Patent Application
20110088848
This invention relates to a microwave plasma-treating apparatus for treatment of a substrate, such as chemical vapor deposition (CVD), etching or asking (resist-asking treatment) using plasma generated by microwave electric power.
In the processes for producing semiconductors and micro machines, plasma has been widely used that is generated when energy is supplied from an external source to a reaction gas. In particular in recent years, it is necessary to develop an apparatus capable of treating substrates of large areas with the plasma to meet the demands for mass-production of wafers of a large diameter.
Under such circumstances, attention has now been given to a plasma-treating apparatus that generates and sustains plasma by using surface waves generated by introducing microwave electric power into a discharge chamber on account of the following characteristics (1)-(3). The surface wave is an electromagnetic wave propagating along interfaces between two media with different permittivities. In surface waves propagating along the interface between a dielectric and a overdense plasma with densities higher than the cut-off density (7.4×1010 cm−3 for 2.45 GHz), the energy of microwaves is concentrated in a region near the surface of the plasma and attenuates in a direction toward the interior of the plasma.
(1) Overdense plasma with densities higher than the cut of density is possible without the application of an external magnetic field.
(2) Microwaves (2.45 GHZ) can be utilized, magnetron is inexpensive, impedance matching is easy, no electrode is necessary for generating plasma, and electrodeless operation is possible.
(3) Because the plasma is generated by the surface waves, i.e., surface wave plasma (SWP), a large-area plasma with the high uniformity and highly density (˜1012 cm−3) can be obtained even under low pressure (<1 Torr (1 Torr=133 Pa)).
The microwave plasma-treating apparatus 60 shown in
The coupling holes are formed as circular radiation holes for radiating the microwave electric power into the plasma-treating chamber 16. Referring to
Though
On the other hand, circular holes or square holes are formed in the upper wall of the plasma-generating chamber 16 so as to be communicated with each of the coupling holes. Dielectric windows (quartz plates, alumina plates, etc.) are fitted to the hole edges of each of the circular holes to constitute dielectric windows 50 that shut off the plasma-generating chamber 16 from the atmosphere.
In a metallic rectangular discharge chamber 12, there are arranged a substrate stage 10 on which a substrate 13 is placed, a gas exhaust port 15 for setting the interior of the treating chamber 16 to a required degree of vacuum, and a gas introduction port 14 for feeding a required reaction gas.
[Patent document 1] JP-A-2002-280196
[Patent document 2] JP-A-10-60657
To realize a high-speed processing, a plasma-treating apparatus is necessary that is capable of providing a high plasma density and a high radical density. For this purpose, it is desired that the plasma apparatus can be operated with large electric power. However, according to the conventional microwave plasma-treating apparatus, an increase in the microwave electric power was accompanied by a problem in that the dielectric windows 50 were broken for the following reasons.
(Reason 1) Heat of the dielectric windows 50 radiates due to natural cooling. If the microwave electric power becomes strong, the temperature of the dielectric windows 50 increases due to heat of the plasma generated in the plasma-treating chamber 16, and the dielectric windows 50 are thermally broken.
(Reason 2) Conventional coupling holes have a circular or square shape, and the microwave electric power intensely concentrate just under the coupling holes causing the dielectric windows 50 to be locally heated and thermally broken.
(Reason 3) As the microwave electric power becomes intense, abnormal discharge (arc discharge) occurs in the coupling holes locally causing thermal damage to the dielectric windows 50 fitted to the edges of the coupling holes and, eventually, causing the dielectric windows 50 to be broken.
(Reason 4) Heat of the waveguides 20, 30 and 40 and heat of the rectangular discharge chamber 12 are removed by cooling water, etc. In the dielectric windows 50, therefore, the portions where the waveguides 20, 30, 40 are contacted to the rectangular discharge chamber 12 radiate heat well and assume a low temperature. However, at portions of the dielectrics just under the coupling holes, the microwaves are concentrated and the temperature is high. Accordingly, the dielectric windows 50 develop a large temperature gradient and are thermally broken.
