JP2011085412A - Terahertz focusing method, terahertz focusing device, and terahertz focusing program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a terahertz focusing method capable of enhancing the image quality of tomographic images, and to provide a terahertz focusing device and a terahertz focusing program. <P>SOLUTION: The refractive index and thickness of a layer, in the depth direction from the interface with a medium of terahertz waves and the incident angle of the terahertz waves to the interface are used, to calculate the moving distance in the depth direction of the focus in the layer, and the moving distance calculated is used to make focusing performed, from one interface of the layer to the other interface of the layer. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a technique using electromagnetic waves (terahertz waves) in a band of approximately 0.1 × 10 ¹² [Hz] to 100 × 10 ¹² [Hz].

  Conventionally, there is terahertz time-domain spectroscopy (THz-TDS) as a method for generating or detecting terahertz waves. This terahertz time domain spectroscopy is attracting attention in various technical fields such as industrial, medical, bio, agricultural or security.

  In this terahertz time domain spectroscopy, pulse light of about 100 fetom seconds is split into pump light and probe light from an ultrashort laser light source, and the pump light is collected on a terahertz wave generating element. Thereby, in the terahertz wave generating element, a current of about sub-picosecond is generated, and a terahertz wave proportional to the time differentiation is generated.

  This terahertz wave is applied to the terahertz wave detection element via the sample. When the terahertz wave detection element is irradiated with the probe light at this time, carriers are generated and accelerated by the electric field of the terahertz wave, and an electric signal is generated. It becomes.

  By shifting the timing at which the probe light reaches the terahertz wave detection element, the time waveform of the amplitude electric field of the terahertz wave is measured, and the spectrum of the terahertz wave can be obtained by Fourier transforming the time waveform.

  By the way, imaging using terahertz time-domain spectroscopy is being examined in order to inspect luggage such as airport customs, human clothes, and prohibited drugs in the body. Further, since the sample can be inspected in a non-destructive state, application to an internal inspection of a structure or an IC or a defect inspection of a molded product is being studied.

  In such an application, acquiring a tomographic image is one of the important matters. As a technique for acquiring this tomographic image, a technique has been proposed in which a terahertz wave signal reflected from a sample is actually measured with respect to the depth of the sample, and a tomographic image is acquired using the result (for example, Patent Document 1).

Special table 2006-516722 gazette

  However, the acquisition method disclosed in Patent Document 1 has a problem in that the image quality of a tomographic image is deteriorated because the signal amount of the terahertz wave signal actually measured at the position becomes smaller as the depth of the sample becomes deeper.

  The present invention has been made in consideration of the above points, and intends to propose a terahertz focusing method, a terahertz focusing apparatus, and a terahertz focusing program that can improve the image quality of a tomographic image.

  In order to solve such a problem, the present invention is a terahertz focusing method using a refractive index and a thickness in a layer in a depth direction from an interface with a terahertz wave medium, and an incident angle of the terahertz wave with respect to the interface. A calculation step for calculating a movement distance in the depth direction of the focal point in the layer, and a focusing step for focusing from one interface of the layer to the other interface using the movement distance calculated in the calculation step.

  The present invention is also a terahertz focusing device, a lens that collects a terahertz wave generated from a generating element toward a surface on which a sample is arranged, and a focal point of the terahertz wave collected by the lens in the normal direction of the surface Using the refractive index and thickness of the layer in the depth direction from the interface between the moving means for moving the light to the medium and the medium of the terahertz wave, and the incident angle of the terahertz wave through the lens, the moving distance of the focal point in the layer in the depth direction is determined. And calculating means for calculating, and control means for controlling the moving means so as to focus from one interface of the layer to the other interface using the moving distance calculated by the calculating means.

  The present invention is also a terahertz focusing program using a refractive index and a thickness in a layer in a depth direction relative to an interface with a terahertz wave medium, and an incident angle of the terahertz wave with respect to the interface. , Calculating the moving distance of the focal point in the depth direction in the layer, and using the calculated moving distance to perform focusing from one interface of the layer to the other interface.

