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 O 2 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 O 2 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 SiO 2 layer formed by irradiation of O 2 plasma should be large enough as a protection layer. However, because of insufficient SiO 2 thickness formed by O 2 plasma, the aspect ratio has been limited in previous work. Furthermore, mask material (SiO 2 ) erosion which is another limitation factor of the aspect ratio is increased by the insertion of O 2 plasma irradiation. In this paper, we have investigated optimum O 2 plasma source conditions that allow an increase in SiO 2 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 SiO 2 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).