Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 306

+ 1.3 g pyrazine + 0.26ml FC129 (3M) surfactant. The typical etch rate of (100) silicon
in this etchant at 118 °C is about 300 Å/s. It has high boron selectivity and good uni-
formity. 221 Also, the etch rate ratios for silicon to other materials such as carbide, nitride,
oxides, and most metals in ethanolamine etchants are very large. 506
Gallic acid functions as an effective silicon complexing agent in aqueous
ethanolamine solutions. In its absence the etch rate is zero. Etch rate increases with
water content. The additions of pyrazines, pyridazines, and triazoles show various
catalytic effects on the etching process. 520 The catalysts that lead to faster oxidation
result in faster etch rates, and the difference among the catalysts is due to a steric
effect. Oxidative catalysts tend to influence the etching selectivity of the major crystal
orientations. 519
Tetramethyl Ammonium Hydroxide (TMAH). TMAH, etchant
has been developed more recently for anisotropic etching of silicon. 505,518,996 It is rela-
tively safe to use and presents no special disposal issues. It has a very low oxide etch
rate and does not attack aluminum if the solution contains a certain amount of silicates.
However, hillocks tends to develop in this solution. 518,1005
Figure 7.26 shows the etch rate as a function of TMAH. 505 At a concentration
above about 4% TMAH the etch rate is basically independent of the concentration.
Figure 7.27 shows that the etch rates of (100) and (110) planes decrease as a function
of TMAH concentration at higher concentreations. 1005 The etch rate tends to increase
with increasing amount of dissolved silicon in the solution as shown in Fig. 7.28. 518
The selectivity in etching rate between (111) and other planes is poor compared with
that in KOH solutions. 1001 The etch rate ratio of (100)/(111) planes varies with TMAH
concentration and temperature with a minimum at about 25% TMAH. 1005 The slowest
etching plane is found to be (111), 1.5 Å/s, and the fastest one (210), 99.5 Å/s. 1001 The
etch rate of n -Si is slightly higher than that of p -Si. 505 Addition of IPA to TMAH solu-
tion slows down the etch rate linearly with increasing IPA concentration but changes
very little with the etch rate ratio of (100)/(111). 1012 Addition of -4g/liter pyrazine

to 25% TMAH solution increases slightly the etch rate and improves the surface
smoothness. 594

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