Development of Wide Bandgap Semiconductor Photonic Device Structures by Excimer Laser Micromachining
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ABSTRACT Excimer laser ablation rates of Si (111) and AlN films grown on Si (111) and rplane sapphire substrates were determined. Linear dependence of ablation rate of Si (111) substrate, sapphire and AlN thin films were observed. Excimer laser micromachining of the AlN thin films on silicon (111) and SiC substrates were micromachined to fabricate a waveguide structure and a pixilated structure. This technique resulted in clean precise machining of AlN with high aspect ratios and straight walls.
INTRODUCTION Laser micromachining is a noncontact process offering selective material removal with high precision and repeatability, and a minimal heat-affected zone (HAZ.) It can also produce flexible feature size and shape using mask-imaging methods and a computer controlled multi-dimensional scanning stage etc1. Excimer lasers with 20~30ns pulse duration have been demonstrated to be suitable for various materials.2, 3 Excimer laser machining provides the advantage of greater photon energy to break bonds, clean cutting with less thermal effects. For example, Behrmann 4 and Wang 5 fabricated diffractive optical elements (DOE) on poly(imide) micromachined by Eximer laser. Han6 fabricated Fabry-Perot cavity chemical sensors by silicon Eximer laser micormachining. We are interested in wide bandgap materials, particularly AlN, since it has great potential in an array of electro-optical and photonic devices. Conventional micromachining techniques, such as wet chemical, are not suitable for processing wide bandgap semiconductors. Although plasma etching is a popular method of micromachining, the possibly introduced damage makes it less suitable for precise micromachining. AlN is not etched by traditional chemical methods. Only concentrated KOH and analogous base etchants are reasonable. Furthermore, the chemical etching process is highly isotropic. Even ion beam and other plasma etching techniques are not viable solutions due to parasitic (with respect to waveguide performance) damage of the structure. With its advantages of direct writing and greater photon energy for cool ablation, we hope to utilize Excimer laser micromachining technique to fabricate photonic device structures on AlN in a one-step process. An image-based pulse KrF Excimer laser micromachining system has been developed. With an ultra-precise 4dimensional (X, Y, Z and Rotation) scanning stage and an in-house developed software package, this system has the capability to produce feature size down to less than 1 micron scale with flexible shaped features possible. Wave guide structures and a pixilated structure of AlN on Silicon (111) and SiC have been fabricated. The focus of this paper is to study the relation between laser ablated hole depth to the incident pulse energy for the thin film and substrate structures.
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Linear dependence within the thin film and substrate has been observed. This quantitative relationship provides a methodology for precise depth control, which is critical in fabrication of electronic and photonic device structures.
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