Effect of driving frequency on the operation of a radiofrequency glow discharge emission source

SPECTROCHIMICA ACTA PART B

ELSEVIER

Spectrochimica Acta Part B 50 (1995) 1125-1141

Effect of driving frequency on the operation of a radiofrequency glow discharge emission source M.J. Heintz, G.M.

Hieftje*

Department of Chemistry, Indiana University, Bloomington, IN 47405, USA Received 8 September 1994; accepted 4 January 1995

Abstract

Several characteristics of the r.f. glow discharge were examined to determine their dependence on driving frequency. The cathode potential exhibited a strong dependence on frequency, as would be expected, because of the capacitive nature of the discharge. As the frequency was dropped from 13 to 6 MHz there was a dramatic rise in the r.f. voltage of the discharge (with a conductive sample), attributed to a change in the mode of power coupling. The sputtering rates of a conductive sample were dramatically greater at the lower frequencies, in part due to higher energy of the sputtering ions. The emission characteristics of the source also changed as the frequency was varied from 6 to 13 MHz. At the higher operating frequencies, atomic emission peaked at a particular r.f. power level, whereas at lower frequencies the neutral-atom signal generally increased monotonically with power. The highest signal levels were found at 20 MHz, the highest frequency studied. Detection limits were determined for both conductors and insulators; in both cases they are detector-noise-limited because of the low throughput of the spectrometer. Detection limits for a conducting sample ranged from 0.1 ixg g-1 at 20 MHz to 20 ~g g-I at 3 MHz. The emission from an insulating sample showed the same trends as those from a conducting sample but required higher r.f. power; the greatest signals were found at 6 and 13 MHz because not enough power was available from the r.f. amplifier to reach the optimum power for the 20 MHz discharge. Detection limits for elements in a Macor ® ceramic sample ranged from 30 to 110 Ixg g-L

Keywords: Driving frequency; Equipment; Glow discharge; Radio frequency

1. I n t r o d u c t i o n

The characteristics of the d.c. glow discharge have been extensively studied [1,2] and their dependence on operational parameters such as current, voltage, pressure, and support-gas composition well established. An added factor in the optimization of r.f.-driven sources is that the applied field is time-dependent and changes in driving (or excitation) frequency can affect the discharge characteristics. The effect of changing the driving frequency o f an r.f. glow discharge has been widely studied, primarily for systems used in semiconductor etching and material deposition [3,4] but also analytical sources [5,6]. Such sources are intended for substrate sput* Corresponding author. 0584-8547/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0584-8547(95)01308-3

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tering rather than for producing atomic emission or mass spectra. Consequently, the effect of driving frequency might be different for a source configuration intended for use in instrumental analysis. The first concern in any study of an r.f.-driven system is whether the delivered power can be accurately measured and, if not, whether a comparison of operation at different frequencies is valid. Previous studies [7,8] have shown that the true delivered power is frequency dependent because of the change in stray impedance. In addition, the effect of the sample capacitance on power loss must also be considered when insulating materials are analyzed; the added impedance generated by a non-conductive sample leads to greater power loss through stray paths. Furthermore, the voltage at the cathode block (the easiest to measure) is not the same as that found on the surface of a non-conductive sample, even though the latter is the relevant potential. Overall, measuring optimal emission signals at each driving frequency, regardless of operational parameters such as power and voltage, is the soundest method of comparison. Frequency-induced changes associated with the capacitive nature of an r.f. glow discharge are typically linear and predictable [4]. In general, as the frequency increases, plasma density (related to discharge current) goes up and the electron energy and sputtering-ion energies (related to the discharge voltage) drop. Because raising pressure affects the discharge in much the same manner as increasing the radio frequency, and because cell pressure is usually easier to control, in many regions of the r.f. spectrum it is preferable to control these fundamental parameters (energies, plasma density) by varying pressure. Several other discharge characteristics are frequency dependent [3]. All fundamental processes involved in sustaining the discharge have a time constant associated with them, the reciprocal of which is the characteristic frequency of the process. Several of these events have characteristic frequencies which lie in the radio frequency range for low-pressure discharges. When the driving frequency is varied across or near this characteristic frequency there occurs a drastic change in the corresponding process, possibly causing large shifts in the analytical characteristics of the source. One very important process, electron heating, or the manner in which the r.f. energy is coupled to electrons in the discharge, has been shown to change dramatically for many etching systems at frequencies between 1 and 10 MHz [9,10]. Above this frequency range, electrons are thought to gain energy directly from the oscillating plasma sheath. Below this frequency secondary electrons from cathodic sputtering are more responsible for sustaining the discharge. This latter process is far less efficient than the former; as a consequence, at lower frequencies the discharge density goes down and there is a sharp rise in discharge voltage. The sputter-atomization process is dependent mainly upon the impinging ion energy, on the ion current to the cathode, and on the physical nature of the sputtered material. It has been shown that there is a large rise in ion energy [ 11 ], and therefore in the etching rates of semiconductors, at frequencies below 2 - 6 MHz [12,13]. In part, this higher energy is due to the previously mentioned greater voltage that exists at lower frequencies. However, the elevated ion energy has been attributed also to a traversing of the fundamental frequency (the ion transit frequency or ITF) associated with ion movement across the cathode sheath. At frequencies above the ITF the ions require several cycles of the r.f. driving waveform to traverse the cathode sheath; during some of this time they are not being accelerated toward the cathode (sample). As a result, at higher operating frequencies there is a narrower ion energy distribution and overall lower sputtering ion energies. Several other processes in the glow region have characteristic frequencies in the radio frequency regime. These processes include free diffusion, charge exchange, and electron attachment [3]; however, their characteristic frequencies fall just below those used in the present studies. In contrast, a major change that occurs in the glow at frequencies between 10 and 60 MHz is bulk electron modulation [14,15]. Above this frequency range, bulk electrons in the discharge cannot respond to alternations in polarity of the driving frequency, so the electrons achieve a pseudo-steady-state energy. Because ionization of the support gas argon (and some high-energy excitation processes) relies on a small number of electrons with relatively high energies, the oscillation of the electron energies at low driving frequencies can have a large effect on the excitation and emission characteristics of the discharge. In the present study we measured the electrical characteristics of an r.f. glow discharge

