JPH039259A - Mass spectrometer by high-repetition laser stimulation - Google Patents

Abstract

PURPOSE:To allow the mass analysis with a high sensitivity and good quantitative characteristic by irradiating the surface of a sample with repeatedly generated pulse laser beams, making mass sepn. of the generated ions and making measurement only when the ions arrive at an ion detector. CONSTITUTION:The sample 5 is irradiated, via a condenser system 4, with the pulse lasers 3 generated from a pulse laser generator 1 by receiving the high-repetition pulse signals form a signal generator 2. The ions 6 evaporated from the sample 5 are accelerated and converged by a leader electrode 7 and are made incident to a mass spectrograph 8. The spectrograph 8 makes the mass sepn. of only the prescribed ion and makes the ion incident to the ion detector 9. The detector 9 has a detecting time limiting means which operates only when the ion arrives at the detector. An ion detecting part 10 generates a signal by detecting the ion and an AND circuit 12 outputs a pulse when this signal and the delay signal from a delay signal generator 11 arrive simultaneously at the AND circuit. The pulses are counted by a pulse counter 13.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention sputters a sample, which is an analyte, with a charged beam, a laser beam, etc., and measures the mass spectrum of ions generated from the surface of the sample. The present invention relates to a micro mass spectrometer that performs mass spectrometry of a sample.

[Conventional technology]

Secondary ion mass spectrometry is a typical method that irradiates a sample (analyte) with a primary ion beam, detects secondary ions sputtered from the sample surface, and measures the element concentration in the sample from the intensity of the secondary ions. It has traditionally been used as a microanalysis method. In addition, by irradiating the sample with a laser beam,
There is also a laser micromass spectrometry method that detects the ions generated thereby. Below, an outline of the apparatus used in the conventional secondary ion mass spectrometry method and the laser micro mass spectrometry method will be explained according to the figures.

FIG. 4 shows a schematic configuration of the secondary ion mass spectrometer. In FIG. 4, 21 is an ion beam generator that ionizes gas such as argon or oxygen or metal gas to generate ions and beams 22.
After converging using the ion lens 23, the scanning electrode 24
The ion beam 22 is scanned to impact the surface of the sample 25. Then, secondary ions 26 are sputtered from the surface of the sample 25 due to the impact of the ion beam 22.

These secondary ions 26 are accelerated by the extraction electrode 2γ and are mass-separated by the mass spectrometer 28, and then each mass-separated ion is counted and measured by the ion detector 29. There is.

Figure 5 shows the mass spectrum of QaAg measured with this device.As shown in the figure, secondary ion mass spectrometry can detect individual pulses of ions generated by sputtering. This is an extremely sensitive microanalysis method.

However, the generation efficiency of the secondary ions 26 sputtered by the ion beam 22 varies depending on the element of the ions.

Therefore, the secondary ion strengths of Ga and As contained in the same amount in GaAs differ by about three orders of magnitude, as shown in FIG. Furthermore, the generation efficiency of secondary ions varies depending on the composition of the sample, so even if the same element is used, different samples will have different intensities. For these reasons, in secondary ion mass spectrometry, it is not possible to directly determine the element concentration from the secondary ion intensity, and there is a problem in quantitative performance.

Further, FIG. 6 shows a schematic configuration of a laser micro mass spectrometer. Here, 31 is a pulse laser generator that generates a high-intensity ultraviolet pulse laser 32. This pulsed laser 32 is focused by a lens 33 and irradiated onto the surface of a sample 340. When laser evaporated ions 35 are generated from the sample 34 by irradiation with the pulsed laser 32, the laser evaporated ions 35 are accelerated and focused by the extraction electrode 36 and enter the time-of-flight mass spectrometer 3T. At this time, since lighter ions fly faster and heavier ions fly slower inside the time-of-flight mass spectrometer 3T, the time to arrive at the ion detector 38 becomes slower as the mass of the laser-evaporated ions 35 increases.

Therefore, the laser-evaporated ions 35 having the same mass simultaneously enter the ion detector 38 and are converted into electric current. This allows the oscilloscope 39 to monitor changes in current intensity over time.
A mass spectrum is obtained by measuring the mass.

