JPH10334847A - Photoionization mass spectrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the detecting efficiency by repeating a process for making the laser beam focused on a sample by a first optical element, passed through a second optical element, reflected by a third optical element to be deflected in a direction vertical to an optical axis, further passed through the second and first optical elements, and reflected and deflected by a fourth optical element several times, and then discharging the laser beam to the outside rather than through the optical element. SOLUTION: A laser beam 7 from a laser 6 is entered in parallel with an optical axis of a lens 11, focused on a sample 1, returned to an almost parallel optical path by a lens 12, deflected and reflected in a direction vertical to the optical axis by a flat reflection mirror in which two sheets of mirrors are crossed to each other, and focused on the sample 1 by the lens 12. The laser beam is returned to an almost parallel beam by the lens 11, deflected and reflected by a flat reflection mirror 10, and then entered into the lens 11. This process is repeated. The pulses of the laser beam 7 are either overlapped on the sample 1 or the beams are closed to each other. As a result, the intensity of light is increased, or a diameter of the laser beam is increased, which improve the ionization efficiency or the laser dose rate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a photoion mass spectrometer.

[0002]

2. Description of the Related Art Conventionally, a mass spectrometer has been used as a measuring device for determining the type of an element in a sample, and neutral particles sputtered by irradiating the sample with an ion beam are irradiated with a laser. 2. Description of the Related Art A photoionization mass spectrometer that ionizes with light and performs mass analysis of generated ions is known. In this photoionization mass spectrometry, when a sample is irradiated with an ion beam, the number of neutral particles emitted from the sample is at least a few orders of magnitude larger than the number of secondary ions emitted simultaneously from the sample. It is expected that the sensitivity will be significantly improved as compared with the secondary ion mass spectrometry in which the type of element is determined by measuring the mass spectrum of the ion.

FIG. 2 is a view for explaining the principle of a conventional photoionization mass spectrometer. The principle will be described below. When the analysis sample 1 is irradiated with the primary ion beam 3 emitted from the ion gun 2, neutral particles 4 and secondary ions 5 are emitted from the analysis sample 1. When the neutral particles 4 are irradiated with the laser beam 7 from the laser device 6, the neutral particles 4 are ionized by photoexcitation, and photo ions 8 are generated. The sample 1 is subjected to mass analysis by introducing the photoion 8 into the mass spectrometer 9 and measuring the mass spectrum.

In photoionization mass spectrometry, sputtered neutral particles are irradiated with a laser beam to perform ionization. However, neutralizing particles are ionized with only one photon for high-sensitivity analysis and are currently commercially available. Insufficient power is output by the laser. Therefore, a method is employed in which neutral particles are stepwise ionized by laser light through one or more excited states, that is, multi-photon ionization of the neutral particles. At this time, in order to obtain a laser output expected to enable high-sensitivity analysis, a continuous oscillation laser has no sufficient output. Therefore, a pulse oscillation laser having a high peak output is used.

In the photoionization mass spectrometry, the detection efficiency is determined by the ratio of the neutral particles emitted from the sample surface to the laser irradiation (laser irradiation rate) and the ratio of the neutral particles irradiated by the laser to the laser. It is given by the product of the ratio of ionization by light (ionization efficiency) and the ratio of the ionized neutral particles detected by the mass spectrometer. When a pulsed laser is used, the laser irradiation rate increases as the diameter of the laser beam increases, but the ionization efficiency decreases due to the decrease in the output density of the laser beam. It is known that efficiency is maximized. However, depending on the type of element to be analyzed, it is difficult to obtain high detection efficiency because the output is still insufficient even if the laser beam diameter is optimized using a currently available pulse laser with the highest output. is there.

The following three methods have been devised to solve the above problems.

The first method (Japanese Patent Laid-Open No. 62-170841)
Referring to FIG. 3, neutral particles 4 released from sample 1
Is penetrated into the ionization chamber 29 whose inner wall is coated with a substance having a high reflectance, and the laser light 7 is made to enter from the laser entrance hole 30 in the ionization chamber 29. In this method, since the laser light is repeatedly reflected by the inner wall of the ionization chamber 29, the number of neutral particles to be irradiated with the laser is increased, and the ratio of generated photoions is increased.

