WO2010106589A1

WO2010106589A1 - Optical measuring instrument and optical measurement method

Info

Publication number: WO2010106589A1
Application number: PCT/JP2009/005142
Authority: WO
Inventors: 松本直樹
Original assignee: 株式会社村田製作所
Priority date: 2009-03-18
Filing date: 2009-10-05
Publication date: 2010-09-23

Abstract

Provided is an optical measuring instrument capable of switching between transmitted light detection and reflected light detection, with a simple configuration, and capable of accurately measuring a physical property by changing the incident angle of terahertz light made incident according to the type of a sample, and also provided is an optical measurement method. The optical measuring instrument comprises: a terahertz light source (4) that emits a pulse light in the terahertz frequency region; a light detector (8) for detecting the pulse light; and two pairs of mirror groups (10a, 10b) used for guiding the pulse light emitted from the terahertz light source (4) to a sample (6) to be measured and for guiding transmitted light transmitted through the sample (6) or reflected light reflected on the sample (6) to the light detector (8), including at least two mirrors or more, and disposed at positions symmetrical about the sample (6). The incident angle of the pulse light emitted from terahertz light source (4) with respect to the sample (6) is changed by changing the positions and angles of the two pairs of mirror groups (10a, 10b).

Description

Optical measuring device and optical measuring method

The present invention relates to a light measurement apparatus and a light measurement method for measuring physical properties such as dielectric constant, absorption coefficient, electrical conductivity, etc. of dielectric materials, semiconductors, magnetic materials, etc. in a non-contact manner using pulsed light in the terahertz frequency region. .

In a conventional optical measurement device, particularly an optical measurement device using pulsed light in the terahertz frequency region, the sample is irradiated with pulsed light having a frequency component of 0.1 × 10 ¹² to 100 × 10 ¹² hertz and transmitted through the sample. The physical property value of the sample is calculated by detecting the intensity and phase difference of the transmitted light or the reflected light reflected from the sample surface. For example, Patent Document 1 discloses a measuring device in which a device that detects the intensity and phase difference of transmitted light and a device that detects the intensity and phase difference of reflected light are configured as independent devices.

Even in the case of a sample that transmits terahertz light, transmitted light having sufficient intensity cannot be obtained in the case of a sample having a large absorption coefficient. In addition, when the thin film formed on the conductive film is a sample or the like, the terahertz light does not pass through, so it is necessary to switch to an apparatus that detects the intensity and phase difference of the reflected light. Therefore, in Patent Document 2, it is possible to detect both transmitted light and reflected light with a single measuring device by providing a transmitted light detection unit and a reflected light detection unit and switching the wire grid according to the sample. A terahertz light measuring device is disclosed.

In the case of reflection measurement, it is necessary to accurately measure the relative reflectance ratio and phase difference between the reference reflecting surface and the sample. However, there is a problem that it is particularly difficult to accurately measure the phase difference due to a mechanical error or the like that occurs when the position of the reference reflecting surface and the sample is switched. In order to solve such a problem, for example, Patent Document 3 discloses an ellipsometer capable of deriving a complex optical constant spectrum of a sample in the terahertz region without performing a reference measurement. Patent Document 3 employs an ellipsometry method that uses the property that the reflectance and phase of s-polarized light and p-polarized light differ depending on the complex dielectric constant of the sample, and calculates the physical property value based on the polarization change of the reflected light with respect to the incident light. is doing.

JP 2002-277393 A Japanese Patent Laying-Open No. 2005-227021 JP 2003-014620 A

When switching between transmission measurement and reflection measurement according to a measurement sample, the terahertz light measurement device disclosed in Patent Document 2 needs to include a transmitted light detection unit and a reflected light detection unit separately. Since all of these light detection units are expensive, there is a problem that it is difficult to reduce the cost of the entire measuring apparatus.

In addition, in the ellipsometer disclosed in Patent Document 3, the incident angle of the terahertz light incident on the sample is originally based on the principle of the ellipsometry method, and the ratio of the amplitude reflection coefficient between the s-polarized light and the p-polarized light is the maximum. It is necessary to set near the Brewster angle. Since the Brewster angle changes depending on the complex dielectric constant of the sample, it is desirable to make the incident angle to the sample variable when employing the ellipsometry method. However, in the optical system arrangement of Patent Document 3, since the incident angle is fixed by the physical arrangement of the terahertz light source and the light detection unit, depending on the sample, the incident angle of the terahertz light incident on the sample is the Brewster angle. There is a problem that the reliability of the measured physical property value is low.

The present invention has been made in view of such circumstances, and can switch between transmitted light detection and reflected light detection with a simple configuration, and changes the incident angle of the terahertz light incident according to the type of measurement sample. An object of the present invention is to provide a light measuring device and a light measuring method capable of measuring a physical property value with high accuracy.