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operated at driving frequencies between 3 and 20 MHz in order to determine whether nonlinear changes in the plasma were occurring. We also measured sputtering rates and emission characteristics of the discharge in order to determine what effect changes in the fundamental processes of the discharge have on these analytical features. The experiments were carried out with both conducting and insulating samples in order to determine how the voltage drop across the sample affected the source performance. Finally, the system was optimized at each frequency and detection limits were determined for both conducting and non-conducting samples.

2. Experimental 2.1. R.f glow discharge source A schematic representation of the r.f. glow discharge source is shown in Fig. 1. This design is identical to the magnetron cell which has been described previously [16], except for modifications to the insulating spacer. In the new design the sample is isolated from the discharge cell by an 8 m m thick Macor ® spacer having a 12 mm diameter orifice. A removable Macor insert ring restricts the effective sample diameter to 8 mm. The replaceable insert is necessary because of arcing which sometimes occurs at the sample/Macor junction. 2.2. Support systems Argon (Air Products, purity 99.9995%) was used as the discharge support gas. Cell pressure was regulated by varying the argon flow rate, which was controlled in turn by means of a needle valve and measured with a Matheson rotameter. The cell was evacuated with a rotaryvane pump (Balzers, model DUO 060A) and the cell pressure measured with a capacitance manometer (MKS Baratron, model 122A). The cooling block was maintained at 10 °C with deionized water from a recirculating chiller (Neslab, model RTE-5B). 2.3. R.f driving system The r.f. amplifier operates over the frequency range between 0.3 and 40 MHz at a maximum forward power of 150 W. In these studies the source was operated at 3.5, 6.78, 13.56 and

Additional Port [~x\\\\\\\\\\\\~ 2 Vacuum ports ~ 1

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Fig. 1. Schematic drawing of the r.f. glow-dischargecell.

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20.34 MHz. Low-pass filters were placed in the line in order to eliminate harmonics, and the reflected power was held below 1 W in all studies. The components of the r.f. power supply are listed in more detail in Table 1. The RG8U cable between the impedance-matching circuit and the source was a maximum of 30 cm in length, unless otherwise stated, in order to minimize power loss to ground because of stray capacitance. The RG8U cable could handle only 3 5 0 0 - 4 0 0 0 V, which limited the attainable forward power with some of the insulator samples that were studied. The resulting stray capacitance to ground parallel to the source was between 80 and 110 pF. In order to measure the electrical characteristics of the source a high-voltage probe (Tektronix, model P6057, 1 pF input capacitance) was connected to the electrical feedthrough of the cathode block via an MHV T connector. The probe output was fed to a 250 MHz oscilloscope (Tektronix, model 485) for visual display of the source voltage waveforms.

