It will not affect the pattern but it may affect the quality. If it gets too thick you won’t see anything at all. EDIT 5-3-21 It does not affect a double or multiple slit pattern but it does affect a single slit pattern. (Wavelength) X (distance to screen) / (thickness of slit)= first dark fringe and ( 2 x Wavelength) X (distance to screen) / (thickness of slit)= second dark fringe Etc.
