Low sodium, low metals silica polishing slurries

A colloidal silica slurry comprising:

• a low metals ammonium-stabilized silica sol formed by:
• (a) diluting said silica-containing solution with water to obtain a dilute silica-containing solution containing from about 5.0 to about 8.0 weight percent silica, as SiO.sub.2 ;
• (b) exposing said dilute silica-containing solution to a cation exchange resin in the acid form and in sufficient capacity to remove essentially all sodium values, thereby forming a dilute silicic acid solution, said silicic acid solution comprising from about 5.0 to about 8.0 weight percent silicic acid as SiO.sub.2 ;
• (c) adding to said silicic acid solution at least 0.10 weight percent, based on SiO.sub.2, of oxalic acid crystals and at least 0.25 weight percent, based on total silicic acid solution, of an inorganic acid, thereby forming an oxalate-containing, low pH silicic acid solution having a pH ranging from about 0.5 to about 2.5;
• (d) mixing said oxalate-containing, low pH silicic acid solution to recover silicic acid solution which contains from about 5.0 to about 8.0 weight percent silicic acid, as SiO.sub.2 ;
• (e) exposing said silicic acid solution to an anion exchange resin in the hydroxide form, and in sufficient amount and with sufficient capacity to replace essentially all negatively charged species contained therein with hydroxide ions, thereby forming a hydroxide-neutralized silicic acid solution having a pH ranging between about 2.5 to about 4.0;
• (f) exposing said hydroxide-neutralized silicic acid solution to a cation exchange resin in the acid form and in sufficient amount and with sufficient capacity to replace all positively charged species contained therein with hydrated protons, thereby forming a low metals silicic acid solution;
• (g) chilling said low metals silicic acid solution to a temperature ranging between about 40.degree. F. to about 50.degree. F., and storing for from one minute to about fifty hours, thereby forming a chilled low metals silicic acid solution;
• (h) adding up to 50 volume percent of the low metals silicic acid solution to a preformed ammonium hydroxide solution made by adding concentrated ammonium hydroxide to deionized or softened water in sufficient quantity to achieve the preformed ammonium hydroxide solution having a pH ranging between about 8.0 to about 11.2, thereby forming an ammonium-neutralized silicic acid heel solution having a pH ranging between about 8.0 to about 11.2;
• (i) heating said heel solution to a temperature ranging between about 75.degree. C. to about 150.degree. C., under sufficient pressure to prevent boiling, and then maintaining this temperature for from about 0.5 hours to about twenty-four hours thereby forming silica sol particles, while adding, with stirring, the remainder of the low metals silicic acid solution, thereby reacting same with said silica sol particles, while simultaneously adding sufficient ammonium hydroxide solution to maintain a pH ranging from about 8.0 to about 11.2, and finally forming a dilute ammonium-stabilized low metals silica sol admixture;
• (j) reacting this final admixture, at a temperature from 75.degree. C. to about 150.degree. C., at pressure sufficient to prevent boiling, for an additional 0.5 to about 8 hours, thereby forming a dilute, low metals silica sol solution containing from about 2.0 to about 6.0 weight percent silica, as SiO.sub.2 ;
• and
• (k) concentrating the dilute, low metals sol solution thereby forming said low metals ammonium-stabilized silica sol;
• said low metals ammonium-stabilized silica sol having the following characteristics:
• SiO.sub.2 present in the range between about 8.5 to about 11.3;
• a particle diameter in the range between about 4.0 to about 130 nm;
• aluminum, as Al, present in an amount less than about 100 ppm, based on SiO.sub.2 ;
• iron, as Fe, present in an amount less than about 50 ppm, based on SiO.sub.2 ;
• potassium, as K, present in an amount less than about 25 ppm, based on SiO.sub.2 ;
• and sodium, as Na, present in an amount less than about 500 ppm, based on SiO.sub.2 ;
• and
• a polishing rate accelerator present in an amount between about 0.5 to about 5%.

