JP2977862B2 - Plasma generator - Google Patents

Description

The present invention relates to an apparatus for generating plasma in a vacuum chamber, and more particularly to an apparatus for forming a film in ion plating and plasma CVD (chemical vapor deposition). The present invention relates to an apparatus for generating a material plasma.

[Conventional technology]

Conventionally, for example, an ion plating apparatus as shown in FIG. 2 is known. Reference numeral 1 denotes a vacuum chamber, which is connected to an exhaust pump via a discharge pipe 2 and is grounded. A crucible 4 containing an evaporating material 3 and a gas supply nozzle 5 are arranged at a lower portion in the tank, and the crucible 4 is heated by a heater 7 supplied with a heating current from a heating power supply 6 outside the tank. At the upper part in the tank, a holder 9 supporting the object 8 to be processed is provided on the lower surface, and the holder 9 is provided with a heater 10 for heating the holder 9 as necessary.
Numeral 10 is supplied with a heating current from a heating power supply 11 outside the tank. Reference numeral 12 denotes a shutter that covers the front surface (the lower surface in the figure) of the workpiece 8 as necessary.

An anode 13 is arranged near the crucible 4 and the nozzle 5 along the vapor or gas path from the crucible 4 or the nozzle 5 to the workpiece 8, and a thermoelectron emission cathode 14 is arranged slightly away from the anode 13. Anode 13 is 0 to +10 outside the tank
The cathode 14 is connected to a heating power supply 16 and a 0 to -100 V bias power supply 17 outside the tank. The holder 9 is also connected to a negative bias power source 18 outside the tank.

In the above-described device, when the thermoelectron emission cathode 14 is energized and heated, a thermoelectron flow of about 0.5 to 1 mA flows toward the anode 13. Here, when the material gas is supplied from the nozzle 3 or the material in the crucible 4 is heated and evaporated,
The constituent particles of these gases or vapors are ionized by the collision of high-energy thermoelectrons, and the cathode 14 and the anode 14
Plasma is generated in the space between the two.

Thermions that do not lose energy without colliding with plasma electrons and particles generated by this ionization are several to several hundreds of ampere
Discharge current flows into the anode. This discharge is a low-voltage, large-current arc discharge, not a glow discharge.

Here, when the workpiece bias power supply 18 is adjusted to an appropriate value and the shutter 12 is opened, the material gas or the like and the ions thereof reach the workpiece 8 and deposit on the surface thereof to perform ion plating. It is. At this time, if only the vapor of the material 3 generated in the crucible 4 is supplied, a film of the same material is formed on the workpiece 8 and a reaction gas such as nitrogen, oxygen, acetylene, etc. is supplied from the nozzle 5 together with the material vapor. For example, a film of nitride, oxide, carbide, or the like is formed (PVD method). Further, instead of the vapor of the material 3, a material gas such as silane or titanium tetrachloride and a reaction gas such as nitrogen, methane, or hydrogen are mixed with argon,
When supplied together with a discharge gas such as xenon, a film of a substance generated by a chemical reaction is formed (CVD method).

[Problems to be solved by the invention]

In the above-described conventional apparatus, the relationship between the discharge current and the electron density measured by the probe method on the front surface of the object to be processed was proportional as shown in FIG. That is, as the discharge current increases, the plasma density (electron density and ion density) increases, the ionization rate of material vapor and the like increases, and a large amount of ions enter the workpiece.

However, the result of measuring the electron density on the front surface of the work piece shows a constant value as shown in Fig. 4 by changing the voltage applied to the cathode while adjusting the heating current of the thermionic emission cathode so that the discharge current becomes constant. Was. That is, when the voltage applied to the cathode is increased, the energy of thermionic electrons increases, but the plasma density is not affected by the magnitude of thermionic energy.

The result of measuring the space electrons of the plasma with respect to the tank wall (ground potential) while changing the anode applied voltage changed almost proportionally as shown in FIG. 5, and the space potential became almost equal to the anode applied voltage. . When the space potential of the plasma increases,
The plasma electrons cannot escape from the plasma space due to the electrons, but ions in the plasma are accelerated with respect to the tank wall by energy corresponding to a space potential.

