V Lisovskiy - Modes and the alpha-gamma transition in rf capacitive discharges inn2o at different rf frequencies - страница 1

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PHYSICS OF PLASMAS 13, 103505 (2006)

Modes and the alpha-gamma transition in rf capacitive discharges in N2O at different rf frequencies

V. Lisovskiy[1]

Laboratoire de Physique et Technologie des Plasmas, Ecole Polytechnique, Palaiseau 91128, France and Kharkov National University, Kharkov 61077, Ukraine

J.-P. Booth

Laboratoire de Physique et Technologie des Plasmas, Ecole Polytechnique, Palaiseau 91128, France

K. Landry, D. Douai, and V. Cassagne

Unaxis Displays Division France SAS, 5, Rue Leon Blum, Palaiseau 91120, France

V. Yegorenkov

Kharkov National University, Kharkov 61077, Ukraine

(Received 7 August 2006; accepted 25 September 2006; published online 31 October 2006)

This paper reports current-voltage characteristics and pressure-voltage transition curves from the weak-current a-mode to the strong-current y-mode for rf capacitive discharges in N2Oat frequencies of 2 MHz, 13.56 MHz, and 27.12 MHz. At 2 MHz the rf discharge is mostly resistive whereas at 13.56 MHz and 27.12 MHz it is mostly capacitive. The weak-current a-mode was found to exist only above a certain minimum gas pressure for all frequencies studied. N. Yatsenko [Sov. Phys. Tech. Phys. 26, 678 (1981)] previously proposed that the a- y transition corresponds to breakdown of the sheaths. However, we show that this is the case only for sufficiently high gas pressures. At lower pressure there is a smooth transition from the weak-current a-mode to a strong-current y-mode, in which the sheaths produce fast electrons but the sheath has not undergone breakdown. © 2006 American Institute of Physics. [DOI: 10.1063/1.2364135]

I. INTRODUCTION

rf capacitive discharges in N2O are commonly used for cleaning silicon wafers, for oxidizing silicon surfaces,1 as well as for producing oxynitride films on silicon. Oxyni-trides have good electrical properties (high breakdown volt­age, reliability, improved current-voltage, and optical characteristics),2 permitting them to be used as active layers in semiconductors, memory devices, and chemical sensors. These amorphous films (a -SiNxOy and a-SiNxOy :H) are produced in rf discharges in a N2O/SiH4 (silane) mixture,3-7 the oxynitride film being deposited on the substrate surface rather than formed by oxidation and nitridation of the mate­rial. rf discharges in N2O/SiH4 mixtures are also used for

812

depositing SiO2 films. Typically the N2O concentration is 2—40 times higher than that of silane. Consequently, it is of considerable interest to study the properties of rf capacitive discharges in pure N2O. However, published results are scarce,13,14 and present only 3 V W characteristics together with discussions of the molecular dissociation. The process of N2O dissociation was also studied experimentally and theoretically.15

This paper considers the current-voltage characteristics ( Ohmic current) and delivered power in a rf capacitive dis­charge in N2O at rf frequencies of 2 MHz, 13.56 MHz, and 27.12 MHz over a broad range of rf voltage and gas pres­sure. At 2 MHz and low gas pressure (<0.2 Torr) the dis­charge was found to exist only in the strong-current y-mode, whereas the weak-current a-mode can exist at higher N2O pressure. Furthermore, the discharge was relatively resistive, whereas at 13.56 MHz and 27.12 MHz the discharge is more capacitive. In the weak-current mode the dependence of the Ohmic current on frequency f at fixed voltage can be ap­proximated as f5/3. The pressure-voltage curves of the a-y transition were measured at each rf frequency. The weak-current a-mode was found not to exist at the low-pressures side of the point where the a - y transition curve crosses the extinction curve for all three frequencies. We investigated the a-y transition and show that at high gas pressure the a-y transition is accompanied by breakdown of the near-electrode sheath whereas at low gas pressure the breakdown criterion for the sheath below the a-y transition is not met. However, in this mode the near-electrode sheaths are a source of fast electrons.