  In the present invention, the moving distance of the focal point in the layer in the depth direction is calculated using the refractive index and thickness of the layer in the depth direction relative to the interface with the medium of the terahertz wave and the incident angle of the terahertz wave with respect to the interface. . For this reason, even if the interface to be measured is a deep interface in the depth direction in the sample, it is possible to obtain a moving distance at which the focal point is accurately located at the interface to be measured in consideration of the refraction angle in the layer. . As a result, the intensity of the oscillating electric field of the terahertz wave reflected from the interface to be measured is captured sharply, and the image quality of the tomographic image is improved.

It is the schematic which shows a terahertz spectroscopy apparatus. It is the schematic where it uses for description of the sampling of a terahertz wave. It is a block diagram which shows the structure of a computer. It is the schematic where it uses for description of the refraction angle of light. It is the schematic which shows the state of the terahertz wave before and behind the movement of a focus. It is the schematic which shows the relationship between the separation degree of the focus position with respect to an interface, and the size of the said focus. It is the schematic which shows the time waveform of the terahertz wave in each interface observed when focusing on the surface of the sample which consists of multiple layers. It is the schematic which shows the relationship between the movement timing of a stage, and the time waveform of a terahertz wave. It is a flowchart which shows a tomogram generation processing procedure.

Hereinafter, modes for carrying out the present invention will be described. The description will be in the following order.
<1. Embodiment>
[1-1. Configuration of terahertz spectrometer]
[1-2. Specific computer configuration]
[1-3. Details of measurement mode processing]
[1-4. Tomographic image generation procedure]
[1-5. Effect]
<2. Other embodiments>

<1. Embodiment>
[1-1. Configuration of terahertz spectrometer]
FIG. 1 shows an overall configuration of a terahertz spectrometer 10 according to the present embodiment. The terahertz spectrometer 10 includes an ultrashort laser 11, a beam splitter 12, an optical delay unit 13, a terahertz wave generating element 14, a sample stage 15, a terahertz wave detecting element 16, and a computer 17.

  For example, the ultrashort laser 11 emits pulsed light having a pulse width of about 100 [fs], a repetition period of about several tens [MHz], and a center wavelength of about 780 [nm]. Specific examples of the ultrashort laser 11 include a femtosecond pulse laser titanium and a sapphire laser.

  The beam splitter 12 separates the pulsed light emitted from the ultrashort laser 11 into pump light and probe light.

  The optical delay stage 13 has a mirror 13A that guides the probe light coming from the beam splitter 12 to the reflecting mirror ROP, and moves the mirror 13A toward or away from the beam splitter 12 and the reflecting mirror ROP at a predetermined speed. Move. As a result, the optical path length of the probe light with respect to the terahertz wave detection element 16 is varied, and the arrival timing is delayed.

  The terahertz wave generating element 14 generates a terahertz wave based on carriers (electrons and holes) excited by pump light that arrives through the beam splitter 12. Specific examples of the terahertz wave generating element 14 include a photoconductive antenna and a nonlinear optical crystal such as ZnTe.

  Incidentally, the photoconductive antenna is composed of a semiconductor substrate made of semi-insulating gallium arsenide, an electrode disposed on one surface of the semiconductor substrate, and an application unit for applying a voltage between gaps formed in the electrode. It is what. In this photoconductive antenna, carriers generated in the semiconductor substrate by the irradiation of pump light are accelerated by the gap gap voltage, and at this time, current flows between the gaps to generate terahertz waves.

  The terahertz wave radiated from the terahertz wave generating element 14 is collected by the objective lens OP1 in a state where the incident angle is acute with respect to the sample arrangement surface of the colorless and transparent sample arrangement plate BD provided on the sample stage 15. For example, a hemispherical lens is used as the objective lens OP1. When a hemispherical lens is used, the aspherical surface thereof is opposed to one surface of the semiconductor substrate that is a component of the terahertz wave generating element 14 (the surface opposite to the surface on which the pulsed light is collected).

  The sample stage 15 is movable in the direction of the sample arrangement surface (x-axis direction and y-axis direction) and the normal direction of the sample arrangement surface (z-axis direction), and the terahertz wave is controlled according to the control of the computer 17. The focal point is located at a specific part of the sample SPL.

