Composition for forming silicon-containing resist underlayer film and patterning process

A composition for forming a silicon-containing resist underlayer film contains a compound shown by the following general formula (A-1) and a thermally crosslinkable polysiloxane. R1 represents a methyl group, an ethyl group, a propyl group, an allyl group, or a propargyl group. R2 represents a hydrogen atom, an acetyl group, an acryloyl group, a methacryloyl group, a benzoyl group, a naphthoyl group, or an anthranoyl group. R3 represents a methyl group, an ethyl group, a propyl group, an allyl group, a propargyl group, or a group shown by the following general formula (A-2), where a broken line represents a bonding arm, and R1 and R2 are as defined above. An object of the present invention is to provide a silicon-containing resist underlayer film capable of exhibiting high effect of suppressing ultrafine pattern collapse and forming a resist pattern with favorable pattern profile in multilayer resist methods.