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. Surface Contact Surfaces Interfacial Forces –Capillary Forces –Electrostatic Forces –Van der Waals Forces 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 SPIE 1998, 3512, 215-226.

5. Wear Mechanisms Adhesive –Low contact pressures –Augmented asperities Abrasive –High contact pressures –Wear tracks Tanner, D. M.; Peterson, K. A.; Irwin, L. W.; Tangyunyong, P.; Miller, W. M.; Eaton, W. P.; Smith, N. F. Proceedings of SPIE 1998, 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 SPIE 1998, 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. http://www.uwgb.edu/dutchs/PETROLGY/ Diamond%20Structure.HTM Ultrananocrystalline diamond thin films for MEMS and moving mechanical assembly devices A.R. Krauss, et al. Diamond and Related Materials 10(2001) 1952-1961

8. Conventional CVD Methane (CH 4 ) 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. UNCD ultrananocrystalline diamond A “better” method for producing diamond-like films. Grain size is 2-5nm. Unlike conventional diamond film CVD, C 60 is introduced into the reaction along with CH 4. –C 60 collides with itself, creating C 2 (carbon “dimers”) –These C 2 molecules enter the diamond lattice. –An abundance of C 2 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 Surface Finish Comparison Coating Uniformity Lack of internal stress allows for free- standing structures. 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. Self Assembled Monolayers (SAMs) OOOO Si O O OO OOO OOOO OH Deposition Biomaterials 23: 929-935 (2002) 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)

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

15. 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 Beam Structures Cantilever Beam Array Technique J. MEMS 7: 252-260 (1998) J. MEMS 10: 41-49 (2001) Results

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

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

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

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?