Lifetime Characterization of Poly-Silicon Back Sealed Wafers with Bi-Surface Photoconductivity Decay Method
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In this study, we applied the BSPCD to characterization of thermally oxidized silicon wafers with and without a poly-Si back sealed (PBS) surface. It was revealed that the PBS surface has the very fast surface recombination velocity which significantly affects the effective lifetime. It was also shown that the PBS gettering improves the bulk lifetime degraded by the additional impurities introduced in the thermal oxidation. EXPERIMENTAL A 6" (100) n-type (phosphorus doped) Czochralski (CZ) silicon crystal with 10-20 Qcm in resistivity was employed in this study. Half side mirror polished wafers prepared from the crystal separated into two groups. One group has a brightly etched back surface (referred to as BE-wafer) and another has a poly-Si (1.25 pm in thickness) back sealed surface (referred to as PBS-wafer). Both the wafers were identically subjected to two thermal oxidations, respectively. Oxidation-A was performed at 1000 'C for 30 min in dry 02 ambient while oxidation-B at 1000 'C for 120 min in 0i including 5 vol% of HCI. Both the oxidation-A and -B were carried out in an identical diffusion furnace in our Iaboratory. Lifetime measurement system used in our study is shown in Fig.1. The sample wafer mentioned above was placed apart from the coplanar wave guide antenna by 1 mm to couple with the radiated wave of 500 MHzUHF supplied through the stub tuner and the circulator from the oscillator with a output power of 100 mW. The photoconductivity change was induced by photoexciting the front and back surfaces of the wafer using LDs (LD=65, Laser Diode Lab.) with a wavelength of 904 nm, a pulse width of about 60 ns and an optical power of several watts. The induced photoconductivity change was detected as the reflection change of the UHF wave. The detected signal was amplified by 10 to 100 times, A/D converted and eventually displaced on the computer monitor as the photoconductivity decay curve. The reason why we use the probe frequency of 500 MHz lower than the microwave frequency in the ordinary p-PCD measurement equipment is to prevent the influence of so-called the skin effect. Both the surfaces can be also illuminated by the lamp (Manabeam, Matsushita Densi., K.K.) as a bias light to remove the influence of the trapping center. As seen in the theoretically calculated decay curves shown in Fig. 2, the initial portion of the photoconductivity decay is
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Fig. 2 PCD curves calculated theoretically with surface recombination velocity So as a parameter.
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drastically influenced by the photoexcited surface recombination velocity indicated as So in the figure. Then, the intercept of the asymptote of the tail linear decay which is shown by the dashed line in Fig.2 is varied depending on the surface recombination. First, two photoconductivity decays are measured for two photoexcitation directions, i.e., on the front and back wafer surfaces and, in turn, the two intercepts are determined from the ve
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