2.4. Photodiode-array spectrometer Because of the inherently complex spectra produced by a glow discharge source, emission characteristics and detection limits were determined with a medium-resolution photodiodearray spectrometer. The spectrometer (Plasmarray, Leco Corporation) has been described in previously published studies [17] and need not be covered in great detail here. The spectrometer has an aperture ratio off/4.3 and, for these studies, a fixed slit width of 100 txm. The spectrometer examines discrete sections of the spectrum by using a mask to eliminate unused portions. The observed window around each spectral feature of interest is on the order of 1 2 nm; the theoretical resolution on the linear photodiode-array detector varies from 0.02 n m at 200 n m to 0.04 n m at 400 nm. The reciprocal linear dispersion of the spectrometer and its resolution result in each emission line being spread out over several (roughly 10) pixels. The detector consists of a linear photodiode array with 1024 elements; the integration time was varied between 10 and 500 s. The photodiode-array dark current (which was dependent upon integration time and ranged from 400 to 800 counts) was automatically subtracted from each emission spectrum.

2.5. Standard materials Monel and Inconel standards (Inco Europe Limited) were used in the characterization of the source sputtering and emission characteristics with a conducting cathode. Detection limits of the source were calculated from aluminum standards (Alcoa Spectrochemical Standards). For Table 1 Radio-frequency system R.f. power supply R.f. Oscillator: Hewlett Packard model 3325A function generator/frequencysynthesizer (20 MHz high-frequency limit) R.f. amplifier: Kalmus model 170F broadbandamplifier(0.3-40 MHz) Low-pass filters: 13 and 20 MHz: Barker and Williamsonmodel F110/1500 (30 MHz cut-off frequency) 6.78 and 3.5 MHz: CommunicationConcepts Inc. model FLI-40 and model FL1-80 (cut-off frequencies of 7.4 and 4.1 MHz) Power meter: Heathkit model HM2140A Impedance matching L-type Network: laboratory constructed Inductor: air core, locally constructed from 2 nun o.d. insulated copper wire Capacitors: Jennings, vacuum capacitors (10-1000 pF, 5000 V) Additional parallel capacitance: Sprague, ceramic capacitors (500 pF) Network configuration: 20.34 MHz: L~= 2.6 txH C~= 10-1000 pF Co= 10-1000 pF 13.56 MHz: Ls = 2.6 p.H Cs = 10-1000 pF Cp= 10-1000 pF Cp = 10-2000 pF 6.78 MHz: L~= 7.1 o~H Cs = 10-1000 pF Cp= 10-2500 pF 3.5 MHz: Ls = 27.1 ~H C~= 10-1000 pF

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the characterization of insulating materials Pyroquartz ®, non-standardized quartz and Macor ® samples were analyzed. The insulator detection limits were calculated using the Macor ® samples, with the manufacturer supplying representative values for the ceramic composition.

3. Results and discussion

3.1. Impedance matching and power loss

Impedance matching was accomplished in these studies with an L-type network described in Table 1. Several other matching networks could also be used, but the L-network has the advantages of requiring few components and of being easily modified for a wide range of source impedances. Limiting the number of components is important because it often governs the amount of power dissipated parasitically in the network. As the operating frequency is lowered, the matching system becomes more complex, with an increase in the series inductive reactance and a decrease in the parallel capacitive reactance. Consequently, studies of lowerfrequency discharges than those studied here would require a different impedance-matching circuit. In the present studies we were not able to measure directly the power dissipated in the glow because of an unknown fraction lost through stray paths. Accordingly, the first part of the study was intended to determine which trends might be caused by variations in stray power loss. In Fig. 2(a) we see the effect of stray capacitance on the peak-to-peak (p-p) r.f. potential appearing at the cathode block. In Fig. 2(a) it is evident that with a small increase in stray capacitance a much greater power is required to reach the same cathode-block r.f. voltage. The samples that were analyzed were 1.5 mm (labeled "thin" in Fig. 2(a)) and 3 m m (labeled "thick" in Fig. 2(a)) thick specimens of quartz. There is a very limited dependence of the peakto-peak voltage on sample thickness (see Fig. 2(a)) and virtually none on discharge pressure (shown in Fig. 2(b)) (absolute potential values cannot be compared between Fig. 2(a) and either Fig. 2(b) or 2(c) because the discharge was operated at 13 MHz for Fig. 2(a) and 6.78 MHz for the latter two figures). This limited dependence on sample thickness suggests that in the analysis of insulators most of the current (power) passes to ground through paths other than the glow discharge; if the lowest impedance path to ground were through the glow discharge there would be a larger increase in cathode-block r.f. potential as the sample thickness increased (greater sample impedance) or as the pressure decreased (greater glow impedance). This behavior contrasts with that for a conducting sample, shown in Fig. 2(c), in which the potential changes with pressure and where discharge power seems to be dissipated primarily through the discharge. In Fig. 3 is shown the dependence of silicon neutral atom emission on cathode-block r.f. voltage for insulating samples of different thicknesses and using cables with different levels of stray capacitance. Fig. 3 suggests that useful calibration curves might still be able to be generated even if stray capacitance changes, as long as standards and samples are of equal thickness; the analyte emission is directly related to the applied voltage at the sample surface because a glow discharge cell has a characteristic impedance (at constant pressure and driving frequency). In contrast, the magnitude of the emission signal is not the same for the two sample thicknesses, even at an identical applied r.f. voltage or at the respective emission maxima with respect to voltage. Of course, the voltage at the surface of the sample is not known; the voltage plotted in Fig. 3 is measured at the plate behind the sample. This discrepancy would account for the emission maxima occurring at different "apparent" voltages. Furthermore, the two thicknesses displayed in Fig. 3 are from different samples; the thicker sample is pyromatic quartz (a high temperature quartz) and the thinner sample a quartz window. The unlike matrices could therefore account for the different signal levels; if the matrices were identical and only the sample thickness different, the maximum emission levels for the two samples should be equal.

