JP2015161650A - Measuring device and measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a measuring device and a measuring method capable of highly accurately measuring a characteristic value of a sample with a single- or multi-layered structure.SOLUTION: A measuring device includes: means by which a second time waveform is acquired by detecting a terahertz wave reflected by a sample using a detection unit when an irradiation unit irradiates the sample with the terahertz wave; means for setting an initial value of each of parameters indicating characteristic values of respective layers composing the sample, and for estimating a time waveform of each of reflection waves which are generated at respective interfaces of the sample, from the initial value of each of the parameters using a first time waveform; and means for searching for a characteristic value of the sample by sequentially changing the parameters from the respective initial values until an error between the second time waveform and a time waveform acquired by synthesizing the estimated time waveforms of the respective reflection waves becomes less than a predetermined value.

Description

  The present invention relates to a measuring apparatus and a measuring method using terahertz waves.

  In recent years, various technologies applying terahertz waves have been proposed due to advances in quantum electronics and the semiconductor industry. Terahertz waves are mainly electromagnetic waves having a frequency of about 0.1 to 10 THz. As an example of application of such a terahertz wave, a technique for measuring a film thickness as described below has been proposed.

  Japanese Patent Application Laid-Open No. 2004-028618 (Patent Document 1) discloses a high-performance in-process film thickness measurement method having measurement functions such as a wet film, a multilayer film, a film thickness distribution, and a dry state by utilizing the characteristics of terahertz electromagnetic wave pulses. Is disclosed.

  Japanese Unexamined Patent Application Publication No. 2011-196990 (Patent Document 2) and Japanese Unexamined Patent Application Publication No. 2012-225718 (Patent Document 3) disclose a configuration for accurately measuring a film thickness having a curved surface.

  The coating film inspection apparatus disclosed in Patent Document 2 includes a terahertz wave detector that detects a terahertz wave reflected from a sample, and represents the electric field strength of the detected terahertz wave in waveform data on a time axis. And a control unit that calculates the film thickness based on the time difference between the peaks. This control unit detects a plurality of peaks from the waveform data in accordance with a peak pattern inputted in advance.

  The film thickness inspection apparatus disclosed in Patent Document 3 includes a terahertz wave generator that generates a terahertz wave, an irradiation optical system that irradiates the sample on which the film is formed, and a terahertz wave that is reflected by the sample. Based on the terahertz wave detector that detects and outputs the detection signal and the shape information of the reflecting surface of the sample, the electric field intensity of the reflected wave from the reflecting surface to the terahertz wave detector is calculated as a reference signal, and the reference signal And a control device for correcting the detection signal.

  Japanese Patent Laid-Open No. 2013-228330 (Patent Document 4) can measure a film thickness with high accuracy in a non-contact manner by a simple method even when the value of refractive index × film thickness is smaller than 75 in a film to be measured. A film thickness measuring device and a film thickness measuring method are disclosed.

JP 2004-028618 A JP 2011-196990 A JP2012-225718A JP 2013-228330 A

  When trying to measure the characteristic value (for example, film thickness, etc.) of a multilayer film using terahertz waves, noise due to terahertz light emitting elements, noise due to multiple reflections occurring in the multilayer film, etc. are generated and measured. The accuracy cannot be increased. There is a demand for a measuring apparatus and a measuring method that can measure the characteristic value of a sample having a single layer structure or a multilayer structure with higher accuracy.

  According to one aspect of the present invention, a measuring apparatus for measuring a characteristic value of a sample having a single layer structure or a multilayer structure is provided. The measurement device detects the terahertz wave that is reflected by the reference sample when the terahertz wave is emitted from the irradiation unit toward the reference sample. Means for detecting the first time waveform by the detection unit, and the detection unit detects the terahertz wave that is reflected by the sample when the terahertz wave is irradiated from the irradiation unit toward the sample. The initial value is set for each of the means for obtaining the time waveform of 2 and the parameter indicating the characteristic value of each layer constituting the sample, and each interface of the sample is determined from the initial value of each parameter using the first time waveform Means for estimating the time waveform of each reflected wave generated in the step, the time waveform obtained by synthesizing the estimated time waveform of each reflected wave and the second time To below the value an error predetermined between the forms, by sequentially changing each parameter from the initial value, and means for searching a characteristic value of a sample.

  According to another aspect of the present invention, a measurement method for measuring a characteristic value of a sample having a single layer structure or a multilayer structure is provided. In the measurement method, when the terahertz wave is irradiated from the irradiation unit toward the reference sample, the detection unit detects the terahertz wave that is reflected by the reference sample and acquires the first time waveform; When the terahertz wave is irradiated from the unit toward the sample, the detection unit detects the terahertz wave that is reflected by the sample and acquires the second time waveform, and the characteristic value of each layer constituting the sample A step of estimating a time waveform of each reflected wave generated at each interface of the sample using a first time waveform from an initial value set for each of the parameters indicating, and a time waveform of each estimated reflected wave Each parameter is sequentially changed from the initial value until the error between the synthesized time waveform and the second time waveform falls below a predetermined value. In Rukoto, and a step of searching the characteristic values of the sample.

  According to the present embodiment, the characteristic value of a sample having a single layer structure or a multilayer structure can be measured with higher accuracy using terahertz waves.

It is a figure which shows an example of the time waveform of the terahertz pulse wave generated using the photoconductive element (PC element). It is a figure which shows an example of the time waveform of the terahertz pulse wave generated using the DAST element. It is a schematic diagram for demonstrating the measurement principle of the characteristic value of the sample according to this Embodiment. It is a schematic diagram which shows the apparatus structure of the measuring apparatus according to this Embodiment. It is a schematic diagram which shows the apparatus structure of the measuring apparatus according to the modification of this Embodiment. It is a schematic diagram which shows the apparatus structure of the signal processing apparatus of the measuring apparatus according to this Embodiment. It is a schematic diagram which shows the function structure of the signal processing apparatus of the measuring apparatus according to this Embodiment. It is a flowchart which shows the process sequence of the measuring method according to this Embodiment. It is a flowchart which shows the process sequence of the residual calculation in the flowchart shown in FIG. The analysis example which measured the characteristic value of the sample which has a single layer structure using the measuring method according to this Embodiment is shown. It is a flowchart which shows the process sequence of the measuring method according to the modification of embodiment of this invention. It is a figure which shows an example of the search range of the starting position offset in the measuring method according to the modification of embodiment of this invention. It is a figure which shows an example of the evaluation value of the search process in the measuring method according to the modification of embodiment of this invention. It is a figure for demonstrating evaluation of the error which arises by the initial fluctuation of a starting position offset.

  Embodiments of the present invention will be described in detail with reference to the drawings. Note that the same or corresponding parts in the drawings are denoted by the same reference numerals and description thereof will not be repeated.

<A. Issues of prior art>
First, before describing the measuring apparatus and the measuring method according to the present embodiment, problems in the prior art found by the present inventor will be described.

