Matthew Duty & Phillip Sandborn ENEE416 9/29/2011Wet Etch Etch: removal of material from wafer (e.g. removal silicon dioxide) Wet Etch: removal by liquid-phase etchant Dry Etch: removal by plasma-phase etchantAdvantages/Disadvantages + Selectivity + Inexpensive + Speed + Batch process - Isotropic (undercutting) - Temperature sensitivity - Safety - Chemical WasteSelectivity Ability to etch one material but not another (e.g. silicon dioxide but not silicon) Different etch rates for each material Different etch rates for certain crystal orientations Allows anisotropic etchIsotropy Mask Material to etch Si-substrate Undercut Isotropic : etches equally in all directions No Undercut Anisotropic: etches at different rates in different directionsAnisotropic Etching Mask Material to etch <100> <111> 54.74°Anisotropic EtchingEtchants – HNA Hydrofluoric acid; Nitric acid; Acetic acid Redox reaction oxidizes Si; Si2+ reacts to form SiO2 (reaction with nitric acid) SiO2 dissolved by HF acid to become soluble in acetic acid Isotropic Etch rate = 1-3um/min Mask: Si3N4 (not SiO2!) Low cost Simple Process not easily repeatableEtchants – KOH Potassium hydroxide Anisotropic (Si plane selectivity: <110>:<100>:<111> = 600:400:1) Etch rate = 2um/min Mask: Si3N4 or SiO2 (SiO2 will etch quicker, though) Not CMOS-compatible (Ions contaminate gate oxide) Not allowed in some IC cleanroomsEtchants – EDP Ethylene Diamine, Pyrochatechol, and water Anisotropic (Si plane selectivity: <100>:<111> = 35:1) Etch rate = 1um/min Mask: SiO2 Not CMOS-compatible (Ions contaminate gate oxide) Not allowed in some IC cleanrooms DangerousEtchants – TMAH Tetramethylammonium hydroxide No alkali ions (CMOS-compatible) Anisotropic (Si plane selectivity: <100>:<111> = 10-35:1) Mask: SiO2Stops – Controlling Etch Depth Photolithography Anisotropy Heavily-doped etch stopsReferences http://www.mrsec.harvard.edu/education/ap298r2004/Erli%20chen%20Fabrication%20III%20-%20Etching.pdf http://en.wikipedia.org/wiki/Etching_%28microfabrication%29 Jaeger, Richard C. (2002). "Lithography". Introduction to Microelectronic Fabrication. Upper Saddle River: Prentice Hall Schwartz, B., and Robbins, H. “Chemical Etching of Silicon” Journal of the Electrochemical Society, 123 (12), pp. 1903-1909 Collins, Scott D. "Etch Stop Techniques for Micromachining." Journal of the Electrochemical Society 144.6 (1997):
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