Plasma treating method and apparatus therefor

What is claimed is:

1. A plasma treating method comprising the steps of:

rendering a gas having a critical potential plasmic under a reduced pressure, said critical potential being a potential at which an etching action and a deposition action are in equilibrium with each other when a sample is treated with the plasma which is generated by rendering a certain kind of gas plasmic under a predetermined plasma condition and applying an acceleration voltage to ions in said plasma so as to let them be incident to said sample; and

changing said acceleration voltage for accelerating said ions in said plasma towards said sample interposing said critical potential.

2. A plasma treating method according to claim 1, wherein said acceleration voltage is provided by the combination of a D.C voltage and a radio frequency voltage with a frequency having a half cycle time within the time required by said ions in said plasma to pass through an ion sheath.

3. A plasma treating method according to claim 1, wherein said acceleration voltage is provided by a radio frequency voltage with a frequency having a half cycle time from 100 KHz to above the time required by said ions in said plasma to pass through an ion sheath.

4. A plasma treating method according to claim 1, wherein said acceleration voltage is provided by the combination of an A.C. voltage with a low frequency and a radio frequency voltage with a frequency having a half cycle time within the time required by said ions in said plasma to pass through an ion sheath.

5. A plasma treating method according to claim 1, wherein said acceleration voltage is provided by a D.C. voltage.

6. A plasma treating method according to claim 1, wherein said acceleration voltage is a voltage to be applied to a sample table for said sample or a voltage to be applied to a grid electrode disposed between said sample and said plasma.

7. A plasma treating method according to claim 1, wherein said acceleration voltage is changed in such a manner as to interpose said critical potential and is changed, too, during the etching operation.

8. A plasma treating method comprising the steps of:

rending a gas having a critical potential plasmic under a reduced pressure, said critical potential being a potential at which an etching action and a deposition action are in equilibrium with each other when a sample is treated with the plasma which is generated by rendering a certain kind of gas plasmic under a predetermined plasma condition and applying an acceleration voltage to ions in said plasma so as to let them be incident to said sample; and

changing said acceleration voltage for accelerating ions in said plasma towards said sample interposing said critical potential and stepwise etching said sample in the direction of depth by carrying out alternately said etching action and said deposition action.

9. A plasma treating method according to claim 8, wherein said etching action takes place within the time at which undercut occurring on said sample is within a range of allowable values and said deposition action takes place within the time at which a film can be formed to a film thickness sufficient to act as a protective film on side surfaces of said sample.

10. A plasma treating method according to claim 8, wherein said gas is either a single gas of C.sub.2 Cl.sub.3 F.sub.3, C.sub.2 Cl.sub.4 F.sub.2, CCl.sub.4 or C.sub.4 F.sub.8 or its mixed gas with SF.sub.6 or NF.sub.3 when said sample is polysilicon, and said gas is either a single gas of CCl.sub.4, CF.sub.4, C.sub.2 F.sub.6, C.sub.4 F.sub.8 or SiCl.sub.4 or its mixed gas with Cl.sub.2 when said sample is aluminum.

11. A plasma treating apparatus comprising:

means for rending a gas having a critical potential plasmic under a reduced pressure, said critical potential being a potential at which an etching action and a deposition action are in equilibrium with each other when a sample is treated with the plasma which is generated by rendering a certain kind of gas plasmic under a predetermined plasma condition and applying an acceleration voltage to ions in said plasma so as to let them be incident to said sample; and

means for changing said acceleration voltage for accelerating said ions in said plasma towards said sample interposing said critical potential.

12. A plasma treating apparatus according to claim 11, wherein said acceleration voltage is provided by a D.C. power source and a radio frequency power source with a frequency having a half cycle time within the time required by said ions in said plasma to pass through an ion sheath.

13. A plasma treating apparatus according to claim 11, wherein said acceleration voltage is provided by a radio frequency power source with a frequency having a half cycle time from 100 KHz to above the time required by said ions in said plasma to pass through an ion sheath.

14. A plasma treating apparatus according to claim 11, wherein said acceleration voltage is provided by an A.C. power supply with a low frequency and a radio frequency power source with a frequency having a half cycle time within the time required by said ions in said plasma to pass through an ion sheath.

15. A plasma treating apparatus according to claim 11, wherein said acceleration voltage is provided by a D.C. power source.

