ECR Plasma Oxidation: Dependence on Energy of Argon Ion
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171 Mat. Res. Soc. Symp. Proc. Vol. 585 © 2000 Materials Research Society
EXPERIMENTAL PROCEDURE The substrate used was an n-type (100) silicon crystal with a resistivity of 8-12 Qcm. The thickness of the wafer was 0.55 mm. After a standard chemical cleaning of the chips, the chips were subsequently set on the sample holder of the ECR plasma exposure system [4]. The process gas used was a mixture of argon and oxygen. The plasma parameters were evaluated by the Langmuir single probe method. The thickness of the oxide films was measured by using ellipsometry with a single wavelength of 632.8 nm. The electrical characterization was performed for metaloxide-semiconductor capacitors. The gate electrodes with area of 0.2 mm 2 were formed by evaporation of aluminum in a vacuum. The dielectric breakdown characteristics were measured by applying a ramp positive voltage on the gate. RESULTS AND DISCUSSION Energy and Flux of Incident Ions First, the plasma parameters have been evaluated. Figure 1 shows the difference between the plasma potential VP and the floating potential Vf as a function of the flow rate of argon for the microwave power of 300 and 500 W. In the experiment, only the argon gas was introduced in the chamber. Since the Langmuir probe was inserted to the resonance zone during the measurement of V,, the difference is almost equal to the incident energy of ions at the surface of the substrate. It is found that the ion energy decreases from 50 eV to 20 eV with increasing total flow rate. On the other hand, the energy is almost independent of the microwave power. In order to estimate the projected range of ions, a simulation was performed by using TRIM:SRIM-97 code [5]. The result of the simulation suggested that the range was less than 0.7 nm for argon and oxygen ions with incident energy below 20 eV. 100 '20
02=0 sccm 80 -
5,60
L2
PM =300 W
Ar/0 2=10/0SCCM
PtP=500 W •:410
' E•
> 200 0
10
'0( 20
Ar flow rate (sccm)
30
Fig. 1. Dependence of argon ion energy on flow rate of argon gas. The solid circles and open triangles show the ion energy for the microwave power of 300 and 500 W, respectively.
400
600
800
Microwave power (W)
Fig. 2. Dependence of current density of argon ions on microwave power. The argon flow rate is 10 sccm.
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Figure 2 shows ion current density as a function of the microwave power for the argon flow rate of 10 sccm. The current density proportionally increases with increasing microwave power, which is due to the increase in the ionization rate of argon atoms. Thus, it is found that the microwave power changes the ion flux rather than the incident energy of ions. Growth Kinetics of Oxide Film In order to reveal the growth kinetics of silicon dioxide films in the ion-assisted oxidation, the dependence of the film thickness on the oxidation time has been investigated. The oxidation was performed under microwave power of 300 W. flow rates of argon and oxygen of 35 and 0.1 sccm, respectively, gas pressure of 0.2 Pa, and substrate temperatures between 130 and 400cC. The resul
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