Patent document 2, on the other hand, discloses a microwave plasma-treating apparatus in which a gap is formed between the dielectric windows and the microwave coupling holes, and the air is flown through the gap; i.e., even when microwaves of large power is fed to conduct a high-speed processing, damage to the dielectric windows is prevented owing to the supply of a small amount of cooling gas.
In this plasma apparatus, the air is introduced to effect the cooling by the air itself, and heat is transferred in increased amounts from the dielectric windows to the metal walls surrounding the gap due to the flow of air into the gap of a small area. Therefore, the dielectric windows heated by the generation of plasma can be effectively cooled, and the above problems (reason 1) and (reason 4) can be solved to some extent.
However, in conducting the operation with large electric power for extended periods of time, even if the dielectric windows are cooled, the temperature gradient increases in the dielectric windows due to local concentrations of the microwave electric power and heat, and the dielectric windows can be cracked.
The present invention was accomplished in view of the above-mentioned circumstances, and its object is to provide a microwave plasma-treating apparatus which is capable of generating plasma having a high degree of uniformity of density and a high density for executing a large-quantity and high-speed processing, capable of generating plasma of a large area, and is still capable of preventing the dielectric windows from being thermally broken despite the apparatus is operated with large electric power for extended periods of time.
In order to achieve the above object, the present inventors have studied the causes of breakage to the dielectric windows and have learned the following.
When the microwave plasma-treating apparatus is operated with large electric power for extended periods of time, the dielectric windows are quickly and markedly heated at positions near the portions where plasma intensely generated by the microwave electric power. Therefore, the conventional dielectric windows provided for every microwave coupling hole as shown in
From another point of view, another cause of damage to the dielectric windows stems from that in the plasma-treating apparatus, the outer walls of the waveguides and the discharge chamber are radiating heat due to natural air-cooling or water-cooling. In the conventional circular or square coupling holes formed in the lower surfaces of the waveguides, on the other hand, the metal plate around the coupling holes is locally very highly heated due to the concentration of microwaves. Therefore, if the dielectric windows are in contact with the metal portions that are surrounding them, vicinities of the coupling holes are locally very highly heated while other portions radiate the heat. As a result, the temperature gradient becomes great in the dielectrics, and thermal stress generates causing the dielectric windows to be broken.
From another point of view, a further cause of damage to the dielectric windows stems from that when microwaves of large electric power is fed, microwaves are intensely concentrated in the conventional coupling holes of the circular shape or the square shape, abnormal discharge (arc discharge) occurs in the coupling holes, and the dielectric windows fitted to the edges of the coupling holes are locally damaged by heat. As a result, the dielectric windows air-tightly sealing the interior of the discharge chamber to be vacuum can no longer withstand the pressure of the atmosphere and can be broken.
The present inventors, after a great deal of consideration based on the above knowledge, discovered that the heat gradient can be decreased by using dielectric windows of a piece of plate capable of transmitting microwaves arranged in the waveguide in the axial direction thereof under the microwave coupling holes. In order to spatially and widely radiate the microwaves to relax the local concentration of microwaves, further, the inventors have discovered that the microwave coupling holes are better formed in an annular shape. The inventors have, further, discovered that it is better to provide a gap so that the dielectric windows do not come in contact with the metallic wall, and to provide a cooling member for flowing a cooling gas by suitably forming a gas feed port and an exhaust port in the wall of the waveguide or the metallic wall surrounding the dielectric windows.
The present invention was completed based on the above knowledge, and provides a plasma-treating apparatus which generates plasma in a treating chamber by feeding microwave electric power thereto, which, according to the invention of claim 1, includes
a waveguide arranged to feed microwave electric power,
a plurality of microwave coupling holes formed in the waveguide in the axial direction thereof,
a dielectric member of a piece of plate capable of transmitting microwaves arranged in the waveguide in the axial direction thereof under the microwave coupling holes, and
a cooling member for cooling the dielectric member.
