Lambda Resonator Balanced Inductive Plasma Source Jjap 1996 G Vinogradov

Jpn. J. Appl. Phys. Vol. 35 (1996) pp. L 1130-L 1133 Part 2, No. 9A, 1 September 1996

Balanced Inductive Plasma Sources Georgy K. V INOGRADOV* and Shimao YONEYAMA MC Electronics Co. Ltd., 907-8 Shimoimasuwa, Shirane-cho, Nakakomagun, Yamanashi 400-02, Japan (Received June

3, 1996;

accepted for publication July

29, 1996)

An approach is proposed for the design of powerful inductive sources with internally compensated anti-phase RF capacitive currents. It is shown that the total internal balance of both capacitive currents and RF magnetic field exist in a full wave helical resonator source. The new plasma source exhibits peculiar plasma phenomena having a clear physical interpretation. KEYWORDS: inductive plasma source, full wave helical resonator, high-density plasma, radical source

The term "inductive plasma source" refers t o any plasma source which includes an inductive element (inductor) which supplies R F electromagnetic energy into discharges: e.g., cylindrical coil, flat spiral, and helicon antennas. It is usually believed that inductive discharges provide plasmas with low R F and DC potentials. These characteristics are important in numerous industrial applications. However, all inductive plasma sources inevitably have both inductive and capacitive coupling t o plasmas. Moreover, the latter can even determine the kind of generated plasma. An R F voltage distributed along the inductor generates capacitive currents into plasma producing high R F plasma potential in the bulk and a secondary plasma near R F grounded surfaces, i.e., wafers. The simplest way t o suppress capacitive coupling t o plasma is t o electrically shield the plasma from the inductor, as was proposed in the last century.') However, this approach has obvious disadvantages: e.g., low energy efficiency, difficult discharge ignition, and it is cumbersome. We have developed a simple and efficient solution of the capacitive problem of inductive plasma sources. We compensate inductor-plasma capacitive currents inside an inductive source, and hence suppress any exterior capacitive plasma currents. The conventional inductive sources also produce an R F magnetic field which spreads outside the source. This field is potentially harmful for any semiconductor processing since it generates eddy currents in surface structures. We propose here a solution of this problem as well. For simplicity, consider amplitude distributions of R F voltage and current along a cylindrical inductor (Fig. 1). Any inductor can be analyzed this way. For the short inductive coil a) with the electrical length of winding wire much shorter than 1/4 wavelength, we see a monotonically increasing RF voltage and almost uniform current distribution. Certainly, the inductor-plasma capacitive current grows t o the "hot" end of the coil. This current causes fluctuation of plasma potential with the discharge excitation frequency and must be grounded. All other inductive sources, including conventional quarter- and half-wave helical resonators (HR) have the same problem of capacitive coupling t o the ground. However, voltage and current are distributed as standing *E-mail address: 100510,[email protected]

waves (SW) having corresponding local maxima and minima of voltage, current, and R F magnetic fields. The scheme is extended to the full wave HR (A-HR). It is significant that a step from a conventional halfwave t o the A-HR drastically changes inductor-plasma interactions: capacitive phase and anti-phase currents are coupled t o one another in the latter case. Therefore, the fundamental frequency component of the inductorplasma-ground capacitive current is canceled completely. Clearly, the R F fluctuation of plasma potential at fundamental frequency must be zero as well. The last phenomenon is characteristic for symmetric capacitive discharges2) and depends on their degree of s y m m e t r y . 2 , 3 ) Any inductive source with anti-symmetric (anti-phase) R F potentials must function the same way. Ideally, such a plasma source does not have any capacitive coupling to the ground at the excitation frequency, hence, it must not produce any secondary plasma near the grounded surfaces outside the inductor. As for the magnetic compensation, one can see that the A-HR has three inductive parts providing anti-phase currents with exactly zero total magnetic moment outside the inductor. Magnetic compensation occurs in a conventional half-wave structure c) as well. However, this merit of anti-symmetric current inductors has never been taken into consideration. We designed A-HR as shown in Fig. 2. The A-HR has never been used as a plasma source. It was believed t o be devoid of merits, cumbersome, and inconvenient in comparison with, for example, the conventional halfwave structure. However, the reverse seems t o be true in the case of plasma excitation. The R F 50 Ohm coaxial cable is connected directly t o the discharge tybe

I

capacitive balance

....................