A process for polishing a silicon wafer which includes the step of recirculating a colloidal silica slurry comprising:

• a low metals ammonium-stabilized silica sol formed by:
• (a) diluting said silica-containing solution with water to obtain a dilute silica-containing solution containing from about 5.0 to about 8.0 weight percent silica, as SiO.sub.2 ;
• (b) exposing said dilute silica-containing solution to a cation exchange resin in the acid form and in sufficient capacity to remove essentially all sodium values, thereby forming a dilute silicic acid solution, said silicic acid solution comprising from about 5.0 to about 8.0 weight percent silicic acid as SiO.sub.2 ;
• (c) adding to said silicic acid solution at least 0.10 weight percent, based on SiO.sub.2, of oxalic acid crystals and at least 0.25 weight percent, based on total silicic acid solution, of an inorganic acid, thereby forming an oxalate-containing, low pH silicic acid solution having a pH ranging from about 0.5 to about 2.5;
• (d) mixing said oxalate-containing, low pH silicic acid solution to recover silicic acid solution which contains from about 5.0 to about 8.0 weight percent silicic acid, as SiO.sub.2 ;
• (e) exposing said silicic acid solution to an anion exchange resin in the hydroxide form, and in sufficient amount and with sufficient capacity to replace essentially all negatively charged species contained therein with hydroxide ions, thereby forming a hydroxide-neutralized silicic acid solution having a pH ranging between about 2.5 to about 4.0;
• (f) exposing said hydroxide-neutralized silicic acid solution to a cation exchange resin in the acid form and in sufficient amount and with sufficient capacity to replace all positively charged species contained therein with hydrated protons, thereby forming a low metals silicic acid solution;
• (g) chilling said low metal silicic acid solution to a temperature ranging between bout 40.degree. F. to about 50.degree. F., and storing for from one minute to about fifty hours, thereby forming a chilled low metals silicic acid solution;
• (h) adding up to 50 volume percent of the low metals silicic acid solution to a preformed ammonium hydroxide solution made by adding concentrated ammonium hydroxide to deionized or softened water in sufficient quantity to achieve the preformed ammonium hydroxide solution having a pH ranging between about 8.0 to about 11.2, thereby forming an ammonium-neutralized silicic acid heel solution having a pH ranging between about 8.0 to about 11.2;
• (i) heating said heel solution to a temperature ranging between about 75.degree. C. to about 150.degree. C., under sufficient pressure to prevent boiling, and then maintaining this temperature for from about 0.5 hours to about twenty-four hours thereby forming silica sol particles, while adding, with stirring, the remainder of the low metals silicic acid solution, thereby reacting same with said silica sol particles, while simultaneously adding sufficient ammonium hydroxide solution to maintain a pH ranging from about 8.0 to about 11.2, and finally forming a dilute ammonium-stabilized low metals silica sol admixture;
• (j) reacting this final admixture, at a temperature from 75.degree. C. to about 150.degree. C., at pressure sufficient to prevent boiling, for an additional 0.5 to about 8 hours, thereby forming a dilute, low metals silica sol solution containing from about 2.0 to about 6.0 weight percent silica, as SiO.sub.2 ;
• and
• (k) concentrating the dilute, low metals sol solution thereby forming said low metals ammonium-stabilized silica sol;
• said low metals ammonium-stabilized silica sol having the following characteristics:
• SiO.sub.2 present in the range between about 8.5 to about 11.3;
• a particle diameter in the range between about 4.0 to about 130 nm;
• aluminum, as Al, present in an amount less than about 100 ppm, based on SiO.sub.2 ;
• iron, as Fe, present in an amount less than about 50 ppm, based on SiO.sub.2 ;
• potassium, as K, present in an amount less than about 25 ppm, based on SiO.sub.2 ;
• and sodium, as Na, present in an amount less than about 500 ppm, based on SiO.sub.2 ;
• a bactericide present in an amount between about 0.08 to about 5%, said bactericide is at least one compound selected from the group consisting of tetramethylammonium chloride, tetraethylammonium chloride, tetrapropylammonium chloride, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, alkylbenzyldimethylammonium chloride, and alkylbenzyldimethylammonium hydroxide, wherein the alkyl chain ranges from 1 to about 20 carbon atoms;
• and sodium chlorite or sodium hypochlorite present in an amount between about 1 to about 1000 ppm, between a polishing plate containing a polishing pad and said silicon wafer.

