Selective etching bath methods
View Patent ↗An etching method. The method includes etching a first plurality of silicon wafers in a first enchant, each silicon wafer having SiO 2 and Si 3 N 4 deposited thereon, where the etching includes dissolving a quantity of the SiO 2 and a quantity of the Si 3 N 4 in the first echant. A quantity of insoluble SiO 2 precipitates. A ratio of a first etch rate of Si 3 N 4 to a first etch rate of SiO 2 is determined to be less than a predetermined threshold. A portion of the first etchant is combined with a second etchant to form a conditioned etchant. A second plurality of silicon wafers is etched in the conditioned etchant. A ratio of a second etch rate of Si 3 N 4 to a second etch rate of SiO 2 in the conditioned etchant is greater than the threshold. A method for exchanging an etching bath solution and a method for forming a selective etchant are also disclosed.
1. An etching method, comprising:
etching a first plurality of silicon wafers in a first etchant, each silicon wafer of said first plurality of silicon wafers having a first silica layer of SiO 2 and a first silicon nitride layer of Si 3 N 4 deposited thereon, wherein said etching comprises dissolving a quantity of said SiO 2 from the first silica layer at a first etch rate of SiO 2 and a quantity of said Si 3 N 4 from the first silicon nitride layer at a first etch rate of Si 3 N 4 in said first etchant and a quantity of insoluble SiO 2 precipitates in said first echant in response to said dissolving;
after said etching: determining the first etch rate of SiO 2 , determining the first etch rate of Si 3 N 4 , determining a ratio of the determined first etch rate of Si 3 N 4 to the determined first etch rate of SiO 2 from said first plurality of silicon wafers in said first etchant, and ascertaining that said determined ratio is less than a predetermined threshold;
in response to said ascertaining, combining a portion of said first etchant with a second etchant to form a conditioned etchant, wherein said portion is essentially free of insoluble SiO 2 ; and
after said combining, etching in said conditioned etchant a second plurality of silicon wafers having a second silica layer comprising SiO 2 and a second silicon nitride layer comprising Si 3 N 4 disposed thereon, wherein a ratio of a second etch rate of Si 3 N 4 to a second etch rate of SiO 2 from said second plurality of silicon wafers in said conditioned etchant is greater than said predetermined threshold.
2. The method of claim 1 , wherein said second etchant is essentially free of SiO 2 .
3. The method of claim 1 , wherein said first etchant and said second etchant each comprise phosphoric acid.
4. The method of claim 1 , wherein said predetermined threshold is about 50:1.
5. The method of claim 4 , wherein said first etch rate of Si 3 N 4 is less than about 50 angstroms/minute.
6. The method of claim 4 , wherein said first etch rate of said SiO 2 is greater than about 1 angstrom/minute.
7. The method of claim 1 , wherein said conditioned etchant comprises between about 1% and about 30% of said first etchant.
8. The method of claim 1 ,
wherein said determining the first etch rate of SiO 2 comprises measuring a SiO 2 thickness change of the first silica layer as a function of time and utilizing said measured SiO 2 thickness change of the first silica layer as a function of time to determine the first etch rate of SiO 2 ; and
wherein said determining the first etch rate of Si 3 N 4 comprises measuring a Si 3 N 4 thickness change of the first silicon nitride layer as a function of time and utilizing said measured Si 3 N 4 thickness change of the first silicon nitride layer as a function of time to determine the first etch rate of Si 3 N 4 .