Selective etching bath methods

1. A method for forming a selective etchant, comprising:

etching a quantity of SiO 2 and a quantity of Si 3 N 4 from a substrate in a first etchant, said etching comprising dissolving said quantity of SiO 2 and said quantity of Si 3 N 4 in said first etchant, wherein a quantity of insoluble SiO 2 precipitates in said first etchant in response to said dissolving;

experimentally determining an etch rate of Si 3 N 4 in said first etchant;

experimentally determining an etch rate of SiO 2 in said first etchant;

determining a ratio of an etch rate of Si 3 N 4 to an etch rate of SiO 2 in said first etchant from the experimentally determined etch rate of Si 3 N 4 in said first etchant and the experimentally determined etch rate of SiO 2 in said first etchant;

determining that said ratio is less than a predetermined threshold of at least 50:1; and

in response to said determining that said ratio is less than said predetermined threshold, combining a second etchant with a portion of said first etchant to form a selective etchant, said first etchant having said quantity of SiO 2 and said quantity of Si 3 N 4 dissolved therein, wherein said portion is in a sufficient amount such that said selective etchant selectively etches Si 3 N 4 over SiO 2 in a ratio 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 selective etchant comprises between about 1% and about 30% of said first etchant by volume.

5. A method for exchanging an etching bath solution, comprising:

providing an etching system comprising an etching bath having a first etchant disposed therein, a receiving tank connected to said etching bath and configured to collect said first etchant transferred from said bath;

etching Si 3 N 4 and SiO 2 from at least one substrate into said first etchant, said etching comprising dissolving said SiO 2 and said Si 3 N 4 in said first etchant, wherein a quantity of insoluble SiO 2 precipitates in said first etchant in response to said dissolving;

experimentally determining an etch rate of Si 3 N 4 in said first etchant;

experimentally determining an etch rate of SiO 2 in said first etchant;

determining a ratio of an etch rate of Si 3 N 4 to an etch rate of SiO 2 in said first etchant from the experimentally determined etch rate of Si 3 N 4 in said first etchant and the experimentally determined etch rate of SiO 2 in said first etchant;

determining that said ratio is below a predetermined threshold value of at least 50:1;

in response to said determining that said ratio is less than said predetermined threshold value, transferring said first etchant from said bath to said receiving tank;

after said transferring said first etchant, adding a second etchant to said bath; and

after said adding said second etchant, transferring a portion of said first etchant from said receiving tank to said bath, said portion mixing with said second etchant in said bath, said first etchant having said Si 3 N 4 and said SiO 2 dissolved therein.

6. The method of claim 5 , wherein said first etchant and said second etchant each comprise phosphoric acid.

7. The method of claim 5 , wherein said transferring the portion of said first etchant is performed after said adding the second etchant is performed.

8. The method of claim 5 , wherein said transferring the portion of said first etchant comprises transferring the portion of said first etchant from said receiving tank into a valve and then into a pump and then into said bath.

9. The method of claim 5 , wherein said transferring the portion results in said portion mixing in said bath with said second etchant to form in said bath a final volume of etchant consisting of said portion and said second etchant.

10. The method of claim 5 , wherein said transferring the portion of said first etchant consists of transferring the portion of said first etchant from said receiving tank into a valve and then into a pump and then into said bath.

11. 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 etchant in response to said dissolving.

12. The method of claim 11 , said method further comprising:

after said etching: determining a ratio of an etch rate of Si 3 N 4 to an 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; and

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 .

13. The method of claim 12 , said method further comprising:

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.

14. The method of claim 12 ,

wherein said determining the ratio of an etch rate of Si 3 N 4 to the etch rate of SiO 2 comprises determining the first etch rate of SiO 2 , determining the first etch rate of Si 3 N 4 , and determining a ratio of the determined first etch rate of Si 3 N 4 to the determined first etch rate of SiO 2 ; as being the

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 .

15. The method of claim 12 , wherein said second etchant is essentially free of SiO 2 .

16. The method of claim 12 , wherein said predetermined threshold is about 50:1.

17. The method of claim 12 , wherein said first etchant and said second etchant each comprise phosphoric acid.

18. The method of claim 11 , wherein each silicon wafer of said first plurality of silicon wafers consists essentially of the first silica layer of SiO 2 and the first silicon nitride layer of Si 3 N 4 deposited thereon.

19. The method of claim 1 ,

wherein said experimentally determining the etch rate of Si 3 N 4 in said first etchant comprises measuring a thickness change in Si 3 N 4 over time in said first etchant, followed by determining the etch rate of Si 3 N 4 in said first etchant from the measured thickness change in Si 3 N 4 over time in said first etchant, and

wherein said experimentally determining the etch rate of SiO 2 in said first etchant comprises measuring a thickness change in SiO 2 over time in said first etchant, followed by determining the etch rate of SiO 2 in said first etchant from the measured thickness change in SiO 2 over time in said first etchant.

20. The method of claim 5 ,

wherein said experimentally determining the etch rate of Si 3 N 4 in said first etchant comprises measuring a thickness change in Si 3 N 4 over time in said first etchant, followed by determining the etch rate of Si 3 N 4 in said first etchant from the measured thickness change in Si 3 N 4 over time in said first etchant, and

wherein said experimentally determining the etch rate of SiO 2 in said first etchant comprises measuring a thickness change in SiO 2 over time in said first etchant, followed by determining the etch rate of SiO 2 in said first etchant from the measured thickness change in SiO 2 over time in said first etchant.