Composition for forming silicon-containing metal hard mask and patterning process

A composition for forming a silicon-containing metal hard mask, comprising:

• (A) a metal oxide nanoparticle;
• (B) a thermally crosslinkable polysiloxane (Sx) having no aromatic-ring-containing organic group;
• and
• (C) a solvent,
• wherein (B) the thermally crosslinkable polysiloxane (Sx) has a structural unit of any one or more of a repeating unit represented by the following general formula (Sx-1), a repeating unit represented by the following formula (Sx-2), and a partial structure represented by the following general formula (Sx-3),
• wherein Ra represents:
• an organic group represented by any of the following formulae (T1) to (T3);
• and Rb and Rc each independently represent an organic group having no aromatic ring,
• wherein (Si) represents a bonding position to Si.