Plasma Generator

Abstract

[Object] To stably generate plasma at atmospheric pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the structure of a plasma generation apparatus of a particular example according to the present invention.

FIG. 2 is a graph showing the measurement results of optical absorption properties which identifies the temperature of plasma generated in the apparatus.

FIG. 3 is a graph showing the measurement results of a plasma temperature with time from the application of microwave in the apparatus.

FIG. 4 is a graph showing the measurement results of a plasma temperature vs the duty ratio of microwave in the apparatus.

FIG. 5 is a graph showing the measurement results of a plasma temperature vs an electric power of microwave in the apparatus.

FIG. 6 is a graph showing the measurement results of an electrode temperature vs an electric power of microwave in the apparatus.

FIG. 7 is a view showing the structure of a plasma generation apparatus of another particular example according to the present invention.

REFERENCE NUMERALS

10 casing

11 bottom surface

20 electrode

30 hole

60 exhaust chamber

110 casing

300 hole

320 insulating film

120 bottom plate

410 susceptor

420 semiconductor substrate

A minute gap

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the present invention will be described. Embodiments will be described in a particular manner in order to facilitate the understanding of the inventive concept, and hence it is not to be understood that the present invention is limited to the following embodiments.

Example 1

FIG. 1 shows an example of a plasma generation apparatus used for decomposition and synthesis of a CF₄ gas. A tubular casing 10 is formed of copper, and for a bottom surface 11 thereof, an electrode 20 composed of a disc-shaped conductor is provided. A circular hole 30 having a radius of 8 mm is provided in the central portion of the disc-shaped electrode 20. The side-surface cross-section of the electrode 20 is formed to have a taper so that the diameter of the hole 30 is decreased in the outside direction (in the x-axis direction).

An outer surface 20a, an inner surface 20b and a side surface 20c of this electrode 20 are covered with an insulating film 22 composed of Al₂O₃ having a thickness of 150 μm. In addition, the electrode 20 is formed so that cooling water is supplied therein for circulation and reaches the portion forming the hole 30 at the front end, that is, is formed so as to cool the hole 30 of the electrode 20.

A central conductor 40 is provided along the central axis of the casing 10 and is located at the center of the hole 30, and a front end surface 41 of the central conductor 40 is disposed at the same height (the same x axis coordinate) as that of the outer surface 20a of the electrode 20. In addition, an outer surface of a front end portion of the central conductor 40 is covered with an insulating film 42 of Al₂O₃ having a thickness of 150 μm. In this configuration, a minute gap A is formed between a circular contour 23 of a front end portion which forms the hole 30 of the electrode 20 and a circular contour 43 of the front end surface 41 (bottom surface) of the central conductor 40. The width of the minute gap A is in the range of 0.1 to 0.2 mm. In the space inside this central conductor 40 including a front end of the space, cooling water is circulated so as to cool the front end portion and the front end surface 41 of the central conductor 40.

In addition, on the casing 10, a waveguide 50 for guiding a microwave into the casing 10 is provided, and the microwave guided by this waveguide 50 is converted from a waveguide mode to a coaxial mode by a mode converter 52 and is then transmitted to the minute gap A side. The casing 10 and the electrode 20 are both grounded. The microwave supplied by the structure as described above is concentrated at the minute gap A, and as a result, the electric field density at the minute gap A is maximized.

In the side surface of the casing 10, a gas inlet 12 is provided, and from this gas inlet 12, a gas for generating plasma is supplied. In this example, a He gas was used. In the other side surface of the casing 10, a gas induction port 13 is provided, and from this gas induction port 13, a fluorocarbon gas is introduced. In this example, a CF₄ gas was used.

Under the electrode 20, an exhaust chamber 60 is provided and is formed so that gases flowing through the gas inlets 12 and 13 are made to pass through the minute gap A by evacuation from an exhaust hole 61. In addition, a transport device 62, which collects generated particles and transports them outside the exhaust chamber 60, is provided in the exhaust chamber 60 and under the minute gap A. The transport device 62 is formed so that particles are transported in the direction perpendicular to the plane of FIG. 1 (z axis direction) and are recovered from the exhaust chamber 60.

The apparatus described above was operated as described below. Cooling water was circulated inside the central conductor 40 and inside the electrode 20. Next, from the waveguide 50, a microwave was supplied having a frequency of 2.45 GHz, a peak electric power of 300 W, a pulse repetition frequency of 10 kHz, and a duty ratio of 50%. The pressure inside the casing 10 was 1 atom, and the exhaust amount from the exhaust port 61 was controlled so as to introduce a He gas at a flow rate of 2 L/min into the casing 10 from the gas inlet port 12. Under the conditions described above, He plasma was stably generated at the minute gap A. Next, the exhaust amount from the exhaust port 61 was controlled so as to introduce a CF₄ gas at a flow rate of 2 L/min into the casing 10 from the inlet port 13. As a result, at the minute gap A, by decomposition of CF₄ and polymerization reaction, particles of polytetrafluoroethylene were generated, then fell on the transport device 62, and were accumulated. In this step, the generation of carbon dioxide was not observed. The decomposition rate of CF₄ was 80% or more.

