JP2010287415A - Filament and ion source equipped therewith - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the corrosion of a filament and the deflection with its dead weight when heated up to a higher temperature. <P>SOLUTION: The filament 4 includes a filament part 6 to be energized and heated to release thermoelectrons, and a cylindrical thermoelectron releasing part 8 to be heated by the filament part 6 passing therethrough to release the thermoelectrons. A material constituting the thermoelectron releasing part 8 has greater thermoelectron emitting current density than a material constituting the filament part 6, but the material constituting the filament part 6 has a greater rigidity coefficient. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a thermionic emission filament used in, for example, a plasma source, an ion source, a sputtering apparatus, and the like. Furthermore, it is related with the ion source provided with the said filament.

  Plasma sources are used for various sources such as ion sources and sputtering devices. As a thermoelectron emission source necessary for plasma generation in a plasma source, a filament which has a structure in which a wire made of tungsten is bent into a predetermined shape (for example, a U shape) and emits thermoelectrons by energization heating has been conventionally used. Widely used (see, for example, Patent Document 1).

  Patent Document 1 also describes that tantalum can be used in addition to tungsten as a filament material.

JP 2002-334661 A (paragraph 0074, FIG. 1)

  The filament becomes high temperature due to electric heating during use. In particular, higher filament temperatures are required when generating a dense plasma. Therefore, in the case of a filament made of tungsten, there is a problem that the corrosion progresses and the lifetime is shortened.

  In detail, tungsten is often used as a filament because it is relatively inexpensive. However, tungsten is not very high in thermionic emission efficiency, so a large amount of thermionic emission is required to produce a dense plasma. In this case, the filament must be heated to a very high temperature (for example, about 2000K or higher).

As a source gas for generating plasma in an ion source or the like, a corrosive gas such as boron trifluoride (BF ₃ ), phosphine (PH ₃ ), arsine (AsH ₃ ) or the like is often used. When plasma is generated using such a raw material gas, if the filament is heated to a very high temperature as described above, the corrosion of the filament proceeds remarkably and the life of the filament becomes extremely short.

  In order to solve the above-mentioned corrosion problem, it is conceivable to lower the heating temperature of the filament by using a material having high thermionic emission efficiency. As described in Patent Document 1, tantalum is known as such a material having high thermionic emission efficiency.

  However, since tantalum is less rigid than tungsten, when tantalum is used as a filament, the deflection due to its own weight increases when the temperature is raised. There are challenges.

  In detail, tantalum has a lower rigidity than tungsten. Therefore, when the tantalum has a linear shape with a diameter of about several millimeters, when the temperature rises to the heating temperature as described above, Deflection due to increases. When the deflection of the filament is large, for example, the position of the filament is shifted, so that the characteristics of plasma generation change. In extreme cases, it may happen that the filament hits the wall of the plasma generation vessel and normal operation becomes difficult.

  Thus, the main object of the present invention is to suppress filament corrosion and deflection due to its own weight when the temperature is raised by heating.

  The filament according to the present invention is a filament that emits thermoelectrons, and is a filament portion that is heated by energization to emit thermoelectrons, and has a cylindrical shape, and the filament portion passes through the filament portion. And a thermoelectron emission part that is heated by heat from the filament part and emits thermoelectrons, and the material constituting the thermoelectron emission part constitutes the filament part. The thermoelectron emission current density is larger than the material to be used, and the material constituting the filament part has a higher rigidity than the material constituting the thermoelectron emission part.

  In this filament, the thermoelectron emission efficiency of the material constituting the thermoelectron emission portion is high, and specifically, the thermoelectron emission current density is higher in the material constituting the thermoelectron emission portion than in the material constituting the filament portion. Therefore, even if the heating temperature of the filament is lowered, the thermoelectrons can be efficiently emitted from the thermoelectron emission portion. Therefore, it becomes possible to lower the heating temperature of the filament, and even when used in a corrosive gas atmosphere, corrosion of the filament when heated to raise the temperature can be suppressed.

