Study on wet etching of dummy polysilicon in narrow pattern gap using alkaline solution

Abstract

The polysilicon etching in a patterned wafer is an important process. In this process, the polysilicon must be completely removed without damaging the gaps made from SiO2 or Si3N4. In this study, a wet etching method was used to completely remove polysilicon while reducing it to SiO2. Alkaline solutions known as etching polysilicon, i.e., ammonium hydroxide (AH) and tetramethylammonium hydroxide (TMAH), were used. The etch rate was confirmed according to various factors such as temperature, chemical concentration, and rotation speed, but the etch rate was most affected by the chemical concentration. Accordingly, the concentration of hydroxide ions was the main factor for etching polysilicon. The etch rate of TMAH had the same tendency as AH for various factors, but the results indicated a difference of approximately eight times (1744 Å/min for TMAH and 217 Å/min for AH, condition: 6 wt%, 70 °C, 100 RPM). The electrical double layer (EDL) was calculated by using the pH, which was measured at each concentration of AH and TMAH solution, to etch the polysilicon in the narrow gap. The EDL was affected by the hydroxide concentration; thus, the TMAH solution etched the polysilicon in the narrow gap better than AH did. Furthermore, the experiment was conducted using an additive that could decrease the SiO2 etch rate and increase the selectivity between polysilicon and SiO2. Although the etch rate of polysilicon was reduced by 23%, the etch rate of SiO2 was also reduced by 43%.