If you are referring to H3PO4, the standard method for etching SiN is to
maintain a boiling solution at a given temperature. This will also control
the concentration. Straight from the bottle, H3PO4 will boil at
approximately 155°C and provide for an etch rate of approx 45A/min.
Increasing the temperature (boil point) up to 180°C will increase
concentration and the etch rate to around 100A/min.

Special tanks with condensing collars and lids are available for the nitride
etch process that help maintain concentration by refluxing water vapors.
Special temperature controllers are also used that control the heaters and
water addition to ensure that the solution is always boiling at a specific
temperature.

-----Original Message-----
From: kapil kumar jain [mailto:kkjain2002@gmail.com]
Sent: Wednesday, March 23, 2011 10:24 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] Etching of SiN

What is the optimum concentration of SiN etchant ? The thickness is of
the order of 1000 A.