  When measuring various characteristic values using terahertz waves, terahertz time domain spectroscopy (THz-TDS) is generally used. In the method for measuring the characteristic value of a sample having a single layer structure or a multilayer structure using terahertz time domain spectroscopy, a terahertz pulse wave having a pulse width shorter than 1 picosecond is generated from a pulse light source, and a reflected wave from the sample is generated. taking measurement. For the terahertz pulse wave, a photoconductive element (PC (photoconductive element) element), a DAST (4-dimethylamino-N-methyl-4-stilbazolium tosylate) element, or the like is used.

  FIG. 1 is a diagram illustrating an example of a time waveform of a terahertz pulse wave generated using a photoconductive element (PC element). FIG. 2 is a diagram illustrating an example of a time waveform of a terahertz pulse wave generated using a DAST element.

  As the PC element, typically, low-temperature grown gallium arsenide (LT-GaAs) or the like is used (see Patent Documents 1 and 4 described above). As shown in FIG. 1, the time waveform of the terahertz pulse wave generated when the PC element is used is a differential value of the instantaneous current flowing through the electrode. That is, the generated terahertz pulse wave is not a waveform having a symmetric single peak such as a Gaussian function but a complex waveform having positive and negative amplitude components.

  When a DAST element that is a nonlinear crystal is used as a terahertz pulse wave light source, a terahertz pulse waveform having a more complicated time waveform as shown in FIG. 2 is generated.

  When trying to measure the characteristic value of a sample having a single layer structure or a multilayer structure using a terahertz pulse wave, at least two terahertz pulse waveforms are shifted in time and overlapped by the reflected wave generated by the interface between layers. Observed.

  What is observed by terahertz time domain spectroscopy is not the electric field strength (energy value) but the amplitude value. Therefore, as described in FIGS. 10 to 12 of Patent Document 2 described above, the positive and negative directions of the reflected wave are determined depending on the size of the refractive index of the interface, and the magnitude of the difference between the refractive indexes is determined. This is the height of the observed peak. When there are a plurality of peaks having both positive and negative components and there is a difference in amplitude between the peaks, it is not easy to determine the main peak from the plurality of pulses.

  Considering such facts, the technique for obtaining the film thickness from the peak interval of the echo pulse as described in Patent Document 2 is used when the echo pulse is sufficiently separated and has a sufficient amplitude. Only applicable. For example, the pulse half-width of the terahertz pulse wave from the PC element is about 0.4 ps, which corresponds to an optical path length of about 20 μm (when the refractive index is 1.5), but a film thickness of less than that Regarding the value, it is suggested that the accuracy may be significantly reduced. Further, the pulse width of the terahertz pulse wave from the DAST element is narrower than the pulse width of the terahertz pulse wave from the PC element, and it seems effective for lowering the lower limit of the measurable film thickness. As long as the time waveform shown in FIG. 2 is seen, it seems difficult to determine the main peak by the method described in Patent Document 2.

  Even a terahertz pulse wave generated by a reflected wave from a single plane includes a plurality of peaks in its time waveform. In the case of a multilayer film, they are in an overlapping state, so that the peak included in the observed time waveform may not be the original peak of the reflected wave. Therefore, in the method using the peak pattern as described in Patent Document 3, there is a possibility that an erroneous peak is detected.

  The method described in Patent Document 4 requires a mechanism for generating P waves and S waves. A configuration is adopted in which either a P wave or an S wave is selected using a polarizer and an analyzer. However, in the polarizer and the analyzer, the intensity decreases by 1 / √2, respectively. Thus, the received light intensity is reduced to 1/2. In addition, measurement is required for each of the P wave and the S wave, and the measurement time is doubled compared to the case of using one wave. Furthermore, in the method described in Patent Document 4, it is necessary to make the incident angle to the sample close to the Brewster angle. However, since the Brewster angle of the multilayer film depends on the refractive index of the substance on both sides of the interface, It is not suitable for the measurement of film characteristic values.

  The measuring apparatus and measuring method according to the present embodiment solves the problems that occur in the method and apparatus described in the prior art as described above, and measures characteristic values of a sample having a single layer structure or a multilayer structure with higher accuracy. .

<B. Overview>
In the measurement method according to the present embodiment, it is assumed that the observed time waveform is a collection of the time waveforms of the reflected waves generated at each interface of the sample, and is obtained by synthesizing the simulation waveforms of the reflected waves. By fitting the time waveform to the observed time waveform, the characteristic value of the sample having a single layer structure or a multilayer structure is determined.

  FIG. 3 is a schematic diagram for explaining the principle of measuring the characteristic value of the sample according to the present embodiment. FIG. 3 illustrates a sample S in which two layers (a first layer 202 and a second layer 204) are formed on a substrate 206 as a typical example. Here, the air refractive index n0 of the air layer 200 is set, and the incident angle of the incident wave 210 to the sample S (the incident angle from the air layer 200 to the first layer 202) is set to θ0. Here, the refractive index of the first layer 202 is n1, and the film thickness is d1. The refractive index of the second layer 204 is n2, and the film thickness is d2.

  An incident wave 210 is incident on the interface between the air layer 200 and the first layer 202, a part of which is reflected, and is observed as an air layer-first layer interface reflected wave L1. The transmitted wave L12 transmitted through the interface between the air layer 200 and the first layer 202 is incident on the interface between the first layer 202 and the second layer 204. The incident angle to the second layer 204 (incident angle from the first layer 202 to the second layer 204) is θ1. A part of the transmitted wave L12 is reflected at the interface between the first layer 202 and the second layer 204, and the wave transmitted through the interface between the first layer 202 and the air layer 200 is reflected from the first layer to the second layer. Observed as a two-layer interface reflected wave L2.

  In addition, the transmitted wave L23 transmitted through the interface between the first layer 202 and the second layer 204 is incident on the interface between the second layer 204 and the substrate 206. The incident angle to the substrate 206 (incident angle from the second layer 204 to the substrate 206) is θ2. Part of the transmitted wave L23 is reflected at the interface between the second layer 204 and the substrate 206, and among the reflected waves, the interface between the second layer 204 and the first layer 202, and the first layer 202 and the air layer. A wave transmitted through the interface with 200 is observed as a second layer-substrate interface reflected wave L3.

  The interface reflected waves L1, L2, and L3 shown in FIG. 3 are determined depending on the refractive index and film thickness of the air layer 200, the first layer 202, the second layer 204, and the substrate 206. Each interface reflected wave is defined using the characteristic value of the sample S (multilayer film) as a parameter, and a waveform obtained by synthesizing these interface reflected waves is fitted to the actually measured reflected wave from the sample. Thus, the characteristic value of the sample S is determined.

  In the present embodiment, the characteristic value of the sample S is searched by applying a new algorithm to the reflected wave detected by the terahertz time domain spectroscopy. According to this method, although not limited to this, the sample in which the film thickness of the range of 5 micrometers-1 mm was formed can be measured.

  Hereinafter, the apparatus configuration and processing for realizing the measurement method according to the present embodiment will be described in detail.