  The terahertz wave reflected at a specific portion of the sample SPL is guided to the terahertz wave detecting element 16 as a bundle of parallel rays by the objective lens OP2. For example, a hemispherical lens is used as the objective lens OP2.

  As shown in FIG. 2, the terahertz wave detection element 16 samples the intensity of the oscillating electric field of the terahertz wave at every arrival timing of the probe light varied by the optical delay stage 13, and obtains a time waveform at the intensity of the oscillating electric field. measure.

  Specific examples of the terahertz wave detecting element 16 include a photoconductive antenna and a nonlinear optical crystal such as ZnTe as in the case of the terahertz wave generating element 14. However, in the case of a photoconductive antenna employed as the terahertz wave detection element 16, the above-described application unit is changed to an ammeter.

  In this terahertz spectrometer 10, in order to improve the signal-to-noise ratio (S / N ratio), the pump light is modulated by an optical chopper, and the reference signal is input to the lock-in amplifier to perform synchronous detection. Is called.

[1-2. Computer configuration]
Next, a specific configuration of the computer 17 will be described. As shown in FIG. 3, the computer 17 is configured by connecting various hardware to a CPU (Central Processing Unit) 21 that controls the control.

  Specifically, a ROM (Read Only Memory) 22, a RAM (Random Access Memory) 23 serving as a work memory of the CPU 21, an operation input unit 24 for inputting a command according to a user operation, an interface unit 25, a display unit 26, and a storage The unit 27 is connected via the bus 28.

  The ROM 22 stores programs for executing various processes. The interface unit 25 is connected to the ultrashort laser 11, the sample stage 15, the terahertz detection element 15, and the optical delay unit 16 (time delay stage 13B).

  A liquid crystal display, an EL (Electro Luminescence) display, a plasma display, or the like is applied to the display unit 26. For the storage unit 27, a magnetic disk represented by HD (Hard Disk), a semiconductor memory, an optical disk, or the like is applied. A portable memory such as a USB (Universal Serial Bus) memory or a CF (Compact Flash) memory may be applied.

  The CPU 21 develops a program corresponding to a command given from the operation input unit 24 among the plurality of programs stored in the ROM 22, and appropriately controls the display unit 26 and the storage unit 27 according to the developed program. .

  The CPU 21 appropriately controls the ultrashort laser 11, the sample stage 15, the terahertz detecting element 15 or the time delay stage 13B via the interface unit 25 in accordance with the developed program.

[1-3. Details of measurement mode processing]
When the CPU 21 receives a measurement mode execution command from the operation input unit 24, the CPU 21 develops a program corresponding to the measurement mode in the RAM 23.

  In this case, the CPU 21 follows the program corresponding to the measurement mode, as shown in FIG. 3, the reference interface scanning unit 31, the movement distance calculation unit 32, the deep interface focusing unit 33, the sample measurement unit 34, and the image processing unit. It functions as 35.

  The reference interface scanning unit 31 scans an interface with a terahertz wave medium (hereinafter also referred to as a reference interface). Specifically, the sample stage 15 is scanned in a predetermined order from the initial position in units of measurement areas corresponding to one or a plurality of pixels with respect to the direction of the reference interface (x-axis direction and y-axis direction).

  For each measurement region scanned by the reference interface scanning unit 31, the movement distance calculation unit 32 has an interface (hereinafter referred to as “this”) that is present in the depth direction from the interface with the terahertz wave medium (hereinafter also referred to as “reference interface”). Is also referred to as a deep interface). In this embodiment, since the focal point is moved by the sample stage 15, the moving distance of the focal point is the moving distance of the sample stage 15 in the z-axis direction.

As shown in FIG. 4, the moving distance is expressed as follows: the thickness of the layer is e, the refractive index of the layer is n, the incident angle of light is θ, the condensing angle of light passing through the lens is α, and the incident angle θ If the refraction angle corresponding to is θ ₀ and the refraction angle corresponding to the incident angle θ and the light collection angle α is θ ₁ ,

Is calculated by The refraction angles θ ₀ and θ ₁ are

Defined by

  When the incident angle θ is 0 [°], equation (1) is

It becomes.