FIG. 7 shows G measured by such laser micromass spectrometry.
This shows the mass spectrum of aAs.As shown in the figure, it can be seen that in laser micromass spectrometry, the intensities of GaAa are almost equal, and it is a measurement method with good quantitative properties. However, since laser-evaporated ions of all masses are measured with the same ion detector, it becomes difficult to detect trace impurity ions due to interference from constituent element ions that occur in large amounts.

In addition, since ions are generated at IQ cps+ or higher, pulse counting cannot be used, resulting in poor sensitivity compared to secondary ion mass spectrometry.

[Problem to be solved by the invention]

As described above, the conventional secondary ion mass spectrometer described above cannot directly determine the element concentration from the secondary ion intensity, and has a problem in quantitative performance. Furthermore, conventional laser micromass spectrometers are an effective method for microscopic part analysis and organic substance analysis, but they have problems with sensitivity and accuracy.

In view of the above points, the present invention has been made to solve these problems, and its purpose is to realize quantitative intrusive mass spectrometry with a sensitivity comparable to that of secondary ion mass spectrometry. The object of the present invention is to provide a high-repetition laser excitation mass spectrometer.

[Means to solve the problem]

In order to achieve such an object, the high repetition rate laser excitation mass spectrometer of the present invention includes a pulsed laser generator that repeatedly generates a pulsed laser beam in response to a signal generated from a signal generator, and a pulsed laser beam that receives the pulsed laser beam. An irradiation means for irradiating the surface of a sample which is an analyte; a mass separation means for mass-separating ions generated from the surface of the sample by irradiation with the pulsed laser beam and extracting only ions of a desired mass; It consists of an ion detector that detects mass-separated ions, and this ion detector has a detection time limiting means that operates the detection function only at the time when the mass-separated ions are expected to reach the ion detector. The main feature is that

Furthermore, the present invention includes an ion beam generator and a scanning means for sputtering the sample surface with the ion beam generated by the nuclear ion beam generator in such a high repetition laser excitation mass spectrometer, At the same time as performing analysis using a pulsed laser, an ion beam is generated,
Another feature is that the sample surface is sputtered while condensing and scanning the light.

[Effect]

Therefore, the present invention has the following effects. First of all, since a laser beam is used as the means for generating ions, it is possible to perform measurements with good quantitative properties, similar to conventional laser micromass spectrometry. Second, unlike traditional laser micromass spectrometry, which uses time-of-flight mass spectrometers that detect ions of all masses, single-type laser micromass spectrometry, such as sector-type or quadruple-species mass spectrometers, By using a mass separation means for mass-separating only the ions having the same mass, it is possible to mass-separate only the trace element ions having the mass to be detected. This makes it possible to perform pulse measurements of only trace element ions that are generated in small quantities, excluding the effects of sample constituent elements that occur in large quantities, making highly sensitive analysis possible. Thirdly, in order to obtain sensitivity comparable to secondary ion mass spectrometry, sensitivity and accuracy can be increased by emitting light repeatedly from a laser and integrating data. However, at present, the pulse width of pulsed lasers is several tens of nanoseconds, and the repetition frequency is about several hundred to several thousand Hz, and the actual duration of light emission is short, so the amount of ions emitted per pulse is at most about 10,000 ions per second. The integral quantity is very small. For this reason,
It is no longer possible to ignore the integrated amount of noise generated during the time when the laser is not emitting.

Therefore, as a fourth feature, the SZN ratio can be improved and the sensitivity can be increased by using detection time limiting means to perform measurements only when ions are arriving at the ion detector.

Furthermore, according to the present invention, while performing analysis using a pulsed laser, an ion beam is generated and scanned to sputter the sample surface, thereby repeating evaporation of the sample by the laser beam, thereby increasing the depth from the surface of the sample. Depth direction analysis is possible. but,
Current laser beams have poor uniformity compared to ion beams, resulting in poor depth resolution. Therefore, by uniformly sputtering the sample to a desired depth using an ion beam and analyzing the sputtered straw using a laser beam, analysis at the desired depth becomes possible.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram of a mass spectrometer according to an embodiment of the present invention. In FIG. 4, reference numeral 1 denotes a pulse laser generator, which receives a high repetition pulse signal from a signal generator 2 and generates a high intensity ultraviolet pulse laser 3. A pulsed laser 3 generated from this pulsed laser generator 1 is focused by a laser focusing system 4 serving as an irradiation means, and impacts the surface of a sample 5. The impact of this pulsed laser 3 causes the sample 5 to
When laser-evaporated ions 6 are generated, the laser-evaporated ions 6 are accelerated and collected by the extraction electrode 1 and enter the mass spectrometer 8 . At this time, the mass spectrometer 8 is a selector-type or quadrupole mass spectrometer that can mass-separate only ions of one type of mass. The ions are incident on the ion detector 9. This ion detector 9 is
It has a detection time limiting means for operating the detection function only at the time when the mass separated ions are expected to reach it. In the case of this embodiment, the ion detector 9 having means for limiting the ion detection time includes an ion detection section 1G that collects ions mass-separated by the mass separator 8 and converts them into electric pulses, and a signal generator 2. a delay signal generator 11 that receives a generated signal and generates a signal of a predetermined time width after a predetermined time; and the delay signal generator 11
an AND (logical product) circuit 12 that generates a pulse signal only when it simultaneously receives a delay signal generated by the ion detector 9 and an electric pulse generated from the ion detector 9; and a pulse counter 13 that counts the pulse signals from the AND circuit 12. It consists of