In order to achieve the above object, an optical measurement apparatus according to a first invention measures a terahertz light source that emits pulsed light in the terahertz frequency region, a photodetector that detects the pulsed light, and pulsed light emitted from the terahertz light source. A symmetric position across the sample, which is composed of at least two or more mirrors for guiding the transmitted light transmitted through the sample or the reflected light reflected by the sample to the photodetector. The operation is controlled so as to change the incident angle of the pulsed light emitted from the terahertz light source to the sample by changing the position and angle of the two pairs of mirrors arranged in the mirror and the two pairs of mirrors. And a control mechanism.

In the light measurement device according to a second aspect of the present invention, in the first aspect, the control mechanism is configured to change the position and angle of the two pairs of mirrors by a support arm having a center portion of the sample as a rotation center. It is characterized by being.

In the light measurement device according to a third aspect of the present invention, in the first or second aspect of the invention, the control mechanism changes the position and angle of the two pairs of mirror groups so that the light detector transmits the transmitted light. It is characterized by switching between detection and detection of reflected light.

The optical measurement device according to a fourth aspect of the present invention is the optical measurement device according to any one of the first to third aspects, wherein the mirror group includes at least one curved mirror, and the pulsed light emitted from the terahertz light source in the vicinity of the sample. It is characterized by being collimated.

Further, the light measurement device according to the fifth invention is characterized in that, in the third or fourth invention, a polarizer is provided on an optical path through which the pulsed light emitted from the terahertz light source propagates.

The light measurement device according to the sixth invention is based on the amplitude reflection coefficient ratio and phase difference of the s-polarized light and the p-polarized light of the sample when the control mechanism is switched to detect the reflected light in the fifth invention. And calculating a physical property value.

In the optical measurement device according to a seventh aspect based on the sixth aspect, the control mechanism is configured such that the incident angle of the pulsed light emitted from the terahertz light source to the sample substantially coincides with the Brewster angle of the sample. The operation is controlled.

In the light measurement apparatus according to the eighth invention, in the sixth or seventh invention, the pulse light emitted from the terahertz light source incident on the sample has the same light intensity of s-polarized light and p-polarized light. The amplitude detection coefficient ratio and the phase difference between the s-polarized light and the p-polarized light reflected by the sample are linearly polarized light, and the signal detected when the angle of the polarizer is set to 0 degree or 90 degrees. It is characterized by acquiring based on.

According to a ninth aspect of the present invention, in any one of the sixth to eighth aspects, the terahertz light source is a photoconductive antenna that generates pulsed light in the terahertz frequency region using an optical rectification method. The photoconductive antenna is arranged so that the polarization of the generated pulsed light is inclined by 45 degrees.

Next, in order to achieve the above object, an optical measurement method according to a tenth aspect of the present invention includes a terahertz light source that emits pulsed light in the terahertz frequency region, a photodetector that detects the pulsed light, and pulses emitted from the terahertz light source. The light is guided to the sample to be measured, and the transmitted light transmitted through the sample or the reflected light reflected by the sample is guided to the photodetector. The sample is composed of at least two or more mirrors. Two pairs of mirror groups arranged at symmetrical positions, and changing the incident angle of the pulsed light emitted from the terahertz light source to the sample by changing the position and angle of the two pairs of mirror groups The operation is controlled so that

Further, the light measurement method according to the eleventh invention is characterized in that, in the tenth invention, the positions and angles of the two pairs of mirror groups are changed by a support arm having a central portion of the sample as a rotation center.

The light measurement method according to a twelfth aspect of the present invention is the light measurement method according to the tenth or eleventh aspect, wherein the light detector detects transmitted light or reflects light by changing the position and angle of the two pairs of mirror groups. It is characterized by switching whether to detect light.

In the first invention and the tenth invention, a terahertz light source that emits pulsed light in the terahertz frequency region, a photodetector that detects pulsed light, and the pulsed light emitted from the terahertz light source is guided to the sample to be measured, and the sample And two pairs of mirrors arranged at symmetrical positions with the sample interposed therebetween, which guides the transmitted light that has passed through or reflected light reflected by the sample to the photodetector. ing. The incident angle of the pulsed light emitted from the terahertz light source to the sample can be changed by changing the position and angle of the two pairs of mirror groups. It is possible to switch which of the reflected light is detected and to adjust the incident angle when the reflected light is detected. Therefore, it is not necessary to change the relative positions of the terahertz light source and the photodetector according to the sample, and it is possible to detect transmitted light and reflected light with one photodetector. Further, it is not necessary to perform complicated operations such as optical axis alignment every time the incident angle to the sample is changed.