16. A plasma treating apparatus according to claim 11, wherein said acceleration voltage is provided by a power source applied to a sample table of said sample or a power source applied to a grid electrode disposed between said sample and said plasma.

17. A plasma treating apparatus according to claim 11, wherein said acceleration voltage is changed in such a manner as to interpose said critical potential and is changed, too, during the etching operation.

18. A plasma treating apparatus comprising:

means for rendering a gas having a critical potential plasmic under a reduced pressure, said critical potential being a potential at which an etching action and a deposition action are in equilibrium with each other when a sample is treated with the plasma which is generated by rendering a certain kind of gas plasmic under a predetermined plasma condition and applying an acceleration voltage to ions in said plasma so as to let them be incident to said sample; and

means for changing said acceleration voltage for accelerating ions in said plasma towards said sample interposing said critical potential and carrying out alternately said etching action and said deposition action.

19. A plasma treating apparatus according to claim 18, wherein said gas is either a single gas of C.sub.2 Cl.sub.3 F.sub.3, C.sub.2 Cl.sub.4 F.sub.2, CCl.sub.4 or C.sub.4 F.sub.8 or its mixed gas with SF.sub.6 or NF.sub.3 when said sample is polysilicon, and said gas is either a single gas of CCl.sub.4, CF.sub.4, C.sub.2 F.sub.6, C.sub.4 F.sub.8 or SiCl.sub.4 or its mixed gas with Cl.sub.2 when said sample is aluminum.

20. A plasma treating apparatus comprising:

means for rendering a gas having a critical potential plasmic under a reduced pressure by use of the action of an electric field by microwaves and the action of a magnetic field by magnetic field generation means, said critical potential being a potential at which an etching action and a deposition action are in equilibrium with each other when a sample is treated with the plasma which is generated by rendering a certain kind of gas plasmic under a predetermined plasma condition and applying an acceleration voltage to ions in said plasma so as to let thm be incident to said sample;

means for applying said acceleration voltage to a sample table on which said sample is placed; and

means for changing said acceleration voltage interposing said critical potential.

21. A plasma treating apparatus comprising:

a vacuum treating vessel having a discharge tube disposed at an upper opening thereof;

a discharge space within said discharge tube;

a waveguide disposed so as to encircle said discharge tube, said waveguide including a magnetron for generating microwaves;

an electromagnetic coil disposed around the outer periphery of said discharge tube through said waveguide;

an electrode including a sample table disposed within said vacuum treating vessel so as to allow support of a sample to be treated in said discharge space;

a ground electrode disposed around the outer periphery of said electrode and electrically insulated therefrom;

means to introduce gas into said discharge space, said gas having a critical potential at which an etching action and a deposition action of said gas are in equilibrium; and

means to control an acceleration voltage connected to said electrode, for accelerating ions in a plasma of said gas, so as to alternately etch and deposit said sample.

22. A plasma treating apparatus according to claim 21, wherein said means to control an acceleration voltage, comprises:

a radio frequency power source for supplying radio frequency voltage to said electrode, connected to said electrode through a matching box;

a D.C. power source for supplying D.C. voltage to said electrode, connected to said electrode through a lower pass filter;

an output voltage controller connected to said D.C. power source; and

an output waveform controller connected to said output voltage controller.

23. A plasma treating apparatus according to claim 21, wherein said means to control an acceleration voltage, comprises:

a radio frequency power source connected to said electrode through a matching box; and

an output voltage control means connected to said radio frequency power source.

24. A plasma treating apparatus according to claim 21, wherein said means to control an acceleration voltage, comprises:

a radio frequency power source connected to said electrode through a synthesizer;

an A.C. waveform generator connected to said synthesizer; and

an output waveform controller connected to said A.C. waveform generator.

25. A plasma treating apparatus comprising:

a vacuum treating vessel having a discharge tube disposed at an upper opening thereof;

a discharge space within said discharge tube;

a waveguide disposed so as to encircle said discharge tube, said waveguide including a magnetron for generating microwaves;

an electromagnetic coil disposed around the outer periphery of said discharge tube through said waveguide;

an electrode including a sample table disposed within said vacuum treating vessel so as to allow support of a sample to be treated in said discharge space;

means to introduce gas into said discharge space, said gas having a critical potential at which at etching action and a deposition action of said gas are in equilibrium;

means to control an acceleration voltage for accelerating ions in a plasma of said gas, so as to alternately etch and deposit said sample;

said means to control acceleration voltage including a grid electrode disposed in said discharge space above said sample, D.C. power source connected to said grid electrode, an output voltage controller connected to said D.C. power source, and an output waveform controller connected to said output voltage controller.

