JP2014006998A - Method of driving charged particle source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress fluctuation in a generated ion beam current or a generated electron beam current to a minimum, in a charged particle source adopting an AC power supply as a heating power supply for a hot cathode.SOLUTION: A heating current is supplied to a hot cathode of a charged particle source by an AC power supply, and a DC voltage is applied between an arc chamber and the hot cathode to make electrons emitted from the hot cathode collide with gas in the arc chamber and generate a plasma. Ions or electrons in the plasma are extracted from a slit formed through a wall of the arc chamber to generate an ion beam and an electron beam. In a discharge current flowing between the arc chamber and the hot cathode, a first fluctuation component W1 caused by the heating AC and a second fluctuation component W2 caused by a vibration phenomenon of the plasma are included. When the charged particle source is driven, the AC power supply is driven at a frequency fwhere a value W obtained by adding the first fluctuation component W1 and the second fluctuation component W2 of the discharge current becomes the minimum, or at a frequency in the vicinity of the frequency f.

Description

  The present invention relates to a method for driving a charged particle source that heats a hot cathode using an alternating current.

  An ion source that converts elements into plasma and extracts ions in the plasma as an ion beam, and an electron beam source that extracts electrons in the plasma as an electron beam (hereinafter collectively referred to as “charged particle source”) perform ion implantation. First, it is used in various fields such as ion plating, crystal growth, and ion processing.

  There are various types of charged particle sources such as an RF type (radio frequency type), an ECR type (electron resonance type), a freeman type, etc., but at present, several tens of thermoelectrons are emitted from the hot cathode. A charged particle source of a low voltage arc discharge type that generates ions or electrons by accelerating to a bolt and colliding with a gas particle is frequently used.

  Generally, in a low-voltage arc discharge type charged particle source, a hot cathode is heated using a DC power source. However, if a DC power source is used, the hot cathode is locally consumed by evaporation or plasma sputtering. It is necessary to replace the hot cathode in a short time.

  On the other hand, the method of heating a hot cathode with an alternating current power supply is proposed (refer patent document 1). When the hot cathode is heated with an AC power source, evaporation and plasma sputtering can be prevented from being localized, and the life of the hot cathode is extended by uniform wear, so that the frequency of replacement can be suppressed.

JP-A-3-257748

  On the other hand, when the hot cathode is heated by an AC power supply, the generated plasma fluctuates, and as a result, the ion beam current and electron beam current extracted from the charged particle source fluctuate. In ion implantation, electron beam irradiation, etc., it is necessary to accurately control the amount of ion beam or electron beam current. However, if the ion beam current or electron beam current fluctuates, accurate control in ion implantation, electron beam irradiation, etc. Becomes difficult. For this reason, the method of heating a hot cathode using an AC power supply has not been adopted for commercial charged particle sources.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a charged particle source driving method capable of minimizing fluctuations in ion beam current and electron beam current.

In order to achieve the above object, a method for driving a charged particle source according to the present invention includes an arc chamber in which a gas introduction pipe is attached to a wall and gas is introduced into the interior space from the arc chamber, and a first wall of the arc chamber. A charged particle source having a hot cathode that emits electrons, attached via an insulating support member;
A current for heating is supplied to the hot cathode by an AC power source, and a direct current voltage is applied between the arc chamber and the hot cathode, and electrons emitted from the hot cathode are supplied to the gas in the arc chamber. A charged particle source driving method for generating plasma by colliding to generate ions or electrons by extracting ions or electrons in the plasma from a slit formed in a wall of the arc chamber,
A first oscillating component caused by an alternating current for heating and flowing between the arc chamber and the hot cathode using an ammeter in advance and a second oscillating component caused by a plasma oscillation phenomenon are included. Measure the value of the discharge current,
The AC power supply is driven at a frequency at which a value obtained by adding the first fluctuation component and the second fluctuation component of the discharge current is minimum or a frequency in the vicinity thereof.

  Here, the charged particle source includes a first reflector including a reflective electrode attached via a second insulating support member at a position facing the hot cathode in the wall of the arc chamber, and It is preferable to use a charged particle source in which a magnetic field is formed in a direction connecting the hot cathode and the first reflector.

  Furthermore, the charged particle source further includes a second reflector including a reflective electrode attached to a wall of the arc chamber to which the hot cathode is attached via a third insulating support member as the charged particle source. It is preferred to use a source.

