JPH03291559A - Laser ionization neutral particle mass spectrometer - Google Patents

Abstract

PURPOSE:To obtain the mass spectrometer which can improve a spectral sensitivity and can make analysis in a depth direction by operating the detecting function of an ion detector only in the time when the arrival of the light excitation ions past the mass spectrometer is anticipated. CONSTITUTION:While the secondary ions 27 past the mass spectrograph 31 are a continuous quantity, the light excitation ion 29 is generated at every ionization by a UV laser beam 28. The light excitation ion 29 is, therefore, high in the peak value with respect to the secondary ion 27 but the quantity thereof is desultory. A light emitting signal 35 of the UV laser is generated by a UV laser detector 34. This signal 35 is introduced into a trigger signal generator 36 which generation a detection start signal 37 after the lapse of the prescribed time. The signal 37 is introduced to a signal gate 38 installed between the ion detector 32 and a pulse counter 33 to detect the ion pulse from the time when the signal 37 is introduced. A delay is necessary before the transmission of the signal 37 after the reception of the signal 35 and a detection signal 39 is generated for the prescribed time from the generator 36. The detection is ended by the gate 38. The detection only during the time when the light excitation ion is detected is thus executed.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to the mass vector of photoexcited ions, which is obtained by ionizing neutral particles with an ultraviolet laser among the particles that are sputtered by irradiating an ion beam onto a solid sample. This invention relates to a laser ionization neutral particle mass spectrometer that measures and performs mass spectrometry.

[Conventional technology]

A typical microanalysis method for solid samples detects secondary ions sputtered from the surface by an ion beam.

It is a secondary ion mass spectrometry method. However, the amount of secondary ions has a low generation efficiency and is highly dependent on the element, so it is not proportional to the element concentration in the sample, and there is a problem in quantitative performance. On the other hand, since the amount of neutral particles that are sputtered simultaneously with secondary ions is proportional to the element concentration, neutral particle mass spectrometry, which detects this, is an analytical method with good quantitative properties. In particular, laser ionization mass spectrometry, in which neutral particles are ionized with a high-intensity laser and subjected to mass analysis, is known as a method with high ionization efficiency (C, H).

Becker; Journal of Vacuum Science and Toxicity (J, Vac,
Sci・Technol, ) A5.1181 (19
87) ). However, since the lasers that can be used in current laser ionization neutral particle mass spectrometers are pulsed lasers such as excimer lasers and YAG lasers,
Problems exist in terms of sensitivity and application to depth analysis. Next, the principle configuration of a conventional laser ionization neutral particle mass spectrometer will be explained.

In FIG. 6, reference numeral 1 denotes an ion beam generator, which supplies a gas such as argon or oxygen, ionizes it, and generates a sputtering ion beam 2.

Next, the sputtering ion beam 2 is passed through an electrostatic lens 3.
After convergence using the electrode 4, it is pulsed to impact the surface of the sample 5. Due to the impact of the sputtering ion beam 2, neutral particles 8 and secondary ions 6 are released into the sample 5.
emitted from the surface of

The secondary ions 6 are extracted by the ion extraction electrode 7, but the neutral particles 8 are not accelerated and therefore enter the photoionization region 9 at a slower speed than the secondary ions 6. Here, the neutral particles 8 are photoionized by the ultraviolet laser beam 11 generated from the ultraviolet laser generator 10 and become photoexcited ions 12 . The photoexcited ions 12 are extracted by an ion extraction electrode 7, pass through a time-of-flight mass spectrometer 13, and are converted into a current signal by an ion detector 4. The current signal generated from the ion detector 14 is detected by an electric waveform measuring device such as a digital oscilloscope 15.

In the above laser ionization neutral particle mass spectrometer,
Several techniques are required to detect photoexcited ions. The first is an efficient mass separation means for photoexcited ions generated in a short time.

The ultraviolet laser beam 11 has a pulse width of 10 to 3 Q n sec
The generation time of the photoexcited ions 12 is also about the same. A time-of-flight mass spectrometer 13 is used to measure the photoexcited ions 12 generated in such a short time. The time-of-flight mass spectrometer 13 utilizes the fact that low-mass particles arrive at the detector earlier and high-mass particles arrive at the detector later.
Mass separation. The second technique is a technique for separating secondary ions 6 and photoexcited ions 12. The generated secondary ions 6 and photoexcited ions 12 are similarly detected by the ion detector 14. If the sputtering ion beam 2 is a continuous beam, the secondary ions 6 are generated continuously. On the other hand, the photoexcited ions 12 are stimulated by the laser 11.
Since these ions are generated only for a short period of time when they emit light, their intensity is lower than that of the secondary ions 6 described above. Therefore, secondary ions interfere with the detection of photoexcited ions. To prevent this,
First, the sputtering ion beam 2 is pulsed in synchronization with the laser 11. The pulsed ion beam 2 generates pulsed secondary ions and pulsed neutral particles from the sample surface. The secondary ions 6 are accelerated directly toward the detector 14, but the neutral particles 8 are accelerated after being ionized by the laser, so there is a time gap between the two. Thereby, secondary ions and photoexcited ions can be temporally distinguished and measured. Third
The technique is a means of detecting ions that occur over a short period of time.

