JP5597572B2 - Charged particle detector, time-of-flight mass spectrometer - Google Patents

Description

The present invention charged particle detector for detecting the ions to be incident, and a time-of-flight mass spectrometer for performing mass analysis of the sample by measuring the time of flight of the ions by using the charged particle detector is there.

When a sample is irradiated with a charged particle beam (ion beam, electron beam, X-ray, etc.), ions (hereinafter referred to as secondary ions) are generated from the sample. Conventionally, a time-of-flight type (TOF type) that measures the time of flight until the secondary ions are detected by a charged particle detector separated from the sample by a predetermined distance, and performs mass analysis of the sample based on the time of flight. A mass spectrometer is known (see, for example, Patent Document 1). The charged particle detector uses a microchannel plate (MCP) that amplifies secondary ions generated in the sample as electrons.
FIG. 6 is a schematic sectional view showing an example of a conventional charged particle detector.
The charged particle detector shown in FIG. 6 has MCPs 111 and 112 stacked in two stages along the incident direction of secondary ions, and a secondary detector 113 that detects secondary electrons emitted from the MCP 112. doing.
The MCPs 111 and 112 are made of a glass material 51 as shown in FIG. 7, and a large number of channels 52 (photomultiplier tubes) penetrating the front and back surfaces of the MCPs 111 and 112 and a non-channel portion 53 in which the channels 52 are not formed. And have. Further, the entire surface of the MCPs 111 and 112 has a function as the electrode 54. Here, a potential Vin that is negative with respect to the potential Vout of the exit surface of the secondary electrons from the MCP 112 is applied to the electrode 54 that is the entrance surface of the secondary ions of the MCP 111. Note that the secondary detector 113 is given a potential Vs higher than the potential Vout of the exit surface of the MCP 112.
By the way, in the MCP 111, secondary ions incident on the non-channel portion 53 between the channels 52 do not contribute to the emission of secondary electrons. That is, the detection efficiency indicating the ratio of the incident secondary ions contributing to the amplification of secondary electrons is limited by the aperture ratio (for example, about 55%) of the channel 52 in the MCP 111.
On the other hand, as shown in FIG. 6, it is conceivable to arrange the grid electrode 102 to which the potential Vg lower than the potential Vin of the incident surface of the MCP 111 is provided at a position facing the MCP 111. As a result, secondary ions e ⁻ emitted from the non-channel portion 53 of the MCP 111 in a direction opposite to the MCP 111 due to collision with secondary ions are caused by the potential difference between the grid electrode 102 and the MCP 111. And input to the channel 52. Therefore, the detection efficiency in the MCP 111 can be improved.

JP 2009-289628 A

従って，本発明は上記事情に鑑みてなされたものであり，その目的とするところは，ＭＣＰへのイオンの入射方向上流側に配置されたグリッド電極で発生する二次電子に起因した検出誤差を防止することのできる荷電粒子検出器及び飛行時間型質量分析装置を提供することにある。 Incidentally, as shown in FIG. 8, in the charged particle detector having the grid electrode 102, a part of secondary ions collide with the grid line 121 that forms the grid electrode 102. As a result, secondary electrons are generated from the grid lines 121 of the grid electrode 102.
Furthermore, the cross-sectional shape of the grid line 121 forming the conventional grid electrode 102 is generally circular. In this case, as shown in FIG. 8, secondary electrons e ⁻ generated by collision of secondary ions with the top of the grid line 121 are emitted in a direction away from the MCP 111, but collide with other parts. Secondary electrons e ⁻ generated in this manner are emitted toward the MCP 111.
However, secondary electrons e ⁻ generated in the grid lines 121 should not be detected by the secondary detector 113. Therefore, due to the secondary electrons e ⁻ , for example, an error occurs in the sample mass measurement result in the time-of-flight mass spectrometer.
Specifically, when the distance between the grid electrode 102 and the MCP 111 is d, the mass of secondary ions is M, the energy is Ei, the mass of secondary electrons is m, and the energy is Ee, the flight time of each distance d is d (M / Ei) ^1/2 and d (m / Ee) ^1/2 . However, the mass M differs from the mass m by 3 digits or more, and the flight time of secondary electrons is negligibly small compared to the flight time of secondary ions. Therefore, the deviation ΔM of the mass measurement result caused by the secondary electrons e ⁻ emitted from the grid line 121 is as follows, assuming that the total flight distance of the secondary ions is σ and the valence of the secondary ions is q: (1)

Therefore, the present invention has been made in view of the above circumstances, and the object of the present invention is to detect a detection error caused by secondary electrons generated at a grid electrode arranged on the upstream side of the incident direction of ions to the MCP. It is an object to provide a charged particle detector and a time-of-flight mass spectrometer that can be prevented.

