JP6781616B2 - Atom probe analysis system and atom probe analysis method - Google Patents

Description

The present invention relates to an atom probe analysis system and an atom probe analysis method.

Atom probe analysis is an analysis method that enables direct observation of the atomic arrangement and composition distribution of the tip of a sample processed into a needle shape on an atomic scale. FIG. 1 shows a schematic configuration diagram of the atom probe analysis system. Atoms ionized by electric field evaporation from the surface of the needle-shaped sample 1 in the vacuum chamber 4 fly radially from the surface of the needle and 2 to the detector 3. Dimensionally mapped. Since the atoms are always ionized from the sample surface, the two-dimensional map can be expanded in the depth direction by continuously collecting the atoms from the surface. By accumulating the masses of all detected ions and their coordinates in a computer and displaying them three-dimensionally using the three-dimensional graphic function, the distribution of atoms in the sample can be reproduced in three-dimensional real space with almost atomic-level resolution. ..

In atom probe analysis, a high DC voltage 39 is applied to generate a high electric field at the tip of the needle-shaped sample 1, and a pulse voltage is applied to it or a pulse laser 7 is irradiated to evaporate the electric field of atoms belonging to the first layer of the surface. The element species can be determined by measuring the flight time of the mass of the induced and electroevaporated ions. Since this electric field evaporation proceeds for each atomic layer, atom probe analysis has atomic level resolution in the depth direction.

There are severe restrictions on the sample shape in atom probe analysis, and the tip must be needle-shaped with a radius of curvature of several tens of nanometers. For this reason, it is necessary to process the sample itself to be measured into a needle shape, and conventionally, a needle-shaped sample has been produced by a method of electrolytically polishing a metal sample that has been cut into a prismatic or cylindrical shape. For this reason, the atom probe analysis target has been limited to those capable of obtaining a needle-shaped sample by an electrolytic polishing method. In recent years, due to the development of the focused ion beam (FIB) method, measurement objects such as thin films and specific interfaces in actual devices, which were conventionally very difficult to form into needle shapes by electrolytic polishing, are also processed into needle shapes. I became able to do it.

In atom probe analysis, if the accuracy of flight time measurement is constant, the flight time increases as the flight distance of ions increases, so that the time resolution improves, and as a result, the mass resolution of atom probe analysis improves. At this time, the pulse width of the laser 7 irradiated as the start signal needs to be sufficiently shorter than the flight time of the ions. In addition, by increasing the amount of evaporation per unit time, the time required for atom probe analysis of a certain volume can be shortened, but the laser irradiation cycle is basically shorter than the flight time of electric field evaporation ions. Can't. For example, assuming that the flight distance of ions is 1 m, the voltage applied to the needle-shaped sample is 10 kV, and the mass number of monovalent ions is 20, the flight time from the tip of the ion needle to the detector is about 3 μsec. In this case, if the repetition frequency of the irradiation laser is set to about 300 kHz or more, it becomes impossible to determine when the ions that reach the detector are the ions induced by electric field evaporation by the pulse laser irradiation, so there is an upper limit to the measurable frequency.

It is known that electric field evaporation is affected by temperature, and the higher the temperature, the smaller the electric field required for evaporation. Further, in general, when heat greatly contributes to electric field evaporation, spatial resolution and mass resolution deteriorate due to the effects of surface diffusion, evaporation delay, and the like. At this time, the peak shape in the mass-to-charge ratio spectrum created from the data of the ions that reached the detector within a certain period of time as shown in FIG. 3 has an asymmetric peak shape 41 deviating from the Gaussian distribution. It has a structure 42 with a tail on the high mass-to-charge ratio side called the tail. The heat generated at the tip of the needle-shaped sample 1 by the laser irradiation 7 diffuses from the tip of the needle toward the root, and diffuses to the cooler through the holder 2 that holds the needle-shaped sample. The needle-shaped sample itself usually has a radius of curvature of 100 nm at the tip, a diameter of 0.2 mm or less, and a length of several mm or less, has almost no heat capacity, and is cooled in a short time because it comes into contact with a holder having a large heat capacity.

Special Table 2007-531218 Japanese Unexamined Patent Publication No. 2006-260780

Journal of Applied Physics 110, 094901 (2011)

Conventionally, the pulse width of a pulse laser was on the order of nanoseconds, but in recent years, by shortening the pulse width to picoseconds or femtoseconds, the peak tail is reduced and the mass resolution and spatial resolution are improved (Patent Document 1). It also enables analysis of semiconductor / insulator materials (Patent Document 2, Non-Patent Document 1). However, the most suitable pulse width for atom probe analysis has not been clarified so far.

Generally, in the interaction between a laser and a substance having an energy wavelength of several eV, the substance absorbs the energy of the laser by transitioning the electrons in the substance to an excited state. The time required for this transition is generally on the order of femtoseconds, but since the lifetime of the excited state is on the order of about 1 ps, if the time width of the pulsed laser that is the excitation source is longer than this, it is once. Radiation-free relaxation that occurs repeatedly during pulse irradiation (the energy of the excited state of some particles in a substance causes relaxation that is not radiation that escapes to intramolecular vibrations or adjacent molecules, and finally becomes heat.) It is considered that the temperature of the sample rises due to this. The lifetime of these excited states differs depending on the material and the excited electrons.

In the region where the pulse width is several nsec to several tens of psec, heat is generated by repeating the electronic excitation-relaxation cycle many times during one pulse irradiation, and thermal diffusion continues until the pulse irradiation is completed. .. Therefore, the shorter the pulse width, the more the temperature rise due to the heat generated at the tip of the needle can be suppressed, and the spatial resolution / S / N of the atom probe analysis can be improved.