The second method (Japanese Patent Laid-Open No. 2-119040)
Referring to FIG. 4, a photoionization region 33 is provided between two total reflection mirrors 38 and 40 constituting a laser resonator, and a laser beam 7 inside the laser resonator is used for photoionization. In this method, compared with the conventional method in which laser light is extracted from a laser resonator including one total reflection mirror and one partial transmission mirror through a partial transmission mirror and used for photoionization, Light intensity is extremely high. Further, since the loss due to the resonator mirror is small, the laser light is not easily attenuated by a plurality of round trips of the laser light, so that the pulse width is long. Therefore, the number of generated photoions increases.

Third method (Japanese Patent Laid-Open Publication No. HEI 3-16547)
Referring to FIG. 5, a half mirror 41 that transmits light from the front surface and reflects light from the back surface is disposed between the laser device 6 and the chamber 42 so that the front surface faces the laser device 6. The reflection mirror 40 is arranged to face the half mirror 41 with the chamber 42 interposed therebetween. In this method, the half mirror 41 and the reflection mirror 40
The laser beam reciprocates repeatedly between and, so that the ratio of laser irradiation of the neutral particles increases, and the number of generated photoions increases.

[0010]

The above-mentioned conventional examples shown in FIGS. 3, 4 and 5 for improving the detection efficiency have the following problems.

In the conventional example shown in FIG. 3, the laser beam passes through a different path each time it is reflected by the inner wall, and the laser beam passes through a wide spatial area over a wide time width. Therefore, when using a time-of-flight mass spectrometer with the highest ion transmittance among mass spectrometers, the ion generation time width is widened and the ionization region is wide, so the flight time of the generated ions The dispersion becomes large and the mass resolution becomes extremely poor.

In the conventional example shown in FIG. 4, it is not easy to incorporate a photoionization mass spectrometer into a laser device, and it is not practical.

In the conventional example shown in FIG. 5, when the half mirror 41 and the reflection mirror 40 are arranged perpendicular to the optical axis of the laser beam,
Although the passing position of the laser beam is constant, there is a problem that the laser device 6 is damaged by the return light from the half mirror 41. When the half mirror 41 is arranged so as not to be perpendicular to the optical axis of the laser light, the above problem does not occur. However, the ion generation time width is widened, and the laser light passage position is not constant, and the ionization is not performed. When the time-of-flight mass spectrometer is used, the mass resolution is extremely poor.

An object of the present invention has been devised to solve the above-mentioned problems, and does not have the above-mentioned problems caused by reciprocating a laser beam on a straight line, and can improve the detection efficiency. It is to provide a photoionization mass spectrometer.

[0015]

According to the present invention, neutral particles generated by irradiating a sample with an ion beam are irradiated with a laser beam from a laser device to ionize the neutral particles and generate ions. In a photoionization mass spectrometer for conducting mass spectrometry to a mass spectrometer, a second optical element having a condensing function after a laser beam passes through a first optical element having a condensing function and is condensed on the sample Through
The laser beam is reflected by a third optical element having a function of displacing and reflecting the laser beam from the optical axis of the incident light at least in a direction perpendicular to the optical axis, and is again focused on the sample by the second optical element. After passing through the first optical element, the laser beam is reflected by a fourth optical element having a function of displacing and reflecting the laser beam from the optical axis of the incident light at least in a direction perpendicular to the optical axis. 2nd, 3rd, 2nd
The first and the fourth optical elements are condensed on the sample each time a plurality of times are repeated, and finally, the light exits without passing through any of the above optical elements. It is characterized by.

[0016]

FIG. 1 is a diagram showing the configuration of a photoionization mass spectrometer according to a first embodiment of the present invention. An ion gun 2 for irradiating the surface of the sample 1 with a pulsed primary ion beam 3 for sputtering is provided obliquely above the sample 1.
However, immediately above the sample 1, a mass spectrometer 9 for detecting the photoion 8 is provided. Numeral 4 denotes neutral particles emitted from the surface of the sample 1 when the surface of the sample 1 is irradiated with the pulsed primary ion beam 3, and numeral 5 denotes secondary ions emitted simultaneously. Reference numeral 6 denotes a laser device which emits a substantially parallel pulse laser beam 7. Reference numerals 10 and 13 respectively denote two plane reflecting mirrors which are orthogonally coupled to each other.
Lenses 11 and 12 have a focusing function, and the focal length is selected so that the laser beam 7 is focused on the sample 1.