To accomplish the above object, giving a microchannel plate which ions are incident, said lower potential than the potential of said entrance surface while being arranged opposite to the incident surface of ions into the microchannel plate It was made and a grid electrode function, the grid electrode, to move the secondary electrons generated when the ions in the grid electrode is irradiated in a direction away from the incident surface of the microchannel plate It is comprised as a charged particle detector characterized by having.
According to the present invention, since secondary electrons generated in the grid electrode are prevented from entering the microchannel plate, errors in detection results caused by the secondary electrons can be suppressed.

For example, the grid electrode may be formed by a plurality of grid lines having a cross-sectional shape that tapers from the upstream side to the downstream microchannel plate in the ion incident direction. Specifically, the cross-sectional shape of each grid line may be a triangle such as a regular triangle or an isosceles triangle.
Thereby, when the said ion collides with the said grid line from the incident direction upstream, generation | occurrence | production of the secondary electron radiate | emitted toward the said microchannel plate side can be suppressed. For example, if the end face on the upstream side of the grid line is a plane perpendicular to the incident direction of the ions as in the case where the cross-sectional shape of the grid lines is a triangle, the secondary electrons generated by the collision of the ions are The ions are emitted toward the upstream side in the incident direction. In addition, since the grid line is tapered toward the microchannel plate, generation of secondary electrons is reduced by reducing the possibility that the ions incident obliquely to the grid electrode collide with the side surface of the grid line. Can be suppressed.

Further, the grid electrode has at least two mesh layers arranged so that the opening positions overlap when viewed in the incident direction of the ions, and is arranged to face the microchannel plate in the mesh layer. A potential lower than that of the microchannel plate is applied to the downstreammost mesh layer formed, and a potential that sequentially increases from the downstream mesh layer toward the upstream side in the ion incidence direction is applied to the other mesh layer. Is also possible.
As a result, secondary electrons generated by collision of ions with the mesh layer arranged in the upper stage move toward the upper mesh layer due to a potential difference with the mesh layer arranged in the lower stage of the mesh layer. It will be. That is, the secondary electrons generated in the grid electrode move in a direction away from the incident surface of the microchannel plate, and the incidence on the microchannel plate is suppressed.
By the way, the present invention includes the charged particle detector, and analyzes the mass of the sample based on the time of flight until ions generated by irradiating the sample with a charged particle beam are detected by the charged particle detector. It may also be understood as an invention of a time-of-flight mass spectrometer. In the time-of-flight mass spectrometer, it is possible to suppress a measurement error of the sample mass due to secondary electrons generated at the grid electrode.

  According to the present invention, it is possible to prevent a detection error caused by secondary electrons generated at the grid electrode arranged on the upstream side of the incident direction of ions to the MCP. Specifically, in the time-of-flight mass spectrometer, measurement errors in the sample mass due to secondary electrons generated at the grid electrode can be suppressed.

1 is a schematic cross-sectional view showing a schematic configuration of a charged particle detector according to an embodiment of the present invention. The principal part enlarged view of the charged particle detector X which concerns on embodiment of this invention. The figure which shows an example of the production | generation method of the grid electrode 2 with which the charged particle detector X which concerns on embodiment of this invention is provided. The schematic cross section which shows schematic structure of the charged particle detector Y which concerns on the Example of this invention. The principal part enlarged view of the charged particle detector Y which concerns on the Example of this invention. The schematic cross section which shows schematic structure of the conventional charged particle detector. The figure for demonstrating an example of a microchannel plate. The principal part enlarged view of the conventional charged particle detector.

Embodiments of the present invention will be described below with reference to the accompanying drawings for understanding of the present invention. In addition, the following embodiment is an example which actualized this invention, Comprising: It is not the thing of the character which limits the technical scope of this invention.
First, a schematic configuration of the charged particle detector X according to the embodiment of the present invention will be described with reference to FIG.
As shown in FIG. 1, the charged particle detector X detects secondary electrons emitted from the MCPs 11 and 12 stacked in two stages along the ion incident direction and the emission surface 12 a of the MCP 12. A secondary detector 13, a grid electrode 2 disposed opposite to the incident surface 11 a of the MCP 11, and a grid support portion 3 that supports the grid electrode 2 are included. The charged particle detector X also includes a power source (not shown) that individually applies preset voltages to the MCPs 11 and 12, the secondary detector 13, and the grid electrode 2.
The grid support part 3 supports the grid electrode 2 through a gap of about 2 mm from the MCP 11. Thereby, the grid electrode 2 and the MCP 11 are in an insulated state.
The secondary detector 13 is an anode electrode, a phosphor screen, or the like for detecting secondary electrons output from the MCPs 11 and 12 by ions incident on the charged particle detector X.