However, when the pulse width becomes smaller, heat is not induced by the end of the pulse, and the temperature gradient between the needle tip and the root becomes extremely steep. According to Fourier's law, the heat flux is proportional to the temperature gradient at that coordinate, so the heat generated at the needle tip quickly diffuses toward the root, and the temperature at the needle tip drops. Therefore, most of the energy introduced to the needle tip by laser irradiation moves to the bulk portion at the needle root, and as a result, the proportion of laser energy that contributes to evaporation decreases. Therefore, in order to contribute the laser energy to the electric field evaporation most efficiently, there is an optimum value for the pulse width of the laser.

(Problem 1) However, since this changes slightly depending on the material system to be measured and the measurement conditions, it is difficult to predict the optimum measurement conditions before the measurement. To solve this problem, by using a light source that can continuously change the pulse width, it is possible to perform measurement under the optimum conditions for each measurement target, and it is possible to suppress the peak tail of the obtained mass spectrum as much as possible. The purpose is to construct an atom probe analysis system.

In optimizing the pulse width in order to suppress the peak tail to the maximum, it is necessary to compare the magnitude of the peak tail by waving only the pulse width among the measurement conditions. However, due to the principle of atom probe analysis, the measurement of the sample proceeds even during the adjustment work of the measurement conditions including the pulse width, and the once measured part cannot be measured again under another condition. (Problem 2) Therefore, the optimization of measurement conditions must be completed in the shortest possible time.

Further, in atom probe analysis, in general, short wavelength light in which harmonics are generated by using a nonlinear crystal is often used. Since the intensity of harmonic generation is proportional to the square of the temporal and spatial photon density of the original light, the intensity of harmonic generation changes when the pulse width is changed. (Problem 3) Therefore, if the intensity of the harmonic light irradiating the needle-shaped sample is made constant when the pulse width is changed, it becomes necessary to adjust the intensity of the basic wavelength light input to the nonlinear crystal again. As a result, the user manually performs this, which causes a time lag and takes more work time than necessary. To solve this problem, the intensity of the harmonic light to be irradiated is constantly monitored, and feedback is applied so that the intensity of the harmonic light to be irradiated is as set, so that the intensity of the harmonic light emitted to the tip of the needle-shaped sample is controlled to be constant. The purpose is to construct an atom probe analysis system that can be used.

In order to solve the above problems, the atom probe analysis system of the present invention is output from a sample mount capable of holding a needle-shaped sample to be analyzed, a laser oscillator having a variable pulse width, and the laser oscillator. An irradiation optical system arranged to irradiate the tip of the needle-shaped sample with light when the needle-shaped sample is charged, and ions electro-evaporated from the tip of the needle-shaped sample by irradiation with the laser beam. A two-dimensional detector installed on the sample mount so as to face the tip of the needle-shaped sample and perpendicular to the central axis of the needle-shaped sample, which detects the coordinates and time reached by flying radially. A light intensity detector that reads the spectrum of the laser beam radiated to the tip of the needle-shaped sample and detects the time when the laser pulse that triggered the electric field evaporation of the ions was radiated, and the first pulse width. The height of the peak tail that appears on the high mass charge ratio side of the main peak in the mass charge ratio spectrum obtained by performing measurement for a certain period of time using laser light, and the measurement for a certain period of time using laser light with a second pulse width. Compare the heights of the peak tails appearing on the high mass charge ratio side of the main peak in the mass charge ratio spectrum obtained by performing the above, and instruct the laser oscillator to select the pulse width at which the peak tail height becomes low. It is configured to have a control device for output.

Further, as another feature of the present invention, in the atom probe analysis system, the control device selects a plurality of pulse widths within a user-defined pulse width execution range and sequentially changes the pulse widths to the selected pulse widths. The height of the peak tail that appears on the high mass charge ratio side of the main peak in the mass charge ratio spectrum obtained by instructing the laser oscillator to perform measurement for a certain period of time using laser light of each pulse width is determined. The measurement is performed, the heights of the peak tails at each pulse width are compared, and an instruction for selecting the pulse width having the lowest peak tail height is output to the laser oscillator.

Further, as yet another feature of the present invention, in the atom probe analysis system, the laser oscillator changes the pulse width to output laser light, and irradiates the tip of the needle-shaped sample via the irradiation optical system. The light intensity detector reads the spectrum of the laser light, detects the light intensity and feeds it back to the laser oscillator, and the laser oscillator keeps the intensity of the laser light output when the detected light intensity decreases. Feedback control is performed to increase the amount and decrease the intensity of the output laser light by a certain amount when the detected light intensity increases.

Further, in order to solve the above-mentioned problems, the control device sends an instruction to change the atom probe analysis method of the present invention to the first pulse width specified by the user to the laser oscillator having a variable pulse width. The height of the peak tail that appears on the high mass charge ratio side of the main peak in the mass charge ratio spectrum obtained by performing measurement for a certain period of time using a laser beam with a pulse width of is obtained and recorded, and the control device uses the user. Selects different pulse widths from the second to the Nth in sequence, sends an instruction to change to the selected pulse width to the laser oscillator, and measures for a certain period of time using the laser light of the selected pulse width. The process of finding and recording the height of the peak tail appearing on the high mass charge ratio side of the main peak in the mass charge ratio spectrum obtained by performing the process is repeated for each selected pulse width, and then the controller repeats all pulses. The peak tail heights recorded for each width are compared, and an instruction to select the pulse width with the lowest peak tail height is output to the laser oscillator. After that, the atom probe analysis system selects the pulse width. The measurement should be continued using the laser beam with the pulse width instructed to do so.

