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• Results and Discussion
• Experimental results
• Comparison of experimental cross sections with other measurements
• Comparison with theoretical and empirical models

Chapter THREE

Results and Discussion

3.1 Experimental results

As mentioned in Chapter 2, eight to ten sets of pressures (P) and attenuated current intensities (I) were measured for each energy for the predetermined current (I_o). The variation of ln(I/I_o) was studied by scaling it against the pressure (P). The variation of ln(I/I_o) with the pressure of target gasses CH₄, NH₃, SiH₄ and PH₃ for selected energies from 200 to 3500 eV are shown, respectively, in Figures 3.01 through 3.04. The data points of ln(I/I_o) versus pressure lie on a straight line whose slope is a measure of the scattering cross section. The slopes were calculated by the least square fit method. Using the number density 3.5 X 10¹⁹ m^-3 per mTorr and the interaction length 24.5 cm, these slopes were converted to their respective total cross sections. In this manner total cross sections were calculated for each individual trial. The average cross section was obtained from four to eight independent trials for each electron energy. These averaged cross sections are listed in Table 3.1 as functions of energy, along with their absolute errors. Since these are the first experimental measurements of the cross sections of PH₃, the energy range was extended down to 90 eV. SiH₄ cross sections were also measured at these lower energies to compare with those of PH₃. As described in Chapter 2, the errors in these measurements are: (1) gas-electron interaction length determination (2% or less), (2) pressure measurement (2% or less), (3) contribution from the zero degree elastic scattering (1% or less), (4) possible current fluctuation during the experiment (2% or less), and (5) statistical error in the determination of the slope (1% or less). These random errors combine quadratically to give an overall assignment of 4% or less for electron energies 90 – 4000 eV.

Figure 3.01 Variation of ln(I/I_o) with pressure for CH₄ at selected energies from 400 to 3500 eV.

Figure 3.02 Variation of ln(I/I_o) with pressure for NH₃ at selected energies from 400 to 3000 eV.

Figure 3.03 Variation of ln(I/I_o) with pressure for SiH₄ at selected energies from 200 to 3500 eV.

Figure 3.04 Variation of ln(I/I_o) with pressure for PH₃ at selected energies from 200 to 3500 eV.

Table 3.1 Total cross section for electron scattering for CH₄, NH₃, SiH₄ and PH₃ in units of 10^-20 m².