The invention of claim 2 includes:
a plurality of waveguides arranged in parallel maintaining a predetermined distance to feed microwave electric power,
a plurality of microwave coupling holes formed in the waveguides in the axial direction thereof,
dielectric members of a piece of plate capable of transmitting microwaves arranged in the waveguides in the axial direction thereof under the plurality of microwave coupling holes and having a width nearly equal to the width for introducing microwaves of the waveguides, and
a cooling member for cooling the dielectric members.
The invention of claim 3 relates to the microwave plasma-treating apparatus of claim 1 or 2, wherein the plurality of microwave coupling holes have an annular shape.
The invention of claim 4 relates to the microwave plasma-treating apparatus of claim 3, wherein the plurality of microwave coupling holes are so adjusted that their areas and/or their distance of arrangement work to control the spatial distribution of plasma density near the dielectric members to thereby relax the temperature gradient of the dielectric members in the axial direction of the waveguides and to increase the degree of uniformity of plasma density in the treating apparatus.
The invention of claim 5 relates to the microwave plasma-treating apparatus of claim 3 or 4, wherein the plurality of microwave coupling holes are arranged maintaining a distance of a half wavelength of standing waves of the microwaves formed in the waveguides.
In the invention of claim 6, the cooling member flows a cooling gas into a gap formed between the plurality of microwave coupling holes and the dielectric members.
The invention of claim 7 relates to the microwave plasma-treating apparatus of claim 1 or 2, wherein a gap is formed surrounding the dielectric members, and a metal mesh is provided in the gap.
The present invention greatly suppresses the occurrence of problems that, so far, caused dielectric windows to be thermally broken during the operation with large electric power, i.e., greatly suppresses the occurrence of problems such as local and abnormal electric discharge, overheating of the dielectric windows and large temperature gradient in the dielectric windows, and makes it possible to provide a microwave plasma apparatus that can conduct the operation with large electric power. Therefore, there is provided a plasma-treating apparatus featuring high degree of uniformity of density, high density and large area to realize a large-quantity and high-speed processing.
Embodiments of the invention will now be described with reference to the drawings.
The apparatus shown in
Microwaves introduced from a power source into a waveguide 20 are reflected by an end of the waveguide 20 and form standing waves in the waveguide 20. The microwaves are introduced into a discharge chamber 12 passing through the microwave coupling holes 71a, 71b, 71c, 71d, 71e formed in the lower surface of the waveguide 20, through the gap 77 and through the dielectric windows 72, and forms plasma near the dielectric windows 72. The formed plasma spreads over the whole surfaces of the dielectric windows 72 as the surface waves propagate. Based on this principle, the plasma having a uniform distribution of density, forms on each of the dielectric windows.
The plasma generated near the dielectric windows diffuses and spreads over the whole treating chamber 16. Therefore, the plasma having a high degree of uniformity of density, high density and large area is fed onto the upper surface of the substrate 13.
One of the features of the invention is to use a piece of dielectric windows 72 for a plurality of coupling holes formed in the waveguide in the axial direction thereof.
This makes it possible to relax the temperature gradient in the axial direction of the waveguide 20, i.e., in the axial direction of the dielectric windows 72. With this structure, further, a cooling gas flows well into the gap 77 formed between the dielectric windows 72 and the coupling holes 71 enabling the heat of the dielectric windows to be easily radiated and the cooling efficiency to be improved. As a result, it is successful in greatly decreasing the thermal breakage to the dielectric windows 72.
Moreover, it is possible to improve the problem of non-uniformity in the plasma density caused by the dielectric windows in the waveguide in the axial direction thereof and by the metal frame relative to the dielectric windows inherent in the prior art (patent document 1). That is, the uniformity of plasma density is improved as the surface waves spread over the whole surfaces of the dielectric windows along the boundaries of the dielectric windows in the plasma.
Another feature of the present invention is use of coupling holes of an annular shape as the microwave coupling holes.
