INVESTIGATION OF PYRAMID FORMATION ON TMAH ETCHED SILICON

Abstract

With the development of MEMS, a new anisotropic etchant, Tetramethyl ammonioum hydroxide (TMAH) for silicon has emerged. This etchant has many advantages over some of the more commonly used etchants, such as potassium hydroxide (KOH) and ethylene-diamine-pyrocatechol-water (EDP). Similar to these commonly used etchants, etching with TMAH results in the formation of pyramids at the {100} surface of the silicon. This study aims to provide a better understanding on the etching silicon using TMAH by looking at formation of pyramids at the etched silicon surface. The influence of experimental conditions such as TMAH concentration, temperature, etching time, wafer type and annealing on the formation of pyramids were investigated. The condition of the etched silicon surface is captured using the scanning electron microscopy. An image software is used to estimate the number and size of the pyramids. It was found that a very rough surface was obtained when samples are etched in low TMAH concentrations (4 to 10wt. %). The roughness reduces significantly when etched in 12wt. % TMAH. Pyramids can only be observed when etched in 15wt% TMAH and most of the pyramids were < 0.1 µm2. The number and size of pyramids reduce significantly when TMAH concentration increases from 15wt. % to 20wt. %. It was also found that the number of pyramids (< 0.1 µm2 ) increase significantly as the ambient temperature increased from 70°C to 90°C. These phenomena can be explained by the pH theory which proposed that the formation of pyramids is due to the undissolved SiO2(OH)- which is more resistant to TMAH etching than silicon. We also observe that when silicon was etched in 15wt. % TMAH for 5 min at 90°C, there is a distinct difference in the number of pyramids between the rapid thermal annealed and as-received samples. The difference in the number of pyramids becomes smaller when the rapid thermal annealed and as-received samples were etched for a longer time (15min & 30min). It was found that when etched in 15wt. % TMAH at 90°C for 30 min, the number of pyramids of thermal donor created samples is generally higher than that of the as-received samples. The difference in the number of pyramids decreases with the pyramid size increasing. These experimental results are in accordance to the defect theory which proposed that the pyramids formed are due to the presence of silicon oxide precipitate, arising from a considerable amount of oxygen dissolved in Cz-grown silicon crystals during growth. However, we found that none of the pH and defect theory can fully explain all the results in our experiments. Therefore we suggest that in order to explain the etching mechanism of TMAH of silicon satisfactorily, a combination of pH and defect theory is necessary.