JP2007080784A - Plasma generator and scanning electron microscope - Google Patents

Abstract

The present invention relates to a plasma generator and a low-vacuum scanning electron microscope, and to provide a plasma generator and a scanning electron microscope that are small and hardly affected by noise, and that light generated by plasma does not adversely affect an image detection system. It is an object.
A quartz tube 60 having a cylindrical shape in which an ionizing gas flows, a high-frequency coil 63 wound around the tip of the quartz tube 60, and the quartz tube 60 and the high-frequency coil 63 are provided. It includes a mold 65 that covers the periphery, a metal part 66 that covers the entire torch and the vicinity of the anode part, a throttle 73, a pipe 67, and an anode 74 provided inside the metal part 66.
[Selection] Figure 1

Description

  The present invention relates to a plasma generator and a scanning electron microscope.

  In general, a scanning electron microscope (SEM) has a high vacuum inside, and an additional exhaust system is attached to the sample chamber of such an SEM to keep the electron gun and the electron optical system at a high vacuum. The low-vacuum scanning electron microscope allows only the sample chamber to be controlled to an arbitrary degree of vacuum. If a low vacuum, that is, a state where there are a large amount of gas molecules in the vicinity of the sample, the incident electron beam or electrons generated from the sample ionize the floating gas molecules to neutralize the charge of the sample. As a result, a non-conductive sample can be observed as it is without vapor deposition.

  FIG. 4 is a conceptual diagram of a low vacuum scanning electron microscope. In the figure, reference numeral 1 denotes a sample chamber, which is kept at a low vacuum of 1 to 200 Pa. 2 is a sample disposed in the sample chamber 1, 3 is a secondary electron detector that detects secondary electrons emitted from the surface of the sample 2, and 4 is a reflection that detects reflected electrons emitted from the surface of the sample 1. An electron detector 5 is an X-ray detector that detects energy dispersive X-rays.

6 is an electron gun for generating an electron beam, 6a is an orifice provided between the sample chamber 1 and the lens barrel 7, and 7 is a lens barrel. The electron optical system is maintained in a high vacuum (for example, 10 ⁻⁴ Pa). 8 is an oil rotary pump (RP) for exhaust, 9 is a foreline trap connected to the RP, and the foreline trap 9 is connected to the sample chamber 1. 11 is an oil rotary pump for exhaust, and 12 is an oil diffusion pump (DP) connected to the oil rotary pump 11. The oil diffusion pump 12 is connected to an electron optical system. The oil diffusion pump 12 and the oil rotary pump 11 indicate an exhaust system for pulling the electron optical system to a high vacuum.

  In the apparatus configured as described above, the sample chamber 1 is held in a low vacuum (1 to 200 Pa) by the exhaust system, the sample 2 is irradiated with the electron beam from the electron gun 6, and the sample 2 is detected by the backscattered electron detector 4, for example. Detect reflected electrons emitted from. The detected reflection signal is subjected to image processing by an image processing unit (not shown), and then an image is displayed on an image display unit (not shown).

On the other hand, when observing the sample surface with high resolution and high resolution, a secondary electron detector is used. The secondary electron detector is applied with a positive high voltage in order to efficiently take in secondary electrons from the sample surface. When a high voltage (10 kV) is applied, the vacuum pressure in the sample chamber needs to be a high vacuum of 10 ⁻² Pa or less. Under a vacuum pressure of 10 ⁻² Pa or more, the operation becomes unstable due to discharge or the like, and the function of the secondary electron detector is impaired.

  In the low vacuum scanning electron microscope as described above, when the sample 2 is an insulator, an ion source generator is installed to neutralize the charge charged in the insulator. As the ion source generator, for example, a plasma generator is used.

  As a conventional apparatus of this type, there is known an ion irradiation apparatus that is provided outside a sample chamber and irradiates ionized ions in the sample chamber to neutralize the charge-up of the sample (see, for example, Patent Document 1). ). In addition, a technique is known in which positive ions are bombarded from an ion gun provided in a sample chamber toward the surface of the sample so that the sample surface is not charged to a negative potential (see, for example, Patent Document 2).

