Plasma excitation coil

Details Plasma excitation coil Plasma excitation coil

2002-08-26

2003-11-11

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Discharge device load with fluent material supply to the...

Plasma generating

C315S111210, C118S7230IR, C118S7230AN

Reexamination Certificate

active

06646385

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to plasma excitation coils and, more particularly, to an excitation coil having at least one capacitance connected across a discontinuity between the coil excitation terminals, and to a workpiece processor including such a coil. The invention also relates to a method of operating an excitation coil such that a standing wave has a sudden amplitude and slope change, as well as a sudden slope reversal, between the coil excitation terminals.
BACKGROUND ART
One type of processor for treating workpieces with an RF plasma in a vacuum chamber includes a coil responsive to an RF source. The coil responds to the RF source to produce electromagnetic fields that excite ionizable gas in the chamber to a plasma. Usually the coil is on or adjacent to a dielectric window that extends in a direction generally parallel to a planar horizontally extending surface of the processed workpiece. The excited plasma interacts with the workpiece in the chamber to etch the workpiece or to deposit material on it. The workpiece is typically a semiconductor wafer having a planar circular surface or a solid dielectric plate, e.g., a rectangular glass substrate used in flat panel displays, or a metal plate.
Ogle, U.S. Pat. No. 4,948,458 discloses a multi-turn spiral coil for achieving the above results. The spiral, which is generally of the Archimedes type, extends radially and circumferentially between its interior and exterior terminals connected to the RF source via an impedance matching network. Coils of this general type produce oscillating RF fields having magnetic and capacitive field components that propagate through the dielectric window to heat electrons in the gas in a portion of the plasma in the chamber close to the window. The oscillating RF fields induce in the plasma currents that heat electrons in the plasma. The spatial distribution of the magnetic field in the plasma portion close to the window is a function of the sum of individual magnetic field components produced by each turn of the coil. The magnetic and electric field components produced at each point along the coil are respectively functions of the magnitude of RF current and voltage at each point. The current and voltage differ for different points because of transmission line effects of the coil at the frequency of the RF source.
For spiral designs as disclosed by and based on the Ogle '458 patent, the RF currents in the spiral coil are distributed to produce a torroidal shaped magnetic field region in the portion of the plasma close to the window, which is where power is absorbed by the gas to excite the gas to a plasma. At low pressures, in the 1.0 to 10 mTorr range, diffusion of the plasma from the ring shaped region produces plasma density peaks just above the workpiece in central and peripheral portions of the chamber, so the peak densities of the ions and electrons which process the workpiece are in proximity to the workpiece center line and workpiece periphery. At intermediate pressure ranges, in the 10 to 100 mTorr range, gas phase collisions of electrons, ions, and neutrons in the plasma prevent substantial diffusion of the plasma charged particles outside of the torroidal region. As a result, there is a relatively high plasma flux in a ring like region of the workpiece but low plasma fluxes in the center and peripheral workpiece portions.
These differing operating conditions result in substantially large plasma flux (i.e., plasma density) variations between the ring and the volumes inside and outside of the ring, as well as at different angles with respect to a center line of the chamber that is at right angles to the plane of the workpiece holder. These plasma flux variations result in a substantial standard deviation, i.e., in excess of three, of the plasma flux incident on the workpiece. The substantial standard deviation of the plasma flux incident on the workpiece has a tendency to cause non-uniform workpiece processing, i.e, different portions of the workpiece are etched to different extents and/or have different amounts of molecules deposited on them.
Many coils have been designed to improve the uniformity of the plasma. The commonly assigned U.S. Pat. No. 5,759,280, Holland et al., issued Jun. 2, 1998, discloses a coil which, in the commercial embodiment, has a diameter of 12 inches and is operated in conjunction with a vacuum chamber having a 14.0 inch inner wall circular diameter. The coil applies magnetic and electric fields to the chamber interior via a quartz window having a 14.7 inch diameter and 0.8 inch uniform thickness. Circular semiconductor wafer workpieces are positioned on a workpiece holder about 4.7 inches below a bottom face of the window so the center of each workpiece is coincident with a center line of the coil and the chamber center line.
The coil of the '280 patent produces considerably smaller plasma flux variations across the workpiece than the coil of the '458 patent. The standard deviation of the plasma flux produced by the coil of the '280 patent on a 200 mm wafer in such a chamber operating at 5 milliTorr is a considerable improvement over the standard deviation for a coil of the '458 patent operating under the same conditions. The coil of the '280 patent causes the magnetic field to be such that the plasma density in the center of the workpiece is greater than in an intermediate part of the workpiece, which in turn exceeds the plasma density in the periphery of the workpiece. The plasma density variations in the different portions of the chamber for the coil of the '280 patent are much smaller than those of the coil of the '458 patent for the same operating conditions as produce the lower standard deviation.
Other arrangements directed to improving the uniformity of the plasma density incident on a workpiece have also concentrated on geometric principles, usually concerning coil geometry. See, e.g., U.S. Pat. Nos. 5,304,279; 5,277,751; 5,226,967; 5,368,710; 5,800,619; 5,401,350; 5,558,722 and 5,795,429. However, these coils have generally been designed to provide improved radial plasma flux uniformity and to a large extent have ignored azimuthal plasma flux uniformity. In addition, the fixed geometry of these coils does not permit the plasma flux distribution to be changed for different processing recipes. While we are aware that the commonly assigned copending U.S. application of John Holland for “Plasma Processor with Coil Responsive to Variable Amplitude RF Envelope,” Ser. No. 09/343,246, filed Jun. 30, 1999, and Gates U.S. Pat. No. 5,731,565 disclose electronic arrangements for at will controlling plasma flux uniformity for different recipes, the Holland and Gates inventions are concerned primarily with radial, rather than azimuthal, plasma flux uniformity. In the Holland invention, control of the plasma flux uniformity is achieved by controlling a variable amplitude envelope the RF excitation source applies to the coil. In the Gates invention a switch or a capacitor shunts an interior portion of a spiral-like RF plasma excitation coil.
It is accordingly an object of the present invention to provide a new and improved coil for a vacuum plasma processor and method of operating same wherein plasma flux in the processor is relatively uniform.
An additional object of the present invention to provide a new and improved coil for a vacuum plasma processor and method of operating same wherein the plasma density incident on a workpiece of the processor has relatively high azimuthal uniformity.
A further object of the invention is to provide a new and improved coil for a plasma processor, wherein the amplitude variations of standing waves (voltages and/or currents) in the coil are substantially reduced.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, a coil for a plasma generator of a processor for treating a workpiece includes (1) first and second RF excitation terminals, (2) sufficient length to exhibit transmission line effects for RF excitation of the coil and (

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