Near Interface Oxide Degradation in High Temperature Annealed Si/SiO 2 /Si Structures
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polycrystalline silicon film was deposited on the oxide using low pressure chemical vapour deposition (LPCVD) from SiH4 at 620'C. The samples were then capped with a 500 nm thick oxide film deposited by plasma enhanced CVD using Si(OC2H5)4 and 02 at 300*C. The whole wafers were then annealed in an Ar/l%02 atmosphere at 1320*C for periods up to 6 hours. Although MOSFET gate oxides are usually much thinner than the oxides we have used and the annealing temperatures substantially lower (< 1000QC), the structures we have studied and the temperatures used have a double interest. In the first case, they simulate a structure and processing step analagous to one commonly encountered in silicon on insulator technology involving Separation by IMplantation of OXygen (SIMOX) buried oxide. Secondly, using thicker oxides and higher anneal temperatures, we can simulate effects which are qualitatively similar to those anticipated in thinner oxides with lower anneal temperatures and so obtain a much thicker degraded oxide to make measurement easier. Following annealing, the deposited oxide cap was removed by an HF acid dip and the deposited polycrystalline layer then etched off in XeF2 gas (no plasma assistance involved). Reference samples were subjected to identical treatment except for the annealing step. For infrared absorption studies the wafers were subjected to an oxide removal treatment involving an acid etch using a mixture of 25% HF acid in ethanol, this product leaves the Si surface native oxide free and terminated with hydrogens. Again, reference samples were made using identical treatments except for the anneal phase. Infra-red absorption experiments were made using a Bruker IFS 66 Fourier transform spectrometer; multiple scans were used with a resolution of 4 cm- 1 . Refractive index measurements were carried out at fixed wavelength (X = 632.8 nm) using a Rudolph Research ellipsometer. Electron spin resonance (ESR) measurements were carried out on samples of bared oxides either following 26 Mrad (SiO2) of X-irradiation ( ARACOR 4100 source, W target, 10 keV) or 35 Mrad (NiO2)of 60Co y radiation at a rate of 1.5 Mrad/hour. Some samples were also exposed to the vacuum ultra-violet radiation (Ephoton < 10.4 eV) from a microwave excited, low pressure Kr plasma or injected with holes using a vacuum ultra-violet source together with applied positive corona ions. The ESR measurements were made at room temperature using Bruker X band ESR spectrometers. In the ultra-violet exposure experiments it was not possible to estimate radiation doses. Chemical etching of irradiated oxides to measure the profile of defects was performed using commercial ENERLEC BE 7:1 buffered HF acid etch (HF acid plus a solution of NH4F). The etch rates were determined by a mechanical stylus measurement on a predefined step profile in the oxide. RESULTS AND DISCUSSION In Fig. 1 we show the ESR spectra observed in a) bulk, Suprasil 1 (high OH containing oxide) exposed to 35 Mrad of 60 Co y rays, b) bared, thermal oxide annealed for 6 hours at 1320'C then
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