  Also, a plasma generator that generates stable ultrafine plasma on the premise that it is provided in a sample chamber is known (see, for example, Patent Document 3). FIG. 5 is a diagram illustrating a configuration example of the plasma generation apparatus described in Patent Document 3. In FIG. In the figure, reference numeral 21 denotes a plasma torch, which is composed of a glass tube 22 having a narrowed tip and a metal tube 23 fitted so as to seal the base of the glass tube 22. The tip of the glass tube 22 is processed to be thin. Reference numeral 24 denotes a high frequency coil wound around the distal end portion 29 of the glass tube 22. Reference numeral 25 denotes a high frequency power source for applying a high frequency to the high frequency coil 24 via a matching circuit 24a. Reference numeral 26 denotes an ignition coil. Reference numeral 27 denotes an ignition coil 26. This is a high voltage power supply for applying a high voltage.

  A gas introduction tube 39 is inserted into the metal tube 23 so that a plasma gas (for example, argon gas) can be introduced from the gas cylinder 30 through the tube 39 into the plasma torch 21. In the flow path between the gas cylinder 30 and the plasma torch 21, a compressor 31, a flow rate adjustment valve 32, and a flow meter 33 for applying an appropriate pressure to the plasma gas are disposed.

  In the plasma torch 21, a refractory metal wire (for example, tungsten wire) 28 is disposed along the axial center of the torch. The diameter of the wire 28 is different between the metal tube 23 side and the distal end portion 29. The operation of the apparatus configured as described above will be described as follows.

  First, argon gas is introduced into the plasma torch 21 through the gas introduction tube 39 in advance. As a result of the pressure applied by the compressor 31, the introduced argon gas flows at high speed in the direction of the distal end portion 29 of the glass tube 22 and is ejected from the distal end. Next, the high frequency power supply 25 is operated to supply high frequency power to the high frequency coil 24. As a result, a high-frequency electromagnetic field is generated at the coil winding portion of the glass tube 22, and the tip of the wire 28 is subjected to high-frequency induction heating to be in a heated state. In this state, when the high voltage power supply 27 is operated and a high voltage of, for example, about 15 KV is applied to the ignition coil 26, a discharge is generated at the distal end portion 29 of the glass tube 22, and the plasma generated by this discharge is used as a spark. Inductive plasma is generated by the high-frequency power supplied through the high-frequency coil 24.

  As a conventional apparatus of this type, there is known another plasma generator that generates stable ultrafine plasma on the assumption that it is provided in a sample chamber (for example, see Patent Document 4). FIG. 6 is a diagram showing another conventional configuration example of the plasma generator. In the figure, reference numeral 40 denotes a gas introduction tube having one end connected to a metal tube 53 and the other end connected to a gas supply system 41 including a gas cylinder, a compressor, a flow valve and a flow meter. 42 is a Diegel wire having one end connected to the high voltage power supply 47 and the other end connected to the ignition wire 58. In the figure, S is a shield.

  43 is a coaxial cable having one end connected to the high frequency power supply 45 and the other end connected to the matching circuit MA. The coaxial cable 43 includes a portion 43A on the high frequency power supply 45 side and a portion 43B on the matching circuit side, and the two coaxial cables are connected by a dedicated connector 44 that connects the coaxial cables. After that, the core wire from which the covering portion has been removed extends to the portion of the ignition wire 58 after passing through the hole on the metal tube 53 side.