Example 2

Next, by using the above apparatus, an Ar gas and a N₂ gas were used instead of a He gas. Since a He gas is expensive, when an Ar gas and a N₂ gas can be used, significant industrial advantages can be obtained. Hence, first of all, by using an apparatus in which the insulating film 22 and the insulating film 42 are not formed on the metal electrode 20 and the central conductor 40, respectively, experiments were each carried out by continuous supply of a microwave having an electric power of 200 W. However, the electrode 20 and the central conductor 40 were cooled by circulating cooling water, and the pressure was set to atmospheric pressure. Three experiments, that is, an experiment in which a He gas was supplied at a flow rate of 2 L/min, an experiment in which an Ar gas was supplied at a flow rate of 2 L/min, and an experiment in which a N₂ gas was supplied at a flow rate of 2 L/min, were carried out. As a result, in the case of a He gas, the generation of stable plasma was observed, and on the other hand, in the cases of an Ar gas and a N₂ gas, it was difficult to uniformly generate stable plasma at the ring-shaped minute gap A.

Next, an apparatus was used in which the insulating film 22 was formed on the side surface, the outer surface, and the inner surface of the metal electrode 20, and the insulating film 42 was formed on the surface of the front end portion of the central conductor 40. Next, as described above, three types of gases were separately supplied at a flow rate of 2 L/min. By the three types of gases, stable plasma was observed at the ring-shaped minute gap A. In order to investigate the plasma state, a gas temperature was measured by an ICCD camera and an electrode temperature was measured by FTIR. The emission spectrum was measured by an ICCD camera, and the gas temperature of plasma was obtained from the second positive band emission. That is, the coefficient was determined so that the simulation spectrum coincides with the measured spectrum, and the rotation temperature was obtained. This rotation temperature was regarded as the plasma temperature. In the following results, the values obtained for the rotation temperature are all shown as the plasma temperature. The results are shown in FIG. 2. The plasma temperatures of a He gas, an Ar gas, and a N₂ gas were 350K, 720K, and 900K, respectively, and the relationship thereof was represented by He<Ar<N₂. While the electrode temperature and the plasma temperature are detected, they are preferably maintained constant by controlling the duty ratio of microwave using a feedback circuit.

In addition, after the insulating film 42 was not formed on the surface of the central conductor 40, and the insulating film 22 was only formed on the electrode 20 as described above, an experiment similar to that described above was performed. In this case, the results approximately similar to those described above were obtained although the stability was slightly inferior. In addition, after the insulating film 42 was formed on the surface of the central conductor 40, and the insulating film 22 was not formed on the electrode 20, an experiment similar to that described above was performed. In this case, relatively stable plasma was also observed although the stability was more degraded than that described above. Accordingly, it is most preferable that the insulating films be provided for both the central conductor 40 and the electrode 20.

Example 3

Next, the plasma temperature with time from the application of microwave was measured in a manner similar to that in Example 2. The results are shown in FIG. 3. A microwave having a frequency of 2.45 GHz and an electric power of 300 W was introduced into the casing 10 under the condition similar to that in Example 2. Subsequently, He, Ar, and N₂ gases were separately introduced, and the change in plasma temperature was separately measured. From the results shown in FIG. 3, it is understood that although the temperature increase is not observed in the cases of He and Ar, the temperature is rapidly increased in the case of N₂. From the above measurement results, the inventors of the present invention assumed that in order not to increase the plasma temperature, when a pulse wave is used as the microwave, and the cycle period and the pulse width are controlled so as to control the duty ratio, the plasma is cooled while the microwave is not applied. Accordingly, the inventors of the present invention conceived from this result that when a pulse wave is used as the microwave, and frequency control and duty control are performed, the increase in plasma temperature is suppressed, and stable plasma having a constant temperature can be obtained; hence the following experiments were carried out.

Example 4

Next, by using a microwave having a frequency of 2.45 GHz, an average electric power of 200 W, and a pulse period of 100 kHz, the plasma temperature of each gas was measured with the change of the duty ratio. The other conditions were the same as those in Example 2. The measurement results are shown in FIG. 4. One pulse having a pulse period of 100 KHz and a duty ratio of 50% indicates 5 μs after the application of the microwave in terms of time shown in FIG. 3. In particular, it is understood that the plasma temperature of N₂ is stabilized at approximately 900K. In addition, since the plasma temperature of N₂ is increased to 1,300K when the microwave is applied for 50 μs, as shown in FIG. 3, it is understood that in the case of a N₂ gas, the duty control of the microwave is significantly important in order to control the plasma temperature. In particular, in the case of N₂, since an effect of suppressing the increase in temperature is significant, the combination of the duty control of microwave and a N₂ gas is specific.