26. A plasma treating apparatus comprising:

a vacuum treating vessel having a gas injection port;

an upper electrode disposed in an upper portion of said vacuum treating vessel;

a lower electrode for supporting a sample to be treated, disposed in a lower portion of said vacuum treating vessel so as to face said upper electrode, said lower electrode being insulated from said vacuum treating vessel;

means to introduce gas into said vacuum treating vessel through said injection port, said gas having a critical potential at which an etching action and a deposition action of said gas are in equilibrium; and

means to control an acceleration voltage connected to said lower electrode for accelerating ions in a plasma of said gas, so as to alternately etch and deposit said sample.

27. A plasma treating apparatus according to claim 26, wherein said means to control an acceleration voltage, comprises:

a radio frequency power for supplying radio frequency voltage to said lower electrode, connected to said lower electrode through a matching box;

a D.C. power source for supplying D.C. voltage to said lower electrode, connected to said lower electrode through a low-pass filter;

an output voltage controller connected to said D.C. power source; and

an output waveform controller connected to said output voltage controller.

28. A plasma treating apparatus according to claim 25, wherein said means to control an acceleration voltage, comprises:

a radio frequency power connected to said lower electrode through a matching box; and

an output voltage control means connected to said radio frequency power source.

29. A plasma treating apparatus according to claim 26, wherein said means to control an acceleration voltage, comprises:

a radio frequency power source connected to said lower electrode through a synthesizer;

an A.C. waveform generator connected to said synthesizer; and

an output waveform controller connected to said A.C. waveform generator.

30. A plasma treating apparatus according to claim 26, wherein said means to control an acceleration voltage, comprises:

a radio frequency power source connected to said lower electrode through a matching box and a capacitor; and

an output voltage control means connected to said radio frequency power source.

31. A plasma treating apparatus according to claim 26, wherein said means to introduce gas includes a discharge tube connected to said injection port and a coil wound around a periphery of said discharge tube and connected to a radio frequency power source.

32. A plasma treating apparatus comprising:

a vacuum treating vessel having a gas injection port;

an upper electrode disposed in an upper portion of said vacuum treating vessel;

a lower electrode for supporting a sample to be treated, disposed in a lower portion of said vacuum treating vessel so as to face said upper electrode, said lower electrode being insulated from said vacuum treating vessel;

means to introduce gas into said vacuum treating vessel through said injection port, said gas having a critical potential at which an etching action and a deposition action of said gas are in equilibrium; and

means to control an acceleration voltage for accelerating ions in a plasma of said gas, so as to alternately etch and deposit said sample, said means to control an acceleration voltage including a grid electrode disposed in said vacuum treating vessel above said sample, a D.C. power source connected to said grid electrode, an output voltage controller connected to said D.C. power source, an output waveform controller connected to said output voltage controller, and a radio frequency power source connected to said lower electrode through a matching box.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a plasma treating method and apparatus therefor, and more particularly to a plasma treating method and apparatus therefor which will be suitable for carrying out the treatment by effecting alternately etching and deposition.

2. Description of the Prior Art

With a progress in miniaturization of semiconductor devices, dimensional machining accuracy of circuit patterns and a low damage machining method have become more and more important. Particularly in devices of a submicron range, the device structure has become three-dimensional owing to limitation to a chip area. For this reason, it has become necessary to form a film having a large ratio of a machining depth to a machining width, that is, an aspect ratio, with high dimensional accuracy.

An example of prior art techniques for satisfying such a requirement is disclosed in Japanese Patent Laid-Open No. 50923/1985, for example. This prior art technique uses a mixed gas consisting of an SF.sub.6 gas which contributes to etching, an N.sub.2 gas which contributes to the formation of a protective film of silicon nitride and other gases as an etching gas for etching poly-Si, and changes periodically the composition and concentration of the treating gas during the etching treatment. In this manner, this prior art technique carries out etching at a high speed and with high dimensional accuracy by repeating alternately the etching step and the formation step of the silicon nitride protective film.

