JP5545922B2 - Sample analysis method and apparatus - Google Patents

Description

  The present invention relates to a method and apparatus for analyzing secondary particles obtained by irradiating a sample with cluster ions with a time-of-flight mass spectrometer.

  Secondary ion mass spectrometry that performs mass analysis of secondary ions generated by the collision phenomenon between incident primary particles and the sample, or mass analysis of secondary ions obtained by ionizing sputtered neutral particles Sputtered neutral particle mass spectrometry is widely used in the industry such as the semiconductor industry, the pharmaceutical industry, and the material manufacturing industry. In secondary ion mass spectrometry and sputter neutral particle mass spectrometry, a time-of-flight (TOF) mass spectrometer is widely used as a method for analyzing generated particles.

  The TOF mass analyzer gives the same kinetic energy to the measurement target particle, and measures the time required for the analysis target particle flying in the analyzer to pass through the analyzer by the kinetic energy. Is a device for performing mass spectrometry. In the case of secondary ion mass spectrometry, it is necessary to give an appropriate potential to the secondary ions generated by the collision of the primary particles with the sample. In the sputtering neutral particle mass spectrometry, neutral particles sputtered by collision of primary particles with a sample are ionized by irradiation with a laser, an electron beam or the like, and then a potential is applied to increase the speed. Depending on the flight method of the particles to be analyzed, there are various variations, such as a linear type in which particles fly almost linearly, a reflective type in which particles are reflected by a reflector, and a method in which resolution is increased in combination with an electrostatic analyzer.

  In order to measure the time (Δt) during which particles fly in the analyzer, the time (t1) when the measurement target particle enters the analyzer and the time (t2) when the particle has passed through the analyzer are measured. There is a need. As a signal related to the time when the measurement target particle enters the analyzer, the signal of the pulse generator used when pulsing the primary particle irradiated to the sample and the probability that the primary incident particle collides with the sample is high. In many cases, the detection signal of the secondary electrons to be emitted is used, and the signal related to the time when it has passed through the analyzer includes the detection signal of the particle detector installed near the exit of the mass analyzer. In many cases, the flight time is obtained from the time difference. Conventionally, when measuring the time of flight of a measurement target particle flying through the analyzer, a pulse related to the time when the measurement target particle enters the analyzer and a pulse related to the time when the particle finishes passing through the analyzer The measurement is performed using a device (time wave height analyzer, TAC) that generates an output pulse having a wave height proportional to the time difference between the two.

Japanese Patent No. 4111270 JP 2006-023251 A

  Usually, in secondary particle mass spectrometry using a TOF mass analyzer, Cs ions having an incident energy of about several hundreds eV to 30 keV are used as primary irradiation particles to generate secondary ions or sputtered neutral particles. Irradiation with a single atom ion composed of one atom such as Ga ion, O ion, Ar ion or the like. In this case, for example, the number of secondary ions generated per 10 keV monoatomic ion, which is often used as a primary ion in secondary ion mass spectrometry, is about 0.01 at most. The number of secondary particles obtained per incident particle is very small, and when a pulsed primary beam is used, the number of primary particles per pulse is very small, or pulsed When a primary beam that is not used is used, the incident particle intensity (the number of primary particles that collide with the sample per unit time) is very small. There will be a pulse associated with the time and a pulse associated with the time when the particles have passed through the analyzer and the time of flight can be measured. In this case, the premise of the measurement is that only one pulse related to the time when the particle has passed through the analyzer is generated for one pulse related to the time when the particle to be measured enters the analyzer. It has become.

  If the amount of secondary particles released from the sample is small, the measurement takes time, the signal to noise ratio becomes high, the data quality deteriorates, or the measurement is below the detection limit, and measurement is impossible. In order to improve this and increase the accuracy and sensitivity of analysis, it is necessary to detect as many secondary particles as possible and reflect them in the spectrum, and it is necessary to increase the generation amount and generation efficiency of secondary particles. .

