Effect of Nitrogen Implants on Boron Transient Enhanced Diffusion
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EFFECT OF NITROGEN IMPLANTS ON BORON TRANSIENT ENHANCED DIFFUSION Omer Dokumaci, Paul Ronsheim, Suri Hegde, Dureseti Chidambarrao, Lahir Shaik-Adam*, and Mark E. Law* IBM SRDC, Hopewell Junction, NY 12533 * Electrical Engineering Dept. , University of Florida, Gainesville, FL 32611 ABSTRACT The effect of nitrogen implants on boron transient enhanced diffusion was studied for nitrogen-only, boron-only, and boron plus nitrogen implants. A boron buried layer was used as a detector for interstitial supersaturation in the samples. Boron dose ranged from 1x1014 to 1x1015 cm-2 and N2+ dose from 5x1013 and 5x1014 cm-2. The energies were chosen such that the location of the nitrogen and boron peaks matched. After the implants, RTA and low temperature furnace anneals were carried out. The diffusivity enhancements were extracted from the buried layer profiles by simulation. Nitrogen-only implants were found to cause significant enhanced diffusion on the buried boron layer. For lower doses, the enhancement of the nitrogen implant is about half as that of boron whereas the enhancements are equal at higher doses. Nitrogen coimplant with boron increases the transient enhanced diffusion of boron at low boron doses, which implies that nitrogen does not act as a strong sink for excess interstitials unlike carbon. At high boron doses, nitrogen co-implant does not significantly change boron diffusion. Sheet resistance measurements indicate that low nitrogen doses do not affect the activation of boron whereas high nitrogen doses either reduce the activation of boron or the mobility of the holes. INTRODUCTION Implanted nitrogen has been observed to retard the oxidation rates [1]. This feature of nitrogen is utilized to obtain multiple oxide thickness on the same wafer. Nitrogen implantation into the channel of an NFET has been found to significantly enhance the channel dopant diffusion due to the annealing of the nitrogen implant damage [2]. This diffusivity enhancement causes a significant shift in the device characteristics and should be understood better. Nitrogen co-implant has been found to decrease transient enhanced diffusion of boron when the nitrogen dose is between 1x1015-1x1016 cm-2 and nitrogen implant is significantly deeper than the boron implant [3]. However, it is not clear whether suppressed diffusion of boron is due to the amorphization caused by nitrogen or nitrogen-interstitial interactions during annealing. Nitrogen co-implant into the p+ gates has been observed to reduce boron penetration through gate oxides [4,5]. This has also been attributed to the suppression of boron diffusion due to the presence of nitrogen. Nitrogen also exhibits “backwards diffusion” in single crystalline silicon [6]. The peak of the nitrogen profile shifts towards the surface during annealing and nitrogen piles up at the silicon-oxide interface. Nitrogen atoms which are diffusing to the surface can effectively remove the interstitials from the implant damage region, therefore changing the annealing characteristics of the implant damage. Thi
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