be controlled similar to fig2 a fig3 a illustrates a portion of a silicon substrate 41 where a shallow trench will be formed on the silicon substrate 41 is a pad oxide silicon dioxide layer 42 of thickness 5 to 20 nm formed by the thermal oxidation method a pad nitride silicon nitride layer 44 of thickness 100 to 300 nm is formed on the pad oxide layer 42 on the pad nitride layer 44 is a silicon dioxide layer 45 of thickness 30 to 60 nm on the silicon dioxide layer 45 is a patterned resist 46 having an aperture greater than the area in which the trench is to be formed anisotropic dry etching method is used to remove portions of the pad oxide layer 42 pad nitride layer 44 and silicon dioxide layer 45 until a portion of the substrate 41 is exposed to the outside through the aperture forming a recess 46 a the patterned resist 46 is then stripped and a silicon dioxide layer is deposited over the wafer this silicon dioxide layer is anisotropically etched until the silicon dioxide layer 45 is exposed as a result here remains a spacer 47 attached to the sidewall of recess 46 b spacer 47 defines a mask window 47 a fig3 b illustrates the results of this anisotropically etching process depending on the thickness of the silicon dioxide layer 45 the spacer 47 has a desired maximum wall thickness 0026 fig3 c illustrates the results of an anisotropic dry etching step of the silicon substrate 41 for etching the.