Simulations of silicon diffractive lenses for terahertz frequencies have been performed using a Fraunhofer wave propagation model to show how the lens efficiency can be affected by different fabrication defects. A general model is presented to illustrate the operating bounds which must be adhered to when fabricating a lens using a multiple level process. From these simulations it is possible to ascertain for a specific lens design with known processing inaccuracies what level of design complexity will gain the most benefit. It is shown that for a 1 THz lens fabricated using an eight level process the lens efficiency is reduced by less than 20% for realistic values of etch non-uniformity, inter-level misalignment and surface roughness. (C) 2003 Elsevier B.V. All rights reserved.