  There is a method of increasing the number of secondary charged particles to be emitted by irradiating incident particles as an atomic group consisting of two or more atoms or a charged atom group to impart high density energy to the sample surface. For example, in FIG. 3 of Patent Document 1, when a sample is irradiated as a charged particle group, the current caused by secondary particles emitted from the sample surface is normalized by the beam current of the incident particle and displayed as a specific current. . In this figure, the specific current greatly exceeds 1 when the dose is small. Considering that the charge of incident charged particles is monovalent and that most of the positive secondary charged particles are monovalent ions, at least several times the number of incident charged particles It can be seen that positive secondary charged particles are emitted. In addition, since secondary particles can be generated more efficiently by irradiating such a charged particle population, the sensitivity of surface analysis can be improved as in Patent Document 2.

  As described above, when an atomic group is used as the primary incident particle, a plurality of secondary particles may be obtained from one group. This indicates that a plurality of secondary charged particles may be detected regardless of how weak the incident particle intensity is. When the time of flight is measured by TAC when multiple secondary charged particles reach the detector in one atomic group or charged atomic group, the counted secondary charged particles reach the detector first. The signal due to the secondary particles reaching the detector is not observed after that. For this reason, not only the counting down increases, but the measurement result is biased toward the secondary particles that first reach the detector, so that a secondary particle spectrum with an accurate intensity relative ratio cannot be obtained. The fact that only the signal from the secondary particles that have reached the detector first is observed indicates that the probability of reflection in the spectrum differs depending on the flight time of the secondary particles. In this way, when a plurality of secondary charged particles are generated by the incidence of one atomic group or charged atomic group, an accurate spectrum cannot be obtained using TAC, no matter how weak the incident particle intensity is.

  In order to obtain an accurate spectrum in a time-of-flight secondary particle analysis method in which the amount of secondary particles is increased by irradiating the sample with primary incident particles as an atomic group or charged atom group to increase the analysis sensitivity, It is necessary to optimize the secondary particle counting method. SUMMARY OF THE INVENTION An object of the present invention is to provide a sample analysis method and an analysis apparatus that achieve high sensitivity and high accuracy in analysis using secondary particles in combination with atomic group irradiation and charged atom group irradiation.

  The present invention relates to the counting of secondary charged particles in time-of-flight secondary ion mass spectrometry using atomic populations or charged particle populations as primary incident particles. By detecting the secondary charged particles with an equal probability regardless of the mass / charge ratio), it is possible to perform highly sensitive and accurate secondary particle analysis.

  In the present invention, a sample is irradiated with a group of atoms composed of two or more atoms or a group of charged atoms composed of two or more atoms (hereinafter collectively referred to as cluster particles) as primary incident particles. When two or more secondary charged particles per secondary incident particle are obtained, when analyzing the mass analysis of secondary charged particles with a time-of-flight analyzer, the probability is equal regardless of the time of flight of the secondary charged particles The accurate spectrum is obtained by counting the detection signal of the secondary charged particles. As a method of collecting the detection signals of secondary charged particles with an equal probability regardless of the flight time, a case where all the detection signals of secondary charged particles are collected, and a case where the detection signals are collected at a certain rate unrelated to the flight time, There is. When all the samples are collected, if a detection signal is collected together with a signal related to the time when the primary incident particles enter the sample, an accurate spectrum of the intensity of the detection signal of the secondary charged particles can be obtained regardless of the flight time. it can. Even if all signals cannot be acquired due to signal capture, if the signal capture is captured at the same rate regardless of the time of flight, the relative ratio of secondary particle types in the spectrum is correct and an accurate spectrum is obtained. be able to.