Therefore, the laser evaporated ions 6 generated from the surface of the sample 5 by the impact of the pulsed laser 3 have an initial energy of several to several tens of volts. Furthermore, it is accelerated by the potential difference between the sample 5 and the extraction electrode γ.

Then, since this potential difference is constant, ions with smaller mass move faster and ions with larger mass move slower. Therefore, the time required for laser-evaporated ions 6 of a predetermined mass to reach the ion detection section 10 is determined in accordance with the mass. After receiving the signal from the signal generator 2, the delay signal generator 11 generates a delay signal delayed by a time necessary for a predetermined ion to reach the ion detection section 10. At this time, the velocity distribution of the laser-evaporated ions is widened by the variation in initial energy, so the delay signal width is set to a width corresponding to this. Therefore, by ANDing this delay signal and the electric pulse from the ion detection section 10, it is possible to detect only when the laser-evaporated ions 6 are being generated. As a result, background noise that occurs when laser-evaporated ions 6 are not generated can be removed, and the S/N ratio can be significantly improved. In this case, the S/N ratio is improved as the time width of the delay signal generated by the delay signal generator 11 is made smaller. cannot be detected. For this reason, the delay signal needs to have a certain time width.

In addition, among the means for limiting the nine ion measurement time used in this embodiment, instead of the AND circuit 12, the pulse counter 13 is provided with a counting start/stop function controlled by an external signal, and the delay signal generator 11 generates a count. Even if the pulse counter 13 starts and ends counting using a signal, the same effect as described above can be obtained. Furthermore, without using the delay signal generator 11 and the AND circuit 12, the ion detector 1
Even if all the signals are stored in a memory that time-resolves the signals from 0 to 0 and stores them, and after measurement, only the measured values of the portion corresponding to the time when the laser-evaporated ions 6 are generated are used,
The functionality of the time limiting means for ion detection is equivalent.

FIG. 2 shows an example of a mass spectrum of 90aAm measured by the apparatus of the present invention, and this mass spectrum has two advantages over secondary ion mass spectrometry. Fourth, the intensities of Ga and As are almost equal, and the laser evaporation ion intensity corresponds well to the concentration of the sample.

Second, when comparing the S/N ratio in secondary ion mass spectrometry, the Qa intensity is almost the same in both methods, whereas the background level is low and the SZN ratio is good in the present invention.

This is because in secondary ion mass spectrometry, secondary ions are generated continuously and weakly, whereas laser-evaporated ions are
This is because ions are strongly generated during a very short period of time when the laser is irradiated, so by measuring only when ions are generated as in the present invention, the background level can be reduced. From the above two points, it is clear that this measurement has a sensitivity equal to or higher than that of secondary ion mass spectrometry and has dramatically improved quantitative performance.

FIG. 3 shows another embodiment of the mass spectrometer according to the present invention, which enables analysis in the depth direction. In this embodiment, an ion beam is applied to the configuration of the embodiment shown in FIG.
6 is an ion lens for focusing this ion beam 15, and 1T is a scanning electrode for scanning the focused ion beam 15 and irradiating it onto the sample 5.