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Fig. 2. (a) Dependence of the applied r.f. potential (peak-to-peak) on r.f. power for quartz samples of different thicknesses. The "thin" sample is 1.5 mm thick and the "thick" sample is 3 mm thick. C is the stray capacitance of the coaxial cable. Discharge operated at 13 MHz. (b) Dependence of the applied r.f. potential (peak-to-peak) on r.f. power for an insulating sample measured at different discharge pressures. Discharge operated at 6.78 MHz. (c) Dependence of the applied r.f. potential (peak-to-peak) on r.f. power for a conducting sample. Notice that Fig. 2(c) has different horizontal and vertical scales. Discharge operated at 6.78 MHz.

3.2. Dependence of electrical characteristics on operating frequency In Figs. 4(a) and 4(b) is s h o w n the d e p e n d e n c e o f the m e a s u r e d r.f. potential on f r e q u e n c y and r.f. p o w e r for a c o n d u c t i n g and an insulating sample, respectively. The general rise in r.f.

M.J. Heintz, G.M. Hieftje/Spectrochimica Acta Part B 50 (1995) 1125-1141 1000 .~

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Fig. 4. (a) Dependence of the r.f. potential (peak-to-pe,ak) on r.f. power and frequency for a conducting sample (Monel). (b) Dependence of the measured r.f. potential (peak-to-peak) on r.f. power and frequency for an insulating sample (1.5 mm thick quartz). Notice different horizontal and vertical scales on (a) and (b). voltage as the frequency is lowered is probably due to the capacitive nature o f the discharge. The abrupt rise in r.f. potential as the operating frequency was dropped from 13 M H z to 6 M H z is probably a result o f crossing the ITF, as m e n t i o n e d in the Introduction. The m u c h greater voltage o n the cathode block w h e n an insulator is studied (Fig. 4(b)) is again due to the capacitive reactance o f the sample. Because o f the r.f. voltage drop across such a sample, the potential

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at its surface is only a fraction of the cathode voltage shown in Fig. 4(b). Consequently, greater power is needed to start the discharge and optimize it when a non-conductive sample is analyzed. Of course, the voltage drop across an insulator is greater at lower applied frequencies. 3.3. Dependence of sputtering characteristics on operating frequency

