A method of forming features, e.g. contact holes, at a higher density than is possible with conventional lithographic techniques involves forming an array of sacrificial positive features, conformally depositing a sacrificial layer so that negative features are formed interleaved with the positive features, directionally etching the sacrificial layer and removing the sacrificial features. The result is an array of holes at a higher density than the original sacrificial features. These may then be transferred into the underlying substrate using a desired process. Also, the method may be repeated to create arrays at even higher densities.