<C. Configuration of measuring device>
FIG. 4 is a schematic diagram showing a device configuration of measuring apparatus 1 according to the present embodiment. Referring to FIG. 4, measurement apparatus 1 irradiates sample S with a terahertz wave, and measures a physical or optical characteristic value of sample S using a reflected wave from sample S.

  The measurement apparatus 1 employs a reflective measurement system using terahertz time domain spectroscopy, and includes an irradiation unit that generates a terahertz wave and a detection unit that detects the terahertz wave reflected by the sample S. The measuring apparatus 1 irradiates the reference sample with terahertz waves in addition to the sample.

  The measuring device 1 includes a pulse light source 2, a bandpass filter 4, a beam splitter 6, an emitter unit 10, an antenna unit 20, convex lenses 12 and 22, off-axis paraboloidal mirrors 14 and 24, and a mirror 8. , 18, 32, 34, 36, delay stage 30, and signal processing apparatus 100.

  The terahertz wave is applied to the sample S through the path of the emitter unit 10, the off-axis parabolic mirror 14, and the convex lens 12. The terahertz wave irradiated by the sample S is received through the path of the convex lens 22, the off-axis parabolic mirror 24, and the antenna unit 20. A signal indicating the time waveform of the amplitude of the reflected wave detected by the antenna unit 20 is output to the signal processing apparatus 100.

The pulse light source 2 generates pulsed light for driving the emitter unit 10 and the antenna unit 20. The pulsed light source 2 is a laser light source, and generates and emits femtosecond pulsed light having a pulse width on the order of femtoseconds ( ^10-15 seconds). The oscillation period of the pulsed light in the pulse light source 2 is on the order of several tens of MHz (period of several seconds).

  The band pass filter 4 allows only pulse light having a target wavelength to pass among a plurality of pulse lights having different wavelengths generated by the pulse light source 2. For example, it is assumed that the pulsed light source 2 generates 760 nm pulsed light that is a half-long wave in addition to 1560 nm pulsed light having a fundamental wavelength. In such a case, the band-pass filter 4 transmits only 1560 nm pulsed light suitable for driving the emitter unit 10 and the antenna unit 20.

  The beam splitter 6 distributes the pulsed light transmitted through the bandpass filter 4 into two pulsed lights at a predetermined ratio, and guides them to the emitter unit 10 and the antenna unit 20, respectively. The pulsed light guided to the emitter unit 10 is used as pump light for exciting the emitter unit 10. The pulsed light guided to the antenna unit 20 is used as probe light for exciting the antenna unit 20.

  The emitter unit 10 is a terahertz wave generation source (irradiation unit) and generates terahertz waves by receiving pulsed light. The emitter unit 10 is typically a PC element made of low-temperature grown gallium arsenide (LT-GaAs) or a DAST element that is a nonlinear crystal.

  The terahertz wave generated by the emitter unit 10 is condensed by the convex lens 12 after being irradiated by the off-axis paraboloidal mirror 14 and irradiated to the target position of the sample S. A part of the terahertz wave incident on the sample S is reflected by the sample S, converted into parallel light by the convex lens 22, and then guided to the antenna unit 20 by the off-axis parabolic mirror 24.

  The antenna unit 20 corresponds to a detection unit, receives the terahertz wave reflected by the sample S, and outputs a signal indicating a time waveform of the received light intensity of the terahertz wave (reflected wave) to the signal processing device 100. While receiving the pulse light (probe light) from the pulse light source 2, the antenna unit 20 outputs a signal indicating the time waveform of the received terahertz wave. The antenna unit 20 can also be realized using low-temperature grown gallium arsenide (LT-GaAs). Alternatively, the antenna unit 20 may be configured using a substrate made of InGaAs.

  The emitter unit 10 and the antenna unit 20 are configured to receive pulsed light from the common pulsed light source 2, and synchronization of terahertz light irradiation and light reception is realized by this pulsed light.

  The measurement apparatus 1 includes a delay stage 30 as a configuration related to terahertz time domain spectroscopy. The delay stage 30 is configured to reciprocate at a predetermined speed in a vertical direction on the paper surface by a drive mechanism (not shown). The pulse light from the beam splitter 6 is guided to the delay stage 30 by the mirror 36, is sequentially reflected by the mirror 32 and the mirror 34 that moves integrally with the delay stage 30, and is guided to the mirror 18. As the delay stage 30 reciprocates at a constant speed in the vertical direction of the paper, the optical path length from the beam splitter 6 to the antenna unit 20 changes with time. Such a temporal change in the optical path length causes a temporal shift in the phase at which the terahertz wave generated from the sample S is detected. That is, in the terahertz time domain spectroscopy, a time waveform of a terahertz wave is synthesized using a result obtained by measuring a plurality of times while shifting the phase. Thereby, the time waveform of the terahertz wave can be measured by expanding the time region. In the measuring apparatus 1, it is not necessary to switch between the P wave and the S wave for measurement.

  It is preferable that the distance between the convex lenses 12 and 22 and the sample S is about 10 mm. By adopting such a configuration, it is not necessary to bring the measurement apparatus 1 close to the sample S, and the measurement procedure can be made easier.

  Further, the off-axis parabolic mirrors 14 and 24 are configured such that the incident angle of the terahertz wave irradiated to the sample S and the emission angle of the terahertz light reflected by the sample S are both about 15 °. Is preferred. By maintaining both the incident angle and the outgoing angle at a relatively small value, it is possible to reduce the influence on the measurement error due to the unevenness of the surface of the sample S.

  Instead of the measuring apparatus 1 shown in FIG. 4, a measuring apparatus 1A shown in FIG. 5 may be adopted. FIG. 5 is a schematic diagram showing a device configuration of measuring apparatus 1A according to the modification of the present embodiment.

  In the measurement apparatus 1 </ b> A shown in FIG. 5, the optical axis of the terahertz wave irradiated toward the sample S and the optical axis of the terahertz wave reflected by the sample S are the same. The measuring device 1 </ b> A has a convex lens 28 and a half mirror 26 instead of the convex lenses 12 and 22 as compared with the measuring device 1. In addition, the orientation of off-axis parabolic mirrors 14 and 24 has been changed. That is, the terahertz wave generated by the emitter unit 10 is reflected by the off-axis parabolic mirror 14 and propagates leftward on the paper surface, and then reflected by the half mirror 26 in the vertical direction on the paper surface. Then, the terahertz wave is applied to the sample S through the convex lens 28. The terahertz wave reflected by the sample S is reflected by the half mirror 26 and propagates in the left direction of the paper, and then reflected by the off-axis parabolic mirror 24 and enters the antenna unit 20.

  As the half mirror 26 for the terahertz wave, a wire grid or the like can be used. In this case, the antenna unit 20 is rotated by 90 ° with respect to the emitter unit 10 so that the terahertz wave can be separated using polarized light.

  Since other configurations are the same as those of measuring apparatus 1 shown in FIG. 4, detailed description will not be repeated.

