Planarization of GaN by the Etch-Back Method
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0892-FF14-11.1
Planarization of GaN by the Etch-Back Method
Adrian D. Williams, Theodore D. Moustakas Department of Electrical and Computer Engineering, Boston University, Boston MA ABSTRACT The planarization of GaN has been mainly addressed by chemo-mechanical polishing methods in the literature. Such techniques were found to be successful in planarizing the nitrogen polar (000-1) surface of GaN. However, planarization of the gallium polar (0001) surface presents a challenge due to its higher degree of chemical inertness. Moreover, studies on the planarization of the remaining crystal orientations remain a topic with sparse representation in the literature. In this paper, we report our planarization studies of GaN films using an etch-back technique. Photoresist is employed as the sacrificial layer and a chlorine inductively coupled plasma (ICP) used for etching. We demonstrate the planarization of rough C-plane and A-plane GaN films grown by hydride vapor phase epitaxy (HVPE). Specifically, an rms roughness of approximately 2.1nm and 4.1nm was realized after iterative etch-back for a representative C-plane and A-plane sample with initial rms roughness of 134nm and 414nm respectively. INTRODUCTION The development of robust GaN planarization techniques will undoubtedly play a crucial role in the continued advancement of nitride based semiconductor devices. With the inevitable emergence of commercially available GaN wafers, manufactures of such have an unquestionable need to develop methods to planarize them prior to use. Such substrates are anticipated to open the door to the next generation of nitride semiconductor devices since problems such as cracking and high dislocation densities stemming from the epitaxial growth of nitrides on non-native substrates would be bypassed. The surface condition of wire-sawn wafers is unsuitable for device growth due to its high degree of roughness and requires further processing to be made atomically smooth. To date, the volume of literature addressing such planarization processes for GaN remains thin and therefore motivates us to address the issue. The traditional method of planarizing semiconductor materials is lapping followed by chemo-mechanical polishing (CMP). Lapping is a coarse planarization technique utilizing successively smaller sizes of pad-embedded grits of very hard materials such as diamond, cubic boron nitride, or alumina to achieve bulk removal and flattening of a material through grinding action. Lapped surfaces tend to have micronscale roughness. Chemo-mechanical polishing is a planarization technique that simultaneously uses both chemical and mechanical processes to achieve a finer degree of planarization. This is accomplished by a far gentler grinding of the semiconductor with nanoscale grits embedded in an etching solution. Mature CMP techniques for various materials such as silicon, germanium, arsenide and phosphide family semiconductors, allow for the generation of atomically smooth surfaces [1,2,3]. However, GaN does not lend itself so readily to this pro
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