Surface morphology of oxygen-implanted wafers
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. INTRODUCTION
SIMOX,1 one of the silicon-on-insulator (SOI) technologies, has been applied to high-speed CMOS, highvoltage CMOS, and radiation-hardened CMOS LSIs.2"5 It is emerging as one of the most promising technologies for fabricating high performance VLSIs using ultrathinfilm MOSFETs/SOI.6 The development of a 100-mAclass oxygen implanter7 has promoted the research and development of SIMOX LSI technology. A rough surface causes degradation of electrical characteristics of MOSFETs. In SIMOX technology, an enormous number of oxygen ions penetrate the top silicon layer of a silicon wafer. This penetration results in degradation of the crystallinity of the top silicon layer.8 The surface morphology is also assumed to be strongly affected by implantation conditions. So far, however, few reports have addressed the issue of the surface morphology. This study describes the surface morphology of SIMOX wafers prepared under different implantation conditions. Particular attention has been focused on asimplanted wafers to clarify the mechanism of surface morphology degradation. Methods to improve the morphology are also described. Finally, a model explaining surface morphology degradation is proposed. II. EXPERIMENTAL
A 100-mA-class high-current oxygen implanter, NV-200,7 was used to produce SIMOX wafers. It has the capability of implanting twenty-five 100-mm wafers in a batch. Oxygen ions (16O+) were implanted into (100) silicon wafers with a resistivity of 1000 ohm-cm at beam currents of 40-80 mA with doses of 0.4-2.2 x 1018 cm 2. The acceleration energy was kept constant at 180 keV. The wafer temperature during the implantation ranged from 400 to 700 °C, with all the wafers being preheated from the front by halogen lamps to 1918
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 9, Sep 1990
Downloaded: 24 Mar 2015
500 °C. The wafer temperature was concurrently measured using an optical pyrometer and a thermocouple. The readings were calibrated with a temperature measurement technique using Pt diffusion into Au.9 The beam was scanned using a magnet to improve the dose uniformity across a wafer,10 and the implant dose was measured using a Faraday cup.10 In most cases, oxygen ions were implanted into wafers without screen oxide. Wafers with screen oxide films ranging from 14 to 1000 nm were also examined. Some of the SIMOX wafers were annealed in a temperature range of 8001150 °C in an N2 ambient for 2 h, at 1260 °C in an Ar ambient for 2 h, and at 1300 °C in an Ar ambient containing 0.5% oxygen for 4 h to investigate the effects of the annealing treatment. A 500-nm-thick epitaxial silicon layer was grown at 1050 °C on one of the wafers annealed at 1150 °C, to clarify the effects such a layer might have on surface morphology. The surface morphology of SIMOX wafers was observed using a TEM replica technique, a particularly useful technique for observing microscopic surface roughness.11 A two-step replica method12 was employed for sample preparation. Platinum-shadowed carbon replicas of the surfaces of the wafer
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