  In addition, since the thermoelectron emitting portion is supported by a filament portion made of a material having a higher rigidity than the material, it is possible to suppress bending due to its own weight when the temperature is increased by heating. Can do.

  For example, the filament portion is made of tungsten, and the thermal electron emission portion is made of tantalum.

  The thermoelectron emission unit may have a structure in which a thin plate is rolled up into a cylindrical shape. Moreover, you may fix to the said filament part by caulking.

  An ion source according to the present invention is an ion source that ionizes a source gas in a plasma generation container to generate plasma and draws an ion beam from the plasma, and serves as a hot cathode that emits thermoelectrons into the plasma generation container. The above-mentioned filament is provided.

  According to the first and second aspects of the present invention, since the thermoelectron emission efficiency of the material constituting the thermoelectron emission portion is high, specifically, the material constituting the thermoelectron emission portion constitutes the filament portion. Since the thermoelectron emission current density is larger than that of the material, thermoelectrons can be efficiently emitted from the thermoelectron emission portion even if the heating temperature of the filament is lowered. Therefore, it becomes possible to lower the heating temperature of the filament, and even when used in a corrosive gas atmosphere, corrosion of the filament when heated to raise the temperature can be suppressed. As a result, the lifetime of the filament can be extended.

  In addition, since the thermoelectron emitting portion is supported by a filament portion made of a material having a higher rigidity than the material, it is possible to suppress bending due to its own weight when the temperature is increased by heating. Can do. As a result, for example, a change in characteristics due to bending of the filament, such as a plasma source using the filament, can be suppressed.

  According to invention of Claim 3, there exists the following further effect. That is, since the filament part is provided with a bent part having a function of limiting the movement range of the thermoelectron emission part, even if the position of the thermoelectron emission part is shifted for some reason, the position shift The range can be limited.

  According to invention of Claim 4, there exists the following further effect. That is, since the thermoelectron emission portion has a structure in which a thin plate is rounded into a cylindrical shape, the thermoelectron emission portion is easier to manufacture than those made by other cutting processes or the like. The cost of the filament can be reduced.

  According to invention of Claim 5, there exists the following further effect. That is, since the thermoelectron emission portion is fixed to the filament portion by caulking, it is possible to more reliably prevent the position shift of the thermoelectron emission portion. As a result, for example, it is possible to suppress a change in characteristics due to a displacement of the thermionic emission portion such as a plasma source using the filament.

  According to the invention described in claim 6, since the filament as described above is provided as the hot cathode, the same effects as those described above for the inventions described in claims 1-5 can be obtained.

  According to the seventh aspect of the present invention, the following further effect is obtained. That is, the thermoelectron emission part of the filament covers the entire area of the filament part located in the plasma generation container, so that the filament part also serving as the support part of the thermoelectron emission part exists in the plasma generation container. Therefore, it is possible to prevent exposure to the raw material gas that causes corrosion. As a result, it is possible to suppress the filament portion from being corroded and disconnected by the raw material gas, so that the life of the filament can be further extended.

It is a top view which shows one Embodiment of the filament which concerns on this invention. It is a figure which shows an example of the expanded cross section along line AA of FIG. It is a perspective view which shows the other example of the thermoelectron emission part of a filament, and is equivalent to what was seen in the line AA direction of FIG. It is a top view which shows other embodiment of the filament which concerns on this invention. It is a top view which shows other embodiment of the filament which concerns on this invention. It is a top view which shows other embodiment of the filament which concerns on this invention. It is a top view which shows other embodiment of the filament which concerns on this invention. It is a top view which shows other embodiment of the filament which concerns on this invention. It is a top view which shows other embodiment of the filament which concerns on this invention. It is a top view which shows other embodiment of the filament which concerns on this invention. It is a top view which shows other embodiment of the filament which concerns on this invention. It is a schematic sectional drawing which shows one Embodiment of an ion source provided with the filament which concerns on this invention. It is a schematic sectional drawing which shows other embodiment of an ion source provided with the filament which concerns on this invention.

