Merging focused ion beam patterning and optical lithography in device and circuit fabrication

Abstract

Combining optical lithography and focused ion beam (FIB) patterning in direct-write device and circuit fabrication by generating large features optically and small features with the FIB can significantly reduce beam writing time. Our approach to FIB pattern definition is ideally suited for alignment to optically patterned features and takes full advantage of the lithographic and implantation capabilities of the FIB. Optical and FIB patterns are designed simultaneously in the very large scale integrated layout tool magic, [magic, Report No. UCB/CSD 85/225 University of California, Berkley (March 1985)]. We have developed a software package magtofib [James E. Murguia, Christian Musil, Mark Shepard, Sasan Zamani, magtofib, Massachusetts Institute of Technology (1989)] which takes its input from the magic data file and converts it to FIB stage and beam commands. The FIB is aligned by operating in scanning ion microscope mode and finding the centroid of an optically produced die alignment mark. Since the coordinate systems of a typical optical reticle, stepper, and of the FIB are, in general, nonorthogonal and rotated with respect to the wafer, the alignment error increases as one moves away from the alignment point. To place features 1 cm apart to an accuracy of 0.1 μm, calibration must be better than one part in 105. This precision is achieved with a linear transformation calculated from three alignment crosses on the pattern perimeter which transform computer aided design layout coordinates into wafer coordinates. The transformation is calculated once per wafer. The patterning tools have been demonstrated using three types of alignment marks and two resists, on a variety of devices, as well as x-ray masks. The alignment accuracy is ±0.1 μm. In addition, we report a process to combine optical and FIB lithography on the same layer using the same positive UV resist (KTI 820) as a negative FIB resist.