Multilayer resist pattern forming method

A pattern forming method employing a multilayer resist, said method comprising the steps of:

• providing a substrate having a surface;
• forming a lower resist layer of an organic material on said surface, said lower resist layer having a thickness large enough to absorb irregularities in said substrate surface and provide a flat surface on said lower resist layer opposite sad substrate;
• forming an intermediate resist layer having a thickness of about 0.2 .mu.m and comprising SiO.sub.2, Si.sub.3 N.sub.4, Al.sub.2 O.sub.2, SnO.sub.2, In.sub.2 O.sub.3 or ITO on the lower resist layer by a high density plasma processing at a plasma density of between about 1.times.10.sup.11 /cm.sup.3 and 1.times.10.sup.14 /cm.sup.3 while maintaining the substrate at a temperature of between about 20.degree. C. to about 200.degree. C. to provide intermediate resist layer having a film density sufficient to be resistant to ion bombardment, said forming an intermediate resist layer is carrier out using a plasma processing system selected from the group consisting of inductive coil plasma CVD and helicon wave plasma CVD;
• forming an upp resist layer on said intermediate resist layer;
• exposing the upper resist layer to laser light and developing the exposed upper resist layer to define an upper resist pattern;
• etching the intermediate resist layer by reactive ion etching methods using the upper resist pattern as a mask to define an intermediate resist pattern;
• anistropically etching the lower resist layer at a high selective etching ratio using the upper resist pattern and intermediate resist pattern as a mask to define a substantially critically-dimensioned lower resist pattern useful for further pattern processing, the etching of the lower resist pattern being carried out using an etching gas selected from CO.sub.2 and a mixture of O.sub.2 and a sulfur halide, the etching of the lower resist pattern being carried out without contraction or reduction of the intermediate resist pattern.