The Etch Rate Variations of p+ Silicon Wafers in Aqueous KOH Solutions as a Function of Processing Conditions
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ABSTRACT The etching behaviour of highly boron doped (20 - 25 mQ-cm) silicon wafers in aqueous KOH solutions were investigated in the present study. The etch rate of (100) crystal plane and the fastest etch plane projections (31X) and (41X), where X can be 0,1,2..., were measured as a function of processing conditions. The experimental temperatures of solutions were 65 - 75 'C and the concentrations 5 - 40 wt-% KOH. The maximum etch rate of (100) crystal plane was measured to locate between 10 - 15 wt-% KOH and the maximum etch rates of fastest etch plane projections were measured to locate between 0 - 10 wt-% KOH in the whole temperature range. In lower concentrations (5 - 20 wt-% KOH) the fastest etch plane projection was determined to be (31X) crystal plane projection. In higher concentrations (25 - 40 wt-% KOH) it was determined to be (41X) crystal plane projection. The results of the present study indicate that there are differences between etch reactions of different etch planes. The difference may be connected to transferring electrons. This could explain why the different etch planes have the position of etch rate maximums as a function of KOH concentration in different places. Furthermore, this could also explain why fastest etch planes are sensitive to experimental conditions. Keywords: KOH, Silicon, Anisotropic etching.
INTRODUCTION Anisotropic etching of single crystalline silicon in aqueous KOH solutions is a technically important process in silicon micromachining. Differences in etch rates between the (100) and other crystal planes are utilized in the processes. However, in the literature different results of the positions of etch rate maximums as a function of KOH concentrations are reported. The position of (100) crystal plane etch rate maximum is reported to locate between 10 - 15 wt-% KOH [1], or near 20 wt-% KOH [2]. Furthermore, different etch planes are reported to be as the fastest etch plane. Reported crystal planes are (212)[3], (130)[4], (411)[5] and {((110), (320) and (540)}[6], respectively. In the present study the etching behaviour of highly boron doped (20 - 25 mQ-cm) silicon wafers in aqueous KOH solutions were investigated.
293 Mat. Res. Soc. Symp. Proc. Vol. 605 © 2000 Materials Research Society
EXPERIMENTAL Wafers used in experiments were double side polished P+(100) Czochralski wafers. These wafers were oxidized by wet oxidation in 1050 °C and the thickness of the grown oxide was approximately 1.5 Jim. A test mask pattern of rectangular windows with subdivision into angles of 3' on outer circle of the wafer was lithographically transferred to the oxidized wafers (figure 1).
Figure 1.
A test mask pattern of rectangularwindows with subdivision into angles of 30 on outer circle of the wafer. An optical microscope picture of under etched rectangularwindow in the blow-up picture.
Before etching the wafers were cleaned by RCA 1 cleaning process and after etching the wafers were rinsed in DI water. The etching solutions were mixed from Merck's high purity, p.a. quality KOH pellets
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