<D. Device Configuration of Signal Processing Device>
Next, the device configuration of the signal processing device 100 will be described. FIG. 6 is a schematic diagram showing a device configuration of the signal processing device 100 of the measuring device 1 according to the present embodiment.

  Referring to FIG. 6, signal processing apparatus 100 is typically realized by a general-purpose computer, and includes a CPU (Central Processing Unit) 102 that executes various programs including an operating system (OS), and a CPU 102. A memory 104 that temporarily stores data necessary for execution of the program in the computer and a hard disk 106 that stores the program executed by the CPU 102 in a nonvolatile manner. Each component constituting the signal processing apparatus 100 is connected to be communicable with each other via a bus 120.

  Hard disk 106 stores in advance a measurement program 108 for realizing the measurement method according to the present embodiment. Such a measurement program 108 is read from a CD-ROM 112, which is an example of a recording medium, by a CD-ROM (Compact Disk-Read Only Memory) drive 110. That is, the measurement program 108 for realizing the measurement method according to the present embodiment is stored and distributed in a recording medium such as the CD-ROM 112. Alternatively, the measurement program 108 may be distributed via a network. In such a case, the measurement program 108 is received via the network interface 114 of the signal processing apparatus 100 and stored in the hard disk 106.

  The display 116 displays measurement results and the like to the user. The input unit 118 typically includes a keyboard, a mouse, and the like, and accepts user operations.

  Note that some or all of the above functions may be realized by dedicated hardware. Further, the signal processing device 100 may be provided as a separate body from the measuring device 1. That is, the time waveform of the amplitude of the reflected wave detected by the antenna unit 20 of the measuring apparatus 1 may be stored using some means, and the stored time waveform may be read and processed.

<E. Functional configuration of signal processing device>
Next, a functional configuration in signal processing apparatus 100 of measuring apparatus 1 according to the present embodiment will be described. As described above, the measurement method according to the present embodiment is obtained by synthesizing the time waveforms of the reflected waves generated at each interface of the sample and combining the simulation waveforms of the reflected waves. Fit the time waveform to the observed time waveform.

  In this embodiment, time waveform data measured from a reference sample is set as reference data, and the time waveform data measured from the sample is assumed to be a set of reference data reflected at each interface. Fitting (typically, the method of least squares) is performed with the interval between the reference data reflected at each interface and the refractive index of each layer as parameters. Within the range of residual calculation, all the transmitted waves and reflected waves at each interface are simulated, and the characteristic values of each layer of the sample S are obtained from the interval and refractive index of the reference data as the fitting results.

  FIG. 7 is a schematic diagram showing a functional configuration of the signal processing device 100 of the measuring device 1 according to the present embodiment. Each module shown in FIG. 7 is typically realized by the CPU 102 of the signal processing apparatus 100 executing the measurement program 108. However, all or part of these functions may be realized using a hard wired circuit such as an ASIC (Application Specific Integrated Circuit).

  Referring to FIG. 7, signal processing apparatus 100 has, as its functional configuration, a reference time waveform data holding unit 152, a sample time waveform data holding unit 154, an evaluation module 156, a conversion module 158, and a reflected wave simulation. Part 160.

  The reference time waveform data holding unit 152 holds reference time waveform data measured by reference measurement. That is, the reference time waveform data holding unit 152 causes the antenna unit 20 (detection unit) to generate a terahertz wave that is reflected by the reference sample when the terahertz wave is irradiated from the emitter unit 10 (irradiation unit) toward the reference sample. By detecting, a reference time waveform (first time waveform) is acquired. The reference sample is for obtaining a (raw) time waveform of the terahertz wave irradiated from the emitter unit 10, and is arranged in place of the sample S in the configuration of FIG. 4 or FIG. That is, in the reference measurement, a time waveform as shown in FIG. 1 or FIG. 2 is acquired. As the reference sample, a structure that generates a single reflected wave with respect to the irradiated terahertz wave is preferable. For example, a flat plate whose surface is plated with gold having high conductivity is used.

  The sample time waveform data holding unit 154 holds sample time waveform data measured by sample measurement. That is, the sample time waveform data holding unit 154 receives the terahertz wave reflected by the sample S when the terahertz wave is irradiated from the emitter unit 10 (irradiation unit) toward the sample S to be measured. The sample time waveform (second time waveform) is acquired by the detection of the (part). In the present embodiment, it is assumed that the layer structure of the sample S is known. In order to increase the accuracy of the fitting described later, the position of the first peak in the time waveform acquired by the sample measurement is close to the position of the reflected wave peak in the time waveform acquired by the reference measurement. It is preferred that sample measurement and post-processing be performed.

  The reflected wave simulating unit 160 sets initial values for parameters indicating the characteristic values of the layers constituting the sample S, and uses the reference time waveform (first time waveform) from the initial values of the parameters. A time waveform (hereinafter also referred to as “simulation waveform”) of each reflected wave generated at each interface of the sample S is estimated. More specifically, the reflected wave simulation unit 160 includes each layer information holding unit 162, a start position offset holding unit 164, a simulation waveform generation module 166, a simulation waveform holding unit 168, and an addition module 169.

  Each layer information holding unit 162 holds the characteristic value of each layer of the sample S. As long as the behavior of reflection and transmission of the incident terahertz wave can be specified, any information may be used as the characteristic value of each layer. In the present embodiment, as an example, it is assumed that each layer of the sample S has a set of an optical path length D and a refractive index n. However, for convenience of calculation, the optical path length D uses the same number of data on the time axis as the time waveform data. That is, the number of data on the time axis corresponding to the time required for the terahertz wave to propagate through the film thickness (optical path) is used as the optical path length of each layer (that is, the time interval between reflected waves). Thus, the characteristic value of each layer constituting the sample S includes the optical path length corresponding to the refractive index and film thickness of each layer.

  Alternatively, each layer of the sample S may have a set of optical path length D and reflectance R. Since the reflectance R is determined depending on the refractive index of the layer constituting each interface, a substantially equivalent process can be performed even if the reflectance R is used instead of the refractive index of each layer. Thus, the characteristic value of each layer constituting the sample S may include an optical path length corresponding to the reflectance and film thickness of each layer.

  The characteristic value of each layer of the sample S held by each layer information holding unit 162 is set to some initial value based on known information at the initial stage of fitting. As the fitting proceeds, the characteristic values of each layer of the sample S are sequentially updated in accordance with the correction instruction from the evaluation module 156.

  The start position offset holding unit 164 holds a start position offset for correcting a deviation on the time axis between the reference time waveform and the sample time waveform. For convenience of calculation, the start position offset is defined as the number of data on the time axis.

  The simulation waveform generation module 166 uses the parameter indicating the characteristic value of each layer of the sample S held by each layer information holding unit 162, the start position offset held by the start position offset holding unit 164, and the like to generate a terahertz wave to the sample S. A time waveform of a reflected wave that will occur at each interface when incident, that is, a simulation waveform is generated. That is, the simulation waveform generation module 166 delays the reference time waveform held by the reference time waveform data holding unit 152 by a time (number of data) corresponding to the propagation path (optical path) of each reflected wave and exists in the propagation path. The amplitude is corrected by multiplying the value according to the refractive index and reflectance of the interface.