In the plasma generator having such a configuration, when the high voltage power supply 47 is operated and a high voltage of about 15 KV, for example, is applied to the ignition wire 58, a discharge is generated at the tip 52F in the glass tube, and this discharge is seeded. Inductive plasma is generated by high-frequency power supplied through the high-frequency coil 54 as fire. The generated induction plasma is supplied with thermoelectrons generated from the wire 58 in a high temperature state and is stably maintained. As a result, a plasma flame having a minute diameter is ejected from the tip of the glass tube.
JP-A-8-273579 (2nd page, FIG. 1) JP-A-5-275046 (second page, FIG. 1) JP 2002-343599 A (2nd page, 3rd page, FIG. 1) JP 2003-173898 A (page 4, page 5, FIG. 2)

  Since the above-described plasma generator described in Patent Document 1 is provided outside the scanning electron microscope, there is a problem that the configuration is complicated, large, and expensive. The plasma generator described in Patent Document 2 also has a problem that the apparatus is large and expensive. Moreover, although the plasma generators described in Patent Document 3 and Patent Document 4 are suitable for miniaturization, there is a problem that discharge occurs outside the high-frequency coil. In addition, there is a problem that external noise is generated in the space between the objective lens and the sample, and the resolution is adversely affected. In addition, when light generated by plasma enters the backscattered electron detector, the entire image becomes white and the image contrast is impaired.

  On the other hand, when performing high-resolution SEM observation, it is necessary to use a secondary electron detector having a positive high voltage, but a large amount of electrons in the plasma generated by the plasma generator are taken into the secondary electron detector. Therefore, there is a problem that it is impossible to observe the secondary electron image of the sample.

  The present invention has been made in view of the above problems, and is small and hardly affected by noise. The function of blocking plasma electrons in the secondary electron detector and the light generated by the plasma generation have an adverse effect on the image detection system. An object of the present invention is to provide a plasma generating apparatus that does not give the above, and to provide a scanning electron microscope using the plasma generating apparatus.

  The invention described in claim 1 has a cylindrical shape, a quartz tube in which an ionizing gas flows, a high-frequency coil wound around the tip of the quartz tube, and the periphery of the quartz tube and the high-frequency coil And a cylindrical pipe portion covering the entire torch and the vicinity of the anode, a throttle provided at the tip of the quartz tube, and an anode provided at the tip of the torch. It is characterized by that.

According to the second aspect of the present invention, when the atmospheric pressure in the sample chamber is about 200 Pa to 10 ⁻⁴ Pa, the sample is irradiated with an electron beam, and an electron signal emitted from the sample surface is detected to obtain a sample image. In a scanning electron microscope, when the sample is an insulator, the plasma generator according to claim 1 is arranged in the vicinity of the sample, and the charge charged to the sample by ions generated from the plasma generator It is characterized by canceling out.

The invention described in claim 3 is characterized in that cooling water is allowed to flow inside the plasma generator.
According to a fourth aspect of the present invention, cooling water is allowed to flow inside the plasma generator.

According to a fifth aspect of the present invention, there is further provided a metal portion covering a cable for applying a high frequency to the high frequency coil from the plasma generator to the lens barrel wall.
The invention according to claim 6 is characterized in that only electrons from the sample are detected by the secondary electron detector by completely taking electrons in the plasma state into the anode by the anode voltage of the plasma generator. And

  According to the seventh aspect of the present invention, the appropriate ion amount commensurate with the amount of electron beam applied to a predetermined sample surface can be easily adjusted by the anode voltage of the plasma generator, the high frequency power and the ionization gas amount. It is characterized by that.

  According to the plasma generator according to the invention of claim 1, by covering the periphery of the high-frequency coil with a mold, it is possible to prevent discharge in the high-frequency coil, and by covering the entire torch and the vicinity of the anode portion with a metal part, By eliminating the influence of noise and providing an aperture in the interior, it is possible to reduce light due to discharge entering the detector. In addition, by applying a positive voltage to the anode part, electrons in the plasma state are completely taken into the anode, thereby preventing deterioration of the S / N ratio of the secondary electron detector.

According to the invention described in claim 2, by using the plasma generator of claim 1 for ion generation in the sample chamber, an image of the insulator sample can be accurately obtained.
According to invention of Claim 3, the temperature of a plasma generation part can be lowered | hung and stable plasma discharge can be maintained.

According to invention of Claim 4, the temperature of a plasma generation part can be lowered | hung and stable plasma discharge can be maintained.
According to invention of Claim 5, it becomes difficult to receive to the influence of electromagnetic noise.

