Mohan:

We have been doing a variety of KOH etches here and I would recommend using a
combination thermal oxide SiO2 at around 3500 angstroms followed by a 1500
angstrom LPCVD Nitride. Even though we have PECVD nitride in house the above
described stack has worked much better for long etches like you are describing.
Very few pinholes assuming quality film deposit. Bob Henderson