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.
Particulate generation has been studied during reactive-ion etching of oxide wafers in C, F, -CHF, and CF. , +HF, plasmas using both a commercial etch tool and the GEC reference cell modified to resemble the commercial tool. Under certain discharge process conditions, copious amounts of submicrometresized particles are shown to form due to plasma interactions with the oxide substrate. In the commercial tool. particles were detected only by a downstream particle flux monitor, whereas in the reference cell, particles were observed by both in situ laser light scattering and downstream monitoring. in the commercial tool, wafers etched to end-point were shown by post-process surface analysis to be contaminated by submicrometre-sized columnar structures. Previous reports of similar such columnar structures formed during reactive-ion etching of oxide films have attributed the phenomenon to polymer micromasking. However, the results of this study clearly contradict this conclusion and suggest that the presence of columnar oxide etch residues Is ilnked to process-induced particulate contamination. Laser iight scattering measurements were made in the reference cell during reactive-Ion etching of blanket oxide wafers and used to help clarify the complex processes of parb'culate nucleation, growth and deposition during oxide etching. Polarization coagulation of spherical particles formed in the reference cell Is shown to occur, presumably in the high-field regions of the sheath, forming filamentous rod-like particle aggregates. The implications of this observation for wafer contamination are explored.
The rate of particle generation in a SiH4/NH3 rf discharge has been studied as a function of the discharge operating parameter space, electrode geometry, and power supply coupling mode. Measurements of the bulk quantity of particles produced in the discharge reveal that the mode of coupling (capacitive or dc) as well as the electrode temperature significantly affects particle generation rates. Laser light scattering measurements made as a function of the plasma power density indicate that particle generation abruptly ceases at a threshold value sufficient to induce spark breakdown at the cathode. Based on these observations, it is shown that particle growth in plasmas can be modeled entirely as a heterogeneous process. The initiation of particle growth is shown to be consistent with an electron surface desorption model involving vibrational excitation of surface clusters. Propagation of growth in the gas phase is shown to be consistent with an eliminative ion-molecular condensation reaction, and the pressure dependence of this mechanism is exploited to estimate a value for the rate constants for SiH4 and NH3 condensation in SiN:H particle growth.
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