Due to the recent development in higher density and higher precision of highly integrated circuits such as super-LSI devices, requirements for smoothness and surface defects of a substrate for electronic devices such as a mask blank substrate are becoming stricter year by year.
As an example of a conventional method to reduce minute convex surface defects on a principal surface of a mask blank substrate, a so-called light etching method is known, in which, after local chemical mechanical polishing (CMP) for polishing a principal surface of a quartz glass substrate for mask blank with a polishing solution including colloidal silica grains, the principal surface is cleaned with low-concentration hydrofluoric acid solution (Patent Document 1). Light etching in this cleaning method is for the purpose of enhancing cleaning effect by lifting and removing (lift-off) foreign matters attached to the principal surface of the substrate.
On the other hand, in the cleaning step of a substrate before application of a resist in a lithography step, a cleaning process such as RCA cleaning is employed using a combination of physical cleaning tools such as a megasonic nozzle and a brush and chemical cleaning tools such as APM (ammonium hydrogen peroxide mixture) and SPM (sulfuric acid hydrogen peroxide mixture) to remove foreign matters attached to a principal surface of the substrate.
Further, as an example of a cleaning method of a photomask (transfer mask), a method is known in which foreign matters attached to a surface of a photomask are removed by applying physical energy such as megasonics in hydrogen gas dissolved water (Patent Document 2).
Recently, Catalyst Referred Etching (also referred to as CARE hereafter) method has been suggested (Patent Documents 3, 4).
The CARE method is conducted by causing a principal surface of a crystalline substrate such as SiC and a catalyst to be close to, or in contact with each other with a processing liquid such as an acidic liquid therebetween, and using active species produced from molecules in the processing liquid adsorbed to the catalyst, selectively removing microscopic protrusions generated as crystal defects on the principal surface by mechanical processing and polishing, to thereby achieve flattening and smoothening of the principal surface.
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