One of the problems in defining LSI patterns by the lift‐off technique is burrs, which appear at film pattern edges. The cause of burrs formed at the SiO film pattern edges is discussed. Burrs result from indirectly deposited thin layer residues on the resist pattern side walls. This layer is called a ‘‘side wall layer.’’ A model experiment is carried out and it is shown that the reflection of evaporated SiO molecules is the cause of indirect deposition. SiO molecule reflection probability during deposition is measured. The side wall layer is simulated using the reflection probability. It is concluded that reflection at the substrate surface and reflection at the vacuum chamber wall contribute to an equal degree to the side wall layer formation. Several methods of suppressing the side wall layer, which are also applicable to materials like Pb with higher reflection probability, are discussed. Cooling the substrate or the vacuum chamber, or setting up a shielding board in the chamber are predicted to be effective.