Method to pattern etch masks in two inclined planes for three-dimensional nano- and microfabrication

The authors present a method to pattern etch masks for arbitrary nano- and microstructures on different, inclined planes of a sample. Our method allows standard CMOS fabrication techniques to be used in different inclined planes; thus yielding three-dimensional structures with a network topology. The method involves processing of the sample in a first plane, followed by mounting the prepared sample in a specially designed silicon holder wafer such that the second, inclined plane is exposed to continued processing. As a proof of principle we demonstrate the fabrication of a patterned chromium etch mask for three-dimensional photonic crystals in silicon. The etch mask is made on the 90° inclined plane of a silicon sample that already contains high aspect ratio nanopores. The etch mask is carefully aligned with respect to these pores, with a high translational accuracy of <30 nm along the y-axis and a high rotational accuracy of 0.71° around the z-axis of the crystal. Such high alignment precisions are crucial for nanophotonics and for sub-micrometer applications in general. Although we limit ourselves to processing on two planes of a sample, it is in principle possible to repeat the presented method on more planes. The authors foresee potential applications of this technique in, e.g., microfluidics, photonics, and three-dimensional silicon electronics.