  For example, for the sample S with the number L of layers, the simulation waveform generation module 166 starts the parameters (D1, n1), (D2, n2),. A time waveform of each estimated reflected wave is generated using the position offset (number of data).

  Alternatively, for example, for the sample S having the number L of layers, the simulation waveform generation module 166 includes layer parameters (D1, R1), (D2, R2),..., (DL, RL), and the substrate reflectivity R0. Then, the estimated time waveform of each reflected wave is generated using the start position offset (number of data).

  The simulation waveform holding unit 168 holds the simulation waveforms generated by the simulation waveform generation module 166, respectively.

  The addition module 169 synthesizes each simulation waveform held in the simulation waveform holding unit 168.

  The evaluation module 156 evaluates the similarity between the time waveform output from the reflected wave simulating unit 160, which is a combination of the respective simulation waveforms, and the sample time waveform held by the sample time waveform data holding unit 154. If the waveforms are not similar, a correction instruction is given to each layer information holding unit 162 and / or the start position offset holding unit 164. The evaluation module 156 repeats such evaluation and correction, thereby determining the most probable characteristic value of each layer of the sample S with respect to the actually detected sample time waveform data.

  That is, the evaluation module 156 determines that the error between the time waveform obtained by synthesizing the respective simulation waveforms (estimated reflected wave time waveform) and the sample time waveform (second time waveform) is less than a predetermined value. The characteristic value of the sample S is searched by sequentially changing each parameter from the initial value. In addition to the characteristic values of each layer constituting the sample S (a set of the optical path length D and the refractive index n), the evaluation module 156 combines each simulation waveform (estimated reflected wave time waveform) with the time waveform and the sample. The magnitude of the offset (start position offset) on the time axis between the time waveform (second time waveform) is also changed.

  Various known methods can be employed as the search method of the evaluation module 156, but in the following description, an example using the least square method is shown as a typical example.

  The conversion module 158 uses parameters (for example, (D1, n1), (D2, n2),..., (DL, nL)) indicating the characteristic values of each layer of the sample S determined as a result of the search by the evaluation module 156. Calculate the actual film thickness of each layer.

  As described above, in measuring apparatus 1 according to the present embodiment, the time waveform of each reflected wave generated in sample S is estimated, and the synthesized wave of each estimated reflected wave is compared with the actually measured time waveform. And the characteristic value of the sample S is determined.

<F. Processing procedure>
Next, the procedure of the measurement method according to the present embodiment will be described.

(F1: Overall processing procedure)
FIG. 8 is a flowchart showing a processing procedure of the measurement method according to the present embodiment. Referring to FIG. 8, first, the user performs reference measurement to acquire reference time waveform data (step S2). That is, the user measures the reference time waveform data by placing the reference sample at a predetermined position of the measuring apparatus 1 and irradiating the terahertz wave toward the reference sample.

  Subsequently, the user performs sample measurement and acquires sample time waveform data (step S4). That is, the user measures sample time waveform data by placing a sample at a predetermined position of the measuring apparatus 1 and irradiating the sample with a terahertz wave.

  In addition, the execution order of step S2 and step S4 is not ask | required. Further, when measuring a plurality of samples S, the characteristic value of the multilayer film constituting each sample S may be determined using the reference time waveform acquired by the reference measurement in common.

  Subsequently, the user sets an initial value as a start position offset for correcting a deviation on the time axis between the reference time waveform and the sample time waveform (step S6). Note that if time adjustment is appropriately performed between the sample measurement and the reference measurement, zero may be set as the initial value of the start position offset.

  Subsequently, fitting shown in steps S10 to S32 is performed. FIG. 8 shows an example of fitting using the least square method based on the simplex method as an example.

  More specifically, the user first sets an initial value for each parameter indicating the characteristic value of each layer constituting the sample S (step S10). In the following description, parameters (D1, n1), (D2, n2),..., (DL, nL), start position offset (data number), and substrate reflectivity R0 for sample S (number of layers L) Is a parameter vector.

  Subsequently, the signal processing apparatus 100 creates a new parameter vector for giving fluctuation to only one variable based on the initial value of the parameter vector (step S12). At this time, the total number of parameter vectors is the number of parameters included in the parameter vector plus one. Note that the fluctuation value is set to 0 for a parameter whose value is to be fixed.

  Subsequently, the signal processing apparatus 100 calculates a residual based on each parameter vector (step S14). More specifically, the signal processing apparatus 100 determines the time waveform of each reflected wave from the reference time waveform based on each parameter vector, and synthesizes the time waveform of each reflected wave to obtain the sample time. The simulation result of the waveform is calculated, and the residual between the measured sample time waveform is calculated.

  Subsequently, the signal processing apparatus 100 calculates a simplex size using all parameter vectors (step S16). Then, the signal processing apparatus 100 determines whether or not the calculated simplex size is equal to or smaller than a predetermined set value (step S18). If the calculated simplex size is equal to or smaller than a predetermined set value (YES in step S18), the signal processing apparatus 100 ends the least squares process (step S32).

  If the calculated simplex size exceeds a predetermined set value (NO in step S18), the signal processing apparatus 100 rearranges the residuals for each parameter / vector in descending order of the value. At the same time, a parameter / vector for mirroring / enlarging / reducing mirroring / reducing is calculated using all parameter vectors (step S20).

  The signal processing apparatus 100 determines whether or not the residual by the calculated parameter vector is smaller than the residual of the first parameter vector (step S22). When the calculated parameter vector residual is smaller than the first parameter vector residual (YES in step S22), the signal processing apparatus 100 replaces the vector values (step S24). Then, the signal processing apparatus 100 determines whether or not the calculated minimum value of the residual is equal to or less than a predetermined set value (step S26). If the calculated minimum value of the residual is equal to or less than a predetermined set value (YES in step S26), the signal processing apparatus 100 ends the process of the least square method (step S32).

  On the other hand, when the calculated minimum value of the residual exceeds a predetermined set value (NO in step S26), signal processing apparatus 100 has reached the preset set value for the number of repetitions. Whether or not (step S28). If the number of repetitions has reached a preset set value (YES in step S28), signal processing apparatus 100 ends the least squares process (step S32). If the number of repetitions has not reached the preset set value (NO in step S28), the processes in and after step S14 are repeated.

  On the other hand, when the residual by the calculated parameter vector is equal to or larger than the residual of the first parameter vector (in the case of NO in step S22), the signal processing apparatus 100 uses the original parameter vector. Is reduced (step S30). And the process after step S14 is repeated.

  When the fitting shown in steps S10 to S32 is completed, as a result, a set of the optical path length D and the refractive index n is calculated for each layer of the sample S. Finally, the signal processing apparatus 100 calculates the film thickness of each layer of the sample S from the set of the calculated optical path length D and refractive index n (step S40). In other words, the signal processing apparatus 100 is configured until the error between the time waveform obtained by synthesizing the estimated time waveforms of the reflected waves and the sample time waveform (second time waveform) falls below a predetermined value. The characteristic value of the sample S is searched by sequentially changing the parameters of the sample S from the initial value.