(1) Filament FIG. 1 is a plan view showing an embodiment of a filament according to the present invention. The filament 4 emits thermoelectrons and is used in, for example, a plasma source, an ion source, and a sputtering apparatus as described above.

  The filament 4 includes a filament part 6 that is heated by energization and emits thermoelectrons, and a thermoelectron emission part 8 that has a cylindrical shape and through which the filament part 6 penetrates. The thermoelectron emission unit 8 is supported by the filament unit 6 and is heated by heat (mainly radiant heat) from the filament unit 6 to emit thermoelectrons. That is, the filament part 6 also serves as a support part for the thermoelectron emission part 8.

  Near both ends of the filament portion 6 is a connection end portion 14 for connection to a power source (see, for example, the filament power source 36 shown in FIGS. 12 and 13).

  The thermoelectron emission portion 8 may be a pure cylinder (also referred to as a tube) as shown in FIG. 2, for example, or a thickness of 0. A structure in which a thin plate of about 3 mm is rounded into a cylindrical shape may be used. Reference numeral 12 in FIG. 3 is a joint when rounded. In addition, when a thin plate is rolled up into a cylindrical shape, a very thin thin plate may be wound a plurality of times into a cylindrical shape. The above also applies to other embodiments described later.

  If the thermoelectron emitting portion 8 is formed into a cylindrical shape by rounding a thin plate as described above, the thermoelectron emitting portion 8 is easier to manufacture than those made cylindrical by other cutting processes or the like. The cost of the filament 4 can be reduced. In particular, when the thermionic emission portion 8 is made of tantalum, as will be described later, tantalum is difficult to process such as cutting, so that the above-described effect obtained by forming a cylindrical structure by rounding a thin plate is great.

  Further, in the filament 4, the material constituting the thermoelectron emission portion 8 has a higher thermionic emission current density than the material constituting the filament portion 6, and the material constituting the filament portion 6 is the thermoelectron. The rigidity is greater than the material constituting the discharge part 8.

  The rigidity is a kind of elastic modulus, and is the ratio of the shear stress to the strain due to the shear when the shear stress is applied to the elastic body within the elastic limit. Also called shear modulus or shear modulus.

  For example, the filament portion 6 is made of tungsten (W), and the thermionic emission portion 8 is made of tantalum (Ta).

Thermionic emission current density of the tungsten, for example, the temperature is about 2.6 × 10 ³ A / m ² at about 8.7 A / m ^2, 2500 K at 2000 K. Thermionic emission current density of tantalum, for example, the temperature is about 1.9 × 10 ⁴ A / m ² to about 990A / m ^2, 2500 K at 2000 K. At the same temperature, tantalum is about an order of magnitude larger.

  The rigidity of tungsten is about 157 GPa (27 ° C.), and the rigidity of tantalum is about 69.1 GPa (27 ° C.). Tungsten is about twice as large.

  In this embodiment, the planar shape of the filament portion 6 is U-shaped (in other words, a hairpin shape; the same applies hereinafter), but is not limited thereto, and may be in other shapes. What is necessary is just to decide according to a purpose, a use, etc. Some examples of other shapes are shown in the embodiments of FIGS. The filament portion 6 having such a predetermined planar shape can be formed, for example, by bending a linear or rod-shaped material.

  In this embodiment, the number of thermionic emission portions 8 provided in the filament portion 6 is two as an example. However, the number of thermionic emission portions 8 is not limited thereto, and may be one or three or more. What is necessary is just to decide according to a purpose, a use, etc. The same applies to other embodiments described later.

  In this embodiment, as an example of the arrangement of the thermoelectron emission portions 8 with respect to the filament portion 6, one thermoelectron emission portion 8 is arranged on each of the straight portions of the U-shaped filament portion 6, and the filament The thermionic emission portions 8 are arranged substantially symmetrically (laterally symmetrical) with respect to the center 7 of the portion 6. By doing so, although the thermoelectrons can be emitted from the filament 4 with good symmetry, they are not necessarily arranged symmetrically, and they may be arranged asymmetrically by arranging them only on one side. What is necessary is just to decide according to a purpose, a use, etc. The same applies to other embodiments described later.

