In order to extend the operating range of the GEC RF Reference Cell, we developed an inductively coupled plasma source that replaced the standard parallel-plate upper-electrode assembly. Voltage and current probes, Langmuir probes, and an 80 GHz interferometer provided information on plasmas formed in argon, chlorine, and nitrogen at pressures from 0.1 Pa to 3 Pa. For powers deposited in the plasma from 20 W to 300 W, the source produced peak electron densities between 1010/cm3 and 1012/cm3 and electron temperatures near 4 eV. The electron density peaked on axis with typical full-width at half maximum of 7 cm to 9 cm. Discharges in chlorine and nitrogen had bimodal operation that was clearly evident from optical emission intensity. A dim mode occurred at low power and a bright mode at high power. The transition between modes had hysteresis. After many hours of high-power operation, films formed on electrodes and walls of one Cell. These deposits affected the dim-to-bright mode transition, and also apparently caused generation of hot electrons and increased the plasma potential.
Spatially resolved, line integrated, excited state densities, and neutral and ion temperatures have been measured in inductively coupled argon plasmas. Absorption spectroscopy was used to measure the line integrated density and temperature of the argon 1s5, 1s4, 1s3, and 1s2 energy levels. Laser-induced fluorescence was used to confirm the neutral temperatures and to measure argon metastable ion temperatures. For rf powers between 50 and 300 W and pressures of 4–50 mTorr, the line integrated density of the 1s5 energy level varied between 1×1016 and 2×1016 m−2. The densities of the 1s4, 1s3, and 1s2 levels were approximately 4–10 times smaller. In the center of the plasma, the ion and neutral temperatures were identical, between 550 and 1000 K for plasma powers between 30 and 240 W and pressures between 4 and 50 mTorr. The neutral temperature had a maximum in the center of the discharge and decreased towards the edge of the discharge. However, the ion temperature increased to between 3000 and 4000 K at the edge of the discharge. Ion drift velocity in the radial direction was between 1×105 and 2×105 cm/s at the edge of the plasma. No significant changes in the spatial density distribution or temperature were observed when either a rf bias was applied to the lower electrode or when the stainless-steel lower electrode was covered with a bare silicon wafer. The addition of nitrogen to the argon discharge resulted in the density of the 1s5 state decreasing by a factor of 2 and the density of the 1s4 state decreasing by a factor of 10. Implications of these measurements on the radial electric fields, radiation trapping, and the energy transport in the plasma are discussed.
A "reference cell" for generating radio-frequency (rf) glow discharges in gases at a frequency of 13.56 MHz is described. The reference cell provides an experimental platform for comparing plasma measurements carried out in a common reactor geometry by different experimental groups, thereby enhancing the transfer of knowledge and insight gained in rf discharge studies. The results of performing ostensibly identical measurements on six of these cells in five different laboratories are analyzed and discussed. Measurements were made of plasma voltage and current characteristics for discharges in pure argon at specified values of applied voltages, gas pressures, and gas flow rates. Data are presented on relevant electrical quantities derived from Fourier analysis of the voltage and current wave forms. Amplitudes, phase shifts, self-bias voltages, and power dissipation were measured. Each of the cells was characterized in terms of its measured internal reactive components. Comparing results from different cells provides an indication of the degree of precision needed to define the electrical configuration and operating parameters in order to achieve identical performance at various laboratories. The results show, for example, that the external circuit, including the reactive components of the rf power source, can significantly influence the discharge. Results obtained in reference cells with identical rf power sources demonstrate that considerable progress has been made in developing a phenomenological understanding of the conditions needed to obtain reproducible discharge conditions in independent reference cells.
Argon plasma characteristics in a dual-frequency, capacitively coupled, 300 mm-wafer plasma processing system were investigated for rf drive frequencies between 10 and 190 MHz. We report spatial and frequency dependent changes in plasma parameters such as line-integrated electron density, ion saturation current, optical emission and argon metastable density. For the conditions investigated, the line-integrated electron density was a nonlinear function of drive frequency at constant rf power. In addition, the spatial distribution of the positive ions changed from uniform to peaked in the centre as the frequency was increased. Spatially resolved optical emission increased with frequency and the relative optical emission at several spectral lines depended on frequency. Argon metastable density and spatial distribution were not a strong function of drive frequency. Metastable temperature was approximately 400 K.
The characteristics of pulse-modulated inductively coupled plasmas in argon and chlorine have been experimentally investigated. Measurements were performed for peak rf powers between 150 and 400 W at 13.56 MHz, duty cycles between 10 and 70%, and pulse repetition frequencies between 3 and 20 kHz. Over this parameter space, measurements were performed of the time dependent forward and reflected rf powers into the matching network, coil voltage, rf variation of the plasma potential, electron density, and Cl− density. These measurements indicated that for the first 5–30 rf cycles of each pulse, the discharges probably were operating in a capacitively coupled discharge mode with rf variations in the plasma potential of several hundreds of volts and relatively low electron density. Measurements of the electron density in pulse-modulated chlorine discharges indicated that the plateau electron density was a function of the duty cycle; the plateau electron density was lower for higher duty cycles. This may indicate that the ratio of Cl to Cl2was changing with duty cycle. In addition, a microwave radiometer was used to provide an indication of the time-dependent electron temperature. Large spikes in the microwave radiation temperature were noted at the turn-on of the rf power pulses and, in some cases, at the transition from a capacitively coupled to an inductively coupled plasma.
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