Arrays of monocrystalline silicon micromirrors fabricated using CMOS compatible transfer bonding
Researchers Sjoerd Haasl Frank Niklaus Göran Stemme	Sponsors
Micromirror arrays are light modulators that can modulate the phase or amplitude of incident light. The main application areas of micromirror arrays are projection display systems, pattern generators in maskless lithography systems, optical scanners, printers, optical spectroscopy, aberration correction, adaptive optical systems and switches and cross connectors in optical communication systems. Some of these applications require large two-dimensional arrays of individually addressable micromirrors. To achieve this, it is necessary to combine the micromirrors with on-chip electronics. Micromirrors that are made of monocrystalline silicon or other monocrystalline materials have several advantages. The mechanical stability of monocrystalline silicon is superior over metals. There are no recrystallization effects due to material deformation in monocrystalline silicon hinges. This minimizes hysteresis and memory effects as a result of repeated or prolonged micromirror actuation. Also, the achievable optical quality, the surface roughness and the uniformity of monocrystalline silicon surfaces is superior compared to most other surfaces. However, monocrystalline silicon mirrors can not be monolithically integrated on CMOS wafers. We have developed a technique that allows wafer-level integration of monocrystalline silicon membranes with standard CMOS circuits. Arrays of 4x4 monocrystalline silicon micromirrors have been fabricated using transfer bonding that is compatible with CMOS substrate wafers. The SEM image shows an array of the micromirrors with the underlying aluminium addressing electrodes. The mirror membranes are 0.34 mm thick and measure 16 mm x 16 mm. The hinges are 4 mm long, 0.6 mm wide and the posts are 3 mm x 3 mm in size. Transfer bonding allows thin monocrystalline silicon films from commercially available SOI wafers to be used for the fabrication of very flat, uniform and low stress silicon micromirror membranes.
Figure: CMOS compatible fabrication of monocrystalline silicon micromirror arrays using transfer bonding.
The torsional micromirror arrays have potential applications as spatial light modulators in projection display systems, in maskless lithography systems and in optical communication systems. In principle, the CMOS compatible membrane transfer bonding technique can be applied to any type of transducer that consists of membranes and that benefits from the use of high temperature annealed or monocrystalline materials.
Figure: A 4 by 4 array of micromirrors
Publications F. Niklaus, S. Haasl and G. Stemme, "Arrays of monocrystalline silicon micromirrors fabricated using CMOS compatible transfer bonding," Journal of Microelectromechanical Systems, v 12, n 4, Aug. 2003, p 465-469 S. Haasl, F. Niklaus and G. Stemme, "Arrays of monocrystalline silicon micromirrors fabricated using CMOS compatible transfer bonding," 16th Annual International Conference on Micro Electro Mechanical Systems, pp. 271-Kyoto, Japan, 2003, p 271-274.