  In this filament 4, the thermoelectron emission efficiency of the material constituting the thermoelectron emission portion 8 is high. Specifically, the material constituting the thermoelectron emission portion 8 is more heat-resistant than the material constituting the filament portion 6. Since the electron emission current density is large, even when the heating temperature of the filament 4 is lowered, the thermal electrons can be efficiently emitted from the thermoelectron emission portion 8. Therefore, it becomes possible to lower the heating temperature of the filament 4, and even when used in a corrosive gas atmosphere, corrosion of the filament 4 when the temperature is raised by heating can be suppressed. This is because the reaction rate between the corrosive gas and the filament part 6 or the thermoelectron emission part 8 is greatly reduced when the temperature is lowered. As a result, the life of the filament 4 can be extended. The experimental result about lowering the heating temperature will be described later.

  Moreover, the thermoelectron emission portion 8 is supported by the filament portion 6 having high rigidity, more specifically, the filament portion 6 made of a material having a higher rigidity than the material of the thermoelectron emission portion 8. Since it is a structure, the bending by the self-weight of the said filament 4 when heating and raising temperature can be suppressed. For example, as compared with the case where the entire filament 4 is formed of tantalum, bending due to its own weight when heated to a high temperature can be suppressed. As a result, for example, it is possible to suppress a change in characteristics due to bending of the filament, such as a plasma source or an ion source using the filament 4.

  Further, since the thermoelectron emitting portion 8 of the filament 4 is cylindrical, it can receive radiant heat from the entire circumference of the filament portion 6 located inside thereof, and thus can be easily heated to a high temperature.

  In addition, a gap 10 is usually present between the tubular thermoelectron emitting portion 8 and the filament portion 6 inside the tubular thermoelectron emitting portion 8. In FIG. 2, since the enlarged view is shown, the gap 10 is displayed large, but actually it is not so large. In the thermoelectron emission portion 8 that is formed by rolling a thin plate into a cylindrical shape as in the example of FIG. 3, the gap 10 is usually very small. However, in any case, since there is a slight gap 10 and the thermoelectron emission portion 8 is a cylindrical unit, if the thermoelectron emission portion 8 is not caulked as will be described later, or Prior to crimping, the thermionic emission part 8 can be moved along the filament part 6. Thereby, for example, it can be used for adjusting the plasma distribution in a plasma source, an ion source, or the like.

  More specifically, the thermoelectrons are also emitted from the filament part 6, but more are emitted from the thermoelectron emission part 8. This is because, as described above, the material constituting the thermoelectron emission portion 8 has a higher thermoelectron emission current density. By utilizing this, the state of the plasma distribution can be changed. In other words, depending on the volume of the plasma generation vessel of the plasma source or ion source, the position of the inlet of the source gas introduced into it, the state of the thermoelectrons emitted from the filament can be changed in order to obtain a desired plasma distribution. However, this change can be made relatively easily by moving the thermionic emission portion 8 along the filament portion 6, which can also be useful for adjusting the plasma distribution state. Is possible.

  Next, other embodiments of the filament 4 are shown in FIGS. Parts that are the same as or equivalent to those in the embodiment shown in FIG. 1 are denoted by the same reference numerals, and differences from the filament 4 shown in FIG. 1 will be mainly described below.

  As in the filament 4 shown in FIG. 4, the planar shape of the filament portion 6 may be generally U-shaped as a whole, but the interval between the straight portions following the connection end portion 14 may be increased.

  Like the filament 4 shown in FIG. 5, the planar shape of the filament portion 6 may be M-shaped.

  Like the filament 4 shown in FIG. 6, the planar shape of the filament portion 6 may be a U shape.

