The effect of BCl 3 pretreatment on the etching of AlN in Cl 2 -based plasma
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1108-A09-34
The effect of BCl3 pretreatment on the etching of AlN in Cl2-based plasma X. Xu, V. Kuryatkov, B. Borisov, M. Pandikunta, S. A. Nikishin, and M. Holtz Nano Tech Center, Texas Tech University, Lubbock, TX 79409 ABSTRACT The effect of BCl3 and BCl3/Ar pretreatment on Cl2/Ar and Cl2/Ar/BCl3 dry etching of AlN is investigated using inductively coupled plasma reactive ion etching. The native AlN oxide can be effectively removed by a short exposure to BCl3 or BCl3/Ar plasma. Compared to the chlorine based plasma etching, BCl3/Ar is found to have the highest etch rate for both AlN and its native oxide. Following removal of the native oxide, Cl2/Ar/BCl3 plasma etching with 15% BCl3 fraction results in a high etch rate ~ 87 nm/min and modest increases in the surface roughness. INTRODUCTION AlN is an important material for production of III-Nitride based optoelectronic devices. Because of the high chemical stability of AlN, conventional wet etching is unsuitable for device processing. Plasma or dry etching is preferred, with numerous approaches available [1]. Dual power inductively coupled plasma – reactive ion etching (ICP-RIE) is popular because it provides a high-density plasma and independently controlled substrate bias voltage. The chlorides of Al and N are relatively volatile [2, 3, 4] making Cl2, BCl3, and SiCl4 the primary reagents used to etch AlN and others III-N group materials. An important issue in etching AlN and its alloys is a delay in the onset of material removal. This delay is generally attributed to the formation of an etch-resistant native oxide. One approach for mitigating the etch delay of AlN [1, 5] and AlGaN [6] is the use of a GaN cap layer, since GaN forms a less robust oxide. Exposed layers of AlGaN, with Al mole fraction as high as 35%, have been etched without delay using a BCl3 breakthrough (pretreatment) approach [7]. Khan et al. [8] observed that adding of BCl3 into the Cl2/Ar mixture help to improve a surface smoothness of plasma etched polycrystalline AlN. Recently, we have shown that the native oxide forming on AlN, due to exposure to air, is primarily Al2O3 [9]. The thickness of this oxide is not uniform across a wafer [9] and has been reported to vary from 0.5 to 1.2 nm [10] from sample to sample. The etch rates of Al2O3 and AlN are different [11]. Thus, precise control of Al2O3 etch rate is needed to develop reproducible plasma processing of AlN. In this paper, we describe experiments on BCl3-based plasma etching (pretreatments in BCl3 and BCl3/Ar) of Al2O3 formed on the surface of AlN thin layers. We also report the results on Cl2/Ar and Cl2/Ar/BCl3 plasma etching of oxide-free AlN. We describe a new sequence of the etching steps yielding low root-mean square (RMS) surface roughness and delay-free etch processing of oxidized AlN films. EXPERIMENTAL DETAILS The AlN epitaxial layers were grown on 50 mm (0001) sapphire substrates by gas source MBE with ammonia. We found that AlN layer quality strongly depends on substrate nitridation time. The optimal nitridation time was 20
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