Diffusion of Implanted Nitrogen in Silicon at High Doses
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DIFFUSION OF IMPLANTED NITROGEN IN SILICON AT HIGH DOSES Lahir Shaik Adam, Lance Robertson, Mark E. Law, Kevin Jones, Kevin Gable, SWAMP Center, Univ. of Florida, Gainesville, FL 32611 USA Suri Hegde, Omer Dokumaci, SRDC, IBM Corp., East Fishkill, NY 12533 ABSTRACT Nitrogen implantation is used to retard gate oxide growth thereby making it particularly useful for dual- VT and System On A chip technologies. This paper discusses the diffusion behavior and the concomitant defect evolution at high doses of implanted nitrogen in silicon. This paper shows that as the nitrogen implant dose is increased, the extent of nitrogen diffusion reduces. This paper also reports based on TEM studies, that upon annealing at 750OC , 5 * 1014 N2+/cm2, 40 keV implant produces Type I extended defects. However, 2 * 1015 N2+/cm2, 40 keV implant, produces a continuous amorphous layer to a depth of about 800 to 900 Å from the surface. In addition, upon annealing at 750OC, the 2 * 1015 N2+/cm2, 40 keV implant produces Type V or solid solubility defects in addition to End of Range or Type II defects. INTRODUCTION Nitrogen implantation is used to control gate oxide thickness [1]. Ref 1 shows that upon implantation and subsequent oxidation, nitrogen retards gate oxide growth. Therefore, by varying the dose of the nitrogen implant across the wafer, it is possible to vary the gate oxide thickness across the wafer. This ability to have variable gate oxide thickness across the wafer is of particular importance in dual –VT and System On a Chip technologies. Previous studies done by us reported the diffusion behavior of implanted nitrogen in silicon at low doses of 5 * 1013 N2+/cm2, 40 keV [2]. By “low” doses, we mean doses low enough so as not to create extended defects. That study showed that nitrogen diffused towards the surface rapidly with time at 750OC. In this paper, we discuss the diffusion behavior of implanted nitrogen in silicon at doses high enough to create extended defects. Through TEM studies, we also discuss the concomitant extended defect evolution for high doses of nitrogen implants. DIFFUSION AND DEFECT STUDIES ON 5 * 1014 N2+/cm2, 40 keV IMPLANTS Nitrogen at a dose of 5 * 1014 N2+/cm2 was implanted at 40 keV into Czocharlski silicon through a 50 Å screen oxide. After implantation, the samples were furnace annealed at 750OC for various times. Figure 1 shows the diffusion profiles. As can be observed from Figure 1, the nitrogen profile shrinks with time. To that extent, there is qualitative similarity between the diffusion behavior of nitrogen implanted at the lower dose of 5 * 1013 N2+/cm2 at 40 keV and the present case. However, we can observe a couple of distinct differences in the diffusion behavior between the lower dose implant of 5 * 1013 N2+/cm2, 40 keV and the present case.
J3.10.1
Concentration (N
+/cm 3
)
1022 As Implanted 750, 12 min 750, 30 min 750, 120 min 750, 240 min
1021 1020 1019 1018 1017 1016 0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Depth (microns) Figure 1: Diffusion of nitrogen implanted into silicon
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