The sputtering characteristics of the source are influenced by the capacitive nature of the discharge and by a change that occurs in the sputtering mechanism at lower frequencies. In Fig. 5 the dependence of the sputtering rate on cell pressure and operating frequency is shown. At all frequencies the sputtering rate goes up as the pressure is raised, due to an increase in discharge density, and consequently to a greater frequency of atoms or ions striking the sample surface. The density of sputtering atoms and ions at higher pressures is also elevated by more efficient charge exchange in the sheath. The lower energy of these ions at the higher pressure is more than compensated by the greater number of impinging atoms and ions. There is also an increase in sputtering as the frequency is lowered, which at first seems surprising since higher-frequency discharges probably have higher ion densities because of the capacitive component of the discharge impedance. The greater sputtering rate at lower frequencies might be explained by a change in the energy distribution of the impinging ions. In previous work [12,13] more rapid etching was observed at lower frequencies, attributable to a drastic change in both the average ion energy and the ion energy distribution at operating frequencies on opposite sides of the ion crossing rate (i.e. the ITF discussed in the Introduction). The ITF for a chlorine discharge at approximately 0.5 Torr was found to be between 1 and 10 MHz [13]. At operating frequencies below this value, an ion can cross the sheath in only a couple of cycles; this leads to a broader energy distribution of ions, greater average ion energies (less "drift" in the sheath), and fewer collisions in the sheath (which also causes the ion energy to be greater). Etching and sputtering are, of course, much different processes, but the fundamental change in the ion energies should occur for both rare gas and molecular discharges. The much more rapid sputtering as the frequency is dropped from 13 to 3 MHz might be explained by a greater overlap of the ion energy distribution with the sample-sputtering cross section, so any decline in sputtering rate caused by fewer impinging ions is more than compensated by the greater efficiency of sputtering. The added sputtering at lower operating frequencies may also be due to changes in the directionality of the impinging ions, or by alterations in the structure of the discharge. These structural changes were discussed in detail in a previous paper [18]. In brief, the local ion density near the sheath may be greater at the lower frequency. The sputtering rate was measured also when r.f. voltage and r.f. power were both kept

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M.J. Heintz, G.M. Hiefije/Spectrochimica Acta Part B 50 (1995) 1125-1141

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constant by adjusting the pressure at each applied, r.f. power; the result is shown in Fig. 6. It is apparent that if the discharge has roughly the same power density (constant power and voltage), the sputtering efficiency is much greater at lower frequencies. Furthermore, the dependence of sputtering on frequency at constant pressure and voltage (achieved by adjusting the applied r.f. power), also shown in Fig. 6, indicates that two effects oppose each other. The greater plasma density at higher operating frequencies and the greater efficiency of sputtering at lower frequencies largely offset each other, so sputtering is nearly equivalent at all frequencies. The sputtering rate of an insulating sample was too slow at low pressures and powers to be accurately determined. In Table 2 are listed the measured sputtering rates at the maximum voltage or power (the latter in the case of the 20 MHz discharge because of its high currentto-voltage ratio) of the glow discharge source at four operating frequencies. The sputtering rates for Macor ® appear to be at least two orders of magnitude lower than those for conducting samples such as Inconel (cf. Fig. 5). This lower rate is due partially to parasitic power losses, but depends also on the sputtering characteristics of the sample, particularly the binding energy. In general, the data agree with what was shown in Fig. 6, in that there is not a large change in sputtering rate with driving frequency if voltage and pressure are held constant. The greater sputtering rate at 13 MHz may be due to the true voltage at the insulator surface being higher than that for the two lower frequencies (due to the capacitive influence of the sample). The difference between the 20 and 13 MHz discharges agrees with what is shown in Fig. 5; when the pressure and power are held constant there is more rapid sputtering at the lower frequency. Nonetheless, sputtering rates have to be increased dramatically before a thorough study of insulator sputtering can be carried out.

Table 2 Sputtering rates of an insulating sample (Macor®)a Driving frequency MHz

Peak-to-peak voltage Vb

Forward power W

Sputter rate (p.g/min-l)

20.34 13.56 6.78 3.5

1800 3300 3500 3500

110 110 50 35

0.6 + 0.6 3.5 + 5 1 5-0.2 1 + 0.6

a Pressure, 2 Torr. b Measured at the cathode block, not at the sample surface.

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3.4. Dependence of source emission on operating frequency, power, and pressure 3.4.1. Conducting samples: atomic emission The emission characteristics of the source for a conducting standard (Monel) at three pressures and four operating frequencies are shown in Fig. 7(a)-(d). Nickel emission at 352 nm (205-28 569 cm -1) was chosen as a representative spectral line because it is not a resonance transition (thus avoiding self absorption) and because the energy required for excitation to the upper energy level is not very great. Additional studies, such as the determination of detection limits in the next section, have shown that many strong emission lines exhibit the same trends as will be discussed below. Three general trends are evident in the plots of Fig. 7. First, the higher the discharge frequency (from (a) to (d)), the higher the initial optimum power (defined as the first signal maximum that occurs as power is increased) becomes, and the greater the optimal signal (note different vertical scales on Figs. 7(a)-(d)). This initial emission maximum with respect to r.f. power is probably caused by an optimal overlap between the electron energy distribution and the Ni excitation cross section, although this hypothesis will have to be verified with future electrical-probe measurements. The two higher-frequency discharges (Figs. 7(c) and 7(d)) exhibit clear maxima with respect to applied r.f. power, whereas the discharges sustained at 3 and 6 MHz (Figs. 7(a) and 7(b)) show only local maxima but with a general trend toward greater signal with increased power. From this behavior, the optimal pressure is 2 Torr for the higher-frequency discharges (13 and 20 MHz) and 3 Tort (or possibly higher) for the lower-frequency discharges (3 and 6 MHz). The true difference in emission behavior between the high-frequency (Fig. 7(c) and 7(d)) and low-frequency (Figs. 7(a) and 7(b)) discharges may not be as great as the plots initially suggest. The disparity among the discharges operated at different frequencies might be only a matter of required r.f. power. Focusing on Figs. 7(c) and 7(d), one notes that the higher frequency discharge (Fig. 7(d)) exhibits an emission maximum that peaks at higher r.f. power

