A pattern forming material contains a siloxane polymer having the general formula: ##STR1## [wherein R, R' and R" are the same or different and are respectively one member selected from hydrogen, an alkyl group or a phenyl group; X is one member selected from fluorine, chlorine, bromine, iodine and a --CH.sub.2 Y group (wherein Y is one member selected from chlorine, fluorine, bromine, iodine, an acryloyloxy group, a methacryloyloxy group, and a cinnamoyloxy group); and l, m and n are respectively 0 or a positive integer, l and m not being simultaneously 0]. The material has a high sensitivity to high-energy radiation, a high contrast, and an excellent resistance to reactive ion etching under oxygen gas. The material is conveniently used as a negative resist for forming a submicron pattern having a high aspect ratio.