Thin film front protection of CMOS wafers against KOH

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Abstract
KOH based silicon bulk micromachining of fully processed CMOS wafers is still a challenge since KOH heavily attacks the aluminum metallizations. At present, the protection of the front of such wafers is still accomplished using mechanical fixtures. These fixtures prevent batch processing. This paper reports a novel protection scheme for the front side of fully processed CMOS wafers against KOH solutions. Since no mechanical fixture is required the new scheme allows batch micromachining. The protection method uses standard CMOS equipment and materials only and is therefore fully CMOS compatible. Different protection schemes based on silicon nitride and oxynitride PECVD thin films are investigated. With a 4 hour etch in a 27 weight-% (wt%) KOH solution at 95 degree(s)C, membranes consisting of the CMOS dielectrics were successfully produced. After KOH etching the protection layers are removed in an reactive ion etcher (RIE). Two aspects of the protection schemes were investigated in detail. First, we analyzed the influence of the stress in the nitride layer on the fabrication yield. With an optimized recipe with a compressive stress of -150 MPa, more than 99% of all contact pads remain intact after the KOH etching. Secondly, potassium contamination of the RIE etcher is negligible if the wafers undergo an RCA cleaning procedure. Secondary ion mass spectroscopy showed that the total alkaline contaminations in thermal oxide, silicon, silicon nitride and silicon oxynitride after the RCA cleaning are not higher than those in reference samples not exposed to the KOH solution.

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