From these measurement results, A. The plasma density is proportional to the discharge current and does not depend at all on the anode applied voltage.

B. The energy of ions incident on the grounded tank wall is almost proportional to the applied voltage in the anode.

The law of the thing was found.

On the other hand, in order to efficiently form a film on a workpiece, a large amount of ions are required, and from the viewpoint of suppressing temperature rise due to ion bombardment, the energy of ions incident on the workpiece must be kept as low as possible. Must. However, when attempting to obtain a large amount of ions in the above-described apparatus, not only raise the temperature of the thermionic emission cathode, but also increase the anode applied voltage to extract more thermoelectrons from the cathode and increase the discharge current. I have to do it. As a result, the ions have high energy, and it becomes impossible to avoid a rise in the temperature of the object.

Further, in order to strongly adhere the film to the object to be processed, it is necessary to apply considerably high energy to the ions in the early stage of the film formation. However, many particles (gas, metal atoms)
Shows the maximum value when the electron energy is two to three times the first ionization voltage (about 15 to 25 eV for gas and about 7 to 10 eV for metal atoms), and gradually decreases above that. In the apparatus, the voltage applied to the anode is about +100 V at the maximum with respect to the ground, and practically around +50 V is used. But the +50 to + 100V provided by this
The degree of space potential is extremely low for the purpose of enhancing the adhesion of the coating, and therefore, the effect of increasing the adhesion is hardly obtained.

As described above, in the conventional apparatus, it is difficult to separately control the density of the plasma and the space electrons, and therefore, the amount and energy of ions applied to the film formation cannot be appropriately controlled. .

An object of the present invention is to realize a plasma source that can separately control the plasma density and the space potential.

[Means for solving the problem]

The plasma generating apparatus according to the present invention comprises a thermionic emission cathode provided with a heating means and an anode arranged facing a radiated thermoelectron flow of the cathode in a vacuum chamber provided with an exhaust means and a wall of the chamber is grounded. And means for supplying a material gas or a material vapor and, if necessary, one or both of a reaction gas and a discharge gas in the path of the thermionic electron flow from the cathode to the anode.

The material gas referred to here is a gas such as silane or titanium tetrachloride that generates a film-forming substance by a chemical change, and the reaction gas reacts chemically with a material gas or a material vapor such as nitrogen, methane, or hydrogen to form a film. The discharge gas is a gas for assisting the discharge, such as argon and xenon.

The features of the present invention are that the cathode is kept at a positive potential with respect to the cathode while the thermionic emission cathode and the anode are electrically floated from the vacuum vessel wall, and that the anode is kept close to the vessel wall near the anode.
~ By providing a space potential control electrode maintained at a positive potential,
The configuration is such that plasma electrons are removed from the plasma generated between the cathode and the anode.

Further, in the present invention, means for forming a magnetic field parallel to the electric field between the cathode and the anode may be provided in the space between the cathode and the anode. The magnitude of this magnetic field may be on the order of tens to hundreds of gauss.

[Action]

In the above-described apparatus, when the thermoelectron emission cathode is heated to an appropriate temperature and an appropriate voltage is applied to the anode, a thermoelectron current (space charge limited current) of about 0.5 to 1.0 mA flows, and the thermoelectrons attempt to ionize. Energy below the ionization voltage of gas, steam, etc. is given by the anode voltage.

Then, when gas, vapor, or the like is introduced into the thermoelectron flow, thermoelectrons having the above-mentioned energy collide with these particles and ionize them. The plasma electrons generated by the ionization and thermions which do not lose energy without colliding with the particles flow into the anode as a discharge current of several to about 100A. This type of discharge is a low voltage, large current arc discharge. When the pressure of the gas introduced into the thermoelectron flow space, the amount of vaporization of the vapor, etc. are low and the particle density is low, ionization is unlikely to occur because the particles have little chance to collide with thermionic electrons. By giving a magnetic field, the chance of collision between particles and thermionic electrons is increased, and good ionization can be achieved.

