Micro Electro Mechanical Systems (MEMS) Device Fabrication What is a MEM? How are they used? How are they made? What are the challenges encountered during fabrication? EE4611 04/13/2016 David Niska

Preview MEM applications Fabrication Techniques MEM design challenges Lithography Sacrificial layers Micromachining Surface Bulk HAR Material deposition MEM design challenges MEM industry challenges

What is a MEM? Applications Sensors Actuators Sensor Animation http://www.sandia.gov/mstc/mems/ http://electroiq.com/blog/2013/10/mems-devices-for-biomedical-applications/

What’s so special about that?? Grain of pollen

Device Fabrication Basics Substrate (Si Wafer) Photoresist applied Photoresist masked and exposed Photoresist removal Further processing Materials etched/dissolved And/or Material deposition Rinse/Repeat http://www.mdpi.com/1424-8220/8/9/6077/htm

Keystone Bridge Sacrificial Layers http://www.keystonefinancialadvisory.com/ http://www.technologystudent.com/struct1/arch1.htm Sacrificial Layers https://www.studyblue.com/notes/note/n/ancient-etruscan-and-rome-ca2/deck/15636392

Material Removal (Micromachining) Surface micromachining Bulk micromachining High Aspect Ratio (HAR) Micromachining http://www.thinfilmmfg.com/subscribers/Subscriber02/mems10Apr02.htm https://www.mems-exchange.org/MEMS/fabrication.html https://www.mems-exchange.org/MEMS/fabrication.html

Material Deposition Spin-on deposition Thermal oxidation No chemical reactions occur Similar to frost on a window Poly Silicon, Silicon Nitride, Silicon Dioxide, metals Photoresist, glass Spin-on deposition Thermal oxidation Chemical vapor deposition Physical vapor deposition Electroplating http://chemwiki.ucdavis.edu/@api/deki/files/57994/=figure_2.jpg?revision=1 http://www2.ece.gatech.edu/research/labs/vc/theory/oxide.html http://cnx.org/contents/m7vjnKhA@2/Chemical-Vapor-Deposition https://en.wikibooks.org/wiki/Microtechnology/Additive_Processes https://www.youtube.com/watch?v=ljbESGs1XrM&list=PLWbWYEQLbAEaN1VdH9KF1G3TPQE06AbWd&index=2&nohtml5=False

Surface micromachining - planar - 2.5-d Advantages: Use existing MOS equipment Batch Fabrication Integration with logic components Silicon has good mechanical properties Photoresist Selective etching SiO2 Sacrificial layers Insulators Poly crystalline Si Structural Layers http://81.161.252.57/ipci/courses/technology/files/tutorial/images/9/sm2.gif

Bulk micromachining Isotropic etching Anisotropic etching Long etching processes Silicon Nitride used to mask Wet etching Isotropic Anisotropic Isotropic etching Anisotropic etching

HAR (high aspect ratio) Micromachining LIGA Deep Reactive Ion Etching (DRIE) “Bosch” process High etching rate Alternate between etching cycles and passivation cycles Ions “bombard” the bottom, and not the sides Both processes use electroplating to build up the final component

Other Types of Micromachining Xenon Difluoride dry phase etching Electro-Discharge micromachining Laser micromachining Focused Ion Beam micromachining

Design Issues/mitigation Examples Stiction Planarization Imaging Interference parts https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwi5raq4-ofMAhWkvoMKHWfIBJgQjRwIBw&url=http%3A%2F%2Fmemscyclopedia.org%2FintroMEMS.html&psig=AFQjCNGGxIi5-bVKmAcQkox3hqiYeVlNTQ&ust=1460511201959031 https://www.youtube.com/watch?v=sagmAHX4tcg

Current Industry Challenges Cost Standardization between manufacturers to ensure compatibility Also important for packaging Integration with electronics “system on a chip”

WORKS CITED Alexandre Reis, Raj Bhattacharya. 2004. Deep Reactive Ion Etching (DRIE): https://ece.umd.edu/class/enee416.S2004/report6.pdf “A (not so) short Introduction to Micro Electromechanical Systems”, F. Chollet, HB. Liu, version 5.2, 2015: http://memscyclopedia.org/Document/IntroMEMS.pdf “MEMS devices for biomedical applications”, Dr. Ramesh Ramadoss, 2013: http://electroiq.com/blog/2013/10/mems-devices-for-biomedical-applications/ https://www.mems-exchange.org/MEMS/fabrication.html https://en.wikipedia.org/wiki/Microelectromechanical_systems#MEMS_basic_processes https://en.wikipedia.org/wiki/Deep_reactive-ion_etching http://www.sandia.gov/mstc/mems_info/ https://www.youtube.com/watch?v=JJElLA8k6Qg&list=PLWbWYEQLbAEaN1VdH9KF1G3T PQE06AbWd&nohtml5=False https://www.youtube.com/watch?v=sagmAHX4tcg

5 Key Points Two main applications for mems are sensors and actuators. Isotropic etching attacks a material in all directions, anisotropic etching attacks at different rates in different directions depending on the material’s properties. The three main fabrication techniques are surface micromachining, bulk micromachining, and HAR micromachining. In bulk micromachining, the etching process attacks along the crystalline planes, making orientation of the Si substrate very important. MEMs fabrication differs from MOSFET fabrication mainly by the use of sacrificial layers.