In atom probe analysis, it becomes possible to measure with the optimum pulse width for each measurement target, and the peak tail in the mass spectrum can be minimized in a short time. In addition, the feedback mechanism can suppress the increase or decrease in the intensity of harmonic generation due to the variable pulse width, and it is possible to continuously shake only the light pulse width condition in atom probe analysis for measurement. become able to. From these facts, it becomes possible to perform atom probe analysis with a pulse width that maximizes the S / N and resolution in any measurement sample, and electric field evaporation progresses during the setting of measurement condition optimization conditions. It becomes possible to keep the sample volume lost by the method small.

It is the schematic of the whole structure of the pulse width control atom probe analysis system of this invention. It is a block diagram of the control device 100 of the atom probe analysis system of this invention. This is an example of a histogram (mass spectrum) obtained by plotting the mass-to-charge ratio on the horizontal axis and the counts obtained on the vertical axis. It is an example of the graph of the change of the peak tail height h with respect to the pulse width Δt. It is a flowchart which shows the flow of the general atom probe analysis. It is a flowchart which shows the flow of the atom probe analysis controlled by the atom probe analysis control part of this Example. It is a flowchart which shows the flow of a pulse width optimization process. This is an example of a graphic user interface (GUI) presented to the user by the control parameter setting unit of the control device. It is a figure explaining the method of defining the peak tail height h with respect to the peak tail evaluation of a mass spectrum. It is a figure which shows an example of the structure of the function which makes the pulse width included in a light source variable.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic view of the overall configuration of the pulse width control atom probe analysis system of the present invention. First, a part related to atom probe analysis as a prerequisite technique of the present invention will be described, and then a part related to the present invention will be described.

《Prerequisite technology》
The needle-shaped sample 1 to be analyzed is held by the sample mount 2, and the sample mount 2 can be cooled to about 40 Kelvin or less by a cooling mechanism (not shown). The sample mount 2 is made of a conductive material, and by applying a voltage 39 of several kV from the DC power supply 6 to the sample mount 2, the tip of the needle-shaped sample 1 held is as high as several tens of V / nm. Generates an electric field. In addition to this high electric field, by irradiating the tip of the needle-shaped sample with a laser pulse 7, the elements constituting the needle-shaped sample 1 are electrovaporated from the tip of the needle-shaped sample one atom at a time. Each field-evaporated ion flies toward the Multi Channel Plate (MCP) delay line detector 3, and the xy coordinates of the ion collision and the ion flight time are recorded. The energy that Ion had at this time was

The mass-to-charge ratio m / n can be obtained from the flight time t, and the element species can be identified. However, m is the mass of the ion, v is the flight speed of the ion, L is the flight distance of each ion from the tip of the needle-shaped sample to the MCP delay line detector 3, n is the valence of the ion, e is the elementary charge, and V is This is the voltage applied to the needle-shaped sample.

The xy coordinates of the ions colliding on the MCP delay line detector 3 are the magnified projection coordinates at the tip of the needle-shaped sample 1 where the electrically evaporated ions originally existed, and the xy coordinates of the ions can be obtained from this. The flight distance of ions is about several tens to several hundreds of cm, but if they collide with other atoms or molecules during this flight, the xy coordinate information of the ions evaporated from the tip of the needle-shaped sample 1 will not be saved, so at least The flight path of ions needs to be contained in the vacuum chamber 4.

The flight time t of the ion is obtained from the difference between the time of the start signal at the timing when the laser 7 is irradiated and the time of the detection signal at the timing when the ion reaches the detector 3. FIG. 1 is a laser pulse type atom probe analysis system in which electric field evaporation is triggered by laser assist. That is, _assuming that the time when the tip of the needle-shaped sample 1 is irradiated with the laser pulse 7 is t _s and the time when the ion evaporated in the electric field collides with the MCP delay line detector 3 is t _g , the flight time t = t _g. −t _s . The time when the laser pulse is irradiated is recorded by reading a fixed ratio of the irradiated light intensity sampled by, for example, a wave plate and a polarizing beam splitter or a dichroic mirror 13 with a light intensity detector 14. The light intensity detector 14 is required to have a response speed capable of analyzing the flight time of ions, and for example, a photodiode is used. Photodiodes convert the energy of light pulses into voltage pulses. The time when this voltage pulse is generated is defined as the time t _s when the laser pulse is irradiated. The time t _{g when the} ion collides with the MCP delay line detector 3 is recorded by reading the voltage pulse multiplied by the MCP section.

When the atom probe analysis is continued, the tip curvature diameter of the needle-shaped sample 1 gradually increases due to electric field evaporation. If the curvature of the tip of the needle-shaped sample 1 increases while the voltage applied to the needle-shaped sample 1 through the sample mount 2 remains constant, the electric field generated at the tip of the needle decreases, and the number of atoms that evaporate per unit time decreases. .. Since it is necessary to perform the measurement so that the electric field generated at the tip of the needle is constant during the measurement, the intensity of the laser irradiated during the measurement is always constant, and the needle-shaped sample 1 becomes larger according to the curvature of the tip of the needle-shaped sample 1. It is necessary to increase the voltage applied to. However, since it is difficult to directly observe the curvature of the tip of the needle-shaped sample, the number of ions detected by electric field evaporation per unit time is usually used and fed back to the voltage so that the number of detected ions becomes constant. Take the method.