The manner of propagation of the laser beam in the first embodiment will be described with reference to FIG. An almost parallel laser beam 7 emitted from the laser device 6 has an optical path so as to be off-center and incident on the lens 11 in parallel with the optical axis of the lens 11, and is collected on the sample 1 by the lens 11. It is illuminated and returned by the lens 12 to a substantially parallel beam. The laser beam 7 is displaced in the direction perpendicular to the optical axis and reflected by a combination 13 of two plane reflecting mirrors orthogonally coupled, condensed on the sample 1 by a lens 12, and substantially parallel by a lens 11. Back to the perfect beam.

The laser beam 7 passing through the lens 11 is
The light is reflected by being displaced in the direction perpendicular to the optical axis by a mirror 10 in which two plane reflecting mirrors are orthogonally coupled, and is incident on the lens 11 again. At this time, the mirrors 10 and 13 in which the two reflection mirrors are orthogonally coupled are the lens 1
Since the beams parallel to the optical axes 1 and 12 are reflected in parallel to the optical axis, two reflecting mirrors 10 and 13 are coupled at right angles to each other as shown in FIG. If the laser beam 7 does not lie on a straight line passing through the centers of the lenses 11 and 12, the laser beam 7 reciprocates once on the sample 1, and then the light of the lens 11 is shifted from the position where the laser beam 7 first enters the lens 11. Incident parallel to the axis.

Therefore, these light collection, reflection, light collection,
The reflection is repeated until the laser beam 7 does not pass through any of the optical elements and goes out of the optical system. Moreover,
If the laser beam is a strictly parallel beam, ignoring the spherical aberration of the lens, a parallel beam incident parallel to the optical axis of the lens will be focused at the same point no matter where it is incident on the lens, Each time the laser beam 7 passes over the sample 1, it is focused on the same point.

In practice, since the laser beam 7 has a finite beam diameter, it has a finite divergence angle or convergence angle. Therefore, the condensing point after passing through the lens is a focusing point when a parallel beam is incident. It deviates from the light spot. For example, when the laser beam 7 diverges at a slight angle, the laser beam 7 is converged at a position farther from the lens than the converging point in the case of the parallel beam.
Each time the light passes over the sample 1, the focal point shifts. However, the laser beam 7 can be focused near one straight line on the sample 1 by adjusting the arrangement of the optical elements, particularly the orientation, and
Light can be collected on substantially the same straight line. Since the optical path shifts each time the laser beam 7 reciprocates, the laser beam does not reverse the optical path.

The operation of the first embodiment configured as described above will be described. Even if the laser light is condensed by a lens, a beam waist of a finite size is formed near the converging point due to the diffractive property of the light. Therefore, when the distance between the two reflecting mirrors 10 and 13 which are coupled orthogonally is shorter than the distance that light travels within the time of the laser pulse width, the position of the focal point of the laser beam 7 is Even if it changes each time it passes over the sample 1, if it is near a certain straight line or is almost co-linear, the laser beam 7 has pulses overlapping on the sample 1 or the beams are close to each other .

This means that the light intensity increases or the laser beam diameter increases as compared with the conventional case where the pulse is not reciprocated, so that the ionization efficiency or the laser irradiation rate increases. Therefore, while the detection efficiency is greatly improved,
Unlike the conventional case, the photoion generation position is fixed, so that when a time-of-flight mass spectrometer is used as the mass spectrometer, the mass resolution is improved as compared with the conventional case. Further, since the laser beam 7 does not reverse, there is no problem that the laser device is damaged.

The second embodiment is shown in FIG. 7. The difference from the first embodiment is that a right angle prism 14 is used instead of the plane reflection mirrors 10 and 13 which are orthogonally coupled. , 15 are used. Since the right-angle prism shifts and turns the incident light in parallel, this embodiment also has the same operation as the first embodiment.