Here, when the anode applied voltage is changed, the discharge current starts flowing to the anode when the voltage reaches the vicinity of the ionization voltage of the introduced particles, and the discharge current rapidly increases up to 20 to 30 V, and becomes 40 to 50 V.
As a result, the state is almost saturated. And this discharge current is
It changes when the temperature of the thermionic emission cathode is changed.

In the conventional apparatus shown in FIG. 2, even when the anode voltage was changed while the discharge current was kept constant by adjusting the temperature of the thermionic emission cathode, the plasma density did not change at all. However, in this invention, since the thermionic emission cathode and the anode are electrically floating from the tank wall, the plasma density changes when the voltage applied to the anode is changed while the discharge current is kept constant.

Plasma generated between the thermionic emission cathode and anode electrically separated from the cell wall maintains the volume of the plasma itself using the cell wall at the ground potential as a barrier, It has a space potential of about + 10V. Therefore, this spatial potential hardly changes even if the voltage applied to the anode is changed or the current flowing into the anode is changed.

However, when electrons are removed from the plasma, the plasma potential increases significantly. That is, when a voltage exceeding the space potential (about +10 V) which the above-mentioned plasma naturally has is applied to the space potential control electrode to the tank wall, particles (electrons, ions, neutral particles) constituting the plasma Some of the electrons flow into the space potential control electrode, and the space potential of the plasma increases to a value substantially equal to the voltage applied to the space potential control electrode. It should be noted that even if the electrons are removed from the plasma, the discharge current does not change, and the plasma density hardly changes.

Therefore, using the plasma generator according to the present invention,
When a film is formed on a workpiece, the density and energy of ions arriving at the workpiece can be freely changed independently of each other.

〔Example〕

In FIG. 1, a vacuum chamber 1 is connected to an exhaust pump via an exhaust pipe 2 and is grounded. A crucible 4 containing an evaporating material 3 and a gas supply nozzle 5 are arranged at a lower portion in the tank. The crucible 4 receives a heating current from a heating power source 6 outside the tank, and becomes a heating element by itself and becomes a heating element. Is heated and it is evaporated.

At the upper part in the tank, a holder 9 supporting the object 8 is provided on the lower surface, and the holder 9 is provided with a heater 10 for heating the object 8 as necessary.
Is supplied with a heating current from a heating power supply 11 outside the tank.
The holder 9 is connected to a bias power supply 18 outside the tank. Reference numeral 12 denotes a shutter that covers the front surface of the workpiece 8 as necessary.

An anode 13 is arranged near the crucible 4 and the nozzle 5 along the path of the vapor or gas from the crucible 4 or the nozzle 5 toward the workpiece 8, and a hot cathode 14 is arranged slightly away from the crucible 4 and the nozzle 5. The anode 13 and the cathode 14 are both insulated from the tank wall 1 and between them, an anode power supply is provided outside the tank.
15 are connected. Further, a heating power supply 16 is connected to the cathode 14 outside the bath.

There is a cylindrical space potential control electrode 20 surrounding the space 19 between the thermionic emission cathode 14 and the anode 13, and the electrode 20 is connected to a positive space potential control power supply 21 with respect to ground outside the tank. I have. Further, outside the tank, a coil 22 is formed in a space 19 so that a magnetic field parallel to the electron flow from the cathode 14 to the anode 13 is generated.
Are arranged, and the coil 22 is connected to the excitation power supply 23.

The crucible 4 is made of a high melting point metal such as tungsten, tantalum, molybdenum, etc., and its heating power source 6 has a capacity of 5V400A. Thermionic emission cathode 14 is a filament made of a high melting point metal such as tungsten, molybdenum, or tantalum, has a wire diameter of 1.0 mm and a length of 100 mm, and its heating power supply 16 has a capacity of 10 V 100 A. The anode power supply 15 has a direct current of 0 to +100 V and 100 A.
The distance between the thermionic emission cathode 14 and the anode 13 is 10 to 100 mm, preferably 50 mm. The distance between the space potential control electrode 20 and the anode 13 is 50 to 300 mm, preferably 100 mm, and the space potential control power supply 21 is 0 to +500 VDC. The strength of the magnetic field φ generated by the coil 21 is several tens to several hundreds gauss.

