Multiple coil antenna for inductively-coupled plasma generation systems

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FIELD OF THE INVENTION

The present invention relates to plasma reactors for processing materials such as semiconductor substrates. More particularly, the present invention relates to a system for improving the inductive coupling uniformity within plasma reactors.

BACKGROUND OF THE INVENTION

Plasma generation is useful in a variety of semiconductor fabrication processes, for example plasma enhanced etching and deposition. Plasmas are generally produced from a low pressure gas by electric field ionization and generation of free electrons which ionize individual gas molecules through the transfer of kinetic energy via individual electron-gas molecule collisions. The electrons are commonly accelerated in an electric field, typically a radio frequency electric field.

Numerous techniques have been proposed to accelerate the electrons in an RF electric field. For example, U.S. Pat. No. 4,948,458 discloses a plasma generating device in which electrons are excited in a radio frequency field within a chamber using a planar antenna coil that is situated parallel to the plane of a semiconductor wafer to be processed. FIG. 1 schematically illustrates a plasma generating device 100 which includes an antenna system 105, a dielectric window 120, a gas distribution plate 130, a wafer to be processed 140, a vacuum chamber 150, an electrostatic chuck 160, and a lower electrode 170.

In operation, a radio frequency source (not shown) is used to provide a radio frequency current to the antenna system 105, typically via a radio frequency matching circuit (also not shown). The radio frequency current resonates through the antenna system 105, inducing an azimuthal electric field within the vacuum chamber 150. At the same time, a process gas is introduced into the vacuum chamber 150 via the gas distribution plate 130, and the induced electric field ionizes the process gas to produce a plasma within the chamber 150. The plasma then impinges upon the wafer 140 (which (which is held in place by way of the electrostatic chuck 160) and processes (e.g., etching) the wafer 140 as desired. Another radio frequency, at a frequency which is different from that applied to the antenna coil, is typically applied to the lower electrode 170 to provide a negative DC bias voltage for ion bombardment.

FIGS. 2A and 2B depict two planar spiral coils 110a, 110b which make up the antenna system illustrated in the '458 patent. As shown in FIG. 2A, a first planar coil 110a is constructed as a singular conductive element formed into a planar spiral and connected to radio frequency taps 205, 215 for connection to radio frequency circuitry. In FIG. 2B, an alternative planar coil 110b is constructed as a plurality of concentric rings 220 connected in series via inter-connectors 225 and coupled at each end to radio frequency taps 205, 215.

As is well known in the art, the circular current pattern provided by such spiral coils creates toroidal-shaped plasmas which can in turn cause radial non-uniformity in the etch rate at the wafer 140. In other words, the E-field inductively generated by the planar coil antenna 110 is generally azimuthal (having a radial component E.sub.r =0 and an azimuthal component E.sub..theta..noteq.0), but zero at the center (E.sub.r =0 and E.sub..theta. =0). Thus, the coil antenna 110 produces a toroidal plasma having a lower density in the center, and must rely on plasma diffusion (i.e., the diffusion of electrons and ions into the center) in order to provide reasonable uniformity at the center of the toroid. In certain applications, however, the uniformity provided by plasma diffusion is insufficient.

Further, such spiral coil antennas tend to make azimuthal non-uniform plasma. This results from the fact that the relatively long lengths of coupling lines used to construct the planar antenna coils have significant electrical lengths at the radio frequency at which they typically operate. The voltage and current waves travel forward from the input end to the terminal end, and will be reflected back at the terminal end. The superposition of the forward and reflected waves results in a standing wave on the coil (i.e., the voltage and current vary periodically along the length of the coil). If the coil is grounded at the terminal end, the current at the terminal end is at a maximum value, and the voltage at the terminal end is zero. Proceeding along the coil toward the input, the voltage increases and the current decreases until, at 90 degrees of electrical length, the voltage is at a maximum and the current is at a minimum. Such a degree of variation results in a highly non-uniform plasma. Consequently, the planar coil is typically terminated with a capacitance such that the current in the coil is similar at both ends of the coil and increases to a maximum near the middle of the coil. Doing so can improve plasma uniformity, but azimuthal non-uniformity still exists because the current varies in the azimuthal direction along the length of the coil. For example, point P in FIG. 2A is the current maximum. On either side of point P the current drops off. Therefore, the power coupling to the plasma is higher beneath P and the corresponding plasma is denser. In contrast, the plasma density at point P' is relatively lower.

Note that, although the terminating capacitor value can be varied, doing so only changes the position of the voltage null along the coil. Further, although the coil can be terminated with an inductance in order to provide the same polarity voltage along the coil length, a current null will exist somewhere in the central portion of the coil (with the current traveling in opposite directions on either side of the null), and the resulting plasma density can be unacceptably low and non-uniform. U.S. Pat. No. 5,401,350 to Patrick et al. attempts to overcome the above-described deficiencies. Therein, a multiple planar coil configuration is set forth in order to improve plasma uniformity. The RF power to the individual coils is independently controlled, requiring separate power supplies and separate matching networks which allow for independent adjustment of the power and phase.