It is difficult to form a completely annular shape or, in other words, island. Therefore, in practice the shape of the coupling hole is produced being divided into a plurality of holes of not less than two. As a concrete example,
In the present invention, the microwave coupling holes are formed in an annular shape making it possible to uniformly radiate the microwave electric field spatially in all directions as compared to the circular or square coupling holes in the conventional microwave-treating apparatus. As a result, it is made possible to prevent the microwave electric field from being locally concentrated even in the operation with large electric power, and the microwave coupling holes and the dielectric windows are not damaged by abnormal discharge (arc discharge).
Further, the plurality of microwave coupling holes are so adjusted that their distance of arrangement work to control the spatial distribution of plasma density near the dielectric members to thereby relax the temperature gradient of the dielectric members in the axial direction of the waveguides and, therefore, to increase the degree of uniformity of plasma density in the treating apparatus.
For example, the ring-shaped coupling holes 71a, 71b, 71c, 71d and 71e are arranged maintaining a distance of a half wavelength of the standing waves of the microwaves formed in the waveguide 20. Namely, the center of ring of each coupling hole is arranged at a position where the electric field of the standing wave becomes a peak so that the ring-shaped coupling hole cuts the surface current flowing into the lower surface of the waveguide in a vertical direction.
Further, the areas of the plurality of coupling holes 71a, 71b, 71c, 71d and 71e formed in the waveguide in the axial direction thereof may all be the same but may be varied depending on the operating conditions (kind of gas, pressure, microwave electric power, etc.) of the plasma-treating apparatus to vary the coupling coefficients between the microwave electric power and the plasma in the coupling holes and to control the spatial distribution of plasma density near the dielectric windows 72. This makes it possible to improve the uniformity of plasma density in the treating chamber 16. The areas of the coupling holes can be easily controlled by controlling the outer diameters or the inner diameters (or both diameters) of the rings.
Table 1 shows examples of combinations of the coupling holes. For the microwave frequency of 2.45 GHz, the half wavelength is 80 mm in a rectangular waveguide of which the interior is a cavity (specific inductivity is assumed to be 1) having a width of 96 mm and a height of 27 mm in cross section. Therefore, the outer diameters and inner diameters of the coupling holes shown in Table have been determined by taking these values into consideration.
Combination types I to VI of the microwave coupling holes shown in Table 1 are schematically shown in
The cooling gas is fed by the fan motor 73 into the waveguide 20 passing through the cooling gas feed hole 75, and partly flows in the waveguide 20, partly introduced into the gap 77 through the coupling holes and flows therein in the axial direction of the waveguide 20, and is exhausted to the exterior by the exhaust fan motor passing through the cooling gas exhaust hole 76.
Therefore, the interior of the waveguide 20 and the heat of the dielectric windows 72 can be forcibly cooled preventing the dielectric windows from being thermally broken even during the operation with large electric power. The air, nitrogen or neon can be used as the cooling gas. As a mechanism for blowing the cooling gas into the waveguide, further, there can be used an air compressor or the like instead of the fan motor.
In the present invention, the gap 77 formed between the dielectric windows 72 and the coupling holes 71 also plays the role of a heat-insulating layer between the waveguide 20 and the dielectric windows 72 in addition to the role of the cooling gas flow passage. Further, a gap of 2 mm is provided on the side surfaces of the dielectric windows 72 so will not to come in direct contact with the discharge chamber. This relaxes the temperature gradient of the dielectric windows 72 and, therefore, greatly suppresses the thermal breakage to the dielectric windows.
In the following embodiment, the gap 77 has a height of 2 mm which, however, is better suitably determined by taking the flow of the cooling gas and the mode of electric field of microwaves into consideration.
In the drawings, reference numeral 10 denotes a substrate stage, 13 denotes a substrate, 14 denotes a gas introduction port, 15 denotes a gas exhaust port, 16 denotes a treating chamber, 20 denotes a waveguide, 22 denotes a cylindrical discharge chamber, 73 denotes a fan motor, 75 denotes a cooling gas feed hole, 76 denotes a cooling gas exhaust hole, 87 denotes a gap, 81a to 81d denote coupling holes, 82 denotes circular dielectric windows and 88 denotes dielectric window covers.
