Dear MEMS researchers,

I am using TMAH (20wt%, 90 degree C, magnetic stirrer 250rpm) to etch a
through-wafer cavity (525um) of a Si (100) chip. The mask layer is a
thermally grown SiO2 (600nm). To protect back side, I use an etching jig
made of stainless steel with O-ring around the chip. I found that the
etching process is extremely unstable and unreproducible.

Initially, after SiO2 etching window was patterned by BHF (in 15 min) around
one week, I assembled the chip into the jig and started the etching. At
first, there was no etching at all since I could not see the bubbles come
out. And this was also confirmed by surface profiler (alpha step). So I
thought there might be still some SiO2 left on the etching window. To get
over it, I did BHF etching again for about 1 min. And right after that, I
put the chip into to TMAH etching beaker without assembling with the jig. I
immediately saw the bubble come out . I let it etch for 3 min and the
measurement by surface profiler confirmed an etching depth of 2.6 um
(0.86um/min rate). However, when I rinsed the chip with DI water and
assembled
back with the etching jig. After the putting in the TMAH etching beaker,
there was, again, not etching at all (i.e. no bubbles come out). Also
confirmed by surface profiler.

By chance, should anyone experienced this before, please explain to me why
was it? Does the etching jig or O-ring or anything else have to do with this
sudden stop in etching? As I can not find anything like this in literature,
any help would be really appreciated.

P/S: It's weird that with the same etching jig and the same procedure, last
time I succeeded in etching through wafer and I didn't experience the sudden
stop of etching like this time. I have tried several chips, but it doesn' t
work.

Thanks and best regards,

Nam Le
Graduate student
Ritsumeikan University, Japan