The mass spectrometry method in this embodiment will be explained next. First, an ion beam 15 generated by an ion beam generator 14 is focused by an ion lens 16 and scanned by a scanning electrode 11 to scan the surface of a sample 5. Shock. Then, the surface of the sample 5 is sputtered by the impact of the ion beam 15, so the pulsed laser 3 is irradiated onto the sputtered surface of the sample 5 to a desired depth as in the embodiment shown in FIG. Laser vaporized ions generated from the surface 6
Perform mass spectrometry. As described above, according to the present invention, by combining the embodiment shown in FIG. 1 with the conventional secondary ion mass spectrometer, it becomes possible to analyze the sample 5 in the depth direction with good quantitative properties.

〔Effect of the invention〕

As explained above, according to the high repetition rate laser excitation mass spectrometer of the present invention, the surface of a sample, which is an analyte, is irradiated with a repeatedly generated pulsed laser beam, and the When detecting the mass-separated ions by mass-separating the ions to be detected, a means for limiting the detection time is used to measure only when the ions reach the ion detector. By doing so, it is possible to perform trace analysis with a sensitivity equal to or higher than that of secondary ion mass spectrometry and with good quantitative properties.

Furthermore, according to the present invention, while performing analysis using pulsed laser beam AK, it is also possible to perform analysis at a position in the depth direction from the surface of the sample by sputtering the sample surface while scanning the ion beam. can.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of a mass spectrometer according to an embodiment of the present invention, and Fig. 2 shows G1As measured by this embodiment.
3 is a schematic configuration diagram showing another embodiment of the present invention, FIG. 4 is a schematic diagram of a conventional secondary ion mass spectrometer, and FIG. 5 is a diagram showing the secondary ion mass spectrometer of FIG. A diagram showing the mass spectrum of 90a As measured with an ion mass spectrometer,
Figure 6 is a schematic diagram of a conventional laser micro mass spectrometer;
FIG. 7 is a diagram showing a mass spectrum of 90aAs measured by a conventional laser micro mass spectrometer. 1...Pulse laser generator, 2...Signal generator, 3...Fist/pulse laser, 4...Laser focusing system, 5...Sample, 6...Laser Evaporated ion, T.
...Extraction electrode, 8...Mass spectrometer, 9...
・Ion detector, 10...Fist ion detection section, 1111
...-Delay signal generator, 12 races...AND circuit,
13--Pulse counter, 14--O ion beam generator, 15--Ion beam, 16--Ion lens, 17--Scanning electrode.

Claims (5)

[Claims] (1) A signal generator that generates a repetitive signal, a pulsed laser generator that generates a pulsed laser beam in response to the signal generated by the signal generator, and irradiates the surface of a sample, which is an analyte, with the pulsed laser beam. An irradiation means, a mass separation means for mass-separating ions generated from the surface of the sample by irradiation with the pulsed laser beam and extracting only ions of a desired mass, and an ion detector for detecting the ions mass-separated by the mass separation means. and the ion detector has a detection time limiting means that operates the detection function only at the time when the mass-separated ions are expected to reach the ion detector. Analysis equipment. (2) Ion detectors with detection time limiting means include a delay signal generator that generates a signal with a predetermined time width after receiving a signal generated from a signal generator, and a delay signal generator that generates a signal with a predetermined time width after receiving a signal generated from a signal generator, and ions that have been mass separated by a mass separation means. an ion detection section that collects and converts it into an electric pulse; an AND circuit that generates a pulse signal only when receiving a delay signal generated from the delay signal generator and an electric pulse generated from the ion detection section at the same time; 2. The high repetition rate laser excitation mass spectrometer according to claim 1, further comprising a pulse counter that counts pulse signals from the circuit. (3) Ion detectors with detection time limiting means include a delay signal generator that generates a signal with a predetermined time width after receiving a signal generated from a signal generator, and a delay signal generator that generates a signal with a predetermined time width after receiving a signal generated from a signal generator, and ions that have been mass separated by a mass separation means. and a pulse counter that operates to count the electrical pulses from the ion detection section using a delay signal from the delay signal generator. High repetition rate laser excitation mass spectrometer as described. (4) An ion detector with detection time limiting means includes an ion detection section that collects ions that have been mass separated by a mass separation means and converts them into electrical pulses, and a time-resolved signal from the ion detection section that is stored. 2. The high repetition rate laser excitation mass spectrometer according to claim 1, further comprising a memory for storing data. (5) High repetition laser excitation according to any one of claims 1 to 4, further comprising an ion beam generator and a scanning means for sputtering the sample surface with the ion beam generated by the ion beam generator. Mass spectrometer.