Improvement of high aspect ratio Si etching by optimized oxygen plasma irradiation inserted DRIE

This paper describes an advanced Si-deep etching process achieving a high aspect ratio with excellent verticality by the improvement of the O2 plasma source condition in the oxygen plasma irradiation inserted deep reactive ion etching (OP-DRIE) process that we have developed. The conventional DRIE process which we call the Bosch process has a trade-off relation between the high aspect ratio and verticality in the trench profile. Our developed process technique, repeating the conventional DRIE and the O2 plasma irradiation process alternately, can achieve the vertical trench profile with a higher aspect ratio than that of the conventional DRIE process. In order to maximize an advantage of the developed process, a thickness of the SiO2 layer formed by irradiation of O2 plasma should be large enough as a protection layer. However, because of insufficient SiO2 thickness formed by O2 plasma, the aspect ratio has been limited in previous work. Furthermore, mask material (SiO2) erosion which is another limitation factor of the aspect ratio is increased by the insertion of O2 plasma irradiation. In this paper, we have investigated optimum O2 plasma source conditions that allow an increase in SiO2 thickness with a high oxidation rate, and at the same time, with less mask erosion on the top of the wafer. We have clarified the effects of frequency and pulsed/CW modes of the plasma source on the effectiveness in SiO2 formation. From the obtained oxygen plasma source condition, we achieved the etched Si trench having an aspect ratio of over 70 with excellent verticality (uniform trench width).