When the atom probe analysis is continued and a certain amount of data is obtained, the mass-to-charge ratio is plotted on the horizontal axis and the number of counts obtained is plotted on the vertical axis. It is called a spectrum. FIG. 3 shows an example of a mass spectrum. The shape of the peak in the mass spectrum may take a left-right asymmetric shape deviating from the Gaussian function, and the peak structure with a tail on the right side of the peak that is the asymmetric component on the high mass-to-charge ratio side is called the peak tail.

The flowchart of FIG. 5 shows a general flow of atom probe analysis.
First, in step S101, the needle-shaped sample 1 is fixed to the sample mount 2 in the vacuum chamber 4 and the sample is cooled in preparation for measurement. Samples for atom probe analysis generally need to be cooled to tens of Kelvin or less. Further, if the sample is cooled in the atmosphere before being introduced into the vacuum chamber, contamination such as frost on the surface may occur. Therefore, the sample is often cooled for the first time inside the vacuum chamber. For these reasons, it is necessary to provide a sufficient cooling time to sufficiently lower the temperature to the tip of the needle.

After sufficient cooling, in step S102, the DC voltage applied to the needle-shaped sample 1 is increased until electric field evaporation is confirmed. An electric field ionization observation (FIM observation) means by introducing an inert gas may be used to confirm the electric field evaporation. When the inert gas is introduced, the inert gas becomes electric field evaporation ions, and there is a margin for confirmation. After introducing the inert gas, after confirmation, the inert gas is exhausted to create a vacuum again.

In this state, in step S103, laser irradiation is started on the tip of the needle-shaped sample 1. If the tip of the needle-shaped sample 1 is properly irradiated with the laser, the number of atoms that evaporate in the electric field will increase. In addition, since the electric field evaporation synchronized with the timing of the laser pulse can be confirmed, the mass spectrum is continuously updated on the user interface by the flight time analysis.

After that, in step S104, measurement conditions such as pulse frequency and voltage control parameters are set. This setting work may be set in advance at a stage earlier than this, or may be set again during the subsequent measurement condition setting. By making this setting, the DC voltage applied to the needle-shaped sample 1 is reduced when the electric field evaporation amount increases too much, and the DC applied to the needle-shaped sample 1 when the electric field evaporation amount decreases. Feedback that increases the voltage works.

Then, in step S105, the laser irradiation position is optimized. Since the laser beam is usually focused on the diameter of several μm to several tens of μm through the window 5 of the vacuum chamber 4, it is often not correctly focused on the tip of the needle-shaped sample 1 in the optical axis adjustment such as visual inspection, and the temperature changes. The position of the tip of the needle-shaped sample 1 may shift due to the vibration of the refrigerator. Therefore, the focus position is shaken within the x, y, z scanning range set by the user, and optimization is performed based on the amount of electric field evaporation (Patent Document 1). As a method of swinging the focus position on three axes, there is a method of translating the final stage condenser lens 17 installed outside the vacuum chamber 4 on three axes (the movement mechanism is not shown).

When the focus position is adjusted correctly, the light intensity is optimized in step S106. The higher the light intensity, the greater the contribution of laser evaporation, the lower the voltage, and the lower the risk of fracture due to the electric field of the sample. However, increasing the light intensity results in an increase in peak tail and a decrease in resolution. Since there is a trade-off, the user will decide according to the sample.

In step S107, the optimization of the light intensity is completed, the mass spectrum is confirmed, and if there is no problem under the conditions, the process proceeds to S108, and the measurement is continued until the set analytical amount is obtained (S108). If it is necessary to correct the setting conditions from the mass spectrum shape, etc., move to S104 and set the measurement conditions again.
In step S109, when the needle-shaped sample 1 contains a grain boundary or the like to be observed and the user determines that the measurement of that region has been completed, or when the user determines in advance the end condition for terminating the measurement, the end condition is set. If the condition is satisfied, the measurement is terminated.

<< Configuration of control device 100 >>
FIG. 2 is a diagram illustrating a configuration of a control device 100 of an atom probe analysis system according to the present invention.
The control device 100 can be configured on a general-purpose computer, and its hardware configuration includes a calculation unit 110 composed of a CPU (Central Processing Unit), a RAM (Random Access Memory), and a ROM (Read Only Memory). ), HDD (Hard Disk Drive), storage unit 120 composed of SSD (Solid State Drive) using flash memory, input unit 130 composed of input devices such as keyboard and mouse, CRT display, LCD ( Liquid Crystal Display), a display unit 140 composed of display devices such as an organic EL display, an input / output interface 150 composed of a parallel interface type or serial interface type connection device, communication composed of a NIC (Network Interface Card), etc. It has a part 160, etc.
The input / output interface 150 is connected to the MCP delay line detector 3, the DC power supply 6, the light source 11, the light intensity detector 14, and the like. Further, the communication unit 160 is connected to an external device such as a graphic workstation 180 via a network 170.

The storage unit 120 displays the atom probe analysis program storage area 121 that controls the pulse width control atom probe analysis of the present invention, the control parameter storage area 122 that controls the atom probe analysis, and the user interface on the display unit 140. User interface to be used Data storage area 123, mass spectrum storage area 124 to store newly created or updated mass spectra, and flying ions detected by the detector after evaporating from the tip of the needle-shaped sample are recorded. It is provided with an impact detection ion data storage area 125.

