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
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 Micromachining
Advantages/Disadvantages
• Not easily integrated with
microelectronics
• Can be done much faster
complexity must be
• Can make high aspect ratio parts • Part
relatively simple
• Cheaper
• 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 • Mechanical properties of most thinelectrical components on same
substrate
• Can create structures that Bulk
Micromachining cannot
• Cheaper glass or plastic substrates
can be used
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-microelectromechanicalsystems-mems>.
Key Concepts
• Advantages and Disadvantages of Bulk and Surface Micromachining
• Anisotropic and Isotropic etching
• XeF2 etching
• Wafer Bonding
• Device Packaging considerations