  As the incident energy of the primary cluster particles increases, the amount of secondary particles emitted increases, and the probability that a plurality of secondary particles are emitted with respect to one incident particle becomes higher. In particular, when the incident energy is 30 keV or higher, the frequency of obtaining two or more secondary charged particles per atomic group or charged atomic group is increased, and the usefulness of the present invention is increased. Further, as the size of the primary cluster particles increases, the amount of secondary particles emitted increases, and the probability that a plurality of secondary particles are emitted with respect to one incident particle becomes higher. When the number of atoms constituting the cluster particles is 4 or more, the frequency with which two or more secondary charged particles are obtained per cluster particle increases, and the usefulness of the present invention increases.

  FIG. 1 is a schematic view showing an embodiment of a sample analyzer according to the present invention. Primary ions generated in the ion source 10 and having a specific mass / charge ratio separated through the analysis magnet 11 are accelerated by the accelerating tube 12, pass through the collimator 13, become a thin beam, and are held in the holder 15. The sample 16 is irradiated. The secondary ions 17 emitted from the surface of the sample 16 by the incidence of the primary ions 14 are accelerated in the electric field formed on the sample surface and enter the TOF tube 18. Secondary ions flying in the TOF tube enter the multichannel plate 19 and are detected. The detection signal is amplified by the preamplifier 20, the signal waveform is adjusted by a CFD (Constant Fraction Discriminator) 21, and input to the digital oscilloscope 25. The pulses generated from the pulse generator 22 are supplied simultaneously to the high voltage source 23 and the digital oscilloscope 25. The high voltage source 23 controls the deflection voltage applied to the deflection plate 24 in synchronization with the pulse generated from the pulse generator 22, and pulsates primary ions so as to enter the sample 16. 1 shows a linear mass analyzer as a time-of-flight mass analyzer, the type of mass analyzer is not limited to this. The CFD is installed in order to facilitate the setting of signal acquisition into the digital oscilloscope, but its role can be performed in the digital oscilloscope and is not always necessary.

  In this embodiment, a charged atom group (cluster ion) is used as a primary ion, but an atom group composed of two or more atoms may be used instead of the cluster ion. The energy of incident ions is 30 keV or more. When irradiating a sample with cluster ions, depending on the incident conditions (incident ions, energy, sample type, etc.), secondary ions may be generated with a probability exceeding one per incident cluster ion. In this case, when analyzing the mass analysis of secondary charged particles with a time-of-flight analyzer, it is detected in a single-stop format (generally used to detect only secondary ions that have first reached the detector). In this case, only the first detected secondary ion, that is, the one having the smallest mass / charge ratio among the released secondary ions is detected, and an accurate secondary ion spectrum cannot be collected. This is a phenomenon that always occurs when the irradiation intensity of incident particles is weakened when secondary ions are detected with a probability of exceeding one per cluster ion. There is no.

  Therefore, in a time-of-flight analyzer using cluster ions as primary ions, it is necessary to use a counting method that detects secondary ions with an equal probability regardless of the time of flight (or mass / charge ratio). . The counting method of the present invention can meet such a demand.

  FIG. 2 is an explanatory diagram schematically showing a signal from the pulse generator 22 input to the digital oscilloscope 25 and a secondary particle detection signal from the CFD 21. 2A shows a signal (pulsed timing signal) 31 generated from the pulse generator 22, and FIG. 2B shows secondary particle detection signals 32 and 33 output from the CFD 21. When the mass of secondary ions emitted from the sample by primary ion irradiation is m and the charge is z, secondary particles with a small mass-to-charge ratio (m / z) reach the detector early because the flight speed is fast. Secondary particles having a large mass-to-charge ratio (m / z) reach the detector with a delay because of their slow flight speed.

  In the digital oscilloscope 25, as shown in FIG. 2 (c), the pulsed timing signal and the secondary particle detection signal are captured, and the time difference between each secondary particle detection signal and the corresponding pulsed timing signal is measured. A TOF spectrum is obtained by plotting the time difference. Data stored in the digital oscilloscope 25 is transmitted to the computer 26.

