LOW-TEMPERATURE MICROWAVE PLASMA-ETCHING OF CRYSTALLINE SILICON

Low-temperature microwave plasma etching of crystalline silicon is described. Vertical and lateral etch rates of Si and the selectivities of Si to photoresist are measured as a function of wafer temperature within a range of -150 to +30-degrees-C for SF6, CF4, and NF3. The results indicate that (1) dramatic reductions in lateral etch rate are obtained below -130-degrees-C, (2) vertical etch rates remain high up to 1.4-mu-m/min in the range of -140 to +30-degrees-C for SF6, and (3) the selectivities become high at low temperatures (e.g., > 40 at -90-degrees-C and 2.3 Pa). This etching enables highly anisotropic Si etching at a high etch rate and high selectivity with fluoride gases. Less-polymerizing-type gases can provide high etch rates. An etching model of the ion-bombarded surfaces is discussed. The model implies that separate control of the side wall reaction and the horizontal surface reaction is achieved by the low-temperature etching.