Reactive ion etching of crystalline silicon using NF3 diluted with H2

Abstract

Experimental and modeling work was performed to examine the etch rates and etch profiles during the reactive ion etching of crystalline silicon using NF3 diluted with H2 as a function of reactor pressure, etchant gas flow, rf power, and external dc voltage. All experiments were carried out at a frequency of 13.56 MHz. A regression model for the etch rate was developed using a statistical analysis program and the effects of the variables were studied using the e−chip computer program. In the early work on plasma etching of silicon using NF3 diluted with H2, a deposited layer was detected at high H2 dilutions. After a thorough study, it was determined by infrared spectroscopy that the deposit was ammonium hexafluorosilicate, (NH4)2SiF6. Utilizing the ammonium hexafluorosilicate as a side wall passivation layer and by a judicious choice of etching parameters, anisotropic etching of silicon was obtained. The gas phase concentration of NF3 was varied from 10% to 90%. No etching of silicon was obtained for 10%–40% NF3. By increasing the rf power, the self-bias increases but the deposit formation decreases. Low rf power was used to assist the formation of the deposit which acts as a side wall passivation layer during the etching process. A high external dc voltage (−500 V) promotes anisotropic etching at high etch rates.