  When there are a plurality of deep interfaces, the equation (1) is applied to each layer of the i (i = 2, 3, 4,...) Deep interface and the (i−1) th interface from the reference interface. Then, the moving distance d of the focal point in the layer is calculated.

  Here, the experimental results are shown in FIGS. FIG. 5 shows the case (FIG. 5A) when focusing on a surface (reference interface) opposite to the sample arrangement surface in the sample arrangement plate BD, and the interface (deep interface) between the sample arrangement surface and the sample SPL. The state of the terahertz wave in the case of focusing (FIG. 5 (B)) is shown. FIG. 6 is a graph showing the relationship between the degree of focus position separation with respect to the interface between the sample placement plate BD and the sample SPL and the size of the focus. Incidentally, the broken line in FIG. 6 indicates the intensity (signal amount) of the oscillating electric field of the terahertz wave detected by the terahertz wave detecting element 16.

  In this experiment, the thickness of the sample arrangement plate BD (FIG. 1) is 20 [mm], the refractive index of the sample arrangement plate BD is 2, the incident angle θ of the terahertz wave is 40 [°], and the collection angle of the terahertz wave is α was set to 11.3 [°]. Under this condition, the moving distance d is calculated as 8.7 [mm] (FIG. 5) by the equation (1).

As is clear from FIGS. 5 and 6, in the formula (1), not only the refraction angle θ ₀ corresponding to the incident angle θ but also the converging angle α (divergence of the terahertz wave) is added to the incident angle θ. Since θ ₁ is taken into consideration for the refraction angle, the amount of distortion of the focus due to aberration is removed.

  Therefore, the movement distance calculated by the equation (1) is calculated as the focal point of the terahertz wave that has passed through the sample arrangement plate BD is accurately positioned with respect to the interface between the sample arrangement surface and the sample SPL. As a result, the intensity of the oscillating electric field of the terahertz wave reflected at the deep interface located in the depth direction with respect to the reference interface is captured most sensitively.

  Each parameter of the thickness e and the refractive index n of the sample arrangement plate BD and the incident angle θ and the converging angle α of the terahertz wave that has passed through the objective lens OP1 is information necessary for calculating the moving distance d (hereinafter referred to as the distance). And is also stored in the storage unit 27. In addition, when the sample SPL itself constitutes a plurality of layers and the thickness and refractive index of each layer are known, those parameters are also acquired as distance calculation information.

  The parameter acquisition method includes, for example, a method of displaying an input screen on the display unit 26 and inputting from the operation input unit 24, or a method of acquiring from a barcode attached to the sample arrangement board BD set on the sample stage 15. Applicable. In addition, a method of reading and acquiring from the ROM 22 or the storage unit 27 in which parameters are stored in advance, or a method of acquiring via a network from another device in which parameters are stored in advance is applicable. However, the parameter acquisition method is not limited to these examples.

  By the way, when the sample SPL itself is configured as a plurality of layers, the thickness and refractive index of the layers are often not known. In this case, the movement distance of the layer between the interface between the sample placement plate BD and the sample SPL and the interface in the sample SPL or between the two interfaces in the sample SPL cannot be calculated using the equation (1). become.

  However, in the moving distance calculation unit 32 in this embodiment, even if the thicknesses and refractive indexes of the plurality of layers that the sample SPL itself constitutes are not known, the equation (1) is used to calculate 2 in the sample SPL. The moving distance of the layer between the two deep interfaces is calculated.

  Specifically, when the thickness and refractive index of the plurality of layers that the sample SPL itself constitutes are not known, the movement distance calculation unit 32 first uses a database in which the type of layer and the refractive index are associated with each other. The refractive index is detected from the type of layer in the sample SPL. For example, the same method as the above-described parameter acquisition method is used as the acquisition method of the database and the layer type in the sample SPL.

  In general, even when the refractive indexes of a plurality of layers formed by the sample SPL itself are not known, the types of the layers are known. For example, when the sample SPL is a tissue cut out from a living body, the type of the cut-out tissue layer is known such as an epithelial tissue layer, a connective tissue layer, or a muscle tissue layer. When the sample SPL is a semiconductor, the type of the semiconductor layer is known such as a substrate layer, a P layer, or an N layer. It is also known that the refractive index of the layer becomes unique depending on the type of the layer. Therefore, the refractive index of the layer can be detected from the type of the layer.