The arithmetic unit 110 realizes the following functional units by loading the atom probe analysis program stored in the storage unit 120 into the RAM and executing it on the CPU. The calculation unit 110 receives the atom probe analysis control unit 111 that controls each step of the atom probe analysis in the present invention, and the control parameters input from the user via the user interface prior to the measurement, or by automatic measurement. A control parameter setting unit 112 that stores the created control parameters in the control parameter storage area 122, and a measurement continuation unit that continues measurement according to the control parameters set for the needle-shaped sample 1 in the atom probe analysis system of this embodiment. 113, the pulse width optimization processing unit 114 that executes the pulse width optimization processing of the laser, and the atom probe analysis are continued, and when a certain amount of landing detection ion data is obtained, a mass spectrum is created. , The mass spectrum storage area 124 is stored, and the mass spectrum creation unit 115 is displayed on the display unit 140.

The flowchart of FIG. 6 shows the flow of atom probe analysis controlled by the atom probe analysis control unit 111 of this embodiment. The steps with the same step numbers as those in the flowchart of FIG. 5 are subjected to the same processing. In particular, in S101 to S107 and S109, in the steps in which the user mainly performs work / setting actions and data entry actions, the user The process of displaying the user interface that supports actions / judgments, accepting the data set / input by the user, and reflecting it in the corresponding control parameters is performed by atom probe analysis together with the control of each component in the atom probe analysis system. This is the control content of the control unit 111.

In S103 of FIG. 6, after the start of laser irradiation, the atom probe analysis control unit 111 evaporates the electric field from the tip of the needle-shaped sample 1 and detects the ions detected by the MCP delay line detector 3 from the detector 3 at the ion detection time. Input t _g , ion detection coordinates x (i), y (i) (33), input the light pulse irradiation time t _s from the light intensity detector 14 (34), and mass from both input data (32). The process of calculating the charge ratio m / n and recording it in the landing detection ion data storage area 125 together with the time data and the coordinate data is continued until the end of the analysis process.

In S104 of FIG. 6, the atom probe analysis control unit 111 activates the control parameter setting unit 112, and the control parameter setting unit 112 displays the graphic user on the display unit 140 according to the data registered in advance in the user interface data storage area 123. Display the interface (GUI) and request the user to input measurement conditions such as frequency and voltage control parameters. The user inputs parameters as appropriate.
The information 31 output from the control parameter setting unit 112 to the light source 11 is ON / OFF, pulse width, frequency, and light intensity of the laser pulse 36 oscillated from the light source 11.

The difference between the flow of atom probe analysis in this example (Fig. 6) and the flow of conventional atom probe analysis shown in FIG. 5 is that the peak tail is minimized by adjusting the pulse width during optimization of measurement conditions. To include the step (S200). In step S200, the user can, for example, find the optimum pulse width of the laser beam with the lowest peak tail height.

In S200 of FIG. 6, the atom probe analysis control unit 111 activates the pulse width optimization processing unit 114 if the user defines the control parameters of the pulse width optimization processing described later in advance, and the flowchart of FIG. 7 shows. The pulse width optimization process shown in is executed. If the user has not yet defined the control parameters for the pulse width optimization process, the graphic user interface below is displayed to prompt the user to enter the definition.

Prior to the pulse width optimization process, the user controls the "pulse width optimization" column 51 in the graphic user interface (GUI) 50 displayed on the display unit 140 by the control parameter setting unit 112, for example, as shown in FIG. Enter the parameters. Examples of control parameters include pulse width optimization range 60,61, its step width (step width 62), and how many atoms are detected per pulse width (detection number 64). specify. For example, if the execution range is set to 500 to 2500 fs and the step width is specified as 200 fs, the number of measurement points 63 is calculated as 11 points, which is automatically displayed on the GUI. If the number of measurements per point is set to 2000 (64), for example, when the detection rate is 1000 / sec, it takes a measurement time of about 2 seconds per pulse width of 65, and this is repeated 11 times. Become.

At this time, it is necessary to correctly define (specify) the peak tail height h to be optimized. The peak tail evaluation of the mass spectrum will be described with reference to FIG. The peak tail height h, which is the target of mass spectrum optimization, is the height of the skirt spread of the peak appearing on the high mass-to-charge ratio side of the peak X1, and is newly defined in the present invention. The highest peak 41 (main peak) in the mass spectrum is automatically determined and displayed as X1 on the GUI (“mass spectrum” column 55 in FIG. 8), but X1 (66) is manually input by the user on the GUI. You can also choose freely. From the count number N (X1 + D) at the point X1 + D where the mass-to-charge ratio is larger by D than the peak position X1, the background height B (the definition of the background height B is the pulse width optimization in FIG. 8). By specifying the pre-peak range 68,69 in column 51 by the user, the height of the background line 74 connecting the two points 68,69 in the corresponding range on the mass spectrum displayed in the mass spectrum column 55 can be set. The value obtained by subtracting the background height B) is defined as the peak tail height h. At this time, the size of the value of D (67) must also be specified by the user through the GUI.
For example, in the case of Fe-based magnet materials and steel materials, the peak of Fe having a mass-to-charge ratio of 28 or 56 is often the main peak. However, in this case, if Co is contained in the same material, Co has a peak at 29.5 or 59, so when trying to evaluate the peak tail height of pure Fe only, X1 + D is Co. It may be necessary to take measures such as setting D small so that the mass-to-charge ratio is smaller than the peak position. Alternatively, it is conceivable to evaluate the peak tail using an element other than Fe. This point needs to be judged by looking at the measurement material and the obtained mass spectrum.