Coil ends can be open, grounded, or RF fed.

Fig. 1. RF current (I) and voltage (V) amplitude distributions on cylindrical inductors: (a) "short" inductor; (b) quarter-wave HR; (c) half-wave HR; (d) full-wave A-HR; (e) half-wave HR with open ends. Coil ends are connected to the shield (grounded), open and/or RF fed.

-

Jpn. J. Appl. Phys. Vol. 35 (1996) Pt. 2, No. 9A

G. K. VINOGRADOV and S. YONEYAMA

L 1131

235 rnm ID

Al flange

50 ohm coaxial from RFgenerator

1 gas input

Al chamber /

stage Fig. 2. Schematic diagram of the simplest full-wave helical resonator (A-HR) reactor.

corresponding point of the inductor without matching elements. The plasma source is mounted on the chamber above the movable grounded A1 electrode. Pure Ar and O2were used as feed gases a t flow rates up to 3 slm and in the 0.001-50Torr pressure range. The chamber was pumped with a Roots blower and rotary pumps. R F power at 26-28 MHz frequency up t o 2.25 kW was supplied from the wide band tube amplifier IFI-410 with the IFI-5300 pre-amplifier or from the Comdel 3000127 R F generator up to 3 kW at 27.12 MHz. The R F frequency in the first case was fed from a signal generator. The A-HR represents a highly symmetric R F structure by its very nature, hence it does not need symmetric excitation to provide highly symmetric current and anti-symmetric voltage distributions. The first factor guarantees minimum RF magnetic and the second one minimum electric, fields outside the source. Since the R F discharge in A-HR has never been observed before, we describe it here in much details as~possible.The A-HR discharge gives a very clear picture of capacitive and inductive plasma phenomena, many of which can be understood and interpreted with a fair degree of confidence. This is because of the strictly defined localization of inductive and capacitive interactions (SW structure) and an optimum set of experimental conditions. A clearly visible gas discharge can be initiated a t a power of about 2-3 W and 0.001-0.01 Torr pressure in air. Such an easy ignition is typical for HR due t o its high quality factor Q (N l o 3 ) , and hence, very high voltage on the coil even under the low power condition. The low pressure discharge occupies all the free volume with the power increase. Even low power discharge is very stable since the HR interacts with plasma in the regime of a high impedance generator of the capacitive current. The low pressure discharge, which is important from a practical point of view, provides no means of revealing the internal structure of discharge or the distribution of R F power dissipation due to fast diffusional mixing of plasma components. Therefore, we intentionally used moderate- and high-pressure conditions when the characteristic diffusional lengths of the majority of plasma

Fig. 3. Evolution of visible Ar plasma patterns a t 15-20Torr pressure with RF power increase from about 50 t o 2000W in a full-wave helical resonator: 1-low-power capacitive excitation, clearly visible distribution of capacitive currents; 2-localized ball plasmoids appear at the central plane; 3-central inductive toroid is formed; 4-secondary ball plasmoids appear above the central toroid; 5-ball plasmoids develop more complex structure; 6-iongated capacitive current structures appear; 7-just two toroids exist; 8-pre-toroid plasma structure appears; 9-three toroids are finally formed.

processes were essentially decreased. The transition from diffusional t o volume charge loss causes the plasma to be localized a t the place of its generation. Such conditions are favorable for visualizing the spatial distribution and features of RF power dissipation. It provides solid experimental evidence of the mechanism of plasma generation. Figure 3 shows the consequence of plasma phenomena which accompany power increase in the A-HR Ar discharge in the 10-20 Torr pressure range. After the initial breakdown, which occurs under below about 100 watts, a weak glow discharge appears. The plasma trends to the tube surface and looks like a body of rotation. It never spreads outside the inductor. Such a plasma shape could, in principle, appear as a result of either capacitive or inductive discharge. However, under the high-pressure condition we can clearly discriminate these cases since the plasma cannot diffuse far from the place of generation. Hence the inductive plasma cannot exist in the cross-sectional planes of zero coil currents on both sides of the inductor. That is, initially, only capacitive discharge exists in the A-HR excited by the push-pull longitudinal currents flowing between the high voltage parts of the coil. The further power increase causes a sudden appearance of a 2-3-cm-diameter stable plasma ball at the central plane of the A-HR. This plasmoid remains a t about 3-4cm from the tube wall. Then two more ball plasmoids appear, and suddenly, a bright central plasma