Example 5

Next, in Example 4, in the case in which the duty ratio was set to 100% (continuous electricity supply) and cooling was not performed for the electrode 20 and the central conductor 40, the plasma temperature was measured by introducing a N₂ gas. As shown in FIG. 4, when the electrode 20 and the central conductor 40 are cooled, the temperature was 900K; however, when cooling was not performed, the temperature was increased to 1,250K. From this result, it is understood that cooling of the central conductor 40 and the electrode 20 is effective for the control of the plasma temperature. In particular, in the case of N₂, since an effect of suppressing the increase in temperature is significant, the combination of the duty control of microwave and a N₂ gas is specific. In addition, the cooling structure of the electrode, the duty control of microwave, and the coating of the minute gap portion with the insulating film are particularly effective to control the plasma temperature, and hence these three elements form a specific combination.

Example 6

According to Example 4, it is understood that water cooling of the central conductor 40 and the electrode 20 is effective for the control of the plasma temperature; hence, for further investigation, the relationship between the plasma temperature and the temperatures (when the temperatures of the two described below are not necessarily discriminated from each other, the temperatures are simply referred to as “electrode temperature”) of the central conductor 40 and the electrode 20 (when the above two are not necessarily discriminated from each other, they are simply referred to as “electrode”) was measured. However, in this experiment, gases were not made to flow and were enclosed in a closed space. That is, the experiment was performed while the exhaust chamber 60 shown in FIG. 1 was insolated from the outside. A microwave was continuously supplied under the conditions in which a He gas was enclosed in a chamber (formed by the casing 10 and the exhaust chamber 60) at 1 atmosphere. The plasma temperature and the electrode temperature were measured with the change in electric power of the microwave. The plasma temperature is shown in FIG. 5, and the electrode temperature is shown in FIG. 6. Three measurements were performed in which cooling was performed at a water temperature of 280K and 300K, and cooling was not performed at all, and it is understood that even when the electric power of the microwave is changed, the plasma temperature well coincides with the electrode temperature. In addition, it is understood that when the electrode is cooled, the plasma temperature is decreased by 200K or more as compared to that obtained when the electrode is not cooled. The reason the plasma temperature coincides with the electrode temperature even when the electrode is not cooled is believed that the increase in plasma temperature of a He gas by electric power of the microwave is relatively small. From these measurement results, it is understood that the cooling of the electrode is significantly effective to control the plasma temperature.

Example 7

The plasma generation apparatus can be designed to have the structure shown in FIG. 7. A resonator is composed of a casing 110 formed from a tubular conductor having a diameter of 100 mm and a bottom plate 120 formed from a conductor. In the central portion of the bottom plate 120, a rectangular hole (slit) 300 having a width of 0.1 to 0.2 mm and a length of 30 mm is formed. This hole 300 has a tapered cross-section as shown in the figure. Cooling water 122 is circulated inside the bottom plate 120 including a part thereof forming the hole 300. The cooling water 122 is circulated and reaches the tapered side wall forming the hole 300. In addition, an insulating film 320 is formed on an outer surface 120a, an inner surface 120b, and a side surface 120c of the bottom plate 120. A material for the insulating film is similar to that described in the above example. The upper end surface of the casing 110 is sealed by a quartz plate 130 so that a gas introduced into the casing 110 is not allowed to flow backward. The microwave passes through this quartz plate 130 and is then introduced inside the casing 110 which is the resonator, and the power density is increased at the minute gap A formed by the hole 300 provided in the bottom plate 120. A NF₃ gas and a He gas passing through H₂O are introduced into the casing 110 via a gas inlet 125 and reach the minute gap A.

In this step, by the electric power of the microwave, He plasma is generated at the minute gap A portion, NF₃ and H₂O are decomposed, and F radicals, H radicals, OH radicals, F ions, F₂ molecules, HF molecules, and the like are generated. By the radicals and the like, a semiconductor substrate placed on a rotary susceptor 410 provided under the hole 300 is etched. The plasma thus generated is observed by an absorption spectroscopy using laser and is controlled to be placed in a most preferable state.

As is the case of the above examples, for example, the microwave may be a continuous wave or a pulse wave, and in the case of a pulse wave, the plasma temperature can be controlled by the cycle period and the duty ratio of the pulse.

