The photo-induced etching effect．or development．free vapor photolithvgraphy is a very interesting phenomenon．For determining the mechanism of this phenomenon of cinnamate-type photopolymers，the effects of some factors Dn etching rate have been observed and other experiments have also been made．The results obtained lead to the following hypothesis：When the film of a cinnamate-type photopolymer is irradiated by UV light，a photochemical reaction occurs and the properties of the film are changed．This change enables the film to absorb both HF and H2O from the etching vapor．and a “polymer-hydrofluoric acid solution”can be formed．The solution，which is very different from the mixed gas of HF and H2O，can react with SiO2 rapidly，but the unirradiated film can not absorb the mixed gas easily

hence the rate of etching SiO2 of unirradiated areas is much slower than that of irradiated areas．It is the difference of etching rate be-tween two areas that makes a pattern．The results of our experiments are explained by this hypothesis．