In the second invention and the eleventh invention, the position and angle of the two pairs of mirrors are changed by the support arm having the center portion of the sample as the center of rotation, so the relative positions of the terahertz light source and the photodetector are changed according to the sample. Therefore, it is possible to detect transmitted light and reflected light with a single photodetector. Further, each time the incident angle to the sample is changed, it is not necessary to perform complicated operations such as optical axis alignment, and pulse light can be reliably guided to the photodetector.

In the third and twelfth inventions, by changing the position and angle of the two pairs of mirror groups, the photodetector switches between detecting transmitted light and reflected light. The transmitted light and the reflected light can be detected by the instrument, so that the structure of the entire optical measuring device can be simplified and the cost can be reduced.

In the fourth invention, the mirror group includes at least one curved mirror, and collimates the pulsed light emitted from the terahertz light source in the vicinity of the sample, so that the physical property value is reliably measured even when the light receiving area of the sample is small. It becomes possible.

In the fifth invention, by providing a polarizer on the optical path through which the pulsed light emitted from the terahertz light source propagates, a polarization measurement function can be provided, and polarization measurement using the ellipsometry method can be performed. . It is also possible to measure an anisotropic sample.

In the sixth invention, when switching to detect reflected light, a reference measurement is performed by using a so-called ellipsometry method that calculates a physical property value based on the amplitude reflection coefficient ratio and phase difference of s-polarized light and p-polarized light of a sample. Therefore, it is not necessary to execute the measurement every measurement, and the measurement accuracy can be maintained high.

In the seventh invention, by controlling the operation so that the incident angle of the pulsed light emitted from the terahertz light source substantially coincides with the Brewster angle of the sample, the pulsed light is applied to the sample near the Brewster angle of the sample. The incident angle can be adjusted. Therefore, measurement can be performed at an incident angle at which the ratio of the s-polarized light and p-polarized light to the amplitude reflection coefficient is maximized, and the physical property value can be measured with high accuracy.

In the eighth invention, the pulsed light emitted from the terahertz light source incident on the sample is 45-degree linearly polarized light having the same light intensity of s-polarized light and p-polarized light, and the s-polarized light and p of reflected light reflected by the sample. The amplitude reflection coefficient ratio and the phase difference of the polarized light are acquired based on a signal detected when the angle of the polarizer is set to 0 degree or 90 degrees. As a result, a slight change in polarization due to the complex dielectric constant of the sample can be detected, and the physical property value can be measured with high accuracy. Further, since the rotation angle of the polarizer is fixed at the set angle during the measurement, it is possible to reduce the mechanical error as compared with the conventional measurement method in which the polarizer is continuously rotated.

In the ninth invention, the terahertz light source includes a photoconductive antenna that generates pulsed light in the terahertz frequency region using an optical rectification method. Since the photoconductive antenna is arranged so that the polarization of the generated pulsed light is inclined by 45 degrees, it is possible to increase the utilization efficiency of the pulsed light emitted from the terahertz light source.

According to the above configuration, the incident angle of the pulsed light emitted from the terahertz light source to the sample can be changed by changing the position and angle of the two pairs of mirror groups, so the terahertz light source and the photodetector are fixed. In this state, it is possible to switch between detection of transmitted light / reflected light and to adjust the incident angle when detecting reflected light. Therefore, it is not necessary to change the relative positions of the terahertz light source and the photodetector according to the sample, and it is possible to detect transmitted light and reflected light with one photodetector. Further, it is not necessary to perform complicated operations such as optical axis alignment every time the incident angle to the sample is changed.

It is a schematic diagram which shows the structure at the time of the transmitted light measurement of the optical measurement apparatus which concerns on Embodiment 1 of this invention. It is a schematic diagram which shows the structure at the time of the reflected light measurement of the optical measurement apparatus which concerns on Embodiment 1 of this invention. It is an illustration figure which shows the structure of an antenna module in case the terahertz light source element 4 is provided with a photoconductive antenna. It is explanatory drawing for calculating the relationship of the position of the horizontal axis direction of a plane mirror, an inclination angle, and the rotation position of a support arm. It is a schematic diagram which shows the structure at the time of the reflected light measurement of the optical measurement apparatus which concerns on Embodiment 2 of this invention. It is a schematic diagram which shows the mode of the polarization change of the pulsed light in the optical measurement apparatus which concerns on this Embodiment 2. FIG. It is a graph which shows the result of having measured the amplitude reflection coefficient ratio of the p-type Si substrate with the optical measurement apparatus which concerns on this Embodiment 2. FIG. It is a graph which shows the result of having measured the phase difference of the p-type Si substrate with the optical measurement apparatus which concerns on this Embodiment 2. FIG. It is the graph which compared the result of having calculated the complex dielectric constant with the result of theoretical calculation based on the amplitude reflection coefficient ratio and phase difference of the p-type Si substrate which were measured with the optical measurement apparatus which concerns on this Embodiment 2. FIG. It is a graph which shows the example which computed complex electrical conductivity in the optical measurement apparatus which concerns on Embodiment 2 of this invention.

