Method for polishing silicon substrate and polishing composition set

A method for polishing silicon substrates consisting of a high-resistivity silicon substrate having a resistivity of 1 Ω·cm or greater and a low-resistivity silicon substrate having a resistivity less than 0.005 Ω·cm, the method comprising a stock polishing step and a final polishing step for each of the high-resistivity silicon substrate and the low-resistivity silicon substrate, wherein the final polishing step is a step polishing only a single side of each of the high-resistivity silicon substrate and the low-resistivity silicon substrate, the final polishing step comprising:

• supplying a first polishing slurry S1 and a second polishing slurry S2 to the silicon substrate to be polished, switching them in this order midway through polishing a single side of the silicon substrate,
• wherein the first polishing slurry S1 comprises an abrasive A1, a water-soluble polymer P1, and a basic compound B1, wherein:
• the abrasive A1 has a BET diameter of 5 nm or greater and 100 nm or less and a true specific gravity of 1.5 or higher and 2.3 or lower, and a content of the abrasive A1 in the first polishing slurry S1 is 0.05% by weight or higher and 10% by weight or lower;
• the water-soluble polymer P1 is a mixture of a water-soluble polymer having a vinyl alcohol-based polymer chain and a water-soluble polymer having an N-vinyl-based polymer chain, or the water-soluble polymer P1 is a copolymer having the vinyl alcohol-based polymer chain and the N-vinyl-based polymer chain in the same molecule, a content of the vinyl alcohol-based polymer chain in the water-soluble polymer P1 is 50% by weight or higher and an amount of the water-soluble polymer P1 per 1 g of abrasive A1 contained in the first polishing slurry S1 is 0.03 g or less;
• the basic compound B1 is an alkali metal hydroxide comprising potassium hydroxide;
• the first polishing slurry has a pH of 8.0 or higher and 12.0 or lower;
• and
• the first polishing slurry S1 shows higher polishing removal rate than the second polishing slurry S2;
• and
• wherein:
• the first polishing slurry S1 for polishing the low-resistivity silicon substrate in the final polishing step has the same composition as the first polishing slurry S1 for polishing the high-resistivity silicon substrate in the final polishing step;
• the second polishing slurry S2 for polishing the low-resistivity silicon substrate in the final polishing step has the same composition as the second polishing slurry S2 for polishing the high-resistivity silicon substrate in the final polishing step;
• and
• the first polishing slurry S1 shows a first polishing removal rate for polishing the high-resistivity silicon substrate and a second polishing removal rate for polishing the low-resistivity silicon substrate, wherein the first polishing removal rate is less than or equal to 6.2 times the second polishing removal rate.