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Fig. 7. Dependenceon pressureand powerof nickel neutral-atomemissionat 352 nm for a conducting(Monel) sample: (a) 3.5 MHz; (b) 6.78 MHz; (c) 13.56MHz; (d) 20.34 MHz. Note the different vertical scales.

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1135

than does the 13 MHz discharge (Fig. 7(c)). This higher optimal power for the 20 MHz source generates a greater current density and an understandably stronger emission signal. Similar conclusions can be reached at lower frequencies (cf. curve for 2 Torr in 6 MHz discharge; Fig. 7(b)), but the first maximum of emission occurs at very low applied r.f. power levels in some cases, and does not appear clearly in these plots. Perhaps even higher frequencies such as 27 or 40 MHz might produce even stronger emission levels, but would require greater applied r.f. power. There would be an expected increase in the current to voltage ratio of higher-frequency discharges because of the reduced capacitive reactance of the discharge. In addition, there might be modulation of the plasma electron energy in the lower-frequency discharges [3]; over the range of frequencies (and pressures) used in these studies it has been shown [3] that the electron energy can "follow" oscillations in r.f. potential at the lower frequencies, leading to a bimodal electron energy distribution. The more well defined electron energy distribution function (EEDF) at the higher operating frequency might then lead to a greater maximum emission because of the greater overlap of the EEDF with the excitation function of the atoms. As mentioned in the Introduction, lower-frequency discharges are sustained largely by highenergy secondary electrons that stem from sputtering events [9, 10]. These electrons are much higher in energy than those produced in the glow itself and their energy overlap with the excitation function of most atomic transitions is very poor. The elevated signals at higher r.f. power in the low-frequency discharges is therefore probably due to increased sample sputtering (see Fig. 5) and to greater glow density and not to more efficient atomic excitation. This increased plasma density might also lead to more efficient Penning excitation. However, the abundance of high-energy electrons at low frequencies would work against Penning processes by reducing the efficiency of metastable formation and by depopulating existing metastable argon levels. In many theoretical treatments based on an equivalent-circuit model of the r.f. glow discharge (for example see Ref. [19]), frequency and pressure are considered interchangeable operating parameters. That is, the same change in discharge characteristics would be predicted by dropping either the pressure or operating frequency of the source. However, the foregoing findings suggest otherwise. They argue that a model based upon the fundamental processes of the plasma is necessary if the operating frequency of the source is in a region of the r.f. spectrum that is near the fundamental frequency of any discharge-sustaining event. Optimal conditions for the later study on detection limits were easy to determine for discharges operated at 13 and 20 MHz, because maximum signals for Ni atom emission lines were readily evident (cf. Figs. 7(c) and 7(d)). Other atomic emission lines displayed emission maxima under roughly the same plasma conditions. In contrast, the discharges operated at the two lower frequencies showed no true optima (cf. Figs. 7(a) and 7(b)), so it was decided to operate the 6 MHz (Fig. 7(b)) discharge at the local maximum obtained at roughly 12 W and 2 Torr, and to operate the 3 MHz discharge in the region where the emission increases monotonically with power. The difference in emission characteristics between operating in these two discharge "modes" can then be explored. The 3 MHz discharge was operated at 3 Torr and 25 W, in part because higher-power discharges arced more frequently.