According to the sixth aspect of the present invention, only the electrons from the sample can be detected by the secondary electron detector.
According to the seventh aspect of the invention, it is possible to obtain an appropriate ion amount commensurate with the amount of electron beam applied to the sample surface.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a block diagram showing an embodiment of a plasma generator according to the present invention. In the figure, 60 is a quartz tube through which an ionizing gas (eg, argon) flows, 61 is a tungsten wire disposed in the quartz tube 60, and 63 is wound around the tip of the quartz tube 60. It is a high frequency coil. As the number of turns of the high-frequency coil 63, for example, about 5 turns is used. 64 is a glass tube attached to the tip of the quartz tube 60, 65 is a mold that covers the periphery of the quartz tube 60 and the high-frequency coil 63, 66 is an aluminum foil as a metal part that covers the entire torch and the vicinity of the anode, 67 Is a pipe as a cylindrical insulator for holding the anode, and 74 is an anode made of copper, which is provided at the tip of the torch. Reference numeral 73 denotes a diaphragm in which an antireflection film is applied to quartz glass connected to the glass tube 64. Such a configuration is sometimes referred to as a torch.

  Reference numeral 68 denotes a coaxial cable that allows high frequency to pass therethrough. As the coaxial cable 68, for example, 3D-2V is used. The coaxial cable 68 contains lead wires, and a high frequency is supplied to the high frequency coil 63. A matching capacitor 69 connects the coaxial cable 68 and the high-frequency coil 63. A trigger wire 70 supplies a trigger signal to the tungsten wire 61. Reference numeral 93 denotes an aluminum foil serving as a metal part covering a coaxial cable from the plasma generator to the lens barrel wall surface. 71 and 72 are pipes through which cooling water passes. The water that has passed through the pipe 71 passes through the high-frequency induction heating unit and exits from the pipe 72. In this way, the plasma generator is cooled by passing water through the plasma generator. When the plasma generating part is cooled, the temperature of the plasma generating part can be lowered and stable plasma discharge can be maintained. The length of the torch part is, for example, approximately 20φ (mm) × 70 (mm). A predetermined voltage is supplied to the anode 74 by a withstand voltage cable (not shown).

The operation of the apparatus configured as described above will be described as follows.
First, argon gas is introduced into the quartz tube 60 in advance. Here, when a pressure is applied to the argon gas, the introduced argon gas flows at high speed toward the tip of the quartz tube 60 and is ejected from the tip. Next, high frequency power is input from the coaxial cable 68. The input high frequency power is applied to the high frequency coil 63 via the matching capacitor 69. Next, a trigger signal is input from the trigger wire 70. As a result, a high-frequency electromagnetic field is generated in the coil winding portion of the quartz tube 60, and a discharge is generated at the tip of the quartz tube 60. Inductive plasma is generated by high frequency power.

  In a normal plasma generator, when a high frequency coil is excited at a high frequency of, for example, four hundred and several tens of MHz, glow discharge is generated outside the high frequency coil 63 without discharging in a quartz tube and an argon atmosphere. In order to prevent such discharge, the periphery of the quartz tube 60 and the high-frequency coil 63 is covered with a mold 65. As a result, glow discharge does not occur outside the high-frequency coil 63, and plasma can be generated stably by exciting the torch.

  Further, it is known that the scanning electron microscope is vulnerable to external noise. When the torch was excited at several W, it was found that external noise adversely affects the resolution in the space between the objective lens and the sample. Therefore, it has been experimentally confirmed that if the space between the objective lens and the sample is surrounded by a copper plate as a countermeasure, the influence of torch noise is almost eliminated.

  Since the sample stage has a function of moving in the sample chamber, it is not practical to shield it with a copper plate in order to remove the adverse effect on the resolution. Therefore, in the experiment, an aluminum foil was used to cover the rear part of the torch to the flange part. In this case, if the coaxial cable 68 or the like is directly covered, matching (matching) is shifted, which is not appropriate. Therefore, the aluminum foil 93 needs to be covered with a slight gap. Specifically, it is considered that a favorable result of noise reduction can be obtained when a metal case is employed.

