SPRAY ETCHING OF SILICON IN THE HNO3/HF/H2O SYSTEM

Etching silicon with the standard HNO3/HF/H2O solution in the low HF regime is investigated using a single-wafer spray-etching machine. The single-crystal silicon and undoped polysilicon etch rate is measured for different percentages of HF between 0.13 and 0.18% (by weight). The polysilicon etched about five times faster than single-crystal silicon. Spray-etching damaged n- silicon out of isolation trench structures is examined at spin speeds of 10 and 1000 rpm by scanning electron microscopy (SEM) photographs. The etch rates are measured from the photographs and compared to the etching of undoped polysilicon at the same spin speeds. Both of these have a linear etch rate. The damaged silicon etch rate varies between one-half and one-third of the poly etch rate depending on the spin speed. The spray-etching at 1000 rpm yields the best uniformity f or both the polysilicon and the damaged silicon in the trench. Furthermore, the etch rate of the silicon in the n+ buried layer in the trench is measured and compared with the etch rate on then silicon in the trench. The results show a clear etch-rate dependence on doping. The etch rate of the n+ silicon is about three times as fast as the etch rate of the n- silicon in the trench.