I have "inherited" a TMAH process for fabrication of micro cavities and
cavities for MEMS devices.  The process uses a 10% Wt. TMAH solution at
95c.  Both the micro-cavity and cavity etch processes are extremely
unstable, where a bath will work for some time and suddendly it will
stop working!  We control the temperature and maintain the concentration
fairly well.  There does not appear to be a relationship to these
parameters.  We process 5" wafers with a 500 A hard OX mask, the wafers
are placed in the bath in a vertical orentation.  We have looked for
fluorides in the bath, found to be <15 ppm by IC.  We have tried Si
loading, with no improvement.  It is the unpredictability of the bath
that is baffeling!  Any suggestions as to what to look for?

Thanks
Hemant Desai
Group Leader
MOS5 Die Manufacturing
Motorola