In the above-described embodiment, 1 × argon gas is supplied into the tank 1.
FIG. 6 shows the relationship between the voltage applied to the anode and the discharging current when the heating power is supplied at 10 ⁻² Torr and the heating power supplied to the thermionic emission cathode 14 is 9 V 65 A or 8 V 60 A. The voltage of the space electric control electrode 20 is 0 V, and no magnetic flux φ is given.

As the voltage applied to the anode increases, the first
Discharge starts around 15.7 eV and gradually increases,
The discharge current is saturated around +40 to + 60V. And
The discharge current at the time of this saturation changes depending on the temperature of the thermionic emission cathode. That is, when the temperature of the cathode is increased, the emission amount of thermionic electrons increases, and the frequency of collision with the argon gas particles and ionization thereof increases, so that the discharge current increases.

This discharge is a low-pressure, large-current arc discharge and a magnetic field φ
If no pressure is applied, arc discharge can be caused in the pressure range of the latter half of the 10 ^-4 Torr range to the 10 Torr range. And
If a magnetic field φ is applied, an arc discharge can be generated even in the order of 10 ⁻⁵ Torr.

FIG. 7 shows the change in plasma electron density under the same conditions as in FIG. 6 by changing the voltage applied to the cathode while adjusting the temperature of the thermionic emission cathode so that the discharge current is always 10 A. It is a thing that caught. The plasma electron density is
It was measured by a probe method at a position 100 mm from the thermionic emission cathode in the direction of the object to be processed.

According to this, in the conventional method shown in FIG. 2, as shown in FIG. 4, the plasma electron density did not change even when the anode applied voltage was changed. May change. That is, in the present invention, the plasma electron density depends on both the voltage applied to the anode and the discharge current.

FIG. 8 shows the relationship between the anode applied voltage and the plasma space potential under the same conditions as in FIGS. 6 and 7. According to this, in the conventional method shown in FIG.
As shown in FIG. 5, when the anode applied voltage is changed, the plasma space potential changes greatly, whereas in the present invention, the plasma space potential does not depend at all on the anode applied voltage.
It is almost constant at about 10V. That is, it is understood that the plasma characteristics do not depend at all on the acceleration energy of thermoelectrons.

FIG. 9 shows 1 × 10 as in the case of FIG. 6 to FIG.
^{Using a -2} Torr argon gas, the anode applied voltage and the discharge current were set to 100V10A, the voltage of the space potential control electrode was changed, and the change in plasma density was examined. The plasma electron density was measured by a probe method at a position 100 mm from the thermionic emission cathode toward the object to be processed. Note that no magnetic field was used.

As a result of this measurement, even if the voltage applied to the space potential control electrode was changed, the plasma electron density was 5 to 7 × 10 ¹¹
It was found that the plasma density was substantially constant within the range of cm ^-3 . During this period, the initially set discharge current 10A did not change.

FIG. 10 shows a measurement of a change in space potential during a festival in which the potential of the space potential control electrode was changed under the same conditions as in FIG. Thus, it can be seen that the space potential of the plasma increases due to the positive voltage applied to the space potential control electrode with respect to the tank wall (ground), and its value is substantially equal to the voltage applied to the space potential control electrode.

FIG. 11 shows the change of the electron current flowing into the space potential control electrode when the voltage applied to the space potential control electrode is changed under the same conditions as in FIGS. This is a measurement of the change state of the inflowing ion current.

According to this, as the voltage of the space potential control electrode is increased, the electron current taken away from the plasma and flowing into the electrode increases with the increase of the voltage, but this electron current is increased as shown in FIG. It was found that it hardly affected the plasma density and did not affect the discharge current of the anode. Further, the ion current flowing into the object to be processed changes in a manner very similar to the above-mentioned electron current, although there is no change in the plasma density as shown in FIG. These reasons are explained as follows. When the space potential control electrode steals electrons from the plasma in the form of the above-mentioned electron current, the plasma becomes insufficient as a whole, but because the plasma has the property of maintaining electrical neutrality, ions are exposed to the outside. Try to eject. As a result, the ions are accelerated by obtaining energy corresponding to the space potential in the direction of the object without changing the plasma density, whereby the ion current incident on the object increases. Therefore, the ion current incident on the object increases in a manner similar to the electron current.

