Etching a substrate: processes – Gas phase etching of substrate – Application of energy to the gaseous etchant or to the...
Reexamination Certificate
1999-02-19
2001-06-19
McDonald, Rodney G. (Department: 1753)
Etching a substrate: processes
Gas phase etching of substrate
Application of energy to the gaseous etchant or to the...
C118S7230AN, C118S7230IR, C118S7230IR, C156S345420, C204S192320, C204S298310, C204S298340, C427S569000
Reexamination Certificate
active
06248251
ABSTRACT:
This invention relates to plasma processing and particularly to plasma etching with an inductively coupled plasma.
BACKGROUND OF THE INVENTION
In semiconductor wafer processing, inductively coupled plasmas (ICPS) have been used with success in plasma processing, particularly plasma etching, of semiconductor wafer substrates. One processing apparatus for etching semiconductor wafers with an ICP is disclosed in Bayer et al. U.S. Pat. No. 5,569,363, hereby expressly incorporated by reference herein. In the Bayer et al. patent, RF energy is applied to a coil that surrounds a vacuum chamber outside of a chamber wall made of quartz. The coil generates a magnetic field within the chamber which excites electrons and forms a plasma. While the field is largely inductively coupled, the development of a high peak to peak voltage across the coil causes some degree of capacitive coupling of energy into the chamber. Such voltage has, in situations, developed a sheath of 900 to 1000 volts, for example, which imparts substantial energy to ions in the plasma. Such energy is enough to cause an undesirable degree of sputtering of the inside of the chamber wall and the sheath reduces the useful volume of the plasma.
It has been proposed to add a metal Faraday shield around the outside of the chamber, between the coil and the chamber wall. The proposed shield is provided with slits that extend axially but which leave a complete circumferential band around each edge of the shield. The shield is grounded to prevent capacitive coupling of voltage into the chamber while the slits prevent the inductive shielding of magnetic energy that is required to energize the plasma. Such a proposed shield, unfortunately, has the effect of preventing initial ignition of the plasma by the coil, requiring a separate electrode or discharge element to start the plasma.
There is a need, therefore, to provide for the shielding of a chamber from capacitive coupling effects when a coil is employed to inductively couple energy into a chamber, and to do so without interfering with plasma ignition.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide shielding of a chamber from capacitive coupling. A particular objective of the invention is to provide efficient shielding from an RF coil that is provided around the chamber to inductively couple RF energy into plasma within the chamber. It is a further objective of the present invention to provide shielding that prevents capacitive coupling between a chamber and an exterior coil while facilitating plasma ignition.
In accordance with the principles of the present invention, a plasma processing chamber, particularly an etching chamber, is provided having an external loop-shaped RF inductor. The inductor is preferably a helical coil for inductively coupling RF energy into the chamber to form a plasma within the chamber. In addition, a Faraday shield is provided outside of the chamber wall between the wall and the inductor. The shield is grounded so as to provide effective shielding of capacitive coupling of voltage from the coil to the plasma within the chamber and is slitted to allow inductive coupling of the RF energy into the chamber. The ground connection is localized to a limited section of the inductor so as to allow a peak-to-peak voltage to develop across the gap when the RF current is initially induced into the shield from the inductor.
In the preferred embodiment of the invention, a cylindrical shield is positioned against the outside of a quartz chamber wall between the wall and a helical coil that encircles the wall and is spaced slightly from the wall. The shield preferably has a plurality of closely spaced internal slits extending axially almost the height of the shield, but less than the entire height of the shield, leaving at least one edge of the shield uninterrupted by the slits, thereby leaving at least one circumferential path of uninterrupted metal conductor around almost the entire circumference of the chamber. At one point in the circumference, a full height slit or gap interrupts the entire height of the shield, thereby avoiding a continuous 360° circumferential conductive path formed by the shield around the chamber.
In the preferred embodiment, the coil, which surrounds the chamber outside of the shield, inductively couples RF energy into the chamber to energize a plasma. The shield is preferably grounded at only one point on its circumference, for example, at a point directly opposite or 180° from the full height gap. Preferably, the gap, as well as the internal slits, are approximately ⅛ inch wide, with strips of the metallic portion of the shield being about ⅛ inch wide between the slits and gap.
When the coil is initially energized with RF energy and before a plasma exists in the chamber, a peak-to-peak voltage develops across the coil. The energization of the coil couples an RF voltage onto the shield which results in a peak-to-peak voltage developing across the narrow full height gap. When the shield is grounded diametrically opposite the gap, the voltages on the opposite edges of the shield bordering the gap are equal and opposite relative to ground potential. This peak-to-peak voltage may be several thousand volts, for example, 5000 volts RF, peak-to-peak. This voltage produces a strong RF electric field that extends through the quartz chamber wall and into the chamber where, within a time interval in the order of one or a few milli-seconds, it ignites a plasma opposite the gap inside the chamber. The plasma, once ignited, propagates, also in a time interval in the order of a milli-second, around the perimeter of the chamber inside of and close to the chamber wall, where the plasma forms a belt of conductive ionized gas around the circumference of the chamber inside of the chamber. When this plasma belt forms, the voltage across the gap of the shield quickly drops to a nominal voltage in the order of 10 to 20 volts, for example, 14 volts, peak-to-peak.
With the present invention, a plasma is formed that fills the chamber with very low capacitive coupling of voltage from the coil to the plasma, resulting in a low plasma potential and small plasma sheath between the plasma and the circumferential quartz side wall of the chamber. At the same time, the shield couples energy form the coil to quickly ignite a plasma upon the initial energization of the coil.
These and other objectives of the present invention will be readily apparent from the following detailed description of the present invention in which:
REFERENCES:
patent: 5468296 (1995-11-01), Patrick et al.
patent: 5569363 (1996-10-01), Bayer et al.
patent: 5811022 (1998-09-01), Savas et al.
patent: 5903106 (1999-05-01), Young et al.
patent: 0 836 218 A2 (1998-04-01), None
McDonald Rodney G.
Tokyo Electron Limited
Wood Herron & Evans L.L.P.
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