A spin-on polymeric material has been developed to replace the silicon nitride mask used in the MEMS industry for
silicon wet-etch processing. Built-in photosensitivity eliminates the need for additional photoresists in the system. The
process consists of applying an organosilane-based primer layer onto a silicon wafer, followed by spin coating the
photosensitive layer. After a soft bake, the coating is imaged by exposing it to ultraviolet light. After a post-exposure
bake, the coating is developed by a solvent. After a final bake, the prepared wafer is then etched in a hot concentrated
alkaline solution to complete the pattern transfer. The polymer-coated area remains protected with insignificant and
controllable undercut after extended hours of wet etching. Etch protection performance was characterized as a ratio of
undercut (u) to etch depth (h). The polymeric mask allows silicon substrates to be etched anisotropically in the same way
as silicon nitride masks although more undercut occurs when KOH or NaOH are used as etchants. With use of
tetramethylammonium hydroxide (TMAH) as an etchant, a consistent 1-2% undercut ratio (u/h×100%) was obtained.
The effects of various parameters such as use of different etchants and the effects of etchant concentration and delayed
processing on undercut ratio are investigated.
Hydrolyzed silane primer solutions were made of an organosilane in glycolether diluted with a large amount of water
with or without an acid as a catalyst. The newly developed primer compositions exhibited an extended shelf life of
3 months or more. The compositions were specially designed to accommodate ProTEKTM. layer adhesion in the TALON
Wrap. process. In this application, a spin-coatable polymeric material, ProTEKTM., is applied as the protective coating
to coat the top, edge, and underside rim of the wafer in preparation for backside etching. By applying an underlayer of
primer and an overlayer of ProTEKTM. coating to the top, edge and the bottom side rim of the wafer, an effective
encapsulation of the wafer was achieved by using a custom-designed baffle. Each layer was applied by spin coating
followed by baking at a wide temperature range. Thermal processing was followed by wet etching in KOH at an elevated
temperature for . 10 hr. Post-etched wafers were rinsed with deionized (DI) water. Excellent edge profiles without
"knife-edges" were obtained after etching the unprotected areas of the wafer. The process is fully automated because it is
carried out in the TALONTM automated wafer-processing tool. Intact films with no lifting or peeling were obtained
during or after the KOH etch process/DI rinse for silicon substrates.
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