Application Fields of the Invention

The present invention provides an apparatus stably generating plasma. In particular, the apparatus can be advantageously used at atmospheric pressure. In semiconductor etching, film-forming process, machining, cleaning, surface reforming, and the like, which use plasma, it is not necessary to evacuate a process chamber, and hence this apparatus is particularly advantageous. Since the electron density of atmospheric plasma is approximately 10¹⁵/cm³, which is approximately 3 orders of magnitude larger than that of low-pressure high density plasma, high-density radicals and ions can be generated, and hence a high rate process can be performed. In addition, since gases can be decomposed and polymerized by plasma, the recovery of exhaust gas in the form of particles and the formation of a fluorocarbon gas and radicals thereof from graphite and a F₂ gas can be advantageously performed.

INDUSTRIAL APPLICABILITY

According to the present invention, plasma effectively used for semiconductor processes and the like can be stably supplied. Hence, in a semiconductor device manufacturing plant, this technique is significantly effective.

As has thus been described, since individual constituent elements can be separated and extracted, when extracted constituent elements are independently used in combination, one aspect of the invention may be formed. When an optional constituent element disclosed in Claims is eliminated, one aspect of the present invention may also be formed.

Claims

1. A plasma generation apparatus comprising: an electrode composed of a conductor forming a minute gap which allows a gas generating plasma to pass therethrough and which increases an electron density of a guided microwave, wherein an insulating film is formed on a surface of at least a portion of the electrode, which forms the minute gap.

2. The plasma generation apparatus according to claim 1, further comprising: a casing composed of a conductor into which the microwave is introduced; and a bottom plate composed of a conductor which performs electromagnetic shielding at an end face of the casing opposite to that at which the microwave is introduced, wherein the minute gap is formed in the bottom plate.

3. The plasma generation apparatus according to claim 1, further comprising: a casing composed of a conductor into which the microwave is introduced; and a bottom plate composed of a conductor which performs electromagnetic shielding at an end face of the casing opposite to that at which the microwave is introduced, wherein the bottom plate is provided with a window, and the electrode is disposed at the bottom plate so as to close the window, whereby the minute gap is formed.

4. The plasma generation apparatus according to claim 1, wherein the electrode has a structure in which the electrode including a portion forming the minute gap is cooled from the inside of the electrode by a cooling medium.

5. A plasma generation apparatus comprising: a tubular casing into which a gas and a microwave are introduced;a hole provided in a bottom surface of the casing;a columnar conductor provided in an axis direction of the casing and having a bottom surface contour inside a contour of the hole;a minute gap formed between the contour of the bottom surface of the conductor and the contour of the hole;a coaxial waveguide formed by the conductor and the casing; andan insulating film formed at least on a contour portion forming the hole which forms the minute gap,wherein the microwave is introduced into the minute gap by the coaxial waveguide, and the gas is allowed to pass through the minute gap, whereby the gas is placed in a plasma state at the minute gap.

6. The plasma generation apparatus according to claim 5, further comprising an insulating film which is formed at least on a portion, which forms the minute gap, of the conductor.

7. The plasma generation apparatus according to claim 1, wherein the bottom surface of the conductor is cooled from the inside thereof.

8. The plasma generation apparatus according to claim 1, wherein a hole portion of the bottom surface of the casing is cooled.

9. The plasma generation apparatus according to claim 1, wherein the microwave is applied in the form of periodic pulses.

10. The plasma generation apparatus according to claim 1, wherein the plasma is plasma of argon gas or plasma of nitrogen gas.

11. The plasma generation apparatus according to claim 2, further comprising: a casing composed of a conductor into which the microwave is introduced; and a bottom plate composed of a conductor which performs electromagnetic shielding at an end face of the casing opposite to that at which the microwave is introduced, wherein the bottom plate is provided with a window, and the electrode is disposed at the bottom plate so as to close the window, whereby the minute gap is formed.

12. The plasma generation apparatus according to claim 2, wherein the electrode has a structure in which the electrode including a portion forming the minute gap is cooled from the inside of the electrode by a cooling medium.

13. The plasma generation apparatus according to claim 3, wherein the electrode has a structure in which the electrode including a portion forming the minute gap is cooled from the inside of the electrode by a cooling medium.

14. The plasma generation apparatus according to claim 2, wherein the bottom surface of the conductor is cooled from the inside thereof.

15. The plasma generation apparatus according to claim 3, wherein the bottom surface of the conductor is cooled from the inside thereof.

16. The plasma generation apparatus according to claim 4, wherein the bottom surface of the conductor is cooled from the inside thereof.

17. The plasma generation apparatus according to claim 5, wherein the bottom surface of the conductor is cooled from the inside thereof.

18. The plasma generation apparatus according to claim 6, wherein the bottom surface of the conductor is cooled from the inside thereof.

19. The plasma generation apparatus according to claim 6, wherein a hole portion of the bottom surface of the casing is cooled.

20. The plasma generation apparatus according to claim 7, wherein a hole portion of the bottom surface of the casing is cooled.