  Next, the movement distance calculation unit 32 detects the appearance time difference of the time waveform observed in a state focused on the back surface of the sample arrangement plate BD, for example, and the refraction of the layer corresponding to the appearance time difference and the appearance time difference. The thickness of the layer is detected from the rate.

  For example, as shown in FIG. 7, when the sample SPL is composed of two layers LY1, LY2, time waveforms corresponding to the interfaces B1, B2, B3 of the layers are observed. The appearance time differences Δta and Δtb of these time waveforms satisfy a certain relationship between the thicknesses of the layers LY1 and LY2 and the refractive indexes in the layers LY1 and LY2. Therefore, the layer thickness can be detected from the appearance time differences Δta, Δtb and the refractive indexes of the corresponding layers LY1, LY2.

  Finally, the movement distance calculation unit 32 calculates the movement distance using the detected thickness and refractive index of the layer, other distance calculation information (incident angle, condensing angle), and equation (1). Has been made. As a result, in addition to the deep interface between the sample placement plate BD and the sample SPL, the intensity of the oscillating electric field of the terahertz wave with respect to the deep interface inside the sample SPL can be grasped sharply.

  Each time the reference interface scanning unit 31 changes the measurement region with respect to the reference interface, the deep interface focusing unit 33 focuses all deep interfaces in the depth direction corresponding to the measurement region as measurement target interfaces in a predetermined order. Adjust.

  Here, an example of a specific focusing method will be described with reference to FIG. When the focal point is scanned in a measurement region where the reference interface B1 is present by the reference interface scanning unit 31, the deep interface focusing unit 33 determines the point of time when the zero crossing has occurred after passing through the peak of the time waveform observed for the measurement region. As a trigger, the optical delay stage 13 is stopped.

  The deep interface focusing unit 33 moves the sample stage 15 by the movement distance calculated by the movement distance calculation unit 32 with respect to the layer LY1 between the reference interface B1 and the deep interface B2 between the sample placement plate BD and the sample SPL. After that, the optical delay stage 13 is driven again.

  As a result, as shown in FIG. 8, when the reference interface B1 is focused, the sampling result of the time waveform of the terahertz wave with respect to the unfocused deep interface B2 in the depth direction with respect to the reference interface B1 is the terahertz wave detection element 16. Before being obtained in step (b), the deep interface B2 is focused.

  Further, when the deep interface focusing unit 33 detects a time point after passing through the peak of the time waveform observed with respect to the deep interface B2 and then passing through the zero cross, the optical delay stage 13 and the sample stage are the same as in the case of the deep interface B2. 15 is controlled to focus on the next deep interface B3.

  When the deep interface focusing unit 33 detects a time point after passing through a time waveform peak observed with respect to the deep interface B3 and then passing through the zero crossing, the deep interface focusing unit 33 stops the optical delay stage 13 and then focuses on the reference interface B1. Then, the sample stage 15 is moved. Thereafter, scanning is performed by the reference interface scanning unit 31 so that the focal point is located in the next measurement region.

  In this embodiment, the stage position in the z-axis direction on the sample stage 15 when the focal point is focused on the reference interface is set as the initial stage position, and the position information of this stage position is stored in the storage unit 27.

  As shown in FIG. 2, the sample measurement unit 34 measures the time waveform of the terahertz wave using the sampling result detected by the terahertz wave detection element 16. The sample measurement unit 34 executes a predetermined analysis process using the measured time waveform, and stores the analysis process result as data in the storage unit 27.

  For example, processing such as obtaining an absorption spectrum from a spectral ratio between a time waveform to be measured and a reference waveform with respect to the measurement waveform is applied to the analysis processing. Note that, compared with Fourier spectroscopy using far-infrared light, the S / N ratio is high, and amplitude information and phase information can be obtained at the same time, so information about the sample SPL can be obtained with high accuracy. Is possible.

