Tribology and MEMS

For the convenience of our readers who are not familiar with MEMS technology, we have first given a brief introduction to MEMS.

Defining Microelectromechanical Systems (MEMS)  

In a simpler way, MEMS can be defined as the technology of microscopic devices, particularly those with moving parts. [1]

In engineering language it can be defined as a technology that integrates micro-mechanical and micro-electronic/micro-electrical devices and are fabricated using micro-fabrication techniques. [2]

History of MEMS

The physicist Richard Feynman delivered a talk at Caltech in December 1959 with the title “There’s Plenty of Room at the Bottom”. Feynman said that he wanted to talk about the problem of manipulating and controlling things on a small scale. In one way, Feynman laid the roots for today’s MEMS industry. [8][3]

In the early 1970s developments in micromachining and improvements to silicon processing led to pressure sensors with non-planar diaphragm geometries which yielded superior performance. These were arguably the first true MEMS sensors. [3]

Fabrication processes for MEMS [4][5]

  1. Deposition: Deposition of thin films of material on a substrate via chemical reaction (CVD) or physical reaction (PVD)
  2. Lithography: To apply patterned mask on top of the films by photolithographic imaging. Types of lithography include electron beam lithography, ion-beam lithography and x-ray lithography.
  3. Etching: Etching is the process of using strong acid to cut into the unprotected parts of a metal surface to create a design. There are two classes of etching processes: wet etching and dry etching.

 Practical applications of MEMS [6][7][8]

  1. To deposit ink on paper inkjet printers use piezo-electrics or thermal bubble ejection.
  2. In case of a car collision, accelerometers trigger airbag deployment. Cars also have Silicon pressure sensors to detect car tire pressure and inform the driver if it’s less by indicate light on dashboard.
  3. Microwave oven uses MEMS for heat adjustment and to detect whether the food is cooked or not.
  4. Laptops have an accelerometers for “drop detection” to save the hard disk head when free-fall is detected which prevents loss of data.
  5. Washing machines have vibration pressure sensor to determine load imbalance.
  6. Health bands and watches uses accelerometers, altimeter and other sensors to track steps, burnt calories, distance, activity and stationary time.
  7. Smartphones uses accelerometers, gyroscopes, motion sensors for landscape-to-portrait rotation, image Stabilization, auto focus, noise cancellation and other purposes.

Advantages of MEMS: [6][7][8]

  1. Easy to integrate into systems
  2. Ease of production
  3. Ease of parts alteration
  4. Inexpensive to make since produced in huge mass

Disadvantages of MEMS: [6][7][8]

  1. Since the size of MEMS is very small, it is physically impossible for MEMS to transfer any significant power.
  2. Materials used to make MEMS such as Poly-Si are brittle and hence cannot be loaded with large forces
  3. Designing and integrating MEMS devices is complex and require prior knowledge

Tribological failures in MEMS devices

Though there are many causes through which a MEMS device can fail, our focus would be mainly on failures related to tribology.

Stiction: Also known as adhesion, this kind of failure can be commonly found in non-moving internal parts of MEMS devices. Due to very small sizes of MEMS internal structures, surface forces like capillary condensation, van der Wall, electrostatic, forces due to chemical bonds between surfaces, cause microstructure surfaces to stick together when their surfaces are in contact. Apart from surface forces, environment can also effect the magnitude of stiction. In case of rough surfaces, higher number of asperities can be found in comparison to smooth surfaces. In humid environments, the gaps between asperities or valleys get filled up by water molecules which and hence there is an increase in surface contact between sliding surfaces which gives rise to adhesion.

Some of the solutions that can prove to be beneficial for avoiding/minimizing stiction are: [9][10][11][12][13]

  • Texturing the surface to reduce the contact area (dimples can be introduced)
  • Use of surface coating of materials having low surface energy
  • Proper insulation to avoid any disturbance from environment.

Wear: Four kinds of wears are generally witnessed in MEMS devices [9][10][11][12][13]

  • Adhesive wear: Materials bonded by surface forces when separate, they don’t separate at original interface and cause fracture of one of the material.
  • Abrasive wear: Abrasive wear occurs when asperities of a harder or rough surface ploughs a groove into softer surface during sliding. Particulates or particles present in the environment can get absorbed on to surface or slide between interfaces and cause 3-body abrasive wear.
  • Corrosive wear: When surfaces of MEMS devices interact in a chemical environment, degradation of surface occurs. Sliding can disintegrate and scatter rust particles or corrosive layer which can cause severe damage to MEMS internal structures. This kind of wear is common in microfluidic systems and biological MEMS.
  • Surface fatigue: This type of wear generally occurs in rolling elements such as gears and bearings. Repeated cyclic loading triggers development of cracks which by time propagate along the surfaces of structures causing partial or complete failure of MEMS device.

Some of the solutions that can prove to be beneficial for avoiding/minimizing wear are: [9][10][11][12][13]

  • Use of solid lubricants such as graphene or carbon-nanomaterial (fullerence C60)
  • Diamond like Carbon Coatings have shown significant reduction in friction and wear.
  • Self-assembled monolayer coatings also show better friction and wear resistance.