II. EXPERIMENTAL SETUP

rf discharges were ignited in N2O over the pressure range p ~ 0.004—5.2 Torr with rf field frequencies f=2 MHz, f =13.56 MHz, and f =27.12 MHz. The distance between the flat circular aluminum electrodes (143 mm in diameter) was equal to L=20.4 mm. The rf voltage (ampli­tude Urf< 1500 V) was fed to one of the electrodes, while the other was grounded. The electrodes were located inside a fused silica tube with an inner diameter of 145 mm (see Fig. 1). The gas was supplied through small orifices in the pow­ered electrode and then pumped out via the gap between the second electrode and the wall of the fused silica tube.

The gas pressure was monitored with 10 and 1000 Torr capacitive manometers (MKS Instruments). The gas flow

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Phys.Plasmas 13, 103505(2006)

Pumping

FIG. 1. Schematic of the symmetric rf discharge.

was fixed with a mass flow controller to 5 sccm, and the pressure regulated by throttling the outlet to the pump. A pressure controller (adaptive pressure controller) maintained constant gas pressure.

The rf voltage was measured with a rf current-voltage probe (Advanced Energy Z'SCAN). This rf probe was lo­cated at the minimum possible distance from the rf electrode. This probe gave measurements of the rf voltage, rf current, phase shift angle p between current and voltage and deliv­ered power. The rf voltage was delivered by a 500 W rf generator rf5S (rf Power Products Inc.) via a matching box

PFM (Huttinger Elektronik GmbH) of the L-type.

FIG. 2. The phase shift angle p between the rf current and voltage (a), the Ohmic rf current lrS cos p (b) and delivered power Pdlv (c) against the rf voltage applied Urf for the frequency value of f =2 MHz.

FIG. 3. The phase shift angle p between the rf current and voltage (a),the Ohmic rf current Irf cos p (b) and delivered power Pdlv (c) against the rf voltage applied Urf for the frequency value of f =13.56 MHz.

III. EXPERIMENTAL RESULTS

First consider the current-voltage characteristics (CVC) and the delivered rf power at different frequencies. Figure 2 shows the phase angle, p, between the rf current and voltage, the Ohmic rf current, /rf cos p, and the delivered power, Pdlv, as a function of the applied rf voltage, Urf,at f =2 MHz. At this frequency, at low voltages (close to discharge extinction) the phase angle was p~-85° to -90°, but for higher rf voltages the absolute value of p decreases quickly, and the discharge becomes more resistive. We also observe a fast growth of the Ohmic rf current and the delivered power, indicating that at low gas pressure (p< 0.2 Torr) the dis­charge is always in the strong-current y-mode. At pressures above 0.22 Torr the CVC, on a logarithmic scale, shows a characteristic dogleg feature, indicating the existence of the weak-current a-mode at low voltage. Over the entire range of gas pressure studied the discharge was in the abnormal glow regime, with the discharge luminosity completely cov­ering the surface of the electrodes. Perhaps the normal re­gime would appear at higher gas pressure.

Figure 3 shows the phase angle, the Ohmic current, and the delivered power as a function of applied rf voltage at f= 13.56 MHz. As the rf voltage is increased the absolute value of p first decreases, passes a minimum, then increases. After the transition from a-toy-mode the absolute value of p decreases again. In contrast to the case of f=2 MHz (Fig. 2),at f = 13.56 MHz the phase angle only varies over a nar­row range p ~ -83 ° to -90°, indicating the capacitive na­ture of the discharge. At pressures below 0.6 Torr the dis­charge was always in the abnormal regime. However, at higher gas pressure the electrode area occupied by the dis­

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FIG. 4. The phase shift angle p between the rf current and voltage (a), the Ohmic rf current Irf cos p (b) and delivered power Pdlv (c) against the rf voltage applied Urf for the frequency value of f=27.12 MHz.

charge decreased as the current was decreased towards ex­tinction, i.e., the normal regime was observed. This decrease in the discharge current and delivered power occurs at ap­proximately constant rf voltage.