  The image processing unit 35 uses the time waveform of the terahertz wave in each measurement region of the reference interface scanned by the reference interface scanning unit 31 and the time waveform of the terahertz wave at the deep interface corresponding to the measurement region. An SPL tomographic image is generated. For example, the spectrum of the terahertz wave time waveform in each measurement region is expressed by coloring as corresponding pixels of the display image.

  Further, the image processing unit 35 displays the generated tomographic image on the display unit 26 and appropriately stores the data of the tomographic image in the storage unit 27 when a storage command is given from the operation input unit 24. Yes.

[1-4. Tomographic image generation procedure]
Next, the tomographic image generation processing procedure will be described with reference to the flowchart shown in FIG. That is, for example, when the CPU 21 receives a measurement mode execution command from the operation input unit 24, the CPU 21 starts this tomographic image generation processing procedure and proceeds to step SP1.

  In step SP1, the CPU 21 focuses on the surface (reference interface) opposite to the sample arrangement surface of the sample arrangement plate BD, and proceeds to step SP2 to adjust the measurement area to be the initial scanning point in the reference interface. The process proceeds to step SP3.

  In step SP3, the CPU 21 calculates the focal distance (the movement distance of the sample stage 15) between the i-th deep interface and the (i-1) -th deep interface and the layer from the reference interface, and proceeds to step SP4. .

  In step SP4, the CPU 21 starts measuring the time waveform of the terahertz wave with respect to the measurement region, and continues to measure until detecting the time point after passing through the peak of the time waveform and then passing through the zero cross in the next step SP5.

  When the CPU 21 detects the time when the zero crossing has been performed, the CPU 21 proceeds to step SP6 to determine whether or not measurement results have been obtained from all the deep interfaces in the depth direction corresponding to the current measurement region. Proceed to SP7.

  In step SP7, the CPU 21 stops the movement of the optical delay stage 13 and moves the sample stage 15 in the z-axis direction using the movement distance calculated in step SP3 so as to focus on the deep interface to be measured. Proceed to SP8.

  In step SP8, the CPU 21 restarts the movement of the optical delay stage 13, returns to step SP4, and starts measuring the time waveform of the terahertz wave with respect to the deep interface focused in step SP7.

  When the CPU 21 obtains measurement results from all the deep interfaces in the depth direction corresponding to the current measurement region through the processing loop of step SP4 to step SP8, the process proceeds to step SP9.

  In step SP9, the CPU 21 determines whether or not the reference interface has been scanned. If not, the CPU 21 proceeds to step SP10 to focus on the measurement area to be the next scanning point on the reference interface. After that, the process returns to step SP3 and the above processing is repeated.

  On the other hand, when the CPU 21 finishes scanning the reference interface through the processing loop of step SP3 to step SP9, the process proceeds to the last step SP11.

  In step SP11, the CPU 21 generates a tomographic image of the sample SPL using the time waveform of the terahertz wave in each measurement region of the reference interface and the time waveform of the terahertz wave in the deep interface corresponding to the measurement region, Thereafter, this tomographic image generation processing procedure is terminated.

[1-5. Effect]
In the above configuration, the terahertz spectrometer 10 calculates the movement distance in the depth direction of the focal point in the layer in the depth direction (for example, LY1 and LY2 in FIG. 7) from the reference interface, and uses the movement distance to Focus from one interface to the other (see FIG. 5).

  In calculating the focal distance, the refractive index n and thickness e of the layer and the incident angle θ of the terahertz wave with respect to the reference interface are used (see FIG. 4).

  Therefore, even if the interface to be measured is a deep interface in the depth direction in the sample SPL, the terahertz spectrometer 10 has a moving distance at which the focal point is accurately located at the interface to be measured in consideration of the refraction angle in the layer. Can be obtained. As a result, in the terahertz spectrometer 10, since the intensity of the oscillating electric field of the terahertz wave reflected at the interface to be measured is most sensitively captured (see FIG. 6), the image quality of the tomographic image is improved.

  In the calculation of the movement distance in this embodiment, in addition to the refractive index n and thickness e of the layer and the incident angle θ of the terahertz wave with respect to the reference interface, the condensing angle α is also taken into consideration (see FIG. 4).