Since the prior art technique carries out the etching treatment by changing the gas composition and concentration, however, the condition of plasma changes whenever the gas composition and concentration are changed. When the gas composition and concentration are changed, a previous plasma condition is changed to a new plasma condition. In other words, any remaining ions and radicals must be exhausted rapidly. Since a treating vessel has a certain inner capacity, a certain period of time is necessary before the plasma condition is changed over so that the overall treating time becomes elongated. If this problem is to be somewhat alleviated, an exhaust device becomes greater in size to reduce the exhaust time. In addition, it becomes necessary to control the exhaust quantity during the treatment and the exhaust quantity at the time of change-over so that a controller and control technique become complicated.

On the other hand, a technique which changes the voltage to be impressed upon an electrode is disclosed in Japanese Patent Publication No. 41132/1986, Japanese Patent Laid-open No. 13625/1986, and the like. These prior arts carry out the plasma treatment by changing the voltage to be applied to the electrode, and merely improve plasma characteristics such as an etching rate, a selection ratio, and the like, by controlling the incidence energy of ions in the plasma.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a plasma treating method and apparatus therefor which carries out alternately the etching step and the film formation step and can shorten the plasma treating time without changing over the treating gas.

As a result of intensive studies and experiments, the inventors of the present invention have found out for the first time the following observation as the basis of the present invention. A certain kind of component gas is rendered plasmic under a predetermined plasma condition. An acceleration voltage is applied to ions inside this plasma and arranged in such a manner that the ions are incident to a sample. The present invention treats the sample by use of such a plasma. The inventors of the invention have found out that if the value of the acceleration voltage is changed in this case, there exist a voltage at which the etching action occurs preferentially and a voltage at which the deposition action occurs preferentially. It has been found also that there is a voltage at which the etching action is in equilibrium with the deposition action. Hereinafter, this equilibrium voltage will be referred to as a "critical potential".

Namely, the present invention provides an apparatus comprising means for rendering plasmic a gas having a critical potential under a reduced pressure and means for changing an acceleration voltage for accelerating ions in the plasma towards a sample while interposing the critical potential, and a treating method including a step of rendering plasmic a gas having a critical potential under a reduced pressure and a step of changing an acceleration voltage for accelerating ions in the plasma towards a sample interposing the critical potential. Thus, the present invention can shorten the plasma treating time by alternately carrying out the etching step and the film formation step without changing over the treating gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural view showing a plasma treating apparatus in accordance with one embodiment of the present invention;

FIG. 2 is a diagram showing the relationship between a bias voltage in a treating gas and an etching speed or a deposition speed;

FIG. 3 is a diagram showing the impressing pattern of an acceleration voltage in the apparatus shown in FIG. 1;

FIG. 4 is a schematic view showing the etching state when a bias voltage is not changed;

FIGS. 5 and 6 are schematic views showing the etching state in accordance with the present invention;

FIG. 7 is a structural view showing a plasma treating apparatus in accordance with the second embodiment of the present invention;

FIG. 8 is a diagram showing the impressing pattern of an acceleration voltage in the apparatus shown in FIG. 7;

FIG. 9 is a structural view showing the plasma treating apparatus in accordance with the third embodiment of the present invention;

FIG. 10 is a diagram showing the impressing pattern of an acceleration voltage in the apparatus shown in FIG. 9;

FIG. 11 is a structural view showing the plasma treating apparatus in accordance wihh the fourth embodiment of the present invention;

FIG. 12 is a diagram showing the impressing pattern of an acceleration voltage in the apparatus shown in FIG. 11;

FIG. 13 is a structural view of the plasma treating apparatus in accordance with the fifth embodiment of the present invention;

FIG. 14 is a structural view showing the plasma treating apparatus in accordance with the sixth embodiment of the present invention;

FIG. 15 is a structural view showing the plasma treating apparatus in accordance with the seventh embodiment of the present invention;

FIG. 16 is a structural view showing the plasma treating apparatus in accordance with the eighth embodiment of the present invention;

FIG. 17 is a diagram showing the impressing pattern of an acceleration voltage in the apparatus shown in FIG. 16;

FIG. 18 is a structural view showing the plasma treating apparatus in accordance with the ninth embodiment of the present invention; and

FIG. 19 is a structural view showing the plasma treating apparatus in accordacce with the tenth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First of all, the first embodiment of the present invention will be described with reference to FIGS. 1 to 6.