  Note that the characteristic value of the sample S is not limited to the film thickness of each layer, and the reflectance of the interface may be calculated together. Or you may output the refractive index of each layer as a measurement result as it is.

(F2: residual calculation)
FIG. 9 is a flowchart showing a processing procedure of residual calculation in the flowchart shown in FIG. In the flowchart shown in FIG. 9, the reflectance and transmittance of each interface of the sample S are calculated based on the Fresnel equation using the refractive index of each layer included in the parameter vector. In order to adjust the peak height between the time waveform of the reflected wave calculated by simulation and the actually measured sample time waveform, it is preferable to use a constant calculated in advance for a sample with a known reflectance. .

  First, the signal processing apparatus 100 calculates behavior data of transmitted waves (waves moving to the next interface) and reflected waves (waves reflected to the air layer) on the surface where the terahertz wave enters the sample S (step S100). . The behavior data includes information such as reflectance, the number of data, the boundary that has been passed so far, and the positive / negative direction. For example, at the initial position indicated by a circle in FIG. 3 (for convenience of calculation, the interface number is “0”), the air refractive index n0 of the air layer 200 and the refractive index n1 of the first layer 202 are used for transmission. Determine the ratio of waves and reflected waves. The signal processing apparatus 100 stores the reflected wave data as a simulation waveform (a wave that should be actually measured). On the other hand, the signal processing apparatus 100 uses transmitted light data as behavior data of waves incident on the next interface.

  Subsequently, the signal processing apparatus 100 calculates behavior data of a transmitted wave (wave moving to the next interface) and a reflected wave (wave reflected to the air layer) at the next interface (step S102). That is, the signal processing apparatus 100 evaluates the behavior of the terahertz wave incident on the target interface using the transmitted wave behavior data calculated in the calculation at the previous interface.

  Then, the signal processing apparatus 100 determines whether or not the transmitted light is in the following state based on the calculated transmitted wave behavior data (step S104). Here, “the state in which the transmitted light continues” indicates that it is meaningful to calculate the behavior in the fitting according to the present embodiment. For example, the reflectance (intensity) at the target interface is previously determined. It means a state that satisfies the conditions such as being equal to or greater than a predetermined set value and the number of data indicating a time waveform being within the calculation range.

  When the transmitted light is in the next state (YES in step S104), the signal processing apparatus 100 uses the transmitted light data as behavior data of the wave incident on the next interface (step S106).

  Further, the signal processing apparatus 100 determines whether or not the reflected light is in the next state based on the calculated behavior data of the reflected wave (step S108). Here, “when the reflected light is in the following state” means that it is meaningful to calculate the behavior in the fitting according to the present embodiment. For example, the reflectance (intensity) at the target interface is It means a state that satisfies the conditions such as being equal to or greater than a predetermined set value and the number of data indicating a time waveform being within the calculation range.

  When the reflected light is in the following state (YES in step S108), the signal processing apparatus 100 stores the reflected light data as internal light data (step S110).

  If the reflected light is not in the following state (NO in step S108), or after execution of step S110, the processes in and after step S102 are repeated.

  On the other hand, when the transmitted light is not in the following state (NO in step S104), the signal processing apparatus 100 causes the transmitted light data to pass through the interface with the air layer on the surface of the sample S. It is determined whether or not it is a state of propagation toward the antenna unit 20 (step S112). When the transmitted light data is in a state of propagating toward the antenna unit 20 through the interface with the air layer on the surface of the sample S (in the case of YES in step S112), the signal processing apparatus 100 Data is stored as a simulation waveform (step S114).

  When the transmitted light data is not in a state of propagating through the interface with the air layer on the surface of the sample S toward the antenna unit 20 (NO in step S112), or after executing step S114, the signal processing apparatus 100 Based on the calculated behavior data of the reflected wave, it is determined whether or not the reflected light is in the following state (step S116). When the reflected light is in the next state (YES in step S116), the signal processing apparatus 100 uses the data of the reflected light as behavior data of a wave incident on the next interface (step S118). And the process after step S102 is repeated.

  On the other hand, when the reflected light is not in the following state (NO in step S116), the signal processing apparatus 100 determines whether or not the previously stored internal light data exists (step S120). . If the previously stored internal light data exists (YES in step S120), the internal light data is used as behavior data of the wave incident on the next interface (step S122). And the process after step S102 is repeated.

  If the previously stored internal light data does not exist (NO in step S120), the simulation waveform generation process ends. And the signal processing apparatus 100 produces | generates the time waveform which synthesize | combined the produced | generated simulation waveform (step S130). That is, the signal processing apparatus 100 generates each of the interfaces of the sample S using the reference time waveform (first time waveform) from the initial value set for each parameter indicating the characteristic value of each layer constituting the sample S. The time waveform of the reflected wave is estimated. Note that simulation data obtained by summing up the simulation waveforms is calculated based on the number of data and the reflectance included in the behavior data of the stored simulation waveforms. Then, the signal processing apparatus 100 outputs, as a residual, a square sum average of the difference between the actually measured sample time waveform and the calculated simulation data for a predetermined range (step S132).

<G. Analysis example>
Next, an analysis example will be described using the measurement method according to the present embodiment. FIG. 10 shows an analysis example in which the characteristic value of a sample having a single layer structure is measured using the measurement method according to the present embodiment. In the analysis example shown in FIG. 10, the sample which used the copper plate as the board | substrate and stuck the thin film was used. In addition, in FIG. 10, the average value which performed the measurement which made the measurement time per time 3.1 seconds was performed 40 times is shown. FIG. 10A shows sample time waveform data (actual measurement data) measured by the sample measurement and simulation waveforms L1 to L5 that can occur in the sample. FIG. 10A also shows a time waveform obtained by synthesizing the simulation waveforms L1 to L5 and an error with respect to the actually measured data of the synthesized time waveform. FIG. 10B shows only the time waveforms of the simulation waveforms L1 to L5.

  In addition, it is as follows when the propagation path (optical path) of simulation waveform L1-L5 is shown, referring the schematic diagram shown in FIG.

L1: R [0]
L2: T [0] ⇒R [1] ⇒T [0]
L3: T [0] ⇒R [1] ⇒R [0] ⇒R [1] ⇒T [0]
L4: T [0] ⇒R [1] ⇒R [0] ⇒R [1] ⇒R [0] ⇒R [1] ⇒T [0]
L5: T [0] ⇒R [1] ⇒R [0] ⇒R [1] ⇒R [0] ⇒R [1] ⇒R [0] ⇒R [1] ⇒T [0]
As shown in FIG. 10 (a), according to the measurement method according to the present embodiment, it can be seen that there is only a slight error with respect to the actual measurement data. Specifically, the thickness of the thin film formed on the sample was 63.2 μm (average of 10 times) by micrometer caliper measurement, whereas the same sample was obtained by the measurement method according to the present embodiment. The film thickness obtained by the measurement was 63.3 μm. That is, the error is 1.5 μm and the error rate is about 2.37%, which indicates that sufficiently high measurement accuracy is obtained.

