The Materials and Fabrication Techniques Used to Reduce Them

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The Materials and Fabrication Techniques Used to
Reduce Them
TRIBOLOGY CONCERNS IN MEMS DEVICES
Guide-PROF.S.CHAKKA presented
by-SHAIKH JAWED
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TRIBOLOGY ON THE MICROSCALE

• Surface Contact
• Surface Roughness
• Interfacial Forces
• Adhesion
• Friction
• Wear
• Models
• Environmental Effects

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Surface Contact

• Surfaces

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

asperities

• Adhesion

• Friction

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Wear
Wear debris identified as amorphous oxidized
silicon with no polysilicon.
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Wear Mechanisms

• Abrasive
• High contact pressures
• Wear tracks

• Adhesive
• Low contact pressures
• Augmented asperities

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Environmental Effects on Wear

• Humidity
• Volume of wear debris
• Morphology of wear debris

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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.

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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.

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Results/Problems of Conventional PECVD Diamond
Films

• Tribological properties better than silicon are
  achieved, but its not an ideal solution
• Low uniformity
• Non-constant density
• Amount of impurities and crystal growth suffers
  if dissociation is incomplete.

If coating isnt uniform, predicting failure is
difficult and surface finish suffers.
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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.

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Benefits of UNCD vs. Conventional CVD

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

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Demonstration
Coating Uniformity
Lack of internal stress allows for free-standing
structures.
Surface Finish Comparison
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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)

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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

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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
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Proof Mass Wear
Silicon Oxide
Post
Apparatus
Proof Mass
FDTS Covered
Post
Results
Proof Mass
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Electrostatic Lateral Output Motor
Relative humidity can determine if failure occurs
from Wear or Stiction
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Cantilevers in Contact Mode
Friction Test
Adhesion Test

• Tests materials at the nanoscale
• Cantilever tips are silicon nitride

Results
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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.

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Reference

• www.falexint.com
• www.wikipedia.org
• lqme.sjtu.edu.cn
• www.tribology-abc.com
• Design of machine elements- V.B BHANDARI
• Fundamentals of Tribology-Basu, Sengupta, Ahuja

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• THANK YOU