  The charged particle detector X accelerates ions (hereinafter referred to as “secondary ions”) generated from the sample when the sample is irradiated with a charged particle beam (ion beam, electron beam, X-ray, etc.) The time of flight until the secondary ions are detected by the charged particle detector X arranged at a position away from the sample by a predetermined distance, and the mass of the sample is analyzed based on the time of flight Mounted on a mass spectrometer. The present invention can also be understood as an invention of the time-of-flight mass spectrometer. Note that the basic configuration of the time-of-flight mass spectrometer may be the same as that of the prior art, and the description thereof is omitted here. Of course, the use of the charged particle detector X is not limited to the time-of-flight mass spectrometer.

Each of the MCPs 11 and 12 is formed of the glass material 51, as already described with reference to FIG. 7, and a plurality of channels 52 (photomultiplier tubes) penetrating the front and back surfaces are formed. There is no non-channel portion 53 or the like. For example, the aperture ratio due to the channel 52 in the MCPs 11 and 12 is about 55%. Further, as shown in FIG. 1, each of the channels 52 is formed to be inclined with respect to the front and back surfaces of the MCPs 11 and 12 by a predetermined angle (for example, about 5 ° to 20 ° depending on the application).
In the MCPs 11 and 12, secondary ions are input into the channel 52 from a direction substantially perpendicular to the incident surface 11 a of the MCP 11. As a result, in the channel 52, the input secondary ions collide with the inner wall of the channel 52 to generate new secondary electrons, and a large number of secondary ions amplified by repeated collisions. Secondary electrons are output. In the charged particle detector X, the MCPs 11 and 12 are overlapped so that the inclination directions of the channels 52 are reversed in order to increase the amplification factor of secondary electrons in the MCPs 11 and 12.
The entrance surface 11a of the MCP 11 and the exit surface 12a of the MCP 12 serve as the electrodes 54 to which a voltage is applied from a power source (not shown). A voltage that is set in advance so that the potential Vout of the exit surface 12a is higher than the potential Vin of the entrance surface 11a is not shown in each of the entrance surface 11a and the exit surface 12a acting as the electrode 54. It is applied from the power source. As a result, the secondary electrons are accelerated in the direction of the secondary detector 13 in the MCPs 11 and 12. The secondary detector 13 is supplied with a potential Vs higher than the potential Vout of the exit surface 12a of the MCP 12 from a power source (not shown). That is, the potentials Vin, Vout, and Vs have a relationship of Vin <Vout <Vs. For example, the potential Vin is −2200 [V], the potential Vout is −200 [V], and the potential Vs is 0 [V].

Next, the detailed shape of the grid electrode 2 will be described with reference to FIG. FIG. 2 is an enlarged view of a main part of the charged particle detector X.
As shown in FIG. 2, the grid electrode 2 is an electrode formed by arranging a plurality of grid lines 21 vertically and horizontally and arranged in a grid pattern. For example, the grid electrode 2 is a stainless mesh having a 90% aperture ratio in which the width of the grid lines 21 is 20 [μm] and the pitch of the grid lines 21 is 400 [μm]. Each grid line 21 of the grid electrode 2 is preferably coated with a material such as carbon (C) or titanium nitride (TiN) having a small secondary electron emission coefficient. Thereby, the generation amount of secondary electrons in the grid line 21 can be suppressed.
In the charged particle detector X, a voltage Vg lower than the potential Vin of the incident surface 11a of the MCP 11 is applied to the grid electrode 2 by applying a voltage from a power source (not shown) ( Vg <Vin). That is, a potential difference is generated between the grid electrode 2 and the incident surface 11a of the MCP 11 so that the incident surface 11a side is positive. Therefore, secondary electrons e ⁻ generated when the secondary ions collide with the non-channel portion 53 of the MCP 11 return to the incident surface 11 a due to a potential difference between the grid electrode 2 and the incident surface 11 a of the MCP 11. Then, the light is incident on the channel 52. Thereby, for example, the detection efficiency limited by the aperture ratio of about 55% of the MCP 11 can be improved to about 70%.

