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 – Models– Environmental Effects

Surface Contact

• Surfaces

• Interfacial Forces – Capillary Forces – Electrostatic Forces – Van der Waals

Forces

asperities

• Adhesion • Friction

Bhusan, B. Handbook of

Micro/Nano Tribology

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.

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.

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.

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 devicesA.R. Krauss, et al.Diamond and Related Materials 10(2001) 1952-1961

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 microactuatorsR. Bandorf, et al.

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 devicesA.R. Krauss, et al.Diamond and Related Materials 10(2001) 1952-1961

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.

Benefits of UNCD vs. Conventional CVD

• Properties more like natural diamond• Method allows for uniform coating• Very little residual stress.

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 devicesA.R. Krauss, et al.Diamond and Related Materials 10(2001) 1952-1961

Self Assembled Monolayers (SAMs)

O O O OSi Si Si SiO O O O

Si Si Si Si

O O O

O O O OSi Si Si SiOH OH OH 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)

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 -SO3H terminus

Step 3:Deposited Ceramic layer

• ZrO2 in the presence of HCl• Y2O3 in the presence of urea

• 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

Proof Mass Wear

Wear 253: 739-745 (2002)

Silicon Oxide

FDTS Covered

Post

Proof Mass

Post

Proof Mass

Apparatus

Results

Electrostatic Lateral Output Motor

Tribology Letters 9: 199-209 (2000)

Relative humidity can determine if failure occurs from Wear or Stiction

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

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.

Tribology and MEMS

• Questions?