The photoexcited ions are converted into an amount of current by an ion detector, and then measured by a detector such as a digital oscilloscope.

[Problem to be solved by the invention]

As described above, the conventional laser ionization neutral particle mass spectrometer requires a pulsed ion beam, a time-of-flight mass spectrometer, a secondary ion separation means, and a current amount detector as components. However, these configurations cause the following problems with respect to high-sensitivity analysis and depth direction analysis.

High-sensitivity analysis requires measuring multiple times and integrating the data, but it takes time to process the data from a current detector such as a digital oscilloscope, and the measurement repetition frequency is at most several tens of Hz. It is. Also.

Conventional current measuring instruments do not have the dynamic range to detect the current of ions originating from sample constituent elements as well as the minute current of ions originating from trace impurities, so as a result, minute currents cannot be detected. Furthermore, in order to separate secondary ions and photoexcited ions, it is necessary to separate the photoionization region from the sample surface to some extent. This reduces the solid angle of photoionization and reduces the amount of neutral particles that can be ionized. Due to the above points, the sensitivity is low and the maximum is ppm.
It was about. In depth direction analysis, the sputtering speed is slow because the ion beam used for sputtering is made into very short pulses and the repetition frequency is not high as described above. Even if this problem were solved by increasing the ion current, the sample surface could not be sputtered uniformly, resulting in a decrease in depth resolution.

As mentioned above, it is difficult to perform high-sensitivity analysis and depth direction analysis with conventional laser ionization neutral particle mass spectrometers.

An object of the present invention is to obtain a laser ionized neutral particle mass spectrometer capable of improving analysis sensitivity and performing depth direction analysis.

[Means to solve the problem]

The above purpose is to irradiate the ion beam onto the solid sample surface.
The pulsed laser that generates photoexcited ions from neutral particles sputtered from the sample surface is an ultraviolet laser that can repeatedly emit light, and the mass separation means for the photoexcited ions is an electric field. An ion detector that detects photoexcited ions has a function that allows only ions of a desired mass to pass through using a magnetic field and a magnetic field. This can be achieved by having a detection time limiting means for operation and means for counting the number of ions.

[Effect]

The present invention is based on a continuous ion beam or a pulsed ion beam, a mass analyzer that separates only ions of a single mass using electric field sweep or magnetic field sweep, a high repetition pulse laser, an ion detection time limiting means, and an ion pulse counting means. be a component of The present invention differs from conventional methods in that it uses a continuous ion beam for sputtering. This makes it possible to obtain an appropriate sputtering rate and to perform uniform sputtering, thereby obtaining high resolution in the depth direction. By employing this sputtering method, secondary ions are generated continuously, which hinders the detection of photoexcited ions. Therefore, as a second means, an ion detection time limiting means is used. This is a means of predicting the time when photoexcited ions arrive at the ion detector after they are generated, and activating the detection function of the ion detector only at that time. As a result, secondary ions are no longer detected when no photoexcited ions are generated, and the intensity of the secondary ions is significantly reduced. According to the present invention, unlike the conventional method, secondary ions are not completely detected, but the intensity is reduced in proportion to the measurement time narrowed by the limiting means. For example, if the measurement time width is set to 1 μsec for repeated measurements of 1 second, the time width becomes 1/10′. As will be described later, the appropriate time width is approximately 1 to 10 μsec. Even if the intensity of the secondary ions is reduced to about 1/105 to 1/10' using a measurement time limiting means, highly sensitive analysis of trace impurities cannot be performed as it is. Therefore, as a third means, we use an electric field sweep or magnetic field sweep type mass analyzer that separates only the ions with the mass we want to detect. Although the performance of the present invention is inferior to the conventional method, which detects photoexcited ions of all masses at once, in actual analysis, the objective can be achieved by detecting only ions of several masses, so this is not a major problem. It won't happen. By adopting this analyzer, it is possible to perform mass separation of secondary ions, so it is possible to measure photoexcited ions of trace impurities without being affected by secondary ions with high intensity of sample constituent elements. If the secondary ions and photo-excited ions have the same mass, the intensity of the photo-excited ions is 2 to 5 orders of magnitude higher, depending on the laser pulse width, the time width of the ion limiting means, the secondary ion yield, etc. does not interfere with the measurement of photoexcited ions.