3.4.2. Conducting sample: ionic emission

The dependence of Cu ion emission at 224 nm on pressure, power and operating frequency for a conducting sample is shown in Figs. 8(a) (6 MHz) and 8(b) (20 MHz). The ion emission behaves quite differently from atom emission shown in Figs. 7(b) and 7(d). Specifically, the ion emission is stronger from the lower-frequency discharge, the optimum pressure is 3 Torr or higher at both operating frequencies, and the higher-frequency discharge does not exhibit a peak in emission at a particular power level. The contrasting behavior between Cu ion (Fig. 8) and Ni atom (Fig. 7) emission may be caused not only by the need to ionize the Cu species but also by the large difference in the energy of the respective excitation process. The upper energy level for the excitation of Cu ion from its ground state is 66419 cm -~ compared to 28569 cm -~ for the Ni atom line. As was discussed earlier, a lower frequency discharge probably produces higher electron energies

1136

M.J. Heintz, G.M. Hiefije/Spectrochimica Acta Part B 50 (1995) 1125-1141 a.

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at the same r.f. input power; as a result, higher-energy transitions are perhaps excited more efficiently. It is not now clear why the ion line (Figs. 8(a) and 8(b)) optimizes at a higher pressure than the atom line (Figs. 7(a)-(d)). It may be that ion lines rely more heavily on excitation processes other than electron collision. For example, Penning excitation (and ionization) depend upon plasma density, which in turn typically increases with pressure. From the Cu ion emission data in Fig. 8 it appears that the optimal plasma operating frequency may be different for an ion source (intended for mass spectrometry) than for an emission source (for emission spectrometry). 3.4.3. Non-conducting sample The dependence of atomic emission on r.f. power, pressure and operating frequency for silicon in a quartz matrix is shown in Figs. 9(a)-(d). The Si neutral atom emission at 288 nm was studied because the 251 nm line suffered spectral interference. The upper-state energy of this transition is 40 992 cm -], significantly higher than that of the Ni atom line studied earlier (see Figs. 7(a)-(d)). The sample was a 1.5 mm thick quartz plate; thicker samples were also examined and produced similar basic trends, although greater power levels were required. Several trends are evident from the data in Figs. 9(a)-(d). The emission reaches a maximum at a much higher r.f. power than did emission from the conducting sample (Figs. 7(a)-(d)), probably because of power losses introduced by the impedance of the insulating sample. Instead of maximizing at 20 MHz, as did the emission from the conducting sample (Fig. 7(d)), atomic emission was greatest for the quartz sample at 13 MHz (Fig. 9(c)). The 20 MHz discharge does not reach a peak with respect to applied power because the source is unstable at power levels above 100 W. The changes in atomic emission behavior with operating frequency are less

M.J. Heintz, G.M. Hieftje/Spectrochimica Acta Part B 50 (1995) 1125-1141 a.

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pronounced with the insulating sample than with the conducting sample (presented in Figs. 7(a)-(d)). The discharge conditions which produced the strongest emission from the 6 MHz discharge (2 Torr and 50-60 W; Fig. 9(b)) were chosen as "optimum" for the later determination of detection limits. The discharge conditions for the 13 MHz and 20 MHz plasmas were chosen to be 100 W and between 1 and 2 Torr, because higher power levels were not possible with the present experimental setup. Detection limits for the discharge operated at 3 MHz were not determined because instability of the discharge (persistent arcing) did not allow prolonged operation at r.f. power levels above 10-20 W. 3.5. Comparison of detection limits obtained at different operating frequencies The detection limits of the source at the accessible operating frequencies were determined utilizing the high-resolution photodiode-array spectrometer and the method described in Ref. [20]. Detection limits for several elements in an aluminum matrix are compiled in Table 3. In all cases, the source noise was lower than the photodiode array readout noise. The detection limits are therefore directly related to the signal levels measured in the previous section and improve with signal integration time. Not surprisingly, the 20 MHz discharge yielded the lowest detection limits. As mentioned previously, the 6 MHz discharge was operated at a low-power local emission maximum, whereas the 3 MHz discharge was operated at a higher power (see Figs. 7(a) and 7(b)). Despite this difference in applied power level, the 6 MHz discharge yielded the better detection limits. The 6 MHz discharge was operated at the lower power because the background noise is less and the background structure simpler when the plasma is operated at this lower-power local maximum. The difference between operating at the initial emission maximum and operating above this maximum is illustrated in Fig. 10, where spectra are shown of 3 MHz and a 20 MHz