<H. Modification>
In the above-described embodiment, the method for measuring the characteristic value of the sample when the refractive index or reflectance of the film constituting the sample is known has been described. However, the measurement method according to the present embodiment has a film thickness. It is also possible to measure an unknown refractive index if known.

  FIG. 11 is a flowchart showing a processing procedure of the measurement method according to the modification of the embodiment of the present invention. In the flowchart shown in FIG. 11, raw sample time waveform data that has not undergone peak separation is taken as data to be analyzed. In this measurement method, by adjusting the start position offset, the peak position is searched so that the difference from the known film thickness (hereinafter also referred to as “film thickness difference”) is minimized. The refractive index is determined using the resulting starting position offset. That is, the search process for the start position offset is executed, and then the search process for the refractive index is executed.

  In the start position offset search process, the start position offset is set to a plurality of values, and the film thickness difference is evaluated for each set start position offset. That is, in the processing procedure shown in FIG. 8 described above, the start position offset is also a variation parameter at the time of fitting. However, in the processing procedure shown in FIG. 8 executed in the processing procedure shown in FIG. A preset fixed value is used. In this modification, the film thickness of each layer is searched as the characteristic value of the sample S in the characteristic value searching process of the sample S shown in FIG.

  More specifically, the signal processing apparatus 100 sets a position (data number) at which the amplitude of the sample time waveform data is positive as an initial value of the start position offset (step S200). Then, the signal processing apparatus 100 executes the processing procedure shown in FIG. 8 using the set value of the start position offset, and calculates the film thickness of each layer of the sample (step S202). The signal processing apparatus 100 stores the calculated film thickness and the start position offset value in association with each other. That is, the signal processing apparatus 100 varies the magnitude of the offset (start position offset) on the time axis between the time waveform obtained by synthesizing each simulation waveform and the sample time waveform (second time waveform). At the same time, the film thickness of the sample S is calculated for each offset size.

  Then, the signal processing apparatus 100 increments the start position offset by the search interval (step S204), and whether or not the incremented start position offset has reached a position (data number) at which the amplitude of the sample time waveform data is maximized. Is determined (step S206). If the start position offset after the increment has not reached the position (data number) at which the amplitude of the sample time waveform data is maximum (NO in step S206), the processes in and after step S202 are repeated.

  On the other hand, when the incremented start position offset has reached the position (data number) where the amplitude of the sample time waveform data is maximum (in the case of YES in step S206), the calculated film thickness Of these, the start position offset associated with the film thickness that minimizes the difference from the known film thickness is determined (step S208). That is, the signal processing apparatus 100 determines the magnitude of the offset (start position offset) that minimizes the difference between the film thickness of the sample S and the known film thickness. This completes the start position offset search process.

  FIG. 12 is a diagram showing an example of a search range for the start position offset in the measurement method according to the modification of the embodiment of the present invention. As shown in FIG. 12, the search range of the start position offset is a range from (position where the amplitude of the sample time waveform data is positive + search interval) to (position where the amplitude of the sample time waveform data is maximum−search interval). It becomes.

  Note that the processing in steps S202 to S208 may be repeatedly executed while the search interval is narrowed in steps. For example, in order to shorten the calculation time, a search interval that divides the search range into 10 equal parts is set in the first search, and a position where the film thickness difference is minimized in the next search ± the search interval range is divided into 10 equal parts. Then, the search interval may be sequentially reduced by the procedure of determining a new search interval, and repeated until the minimum unit (ie, one data) is reached.

  FIG. 13 is a diagram showing an example of the evaluation value of the search process in the measurement method according to the modification of the embodiment of the present invention. FIG. 13A shows the relationship between the start position offset and the film thickness difference, and FIGS. 13B and 13C show the relationship between the refractive index and the film thickness difference.

  In the start position offset search process shown in steps S200 to S208, the value of the start position offset is set in advance. By changing the start position offset, the peak height generated in the time waveform obtained by synthesizing the simulation waveform is reduced. As a result, the calculated film thickness difference and refractive index also change (see FIGS. 13A and 13B).

  Subsequently, a refractive index search process is executed. In this refractive index search process, the refractive index is sequentially changed within the search range to determine the refractive index with the smallest difference in film thickness. That is, in the processing procedure shown in FIG. 8 described above, the refractive index is used as a fluctuation parameter at the time of fitting. However, in the processing procedure shown in FIG. 8 executed in the processing procedure shown in FIG. Use the value.

  More specifically, the signal processing apparatus 100 sets the start position offset determined by the search process for the start position offset as an initial value, and sets the refractive index and the initial value of the search range (step S210). More specifically, the initial value of the search range of the refractive index is the second and third from the smallest in the film thickness value difference calculated in the search process of (1) start position offset. A range of refractive index, or (2) a range of refractive index ± constant (for example, 0.05) in which the difference in film thickness value calculated in the search process of the start position offset is set.

  Subsequently, the signal processing apparatus 100 executes the processing procedure illustrated in FIG. 8 using the initial value of the refractive index, and calculates the film thickness of each layer of the sample (step S212). The signal processing apparatus 100 stores the calculated film thickness and the refractive index value in association with each other. That is, the signal processing apparatus 100 calculates the film thickness of the sample S for each refractive index while varying the refractive index of each layer of the sample S to a plurality of values under the determined offset.

  Then, the signal processing apparatus 100 increments the refractive index by the search interval (step S214), and determines whether or not the incremented refractive index is within a preset search range (step S216). If the incremented refractive index is within the preset search range (YES in step S216), the processes in and after step S212 are repeated.

  On the other hand, when the refractive index after increment does not exist within the preset search range (NO in step S216), the difference between the calculated film thickness and the known film thickness is the minimum. The refractive index associated with the film thickness is determined (step S218). That is, the signal processing apparatus 100 outputs the refractive index that minimizes the difference between the film thickness of the sample S and the known film thickness as the refractive index of the sample S. This completes the refractive index search process.

  Then, the signal processing apparatus 100 outputs the determined refractive index of the sample S as a final result (step S220).

  Note that the processing of steps S212 to S218 may be repeatedly executed while the search interval is narrowed stepwise. For example, in order to shorten the calculation time, a search interval that divides the search range into 10 equal parts is set in the first search, and a position where the film thickness difference is minimized in the next search ± the search interval range is divided into 10 equal parts. Then, the search interval may be sequentially decreased by the procedure of determining a new search interval and repeated until the interval becomes sufficiently small (for example, 0.001). By repeating such a search, the evaluation result as shown in FIG. 13B can be evaluated in detail until it is shown in FIG. 13C.

  When the sample S has a multilayer structure, the process shown in FIG. 11 may be repeated for each layer. If the film thickness of the sample S is unknown, the search process may search for a film thickness and a refractive index that minimize the residual.