The third embodiment is shown in FIG. 8 and is obtained by combining plane reflecting mirrors at right angles to each other.
13 and a combination of parabolic cylindrical mirrors instead of the right-angle prisms 14 and 15 are used. 16,17
9, two parabolic mirrors, each of which has two parabolic mirrors 22 and 23 whose vertical cross-sections share a focal point 18 and an axis 20, respectively, as described with reference to FIG. This is a combination of two parabolic cylindrical mirrors whose vertical cross sections become two parabolas 24 and 25 sharing the focal point 19 and the axis 21. Reference numerals 16 and 17 denote focal points 18 and 19 on a straight line passing through the centers of the lenses 11 and 12, respectively. The distance between the optical elements is defined by points 26 and 19 when the focal point 18 emits light as a virtual point light source. It is assumed that the interval is such that light is collected. The laser device 6 is arranged so that the laser beam 7 propagates as if it were emitted radially with the focal point 18 as a point light source and enters the lens 11.

The manner of propagation of the laser beam in the third embodiment will be described with reference to FIGS. The laser beam 7 is condensed by the lens 11 at one point 26 on the sample 1 and converged by the lens 12 as if it were condensed at the focal point 19 while combining two parabolic mirrors 17 Incident on. By the way, in the parabola,
There is a property that a straight line obtained by reflection of a straight line passing through the focus by a parabola is parallel to the axis of the parabola. Due to this property, the laser beam 7 has two parabolic mirrors.
The combined light 17 is reflected and diverged as if the focal point 19 were a point light source. The laser beam 7 is applied to a point 2 on the sample 1 by a lens 12.
The light is condensed at 6, and is converged by the lens 11 as if it were condensed at the focal point 18, and enters the combination 16 of two parabolic cylindrical mirrors. And 17
In the same manner as the reflection at, the reflection at 16 causes the lens 1 to diverge as if it came out of the point light source 18.
Incident on 1.

At this time, as in the first embodiment, 2
Combination of two parabolic mirrors 16, 17
Both joints at the same time have two focal points 18,1
If the laser beam 7 does not lie on a straight line passing through 9, the incident point on the lens 11 after the laser beam 7 has reciprocated once deviates from the incident point when the laser beam 7 first enters. Therefore, these light collection, reflection, light collection, and reflection are repeated until the laser beam 7 does not pass through any of the optical elements and goes out of the optical system. Moreover, unlike the first embodiment, the laser beam is always focused on one point 26. Therefore, in this case, the same operation as in the first and second embodiments is obtained.

In the first, second, and third embodiments, the laser beam 7 is focused on the sample 1 by the lenses 11, 12, but as shown in FIG.
Instead of 2, concave mirrors 27 and 28 whose focal lengths are selected so that the laser beam 7 is focused on the sample 1 may be used. In this case, the same operation as in the first, second, and third embodiments is obtained.

In the above-described embodiment, the apertures are provided so that the laser beam can pass through the mirrors 10 and 13 in which two flat reflecting mirrors are combined and the mirrors 16 and 17 in which two parabolic mirrors are combined. May be provided. Further, the optical system may be configured by combining two planar reflecting mirrors, combining two parabolic mirrors, or using two rectangular prisms. In this case, the operation is exactly the same as in the above embodiment.

[0029]

As described above, according to the present invention, the pulsed laser beam emitted from the laser device is such that the pulses overlap on the sample, the light intensity increases, and the ionization efficiency increases, Depending on whether the beam width is increased and the laser irradiation rate is increased, the overall detection efficiency is improved and the analysis sensitivity is improved.

[Brief description of the drawings]

FIG. 1 is an apparatus configuration diagram showing an embodiment of the present invention.

FIG. 2 illustrates the principle of a photoionization mass spectrometer.

FIG. 3 is a configuration diagram showing an example of a conventional example that attempts to increase the number of photoions.

FIG. 4 is a configuration diagram showing an example of a conventional example that attempts to increase the number of photoions.

FIG. 5 is a configuration diagram showing an example of a conventional example that attempts to increase the number of photoions.

FIG. 6 is an explanatory diagram showing propagation of a laser beam in one embodiment of the present invention.

FIG. 7 is an apparatus configuration diagram showing one embodiment of the present invention.