L 1132

G. K. VINOGRADOV and S. YONEYAMA

Jpn. J. Appl. Phys. Vol. 35 (1996) Pt. 2 , NO. 9A

toroid forms instead of plasmoids. It is about 2-3 cm in the cross section of the plasma channel, depending on conditions. The capacitive plasma intensity abruptly drops during this transformation. The toroid becomes brighter with increasing power. The new ball plasmoids and elongated structures consisted of small connected balls originate as shown in t h e Fig. 3. The plasmoids remain stationary or rotate around the discharge axis. The rotation of upper and lower plasmoids is in opposite direction in correspondence with Lorenz forces affecting capacitive current plasma structures. The central toroid plays the role of a virtual ground electrode, and the capacitive currents flow from the high voltage part of the inductor t o the toroid which is strongly capacitively coupled t o the central zero voltage part of the coil, since it is highly conductive and strongly pressed t o the tube wall. The further evolution is well shown in Fig. 3. After all three toroids appear no more new structures are formed up to 3 k W R F power. There is no visible plasma above the grounded electrode at any time. New structures almost never appear with power decrease. The whole picture strongly depends on the Ar purity. In order t o check the nature of the toroids we have inserted a bundle of thin Pyrex tubes filling the upper space above the central toroid. There was a nonuniform gap of about 1-12 mm between the bundle and wall. We tried t o extinguish the top toroid. Figure 4 shows this experimental design. However, the top toroid appeared very easily despite the use of the bundle. This toroid was different from a conventional one; it appears very bright yellow only about 1-cm-diameter plasma channel compressed between the bundle and the wall, which was as narrow as about I mm thickness in some places but was never broken. The color of plasma at such narrow gaps was practically white, the Pyrex tubes contacting the plasma in such places were condescend up t o about 700-800•‹C. Clearly, the toroid represents a one turn plasma transformer and its current is constant along the plasma channel. The toroid can be very strongly distorted by t h e bundle, and its length can be increased by vertical bends. One more plasma phenomenon was observed for t h e first time: collapse of the diffuse capacitive current between the top and central toroids into several plasma strings. The bundle displaces capacitive currents to the near-wall area. Consequently, the capacitive current density increases and the plasma becomes divided into the contracted plasma strings. Figure 5 shows equivalent R F schemes of inductive and capacitive coupling of the inductor, divided for the sake of simplicity into four quarter wave parts with the plasma load. The capacitive structure can be represented, at first approximation, as several high potential and grounded electrodes. Consider the discharge phenomena from the initial breakdown. When the R F power is too low t o initiate the discharge, the distributions of electric and magnetic fields in the resonator corresponds to an ideal non-loaded resonator. The magnitude of electric field strength - in the resonator is determined by the characteristic impedance

quartz

bundle of thin

high current density capacitive plasma

toroid bottom inductive toroid

Fig. 4. Simplified schemes of inductive and capacitive interactions in of inductor plasma in a A-HR. Current and voltage distributions and magnetic moments are shown. High-density plasma toroids represent individual one-turn plasma transformers.

inductive structure

/ -

magnetic momentum

v

I

capacitive structure high density inductive plasma

low density capacitive plasma

Fig. 5 . Plasma structures in A-HR discharge with a dielectric insert which constrains the top plasma toroid.

of the resonator and the quality factor, which is typically higher than 1000. Hence, even under 2-3 W of the R F power the electric field strength near the resonator coil can be high enough (> 100V/cm) for electrical breakdown of the low-pressure gas. This breakdown occurs and the quality factor immediately decreases, hence, the R F voltage also decreases. Thus the initial discharge is generated by the resonator operating as a high impedance current generator. This is a typical capacitive breakdown and discharge. The initial weak plasma occupies only an internal volume of the inductor. It is shaped as a body of rotation in correspondence with the longitudinal electric field lines. The capacitive current flows through the plasma between the top and bottom voltage anti-phase halves of the inductor. It is not coupled t o the ground.

