MEMs FabricationAlek Mintz

22 April 2015

AbstractMicroelectromechanical 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

Outline

• Materials

• Bulk and Surface Micromachining

• Etching and Deposition Techniques

• Other Manufacturing Technologies

• Wafer Bonding

• Device Packaging

What are MEMS?

• Small devices that use electrical and mechanical elements

• Common uses include sensors and actuators

• Made using modified semiconductor fabrication technology

Materials

• Silicon

• Polymers

• Metals

• Ceramics

Manufacturing Technologies

• Bulk Micromachining

• Surface Micromachining

Bulk Micromachining

• Oldest technology

• Involves selective removal of substrate material

• Can be done through physical or chemical means

• Etching requires a masking material

Bulk MicromachiningAdvantages/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

Surface Micromachining

• Newer than Bulk Micromachining

• Uses single sided wafer processing

• Involves use of sacrificial and structural layers

• Provides more precise dimensional control

• Involves use of sacrificial and structural layers

Surface Micromachining Advantages/Disadvantages

• 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

• Mechanical properties of most thin-films are usually unknown and must be measured

• Reproducibility of mechanical properties can be difficult

• More expensive

Deposition Techniques

• Sputtering

• Evaporation

• Chemical Vapor Deposition – LPCVD, PECVD

• Thermal Oxidation

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

Anisotropic Etching

• Etch rates are dependent upon crystal orientation

• Used more widely for Silicon micromachining

• Allows for different etching shapes and better dimensional control

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

Plasma Etching

• Gas usually contains molecules rich in Cl or F

• CCl4, CH3F

• Plasma ashing

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

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

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

XeF2 Etching

• Chemical etchant

• High Silicon selectivity

• Stiction-free release

Laser Micromachining

• Lasers generate intense energy quickly

• Focusing optics used to melt or vaporize material

• Can produce very small features

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

Eutectic Bonding

• Bonding of Silicon substrate to another using intermediary level of gold

Packaging

• MEM die is extremely fragile

• Must offer protection, connections to device, heat removal capabilities

• Goal is to minimize size, cost, mass, complexity

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

Packaging Types

• Metal

• Ceramic

• Plastic

• Thin-Film Multilayer

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

References

• "Fabricating MEMS and Nanotechnology." MEMS and Nanotechnology Exchange. Web. 18 Apr. 2015. <https://www.mems-exchange.org/MEMS/fabrication.html>.

• Gerke, R. "MEMS Packaging." University of Pennsylvania. Web. 18 Apr. 2015. <http://www.seas.upenn.edu/~meam550/PackagingJPL.pdf>.

• "Introduction to Microelectromechanical Systems (MEMS)." Brigham Young University. Web. 18 Apr. 2015. <https://compliantmechanisms.byu.edu/content/introduction-microelectromechanical-systems-mems>.

Key Concepts

• Advantages and Disadvantages of Bulk and Surface Micromachining

• Anisotropic and Isotropic etching

• XeF2 etching

• Wafer Bonding

• Device Packaging considerations