When the user's definition input to the pulse width optimization field 51 is completed, the user can execute the pulse width optimization process by pressing the execute button 56. Hereinafter, the flow of the pulse width optimization process shown in FIG. 7 will be described.

In step S201, the pulse width of the laser is changed to the minimum value 60 of the execution range, which is a control parameter of the pulse width optimization process defined by the user.

In step S202, the laser beam 7 to be irradiated to the needle-shaped sample 1 is sampled (37) by a wavelength plate and a polarizing beam splitter or a dichroic mirror 13 before irradiation, and the light intensity is read by the light intensity detector 14. The read light intensity 35 is fed back to the light source 11, and the light source 11 adjusts the output intensity so that the laser light intensity is as set.

In step S203, the measurement is continued by the measurement continuation unit 113, and the record in which the electric field evaporated ions are detected by the detector 3 is recorded in the landing detection ion data storage area 125.

In step S204, the number of landing detection ion data counted in S203 after setting the pulse width in S201 or S207, or the measurement time is the measurement per point of the control parameter of the pulse width optimization process defined by the user. If the number of detected quantities exceeds 64, or the time 65 is exceeded, the transition to S205 is performed, and if not, the transition to S202 is performed.

In step S205, the mass spectrum creation unit 115 reads out the landing detection ion data detected in S203 from the landing detection ion data storage area 125, and creates a mass spectrum based on the data. From the created mass spectrum, the highest (or user-configurable) main peak 41 in the mass spectrum is determined, and according to the peak tail specification data defined by the user, the mass-to-charge ratio is D from this peak position X1. The peak tail height h is calculated by subtracting the background height B from the count number N (X1 + D) at the large point X1 + D.

In step S206, the calculated peak tail height h and the current pulse width set in S201 or S207 are recorded.

In step S207, the current pulse width is increased by the step width 62 of the control parameter of the pulse width optimization process defined by the user. Let the pulse width of the new laser be used.

In step S208, if the new pulse width increased in S207 is larger than the execution range maximum value 61 of the control parameter of the pulse width optimization process defined by the user, the process shifts to S209, and the new pulse width is the maximum execution range value. If it is 61 or less, set a new pulse width to the output 36 of the light source 11 and shift to S202.

In step S209, a graph of pulse width vs. peak tail height h shown in FIG. 4 (described later) is displayed on the display unit 140 based on the peak tail height h and pulse width data recorded in S206.

In step S210, in the graph of pulse width vs. peak tail height h created in S209, the pulse width having the smallest peak tail height h is selected, and the pulse width is set to the output of the light source 11.

FIG. 4 shows a graph of the change in the peak tail height h with respect to the pulse width Δt. For example, when the pulse width optimization process shown in FIG. 7 is executed according to the control parameters of the pulse width optimization process defined by the user shown in FIG. 8 and the measurement of 11 points is completed, the pulse width as shown in FIG. 4 is displayed on the GUI. change in vs peak tail height h is plotted, peak tail height h set pulse width is changed to smallest pulse width Delta] t _p. The pulse width optimization is completed by the above flow.

The process of producing the graph of FIG. 4 described above must be as short as possible. This is because the electric field evaporation is progressing while the spectra are being integrated, and as the pulse width is changed and the measurement is continued, a specific region in the material is transferred to another region having a different composition. This is because the observation area may advance. It is difficult to optimize the pulse width and the like at sites having different compositions and different obtained mass spectrum shapes. Since the present invention minimizes the processing required by the human side to optimize the pulse width, it is possible to adjust the measurement conditions in a short time.

However, in order to make the pulse width variable while keeping the laser output constant or the electric field evaporation rate constant, it is necessary to control the efficiency at the time of wavelength conversion and the input intensity to the nonlinear crystal. In atom probe analysis, it is known that data can be obtained with better resolution when the wavelength of the laser to be irradiated is in the ultraviolet region than in the infrared or visible region (Non-Patent Document 1). When producing ultraviolet light from infrared light, which is the fundamental wave, it is necessary to perform wavelength conversion by passing it through a non-linear crystal. The efficiency of this wavelength conversion is proportional to the square of the photon density. Since the light source of the present invention has a variable pulse width, for example, if the pulse width is shortened even with the same energy, the wavelength conversion efficiency is increased and the intensity of the generated harmonics is increased. A feedback mechanism is provided for the purpose of keeping the output constant while making the pulse width continuously variable.

For example, consider using an Nd-YAG laser as the light source 11 and irradiating the tip of the needle-shaped sample 1 with THG (Third Harmonic Generation) light. The wavelength of the fundamental wave is 1064 nm, and the output laser light 36 is first introduced into the SHG (Second Harmonic Generation) crystal 15, and in the latter stage of the SHG crystal 15, light having a wavelength of 1064 nm and light having a wavelength of 532 nm are mixed. By introducing this into the THG crystal 16 as it is, in the latter stage of the THG crystal 16, the THG light obtained by combining the fundamental wave and the double wave is also added. By sandwiching this with an optical system that selectively extracts only the target THG light, for example, with a dichroic mirror, it is possible to remove unnecessary fundamental waves and SHG light. The obtained THG light is applied to the tip of the needle-shaped sample 1 by setting an appropriate optical system.

The user can continuously change the pulse width of the light output from the light source 11 through the GUI displayed on the display unit 140. FIG. 10 shows an example of the structure included in the light source 11 having a function of making the pulse width variable. The pulse width can be controlled, for example, by continuously changing the angles of the two gratings 21 contained inside the light source 11 by a piezoelectric element, a step motor, or the like according to an electric signal from the outside. The method of FIG. 10 is a general pulse width extension method, and the pulse width output from the short pulse width light source 22 can be continuously extended. In the present invention, the pulse width may be controlled by any pulse width variable means, and the pulse width is not limited by the means for making the pulse width variable. Further, the following feedback mechanism is used to set in advance that the light intensity is constant even if the pulse width is changed.

