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Fabrication of MEMS Devices

Fabrication of MEMS Devices. Drew Sellers ELEC 6750 Spring 2004. Questions. What is the difference between anisotropy and isotropy? What are the two steps involved in the Bosch process?. Introduction.

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Fabrication of MEMS Devices

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  1. Fabrication of MEMS Devices Drew Sellers ELEC 6750 Spring 2004

  2. Questions • What is the difference between anisotropy and isotropy? • What are the two steps involved in the Bosch process?

  3. Introduction • The fabrication of MEMS devices requires the integration of various process techniques to form intricate structures on the micro-scale. The following presentation outlines a simple process for the production of MEMS devices through deep silicon etching.

  4. Outline • Definitions of terms • General process flow • Detailed process steps • Wet vs. Dry etching • Bosch process • Deep reactive ion etching (DRIE) • MEMS applications

  5. Definitions of Terms • MEMS – microelectromechanical systems • Passivation – deposition of protective layer to reduce chemical activity of its surface • Photoresist – organic polymer which becomes soluble when exposed to ultraviolet light, used as a masking layer (positive photoresist) • RIE – reactive ion etcher • DRIE – deep reactive ion etcher

  6. Definitions of Terms • Anisotropic – etched surface is attacked faster in one direction than the other • Isotropic – etched surface is attacked at same rate in all directions

  7. General Process

  8. Photoresist Deposition • Spin coated on to wafer • Thickness controlled by rotational speed and photoresist viscosity • Thicker layers required for certain types of etches

  9. Photoresist Patterning • Photoresist reacts chemically to light emitted from uv source • Mask allows light to expose desired areas only • Resist becomes soluble and rinsed away through aid of developer solution http://www.bidservice.com/browses/NF_detailed_item_view_new.asp?productID=17724

  10. Silicon Etching • Wet and dry etching methods • Removes silicon from “open” areas in masking layer • Many different methods to yield various results http://www.stsystems.com

  11. Wet Etching Vs. Dry Etching

  12. Wet Etching • Etch reactants come from liquid source • Highly selective – only certain chemical solutions etch specific materials • Relatively low accuracy and yield • Mixtures consisting mainly of chemicals such as hydrofluoric acid, nitric acid, and acetic acid

  13. Dry Etching • Etch reactants come from gas or vapor phase source (typically ionized) • Various systems available utilizing different dry etching methods • Most popular techniques involve variations of the Bosch process

  14. Bosch Process • Modified RIE process which produces higher aspect ratio devices than that of earlier RIE systems • Involves utilization of both etching and passivation cycles • Produces aspect ratios of >20 (earlier methods yielded aspect ratios of<12)

  15. Etching/Passivation • Etching/passivation cycles • Passivation deposited through use of C4F8 • Cycle typically lasts <10 sec. • Etching of silicon aided by SF6 • Cycle typically lasts <12 sec.

  16. Deep Reactive Ion Etching (DRIE) • Ions formed in plasma bombard surface to etch silicon • F atoms bond to Si to break down bonds at the wafer surface • Passivation layer etches quickly on horizontal surfaces, but slowly on lateral surfaces http://www.riken.go.jp/lab-www/library/publication/review/pdg/No_31/31_048.pdf

  17. Advantages of DRIE • Capable of achieving etch rates in excess of 3 µm/min (earlier methods etched at 1 µm/min or less) • Selectivities to photoresists greater than 70:1 • Vertical sidewall profiles • High aspect ratios

  18. Biotechnology Communications Accelerometers Ink-jet nozzles Micromirrors Micromotors Microswitches Laser beam deflectors IC heat sinks Microinterconnects Gyroscopes Actuators Microprobes MEMS Applications

  19. MEMS Device Etched Through DRIE

  20. Conclusion • New advances in plasma etching technologies have greatly increased the capabilities of MEMS devices • Through the aid of plasma etching, engineers are able to produce smaller, more precise structures

  21. Answers to Questions • Anisotropic etches exhibit directional etching patterns (i.e. - horizontal surfaces are etched faster than lateral surfaces) • Isotropic etches exhibit uniform etching patterns in all directions • The two steps involved in the Bosch process are etching and passivation

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