Furthermore, in the charged particle detector X, the grid electrode 2 causes the secondary electrons generated when the grid electrode 2 is irradiated with the secondary ions to move away from the incident surface 11a of the MCP 11. It has a function to move.
Specifically, as shown in FIG. 2, each grid line 21 of the grid electrode 2 has a triangular cross-sectional shape that tapers from the upstream side in the incident direction of the secondary ions toward the MCP 11 on the downstream side. doing. In particular, the end face 21a on the upstream side in the incident direction of the secondary ions of the grid line 21 is a plane perpendicular to the incident direction of the secondary ions. For example, the cross-sectional shape of the grid line 21 is an equilateral triangle or an isosceles triangle.
Note that various conventionally known processing techniques may be used as a method of making the cross-sectional shape of each grid line 21 of the grid electrode 2 triangular. For example, as shown in FIGS. 3A to 3F, it is possible to form the mesh-shaped grid electrode 2 having a substantially triangular cross section by anisotropic photomask etching.

In the charged particle detector X configured as described above, most of the secondary ions pass through the mesh opening of the grid electrode 2 and enter the incident surface 11a of the MCP 11 as shown in FIG. become. However, since some of the secondary ions collide with the grid lines 21 of the grid electrode 2, secondary electrons e ⁻ are generated in the grid lines 21.
However, in the charged particle detector X, the grid line 21 has a convex shape that tapers from the upstream side in the incident direction of the secondary ions toward the MCP 11 on the downstream side. That is, as shown in FIG. 2, the end surface 21a of the grid line 21 has the maximum outer diameter in the direction perpendicular to the incident direction of the secondary ions in the grid line 21. Therefore, the secondary electrons e ⁻ generated by the collision of the secondary ions with the end surface 21 a of the grid line 21 are not emitted from the end surface 21 a toward the MCP 11 and are separated from the MCP 11. Will be emitted. Therefore, the incidence of the secondary electrons e ⁻ generated at the grid electrode 2 on the MCP 11 can be suppressed as compared with the conventional case.
Further, the side surface of the grid line 21 is formed with an inclination that tapers toward an end on the downstream side in the incident direction of the secondary ions. Therefore, the possibility that the secondary ions incident on the grid electrode 2 from an oblique direction come into contact with the side surface of the grid line 21 is reduced, and the generation amount of secondary electrons in the grid line 21 is suppressed.

As described above, in the charged particle detector X, since the secondary electrons generated in the grid electrode 2 are prevented from entering the MCP 11, the detection result error caused by the secondary electrons is suppressed. be able to. Specifically, in the time-of-flight mass spectrometer equipped with the charged particle detector X, the measurement error of the sample mass due to the secondary electrons generated at the grid electrode 2 can be suppressed.
The cross-sectional shape of the grid line 21 is not limited to a triangle. Specifically, the cross-sectional shape of the grid line 21 may be any shape that expands from the downstream side to the upstream side in the incident direction of the secondary ions. For example, the grid line 21 may have a semicircular cross section. Further, the end surface 21a of the grid line 21 may have, for example, a concave shape having a depression in the downstream direction of the incident direction of the secondary ions.

In the present embodiment, another method for suppressing the incidence of secondary electrons generated in the grid electrode 2 on the MCP 11 will be described with reference to FIGS. 4 and 5. In addition, about the charged particle detector Y which concerns on a present Example, about the structure similar to the charged particle detector X demonstrated in the said embodiment, the same code | symbol is attached | subjected and the description is abbreviate | omitted.
As shown in FIGS. 4 and 5, the charged particle detector Y according to the present embodiment is arranged in such a manner that the opening positions are overlapped when viewed in the incident direction of the secondary ions, instead of the grid electrode 2. A grid electrode 4 having two mesh layers 41 and 42 is provided. Each of the mesh layers 41 and 42 is a grid-like electrode in which a plurality of grid lines 41 a and 42 a are arranged vertically and horizontally like the grid electrode 2. The cross-sectional shape of each of the grid lines 41a and 42a is circular. Therefore, the grid electrode 4 seems to be formed of one mesh when viewed in the incident direction of the secondary ions.
As a method for arranging the mesh openings of the mesh layers 41 and 42 so that the positions of the mesh openings overlap each other, for example, a positioning member inscribed in the openings of the mesh layers 41 and 42 is provided on the mesh layers 41 and 42. It is possible to insert two or more places. In addition, in a state where the mesh layers 41 and 42 are arranged to overlap each other, secondary ions input to the charged particle detector Y are detected by the secondary detector 13, and the mesh layers are detected based on the detection result. It is conceivable to adjust the positions of 41 and 42 to fix them. For example, at the position where the mesh openings completely overlap, the secondary ions detected by the secondary detector 13 are considered to be the maximum, so that the position may be the fixed position of the mesh layers 41 and 42. .

