Increasing swing and notching effects due to the higher reflectivity of silicon with decreasing exposure wavelength are a major problem in deep UV lithography. Top and/or bottom anti reflective layers (ARL) are necessary for sub-half micron lithography. In this article a simple method describes how optimum values of the refractive index and the thickness of bottom ARLs can be determined. Two classes of bottom ARLs exist: (1) materials with low intrinsic reflectivity in resist; and (2) Bottom ARLs based on the combined effect of absorbance and interference, so that ARL thickness control over topography is critical. The advantages and disadvantages of both ARL types are given. On poly-Si the optimum ARL is a combination of the two types. A quantitative comparison of the usefulness of various materials as ARL on (poly)silicon, oxide and aluminum is made. Materials considered are: spin-on organic ARC, TiW, TiN, and plasma-enhanced CVD deposited amorphous layers such as hydrogenated silicon (a-Si:H), carbon (a-C:H), silicon carbide a-SiCx, oxynitride a-SiNxOy, and nitride a-Si3+xN4.