  Usually, the scanning electron microscope has two functions of image observation with secondary electrons and image observation with reflected electrons. Here, when an insulator sample is observed, image observation with reflected electrons is performed, for example, at a pressure in the sample chamber of about 1 Pa to 200 Pa, and preferably about 10 Pa to 50 Pa.

  In this case, when stray light in the sample chamber enters the backscattered electron detector, the image screen becomes whitish. Generally, light from discharge is emitted from the tip of the plasma generator. As a result, the detected image becomes whitish. Several methods are conceivable as a method for preventing this light from entering the backscattered electron detector. For example, as a method used in the experiment, a diaphragm is provided inside the plasma torch in FIG. Specifically, an antireflection coating is applied to the inside of the quartz glass diaphragm 73 as the antireflection film. Thereby, a stable observation image is obtained. As another method, a method of performing black alumite treatment in a sample chamber or the like can be considered.

  The operation of the apparatus configured as described above will be described as follows. Here, the procedure in the case of observing an insulator sample is shown. FIG. 2 is a flowchart showing an operation procedure of the plasma generator. Hereinafter, it demonstrates along this flowchart.

First, preparation for operation of the scanning electron microscope is performed (step 1). Specifically, argon gas is introduced into the torch. Next, the insulator sample is placed on the sample stage and set at a predetermined position (step 2). Next, the lens barrel and the sample chamber are evacuated (step 3). Specifically, the pressure is set to about 200 Pa to about 10 ⁻⁴ Pa. Preferably, it sets to 10-50 Pa. Next, a high frequency of several W (for example, 430 MHz to 440 MHz) is applied to the plasma generator (step 4).

  As a result, a high-frequency electromagnetic field is generated in the coil winding portion of the quartz tube 60, the tip of the tungsten wire 61 is subjected to high-frequency induction heating, and is heated to generate induction plasma (microplasma) (step 5). Next, image observation is performed by operating the scanning electron microscope (step 6). After the image observation is finished, the high frequency power is cut off and the introduction of argon gas is stopped. Thereafter, image observation is performed by the operation procedure of the low vacuum scanning electron microscope.

  According to the present invention, by covering the periphery of the high-frequency coil with a mold, it is possible to prevent discharge at the high-frequency coil, and by covering from the torch tip to the barrel surface with the metal part, it is not affected by noise. By providing the anti-reflection film aperture 73 inside the plasma torch, it is possible to reduce the light from the discharge from entering the detector (secondary electron detector, reflected electron detector).

FIG. 3 is a diagram showing a configuration example of a low vacuum scanning electron microscope according to the present invention. In the figure, 100 is a lens barrel and 101 is a sample chamber. Here, the inside of the lens barrel 100 is maintained at a high vacuum (for example, 10 ⁻⁴ Pa), and the sample chamber 101 is maintained at, for example, 200 Pa to 10 ⁻⁴ Pa. 80 is a filament, 81 is a grid (Wehnelt electrode) arranged near the filament 80, and 82 is an anode (anode).

  83 is a focusing lens for focusing the electron beam, 84 is a stop disposed below the focusing lens 83, 85 is an objective lens for imaging the electron beam on the sample, and 86 is provided below the objective lens 85. A deflector (deflector) 87 is an astigmatism correction stigma (STIGMA) provided close to the deflector 86.

  Reference numeral 88 denotes a sample stage, 89 denotes an insulator sample placed on the sample stage 88, and 90 denotes a plasma generator (torch unit) according to the present invention disposed in the vicinity of the insulator sample 89. The plasma generator used in the present invention is sometimes called a microplasma generator because the scale of plasma generation is small. 91 is a backscattered electron detector that detects backscattered electrons from the insulator sample 89, and 92 is a secondary electron detector that similarly detects backscattered electrons from the sample 89. Reference numeral 98 denotes a flange. The operation of the apparatus configured as described above will be described as follows.