  When the amount of data is large, the data is transferred to the external storage device at the timing when the memory in the digital oscilloscope 25 is full, and the counting stops during that time. However, when secondary particles of a certain flight time arrive, the transfer is not performed, and the timing of data transfer is not related to the flight time of the secondary particles. . That is, since all signal data other than the data transfer time are stored, a spectrum based on the flight time data of secondary particles obtained with an equal probability regardless of the flight time is obtained, and the accurate relative intensity is obtained. A spectrum having is obtained.

  In the present invention, all signals including the pulsed timing signal and the secondary particle detection signal are stored in the memory in time series. Specifically, the detected pulse waveform signal and the time difference between the time when the pulse signal was observed and the reference time are taken into a storage medium, and analysis is performed during and after the measurement. Therefore, it is necessary to reanalyze from the signal data later (background setting, noise signal removal, etc.), new analysis (how many secondary particles are present for one timing pulse, or released 2 Combinations of secondary particles, correlations, etc.) can also be performed.

  As described above, in the present invention, when the secondary charged particle is subjected to mass analysis by the time-of-flight analyzer, the detection signal of the secondary charged particle released from the sample is obtained with an equal probability regardless of the flight time of the detected particle. It can be analyzed by counting.

  FIG. 3 is an explanatory view schematically showing a single-stop type measuring method that has been widely used for comparison. FIG. 3A shows a pulsed timing signal 35 and FIG. 3B shows a secondary particle detection signal 36. FIG. 3C is a diagram illustrating a state in which a time difference between each secondary particle detection signal and a pulsed timing signal is measured in order to obtain a TOF spectrum. As shown in FIG. 3 (b), in the conventional single-stop measurement method, the mass / charge among the secondary ions that reach the detector first after the pulsed timing signal, that is, among the emitted secondary ions. Only those having the smallest ratio are detected, and secondary ions that have reached the detector after the second are not detected as indicated by broken lines. Therefore, even when many secondary particles are released simultaneously by irradiating a sample with a particle population (neutral cluster) or cluster ions, only the detection signal of the secondary particles that reached the detector first is counted. Since the measured spectrum is biased, an accurate spectrum cannot be obtained.

  FIG. 4 is a schematic view showing another embodiment of the sample analyzer according to the present invention. The difference from the sample analyzer shown in FIG. 1 is that there is no pulse generator and high-voltage power supply, and a secondary electron detector 27 is provided instead. The output of the secondary electron detector 27 is input to the digital oscilloscope 25 via the preamplifier 28 and the CFD 29. In the case of the apparatus shown in FIG. 1, the primary beam is a continuous beam. In order to pulse it, the deflection plate is forcibly pulsed to drive it. Cluster ions, which are primary ions, are emitted in a pulsed manner. Therefore, when the cluster ions are incident on the sample, the secondary electrons generated from the sample are detected by the secondary electron detector 27, and this secondary electron detection signal is used as a reference signal for measuring the time of flight of the secondary ions. . That is, the output of the secondary electron detector 27 is used instead of the pulsed timing signal 31 shown in FIG. Other apparatus configurations and signal processing procedures are the same as those in the above embodiment.

C ₆₀ cluster ions (incident energy: 540 keV) in which 60 carbon atoms gathered as incident primary particles were pulsed and irradiated to an organic polymer (PMMA) sample. FIG. 5 shows the measured negative secondary ion spectrum. The horizontal axis in the figure is m / z, and the vertical axis is the count number. In the figure, the count number of 100 or more m / z is shown as being five times the actual number. FIG. 5 (a) is a spectrum composed of only secondary particles that arrived at the detector earliest, measured by a conventional method. In this case, since the secondary particles having a short flight time are preferentially counted, the probability of being counted is not equal to the flight time. FIG. 5B was obtained by counting the detection signals of the secondary charged particles emitted by the apparatus shown in FIG. 1 with a method of detecting with equal probability regardless of the flight time of the detected particles. It is a spectrum.

