Mark A. Sobolewski

The Gaseous Electronics Conference radio‐frequency reference cell: A defined parallel‐plate radio‐frequency system for experimental and theoretical studies of plasma‐processing discharges

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.

Ion energy distributions and sheath voltages in a radio-frequency-biased, inductively coupled, high-density plasma reactor

Ion energy distributions were measured at a grounded surface in an inductively coupled, high-density plasma reactor for pure argon, argon–helium, and argon–xenon discharges at 1.33 Pa (10 mTorr), as a function of radio-frequency (rf) bias amplitude, rf bias frequency, radial position, inductive source power, and ion mass. The ground sheath voltage which accelerates the ions was also determined using capacitive probe measurements and Langmuir probe data. Together, the measurements provide a complete characterization of ion dynamics in the sheath, allowing ion transit time effects to be distinguished from sheath impedance effects. Models are presented which describe both effects and explain why they are observed in the same range of rf bias frequency.

Electrical characterization of radio‐frequency discharges in the Gaseous Electronics Conference Reference Cell

Measurements of the electrical characteristics of radio‐frequency (rf) discharges can be subject to large errors due to limitations in the measurement instruments and the stray impedance of the discharge cell. This study reports electrical measurements of argon discharges in the GEC Reference Cell in which special care has been taken to identify and minimize these sources of error. Careful calibration of current and voltage probes was found to be essential. In addition, parasitic impedances in the cell were found to be large, sensitive to minor changes in electrical connections, and not adequately described by simple a priori models. A general technique for characterizing the stray impedance, including an analysis of the propagation of errors, is presented here. This technique assures accurate results with specified uncertainties. Error analysis demonstrated that large gains in the precision of the measurements can be obtained using an inductive shunt circuit. Together, these techniques should improve the utility of electrical measurements for gauging the reproducibility of plasma conditions among rf discharge cells, for testing theoretical results, and for monitoring plasma processing.

Measurements and modeling of ion energy distributions in high-density, radio-frequency biased CF4 discharges

Models of ion dynamics in radio-frequency (rf) biased, high-density plasma sheaths are needed to predict ion bombardment energies in plasma simulations. To test these models, we have measured ion energy distributions (IEDs) in pure CF4 discharges at 1.33 Pa (10 mTorr) in a high-density, inductively coupled plasma reactor, using a mass spectrometer equipped with an ion energy analyzer. IEDs of CF3+, CF2+, CF+, and F+ ions were measured as a function of bias frequency, bias amplitude, and inductive source power. Simultaneous measurements by a capacitive probe and a Faraday cup provide enough information to determine the input parameters of sheath models and allow direct comparison of calculated and measured IEDs. A rigorous and comprehensive test of one numerical sheath model was performed. The model, which includes a complete treatment of time-dependent ion dynamics in the sheath, was found to predict the behavior of measured IEDs to good accuracy over the entire range of bias frequency, including complica...

Measuring the ion current in high-density plasmas using radio-frequency current and voltage measurements

The total current or flux of ions striking the substrate is an important parameter that must be tightly controlled during plasma processing. Several methods have recently been proposed for monitoring the ion current in situ. These methods rely on passive, noninvasive measurements of the radio frequency (rf) current and voltage signals that are generated by plasma-processing equipment. The rf measurements are then interpreted by electrical models of the plasma discharge. Here, a rigorous and comprehensive test of such methods was performed for high-density discharges in argon at 1.33 Pa (10 mTorr) in an inductively coupled plasma reactor, at inductive source powers of 60–350 W, rf bias powers up to 150 W, and rf bias frequencies of 0.1–10 MHz. Model-based methods were tested by comparison to direct, independent measurements of the ion current at the substrate electrode made using lower frequency (10 kHz) rf bias and modulated rf bias. Errors in two model-based methods are identified and explained by effect...