FIG. 1 shows a microwave plasma treating apparatus using ECR discharge, which is an etching apparatus in this case. A discharge tube 1 made of silica is disposed at an upper opening of a vacuum treating vessel 4. An exhaust portion 3 which is connected to a vacuum exhaust device, not shown, is disposed at the lower part of the vacuum treating vessel 4. An electrode 5 having a sample table 5a, on which a wafer 6 as a sample is placed, is disposed inside the vacuum treating vessel 4. A discharge space 7 is formed above the sample table 5a inside the discharge tube 1.

A waveguide 9 is disposed above the discharge tube 1 and encircles the tube 1. A magnetron 8 which generates a 2.45 GHz microwave in this case is disposed at the end portion of the waveguide 9. An electromagnetic coil 10 is disposed around the outer periphery of the discharge tube 1 through the waveguide 9.

A gas introduction portion 2 for supplying an etching gas into the discharge space 7 is disposed sideways of the vacuum treating vessel 4 and communicated with a gas source, not shown, through a mass flow controller 18.

A ground electrode 11 is disposed around the outer periphery of the electrode 5 and electrically insulated therefrom. One end of this ground electrode 11 is positioned near the sample table 5a while the other end is grounded. A radio frequency power source 13 which oscillates a 13.56 MHz radio frequency in this case is connected to the electrode 5 through a matching box 12 while a D.C. power source 15 is connected to the electrode 5 through a low-pass filter 14. The other ends of the radio frequency power source 13 and the D.C. power source 15 are each grounded. A voltage controller 16 is connected to the D.C. power source 15 and an output waveform controller 17 is connected to the output voltage controller 16. The matching box 12 consists of capacitor coupling in this case. The low-pass filter 14 is to cut off the radio frequency voltage from the radio frequency power source 13.

The mass flow controller 18 controls the etching gas from the gas source, not shown, to a predetermined flow rate and sends the etching gas into the discharge space 7. The inside of the discharge space 7 is evacuated to a reduced pressure by an exhaust device, not shown, and held at a predetermined pressure.

In this case, a means for rendering plasmic the etching gas introduced into the discharge space consists of the magnetron 8 and the electromagnetic coil 10. The etching gas inside the discharge space 7 is rendered palsmic by the ECR discharge resulting from the action of the electromagnetic field provided by the magnetron 8 and the electromagnetic coil 10.

A means for applying incident energy to the wafer 6 to the ions in the plasma, that is, a means for generating an acceleration voltage in the sample table 5a in this case, consists of the radio frequency power source 13 and the D.C. power source 15. The radio frequency voltage from the radio frequency power source 13 and the D.C. voltage from the D.C. power source 15 are applied to the sample table 5a on which the wafer 6 is placed. When the radio frequency voltage is applied to the sample table 5a through the matching box 12 consisting of the capacitor coupling, the radio frequency voltage is given D.C.-wise and floatingly so that a D.C. bias voltage occurs. This D.C. bias voltage attracts the ions in the plasma towards the sample table 5a, that is, towards the wafer 6. The wafer 6 is sputter-etched by the incidence energy of the ions at this time. Since the D.C. voltage is applied to the sample table 5a, the value of the D.C. bias voltage occurring on the sample table 5a is adjusted. In this case, this D.C. bias voltage becomes the acceleration voltage for accelerating the ions.

Furthermore, a means for changing the value of the acceleration voltage generated on the sample table 5a interposing the critical potential consists of the voltage controller 16 and the output waveform controller 17, and the output voltage controller 16 controls the D.C. voltage value of the D.C. power source 15. The output waveform controller 17 controls the timing for changing the D.C. voltage value which the output voltage controller 16 controls. The timing is controlled periodically in this case.

The wafer 6 in this case is produced by depositing a poly-Si layer that is a wiring forming material on a Si substrate. The etching apparatus of this embodiment uses a mixed gas of sulfur hexafluoride (SF.sub.6) and trichlorotrifluoroethane (C.sub.2 Cl.sub.3 F.sub.3 : "Flon - 113", tradename) as the etching gas and etches the poly-Si layer of the wafer 6.

