Tailoring of the Nitrogen Profile in Thin Gate Oxides Using Substrate Nitridation by Nitric Oxide
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In the present paper, we have successfully tested the nitridation of the silicon substrate using NO before growing the thermal oxide. Previous results [3,8], coupled with already published data on the adsorption properties of NO on Si [9], suggested indeed that nitridation of the native oxide is efficient even at low temperature. This process is expected to avoid defect generation in the silicon substrate. EXPERIMENTAL The process was performed on (100)-oriented, 200 mm in diameter, p-type silicon wafers. The substrates are cleaned using a standard RCA recipe. The chemical oxide which resulted from the RCA cleaning is removed in HF solution and the wafer are rinsed for 10 minutes in ozonated DI water in order to grow a clean and 0.8 nm-thick native oxide. The substrate nitridation is performed in a Rapid Thermal Processing (RTP) reactor [10]. Due to the high reactivity of NO on a silicon surface, the total NO pressure is lowered down to 10 mbars, instead of the standard value of 100 mbars, usually used for nitridation after oxide growth [3] : at this high pressure indeed, the rapid incorporation of nitrogen is difficult to tailor accurately. The oxidation of the nitrided silicon substrate is performed in a vertical furnace using dry oxygen in the 800-850'C temperature range. The dielectric was characterized using SIMS, TEM, XPS and electrical measurements. The dielectric thickness is first measured by ellipsometry (X= 632.8 nm). The determined thickness 159
Mat. Res. Soc. Symp. Proc. Vol. 532 ©1998 Materials Research Society
by fixing the refractive index at 1.459 is found in good agreement with TEM observations[6]. SIMS profiling was used to determine the layers composition with depth. Sputtering was accomplished with 4 KeV Xe' ions at 0.023nm/min. The analyzed species (Si 2160+ and Si 2t 4N÷) reach quickly a steady state equilibrium, which allows the profile resolution in very thin films. This method compares favorably with Cs÷ abrasion [11] since it does not display matrix effect differences between Si and SiO 2. XPS measurements were performed using an Al-KI source. Various analysis angles were used to resolve the nitrogen Nls peak in the depth of the oxide. MOS structures were fabricated using a 200-nm polysilicon deposited at 620"C, phosphorous implanted and annealed at 850'C, 30 min. RESULTS AND DISCUSSION Four groups of samples which differ by the nitridation condition of the substrate were prepared (Table I). Fig. 1 shows the growth kinetics obtained at 850 *C in dry oxygen ambient. The growth rate is significantly reduced, even for the wafers of group B. These results are comparable to those obtained in the nitrogen implant case, where the dose of implanted nitrogen is in the lxlI' 4 to 5xI0' 4 cm-2 range. IGroup A None
Nitridation
40 35
IGroup B 5 sec, 550 'C
Oxidation @ 850*C in Dry
Group C 30 sec, 550 'C
Group D 30 sec, 700 'C
q
3058 25
.70 50
220 -$WC
c••
20
- 30 s
I00C - 30oc. s 5
0
-
2
4
-Control
6
i
10
59-
0C
S
U
10
Oxidation Time (min)
370
37
° oE
E4 `C
150
E
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