A new technique for performing chromeless phase shift optical nanolithography is presented. Phase masks have been wet etched into silicon nitride membranes, using either hot H3PO4 or HF. Contact exposure through these masks results in developed photoresist features as small as 90 nm, even for masks with shallow sidewall slopes. The observed effect is attributed to the presence of an abrupt phase step at some point on the wet etch profile, although this does not need to correspond to a pi phase shift in order to obtain good contrast exposures. Simulations have been performed for typical wet-etching profiles, and it is found that good contrast is expected for a very wide range of etch depths. In addition, the width of the null in the near-field intensity profile varies with etch depth, indicating that control of feature size is possible. This technique is simple and inexpensive, making it an attractive candidate for nanopatterning a wide variety of substrates.