  Preferably, a clamp-type AC current sensor is used as the ammeter, and the clamp-type AC current sensor is attached to a wiring connecting the arc chamber and the hot cathode to measure the discharge current.

  The charged particle source driving method according to the present invention can be applied when positive or negative ions in the plasma are extracted from the slits to generate positive or negative ion beams.

  By employing the charged particle source driving method of the present invention, fluctuations in the ion beam current and electron beam current can be minimized, so that the charged particle source in which the hot cathode is heated by an AC power source can be It can be used for electron beam irradiation.

It is sectional drawing which shows the fundamental structure and wiring state of the Bernas type ion source driven by the driving method of the charged particle source of this invention. It is sectional drawing which looked at the ion source of FIG. 1 from the side surface. It is a graph which shows the waveform of the discharge current of the Ar plasma excited by the voltage waveform applied to a hot cathode, and a heating current. It is a figure explaining the measuring method of ion saturation current. It is a graph which shows the waveform of the ion saturation current measured with the Langmuir probe. It is a conceptual diagram which shows the relationship between the frequency of the alternating current applied to a hot cathode, and the fluctuation width of a discharge current. It is a graph which shows the measurement result of the fluctuation width of a discharge current when changing the frequency of an alternating current.

A charged particle source driving method according to an embodiment of the present invention will be described below with reference to the drawings.
<Configuration and Operation of Charged Particle Source> FIG. 1 is a cross-sectional view showing a basic configuration and wiring state of a Bernas ion source having a hot cathode driven by the driving method of the present invention, and FIG. It is sectional drawing which looked at the ion source from the side. The Bernas type ion source is a kind of low voltage arc discharge type charged particle source. The ion source 1 includes an arc chamber 2, a hot cathode 3, and reflectors 4 and 5.

  The arc chamber 2 has a rectangular parallelepiped shape, and plasma is generated in the internal space surrounded by the flat upper wall 21 and the lower wall 22 and the rectangular tube-shaped side wall 23, and each wall has molybdenum, tungsten, and tantalum. It is made of a refractory metal such as graphite or silicon carbide.

  A pair of holes are formed in the upper wall 21 of the arc chamber 2 at a predetermined interval, and a hot cathode 3 bent in a substantially U shape via a ring-shaped insulating support member 61 is supported in the holes. Has been. The insulating support member 61 is usually made of a heat resistant material such as alumina or boron nitride. The hot cathode 3 may have a spiral shape.

  A cathode heating power source 11 for supplying a heating current is connected between both ends of the hot cathode 3. The cathode heating power source 11 outputs an alternating current, and is designed so that the frequency can be changed between several tens Hz to several tens kHz. A discharge power source 12 that applies a DC voltage is connected between the arc chamber 2 and the hot cathode 3 so that the arc chamber 2 has a positive potential from the hot cathode 3.

A gas introduction pipe 7 for introducing a gas into the chamber is attached to one surface of the side wall 23 of the arc chamber 2. Although not shown, the other end of the gas introduction pipe 7 is connected to an oven for heating and vaporizing a solid sample such as P or As, or a cylinder containing a gas such as Ar, BF ₃ , PH ₃ , AsH ₃ or the like. Has been. Gas particles containing the desired ionic species are introduced into the arc chamber 2 through the gas introduction tube 7.

  The cathode heating electrode 11 is turned on to supply power to the hot cathode to heat the hot cathode 3, and the discharge power supply 12 is turned on to apply a voltage necessary for arc discharge between the hot cathode 3 and the arc chamber 2. Then, thermoelectrons are emitted from the hot cathode 3, collide with gas particles containing a desired ion species introduced into the arc chamber 2 through the gas introduction tube 7, and are ionized to generate plasma P.

  A reflector 4 made of a refractory metal is attached to the lower wall 22 of the arc chamber 2 via a ring-shaped alumina insulating support member 62. The reflector 4 repels the thermoelectrons emitted from the hot cathode 3 to the hot cathode 3 side, and includes a reflective electrode 41 and a support 42 that supports the reflective electrode 41. A negative bias voltage is applied to the reflective electrode 41 by the bias power supply 13 with reference to the potential of the arc chamber 2.

  In the present embodiment, the reflector 5 is also attached to the upper wall 21 of the arc chamber 2 via the ring-shaped insulating support member 63 in order to increase the ionization efficiency of the plasma generating gas. The reflector 5 repels thermoelectrons to the reflector 4 side, and includes a reflective electrode 51 and a support body 52 that supports the reflective electrode 51.