It is evident that a need exists for improved methods and apparatuses for controlling the inductive coupling uniformity within a plasma coupled system.

SUMMARY OF THE INVENTION

The present invention overcomes the above-identified deficiencies in the art by providing a system for improving the inductive coupling uniformity within an antenna system. By controlling the positioning and current distribution of the antenna coils, plasma uniformity can be improved.

According to exemplary embodiments, two or more spiral coils are positioned on a dielectric window of a plasma chamber. Each coil is either planar or a combination of both a planar coil and a vertically stacked helical coil. The input end of each coil is attached to an input variable capacitor and the output end is terminated to the ground through an output variable capacitor. The output capacitor determines where the current is an extreme (i.e., a maximum or a minimum) or the voltage is an extreme, while the input capacitor can change the overall impedance of each coil, and therefore, the ratio of current magnitudes in these multiple coils can be adjusted. By adjusting the magnitude of the current and the location of the maximum current in each coil, plasma density, and therefore, plasma uniformity, can be controlled.

The above-described and other features and advantages of the present invention are explained in detail hereinafter with reference to the illustrative examples shown in the accompanying drawings. Those skilled in the art will appreciate that the described embodiments are provided for purposes of illustration and understanding and that numerous equivalent embodiments are contemplated herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a plasma reactor wherein an antenna system is placed at the top of the dielectric window and is used to couple radio frequency energy into a processing chamber;

FIGS. 2A and 2B depict two conventional planar spiral coil antennas;

FIG. 3 depicts an exemplary arrangement of dual, single-turn planar coils according to a first embodiment of the present invention;

FIG. 4 depicts an exemplary arrangement of dual, multiple-turn planar coils according to a second embodiment of the present invention;

FIG. 5 depicts an exemplary arrangement of dual, multiple-turn planar coils with an inner helical coil according to a third embodiment of the present invention;

FIG. 6 depicts an exemplary arrangement of dual, multiple-turn planar coils, with both inner and outer helical coils according to a fourth embodiment of the present invention; and

FIG. 7 depicts an exemplary arrangement of dual, multiple-turn planar coils with parallel antenna elements according to a fifth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a plasma generating device 100 in which the antenna system of the present invention may be incorporated. As discussed above, the plasma generating device 100 includes a dielectric window 120, a gas distribution plate 130, a wafer 140, a vacuum chamber 150, an electrostatic chuck 160, a lower electrode 170 and an antenna system 105. The antenna system 105 includes a set of coils 110 which is connected to a RF matching network (not shown) and a RF generator (not shown).

According to exemplary embodiments of the present invention, the antenna system is a Transformer-Coupled Plasma (TCP.TM., a registered trademark of Lam Research Corporation) antenna system. FIG. 3 illustrates the TCP.TM. antenna system 300 according to a first embodiment of the present invention. In this embodiment, the TCP.TM. system 300 includes two single-turn coils. Coil 1 is preferably placed near the center while Coil 2 is preferably placed further toward the outer edge of the reactor's top opening. A radio frequency (RF) current is simultaneously provided to one end of Coils 1 and 2 via two tuning capacitors C.sub.1 and C.sub.2. As is well known in the art, the RF input is generated by a RF source 310 and fed to capacitors C.sub.1 and C.sub.2 through a RF matching network 320. Tuning capacitors C.sub.1 and C.sub.2 allow for the magnitude of currents I.sub.1 and I.sub.2 in Coils 1 and 2, respectively, to be adjusted. The opposite ends of Coil 1 and Coil 2 are tied together and terminated to ground through impedance Z.sub.T.

The electric field that is inductively generated by a single-turn, planar coil is azimuthal (the radial component E.sub.r =0 and an azimuthal component E.sub..theta..noteq.0) but zero at the center (E.sub.r =0 and E.sub..theta. =0). Near the dielectric window surface, the induced E-field and induced current (J=.sigma.E) in the plasma are almost mirror images of the driving coil. A planar coil antenna produces a toroidal plasma with a radius which is close to one half of the driving coil's radius. By placing two coils apart, this effectively generates a more gradual plasma toroid having a radius that is approximately equal to one half of the mean radii of the two coils. The power coupling to the plasma from the inner coil is localized in the inner region while the power coupling from the outer coil is localized in the outer region. As a result, plasma diffusion (i.e., the diffusion of electrons and ions) tends to make the plasma density more uniform in the center and elsewhere.

As indicated above, the circuitry associated with the two single-turn coils (i.e., the capacitors C.sub.1 and C.sub.2 and impedance Z.sub.T) is capable of adjusting the ratio of current magnitudes in Coil 1 and Coil 2, i.e., I.sub.1 and I.sub.2, respectively. By a