Wafer bonding for fabrication of three-dimensional photonic band gap crystals

Abstract

<p>Photonic band gap (PBG) crystals are artificially engineered periodic dielectric structures which exhibit forbidden frequency regions where electromagnetic waves cannot propagate. Since the use of three-dimensional PBG crystals was first proposed in 1987 to control optical properties, these structures have generated considerable interest due to their potential applications over a wide frequency range. However, the demonstration of practical three-dimensional PBG crystals has been limited to larger-dimensional structures operating below far-infrared frequencies because of difficulties in fabricating small complex structures;In this work, we have devised techniques for use in fabricating 3-D PBG crystals with micrometer length scales operating in the mid-infrared region. This microfabrication-based approach uses alternating steps of wafer fusion bonding, selective substrate etching, and pattern etching to sequentially build up PBG crystals in a layer-by-layer fashion. The wafer fusion technique was utilized to stack up GaAs thin films. To enhance the bonding, a thin (Ga,In)As "bonding" layer has been incorporated into the structure to improve the bonding strength between two PBG layers;The surfaces and interfaces of the bonded samples have been characterized mechanically and optically to further determine the optimum bonding conditions for PBG crystals. Using (Ga,In)As layers, smooth and uniform bonded surfaces and good adhesion at the interfaces have been achieved at annealing temperatures of ~650°C. By reducing the anneal times and In content in (Ga,In)As alloys, the overall transmission intensities have been improved over the entire spectrum, particularly at higher frequencies;Using wafer fusion bonding techniques, we have successfully constructed multi-layer structures with PBG dimensions at micron length scales. With improved stacking interfaces, wafer bonding and micromachining techniques provide a promising way to realize photonic crystals with stop bands around 10 [mu]m.</p>