Jpn. J. Appl. Phys. Vol. 35 (1996) P t . 2 , No. 9A

Since the plasma exists in the inductor, the inductive current immediately appears, being generated by the circular magnetic field. However, this current is too small t o provide the necessary ionization rate t o generate the self-supporting inductive discharge. It has a negligible effect since the capacitive plasma area has too low electric conductivity for inductive coupling of R F power. As power is increased, the capacitive currents increase in the plasma. This increases plasma conductivity and creates longitudinal heat and ionization instabilities. Naturally, the plasma cylinder can be partitioned into several plasma strings shunting the whole high-voltage area and concentrating the total capacitive current under the high-power and high-pressure conditions. The inductive current increases as well. Under the condition that this current becomes capable of supplying an ionization rate comparable t o that of the capacitive current, inductive "breakdown" suddenly occurs. Such an avalanchelike character is accounted for by the positive feed-back loop between the efficiency of inductive coupling and the rate of ionization in the plasma toroid. Since the plasma transformer is formed, it can efficiently dissipate the R F power from the resonator: the discharge configuration is changed. Clearly, the plasma toroid decreases the efficiency of R F capacitive coupling and the total capacitive plasma power. The term "inductive breakdown'' has a limited sense and means only the formation of the self-supporting inductive plasma toroid which needs a sufficiently high initial level of plasma conductivity t o be formed. The criterion of inductive plasma could therefore be the ratio between the inductive and capacitive current densities. Nevertheless, it is also clear that the presence of inductive plasma in some places does not exclude the existence of capacitive plasma in another part of the discharge. The A-HR can generate one, two or three inductive toroids. The central one is about two times more powerful than the side toroids. The central toroid appears first since the primary capacitive preionization is concentrated near the discharge center. The inductive toroids can easily be scaled up for the condition of constant am-

G. K. VINOGRADOV and S. YONEYAMA

L 1133

plitude of the total current. The final important feature of the A-HR is an internal compensation of not only capacitive currents but RF magnetic fields as well. It is clear from Fig. 5 that side toroids have magnetic moments compensated by the magnetic momentum of the central toroid. Such compensation results from the current distribution along the inductor which has zero total magnetic momentum itself. It can be well understood that such compensation occurs in a conventional short-ended half-wave HR structure as well. Since A-HR has neither capacitive current nor magnetic field effects spreading far outside the plasma source: it can be placed directly above the processed surfaces. Hence, it can be considered as a very powerful plasma source of radicals. It was tested for oxygen ashing of normal and heavily ion implanted polymer resists (P+, As+; dose 1015-1016~ m - 70-80keV). ~ , The ashing rate of normal resist a t 240•‹C was about 11micron/min with an apparent activation energy of about 0.5 eV typical for the chemical ashing by atomic oxygen4) and is much higher than the values typical for a plasma a ~ h i n ~Ion . ~ ) implanted and "after etch" resists were ashed with excellent results as well. There was neither plasma damage for different NMOS (n-channel metaloxide-semiconduictor) antenna structures nor surface metal contamination found. The A-HR creates a new generation of powerful inductive plasma sources with localized plasma structures and compensated capacitive currents and magnetic fields and exhibits all typical features of such sources.

1) H. U. Eckert: Proc. 2nd Annual Int. Conf. on Plasma Chem. and Technology, ed. H. Boenig (Technomic Publ., Lancaster, Pa, 1986) p. 173. 2) R. R. J. Gagne and A. Cantin: J. Appl. Phys. 43 (1972) 2639. 3) V. A . Godyak and R. B. Piejak: J . Appl. Phys. 68 (1990) 3157. 4) J. M. Cook and B. W . Benson: J . Electrochem. Soc. 130 (1980) 2459; E. P. G. T . van der Ven and H. Kalter: Electrochem. Soc. Ext. Abstr. 76-1 (1976) 332. 5) G. N. Tailor and T . M. Wolf: Polym. Eng. Sci. 20 (1980) 1087.