A process for polishing a silicon wafer which includes the step of recirculating a colloidal silica slurry comprising:

• a low metals ammonium-stabilized silica sol formed by:
• (a) diluted said silica-containing solution with water to obtain a dilute silica-containing solution containing from about 5.0 to about 8.0 weight percent silica, as SiO.sub.2 ;
• (b) exposing said dilute silica-containing solution to a cation exchange resin in the acid form and in sufficient capacity to remove essentially all sodium values, thereby forming a dilute silicic acid solution, said silicic acid solution comprising from about 5.0 to about 8.0 weight percent silicic acid as SiO.sub.2 ;
• (c) adding to said silicic acid solution at least 0.10 weight percent, based on SiO.sub.2, of oxalic acid crystals and at least 0.25 weight percent, based on total silicic acid solution, of an inorganic acid, thereby forming an oxalate-containing, low pH silicic acid solution having a pH ranging from about 0.5 to about 2.5;
• (d) mixing said oxalate-containing, low pH silicic acid solution to recover silicic acid solution which contains from about 5.0 to about 8.0 weight percent silicic acid, as SiO.sub.2 ;
• (e) exposing said silicic acid solution to an anion exchange resin in the hydroxide form, and in sufficient amount and with sufficient capacity to replace essentially all negatively charged species contained therein with hydroxide ions, thereby forming a hydroxide-neutralized silicic acid solution having a pH ranging between about 2.5 to about 4.0;
• (f) exposing said hydroxide-neutralized silicic acid solution to a cation exchange resin in the acid form and in sufficient amount and with sufficient capacity to replace all positively charged species contained therein with hydrated protons, thereby forming a low metals silicic acid solution;
• (g) chilling said low metals silicic acid solution to a temperature ranging between about 40.degree. F. to about 50.degree. F., and storing for from one minute to about fifty hours, thereby forming a chilled low metals silicic acid solution;
• (h) adding up to 50 volume percent of the low metals silicic acid solution to a preformed ammonium hydroxide solution made by adding concentrated ammonium hydroxide to deionized or softened water in sufficient quantity to achieve the preformed ammonium hydroxide solution having a pH ranging between about 8.0 to about 11.2, thereby forming an ammonium-neutralized silicic acid heel solution having a pH ranging between about 8.0 to about 11.2;
• (i) heating said heel solution to a temperature ranging between about 75.degree. C. to about 150.degree. C., under sufficient pressure to prevent boiling, and then maintaining this temperature for from about 0.5 hours to about twenty-four hours thereby forming silica sol particles, while adding, with stirring, the remainder of the low metals silicic acid solution, thereby reacting same with said silica sol particles, while simultaneously adding sufficient ammonium hydroxide solution to maintain a pH ranging from about 8.0 to about 11.2, and finally forming a dilute ammonium-stabilized low metals silica sol admixture;
• (j) reacting this final admixture, at a temperature from 75.degree. C. to about 150.degree. C., at pressure sufficient to prevent boiling, for an additional 0.5 to about 8 hours, thereby forming a dilute, low metals silica sol solution containing from about 2.0 to about 6.0 weight percent silica, as SiO.sub.2 ;
• and
• (K) concentrating the dilute, low metals sol solution thereby forming said low metals ammonium-stabilized silica sol;
• said low metals ammonium-stabilized silica sol having the following characteristics:
• SiO.sub.2 present in the range between about 8.5 to about 11.3;
• a particle diameter in the range between about 4.0 to about 130 nm;
• aluminum, as Al, present in an amount less than about 100 ppm, based on SiO.sub.2 ;
• iron, as Fe, present in an amount less than about 50 ppm, based on SiO.sub.2 ;
• potassium, as K, present in an amount less than about 25 ppm, based on SiO.sub.2 ;
• and sodium, as Na, present in an amount less than about 500 ppm, based on SiO.sub.2 ;
• and a polishing rate accelerator present in an amount between about 0.5 to about 5%, between a polishing plate containing a polishing pad and said silicon wafer.