An example of the feedback control mechanism will be described. A certain percentage of the THG light intensity to be irradiated is sampled (37) by a wavelength plate and a polarizing beam splitter 13 and the magnitude of the light pulse energy is determined by the photodiode used as the light intensity detector 14 and the resistor connected to the photodiode. It can be monitored as a voltage value proportional to the light intensity. This voltage value is wired 35 so as to be guided to the electric signal input terminal of the light source 11.

For example, two upper limit threshold values and a lower limit threshold value and a light intensity change step width are set in advance for the light source 11 as light intensity feedback control parameters. When the voltage value 35 read from the electric signal input terminal of the light source 11 is larger than the upper limit threshold value, the pulsed light intensity oscillated thereafter is reduced by the light intensity change step width, and the voltage value read from the electric signal input terminal is also reduced. When 35 is smaller than the lower limit threshold value, the pulsed light intensity oscillated thereafter can be increased by the light intensity changing step width. However, a limit is set on the upper limit of the light intensity so that the photon density does not exceed the damage threshold of SHG crystal 15. The above threshold value and step width for suppressing feedback instability can be arbitrarily input by the user by displaying the user interface on the display unit 140 by the control parameter setting unit 112 (in the example of the user interface shown in FIG. 8). Is not described), it can be adjusted by sending a command from the control device 100 to the light source 11. Furthermore, it is also possible to use the feedforward mechanism together with the pulse width value 72 input by the user. This algorithm allows the laser intensity to remain constant even if the user changes the pulse width to any value.

The frequency of the laser pulse output from the light source 11 is, for example, several tens of kHz to several MHz. The intensity of the irradiated laser pulse is adjusted so that, for example, about one atom evaporates in a pulsed electric field in 100 pulses. Then, dozens to hundreds of atoms per second evaporate in a pulsed electric field. These laser irradiation conditions enable the user to specify and input the frequency 70, intensity 71, and pulse width 72 in the laser irradiation condition field 52 of the user interface 50 displayed on the display unit 140 by the control parameter setting unit 112. There is. Further, the user inputs a scanning range in which the focus position is shaken on three axes in order to optimize the laser irradiation position in the focus position optimization field 73 of the laser irradiation condition field 52.

After the laser irradiation is started, the mass spectrum creation unit 115 continuously performs the flight detection ion from the flight detection ion data storage area 125 for each measurement amount per point defined by the user or during the measurement continuation. The data is read out and continuously displayed / updated as an integrated mass spectrum on the GUI displayed on the display unit 140.

In S108 of FIG. 6, the atom probe analysis control unit 111 activates the measurement continuation unit 113, and the measurement continuation unit 113 continues the measurement according to the control parameters already set. For example, the measurement continuation unit 113 increases the feedback to the voltage when the number of detected atoms / pulse falls below the lower limit 80 according to the control parameter input in the voltage rise control parameter column 54 of the user interface 50 in FIG. When the number of detected atoms / pulse exceeds the upper limit 81, feedback to voltage is reduced. Control to reduce width 83 is performed, and control to DC power supply 6 is performed at intervals of Feedback rate 84 (38), and the electric field generated at the tip of needle-shaped sample 1. Is controlled to be constant.
The measurement continuation unit 113 continuously records the recording of the electric field-evaporated ions detected by the detector 3 in the landing detection ion data storage area 125, and measures the measurement state of, for example, the user interface 50 of FIG. Output to status column 53. The measurement continuation unit 113 ends the atom probe analysis process if the measurement state parameter satisfies the measurement end condition 85 set by the user.

After the atom probe analysis process is completed, the distribution of atoms in the sample can be three-dimensionally distributed by appropriately downloading the data recorded in the flight detection ion data storage area 125 to, for example, an externally connected graphic workstation 180. Can be analyzed.

1 ... Needle-shaped sample, 2 ... Sample mount, 3 ... MCP delay line detector, 4 ... Vacuum chamber, 5 ... Window, 6 ... DC power supply, 7 ... Laser pulse irradiating the tip of needle-shaped sample,
11 ... light source, 12 ... mirror, 13 ... beam splitter or dichroic mirror, 14 ... light intensity detector, 15 ... SHG crystal, 16 ... THG crystal, 17 ... condenser lens,
21 ... Grating, 22 ... Short pulse width light source, 23 ... Lens,
31… Information output from the control device to the light source, 32… Information input from the MCP delay line detector and the light intensity detector to the control device, 33… Information output from the MCP delay line detector to the control device, 34 ... Information output from the light intensity detector to the control device, 35 ... Light intensity information fed back from the light intensity detector to the light source, 36 ... Laser light output from the light source, 37 ... Laser light to irradiate the needle-shaped sample Sampled spectrum, 38 ... Control information from the control device to the DC power supply,
41 ... main peak, 42 ... peak tail,
51 ... Pulse width optimization column, 52 ... Laser irradiation condition column, 53 ... Measurement status column, 54 ... Voltage rise control parameter column, 55 ... Mass spectrum column, 56, 57 ... Execution button,
60 ... Minimum execution range of pulse width optimization, 61 ... Maximum execution range of pulse width optimization, 62 ... Step width of pulse width optimization, 63 ... Number of measurement points for pulse width optimization, 64 ... Pulse width optimization Number of detected measurements per point, 65… Time of measurement per point for pulse width optimization, 66… Peak position specified by peak tail X1, 67… D, specified by peak tail, 68… pre-peak Minimum value of range, 69… Maximum value of pre-peak range,
70 ... Laser frequency, 71 ... Laser intensity, 72 ... Laser pulse width, 73 ... Focus position optimization column, 74 ... Background line
80 ... Lower limit of voltage keep condition, 81 ... Upper limit of voltage keep condition, 82 ... Increase width of feedback to voltage, 83 ... Decrease width of feedback to voltage, 84 ... Feedback rate to voltage, 85 ... Measurement end condition ( Voltage),
100 ... Control device, 110 ... Calculation unit, 111 ... Atom probe analysis control unit, 112 ... Control parameter setting unit, 113 ... Measurement continuation unit, 114 ... Pulse width optimization processing unit, 115 ... Mass spectrum creation unit, 120 ... Memory Unit, 121 ... Atom probe analysis program storage area, 122 ... Control parameter storage area, 123 ... User interface data storage area, 124 ... Mass spectrum storage area, 125 ... Flying detection ion data storage area, 130 ... Input unit, 140 ... Display, 150 ... I / O interface, 160 ... Communication, 170 ... Network, 180 ... Graphic workstation.

