Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 308

and its polymers. 888 The etching mechanism is suggested to be similar to
that in KOH.

7.5. ETCH RATE REDUCTION OF HEAVILY DOPED MATERIALS

For a given type and orientation the etch rate of silicon in alkaline solutions is
207,269 At a doping

largely independent of doping concentration up to about

level of about the etch rate of boron-doped silicon drastically decreases

207,341,711

with increasing dopant concentration. Reduction by as much as three orders of

magnitude can be obtained by varying the boron concentration from about to
above where etching virtually stops. This feature has been widely used as an
etch stop technique for the fabrication of silicon microstructures.
Doping with other elements such as Ge and P ( n -type dopant) also results in etch
rate reduction but at higher concentrations than B as shown in Fig. 7.30. 207 It has
been reported that ion implantation of species such as As to a level of reduces
the etch rate. 385 On the other hand, doping with As, P, and Sb was found to change
little from to in 23.4% KOH + 13.3% IPA. 1026 Carbon implantation to
concentrations greater than causes drastic etch rate reduction as shown in
Fig. 7.30. 214 The effect is more pronounced in EDP than in KOH.
The occurrence of etch rate reduction on highly boron doped materials appears
to be independent of doping methods, whether by solid-source diffusion, epitaxial
growth, or ion implantation. 251 However, the boron concentration at which significant
reduction occurs is different for different methods of doping. The critical boron
concentration for etch rate reduction to occur is affected by the defect density in dif-
ferent doped materials. It is found that for similar boron concentrations the amount of
etch rate reduction in KOH solutions decreases with increasing defect density. 251 In