The behavior of the phase angle can be explained as follows. In the absence of a discharge the rf current is limited by the capacitive reactance of the gap between the parallel-plate electrodes, giving a phase angle p=-7r/2. When the discharge is present and the plasma density (consequently, the conductivity) is high, the rf current is limited by the capacitive reactance of near-electrode sheaths, depending on their thickness dsh. At low plasma density the resistance of the plasma is significant, but as the plasma density increases, the sheath thickness dsh and its capacitive reactance change only a small amount, whereas the Ohmic resistance of the quasineutral plasma decreases considerably. Therefore, on increasing the rf voltage the phase angle again approaches a value -7j72. At moderate values of the rf voltage, when the plasma density and conductivity are small, its resistance is comparable to the capacitive impedance of the sheaths. Fur­thermore, the contribution of the Ohmic current to the total discharge current is at maximum, and the absolute value of the phase angle approaches a minimum value.

Figure 4 shows the phase angle, the Ohmic rf current and the delivered power as a function of the applied rf volt­age at f= 27.12 MHz. Even though the frequency has doubled, the discharge is more resistive than at 13.56 MHz. As a rule, the absolute value of the phase angle was several degrees smaller than at f = 13.56 MHz for a comparable volt­age. At low pressure (p< 0.5 Torr) the discharge was always

Phys. Plasmas 13, 103505 (2006)

0       200      400 600

FIG. 5. The Ohmic rf current Irf cos p against the rf voltage applied Urf for the frequency values of 2 MHz, 13.56 MHz, and 27.12 MHz, p=0.75 Torr.

in the abnormal regime, whereas at higher pressure the nor­mal regime was also observed.

At pressures above 1.5 Torr, as the current is decreased the discharge area decreases, but the voltage increases. Such negative differential resistance is characteristic of the subnor­mal regime of a dc discharge,16 but in this case the discharge

17

area increases as current is decreased. Probably, at high frequency and pressures above 1.5 Torr the normal glow ex­ists only within a narrow range of discharge current. On further decreasing the current, the decrease of the discharge area is accompanied by a reduction of the discharge current density, i.e., the plasma density. At the same time the inten­sity of the discharge glow was reduced a little. It would be of interest to perform detailed probe and optical studies of the normal regime close to rf discharge extinction.

Figure 5 shows the Ohmic current for a gas pressure of 0.75 Torr and frequencies of 2 MHz, 13.56 MHz, and 27.12 MHz. At this pressure the a-y transition occurs smoothly for all three frequency values, without jumps. This transition is manifested by a dogleg feature in the CVC ( logarithmic scale) ; the discharge glow near the sheath boundaries acquires a violet tint in the y-mode (indicating the appearance of high energy electrons accelerated through a high voltage drop across the sheath). Furthermore, the al­most uniform glow of the quasineutral plasma in the a-mode becomes stratified after the transition to the y-mode: an ana­log of a negative glow appears near the boundaries of near-electrode sheaths and a darker region appears in the center, similar to two Faraday dark spaces overlapping at the center of the discharge gap. Such a simultaneous variation of the discharge structure and the glow near the sheath boundaries, together with the dogleg feature in the CVC, were a reliable indication of the a- y mode transition.

Figure 5 also shows that the discharge current in the weak-current mode increases with the frequency. Figure 6 presents the Ohmic rf current against the frequency for p = 0.75 Torr and Urf=300 V. These results are satisfactorily described by the function f5/3, predicted by Raizer et al.18

Figure 7 shows the discharge CVCs at p =1.5 Torr. The a-y transition at f=2 MHz occurs discontinuously, with

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1

1 10 /MHz

FIG. 6. The Ohmic rf current Irf cos p against the frequency for the rf voltage applied Urf=300 V, p=0.75 Torr.

hysteresis (the reverse y-a transition occurs at lower rf volt­age than the a - y transition). The near-electrode sheath is thinner in the y-mode (in the a-mode dsh~ 7.5 mm, whereas in the y-mode dsh~ 6.5 mm). At the frequencies of f=13.56 MHz and f=27.12 MHz the a-y transition is con­tinuous, without jumps in the discharge parameters. It also follows from Fig. 7 that, on decreasing the rf voltage towards extinction (at the smallest rf voltage values of discharge sustainment) the discharge at f=13.56 MHz and f= 27.12 MHz was in the normal regime (the discharge cur­rent decreases at constant rf voltage, with a simultaneous decrease in the discharge area on the electrode). In contrast the discharge at f=2 MHz was in the abnormal regime, and occupied the total area of the electrodes.

