Lecture 2 - ME 601: Manufacturing in Micro- and Nanosystems...

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Unformatted text preview: ME 601: Manufacturing in Micro- and Nanosystems Lecture 2: Photolithography Basics History of Lithography Originally developed for rapid printing of text or artwork onto paper by Bavarian author Alois Senefelder in 1796 Can be used to print inks onto any smooth surface (think screen printing of t-shirts) Uses chemical process to create an image: positive part of image is hydrophobic; ink and water phase separates and ink is preferentially adsorbed to positive part Optical Lithography Indirect method of surface patterning using light of different intensity, typically either completely light or completely dark Optical lithography <-> photolithography used to pattern micro- and nanoscale features, determines minimum feature size Basic process follows many of the same principles across full size range, but finer scale patterning requires several tricks to improve resolution Minimum feature size is continuously shrinking: 4-6 m in 1970s, 1 m in 1980s Current state-of-the-art is 32 nm, but may be possible to go even smaller Main production tool for both VLSI and NEMS/MEMS workhorse! Photolithography: Essential Process * Thin Films Implant Diffusion Etch Test/Sort Polish Photo Patterned wafer Photolithography is at the Center of the Wafer Fabrication Process adou, UC Irvine Basic Photolithography Process 1. Surface Preparation 2. Photoresist Application 3. Soft Bake 4. Align & Expose * 5. Develop 6. Hard Bake 7. Inspection 8. Etch 9. Resist Strip 10.Final Inspection * Some processes may include a Post-exposure Bake 1. Surface Preparation Remove contaminants (chemical, biological), ie. Residual photoresist left from previous operation Remove particulates (dust, microbes, etc.) Reduce pinholes and other defects Improve photoresist adhesion: silicon readily forms native oxide which also attracts water vapor; resist will stick to this layer rather than to wafer if not properly primed Basic steps Chemical clean Rinse Dry Hexamethyldisilazane (HMDS) prime Cleanroom Basics The majority of micro- and nanofabrication operations are carried out in a cleanroom Positive pressure filtration and re- circulating airflow continuously removes dust, particles, and other contaminants Temperature and humidity are also precisely controlled: photoresist chemicals are very sensitive to environmental conditions especially moisture! Special lighting is used to filter wavelengths that will lead to unwanted exposure of resist Other processes that require low particle counts? Cleanroom Basics BS 5295 Definition "Environmental cleanliness in enclosed spaces". "A Clean Room is a room with environmental control of particulate contamination, temperature and humidity, constructed and used in such a way as to minimize the introduction, generation and retention of particles inside the room." Cleanroom Basics...
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Lecture 2 - ME 601: Manufacturing in Micro- and Nanosystems...

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