MEMS Fabrication Techniques and Processes

MEMS, FABRICATION Cody Laudenbach

Overview MEMS overview Electronics Fabrication Bulk Micromachining Chemical Wet Etching Isotropic vs anisotropic Deep Reactive Ion Etching Surface Micromachining Molding LIGA

MEMS Overview Microelectromechanical Systems Micrometer scale devices Can range from microns to millimeters Integrates electrical and mechanical parts Can be merged onto SI substrate with IC s Pressure sensors and gyroscopes https://www.memsnet.org/about/what-is.html

Electronics Fabrication Fabricated using IC processes Etching Wet Dry Lithography Photoresist Coating Masking Light exposure

Bulk vs Surface micromachining Bulk Micromachining Bulk micromachining is the selective removal of substrate material Surface Micromachining Surface micromachining is the deposition of thin film in alternating layers, with the sacrificial layer being removed altogether at the end of the process

Bulk Micromachining Chemical wet etching Isotropic Not dependent on crystallographic orientation Theoretically would etch in all directions at equal rates Silicon dioxide or silicon nitride typically used as masking Anisotropic Dependent on crystallographic orientation Etches much faster along <100> crystalline plane than that of the <111> plane Etch rates dependent on chemicals used such as KOH (Potassium Hydroxide)

Bulk Micromachining (Cont.) http://www.tf.uni-kiel.de/matwis/amat/semitech_en/kap_7/backbone/r7_2_2.html

Bulk Micromachining (Cont.) Deep Reactive Ion Etching Bosch process Process Dry plasma etch Alternates between etching and coating an etch resistant polymer layer on the side walls The layer at the bottom of pit is etched faster than that of the side walls

Deep Reactive Ion Etching https://www.mems-exchange.org/MEMS/fabrication.html

Surface Micromachining One of the most common methods of manufacturing MEMS devices Process Films are deposited on substrate, typically alternating between sacrificial and structural layers Patterned through photolithography Sacrificial parts removed, typically through wet chemical etching Results in parts that are able to move relative to the substrate Early processes used Polysilicon as structural layers and some form of silicon oxide for the sacrificial layer. metals, oxides and nitrides of silicon, and polymers have been used for both structural and sacrificial films.

Surface Micromachining (Cont.) https://www.mems-exchange.org/MEMS/fabrication.html

Molding LIGA German acronym for Lithography, Electroplating, and Molding Originally used x-ray source to expose layers out of photosensitive material Current processes use visible light or UV light sources for creating molds

Summary Microelectromechanical systems are a combination of electrical and mechanical components on the micrometer scale Electronic components created using IC fabrication processes, such as etching and lithography Mechanical components created through bulk micromachining, surface micromachining and molding Bulk micromachining is done through wet chemical etching or Deep Reactive Ion Etching Surface micromachining done through alternating structural and sacrificial film layers, then removing sacrificial layers Molding uses photolithography to etch out molds that are then used to create micromechanical parts

References "7.2.1 Important MEMS Processes." 7.2.1 Important MEMS Processes. N.p., n.d. Web. 07 Apr. 2017. <http://www.tf.unikiel.de/matwis/amat/semitech_en/kap_7/backbone/ r7_2_2.html>. "Deep Reactive-ion Etching." Wikipedia. Wikimedia Foundation, 26 Feb. 2017. Web. 07 Apr. 2017. <https://en.wikipedia.org/wiki/Deep_reactive-ion_etching#Bosch_process>. "Fabricating MEMS and Nanotechnology." MNX. N.p., n.d. Web. 07 Apr.2017.<https:// www.mems-exchange.org/MEMS/fabrication.html>. "Introduction to Microelectromechanical Systems (MEMS)." Introduction to Microelectromechanical Systems (MEMS) | Compliant Mechanisms. BYU, n.d. Web. 07 Apr. 2017.<https://compliantmechanisms.byu.edu/ content/introduction- microelectromechanical-systems-mems>. "Semiconductor Device Fabrication." Wikipedia. Wikimedia Foundation, 30 Mar. 2017. Web. 07 Apr. 2017.<https://en.wikipedia.org/wiki/Semiconductor_device_fabrication>. "What Is MEMS Technology?" What Is MEMS Technology? N.p., n.d. Web. 07 Apr. 2017. <https://www.memsnet.org/about/what-is.html>.

Five takeaways MEMS are a combination of electronic and mechanical components on the scale of micrometers Differences: bulk micromachining is the selective removal of substrate material, while surface micromachining is deposition of thin film in alternating layers, with the sacrificial layer being removed altogether at the end of the process DRIE is the process of a dry plasma etching alternating with an etch resistant polymer which allows for a high aspect ratio Anisotropic wet chemical etching is more widely used as it has a higher resolution etching capability Molding is another process of which photolithography is used to create a mold for micromechanical components to then be formed with