Plasma processor with coil having variable rf coupling

Details Plasma processor with coil having variable rf coupling Plasma processor with coil having variable rf coupling

1999-03-31

2001-05-08

Shingleton, Michael B. (Department: 2817)

Electric lamp and discharge devices: systems

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

Plasma generating

C315S111810, C118S7230AN, C118S7230IR

Reexamination Certificate

active

06229264

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to plasma processors including radio frequency (r.f.) responsive coils for exciting gases in vacuum chambers to plasmas that process workpieces in the chamber and more particularly to such a processor and to a processing method wherein plasma density on the workpiece is controlled by varying coupling coefficients of r.f. fields between different positions of the coil and the plasma, and to such a coil.
BACKGROUND ART
One type of processor for treating workpieces with an r.f. plasma in a vacuum chamber includes a coil responsive to an r.f. source. The coil responds to the r.f. source to produce magnetic and electric 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, i.e., to process the workpiece. 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 r.f. source via an impedance matching network. Coils of this general type produce oscillating r.f. 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 r.f. 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 field component produced by each of the turns is a function of the magnitude of r.f. current in each turn which differs for different turns because of transmission line effects of the coil at the frequency of the r.f. source.
For spiral designs as disclosed by and based on the Ogle '458 patent, the r.f. 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 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, resulting 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.
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 about 2.0, a considerable improvement over the standard deviation of approximately 3.0 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, 5,731,565, 5,401,350, and 5,847,704.
To our knowledge all generally available prior art coils have fixed spatial geometries even though different processes have different recipes requiring differing chamber parameters. The different recipes are associated with different processes performed on the workpiece. The chamber parameters for a particular recipe in the past have generally been limited to gas flow rate, vacuum pressure, gas species, r.f. power applied to the excitation coil and r.f. power applied to an electrode of an electrostatic chuck to produce what is referred to in the art as r.f. bias. Control of a further parameter affecting the plasma while a workpiece is processed in the same chamber is desirable. In addition, sometimes it is desirable to change the r.f. fields the coil couples to the plasma as a function of time during the same recipe step.
While uniform plasma density is usually desirable, there are applications in which it is desirable for the plasma flux density to differ on different parts of the workpiece during a particular processing step. There are other situations where the plasma density desirably has a first particular desired non-uniformity characteristic during a first processing step, i.e., while a first recipe is being performed, and has a second particular desired non-uniformity characteristic during a second processing step. To our knowledge, there is no generally available prior art or method of or apparatus for achieving these types of results in the same processing chamber.
It is accordingly an object of the present invention to provide a new and improved vacuum plasma processor and method of operating same wherein the plasma density incident on the workpiece has relatively high uniformity.
Another object of the invention is to provide a new and improved vacuum plasma processor having an r.f. coil with the same geometry as the prior art coil but which is coupled to the plasma in such a way as to enable the plasma to have relatively high density uniformity characteristics.
A further object of the invention is to provide a new and improved vacuum plasma processor method and apparatus wherein an r.f. excitation coil is arranged so different portions of the coil have differing, changeable r.f.

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