The photo-excited ion measuring means using a continuous ion beam has been described above, but this method can be used when the sputtering speed is too high using a continuous ion beam.

It is possible to pulse the sputtering ion beam to the extent that the depth resolution does not deteriorate.

Furthermore, by adding the following means, highly sensitive analysis becomes possible. The fourth means is a high repetition pulse laser.

Data integration is effective for high-sensitivity measurements. The data obtained by the conventional method is two-dimensional data of changes in current amount over time. Therefore, processing takes a long time, and even when using the highest performance measuring equipment, only an integrated speed of 100 Hz or less can be obtained.

On the other hand, the data obtained by the mass spectrometer used in the present invention is one-dimensional data of only ion intensity, and therefore requires almost no time for processing. The cumulative repetition rate is determined by the emission repetition frequency of the laser pulse, and in current lasers, it ranges from several 100 Hz to about 1 kHz. Also,
In order to increase the generation efficiency of photoexcited ions, it is necessary to bring the photoionization region as close to the neutral particle generation position as possible to increase the solid angle. Therefore, as a fifth measure, the laser is brought as close to the sputtering surface as possible.

Conventional methods have been unable to distinguish between secondary ions and photo-excited ions because they require neutral particles to fly a certain distance. Furthermore, a pulse calculation method that counts the number of ions is effective for high-sensitivity analysis. Therefore, as a sixth means, an ion counting means is used. In the conventional method, the mass of an ion is expressed as the time it takes to arrive at the detector, so in order to improve mass resolution, it is necessary to shorten the time span in which an ion of one mass is detected as much as possible.

This time width depends on the size of the time-of-flight mass spectrometer and the magnitude of the ion accelerating voltage, but it is required to be about several tens of nanoseconds. On the other hand, in normal ion measurement, the pulse width when converting one ion into an amount of current is at least 10 to 20 nsees, so conventional time-of-flight mass spectrometers cannot illuminate one mass. pulses from multiple ions are superimposed. Therefore, the number of ions is expressed as the amount of current. Since the number of ions differs by 6 to 8 orders of magnitude between the sample constituent element and the trace impurity element, the amount of current to be detected also differs by 6 to 8 orders of magnitude. but,
In reality, there is no current measuring device that can accurately detect such vastly different amounts of current. In contrast, in the present invention, a pulse counting method that counts each ion one by one becomes possible for the following two reasons. The first reason is to limit the mass to be measured.

Unlike conventional methods that detect ions of various masses at once, only one type of ion is detected, so by changing the amount of -order ions and adjusting the amount of neutral particles for each mass, the detector It is possible to reduce the amount of incident photoexcited ions to the extent that pulse counting is possible. The second reason is
This method allows the time for measuring ions to be longer than the conventional method.

In the present invention, even if the ion detection time width is lengthened, the mass resolution is not affected. Therefore, superimposition of ion pulses can be prevented. The difference between the present invention and the conventional method is shown in FIG. As mentioned above, since the pulse counting method can be used to detect with high precision both sample constituent elements with a large amount of ions and trace elements with a small amount of ions, the present invention is superior to the conventional method in addition to the high measurement repetition frequency. It can be said that this method allows for highly sensitive analysis compared to other methods.

As explained above, the present invention is an effective method for high-sensitivity analysis. A method of increasing the detection time to enable pulse detection will be described below. In the present invention, when the detection time width is lengthened, the amount of secondary ions detected increases in proportion.

However, if the detection time width must be increased,
Although the amount of photoexcited ions generated is large, it is not affected by secondary ions. On the other hand, when the amount of photoexcited ions is small, ion superposition does not occur even if the detection time width is shortened, so it is sufficient to shorten the detection time width as much as possible to prevent detection of secondary ions. To change the detection time width, the difference in initial kinetic energy of photoexcited ions is used. Each sputtered neutral particle has a different initial energy.

