Just a guess here, but in the time it took you to rinse and reassemble
your jig, the native oxide could have reformed and blocked any further
etching. Try assembling your jig and then do a quick HF dip to strip the
native oxide before you go into the TMAH again.

-Joe Grogan

Le Cao Hoai Nam wrote:
> 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.
>