  Therefore, the terahertz spectrometer 10 can obtain a moving distance at which the focal point is accurately positioned at the interface to be measured in a state where the amount of distortion of the focal point due to aberration is removed. As a result, the terahertz spectrometer 10 can capture the intensity of the oscillating electric field of the terahertz wave reflected at the interface to be measured more sharply.

  If the interlayer distance is too small and reflection cannot be distinguished from both, it is possible to correct the delay line for the optical delay stage 13 by the average value for the refractive index and move the z stage according to the delay line. is there.

  In the terahertz spectrometer 10, when focusing from one interface of the layer to the other interface, the optical delay stage 13 is temporarily stopped and then focused from one interface to the other interface. . The timing at which the optical delay stage 13 is stopped is triggered by the time when a zero cross is passed after passing through the peak of the time waveform indicating the intensity of the oscillating electric field of the terahertz wave corresponding to one interface (see FIG. 8).

  Accordingly, before the terahertz wave detection element 16 obtains the sampling result of the time waveform of the terahertz wave with respect to the interface B2 of the movement destination in the depth direction from the interface B1 when the interface B1 before the movement is in focus, the movement destination Is focused on the interface B2.

  The time waveform of the terahertz wave reflected by the destination interface B2 when the interface B1 before movement is in focus becomes noise in the process of generating a tomographic image. The terahertz spectrometer 10 can reduce the amount of noise components in the tomographic image generation process in order to prevent the time waveform of the terahertz wave from being sampled by the terahertz wave detection element 16 in advance. It is possible to improve the image quality of the tomographic image.

  By the way, when the sample SPL itself is configured as a plurality of layers, the thickness of the layers is often unknown. However, in this terahertz spectrometer 10, based on the refractive index in the layer LY1 and the appearance time difference Δta indicating the intensity of the oscillating electric field of the terahertz wave corresponding to each of the one interface B1 and the other interface B2 of the layer LY1. The thickness of the layer LY1 is obtained (see FIG. 7).

  Therefore, the terahertz spectrometer 10 uses the equation (1) to move a layer between two deep interfaces in the sample SPL even if the thickness of the plurality of layers that the sample SPL itself constitutes is not known. The distance can be calculated.

  According to the above configuration, the moving distance at which the focal point is accurately positioned at the interface to be measured in consideration of the refraction angle in the layer in the depth direction from the reference interface can be calculated. It becomes possible to grasp the intensity of the oscillating electric field of the terahertz wave that is reflected by the sharply. Thus, the terahertz spectrometer 10 that can improve the image quality of the tomographic image is realized.

<2. Other embodiments>
In the above-described embodiment, the focal position is varied by moving the sample stage 15. However, the variable focus method is not limited to this embodiment. For example, it is possible to apply a focus variable method that changes the focus position by moving the stage with respect to the objective lens OP1. When this variable focus method is applied, the moving distance calculated by the above equation (1) is the moving distance of the stage with respect to the objective lens OP1.

  As another example, a variable focus method that changes the focus position by moving both the sample stage 15 and the stage with respect to the objective lens OP1 is applicable. When this variable focus method is applied, the movement distance calculated by the above-described equation (1) is assigned to one or both of the sample stage 15 and the stage with respect to the objective lens OP1.

  Incidentally, the stage driving means includes an electromagnetic linear motor, a stepping motor, a piezo actuator, a MEMS (Micro Electro Mechanical Systems), and the like.

  In the above-described embodiment, the terahertz wave is irradiated to the sample SPL via the sample arrangement plate BD. The sample arrangement plate BD may be omitted. Incidentally, in this embodiment, the above-described reference interface is the surface of the sample SPL.

  In the above-described embodiment, when the refractive index of the layer in the sample SPL is not known, the refractive index is obtained by referring to the database in which the layer type and the refractive index are associated with each other. However, the refractive index of the layer may be obtained from an actually measured waveform of the terahertz wave.