  Similarly to the reflector 4, a negative bias voltage based on the potential of the arc chamber 2 is applied to the reflective electrode 51 by the bias power source 13. The reflective electrode 51 has a hole 53 through which the hot cathode 3 passes. The reflective electrode 51 and the hot cathode 3 are electrically insulated by the hole 53.

  Although not shown, a source magnet having a solenoid coil is installed around the arc chamber 2, and a magnetic field B (see FIG. 2) is formed in the arc chamber 2 in the direction connecting the hot cathode 3 and the reflector 4. It is formed. Due to the magnetic field B formed by the source magnet, the thermoelectrons emitted from the hot cathode 3 drift between the reflector 4 and the reflector 5, are driven back before each reflector, and are confined between the two. This increases the flight distance of thermionics.

  A vertically long beam extraction slit 8 extending along the direction of the magnetic field B is formed on one surface of the side wall 23 of the arc chamber 2. Although not shown, an extraction electrode having a beam passage hole is provided outside the arc chamber 2 at a position facing the beam extraction slit 8. A voltage is applied between the extraction electrode and the arc chamber 2 so that the arc chamber 2 has a positive potential from the extraction electrode. As a result, a strong external electric field is generated between the arc chamber 2 and the extraction electrode, and positive ions in the plasma generated in the arc chamber 2 are extracted from the beam extraction slit 8 by this external electric field, and the ion beam is generated. An IB is generated. In this embodiment, a vertically long slit is used. However, a horizontally long slit or a circular hole may be used, and a plurality of slits may be provided.

As described above, the charged particle source can generate a negative ion ion beam or an electron beam in addition to a positive ion beam. When generating an ion beam or electron beam of negative ions, a voltage is applied so that the extraction electrode has a positive potential from the arc chamber 2.
<Influence on ion beam by alternating current> Next, the influence on the ion beam by using alternating current as the heating current of the hot cathode will be described. As described above, when a direct current power source is used for the cathode heating power source 11, the hot cathode is locally worn by evaporation or plasma sputtering, so that it is necessary to frequently replace the hot cathode.

  On the other hand, when an AC power source is used, it is possible to prevent the localization of evaporation and plasma sputtering and extend the life of the hot cathode, so that the frequency of replacement can be suppressed. There arises a problem that the beam current and the electron beam current fluctuate. Hereinafter, fluctuations in the ion beam current when an alternating current is applied to the hot cathode 3 using the cathode heating power source 11 will be described based on actual measurement examples.

  In order to confirm how the frequency of the cathode heating power source 11 affects the ion beam current, the voltage waveform of the cathode heating power source 11, the waveform of the discharge current flowing between the hot cathode 3 and the arc chamber 2, and the beam extraction Each waveform of the ion beam current emitted from the slit 8 was measured.

  The voltage waveform of the cathode heating power source 11 was measured with a voltmeter and recorded in a commercially available memory recorder. As for the waveform of the discharge current, the current flowing through the wiring between the discharge power source 12 and the arc chamber 2 was measured using a clamp-type AC current sensor, and recorded in a commercially available memory recorder. Note that the AC current sensor is not limited to the clamp type, and a split AC sensor or a penetration AC sensor may be used, or measurement may be performed using a voltage across a resistor inserted in the circuit.

  FIG. 3 shows the waveform of the voltage applied to the hot cathode 3 recorded in the memory recorder and the waveform of the discharge current of the Ar plasma excited by the heating current when the power of the cathode heating power supply 11 is maintained at 100 W. . 3A shows a waveform when the frequency f of the alternating current supplied from the cathode heating power source 11 is 60 Hz, and FIG. 3B shows a waveform when the frequency f of the alternating current is 1 kHz.

  As shown in FIG. 3, when an AC power source is used as the cathode heating power source 11, the discharge current fluctuates at a frequency twice as high as the frequency of the AC current. On the other hand, the amplitude of the discharge current is smaller when the frequency is 1 kHz than when the frequency is 60 Hz.

  The amount of current of the ion beam IB emitted from the beam extraction slit 8 has a correlation with the saturation current of ions in the plasma (hereinafter referred to as “ion saturation current”). The ion saturation current waveform is measured using a Langmuir probe. A method for measuring the ion saturation current will be described with reference to FIG.