In addition, each of the mesh layers 41 and 42 of the grid electrode 4 is disposed in an insulating state by interposing an insulating member 43, and different voltages can be applied to the mesh layers 41 and 42, respectively.
In the charged particle detector Y, the mesh layer 42 (corresponding to the most downstream mesh layer) disposed opposite to the MCP 11 among the mesh layers 41 and 42 is subjected to the potential Vin of the incident surface 11a of the MCP 11. A lower potential Vg2 is applied. On the other hand, the mesh layer 42 is given a potential Vg1 higher than the potential Vg2 of the mesh layer 41. That is, in the charged particle detector Y, the relationship of Vg1> Vg2 <Vin <Vout <Vs is established in the potentials Vg1, Vg2, Vin, Vout, Vs. For example, the potential Vg1 is −2200 [V], the potential Vg2 is −2300 [V], the potential Vin is −2200 [V], the potential Vout is −200 [V], and the potential Vs is 0 [V].

In the charged particle detector Y configured as described above, secondary electrons e ⁻ are generated when a secondary ion collides with the mesh layer 41 of the grid electrode 4 as shown in FIG.
In particular, when a secondary ion collides with a portion other than the top of the grid line 41a of the mesh layer 41, secondary electrons generated by the collision are emitted toward the MCP 11. However, the secondary electron e ⁻ has a small energy (for example, about 0.1 [keV]).
Therefore, secondary electrons e ⁻ generated in the grid line 41a are moved in the direction approaching the mesh layer 41 from the mesh layer 42, that is, in the MCP 11 due to the potential difference (Vg1> Vg2) between the mesh layers 41 and 42. On the other hand, it moves in the direction of separating. The secondary ions move toward the MCP 11 regardless of the potential difference between the mesh layers 41 and 42 because the secondary ions have large energy (for example, about 8 [keV]). In addition, when the secondary ions are incident on the grid electrode 4 at an angle, the secondary ions collide with the grid line 42a. Secondary electrons generated in the grid line 42a Similarly, it moves to the grid line 41a side.

As described above, also in the charged particle detector Y, it is possible to suppress the incidence of the secondary electrons generated when the secondary ions collide with the grid electrode 4 into the MCP 11, and the secondary electrons. The error of the measurement result due to the can be suppressed.
If the grid electrode 4 has at least two or more mesh layers arranged so that the opening positions and grid lines overlap when viewed in the incident direction of the secondary ions, the number of mesh layers is further increased. There may be many. In this case, a potential lower than that of the MCP 11 is applied to the most downstream mesh layer disposed opposite to the MCP 11 in the mesh layer, and the secondary ions from the most downstream mesh layer are applied to the other mesh layers. What is necessary is just to give the electric potential which becomes high in order toward the incident direction upstream.
The cross-sectional shape of the grid lines forming the mesh layer of the grid electrode 4 is not particularly limited to a circle, and may be another shape such as a triangle as described in the above embodiment.

11, 12: MCP (microchannel plate)
11a: entrance surface 12a: exit surface 13: secondary detector 2, 4: grid electrode 3: grid support portions 21, 41a, 42a: grid lines 41, 42: mesh layer 51: glass material 52: channel 53: non-channel Unit 54: Electrodes X, Y: Charged particle detector

Claims (5)

A microchannel plate which ions are incident,
It and a grid electrode potential lower than the potential of said entrance surface is given with disposed to face the incident surface of the ion to the microchannel plate,
Charged particle detection, wherein the grid electrode has a function of moving secondary electrons generated when the grid electrode is irradiated with the ions in a direction away from the incident surface of the microchannel plate. vessel.   2. The charged particle detector according to claim 1, wherein the grid electrode is formed by a plurality of grid lines having a cross-sectional shape that tapers from the upstream side toward the downstream side in the ion incident direction. .     The charged particle detector according to claim 2, wherein a cross-sectional shape of each grid line is a triangle. The grid electrode has at least two mesh layers arranged so that the opening positions overlap when viewed in the incident direction of the ions,
A potential lower than that of the microchannel plate is applied to the most downstream mesh layer disposed facing the microchannel plate in the mesh layer, and the other mesh layer is upstream of the ion incident direction from the most downstream mesh layer. The charged particle detector according to claim 1, wherein a potential increasing in order toward the side is applied. Comprising the charged particle detector according to any one of claims 1 to 4,
A time-of-flight mass spectrometer that analyzes the mass of a sample based on the time of flight until ions generated by irradiating the sample with a charged particle beam are detected by the charged particle detector.

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