1138

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4. C o n c l u s i o n s

The sample-sputtering and analyte emission characteristics of an r.f. glow discharge source are strongly dependent upon the r.f. driving frequency. In addition, because several plasma processes have characteristic frequencies in the r.f. regime the operating frequency and cell pressure are not interchangeable parameters in the operation of the r.f. glow discharge source. At frequencies of 6 MHz and below, plasma support processes begin to change, causing the voltage at the cathode to rise dramatically. At the lower frequencies the power is not as effectively coupled to electrons, causing the discharge impedance (and consequently the cathode potential) to increase [9,10]. In addition, the ion transit frequency (the reciprocal of the time it takes an ion to traverse the cathode sheath) is approached, and the average ion energy per volt applied to the cathode increases [3]. These two factors contribute to greater cathodic sputtering when one operates at 3 MHz, even though the discharge density is probably lower. The dependence of emission on pressure at the four frequencies for a conducting sample

1140

M.J. Heintz, G.M. Hieflje/Spectrochimica Acta Part B 50 (1995) 1125-1141

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follows two trends. At the higher frequencies a clear emission m a x i m u m with applied power is seen at most pressures, whereas at the lower frequencies there is a general increase in signal with both pressure and r.f. power. At lower frequencies, it is hypothesized that the electron energy modulation is greater [14, 15] and this, in conjunction with the greater cathode potentials produced by the lower frequency discharges, may lead to electron energies which are too high to excite many low-energy atomic transitions efficiently. The greatest emission signals and lowest detection limits for a conducting sample were achieved at a driving frequency of 20 MHz. The optimal results for an insulating sample occurred at 6 or 13 MHz (depending on the sample), probably because the power supply could not deliver enough power to reach the projected optimal voltage at the insulator surface for a 20 MHz discharge. It appears that the optimal operational frequency may be higher than the system used in these studies can supply. Ion lines were strongest from the lowest frequency discharges, possibly because of the greater electron energies at these frequencies. In future studies it would be desirable to measure directly the electron energies and discharge densities for the emission source operated at these and additional ISM frequencies (27 and 40 MHz). In addition, the optimal operational frequency is probably much different for a mass spectral source, and the frequency characteristics of the glow discharge as an ion source should also be investigated.

Acknowledgements This work was supported in part by the National Science Foundation through grant CHE 90-20631 and by the National Institutes of Health through grant RO1 GM46853.

References [1] [2] [3] [4] [5]

B.N. Chapman, Glow Discharge Processes, Wiley, New York, (1980). E.Nassar, Fundamentalsof Ionization and Plasma Electronics, Wiley, New York (1971). D.M. Manos and D.L. Flamm (Eds.), Plasma Etching, Academic Press, San Diego, (1989), pp. 104-112. T. Kitamura, N. Nakano, T. Makabe and Y. Yamaguchi, Plasma Sources Sci. Technol., 2, (1993) 40. M.J. Heintz and G.M. Hieftje, Abstract583, 1992 PittsburghConferenceand Exhibition,New Orleans, Louisiana, March 1992. [6] C. Lazik and R.K. Marcus, Spectrochim. Acta Part B, 49, (1994) 649. [7] R. Deltchev, M. Albert, G. Suchaneck and K. Schade, Vacuum, 42, (1991) 33. [8] W.G.M. van den Hoek, C.A.M. deVries and M.G.J. Heijman, J. Vac. Sci. Technol. B, 5, (1987) 647. [9] T.J. Sommerer, W.N.G. Hitchon and J.E. Lawler, Phys. Rev. Lett., 63, (1989) 2361. [10] Y. Ohtsu, Y. Okuno and H. Fujita, J. Appl. Phys., 73, (1993) 2155. [11] D. Field, D.F. Klemperer, P.W. May and Y.P. Song, J. Appl. Phys., 70, (1991) 82. [12] M.F. Toups and D.W. Ernie, J. Appl. Phys., 68, (1990) 6125. [13] V.M. Donnelly,D.L. Flamm and R.H. Bruce, J. Appl. Phys., 58, (1985) 2135. [14] D.L. Flamm and V.M. Donnelly, J. Appl. Phys., 59, (1986) 1052. [15] D.L. Flamm, J. Vac. Sci. Technol.A, 4, (1986) 729. [16] M.J. Heintz and G.M. Hieftje, Spectrochim. Acta Part B, 50 (1995) 1109. [17] G.M. Levy, A. Quaglia, R.E. Lazure, and S.W. McGeorge, Spectrochim. Acta Part B, 42, (1987) 106. [18] M.J. Heintz, P.J. Galley, and G.M. Hieftje, Spectrochim. Acta Part B, 49 (1994) 745. [19] A.J. van Roosmalen, J.A.G. Baggerman, and S.J.H. Brader, Dry Etching for VLSI, Plenum Press, New York, 1991, pp. 34, 109. [20] K. R. Brushwyler and G. M. Hieftje, Appl. Spectrosc., 45, (1991) 682.