  For example, it can be obtained by obtaining the refractive index using Kramers-Kronig transformation from the spectrum of the time waveform of the terahertz wave reflected at one interface and the other interface of the layer. Thus, if the refractive index of the layer is obtained from the actually measured waveform, even if the type of the layer is not known, the moving distance of the focal point in the layer from one deep interface to another deep interface in the sample SPL can be calculated. (1) It can calculate using Formula.

  In the embodiment described above, every time the measurement area to be scanned at the reference interface is changed, the moving distance of the focal point with respect to the deep interface in the depth direction corresponding to the measuring area is calculated, and the focal distance is calculated using the moving distance. Terahertz waves were measured on the moved deep interface.

  However, the terahertz wave measurement timing does not need to be executed every time the measurement region to be scanned at the reference interface is changed. For example, the sample may be scanned while the depth position to be observed is fixed to obtain a two-dimensional image first, and then an image in the depth direction may be accumulated. Specifically, after all of the measurement areas to be scanned at the reference interface have been scanned, the focal point is moved in a predetermined order with respect to the deep interface using the movement distance calculated in each measurement area. Measure terahertz waves at.

  The present invention can be used in industries such as industry, medicine, biotechnology, agriculture, security, information communication, and electronics.

  DESCRIPTION OF SYMBOLS 10 ... Terahertz spectrometer 11 ... Ultrashort laser, 12 ... Beam splitter, 13 ... Optical delay stage, 14 ... Terahertz wave generation element, 15 ... Sample stage, 16 ... Terahertz wave detection element, 17 Computer, 21 ... CPU, 22 ... ROM, 23 ... RAM, 24 ... Operation input unit, 25 ... Interface, 26 ... Display unit, 27 ... Storage unit, 31 ... Reference interface scanning unit , 32... Movement distance calculation unit, 33... Deep interface focusing unit, 34... Sample measurement unit, 35... Image processing unit, BD ... sample placement plate, OP 1, OP 2. sample.

Claims (7)

A calculation step of calculating a moving distance of the focal point in the layer in the depth direction using the refractive index and thickness in the layer in the depth direction from the interface with the medium of the terahertz wave and the incident angle of the terahertz wave with respect to the interface;
A terahertz focusing method comprising: a focusing step of focusing from one interface of the layer to the other interface using the movement distance calculated in the calculation step. In the above focusing step,
The change in the arrival time of the probe light to the terahertz wave detection element was stopped, triggered by the time when the zero crossing was passed after passing through the peak of the time waveform indicating the intensity of the terahertz wave oscillating electric field corresponding to one interface of the above layer. The terahertz focusing method according to claim 1, further comprising focusing on the other interface. In the above calculation step,
The thickness in the layer is obtained based on the refractive index in the layer and the appearance time difference of the time waveform indicating the intensity of the oscillating electric field of the terahertz wave corresponding to each of the one interface and the other interface of the layer. Terahertz focusing method. In the above calculation step,
The terahertz focusing method according to claim 2, wherein a moving distance in the depth direction of the focal point in the layer is calculated using a refractive index and a thickness in the layer, and an incident angle and a converging angle of the terahertz wave with respect to the interface. A scanning step of scanning the surface direction of the interface with the terahertz wave medium for each measurement region;
In the above acquisition step,
The terahertz focusing method according to claim 2, wherein a moving distance of a focal point in a layer in a depth direction of the measurement region is calculated for each measurement region. A lens that collects the terahertz wave generated from the generating element toward the surface on which the sample is disposed;
Moving means for moving the focal point of the terahertz wave collected by the lens in the normal direction of the surface;
A calculating means for calculating a moving distance of the focal point in the layer in the depth direction by using the refractive index and thickness in the layer in the depth direction from the interface with the medium of the terahertz wave and the incident angle of the terahertz wave having passed through the lens. When,
A terahertz focusing apparatus comprising: control means for controlling the moving means so as to focus from one interface of the layer to the other interface using the moving distance calculated by the calculating means. Against the computer,
Using the refractive index and thickness of the layer in the depth direction from the interface with the medium of the terahertz wave and the incident angle of the terahertz wave with respect to the interface, calculating the moving distance of the focal point in the layer in the depth direction;
A terahertz focusing program that executes focusing from one interface of the layer to the other using the calculated moving distance.

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