1. MEMs Fabrication Alek Mintz 22 April 2015 Abstract
Microelectromechanical Systems (MEMS) are devices that integrate mechanical systems with electronic circuits. Fabrication of MEMS involves the use of specialized micromachining technologies. Device packaging aims to protect from outside damage and varies drastically depending upon application. Different materials used, fabrication techniques, and packaging types will be examined.
Key Words: Surface Micromachining, Bulk Micromachining, LIGA Lithography, Isotropic Etching, Anisotropic Etching, XeF2 Etching, MEMS Packaging

2. Outline Materials Bulk and Surface Micromachining
Etching and Deposition Techniques
Other Manufacturing Technologies
Wafer Bonding
Device Packaging

3. What are MEMS?
Small devices that use electrical and mechanical elements
Common uses include sensors and actuators
Made using modified semiconductor fabrication technology

4. Materials
Silicon
Polymers
Metals
Ceramics

5. Manufacturing Technologies
Bulk Micromachining
Surface Micromachining

6. Bulk Micromachining Oldest technology
Involves selective removal of substrate material
Can be done through physical or chemical means
Etching requires a masking material

7. Bulk Micromachining Advantages/Disadvantages
Can be done much faster
Can make high aspect ratio parts
Cheaper
Not easily integrated with microelectronics
Part complexity must be relatively simple
Part size is limited to being larger

9. Surface Micromachining
Newer than Bulk Micromachining
Uses single sided wafer processing
Involves use of sacrificial and structural layers
Provides more precise dimensional control

10. Surface Micromachining Advantages/Disadvantages
Mechanical properties of most thin- films are usually unknown and must be measured
Reproducibility of mechanical properties can be difficult
More expensive
Possible to integrate mechanical and electrical components on same substrate
Can create structures that Bulk Micromachining cannot
Cheaper glass or plastic substrates can be used

12. Deposition Techniques
Sputtering
Evaporation
Chemical Vapor Deposition – LPCVD, PECVD
Thermal Oxidation

13. Isotropic Etching Etching does not depend on crystal orientation
Etch rate of some etching solutions are dependent on dopant concentrations
Solution is stirred to keep homogeneity and allow for optimal etching

14. Anisotropic Etching Etch rates are dependent upon crystal orientation
Used more widely for Silicon micromachining
Allows for different etching shapes and better dimensional control

15. Etching Wet and dry etching
Uniformity of etching can vary across substrate
Timed etches difficult to control
Dopant and Electrochemical etch stops are used to control etch depth

16. Plasma Etching Gas usually contains molecules rich in Cl or F
CCl4, CH3F
Plasma ashing

17. Deep Reactive Ion Etching (DRIE)
Relatively new technology
Enables very high aspect ratio etches
Uses high density plasma to alternately etch and deposit etch resistant polymer on sidewalls

18. Lithographie Galvanoformung Adformung (LIGA)
Popular high aspect ratio micromachining technology
Primarily non-Silicon basted and requires use of x-ray radiation
Special mask and x-ray radiation makes process expensive

20. Hot Embossing Mold insert is made with inverse pattern
Substrate and polymer are heated and force is applied to create structure
Process can replicate complicated, deep features
Part costs very low compared to other technologies

21. XeF2 Etching Chemical etchant High Silicon selectivity
Stiction-free release

22. Laser Micromachining Lasers generate intense energy quickly
Focusing optics used to melt or vaporize material
Can produce very small features

23. Wafer Bonding
Silicon and Glass wafers can be bonded together to create systems using several parts
Bond using high temperatures
Bond using much lower temperature and large voltage

24. Eutectic Bonding
Bonding of Silicon substrate to another using intermediary level of gold

25. Packaging MEM die is extremely fragile
Must offer protection, connections to device, heat removal capabilities
Goal is to minimize size, cost, mass, complexity

26. Packaging Design
Thermal shock, vibration, acceleration, particles, radiation, electric + magnetic fields
Thermal expansion of packaging must be equal or slightly greater than that of Silicon to prevent cracking

27. Packaging Types
Metal
Ceramic
Plastic
Thin-Film Multilayer

28. Conclusion MEMS fabrication uses highly specialized technology
Devices are made using Bulk or Surface micromachining or a combination
Isotropic and Anisotropic etching
Popular etching and microstructure fabrication technologies
Wafer Bonding
Packaging types and considerations

29. References
"Fabricating MEMS and Nanotechnology." MEMS and Nanotechnology Exchange. Web. 18 Apr <
Gerke, R. "MEMS Packaging." University of Pennsylvania. Web. 18 Apr <
"Introduction to Microelectromechanical Systems (MEMS)." Brigham Young University. Web. 18 Apr < systems-mems>.

30. Key Concepts
Advantages and Disadvantages of Bulk and Surface Micromachining
Anisotropic and Isotropic etching
XeF2 etching
Wafer Bonding
Device Packaging considerations