The invention will be more concretely described by experiments using the apparatus shown in
The microwave plasma-treating apparatus 70 includes a metallic rectangular discharge chamber 12 with its upper end open, four rectangular dielectric windows (e.g., quartz, alumina, etc.) 72 capable of transmitting microwaves being air-tightly attached to the upper end of the rectangular discharge chamber 12 and arranged in parallel maintaining a predetermined distance, four rectangular waveguides 20 mounted thereon being arranged in parallel maintaining a predetermined distance serving as a microwave launcher, five ring-shaped coupling holes 71a, 71b, 71c, 71d and 71e capable of transmitting microwaves formed in the lower surfaces of each of the rectangular waveguides 20, and a gap 77 formed between the dielectric windows 72 and the ring-shaped coupling holes 71, 71b, 71c, 71d and 71e. The four waveguides are arranged so that the microwave introduction ports thereof are headed in the same direction but may also be arranged in a zigzag manner as shown in
On the upper surface of each rectangular waveguide 20, there are provided a cooling gas feed port 75 mounted on the coupling hole provided at one end portion and constituted by a plurality of small holes, a cooling gas exhaust hole 76 mounted on the coupling hole provided at the other end portion and constituted by a plurality of small holes, a fan motor 73 mounted on the upper part of the cooling gas feed hole 75 and serving as a mechanism for blowing a cooling gas into the waveguide, and a fan motor 73 mounted on the upper part of the cooling gas exhaust hole 76 and serving as a mechanism for blowing out the cooling gas from the waveguide.
The rectangular discharge chamber 12 has a gas exhaust port 15 for setting the interior of the rectangular discharge chamber 12 to a desired degree of vacuum, a gas introduction port 14 for introducing a required reaction gas, a substrate 13, and a substrate stage 10 for placing the substrate 13 thereon. The substrate stage 10 is reciprocally moved in a direction perpendicular to the axial direction of the waveguide. This improves the uniformity for treating the substrate with plasma.
Experiment 1: Experiment for examining changes in the plasma density depending upon the arrangement types of five ring-shaped coupling holes having different sizes (outer diameters and inner diameters).
Spatial distributions of electron density of plasma were measured by using the combination types I, II, III and IV of coupling holes shown in Table 1. The electron density was measured using a double probe method. Measured were the horizontal distributions of electron density relative to the axial direction of the waveguide at a position 10 mm away from the dielectric window.
(a) Gas: hydrogen
(c) Only one microwave launcher was operated.
Experiment 2: Experiment for synthesizing nanocrystalline diamond films by using the plasma-treating apparatus of the invention with large electric power.
(1) Conditions concerning the constitution of the apparatus.
The plasma-treating apparatus was constituted by applying the present invention.
Where, the gap between the dielectric windows and the lower surfaces of the waveguides was 2 mm. The coupling holes of the combination type IV were employed.
(a) Gas: hydrogen (90%)/methane (5%)/carbon dioxide gas (5%)
(c) Substrate: silicon wafer
(d) Position where the substrate is placed: 100 mm away from the dielectric windows
In this experiment, the operation was continued for an extended period of time (10 hours or longer) by feeding the microwave electric power of 5 kW to each waveguide, i.e., by feeding the microwave electric power of a total of 20 kW. However, no dielectric window was thermally broken. As a result of experiment, a thin uniform film of nanocrystalline diamond was successfully synthesized on the surface of the substrate of an area of 300 mm×600 mm. When a thin film was synthesized with the microwave electric power of 3 kW in a customary manner, the rate of growth of the thin film was 20 nm/hour which according to the invention was markedly increased to 300 nm/hour.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-087100 | Mar 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/056285 | 3/27/2009 | WO | 00 | 12/22/2010 |