  Like the filament 4 shown in FIG. 7, the planar shape of the filament part 6 may be a shape in which the previous part is V-shaped.

  As in the filament 4 shown in FIG. 8, the planar shape of the filament portion 6 may be convex.

  Like the filament 4 shown in FIG. 9, the planar shape of the filament part 6 may be concave.

  Like the filament 4 shown in FIG. 10, a part of the filament part 6 may be spiral.

  Like the filament 4 shown in FIG. 11, the filament part 6 may be linear.

  Also in the filament 4 shown in FIGS. 4 to 11, the position and number of the thermoelectron emission portions 8 shown in the figure are examples, and the fact that the filaments 4 in FIG. It is the same. For example, in addition to or instead of the thermoelectron emission portion 8 indicated by a solid line, a thermoelectron emission portion 8 indicated by a two-dot chain line may be provided.

  Each filament portion 6 of the filament 4 shown in FIGS. 1 to 10 has one or a plurality of bent portions 16. The bent portion 16 has a function of limiting the moving range of the thermoelectron emitting portion 8. This is because the movement of the thermionic emission portion 8 is prevented by the bent portion 16. Therefore, even if the position of the thermoelectron emission portion 8 is shifted for some reason, the range of the position shift can be limited. This is advantageous over the linear filament 4 shown in FIG.

  For example, in the filament 4 shown in FIG. 1, even if a position shift occurs in the thermoelectron emission portion 8, it occurs only within a range R between the bent portion 16 and the connection end portion 14. Similarly, in the filament 4 shown in FIG. 4, the range R between the two bent portions 16 sandwiching the thermoelectron emission portion 8, and in the filament 4 shown in FIG. 5, the range between the two bent portions 16 sandwiching the thermoelectron emission portion 8. Only happens in R. The same applies to the filament 4 shown in FIGS.

  The thermoelectron emission portion 8 may be fixed to the filament portion 6 by caulking (caulking) regardless of whether the filament portion 6 has the bent portion 16 as described above or not. . When the thermionic emission part 8 has a structure in which a thin plate is rolled up into a cylindrical shape, the gap 10 is usually very small as described above. In such a case, the thermoelectron emitting portion 8 may be fixed to the filament portion 6 by caulking. By fixing the thermoelectron emission portion 8 to the filament portion 6 by caulking, it is possible to prevent the displacement of the thermoelectron emission portion 8 more reliably. As a result, for example, it is possible to suppress a change in characteristics due to misalignment of the thermionic emission portion 8, such as a plasma source or an ion source using the filament 4.

(2) About the experimental results Using the filament 4 of the example and the conventional filament, the arc current, the input power to the filament, and the temperature of the filament were measured when helium plasma was generated by arc discharge in the plasma generation vessel. An example of the experimental results is shown in Table 1.

  The filament 4 of the above embodiment has an M shape as shown in FIG. 5, and is a straight line of the filament portion 6 formed by forming a tungsten wire having a diameter of 1.2 mm and a wire length of 220 mm into an M shape. The thermoelectron emission portion 8 having a length of 22 mm, which is formed by rolling a thin tantalum plate having a thickness of 0.3 mm into a cylindrical shape, is provided at two portions of the portion. The conventional filament has a structure consisting only of a filament part having the same configuration as the filament part 6 of the above embodiment.

  The arc current is an index representing the plasma density, and the larger this value, the deeper the plasma is generated. The temperature of the filament is a temperature at a representative location of the filament, and this was estimated by a heat transport equation based on the input power to the filament.

  From the above table, the filament 4 of the embodiment has a lower temperature required to generate the same arc current, that is, to generate the same density plasma as described above, compared to the conventional filament. I understand. This is because, as described above, the tantalum constituting the thermoelectron emission portion 8 has a higher thermionic emission current density than the tungsten constituting the filament portion 6, and the filament 4 of the embodiment has such a thermoelectron emission portion 8. It is because it has. According to the filament 4 of the embodiment, since the heating temperature can be lowered as described above, the corrosion of the filament 4 can be suppressed as described above, and the life of the filament 4 can be extended.

