Patent ID: 7332054

Claim:
An etch apparatus, comprising: an etchant comprising a phosphoric acid solution; a silicon-nitride etching tank containing the etchant, said etchant configured to perform a selective etching of a semiconductor structure in the tank in contact with the etchant, wherein the semiconductor structure comprises a semiconductor wafer, a pad layer comprising silicon dioxide, and a silicon nitride structure comprising silicon nitride, wherein the pad layer is on a surface of the semiconductor wafer, wherein the silicon nitride structure is on the pad layer, wherein the pad layer is disposed between the semiconductor wafer and the silicon nitride structure, wherein said selective etching is characterized by the silicon nitride of the silicon nitride structure being selectively etched by the etchant relative to an etching by the etchant of the silicon dioxide of the pad layer; and a recirculating path coupled to said tank, wherein the recirculating path comprises a chamber, a first conduit, a filtering system, a dissolver, and a second conduit, wherein the chamber is in direct mechanical contact with the tank, wherein a first end of the first conduit is connected to an outlet of the chamber, wherein an inlet of the filtering system is connected to a second end of the first conduit, wherein an outlet of the filtering system is connected to a first end of the second conduit, wherein a second end of the second conduit is connected to the tank or the chamber, wherein the recirculating path is configured to receive an overflow of the etchant from the tank into the chamber and to flow the etchant from the chamber through the first conduit, the filtering system, the second conduit, and back into the tank, wherein the dissolver comprises a porous carbon matrix filter having only silicon nitride coated thereon, wherein the etch apparatus further comprises means for dissolving the silicon nitride coated on the filter into the etchant at a controlled dissolution rate sufficient to cause said selective etching to decrease an etch rate of the silicon dioxide of the pad layer in the etchant in a controlled manner such that an etch rate of silicon nitride of the silicon nitride structure is substantially unchanged, wherein the etch apparatus further comprises means for coating the silicon nitride onto the filter to configure the coated silicon nitrate to facilitate said selective etching, wherein the silicon nitride coated on the filter has a different spatial distribution on the filter than would silicon nitride coated on the filter by being precipitated from the etchant onto the filter while the etchant is passing through the filter.