Measuring the ion current in electrical discharges using radio-frequency current and voltage measurements

This letter describes a technique for measuring the ion current at a semiconductor wafer that is undergoing plasma processing. The technique relies on external measurements of the radio-frequency (rf) current and voltage at the wafer electrode. The rf signals are generated by the rf bias power which is normally applied to wafers during processing. There is no need for any probe inserted into the plasma or for any additional power supplies which might perturb the plasma. To test the technique, comparisons were made with dc measurements of ion current at a bare aluminum electrode, for argon discharges at 1.33 Pa, ion current densities of 1.3–13 mA/cm2, rf bias frequencies of 0.1–10 MHz, and rf bias voltages from 1 to 200 V. Additional tests showed that ion current measurements could be obtained by the rf technique even when electrically insulating wafers were placed on the electrode and when an insulating layer was deposited on the electrode.

The effects of radio-frequency bias on electron density in an inductively coupled plasma reactor

The effect of radio-frequency bias on electron density in an inductively coupled plasma reactor was measured using a wave cutoff probe, over a wide range of conditions in pure Ar, pure CF4, and 50%–50% mixtures of Ar∕CF4, at pressures of 0.7–4.0Pa (5–30mTorr), bias frequencies of 10–30MHz, bias voltages up to 750V, and inductive source powers of 50–300W. Also, at selected experimental conditions, comparisons with Langmuir probe measurements were made. Two types of bias-induced changes in electron density were detected. First, at high source powers, we observed a bias-induced decrease in electron density, which had a slow time response (several minutes), a linear dependence on bias voltage, and little or no dependence on bias frequency or pressure. This decrease is a gas composition effect caused by etch or sputter products liberated from the wafer surface. Second, at low source powers, we observed a faster, bias-induced increase in electron density, which was proportional to the bias frequency and the squ...

Real-time, noninvasive monitoring of ion energy and ion current at a wafer surface during plasma etching

A noninvasive, nonperturbing technique for real-time monitoring of ion energy distributions and total ion current at a wafer surface during plasma processing has been used to monitor rapid changes in CF4∕Ar etching plasmas in an inductively coupled, rf-biased plasma reactor. To mimic the effects of process recipe steps or reactor malfunctions, perturbations were made in the inductive source power, gas flow, and pressure, and the resulting effects on total ion current, sheath voltage, and ion energy were monitored. During etching of a thermal silicon dioxide film, smaller changes, which are caused by the etch process itself, were also observed. Sheath voltages determined by the noninvasive technique were in good agreement with simultaneous measurements made using a capacitive probe. In addition to providing a demonstration of the speed and accuracy of the technique, the results also provide useful information about the relative importance of different types of equipment malfunctions and suggest methods for...

Monitoring sheath voltages and ion energies in high-density plasmas using noninvasive radio-frequency current and voltage measurements

To obtain optimal results from plasma processing, the energy of ions incident on substrate wafers must be carefully controlled. Such control has been difficult to achieve, however, because no practical method exists for monitoring the energy distributions of ions at a wafer surface during processing. To solve this problem, we have developed a noninvasive, model-based method for determining ion energy distributions that is suitable for use during actual processing in commercial plasma reactors. The method relies solely on measurements of the rf current and voltage applied to the reactor. The method was validated by tests performed in argon and CF4 discharges at 1.3 Pa (10 mTorr) in an inductively coupled, high-density plasma reactor, with rf substrate bias at frequencies of 100 kHz to 20 MHz. Plasma potential and sheath voltage wave forms obtained from the noninvasive rf technique agreed well with independent measurements made using a capacitive probe. Ion energy distributions from the rf technique were al...

Absolute Spatially- and Temporally-Resolved Optical Emission Measurements of rf Glow Discharges in Argon

Spatially- and temporally-resolved measurements of optical emission intensities are presented from rf discharges in argon over a wide range of pressures (6.7 to 133 Pa) and applied rf voltages (75 to 200 V). Results of measurements of emission intensities are presented for both an atomic transition (Ar I, 750.4 nm) and an ionic transition (Ar II, 434.8 nm). The absolute scale of these optical emissions has been determined by comparison with the optical emission from a calibrated standard lamp. All measurements were made in a well-defined rf reactor. They provide detailed characterization of local time-resolved plasma conditions suitable for the comparison with results from other experiments and theoretical models. These measurements represent a new level of detail in diagnostic measurements of rf plasmas, and provide insight into the electron transport properties of rf discharges.