The Influence of Ionic Activity on the Electrical Properties of PECVD (TEOS) Silicon Dioxide
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second run ( samples B 1 to B 17) we biased the screen from -500 to 300V and increased the 02 volumetric flow rate. After deposition, the films were characterized by ellipsometry (for refractive index and thickness), by Raman scattering (to determine homogeneity and carbon content), and by electrical means, such as 1KHz to 10 MHz C-V and I-V. For the electrical measurements, we fabricated MOSCAP structures with Imm diam. circular aluminum gates.
Figure 1.Experimental arrangement utilized for the acceleration or suppression of ionic species in a PECVD system. The screen current measured by the ammeter ( figure 1) never exceeded 0.5 4iA in either experimental run. The refractive index seldom showed a value different from 1.44. With a floating grid (no potential applied) the deposition rate was higher, and the refractive index marginally higher. Under this condition we observed no plasma between grid and sample. For the first run the deposition rate slightly diminishes with the magnitude of the applied voltage. For the second run one can observe that the deposition rate tends to a unique value when the applied voltage goes to - 500 V (see figure 2). This suggests that physical processes such as sputtering may be important under these conditions.
Deposition Rate (Al min)
6000
4000
--
7 a 10
-A-Z•11 a 14 --A--15 a 171
-500
.400 Grid Bias (V)
.300
-200
Figure 2. Deposition rate as a function of bias applied to grid. For samples B3-B 14.
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For samples B3-B6, an increase in voltage resulted in a slight increase in the deposition rate. For this group of samples, the ion flux increment translated into an increase in surface reactions. Here most probable the ions are participating or otherwise enhancing the reaction rate with little or no effect on species removal. For samples B7-B 10, where the bombardment is by ions originating from oxygen molecules, the deposition rate remains constant. For samples B 11B 17 increasingly negative voltages diminishes the deposition rate. The strong reduction of the reaction rate for films formed with a high TEOS concentration (samples B 15-B 17) is due to enhanced sputtering. Raman intensities and line profiles suggest poor film formation. In general one can infer that for high negative voltages, physical processes (sputtering) dominate. For positive voltages or less negative ones, chemical processes are favored. This was an expected result, but placing the biasing screen in the deposition chamber allowed us to favor one process or the other. The lack of variation in the deposition rate for voltages between -300 to -500V (samples from B7-B 10) suggest an energy efficient process, that is an enhanced condition of available energy per TEOS molecule is reached. Therefore for low TEOS concentration we expect the best oxide films a matter already corroborated by previous studies [ 1] The film appearance under the Raman scattering microscope (80 Rtm spot size) is a heterogeneous surface. The heterogeneity's correspond to a segregated phase rich in carbon on a silicate background.
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