FIG. 8 is an apparatus configuration diagram showing one embodiment of the present invention.

FIG. 9 is an explanatory diagram showing propagation of a laser beam in one embodiment of the present invention.

FIG. 10 is an apparatus configuration diagram showing one embodiment of the present invention.

[Explanation of symbols]

1 ... sample, 2 ... ion gun, 3 ... primary ion beam, 4
... neutral particles, 5 ... secondary ions, 6 ... laser device, 7 ... laser beam, 8 ... photoion, 9 ... mass spectrometer, 10,1
3: Reflective mirror, 11, 12: Lens, 14, 15: Right-angle prism, 16, 17: Two parabolic mirrors, 2
7, 28 ... concave mirror.

Claims (10)

[Claims] 1. A method for irradiating a laser beam from a laser device to a neutral particle generated by irradiating a sample with an ion beam to ionize the neutral particle, guide the generated ion to a mass spectrometer, and perform mass analysis. In a photoionization mass spectrometer, a laser beam passes through a first optical element having a condensing function, is condensed on the sample, passes through a second optical element having a condensing function, and is incident on the laser beam. The light is reflected by the third optical element having a function of reflecting the light by displacing the light from the optical axis at least in a direction perpendicular to the optical axis, and
After condensing on the sample again by the optical element, the laser beam passes through the first optical element, and reflects the laser beam by displacing the laser beam from the optical axis of the incident light at least in a direction perpendicular to the optical axis. Is reflected by the first optical element, again passes through the first, second, third, second, first, and fourth optical elements.
Photoionization mass spectrometry characterized by comprising an optical system that repeatedly converges on the sample a plurality of times each time and finally goes out without passing through any of the above optical elements apparatus. 2. The photoionization mass spectrometer according to claim 1, wherein twice the distance between said fourth optical element and said third optical element is the distance that light travels within the time of the pulse width of said laser beam. A photoionization mass spectrometer characterized by having an optical system shorter than the above. 3. The photoionization mass spectrometer according to claim 1, wherein said third and fourth optical elements are two or more.
A photoionization mass spectrometer, comprising: an optical system using a plurality of plane reflecting mirrors orthogonal to each other. 4. The photoionization mass spectrometer according to claim 1, wherein the third and fourth optical elements have a vertical cross section having a common axis and a focal point, a direction opposite to each other, and a curvature at a vertex. A parabola whose size is two different parabolas, the reflection surface of which is a reflection mirror composed of two convex surfaces of a parabolic column surface, and a focal point of a parabola which is one vertical cross section, and A photoionization mass spectrometer comprising an optical system in which the center of the second optical element is substantially on a straight line. 5. The photoionization mass spectrometer according to claim 1, wherein said third and fourth optical elements include an optical system using a right-angle prism. . 6. The photoionization mass spectrometer according to claim 1, wherein said third and fourth optical elements are two or more.
A photoionization mass spectrometer, comprising: an optical system in which a plurality of plane reflecting mirrors are orthogonal to each other and provided with an opening for passing a laser beam. 7. The photoionization mass spectrometer according to claim 1, wherein the third and fourth optical elements have a vertical cross section having a common axis and a focal point, a direction opposite to each other, and a curvature at a vertex. It becomes two parabolas of different sizes, the reflection surface is composed of two parabolic columnar convex surfaces,
Using a reflecting mirror provided with an opening through which a laser beam passes, and having an optical system in which the focal point of a parabola, which is one vertical cross section, and the centers of the first and second optical elements are substantially on a straight line A photoionization mass spectrometer characterized by the above. 8. The photoionization mass spectrometer according to claim 1 or 2, wherein the third and fourth optical elements have openings used in any of claims 3, 4, 5, 6, and 7. A photoionization mass spectrometer comprising an optical system using two types of optical elements, one of a reflection mirror, a reflection mirror without an aperture, and a right-angle prism. 9. The photoionization mass spectrometer according to claim 1, further comprising an optical system using a lens as said first and second optical elements. Analysis equipment. 10. The photoionization mass spectrometer according to claim 1, further comprising an optical system using a concave mirror as said first and second optical elements. Mass spectrometer.