Next, the experiment wherein both components of the etching gas described above, that is, SF.sub.6 and C.sub.2 Cl.sub.3 F.sub.3, are rendered palsmic under the same plasma formation condition (microwave power: 400 W, gas flow rate: 70 sccm, pressure: 0.01 Torr, radio frequency power: 100W) in the etching apparatus described above and the D.C. bias voltage applied to the sample table 5a is changed will be explained with reference to FIG. 2.

In the graph of FIG. 2, the etching speed is plotted at the upper part of the ordinate while the deposition speed is plotted at its lower part. The D.C. bias voltage is plotted on the abscissa.

As is obvious from FIG. 2, SF.sub.6 always causes the etching phenomenon irrespective of the magnitude of the D.C. bias voltage and the etching speed increases with an increasing D.C. bias voltage.

On the other hand, C.sub.2 Cl.sub.3 F.sub.3 causes the deposition phenomenon within the range where the D.C. bias voltage is small and the etching phenomenon within the range where D.C. bias is great. Furthermore, it can be seen from the diagram that C.sub.2 Cl.sub.3 F.sub.3 has a critical potential (Vo) exactly at its boundary where neither etching nor deposition occur.

Incidentally, the term "critical potential" used hereby means the potential at which the deposition phenomenon and the etching phenomenon reverse each other when the gas is rendered palsmic and the D.C. bias voltage is changed, and is found out for the first time through the experiments conducted by the inventors of the present invention.

This means the following: When C.sub.2 Cl.sub.3 F.sub.3 is rendered plasmic, the deposition action and the etching action occur concurrently. At this time, if the D.C. bias voltage applied to the sample table 5a is smaller than the critical potential, the deposition action occurs preferentially. If the D.C. bias voltage applied to the sample table 5a is increased within the range where it is smaller than the critical potential, the ions in the plasma are accelerated with the increase of the D.C. bias voltage, the etching action becomes gradually stronger and priority of the deposition action decays gradually. If the D.C. bias voltage increases beyond the critical potential, the ions in the plasma are further accelerated and the etching action occurs more preferentially than the deposition action so that the etching action becomes gradually stronger. If the D.C. bias voltage is equal to the critical potential, the deposition action and the etching action are in equilibrium with each other.

A similar experiment was carried out by use of a mixed gas of SF.sub.6 not having the critical potential described above and C.sub.2 Cl.sub.3 F.sub.3 having the critical potential (1: 9 mixture). The result is represented by dash line curve in FIG. 2. As is obvious from this dash line curve, the critical potential Vo' exists in this mixed gas, the deposition action occurs preferentially at a D.C. bias voltage lower than the critical potential Vo' and the etching action occurs preferentially at a D.C. bias voltage greater than the critical potential Vo'. Moreover, when this mixed gas is used, the etching speed depends more greatly on the D.C. bias voltage than when C.sub.2 Cl.sub.3 F.sub.3 is used alone, as is obvious from the diagram. It is found out therefore that this mixed gas can be used as an etchant whose etching speed has high bias voltage dependence.

Next, the etching treatment using the mixed gas having such characteristics will be described with reference to FIGS. 2 to 6.

First of all, when a poly-Si film having a high aspect ratio is etched at a bias voltage value V1' higher than the critical potential Vo', an undercut C becomes greater as shown in FIG. 4 and dimensional accuracy cannot be secured. Here, reference numeral 19 represents a photoresist, 20 is polysilicon and 21 is the Si substrate.

Accordingly, the D.C. voltage which is to be superposed with the radio frequency voltage which floats D.C.-wise is controlled by the output voltage controller 16 and the output waveform controller 17 and the D.C. bias voltage is set to V.sub.1 (negative potential) greater than the critical potential Vo' of the mixed gas for t.sub.1 seconds and to V.sub.2 (negative potential) smaller than the critical potential Vo' for t.sub.2 second and changed periodically.

Since the D.C. bias voltage is great for the time t.sub.1 seconds, etching can be made while accelerating the ions in the plasma towards the wafer 6 so that relatively anisotropic etching can be carried out. However, an undercut C.sub.0 occurs somewhat due partially to the influences of free radicals. The size of this undercut C.sub.0 is substantially from 1/5 to 1/10 of the etching depth d in the vertical direction. The time t.sub.1 is set within the range which does not exceed the allowance of the undercut C.sub.0.