Claims (5)

A sample mount that can hold, cool, and apply a voltage to the needle-shaped sample to be analyzed,
A laser light source with a variable pulse width and
An irradiation optical system arranged so as to irradiate the tip of the needle-shaped sample with the laser beam output from the laser light source when the needle-shaped sample is charged.
The ions electrically evaporated from the tip of the needle-shaped sample by the irradiation of the laser beam fly radially and detect the coordinates and time of arrival so as to face the tip of the needle-shaped sample held on the sample mount. And a two-dimensional detector installed perpendicular to the central axis of the needle-shaped sample,
Light intensity that reads the spectrum of the laser light emitted to the tip of the needle-shaped sample, feeds back the light intensity to the laser light source, and detects the time when the laser pulse that triggered the electric field evaporation of ions is irradiated. With the detector
The laser light source is instructed to sequentially select the pulse widths of a plurality of points changed at the intervals specified within the pulse width execution range specified by the user according to the analysis target, and change the pulse widths to the respective pulse widths. A command is issued to correct the output intensity of the laser light source toward the set value with the light intensity fed back from the intensity detector, and the laser light of each pulse width is used until the specified number of ion detections is obtained, or the specified time is measured. The mass-charge ratio of each detected ion is calculated to obtain the mass-charge ratio spectrum, and the height of the peak tail appearing on the high-mass-charge ratio side of the main peak in the obtained mass-charge ratio spectrum is determined. Measure and plot the peak tail height at each pulse width in two-dimensional coordinates to create a graph connecting each plot and select the pulse width with the lowest peak tail height on the graph. A control device that outputs instructions to the laser light source,
Atom probe analysis system characterized by having. The control device uses the laser beams of the plurality of pulse widths to obtain the specified number of ion detections for each pulse width, or continues the measurement for a specified time, and the mass charge of each detected ion. The ratio is calculated to obtain the mass-to-charge ratio spectrum, the height of the peak tail appearing on the high-mass-to-charge ratio side of the main peak in the obtained mass-to-charge ratio spectrum is measured, and the height of the peak tail at each pulse width is determined. The atom probe analysis system according to claim 1, wherein the pulse width is plotted on the horizontal axis and the peak tail height is plotted on the vertical axis, and a graph connecting the plots is displayed on the user interface. The laser light source changes the pulse width to output laser light, and the light intensity detector reads the spectrum of the laser light irradiating the tip of the needle-shaped sample via the irradiation optical system to detect the light intensity. When the light intensity detected by the light intensity detector is less than the lower limit threshold value, the control device increases the intensity of the laser light output by the laser light source by a certain amount and detects it. The atom probe analysis system according to claim 1, wherein when the light intensity exceeds the upper limit threshold value, a command is issued to cause the laser light source to perform feedback control for reducing the intensity of the output laser light by a certain amount. The control device displays a graphic user interface on the display unit, and provides information for designating a plurality of pulse width measurement points as control parameters for pulse width optimization processing, a measurement amount per point, and a peak tail height. The atom probe analysis system according to claim 1, wherein the atom probe analysis system according to claim 1, which requires input of specified information for definition, accepts input by a user, and executes a pulse width optimization process according to a control parameter of the user input. .. The control device
Instruct the laser light source to change the pulse width to each pulse width by sequentially selecting the pulse widths of multiple points changed at the specified interval within the pulse width execution range specified by the user according to the analysis target. And put out
A command to read the spectrum of the laser light radiated to the tip of the needle-shaped sample and correct the output intensity of the laser light source in the set value direction by the light intensity from the light intensity detector that feeds back the light intensity to the laser light source. Issued
The mass-to-charge ratio spectrum of each detected ion was calculated until the specified number of detected ions was obtained using the laser beam of each pulse width, or the measurement was continued for the specified time.
The height of the peak tail appearing on the high mass-to-charge ratio side of the main peak in the obtained mass-to-charge ratio spectrum was measured.
The instruction to plot the height of the peak tail at each pulse width in two-dimensional coordinates, create a graph connecting each plot, and select the pulse width having the lowest peak tail height on the graph is described above. Output to a laser light source
An atom probe analysis method characterized in that the selected pulse width is fixed and the measurement of the needle-shaped sample to be analyzed is continued.

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