Figure 8 shows the pressure-voltage extinction curves for a rf discharge in N2Oatf=2 MHz, f=13.56 MHz, and f= 27.12 MHz. As is known,19 rf breakdown curves can pos­sess diffusion-drift, Paschen, and multipactor branches. In a

20

recent paper it was shown that rf discharge extinction curves have shapes that are similar to those of the breakdown curves. Each branch of the breakdown curve has its analog in the extinction curve. At a fixed rf frequency and for narrow gaps (<10 mm) the diffusion-drift branch is weakly ex­pressed or not observed (for pressures below 10 Torr), and

21

the Paschen and multipactor branches dominate. Corre­spondingly, the extinction curve under these conditions also possesses analogs of the Paschen and multipactor branches and the branch corresponding to the diffusion-drift branch of the breakdown curve may be absent. We observe this behav­ior in Fig. 8 at the frequency 2 MHz, where the extinction curve shows only the analog of the Paschen branch and, for the pressures below 0.15 Torr, a transition to the multipactor branch; the equivalent of the diffusion-drift branch was not observed. Increasing the frequency changes the shape of the extinction curves (and of the breakdown curves) in a similar way to increasing the interelectrode distance. In the extinc­tion curves at f =13.56 MHz and f= 27.12 MHz we observe only the equivalent of the diffusion-drift branches of the breakdown curves; the equivalents of the Paschen and mul­

a

FIG. 7. The Ohmic rf current Irf cos p against the rf voltage applied Urf for

the frequency values of 2 MHz (a), 13.56 MHz and 27.12 MHz (b),

p= 1.5 Torr.

tipactor branches occur for much higher rf voltages, and are not observed in Fig. 8. The left-hand branches of the extinc­tion curves (at low gas pressure) show regions of multival­ued dependence of the discharge extinction voltage on gas

21

pressure similar to the breakdown curves. Reference 20 describes why such regions appear. At higher rf frequency the multivalued region of the extinction curves becomes more clearly expressed.

Figure 8 also shows the measured a-y transition curves Uayexp, for all three frequencies. For f =2 MHz the a-y transition curve exhibits a well-expressed minimum at p~ 1 Torr. For pressures below 0.2 Torr the rf discharge can exist only in the strong-current y-mode; the weak-current

mode cannot exist below this gas pressure, in agreement with

24,25

previous work.

The region over which the a-mode can exist is also bounded at the high-pressure side (p ~ 10 Torr: for a fixed distance between the electrodes L there is a pressure pcr, which is such that at p > pcr the rf discharge can burn only

18,22,23

in the strong-current y-mode). At higher frequency

(f = 13.56 MHz and f =27.12 MHz) the a-y transition curves are seen to approach the extinction curves at high pressures. That is, at p ~ 10 Torr (we did not establish the

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FIG. 8. The extinction curve Uext,the a- y transition Uayexp experimental curve, the rf voltage of the a- y transition U aybr, determined from criterion (7), and the quantity M (4) at f =2 MHz (a), f =13.56 MHz (b) and

f=27.12 MHz (c).

exact p value as it was located outside the pressure range studied) the a-y transition curve coincides with the extinc­tion curve. Above this pressure the rf discharge again only exists in the strong-current y-mode, because the rf voltage is sufficiently high to break down the narrow near-electrode sheaths.

On lowering the gas pressure the a-y transition voltage, Uayexp, first decreases slowly then (at p ~ 0.15 Torr for f=13.56 MHz and p ~ 0.3 Torr for f=27.12 MHz) decreases more quickly, ultimately reaching the extinction curve. This point defines the lower pressure limit of the weak-current a-mode (in agreement with Refs. 24 and 25).

Figure 8 also shows that increasing the frequency causes a decrease in the a-y transition voltage. For example, at p = 3 Torr for f =2 MHz we have Uay exp ~ 507 V, for f =13.56 MHz and we obtain Uayexp ~ 305 V, and for f=27.12 MHz the result is Uayexp~254 V. This result is in

agreement with previous experimental theoretical18,28,29 work.

26,27

and

IV. MECHANISMS OF THE a-y TRANSITION

30

Levitskii was the first to observe that rf capacitive dis­charges at intermediate pressures can occur in two distinct stable regimes which he named the a- and y-modes after Townsend's ionization coefficients. Godyak and Khanneh26 proposed that the a-y transition occurs when fast electrons,

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