  In the spectrum of FIG. 5A, the count number is drastically decreased when the m / z of the secondary particles is larger than the spectrum of FIG. When performing time-of-flight mass spectrometry, the smaller m / z reaches the detector first. Therefore, when the m / z increases, the count rapidly decreases, which indicates that the count rate of secondary particles having a long time to reach the detector is rapidly decreased. For example, the peak intensity at m / z = 185 in FIG. 5A is about several percent of FIG. 5B, and in the conventional measurement method, many secondary charged particles at m / z = 185 are in the spectrum. Not contributing. Thus, in FIG. 5 (a), detection is not performed with an equal probability regardless of the flight time of the detected particles, and the counting rate of secondary particles having a long flight time is reduced. The difference occurs. For this reason, the relative intensity of the observed secondary ions is different from the actual spectrum. On the other hand, in FIG. 5B, since the detection signal of the secondary charged particles that are emitted is counted with an equal probability regardless of the flight time of the detected particles, the count number increases rapidly even when m / z increases. There is no significant decline. By counting the secondary particles with an equal probability regardless of the time of flight as in the present invention, the accurate spectrum shown in FIG. 5B can be obtained.

6 to 8, the mass spectrum of secondary particles emitted from the sample is measured by the conventional method and the method of the present invention by changing the cluster ion incidence conditions and the sample type, and the obtained spectra are compared. Showed. FIG. 6 shows a spectrum when the sample is a biological polymer (polytyrosine) sample. FIG. 7 is a mass spectrum when the sample is PMMA and the incident energy of cluster ions is 30 keV. FIG. 8 is a mass spectrum when the sample is PMMA and the incident energy of cluster ions is 120 keV. In any figure, (a) is a spectrum obtained by the conventional method, and (b) is a spectrum obtained by the method of the present invention. In any case, the spectrum measured by the method of the present invention has a higher count in a region where m / z is larger than the spectrum measured by the conventional method, and the effect of the present invention is seen. 5 to 8, C ₆₀ is used as the primary incident particle, but the cluster particles that can be used as the primary incident particle are not limited to C ₆₀ .

  The sample analysis method of the present invention is particularly useful for polymer samples, biopolymer samples, biomaterials, and these thin film samples. That is, the organic sample has a relatively large amount of secondary ions released, and the frequency with which two or more secondary particles are obtained from one primary particle is high, and the usefulness of the present analysis method can be exhibited more strongly.

  In the sample analyzer shown in FIG. 1 or 4, when the surface of the sample is scanned with the primary ions 14 by controlling the voltage applied to the deflection plate 24, the sample surface in-plane distribution measurement (two-dimensional measurement) can be performed. it can. Moreover, the depth direction distribution measurement (one-dimensional measurement) of the sample which measures the depth direction distribution while cutting the sample surface with another incident particle can be performed. Furthermore, three-dimensional measurement combining the depth direction and the in-plane distribution can be performed. By combining the counting method of the present invention and such incident beam scanning, more detailed and highly accurate information can be acquired at high speed.

The schematic diagram which shows one Example of the sample analyzer by this invention. Explanatory drawing of the signal input into a digital oscilloscope. Explanatory drawing of the measuring method of the conventional single stop format. The schematic diagram which shows the other Example of the sample analyzer by this invention. C ₆₀ ion (540KeV) negative secondary ion spectrum when irradiated in PMMA ((a) conventional method, (b) the present invention). Positive secondary ion spectrum when polytyrosine is irradiated with C ₆₀ ion (540 keV) ((a) conventional method, (b) present invention). C ₆₀ ions (30 keV) negative secondary ion spectrum when irradiated in PMMA ((a) conventional method, (b) the present invention). C ₆₀ ion (120 keV) negative secondary ion spectrum when irradiated in PMMA ((a) conventional method, (b) the present invention).