  The Langmuir probe 91 connected in series with the DC power source 92 is arranged so that the tip of the Langmuir probe 91 is separated from the beam extraction slit 8 by a distance L, oscilloscopes 94 are connected to both ends of the resistor 93, and the measured waveform is illustrated. Recorded on a commercially available memory recorder. Here, L = 10 mm, and a voltage was applied by the DC power source 92 so that the Langmuir probe 91 was −74 V with respect to the arc chamber 2.

  The Langmuir probe 91 is a probe tip of a tungsten wire having a diameter of 0.3 mm and a length of 2.0 mm covered with an alumina tube having an inner diameter of 0.4 mm. The surface of the alumina tube is used to shield signals from external noise. Covered with a 0.2 mm thick copper tube. Further, the copper tube is covered with an alumina tube having an inner diameter of 3 mm and an outer diameter of 5 mm in order to avoid direct contact with plasma.

  FIG. 5 shows a waveform of the ion saturation current measured with the Langmuir probe 91 when the frequency f of the alternating current is 1 kHz. For comparison, FIG. 5 shows a waveform of a voltage applied to the hot cathode 3 and a waveform of a discharge current of Ar plasma excited by a heating current.

  As is apparent from FIG. 5, when an AC power source is used as the cathode heating power source 11, the ion saturation current varies. As described above, the frequency of the discharge current is twice the frequency of the heating current of the hot cathode 3, but the frequency of the ion saturation current is the same as the frequency of the heating current.

<Driving method of charged particle source>
Next, a method for driving a charged particle source according to the present invention will be described. The inventors of the ion source 1 shown in FIGS. 1 and 2 have the discharge current flowing between the arc chamber 2 and the hot cathode 3 when the frequency of the alternating current applied to the hot cathode 3 is changed. An experiment was conducted on how the oscillation component changes.

  FIG. 6 is a conceptual diagram showing the relationship between the frequency of the alternating current and the fluctuation width of the discharge current. As a result of repeated experiments by the inventors, as shown in FIG. 6, the fluctuation component W of the discharge current includes a fluctuation component W1 resulting from the use of an alternating current as the heating current of the hot cathode 3 and a plasma. The oscillation component W2 caused by the oscillation phenomenon is included, and the oscillation component W1 is dominant at a low frequency, and the oscillation component W2 is dominant at a high frequency. all right.

  Of the oscillating component W, the oscillating component W1 caused by the alternating current is obtained from a heat transfer equation in the case of heating by directly energizing the hot cathode. The oscillating component W1 becomes smaller as the frequency of the alternating current increases, and the standard is given by the following equation (1).

Where ε is the emissivity of the hot cathode, σ is the Stefan-Boltzmann constant, T is the temperature of the hot cathode, ρ _m is the mass density of the hot cathode, C is the specific heat of the hot cathode material, and a is the wire diameter of the hot cathode. is there.

  On the other hand, the oscillating component W2 is caused by the plasma oscillation phenomenon. Normally, when a high frequency is applied, vibration with a large amplitude is generated in a form close to resonance. Examples of the frequency at which the plasma starts to vibrate include an ion plasma frequency obtained by the following equation (2) and an ion acoustic resonance frequency obtained by the following equation (3).

Here, e is the electron charge, _ne is the electron density, ε ₀ is the vacuum dielectric constant, _mi is the ion mass, k is the Boltzmann constant, and l is the length or width of the plasma.

  In addition, for a resonance phenomenon associated with a drift wave or a discharge at a relatively high pressure, the frequency of ionization waves is the lower limit of the plasma oscillation frequency. These vibration frequencies are clearly observed when the frequency exceeds 1 kHz to 10 kHz.

As can be seen from FIG. 6, the fluctuation component W1 caused by the alternating current decreases as the frequency increases, and conversely, the fluctuation component W2 caused by the plasma oscillation phenomenon increases as the frequency increases. Therefore, the value W obtained by adding the oscillating components W1 and W2 is minimum at the intersection (frequency f ₀ ).

  As described above, if the fluctuation of the discharge current increases, the fluctuation of the ion saturation current also increases. Therefore, if the alternating current is driven at a frequency at which the sum of the fluctuation components W1 and W2 is minimized, the ion saturation current is increased. As a result, an ion beam with few fluctuation components can be realized.

The frequency for driving the alternating current is preferably a frequency f _{0 at} which the fluctuation width of the discharge current is minimized, but even if the fluctuation width of the discharge current is slightly larger (for example, the fluctuation width of the fluctuation current). Within 5% of the minimum value), the fluctuation value of the ion saturation current hardly changes. Therefore, even if the alternating current is driven at a frequency in the vicinity of the frequency f ₀ (a frequency within which the increase in the fluctuation width of the discharge current is within 5% of the minimum value), an ion beam with few fluctuation components can be realized.

