Effects of ECR N 2 O-Plasma Nitridation on Thermal Oxide Characteristics
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ABSTRACT The effects of electron cyclotron resonance (ECR) N20-plasma nitridation on the characteristics of thermal SiO 2 have been investigated. Although the ECR N20-plasma nitridation was performed at low-temperature(_950"C) and amounts of oxidation time to grow oxide layer thicker than 60A since it has self-limited growth characteristics with low oxidation rates. Thus, nitridation of thermal oxide in N20 has been used to achieve a wide range of final oxide thickness at relatively lower process temperature(900°C-950°C) [3]. Also, the N20-nitrided oxide showed suppressed hole and electron trapping rate [3], which was explained to be due to the presence of nitrogen-rich layer located near the Si/Si0 2 interface. However, it still requires high process temperature. We have investigated electron cyclotron resonance (ECR) N20-plasma nitridation as a low temperature (_99.99%) as a source gas at a microwave power of 600W with 143
Mat. Res. Soc. Symp. Proc. Vol. 391 0 1995 Materials Research Society
150 E3OA(bare Si) 100A 0 270A
0o °-*
Fig. 1. Increase of effective oxide thickness as a function of the N20-plasma oxidation time. The initial thermal oxide thickness were
0
50
0A(bare Si), 100A, and 270A. The
0 00
0
ECR N20-plasma nitridation was performed at 400°C and 2mtorr
t
20
40
60
80
100
Nitridation Time (min) varying nitridation time, substrate temperature, and process pressure. Some of the wafers were annealed at 900*C for 30min in N 2 ambient. Al was evaporated and then patterned. The final effective oxide thickness was determined by C-V measurement.
III. RESULTS AND DISCUSSION Figure 1 shows the growth characteristics of ECR N20-plasma nitrided thermal oxide and ECR N20-plasma grown oxide. As can be seen, we could grow 130A oxide from ECR N2 0plasma nitridation only at 400°C for lhour, which is impossible by normal thermal nitridation. The growth characteristics of ECR plasma grown oxide are strongly related to the ion flux density as well as the ion conduction through the growing oxide by the generated self-bias. The growth rate of ECR C2-plasma grown oxide that has been also observed was about 2 times higher than that of ECR N20-plasma grown oxide. Although ion flux density of N20-plasma may be different from that of C2-plasma, the lower growth rate of ECR N20-plasma grown oxide may be attributed to the presence of nitrogen-rich layer near the Si/SiC 2 interface which impedes oxygen ion conduction. From figure 1, we can also observe that oxynitride layer can be grown for the initially thick (270A) thermal oxide. The lower increase of effective oxide thickness for thicker initial oxide can be also explained by the lower ion conduction. From these results, we can conclude that a wide range of final oxide thicknesses can be easily achieved by using different initial oxide thickness on the substrate. It should be noted that the ECR plasma nitridation process has additional control variables such as process pressure and microwave power when compared with plain thermal nitridation. Thus, we investiga
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