As apparent from FIGS. 6 to 11, in the present invention, the plasma density is adjusted by adjusting one or both of the temperature of the thermionic emission cathode and the applied voltage of the anode. Can be set independently. The space potential of the plasma can be freely selected and set within a wide range irrespective of the plasma density.

Therefore, in the film forming apparatus embodying the present invention, by applying high energy to the plasma in the initial stage of film formation, the adhesion of the film to the object to be processed is increased, and thereafter, a large amount of plasma is supplied at a low energy. As a result, a film can be grown with high efficiency while suppressing a temperature rise.

〔The invention's effect〕

As described above, the plasma generator according to the present invention
Not only can the plasma space potential be selected and set in a wide range irrespective of the plasma density, but the plasma density can be increased or adjusted independently of the space potential. Therefore, in the film forming apparatus embodying the present invention, it is possible to form a film having good adhesion at a low temperature and with high efficiency by selecting the plasma density and the space potential according to the situation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a film forming apparatus embodying the present invention, FIG. 2 is a sectional view of a conventional film forming apparatus, and FIG. 3 is discharge current and plasma in the apparatus shown in FIG. FIG. 4 is a diagram showing the relationship between the anode applied voltage and the plasma electron density in the device shown in FIG. 2, and FIG. 5 is a diagram showing the relationship between the anode applied voltage and the space potential in the device shown in FIG. FIG. 6 is a diagram showing the relationship between the applied voltage of the anode and the discharge current in the above embodiment, FIG. 7 is a diagram showing the relationship between the applied anode voltage and the plasma electron density in the above embodiment, FIG. 8 is a diagram showing the relationship between the anode applied voltage and the space potential of the above embodiment, FIG. 9 is a diagram showing the relationship between the space potential control voltage and the plasma electron density of the above embodiment, and FIG. FIG. 11 is a diagram showing the relationship between the space potential control voltage and the space potential in the example, and FIG. Serial is a diagram showing a relationship of the inflow electron current and the object to be treated flowing ion current space potential control potential and spatial potential control electrode of Example. 1 ... Vacuum chamber, 2 ... Exhaust port, 3 ... Evaporation material, 4 ...
Crucible, 5 ... Gas nozzle, 6 ... Crucible heating power supply, 8
…… Workpiece, 13 …… Anode, 14 …… Thermionic emission cathode, 15
... Anode power supply, 16 ... Cathode heating power supply, 20 ... Space potential control electrode, 21 ... Space potential control power supply, 22 ... Magnetic field generating coil.

──────────────────────────────────────────────────の Continuation of front page (56) References JP-A-63-40242 (JP, A) JP-B-56-508 (JP, B2) JP-B-57-2150 (JP, B2) JP-B-57- 1586 (JP, B2) JP 52-32759 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 14/00-14/58 C23C 16/00-16/56 H01J 27/08 H05H 1/42

Claims (2)

(57) [Claims] 1. A thermionic emission cathode comprising a heating means and electrically floating from said vessel wall in a vacuum vessel having an exhaust means in the vessel and a vessel wall grounded, and a radiant thermoelectron of said cathode. An anode, which is placed facing the flow and is maintained at a positive potential with respect to the cathode, and a material gas or material vapor and, if necessary, a reactant gas and a discharge in the path of the thermionic current flowing from the cathode to the anode. Means for supplying at least one of a gas, and disposed near the anode for removing plasma electrons from the plasma generated between the cathode and the anode, and maintained at 0 to a positive potential with respect to the tank wall. And a space potential control electrode. 2. The plasma generating apparatus according to claim 1, further comprising means for forming a magnetic field parallel to an electric field between the cathode and the anode in a space between the cathode and the anode.