Creep:  Creep is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses at elevated temperatures. There are three stages of creep deformation: first rapid rate then uniform rate and at the end accelerated rate till the material breaks. An example of creep phenomenon in MEMS devices can be taken as RF MEMS switches which consist of Aluminum beams. These Aluminum beams under high temperatures and high mechanical stresses become susceptible to creep. [14]

Some of the solutions that can prove to be beneficial for avoiding/minimizing creep are:

  • SiC has been reported to be an excellent creep resistance material for MEMS at elevated temperatures. [15]
  • Using Aluminum alloys instead of pure Aluminum for RF MEMS helps in combating creep. Precipitation hardening of Aluminum alloys further enhances creep resistance. [16]

Delamination: Delamination occurs when a coating or a material looses adhesion to the surface. Delamination can be catastrophic to MEMS devices and can be caused via improper coating deposition onto substrate, cyclic stresses, wafers subjected to uneven temperatures, particulates etc. [9][10][11][12][13]

Some of the solutions that can prove to be beneficial for avoiding/minimizing delamination are:

  • 3-D woven composites and materials are less prone to delamination [17]
  • Interleaving technique or presence of adhesive interleaves in material structure helps in preventing delamination [18]
  • Thickness Stitching also helps in minimizing splitting apart of layers [19]
  • Ensuring a clean surface, free from particulates before coating deposition

References:

  1. Microelectromechanical systems, Wikipedia  https://en.wikipedia.org/wiki/Microelectromechanical_systems
  2. MEMSnet https://www.memsnet.org/mems/what_is.html
  3. History of microelectomechanical systems (mems), History of MEMS learning module, LIGA-micromachined gear for a mini electromagnetic motor [Sandia National Labs] http://scme-nm.org/files/History%20of%20MEMS_Presentation.pdf
  4. Mems (Intro Presentation) by Vinayak Hegde published on slideshare.com http://www.slideshare.net/vinayak.nandi/memsintro-presentation-1115551
  5. Micro Electromechanical System (MEMS) by Navin Kumar published on slideshare.com http://www.slideshare.net/navinec1/micro-electromechanical-system-mems
  6. Micro Electro Mechanical Systems (MEMS), An Overview and Application, Pune Institute of Computer Technology http://www.angelfire.com/indie/alfin/images/mems/What_is_MEMS_Technology.htm
  7. Introduction and application areas for MEMS http://www.eeherald.com/section/design-guide/mems_application_introduction.html
  8. MEMS and sensor industry group; What is MEMS http://www.memsindustrygroup.org/?page=WhatIsMEMS
  9. Fonseca, Daniel J., and Miguel Sequera. “On MEMS Reliability and Failure Mechanisms.” International Journal of Quality, Statistics, and Reliability 2011 (2011): 1-7 Full Paper Available online
  10. Walraven, J.a. “Failure Mechanisms in Mems.” International Test Conference, 2003. Proceedings. ITC 2003 Full Paper Available online
  11. David A. Brass, Dan Fuller, James L. Lovsin, “Tribology Concerns in MEMS devices: The Materials and Fabrication Techniques Used to Reduce Them” Final project report, December 2005 Full Paper Available online
  12. Yunhan Huang, Arvind Sai Sarathi Vasan, Ravi Doraiswami, Michael Osterman and Michael Pecht,” MEMS Reliability Review” IEEE transactions on device and materials reliability, vol. 12, no. 2, June 2012. Full Paper Available online
  13. Handbook of Silicon Based MEMS Materials and Technologies edited by Markku Tilli, Teruaki Motooka, Veli-Matti Airaksinen, Sami Franssila, Mervi Paulasto-Krockel, Veikko Lindroos; Chapter 5: Encapsulation and integration of MEMS, Pg. 746 to 749 Google Books Preview Only
  14. Creep (deformation), Wikipedia https://en.wikipedia.org/wiki/Creep_(deformation)
  15. Sarro, Pasqualina M. “Silicon Carbide as a New MEMS Technology.” Sensors and Actuators A: Physical 82.1-3 (2000): 210-18 Abstract Only
  16. Modlinski, R., P. Ratchev, A. Witvrouw, R. Puers, and I. De Wolf. “Creep-resistant Aluminum Alloys for Use in MEMS.” Journal of Micromechanics and Microengineering 15.7 (2005) Abstract Only
  17. Handbook of Ceramics and Composites: Mechanical Properties and Specialty Applications by Nicholas Cheremisinoff, Chapter 8: Delamination onset and growth in composite laminates, Pg. 350 Google Books Preview Only
  18. Philippidis, T.p., P. S. Theocaris, and G.S. Roupakias. “Delamination Prevention in Composites by Interleaving Techniques.” International Journal of Damage Mechanics 2.4 (1993): 349-63
  19. Through stitching by Professor Julian Ellis, http://www.ellisdev.co.uk/thezarc.html
  20. Image used is taken from pixabay.com which is free for commercial use 

HARSHVARDHAN SINGH
About HARSHVARDHAN SINGH 17 Articles
Harshvardhan Singh is an Automotive Engineer and has good experience in lubrication science and experimental tribology. He loves to write about tribology and related fields such as coating technology, surface engineering and others.

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