Etching a substrate: processes – Gas phase etching of substrate – Etching inorganic substrate
Reexamination Certificate
1997-11-26
2001-01-30
Warden, Sr., Robert J. (Department: 1744)
Etching a substrate: processes
Gas phase etching of substrate
Etching inorganic substrate
C216S080000, C118S7230IR, C118S7230AN, C156S345420
Reexamination Certificate
active
06180019
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a plasma processing apparatus for surface treatment to etch a substrate or to form a thin film with a plasma by supplying a radio-frequency electric field to an antenna, generating an electric field, and thereby generating a plasma by the electric field, and a method of using this apparatus. More particularly, the invention relates to a semiconductor processing apparatus for processing a semiconductor device, and a method of using this apparatus.
In a semiconductor processing apparatus for generating a plasma by induction by feeding an electric current to a coil-shaped antenna, there is a problem that a vacuum chamber wall made of an non-conductive material and enclosing a plasma generating unit so as to establish a vacuum atmosphere is partly removed by the plasma. In order to solve this problem, there has been conceived a method using a field called the “Faraday shield”, as disclosed in Japanese Patent Laid-Open No. 502971/1993. If the Faraday shield is used, however, the plasma ignitability is so deteriorated that the plasma is not ignited unless a voltage as high as tens of KV is applied to the feeding portion of the coil-shaped antenna. This apparatus may fail with a high possibility by the discharge between the antenna and a conductive structure nearby. In order to prevent this discharge, an additional structure is needed to insulate the antenna from the existing structure, causing the apparatus to be complicated.
When a Faraday shield is used to reduce the partial removal of the wall, foreign matters are liable to adhere to the wall and to appear if its sticking rate to the wall from the plasma is accelerated. Therefore the partial removal of the wall must be adjusted according to the process.
The plasma density distribution is determined mainly by the generation rate distribution and by the state of transportation of ions and electrons. In the absence of an external magnetic field, the transportation of the plasma diffuses isotropically in every direction. At this time, electrons instantly escape and tend to reach the wall of the vacuum chamber because the mass is no more than {fraction (1/1,000)} of that of an ion, but they are repelled by the sheath (ion sheath) formed in the vicinity of the wall. As a result, a quasi-neutral condition of the electron and ion densities is always met in the plasma, so that both the ions and electrons are bipolarly diffused toward the wall. At this time, the potential of the plasma takes on its maximum where the plasma density, i.e., the ion density, is the maximum. This potential is termed the plasma potential Vp, approximately expressed by Vp≈Te×ln(mi/me), where Te, mi and me are the electron temperature, the mass of an ion, and the mass of an electron, respectively. In the plasma, the potential distribution is determined by the potential Vp and the wall potential (ordinally at 0 V), so that the density distribution is correspondingly determined. Since, in this case, the plasma is confined by the electrostatic field established by itself, the density distribution is determined by the shape of the apparatus, the place where the induced electric field takes on the maximum, and the ratio of the generation ratio/the bipolar diffusion flux.
When the coil is wound by several turns on the vacuum chamber, for example, the magnetic flux generated by the coil takes on the maximum at the central portion so that the induced electric field takes on the maximum at the central portion. Moreover, the induced electric field cannot penetrate deeper than about the skin depth, e.g., 1 cm, so that both the ionization factor and the dissociation factor take on their maximums at the radially central portion (in the direction of arrow r, e.g., in FIG.
21
(
a
)) and just below the dielectric member (in the direction of arrow z, e.g., in FIG.
21
(
a
)). After this, the plasma diffuses towards the wafer side (downstream side). In the case of an ordinary chamber having a cylindrical shape, therefore, the plasma density is the maximum at the central portion in the direction of arrow r, and the degree of central concentration rises downstream so that the plasma density becomes nonuniform in the region where the wafer is placed.
SUMMARY OF THE INVENTION
A first object of the invention is to control the removal extent of the vacuum chamber wall around the plasma generating portion by the plasma. A second object of the invention is to improve the plasma ignitability.
A third object is to realize a uniform plasma of high density. This object is particularly desired in processing large semiconductor wafers (e.g., large-size semiconductor wafers of 300 mm).
In order to achieve the above-specified objects, according to the invention, there is provided a plasma processing apparatus comprising an antenna (coil) for generating an electric field in a plasma generating portion, a radio-frequency power source for supplying radio-frequency electric power to said antenna, a vacuum chamber enclosing the plasma generating portion to establish a vacuum atmosphere therein, a Faraday shield provided around said plasma generating portion (e.g., around the vacuum chamber), a gas supply unit for supplying gas into said vacuum chamber, a sample stage on which an object to be processed is placed, within the vacuum chamber, and a radio-frequency power source for applying a radio-frequency electric field to said sample stage, a plasma being generated by accelerating electrons and ionizing them by collision with the electric field generated by said antenna, and thereby processing said object; characterized in that a load is provided in the earth portion of said antenna, the average potential of said antenna is adjusted so as to improve the ignitability at a plasma ignition time, and the load is adjusted after the plasma is produced so that the average potential of said antenna may be close to that of the earth, and the removed amount of the wall of said vacuum chamber after the plasma generation may be small. The above-specified objects are also achieved, according to the present invention, by a method of operation of this apparatus whereby the load provided in the earth portion of the antenna is adjusted (that is, the voltage on the ends of the antenna (coil) is controlled) such that ignitability of the plasma at the time of plasma ignition is facilitated, and is then again adjusted to be close to that of the ground to limit the amount of chamber wall removed (e.g., etched) by the plasma.
Here, the phenomenon that the average potential of the antenna comes close to that of the earth means that the potentials
30
a
and
30
b
of
FIG. 4
are mutually opposite in phase but substantially equal to each other, that is, Va≈−Vb.
As another technique and structure to achieve the above-specified objects, the Faraday shield can be provided with at least one switch. When igniting the plasma at a plasma ignition time, the at least one switch is positioned such that the Faraday shield is held in a floating state, to facilitate ignition of the plasma. Thereafter, the at least one switch is thrown to ground the Faraday shield, so as to protect the wall of the plasma chamber from removal by the plasma.
As still another technique and structure to achieve the above-specified objects, the load can be provided in the earth portion of the antenna and a switch or switches can be provided for the Faraday shield. By adjusting the load and positioning the switch as described in the preceding paragraphs, ignition of the plasma is facilitated and removal of the wall in the plasma chamber is avoided.
Means for solving the above-specified problems will be described with reference to FIG.
2
.
FIG. 2
shows an experimental induction type plasma generating apparatus used for verifying the present invention. With this apparatus, the methods for reducing the partial removal of the vacuum chamber wall around the plasma generating portion by the plasma and for improving the ignitability of the plasma are examined by changing the way of groun
Arai Masatsugu
Doi Akira
Edamura Manabu
Kanai Saburo
Kazumi Hideyuki
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Olsen Kaj K.
Warden, Sr. Robert J.
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