  The electrons emitted from the filament 80 pass through the grid 81 and are attracted to the anode 82, and are emitted as an electron beam. The electron beam is focused by the focusing lens 83 and then passes through the stop 84. This electron beam forms an image on the insulator sample 89 by the objective lens 85 that follows. In this case, the electron beam is deflected by the deflector 86, corrected for astigmatism by the stigma 87, and imaged on the insulator sample 89.

The insulator sample 89 is charged. On the other hand, positive ions are generated from the plasma generator (torch unit) 90 according to the present invention disposed in the sample chamber, and neutralizes the charge charged in the insulator sample 89. As a result, correct reflected electrons are emitted from the surface of the insulator sample 89. The reflected electrons are detected by a reflected electron detector 91 disposed in the sample chamber, subjected to image processing by an image processing unit (not shown), and then displayed on a display device (not shown). According to the present invention, a sample image that is not affected by the charge from the insulator sample 89 can be obtained by using the above-described plasma generator for generating ions in the sample chamber. On the other hand, when carrying out SEM image observation of a high-resolution insulator sample, the secondary electron detector 92 is selected by evacuating the vacuum pressure in the sample chamber 101 to 10 ⁻² Pa or less.

  A positive voltage is applied to the anode 74 of the plasma torch so that the plasma electrons do not flow into the secondary electron detector 92, and the plasma electrons are completely taken into the anode. Secondary electrons are emitted from the surface of the sample 89, the secondary electrons are detected by the secondary electron detector 92, and image processing is performed to obtain a sample image.

As described above in detail, when the low-vacuum scanning electron microscope according to the present invention is used, an image of an insulator sample can be accurately obtained by using the plasma generator described above.
In the above-described embodiment, the case where argon is used as the plasma gas is taken as an example, but the present invention is not limited to this, and other gases such as nitrogen gas can also be used.

It is a block diagram which shows one embodiment of the plasma generator concerning this invention. It is a flowchart which shows the operation | movement procedure of a plasma generator. It is a figure which shows the structural example of the low vacuum scanning electron microscope which concerns on this invention. It is a composition conceptual diagram of a scanning electron microscope. It is a figure which shows the example of a conventional structure of a plasma generator. It is a figure which shows the other conventional structural example of a plasma generator.

Explanation of symbols

60 Quartz tube 61 Tungsten wire 63 High frequency coil 64 Glass tube 65 Mold 66 Aluminum foil 67 Anti-reflection film 68 Coaxial cable 69 Matching capacitor 70 Trigger wire 71 Tube 72 Tube 73 Aperture 74 Anode

Claims (7)

A quartz tube that has a cylindrical shape and allows gas for ionization to flow inside.
A high frequency coil wound around the tip of the quartz tube;
A mold covering the periphery of the quartz tube and the high-frequency coil;
A cylindrical pipe part covering the whole torch and the vicinity of the anode,
A diaphragm provided at the tip of the quartz tube;
An anode provided at the tip of the torch;
A plasma generator comprising: In a scanning electron microscope in which the sample chamber pressure is about 200 Pa to 10 ⁻⁴ Pa, the sample is irradiated with an electron beam, and electrons emitted from the sample surface are detected to obtain a sample image.
When the sample is an insulator, the plasma generator according to claim 1 is arranged in the vicinity of the sample so as to cancel the electric charge charged to the sample by positive ions generated from the plasma generator. A scanning electron microscope characterized by that.   2. The plasma generator according to claim 1, wherein cooling water is allowed to flow inside the plasma generator.   The scanning electron microscope according to claim 2, wherein cooling water is allowed to flow inside the plasma generator.   3. The scanning electron microscope according to claim 2, further comprising a metal part covering a cable for applying a high frequency to the high frequency coil from the plasma generator to the lens barrel wall.   3. The scanning according to claim 2, wherein only electrons from the sample are detected by the secondary electron detector by completely taking electrons in the plasma into the anode by the anode voltage of the plasma generator. electronic microscope.   3. An appropriate ion amount commensurate with the amount of electron beam applied to a predetermined sample surface is easily adjusted by the anode voltage, high-frequency power and ionization gas amount of the plasma generator. The scanning electron microscope as described.

Images