In the conventional method, particles with the above-mentioned energies must arrive at the detector at about the same time, so a very high extraction voltage is applied to the ions relative to their initial energy. Conversely, if very little extraction voltage is applied, the difference in initial energy will directly appear as a difference in arrival time. In this way, the detection time is changed by selecting the kinetic energy of the photoexcited ions. A quadrupole mass spectrometer is suitable for a method of performing mass separation without applying much ion extraction voltage. Furthermore, an energy analyzer may be used as a method of selecting the kinetic energy of the ions.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a configuration diagram showing a first embodiment of a laser ionization neutral particle mass spectrometer according to the present invention, FIG. 2 is a diagram showing temporal changes in secondary ions and photoexcited ions, and FIG. 3 is a diagram showing a G a
Figure 4 is a diagram showing a measurement example of an ion-implanted sample of As, Figure 4 is a diagram showing a second example in which the laser ionization region is brought close to the sputtering surface, and Figure 5 is a diagram showing a measurement example using a quadrupole mass spectrometer. It is a figure which shows 3rd Example.

In the first construction drawing of the first embodiment, a continuous ion beam 22 is generated from an ion beam generator 21. Next, after converging the continuous ion beam 22 with an electrostatic lens 23, the scanning electrode 2
4 to cause an impact while scanning the surface of the sample 25. This operation enables uniform sputtering. Neutral particles 26 are generated by the impact of the continuous ion beam 22.
and secondary ions 27 are sputtered. The neutral particles 26 are ionized by the ultraviolet laser beam 28 and become photoexcited ions 29 . The photoexcited ions 29 are extracted from the ionization region by an extraction electrode 30 and subjected to mass separation by a mass spectrometer 31 that uses a magnetic field or an electric field.
The ion detector 32 generates an electrical pulse, and the pulse counter 3
It is counted as 3.

The mass spectrometer 31, which uses an electric field or a magnetic field, cannot temporally separate secondary ions and photoexcited ions, unlike conventional methods. The amount of secondary ions 27 and photoexcited ions 29 after passing through the mass spectrometer 31 is shown as a function of time as shown in FIG. Secondary ions 27 are in continuous quantity. On the other hand, photoexcited ions 29 are generated every time ionization is performed by the ultraviolet laser beam 28. Therefore, although the peak value of the photoexcited ions 29 is higher than that of the secondary ions 27, the amount of the photoexcited ions 29 is scattered.

This difference is used to distinguish between the two. FIG. 1 shows an example of such means. An ultraviolet laser detector 34 generates an ultraviolet laser emission signal 35. The light emission signal 35 is introduced into a trigger signal generator 36, and a detection start signal 37 is generated after a predetermined period of time has elapsed. This detection start signal 37 is introduced into a signal gate 38 installed between the ion detector 32 and the pulse counter 33, so that ion pulses from the time when the detection start signal 37 is introduced are detected. It takes several to several tens of microseconds of time from the generation of photoexcited ions to the time when they are converted into a pulse signal. Therefore, it takes several to several tens of microseconds between the reception of the luminescence signal 35 and the transmission of the detection start signal.
A delay of μSec is required. To end counting, the trigger signal generator 36 generates a detection stop signal 39 after a predetermined time, and upon reception of the detection stop signal, the signal gate 38 ends the detection. Such an operation enables detection only during the time when photoexcited ions are generated.

If the detection time width is set to 10 μsec or less, the amount of detected secondary ions is about 10 −5 when counted continuously, and can be ignored compared to the amount of photoexcited ions. FIG. 3 shows an example of mass spectrometry performed using the above embodiment. FIG. 3 is a depth direction analysis when a Be impurity element is implanted into GaAs. The characteristics of neutral particle mass spectrometry are that the intensities of Ga and As are almost the same,
Analysis of Be impurities could be performed with a sensitivity of less than ppm.

Second Embodiment Neutral particles sputtered from the surface of a sample are emitted in all directions in a vacuum. Since the laser only passes through a portion of space, only a portion of the neutral particles can be photoionized. In order to increase this amount, it is necessary to bring the laser passage position as close as possible to the sputtering surface. Furthermore, since it is better to improve the photoionization efficiency by increasing the photon density, and since sputtering ions have a radius of about 100 μm, the laser is focused to a radius of about several 100 μm.

FIG. 4 shows, as a second embodiment, means for bringing the laser condensing and photoionization region closer to the sputtering surface. The ultraviolet laser beam 28 is condensed by a condensing lens 41. As shown, the sample 25 is made as small as possible. The sample 25 moving mechanism 42 moves the sample directly below the laser condensing position.

The continuous ion beam 22 is adjusted by the scanning electrode 24 so as to irradiate the surface of the sample 25. With the above mechanism, a photoionization region can be set directly above the sputtering surface, and the distance between the sputtering surface and the photoionization region is several tens of times.
It becomes possible to set it to about 0 μm.

