back than the unexposed resist film or undergoes film loss thereby forming a recess portion further a system having the formula 4 may be used str6 the system of the formula 4 is composed of a polymer formed of poly p tert butoxy methoxy and onium salt such as iodonium salt when the system is exposed to light proton acid is generated by such illumination and decomposes tert butoxy methoxy groups to generate phost isobutene and aldehyde such isobutene and aldehyde become gaseous and are discharged to outside the system and only the phost lingers and is left within the system as a result film loss occurs in the present embodiment the resist film is just left after exposure processing to wait for the occurrence of film loss a post exposure bake peb may be carried out to foster reactions such as decomposition for the speedy process of film loss i e change in the film thickness a second embodiment of this invention in which a means for detecting the position of an om overlay measurement pattern is provided independently of a means for detecting the position of an or overlay reference pattern is described referring now to fig1 the structure of the second embodiment is illustrated an illumination optical system 13 a mirror m a mask blind 28 and a mask 14 are all arranged in front of an exposure light source 12.