Explanation of symbols

10 Ion source 11 Analysis magnet 12 Accelerating tube 13 Collimator 14 Primary ion 15 Holder 16 Sample 17 Secondary ion 18 TOF
19 Multichannel plate 20 Preamplifier 21 CFD
22 Pulse generator 23 High voltage source 24 Deflection plate 25 Digital oscilloscope 26 Computer 27 Secondary electron detector 28 Preamplifier 29 CFD
31 Pulsed timing signals 32, 33 Secondary particle detection signal 35 Pulsed timing signal 36 Secondary particle detection signal

Claims (10)

Pulsing cluster particles composed of a plurality of atoms and irradiating the sample as primary incident particles;
Analyzing a secondary particle generated by two or more pulses per pulse of the primary incident particle with a time-of-flight mass analyzer,
In the analyzing step, the sample analysis method is characterized in that the pulsed signal for pulsing the primary incident particles is used as a reference time signal, and the detection signal of the secondary particles is counted with an equal probability regardless of the flight time. . Irradiating the sample with cluster particles composed of a plurality of atoms as primary incident particles;
Analyzing a secondary particle generated by two or more per primary incident particle with a time-of-flight mass analyzer,
In the analysis step, a detection signal of secondary electrons emitted from the sample when the primary incident particle enters the sample is used as a reference time signal, and the detection signal of the secondary particles is equal in probability regardless of the flight time. A sample analysis method characterized in that the sample is counted.   3. The sample analysis method according to claim 1, wherein the energy of the primary incident particles is greater than 30 keV.   The sample analysis method according to any one of claims 1 to 3, wherein the number of atoms constituting the cluster particles is four or more.   5. The sample analysis method according to claim 1, wherein a detection signal waveform of the secondary particles and a time difference between the time when the secondary particles are detected and the reference time signal are recorded. A sample analysis method characterized by the above.   The sample analysis method according to claim 1, wherein the reference time signal and the detection signal are captured using a digital oscilloscope.   The sample analysis method according to any one of claims 1 to 6, wherein the sample is a polymer sample, a biopolymer sample, a biomaterial, or a thin film sample thereof. A cluster particle generating unit that generates cluster particles composed of four or more atoms;
A primary particle incident part for accelerating the cluster particles to 30 keV or more, pulsing and irradiating the sample;
A time-of-flight mass analyzer that analyzes two or more secondary particles generated per pulse from a sample by irradiation of the cluster particles;
A secondary particle detector for detecting two or more secondary particles per pulse flying in the time-of-flight mass analyzer;
A signal for pulsing and a digital oscilloscope to which two or more secondary particle detection signals per pulse detected by the secondary particle detector are input,
The digital oscilloscope, the signal for the pulsed and the reference time signal, uniform regardless of the detected plurality of secondary particle detection signal with respect to one reference time signal to the time of flight of the secondary particles A sample analyzer characterized by counting with probability. A cluster particle generating unit that generates cluster particles composed of four or more atoms;
Accelerating the cluster particles to 30 keV or more and irradiating the sample with primary particle incident portions;
A time-of-flight mass spectrometer that analyzes two or more secondary particles generated from each sample by irradiation of the cluster particles ,
A secondary electron detector for detecting secondary electrons generated from the sample by irradiation of the cluster particles;
A secondary particle detector for detecting two or more secondary particles per one of the cluster particles flying in the time-of-flight mass analyzer;
A detection signal of the secondary electron detector and a digital oscilloscope to which two or more secondary particle detection signals are input per cluster particle detected by the secondary particle detector;
The digital oscilloscope, the detection signal of the secondary electron detector as a reference time signal, regardless of the detected plurality of secondary particle detection signal with respect to one reference time signal to the time of flight of the secondary particles A sample analyzer that counts with an equal probability.   10. The sample analyzer according to claim 8, wherein the digital oscilloscope includes a detection signal waveform from the secondary particle detector, and a time difference between a time when the secondary particle is detected and the reference time signal. A sample analyzer for recording.

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