@misc{oai:ir.soken.ac.jp:00000556, author = {中川, 聰子 and ナカガワ, サトコ and NAKAGAWA, Satoko}, month = {2016-02-17}, note = {Silicon-on-insulator (SOI) wafers have begun to be used as substrates of large-scale integrated circuits (LSIs) for high speed and low power applications. The SOI structure consists of a top silicon layer, a buried oxide (BOX) layer, and a silicon substrate. The thickness of the top silicon layer of the SOI wafers required for metal-oxide-semiconductor field-effect transistor (MOSFET) devices is on the order of 30-200 nm. Defect control for the thin top silicon layer is very important, where the light element impurities play a key role. Oxygen and carbon are main light element impurities included unintentionally in silicon crystals. Oxygen impurities in silicon crystals promote mechanical strength but lead to oxygen microprecipitates after annealing. Carbon impurities act as nucleation sites for oxygen precipitation in the gettering process of LSI production and the carbon impurities precipitate in dislocation loop formation. A quantitative analysis is required for determining these light element impurities in the top silicon layer of SOI wafers. However, the characterization of these impurities in the top silicon layer is lagging because of the difficulty in measuring their concentrations in ultrathin layers.
　　In this study we report on detection of the light element impurities after electron irradiation by photoluminescence (PL) spectroscopy. Irradiation with electrons, protons, or other high-energy particles induces defects that involve light element impurities and emit strong luminescence.
There are two well-known radiative defects: the complex of interstitial carbon and interstitial oxygen which induces the C-line at 0.79 eV and that of interstitial carbon and substitutional carbon which induces the G-line at 0.97 eV. The ultraviolet (UV) excitation enables us to detect these lines from the ultrathin top silicon layer of a SOI wafer because of the shallow penetration depth of UV light and the confinement effect of photo-excited carriers in the top silicon layer.
　　Various commercial SOI wafers (thicknesses of top silicon layers: 60 - 200 nm) were irradiated with 1 MeV electrons by a fluence ranging from 3x10¹⁶ to 1x10¹⁷ electrons/cm² for luminescence activation of light element impurities. We detected the C-line and the G-line from all the samples under the UV light excitation. That demonstrates the presence of oxygen and carbon in the top silicon layers. To the best of my knowledge this is the first report on detection of carbon and oxygen impurities in the ultrathin top silicon layer.
　　As yet we have not observed differences in crystalline quality depending on the SOI wafer fabrication methods. We compared PL spectra under the UV light excitation from UNIBOND, SIMOX, and ELTRAN wafers fabricated in 2002 to find out differences in concentration of the light element impurities. We observed substantial differences in the intensity of the C-line and the G-line among these wafers. The variation in the intensity of the C-line and the G-line reflects the carbon and oxygen concentrations. We also found variation in concentrations of these light element impurities in a wafer.
　　In order to verify that the PL signal under the UV light excitation does not come from the silicon substrate, we implanted xenon ion solely in the top silicon layer by adjusting the accelerating voltage to 50 keV. Although we detected the luminescent activated centers, such as the C-line, under the UV light excitation with a penetration depth of 10 nm, we did not detected the centers under the visible light excitation with a penetration depth of 3μm. This confirmed that PL under UV light excitation detects the C-line not from the substrate but from the top silicon layer.
　　We confirmed the repeatability of this method. Several SOI wafers were separately irradiated with electrons and separately measured by PL spectroscopy under the same conditions. The spectral shape in terms of the relative intensity between the C-line, G-line, and bound exciton line was almost the same for the separate measurements.
We estimated detection limit of our PL measurement for carbon impurities in bulk silicon wafers. We measured the intensity ratios of the C-line and the G-line to the free exciton (FE) line for bulk silicon wafers whose carbon impurity concentration had been determined to be from 1x10¹⁵ to 3x10⁶ atoms/cm³by FT-IR. The quantitative relationships between the intensity ratios (IPL) and the carbon concentration ([C]) were expressed as IPL ∝ [C]ⁿ. We estimated the detection limit to be on the order of 10¹²atoms/cm³ by taking the signal-to-noise ratio into consideration. The detection limit of the carbon impurity in the top silicon layers is supposed to be on the order of 10¹⁴ atoms/cm³ because the thickness of the top silicon layers is two orders of magnitude smaller than the penetration depth of the excitation light. This number is large enough for measuring carbon impurity with practical concentration. Therefore our PL measurement has sufficient accuracy to determine the carbon impurities in the top silicon layers quantitatively.
　　In conclusion, we revealed that differences in the impurity concentration depending on the wafer fabrication methods and variations of the impurity concentration depending on the location of the wafers. Luminescence activation using electron irradiation with PL measurement under the UV light excitation is a very promising tool for the evaluating the light element impurities in the top silicon layer of SOI wafers., 総研大甲第1134号}, title = {電子線照射発光活性化法による極薄SOI層中軽元素不純物の評価}, year = {} }