Matthew Duty Phillip Sandborn ENEE416 9 29 2011 Wet 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 etchant Advantages Disadvantages Selectivity Inexpensive Speed Batch process Isotropic undercutting Temperature sensitivity Safety Chemical Waste Selectivity 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 etch Isotropy Mask Material to etch Si substrate Undercut Isotropic etches equally in all directions No Undercut Anisotropic etches at different rates in different directions Anisotropic Etching Mask Material to etch 100 111 54 74 Anisotropic Etching Etchants 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 repeatable Etchants 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 cleanrooms Etchants 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 Dangerous Etchants TMAH Tetramethylammonium hydroxide No alkali ions CMOS compatible Anisotropic Si plane selectivity 100 111 10 35 1 Mask SiO2 Stops Controlling Etch Depth Photolithography Anisotropy Heavily doped etch stops References http www mrsec harvard edu education ap298r2004 Erli 20chen 20Fabrication 20 III 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 2242 262
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