Since the D.C. bias voltage value is smaller than the critical potential for the period t.sub.2, deposition can be generated so that regression of etching stops, the plasma polymerization products start depositing on the entire surface of the wafer 6 and the protective film is formed on the wall surface on the pattern side of polysilicon 20.

After the protective film is formed, a large D.C. bias voltage V.sub.1 is applied again to the sample table 5a to effect etching. The ions in the plasma accelerated by this large D.C. bias voltage V.sub.1 are incident perpendicularly to the wafer 6. Accordingly, the protective film deposited at the pattern bottom of the polysilicon 20 formed by the photoresist 19 is removed rapidly by the sputter action of the ions and etching proceeds while the pattern bottom of polysilicon 20 is exposed. The protective film deposited on the wall surface on the pattern side of polysilicon 20 is attacked by free radicals having extremely small physical energy and removed gradually by the chemical reaction between the free radicals and the components of the composition of the protective film. The deposition time t.sub.2 of the protective film is set so that the protective film deposited on the wall surface on the pattern side of polysilicon 20 remains even when etching is effected for the period t.sub.1. Incidentally, the time t.sub.1 and t.sub.2 thus set are stored in advance in the output waveform controller 17 and automatically changed over.

The polysilicon film having a high aspect ratio shown in FIG. 6 can be machined with high dimensional accuracy by alternately repeating the etching step and the deposition step, that is, the film formation step, in the manner described above.

Incidentally, the value of the large D.C. bias voltage V.sub.1 is set to be smaller than 1/2 of the total amplitude value V.sub.pp of the radio frequency voltage in order to prevent charge-up of the ions to the sample. For there is a limitation to the D.C. voltage to be superposed with the radio frequency voltage in order to obtain a high etching speed and moreover to effect etching without applying any damage to the device formed on the sample.

Namely, if the D.C. bias voltage value (negative potential) is increased to be greater than 1/2 of the total amplitude value V.sub.pp of the radio frequency voltage, the sample is always at a negative potential, only the positive ions are attracted to the sample surface and the sample is charged. Accordingly, the positive ions (reactive ions) in the plasma become finally repulsive and do not reach the sample, resulting in a drastic drop of the etching speed of the sample: If the charge potential is great at this time, degradation and breakdown of the gate portion of the device formed on the sample will occur.

Therefore, this embodiment applies a negative D.C. voltage smaller than 1/2 of the total amplitude value V.sub.pp of the radio frequency voltage to the sample table 5a so that part of the waveform of the radio frequency voltage remains as the positive potential and this positive potential portion entraps the electrons in the plasma, thereby neutralizing the positive ions charged to the sample.

Furthermore, in order to solve this problem of charge-up, the transmission frequency of the radio frequency voltage must be increased above about 100 KHz, as described in detail in Japanese Patent Publication No. 37311/1981. Though there is no limitation, in particular, to the upper limit of the transmission frequency, an oscillation frequency of up to about 27 MHz will be suitable if oscillators that are commercially available on the market are used.

In accordance with the first embodiment of the invention described above, the radio frequency voltage is applied to the electrode 5 by the radio frequency power source 13 in such a manner as to superpose the D.C. voltage from the D.C. power source 15 with the D.C. bias voltage generated in the sample table 5a by the application of the radio frequency voltage and the D.C. bias voltage having superposed the D.C. voltage therewith is changed in such a manner as to interpose the critical potential by the output voltage controller 16 as well as the output waveform controller 17. In this manner, etching and deposition, that is, film formation, can be effected alternately to the wafer without changing the plasma generated in the discharge portion 7, that is, without supplying the gas by changing it over. Accordingly, in comparison with the conventional technique which changes over the kind of gases and etches the sample by effecting alternately etching and deposition, the present invention eliminates the gas replacement time and can shorten the treating time effectively by at least the elimination of this gas replacement time. For instance, an about 10 seconds time is necessary when a gas of 70 SCCM is supplied to a vacuum treating vessel having a capacity similar to the capacity (20,000 cm.sup.3) of the vacuum treating vessel of this embodiment by use of an exhaust device having an exhaust capacity of 500 l/sec, and the state described above is established by changing over the kind of gases from the state where the pressure is kept at 0.01 Torr. As the number of times of gas switching increases, the effect of the present invention becomes more remarkable. In other words, since it is not necessary in the present invention to change over the gas at a high speed as required by the prior art, the exhaust device can be made compact in size. Since the pressure control and the change-over control for the change-over of the gas kind become unnecessary in accordance with the present invention, the apparatuses and control techniques required therefor become simplified.

