The use of poly-buffered LOCOS processing is a common feature of many sub-micron integrated circuit fabrication processes. However, the silicon layer interposed between the nitride oxidation mask and the pad oxide is often difficult to remove. Different strategies involve dry and/or wet etching of the film. We have demonstrated the utility of adding metals such as Fe or Cu to a conventional phosphoric bath used to etch silicon nitride. Briefly, the presence of the metals is thought to result in a classic oxidation-reduction reaction between the metal and the silicon. Additions of 60ppm of Cu+2 resulted in etch rates of 20Å /min. on undoped polysilicon at a process temperature of 165°C, whereas, the etch rate of SiO2 was less than 1Å/min. Similar results were obtained for additions of Fe+3 and other metals.
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We would like to thank A. Rhodes and P. Chromiak for the polysilicon controls and measurements, J. K. Haas for the thermal oxide controls and measurements.
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Pearce, C.W., Chung, B.C. The use of Metal Additions to Phosphoric Acid to Etch Polysilicon in Poly Buffered Locos Processes. MRS Online Proceedings Library 477, 459–464 (1997). https://doi.org/10.1557/PROC-477-459