  In order to confirm the effectiveness of the driving method of the charged particle source according to the present invention, the fluctuation width of the discharge current when the frequency of the alternating current is changed is measured using the ion source 1 shown in FIGS. did. The result is shown in the graph of FIG. In the figure, ◯, □, Δ, ●, and ■ are measured values when the diameter of the hot cathode is 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, and 0.6 mm. In the graph of FIG. 7, both the vertical axis and the horizontal axis are represented by logarithms.

  For the measurement, an arc chamber 2 made of molybdenum having a volume of 90 mm in height, 36 mm in width, and 29 mm in depth was used. Further, a tungsten wire having a total length of 90 mm deformed into a substantially U shape was used as the hot cathode 3, and 45 mm of this tungsten wire was attached to the arc chamber so as to be exposed to plasma.

  In addition, Ar gas was introduced from the gas inlet 7 and the heating power of the hot cathode 3 was maintained at 100 W. The frequency of the alternating current was changed, and the intensity of the discharge current was measured using a clamp-type alternating current sensor. The value was recorded using a commercially available memory recorder. And the measured value recorded on the memory recorder was read, and the fluctuation width of the discharge current was calculated from the measured value.

  In the graph of FIG. 7, the fluctuation width of the discharge current shows a minimum value at a frequency of 1 kHz for any wire diameter. Therefore, if an alternating current having a frequency of 1 kHz is output from the cathode heating power source 11 and applied to the hot cathode 3, an ion beam with the least fluctuation can be generated, and as a result, more accurate control can be performed in ion implantation or the like. .

  In the above-described embodiment, the case where the charged particle source driving method of the present invention is applied to a Bernas ion source has been described. However, the charged particle source driving method of the present invention is not limited to the Bernas ion source. Needless to say, the present invention can be similarly applied to other charged particle sources using a hot cathode.

DESCRIPTION OF SYMBOLS 1 Ion source 2 Arc chamber 3 Hot cathode 4, 5 Reflector 7 Gas introduction tube 8 Beam extraction slit 11 Cathode heating power source 12 Discharge power source 13 Bias power source 21, 22, 23 Wall 41, 51 Reflective electrode 42, 52 Support body 61, 62 63 Insulating support member 91 Langmuir probe 92 DC power supply 93 Resistance 94 Oscilloscope

Claims (5)

An arc chamber in which a gas introduction tube is attached to a wall and gas is introduced into the internal space therefrom, and a hot cathode that emits electrons and is attached to one wall of the arc chamber via a first insulating support member And a charged particle source with
A current for heating is supplied to the hot cathode by an AC power source, and a direct current voltage is applied between the arc chamber and the hot cathode, and electrons emitted from the hot cathode are supplied to the gas in the arc chamber. A charged particle source driving method for generating plasma by colliding to generate ions or electrons by extracting ions or electrons in the plasma from a slit formed in a wall of the arc chamber,
A first oscillating component caused by an alternating current for heating and flowing between the arc chamber and the hot cathode using an ammeter in advance and a second oscillating component caused by a plasma oscillation phenomenon are included. Measure the value of the discharge current,
A method for driving a charged particle source, wherein the AC power source is driven at a frequency at which a value obtained by adding the first fluctuation component and the second fluctuation component of the discharge current is minimum or a frequency in the vicinity thereof. . As the charged particle source,
A first reflector including a reflective electrode attached via a second insulating support member at a position facing the hot cathode in the wall of the arc chamber;
2. The method of driving a charged particle source according to claim 1, wherein a charged particle source having a magnetic field formed in a direction connecting the hot cathode and the first reflector is used.   As the charged particle source, a charged particle source further including a second reflector including a reflective electrode attached to a wall of the arc chamber to which the hot cathode is attached via a third insulating support member is used. The charged particle source driving method according to claim 2, wherein:   The clamp type alternating current sensor is used as the ammeter, and the discharge current is measured by attaching the clamp type alternating current sensor to a wiring connecting the arc chamber and the hot cathode. 4. The method for driving a charged particle source according to any one of 3 above.   5. The charged particle source driving method according to claim 1, wherein positive or negative ions in the plasma are extracted from the slit to generate an ion beam of positive or negative ions.

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