Temperature Dependence of Wafer Backside Deposition in Single Wafer CVD Reactor
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silicon epitaxial growth 6. Some silicon nodules (silicon islands) will nucleate and grow at the backside edge and other locations where the LTO film is imperfect. Backside LTO removal or any other standard wafer cleaning procedures can not remove these edge and random silicon nodules. These silicon nodules will deteriorate the lithograph process and cause severe yield problems in device fabrication lines. From a generic thin film growth point of view, silicon nodule formation at the backside of the wafer can be divided into two stages; i.e. nucleation and growth7 '8 . Although nucleation sites are one of the important factors in silicon nodule nucleation, silicon vapor or silicon precursor (trichlorosilane) is necessary for both stages. If, in the course of epitaxial deposition, silicon vapor or trichlorosilane can be controlled or eliminated in the space between the wafer backside and the susceptor, the silicon nodule formation process can be controlled at the initial nucleation stage. Optimizing the backside process can suppress the evolution of silicon nuclei into large size silicon nodules. In this paper, the processes occurring at the backside of a wafer in commercial atmospheric single wafer reactor are characterized. Each process and its contribution to the silicon nodule formation were studied. Experimental To quantitatively characterize backside processes under different epitaxial growth conditions, the following experiment was designed. A 10 gtm poly-silicon film was first 209 Mat. Res. Soc. Symp. Proc. Vol. 555 ©1999 Materials Research Society
deposited on the backside of the wafer. The poly-silicon thickness was measured over 21 different locations along the wafer diameter parallel to wafer flat. Following this deposition, a 15 ptm epitaxial silicon layer was grown on the front side of these wafers at different wafer temperatures (Tw). After epitaxial growth, the thickness of silicon layers on both sides was measured. The thickness of backside poly-silicon layer was compared with the initial thickness and the percentage change was calculated. This change was related to both wafer and susceptor temperatures as well as the optical observations of silicon nodules on LTO back-sealed wafers under similar processing conditions. The epitaxial growth rate was calculated from the front side thickness. The actual wafer temperature was deduced from this growth rate 9. The thickness was measured using Fourier-Transfer Infrared spectrometry (FTIR). Silicon film depositions were carried out on Epislon-2000 atmospheric CVD single wafer reactor from Advanced Semiconductor Material (ASM). Wafers were Boron-doped and had a diameter of 150mm. The precursor used in this study was trichlorosilane (TCS) and the carrier gas was hydrogen (H2). All the silicon depositions were carried out at H2 and TCS flows of 65slm and 22grams/min., respectively. The wafer was rotating inside chamber at 45rpm and the backside of the wafer was purged with H2 during epitaxial growth. Two types of wafer positioning were used: (1) wafer in dir
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