The initial energy of the neutral particles emitted from the sample of the third example varies depending on the sputtering conditions, the type of sample, etc., but is approximately a few to several tens of eV, and this kinetic energy does not change even after photoionization. . Since the time to arrive at the ion detector is proportional to the reciprocal of the square root of the kinetic energy, if no extraction voltage is applied after ionization and the photoexcited ions of all the energies generated are detected, the fastest and slowest ions The arrival time can be several times larger than that of ions, making it possible to set extremely long detection times. Furthermore, although low-velocity ions are greatly affected by the application of the extraction voltage, high-speed ions are less affected, so the length of the detection time can be adjusted by adjusting the extraction voltage of about several volts.

A quadrupole mass spectrometer is suitable for detecting ions of several to several tens of ev in this way. Since high-energy ions deteriorate mass resolution, high-energy ions are separated using an energy analyzer. FIG. 5 shows a part of the configuration of this embodiment in which an energy analyzer 51 is installed in front of a quadrupole mass analyzer 52.

〔Effect of the invention〕

As described above, the laser ionization neutral particle mass spectrometer according to the present invention includes a means for irradiating the surface of a solid sample, which is an analyte, with an ion beam in a vacuum, and a means for irradiating the surface of the solid sample, which is an analyte, with an ion beam, and a means for irradiating the surface of the solid sample, which is an analyte, with an ion beam. During laser ionization, the method includes a pulse laser generating means for ionizing sputtered neutral particles to generate photoexcited ions, a mass separation means for the photoexcited ions, and an ion detector for detecting the mass-separated photoexcited ions. In the particle mass spectrometer, the ion beam is either a continuous beam or a pulsed beam, the pulsed laser is an ultraviolet laser that can repeatedly emit light, and the mass separation means uses an electric field or a magnetic field to The ion detector has a function of transmitting only ions of a desired mass, and the ion detector has a detection time limiting means that operates the detection function only at a time when the photoexcited ions that have passed through the mass separation means are expected to arrive. At the same time, by having a means for counting the number of ions, it becomes possible to continuously generate the -order ion beam and improve the sensitivity of the ion detection system, thereby improving the resolution in the depth direction.

Analysis sensitivity can be improved.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing the ↓ embodiment of the laser ionization neutral particle mass spectrometer according to the present invention, Fig. 2 is a diagram showing the time change of secondary ions and photoexcited ions, and Fig. 3 is a diagram showing the temporal change of secondary ions and photoexcited ions.
Fig. 4 shows a second embodiment in which the laser ionization region is brought close to the sputtering surface, and Fig. 5 shows a third embodiment using a quadrupole mass spectrometer.
FIG. 6 is a diagram showing a conventional laser ionization neutral particle mass spectrometer, and FIG. 7 is a diagram showing a comparison between the conventional method of ion signal generation and the present invention. 10... Ultraviolet laser generator 21... Ion beam generator 25... Solid sample 28... Ultraviolet laser beam 31
...Mass spectrometer 32...Ion detector 41...
・Condensing lens 51...Energy analyzer 52...
quadrupole mass spectrometer

Claims (1)

[Claims] 1. Means for irradiating the surface of a solid sample, which is an analyte, with an ion beam in a vacuum, and ionizing neutral particles sputtered from the surface of the solid sample by the impact of the ion beam. In the laser ionization neutral particle mass spectrometer, the laser ionization neutral particle mass spectrometer has a pulsed laser generating means for generating photoexcited ions, a mass separation means for the photoexcited ions, and an ion detector for detecting the mass-separated photoexcited ions. The beam is either a continuous beam or a pulsed beam, the pulsed laser is an ultraviolet laser that can repeatedly emit light, and the mass separation means uses an electric field or a magnetic field to transmit only ions of a desired mass. The ion detector has a detection time limiting means for operating the detection function only at a time when the photoexcited ions that have passed through the mass separation means are expected to arrive, and a means for counting the number of ions. A laser ionization neutral particle mass spectrometer characterized by having: 2. The pulsed laser generating means includes a condensing lens that condenses the pulsed laser, and a condensing position of the pulsed laser.
2. The laser ionization neutral particle mass spectrometer according to claim 1, further comprising means for setting the specimen directly above the sample sputtering surface. 3. Claim 1, wherein the mass separation means is a quadrupole mass spectrometer, and an energy splitter for the photoexcited ions is installed in front or behind the mass separation means. Laser ionization neutral particle mass spectrometer described in .