Thermally Driven In-Situ Removal of Native Oxide Using Anhydrous Hydrogen Fluoride
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ABSTRACT In-situ native-oxide removal is critical for epitaxial single-crystal silicon deposition, for polysilicon emitters and contacts and for ultrathin gate dielectric films in integrated circuit (IC) fabrication. We have developed an in-situ, thermally-driven, anhydrous hydrogen fluoride (AHF)-based native-oxide removal technique in which the wafer is treated by AHF at low temperatures (300-400'C) and a short (10 sec) 9500C 'spike' in AHF-H 2 immediately prior to Si deposition. This process removes native oxides formed by standard wet cleans such as HCl:H 2 0 2 and NH 4 OH:H 2 0 2 , as well as native oxides formed by the clean-room ambient. Further, the technique is an effective pre-clean for both polysilicon and epitaxial silicon deposition. This flexibility, combined with other salient features such as simplicity and a low thermal budget, make the process eminently suited for IC fabrication. INTRODUCTION Atomic-scale wafer cleaning has assumed critical importance over the last few years in the fabrication of integrated circuit (IC) chips. The primary reason for this is the continuous lowering of permissible contaminant levels, in turn caused by shrinking device dimensions. A classic example is the "native oxide" layer that forms on an exposed silicon surface: a 10 A native-oxide layer formed 10% of a 100 A gate oxide in the 0.8-1 pm technology, but forms 20% of a 50 A gate oxide in the 0.25-0.35 pm technology; indicating that the latter technology may be unable to tolerate the contaminant layer. Traditionally cleaning has been performed using wet chemicals, and hence has been an ex-situ process. However, 1-3 atomic layers of native oxide may grow swiftly either due to exposure of the bare Si surface to a standard clean-room ambient, or due to oxidizing agents in the wet chemical clean itself. The key is that if the partial pressure of H2 0 and 02 in the ambient exceeds a threshold value, a native-oxide layer will form [1]. Time for a monolayer of native oxide to form ranges from one hour for the clean-room ambient [2], to a few weeks in a special ambient with moisture < 0.1 ppm [3]. Thus, while native oxide can be controlled using special facilities, the complete es-situ elimination of native-oxide contamination is difficult and in-situ cleaning techniques have become imperative. Further, to be viable, such techniques need to be compatible with the IC processing environment. Several techniques have been tried for in-situ native-oxide removal, including ultra-high vacuum [4], Ar ion plasma [5], NF 3 [6], vapor-phase HF [7] etc. Some problems with these techniques have been the inability to remove native oxides grown by wet chemicals, the requirement of expensive custom equipment, large thermal budgets, and difficulty in integration with standard IC processing. The technique presented here is a dry in-situ native-oxide removal technique that utilizes thermally-driven gas-phase anhydrous hydrogen fluoride (AHF). We have introduced previously some preliminary features of this technique [8, 9]; here we demonstrate
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