High Dose Rate Oxygen Implantation for Formation of Silicon-on-Insulator Structures
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HIGH DOSE RATE OXYGEN IMPLANTATION FOR FORMATION OF SILICON-ON-INSULATOR STRUCTURES E. CORTESIr, F. NAMAVAR', R.F. PINIZZOTrO", AND H. YANG" *Spire Corporation, Patriots Park, Bedford, MA, 01730 University of North Texas, Denton, TX, 76203 ABSTRACT We have studied Separation by IMplantation of OXygen (SIMOX) processes using very high dose rates (40-60 psA/cm'). For a dose of 4 x 10" O'/cm' at 160 keV, the structure formed by implantation at 50 gsA/cm' is very similar to that associated with lower dose rates. The same dose implanted at a dose rate of 60 gsA/cm', however, results in the formation of pits in the silicon surface as well as a somewhat different oxide structure. Implantation through a surface oxide layer appears to result in a structure similar to that associated with lower dose rate implantation. These and higher dose samples suggest that the threshold for pit formation is related to both dose rate and dose. INTRODUCTION The formation of silicon-on-insulator (SOI) material using Separation by IMplantation of OXygen (SIMOX) processes has been extensively studied because of the radiation hardness advantages as well as potential speed advantages of devices fabricated in these substrates. SIMOX is a preferred SOI material because of the high quality of the silicon top layer that can be achieved as compared to other SOI technologies. However, it is still necessary to reduce the density of threading dislocations in the silicon top layer to make the material suitable for a broader range of device applications and it is desirable to reduce implant time to reduce wafer cost. Standard, single implant, SIMOX wafers are generally produced by implantation of 1.5-1.8 x 10 s O÷/cm' at 150-200 keV followed by high temperature annealing. The resultant SIMOX structure has a silicon top layer about 1500-2500 A thick and a buried SiO, layer about 3500-4000 A thick. For a number of years, the usual implantation temperature for SIMOX was about 500-550°C [1-4], but this standard process results in a relatively high density of threading dislocations in the silicon top layer. More recent research by a number of groups has shown that implantation at higher temperatures results in better quality silicon top layers [5-9]. The higher temperatures were generally achieved with substrate heating in addition to beam heating (dose rates up to about 37 gA/cm'). There do appear, however, to be upper process limits. Holland et al. [8], for example, noted that implantation with a dose of 5 x 10" 0/cm' at 180 keV at a substrate temperature, in addition to beam heating, of 650°C, resulted in a pitted surface. The present work sought to study very high dose rate oxygen implantation (no substrate heating) because of the possible advantages of high temperature implantation (reduced density of defects) and of reduced implant time (reduced cost, potentially). Pit formation appears to be a limiting factor for very high dose rate implantation and thus we have focused on the dependence of pit formation on dose rate and on dose. EXPERIMENTAL PROCEDURE
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