(3) Ion source The filament 4 of each of the above embodiments may be used as a hot cathode that emits thermoelectrons into the plasma generation container of the ion source. One embodiment of such an ion source 20 is shown in FIG.

  The ion source 20 is an ion source that ionizes a source gas in the plasma generation container 22 to generate a plasma 28 and draws an ion beam 32 from the plasma 28, and heat that emits thermoelectrons into the plasma generation container 22. As the cathode, the filament 4 as described above is provided.

  In the embodiment of FIG. 12, as an example, the filament 4 is U-shaped as shown in FIG. 1. However, the filament 4 is not limited to this, and the filament 4 is not limited to that shown in FIGS. The embodiment may be used (see also FIG. 13).

A raw material gas such as boron trifluoride (BF ₃ ), phosphine (PH ₃ ), arsine (AsH ₃ ) as described above is introduced into the plasma generation vessel 22 from the gas inlet 24. The filament 4 is energized and heated by a filament power source 36. Reference numeral 34 denotes an insulator. Between the filament 4 and the plasma generation vessel 22 that also serves as an anode, an arc power source 38 that generates an arc discharge is connected. The ion source 20 emits thermoelectrons from the filament 4 into the plasma generation container 22, generates an arc discharge between the filament 4 and the plasma generation container 22, ionizes the source gas, and generates a plasma 28. The ion beam 32 can be extracted from the plasma 28 through the ion extraction port 26 by the extraction electrode system 30.

  Since the ion source 20 includes the filament 4 as described above as a hot cathode, the same effects as those described above for the filament 4 of each embodiment can be obtained.

  When the filament 4 as described above is used, the thermoelectron emission portion 8 of the filament 4 may cover the entire region of the filament portion 6 located in the plasma generation container 22. An example of such an ion source 20 is shown in FIG. Portions that are the same as or correspond to those in the example of FIG. The shape of the filament 4 is not limited to the example shown in FIG.

  In this ion source 20, since the thermoelectron emission portion 8 of the filament 4 covers the entire area of the filament portion 6 located in the plasma generation container 22, the filament also serving as the support portion of the thermoelectron emission portion 8. It is possible to prevent the portion 6 from being exposed to the raw material gas that exists in the plasma generation container 22 and causes corrosion. As a result, the filament portion 6 can be prevented from being corroded and disconnected by the raw material gas, so that the life of the filament 4 can be further extended.

4 Filament 6 Filament part 8 Thermionic emission part 16 Bending part 20 Ion source 22 Plasma generation vessel

Claims (7)

A filament that emits thermoelectrons,
A filament part that is heated by energization and emits thermoelectrons;
And a thermoelectron emission section that is tubular and has a filament portion that passes through and is supported by the filament portion, and is heated by heat from the filament portion to emit thermoelectrons. And
The material constituting the thermoelectron emission portion has a larger thermionic emission current density than the material constituting the filament portion,
The filament is characterized in that the material constituting the filament part has a higher rigidity than the material constituting the thermoelectron emission part.   The filament according to claim 1, wherein the filament portion is made of tungsten, and the thermal electron emission portion is made of tantalum.   The filament according to claim 1 or 2, wherein the filament portion includes a bent portion having a function of limiting a moving range of the thermoelectron emitting portion.   The filament according to claim 1, 2, or 3, wherein the thermoelectron emitting portion has a structure in which a thin plate is rolled up into a cylindrical shape.   The filament according to any one of claims 1 to 4, wherein the thermoelectron emitting portion is fixed to the filament portion by caulking. An ion source that ionizes a source gas in a plasma generation container to generate plasma, and extracts an ion beam from the plasma,
An ion source comprising the filament according to any one of claims 1 to 5 as a hot cathode for emitting thermoelectrons into the plasma generation vessel.   The ion source according to claim 6, wherein the thermoelectron emission portion of the filament covers the entire area of the filament portion located in the plasma generation container.

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