  An error caused by the initial fluctuation of the start position offset may be evaluated. More specifically, the refractive index value determined by the refractive index search process shown in FIG. 11 is used, and the position where the amplitude of the sample time waveform data becomes positive and the position where the amplitude of the sample time waveform data becomes maximum. Is set as the initial value of the start position offset. Then, using the search range of the start position offset × constant (for example, 0.8) as the initial fluctuation of the start position offset, the film thickness of each layer of the sample S is evaluated according to the processing procedure shown in FIG.

  FIG. 14 is a diagram for explaining an evaluation of an error caused by the initial fluctuation of the start position offset. Referring to FIG. 14, by giving some initial fluctuation to a predetermined starting position offset, the film thickness calculated in that case is evaluated.

<I. Advantage>
By using the measurement method according to this embodiment, a reflected wave (reflected pulse) that is time waveform data from a sample that is an opaque film or a multilayer film is acquired using a reflection optical system of terahertz time domain spectroscopy. . Further, time waveform data is acquired as reference data from a reference sample corrected so that the amplitude reflectance is 100%. With respect to the reflected pulse from the sample, the fitting is performed after decomposing the reflected pulse from each interface of the sample. Thereby, the reflectance of each interface and the refractive index and optical path length of each layer are obtained. Furthermore, the incident angle and reflection angle into the layer can be calculated from the refractive index of each layer, and the film thickness value can be determined in combination with the optical path length acquired previously. By adopting such a novel algorithm, a highly reliable film thickness value can be measured even for a thin film.

  In the measurement method according to the present embodiment, the reflectance and optical path length of each interface are set as fitting parameters, the refractive index of each layer is calculated from the determined reflectance, and the refractive index and optical path length of each layer are calculated. Each method proposes to set the fitting parameters, calculate the interface reflectivity from the refractive index of the upper and lower layers, and reduce the residual.

  For example, by adopting as a reference data a pulse waveform having a time width of 10 times or more of the pulse width, it is possible to perform fitting in consideration of fine upper and lower waveforms of terahertz waves before and after the peak, and erroneously determine peak fluctuations. The possibility can be reduced. By fitting to reduce the residual with the actual measurement data, a film thickness of 20 μm or less, which is difficult to obtain by the method using the peak interval as described in the prior art, can be measured with higher accuracy.

  In addition, for samples with unknown refractive index, the reflectance (positive / negative amplitude and amplitude intensity) depends on the refractive index of the two materials forming the interface, and the reflectance is set as the fitting parameter. Calculate the refractive index from the solution, or calculate the refractive index of the sample layer by setting the refractive index to the fitting variable and obtain the amplitude reflectivity that matches the measured data, and use the refractive index The film thickness can be measured with high accuracy.

  In the measurement method according to the present embodiment, since the terahertz wave is irradiated and measurement is performed using the reflected wave, the characteristic value can be accurately measured even for a film made of a material that does not transmit visible light. By using such a measurement method, various films are manufactured and developed, coating films for automobiles are measured and inspected, semiconductors (such as oxide films on IC substrates) are manufactured, measured and inspected, and resin coatings are manufactured and developed. It can be used for

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  1,1A measuring device, 2 pulse light source, 4 band pass filter, 6 beam splitter, 8, 18, 32, 34, 36 mirror, 10 emitter unit, 12, 22, 28 convex lens, 14, 24 off-axis parabolic mirror , 20 antenna unit, 26 half mirror, 30 delay stage, 100 signal processing device, 102 CPU, 104 memory, 106 hard disk, 108 measurement program, 110 CD-ROM drive, 112 CD-ROM, 114 network interface, 116 display, 118 Input unit, 120 bus, 152 reference time waveform data holding unit, 154 sample time waveform data holding unit, 156 evaluation module, 158 conversion module, 160 reflected wave simulation unit, 162 information on each layer Lifting unit, 164 start position offset holding unit, 166 simulation waveform generating module, 168 simulation waveform holding unit, 169 addition module, 200 an air layer, 202 first layer 204 a second layer, 206 a substrate, 210 the incident wave.

Claims (6)

A measuring device for measuring a characteristic value of a sample having a single layer structure or a multilayer structure,
An irradiation unit that generates terahertz waves;
A detection unit for detecting the terahertz wave;
Means for acquiring a first time waveform by detecting the terahertz wave reflected by the reference sample when the terahertz wave is irradiated from the irradiation unit toward the reference sample;
Means for obtaining a second temporal waveform by detecting the terahertz wave reflected by the sample when the terahertz wave is irradiated from the irradiation unit toward the sample;
An initial value is set for each parameter indicating the characteristic value of each layer constituting the sample, and each reflected wave generated at each interface of the sample using the first time waveform is determined from the initial value of each parameter. Means for estimating the time waveform;
By sequentially changing each parameter from the initial value until an error between a time waveform obtained by synthesizing the estimated time waveform of each reflected wave and the second time waveform falls below a predetermined value, And a means for searching for a characteristic value of the sample.   The measuring device according to claim 1, wherein the characteristic value of each layer constituting the sample includes an optical path length corresponding to a refractive index and a film thickness of each layer.   The measurement device according to claim 1, wherein the characteristic value of each layer constituting the sample includes an optical path length corresponding to a reflectance and a film thickness of each layer.   The means for searching includes means for changing the magnitude of an offset on the time axis between the synthesized time waveform and the second time waveform in addition to the characteristic value of each layer constituting the sample. The measuring apparatus of any one of Claims 1-3. The means for searching searches the film thickness of each layer as the characteristic value of the sample,
The measurement apparatus further varies the magnitude of the offset on the time axis between the synthesized time waveform and the second time waveform, and sets the film thickness of the sample for each offset magnitude. Means for calculating each,
Means for determining the magnitude of the offset that minimizes the difference between the thickness of the sample and the known thickness;
Means for differentiating the refractive index of each layer of the sample into a plurality under the determined offset magnitude, and calculating the film thickness of the sample for each refractive index;
The measurement apparatus according to claim 1, further comprising: means for outputting a refractive index that minimizes a difference between a film thickness of the sample and a known film thickness as a refractive index of the sample. A measurement method for measuring a characteristic value of a sample having a single layer structure or a multilayer structure,
A step of acquiring a first time waveform by detecting a terahertz wave generated by reflection from the reference sample when the terahertz wave is irradiated from the irradiation unit toward the reference sample;
Obtaining a second time waveform by detecting the terahertz wave generated by reflection from the sample when the terahertz wave is irradiated from the irradiation unit toward the sample; and
Estimating a time waveform of each reflected wave generated at each interface of the sample from the initial value set for each parameter indicating a characteristic value of each layer constituting the sample, using the first time waveform;
By sequentially changing each parameter from the initial value until an error between a time waveform obtained by synthesizing the estimated time waveform of each reflected wave and the second time waveform falls below a predetermined value, And a step of searching for a characteristic value of the sample.

Images