Lecture 03 Clean Rooms - A true HEPA-rated filter will...

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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)
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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
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