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Unformatted text preview: Fundamentals of Micromachining Clean Rooms How Big of a Particle is Tolerable? –Example: 0.5 µ m CMOS technology •Lateral Features: –pattern size = 0.5 µ m –pattern tolerance = 0.15 µ m –level-level registration = 0.15 µ m •Vertical Features: –gate oxide thickness = 10 nm –field oxide thickness = 20 nm –film thicknesses = 250-500 nm –junction depths = 50-150 nm Filtration Media •Fibers –“depth” filters –many randomly oriented intertangled strands laid into a mat •Fourdrinier process, usually submicron glass fibers –void volume is typically about 85 - 90 % •Membranes –“surface” filters –homogeneous sheet material with holes punched into it –1. cellulose nitrate; void volume is about 70 - 85 % •holes formed by solvent evaporation, irradiation, or stretching –2. polycarbonate sheets; void volume is about 10 - 20 % –3. PTFE sheets; biaxially stretched –4. sintered silver particles Clean Room Air Filters •High Efficiency Particulate Air (HEPA) Filters –most common type of clean room air filter –high efficiency, low pressure drop, good loading characteristics –uses glass fibers in a paper-like medium –are rated by their particle retention: •A true HEPA-rated filter will retain 99.97 % of incident particles of 0.3 µ m or larger. (DEFINITION) HEPA History –developed during WWII atomic bomb research for containment of radioactive aerosols –called “superimpingement” or “superinterception” filters; later referred to as “absolute” filters –first prototype filters used esparto grass as the filter medium –in 1950s glass fibers were introduced into the paper –in 1960s specifications were standardized and called HEPA filters...
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This note was uploaded on 11/16/2011 for the course MSE 5960 taught by Professor Douglas during the Fall '04 term at University of Florida.
- Fall '04