • Diamond (or diamond-like) film must be grown on surface. Ultrananocrystalline diamond thin films for MEMS and moving mechanical assembly devices A.R. Krauss, et al. Diamond and Related Materials 10(2001) 1952-1961 http://www.uwgb.edu/dutchs/PETROLGY/Diamond%20Structure.HTM

  8. Conventional CVD • Methane (CH4) is introduced as a plasma in a PECVD process. • The disassociated carbon ions deposit on the MEMS device. • Under correct conditions, the carbon atoms form a diamond-like film. Influencing factors on microtribology of DLC films for MEMS and microactuators R. Bandorf, et al.

  9. Results/Problems of Conventional PECVD Diamond Films • Tribological properties better than silicon are achieved, but it’s not an ideal solution: • Low uniformity • Non-constant density • Amount of impurities and crystal growth suffers if dissociation is incomplete. If coating isn’t uniform, predicting failure is difficult and surface finish suffers. Ultrananocrystalline diamond thin films for MEMS and moving mechanical assembly devices A.R. Krauss, et al. Diamond and Related Materials 10(2001) 1952-1961

  10. UNCDultrananocrystalline diamond • A “better” method for producing diamond-like films. Grain size is 2-5nm. • Unlike conventional diamond film CVD, C60 is introduced into the reaction along with CH4. • C60 collides with itself, creating C2 (carbon “dimers”) • These C2 molecules enter the diamond lattice. • An abundance of C2 is the goal of the UNCD creation process.

  11. Benefits of UNCD vs. Conventional CVD • Properties more like natural diamond • Method allows for uniform coating • Very little residual stress.

  12. Demonstration Coating Uniformity Lack of internal stress allows for free-standing structures. Surface Finish Comparison Ultrananocrystalline diamond thin films for MEMS and moving mechanical assembly devices A.R. Krauss, et al. Diamond and Related Materials 10(2001) 1952-1961

  13. Deposition O O O Si Si Si Si O O O O OH OH OH OH Si Si Si Si Si Si Si Si O O O O O O O O Self Assembled Monolayers (SAMs) • Two Types • Silane – deposits on silicon • Thiol – deposits on gold • Deposition Formations • Densely Packed • Amorphous Structure • Functional group determines: • applications • hydrophilicity/hydrophobicity • Used as: • binders for subsequent molecules • lubricants • Common hydrophobic SAMS: • OTS (long chain hydrocarbon) • FDTS (long chain fluorocarbon) Biomaterials 23: 929-935 (2002)

  14. Interstitial SAMs for Deposition Step 1: Deposit SAM layer of 3-mercaptopropyl trimethoxysilane (-SH terminus) • Step 2: • Oxidize SAM layer • Forms -SO3H terminus • Step 3: • Deposited Ceramic layer • ZrO2 in the presence of HCl • Y2O3 in the presence of urea Appl. Surf. Sci. 221: 272-280 (2004)

  15. Cantilever Beam Array Technique Beam Structures • Cantilever beams are fabricated of different lengths • Cantilevers are put into contact with surface • Longer beams adhere to surface • Longest beam that does not stick signifies adhesion force • SAM coated beams adhere after longer lengths than oxide surface Results J. MEMS 7: 252-260 (1998) J. MEMS 10: 41-49 (2001)

  16. Proof Mass Wear Silicon Oxide Post Apparatus Proof Mass FDTS Covered Post Results Proof Mass Wear 253: 739-745 (2002)

  17. Electrostatic Lateral Output Motor Relative humidity can determine if failure occurs from Wear or Stiction Tribology Letters 9: 199-209 (2000)

  18. Cantilevers in Contact Mode Friction Test Adhesion Test • Tests materials at the nanoscale • Cantilever tips are silicon nitride Results Wear 254: 974-980 (2003) J. Tribology 126: 583-590 (2004)

  19. Conclusion • Friction and Wear are the biggest issues in blocking advances of MEMS technology • Once SAMS and Diamond Coatings are more fully developed, MEMS technology will be able to more completely realize its potential.

  20. Tribology and MEMS • Questions?

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