Since the output voltage controller 16 is controlled in such a manner as to apply the D.C. bias voltage greater than the critical potential to the sample table 5a, the wafer 6 is etched and if the D.C. bias voltage smaller than the critical potential is applied to the sample table 5a, the protective film is deposited on the wafer 6. Furthermore, the output waveform controller 17 is controlled in order to alternately change over the timings at which the D.C. bias voltage is changed, interposing the critical potential between them. Accordingly, etching can be made step-wise while protecting the etching side surface of the wafer 6 by the protective film and a material to be etched, which has a depth or height greater than the pattern dimensional width can be machined.

Since the embodiment utilizes the treatment by the microwave plasma using ECR discharge, the plasma can be generated at a pressure as low as 10.sup.-2 Torr, and since the ions in the plasma can be attracted by a small acceleration voltage to the wafer 6, anisotropic etching with less damage can be practised and a material to be etched, which has a delicate pattern, can be machined. In other words, the material to be etched having a delicate pattern and a great aspect ratio can be machined in cooperation with the effects described above.

The change-over timing of the etching action and the deposition action is set by the output waveform controller in such a manner that the undercut of the material to be etched is within the allowance at the time of etching and the protective film on the etching side surface remains till next etching at the time of deposition. Accordingly, machining can be made with high dimensional accuracy.

Since the plasma generation means utilizing the electromagnetic action is independent of the D.C. bias voltage application means provided by the radio frequency voltage and the D.C. voltage, that is, the acceleration voltage application means, the state of generation of the plasma, that is, the state of the electrons, ions and free radicals in the plasma, does not change even if the D.C. bias voltage is changed, so that etching can be made while light emission intensity is stable, and the end point of etching can be easily judged by light emission spectroscopy.

Moreover, the radio frequency power source 13 is connected to the sample table 5a to apply the radio frequency voltage and the D.C. voltage of the D.C. power source 15 is controlled so as to set the D.C. bias voltage to be below 1/2 of the total amplitude of the radio frequency voltage generated from the radio frequency power source 13. Accordingly, even if the sample has an insulating material or an insulating film, no charge is built up in the sample and etching can be made without the drop of the etching speed and degradation and breakdown of the gate portion of the device. If a device of a device structure having an MOS gate in a lower layer is etched by the etching method of the first embodiment of the invention, degradation of breakdown voltage and damage of the gate portion do not occur.

Incidentally, the ion sheath width is generally about 0.1 mm in microwave plasma treating apparatuses which cause discharge by the ECR system by utilizing the microwave as in the present embodiment. The time necessary for the ions to pass through this ion sheath (t.sub.i) varies somewhat depending upon the kind of ions but is generally from about 1 to 4.times.10.sup.-7 sec. In contrast, the half cycle time (t.sub.RF) of the voltage waveform of the 13.56 MHz radio frequency that is generally used for industrial applications is 3.7.times.10.sup.-8 sec. For this reason, in the 13.56 MHz radio frequency voltage, the ions cannot follow while passing through the ion sheath. Therefore, the ions can be accelerated by generating the negative D.C. bias voltage as in the present embodiment.

The method which utilizes the D.C. bias voltage is particularly effective when treating a sample which can establish conduction with the electrode 5, such as a Si or a metal film. The method of using the D.C. bias voltage becomes effective when the half cycle time t.sub.RF of the voltage waveform of the radio frequency and the ion sheath passage time of ions t.sub.i have the following relation:

t.sub.RF <t.sub.i

Accordingly, the lower limit of the frequency of the radio frequency power source 13 is about 2 MHz (t.sub.RF =2.5.times.10.sup.-7 sec). If the frequency is lower than this value, the ions follow up the A.C. voltage waveform and are accelerated so that the effect of superposition of the D.