Hereinafter, with respect to the optical measurement apparatus according to the embodiment of the present invention, the sample is irradiated with pulsed light having a frequency component of 0.1 × 10 ¹² to 100 × 10 ¹² Hertz and transmitted through the sample, or the sample surface An example of a so-called terahertz light measuring device that calculates a physical property value of a sample and obtains it as a measured value by detecting the intensity and phase difference of the reflected light reflected by the light source will be specifically described based on the drawings. .

(Embodiment 1)
1 and 2 are schematic views showing the configuration of the light measurement apparatus according to Embodiment 1 of the present invention. FIG. 1 is a schematic diagram showing the configuration of the light measurement device when measuring transmitted light, and FIG. 2 is a schematic diagram showing the configuration of the light measurement device when measuring reflected light.

As shown in FIGS. 1 and 2, the light measurement apparatus according to the first embodiment splits the pulsed light L1 emitted from the pulsed laser 1 with the beam splitter 2, and supplies the one pulsed light L2 to the condenser lens 3a. Induce. The pulsed light L2 condensed on the terahertz light source element (terahertz light source) 4 by the condenser lens 3a is converted into pulsed light in the terahertz frequency region, and the converted terahertz light L4 is mirrored by the non-axial parabolic mirror 5 Guided to group 10a.

As the pulse laser 1, for example, a mode-locked femtosecond (fs) unit ultrashort pulse laser is used, and the pulse width is preferably 10 to 150 femtoseconds. The terahertz light source element 4 includes a photoconductive antenna formed on, for example, a low-temperature grown GaAs thin film.

FIG. 3 is an exemplary diagram showing a configuration of an antenna module when the terahertz light source element 4 includes a photoconductive antenna. FIG. 3A is a schematic perspective view of the antenna module, and FIG. 3B is an enlarged view of the vicinity of the antenna gap. An antenna pattern 43 having an antenna gap 42 is formed on the photoconductive antenna substrate 41, and a bias voltage is applied to both ends of the electrodes forming the antenna gap 42 by a bias power supply 44. When the pulsed light L2 in femtosecond units is incident on the photoconductive antenna substrate 41 with a bias voltage applied, a transient current flows instantaneously, and 0.1 × 10 ¹² to 100 × due to electric dipole radiation. 10 ¹² Hz, i.e. pulsed light in the terahertz frequency range (terahertz light) L4 is generated. The generated terahertz light L4 is radiated into free space by the silicon hemisphere lens 45 that is in close contact with the back surface of the photoconductive antenna substrate 41.

Referring back to FIGS. 1 and 2, the mirror group 10a includes a plane mirror 11a and a non-axis parabolic mirror (curved mirror) 12a. The plane mirror 11a is mounted on a rotation stage 13a that can rotate around the position irradiated with the terahertz light L4, and the rotation stage 13a can linearly move in a direction parallel to the terahertz light L4. For example, a moving rail (not shown) is provided in a direction parallel to the terahertz light L4, and the moving rail can be moved linearly.

The non-axial parabolic mirror 12a is connected to a support arm 14a whose center of rotation is the central portion of the sample 6 to be measured. The support arm 14a is attached to the rotation center of the rotation stage 13b that rotates on the same rotation center axis as the rotation stage 13c on which the sample 6 is mounted and the rotation stage 13d to which the support arm 14b described later is attached. The axial parabolic mirror 12a can be rotated and moved with the effective focal length EFL as the rotation radius. That is, the

rotary stages

13b, 13c, and 13d are provided on the same rotation center axis in the direction perpendicular to the paper surface of FIGS. 1 and 2, and can rotate independently.

The terahertz light L4 is collimated at the center portion of the sample 6, that is, at the rotation center of the non-axial parabolic mirror 12a. For this purpose, the position and tilt angle of the plane mirror 11a are controlled so that the angle formed between the incident light and the reflected light in the non-axis parabolic mirror 12a is always 90 degrees.

At the time of transmitted light measurement shown in FIG. 1, the rotation stage 13c on which the sample 6 is mounted is rotated so that the terahertz light L4 enters the sample 6 at an incident angle of 0 degree, and the inclination angle of the plane mirror 11a is 45 degrees. . In this case, the distance between the center portion of the sample 6 and the plane mirror 11a is equal to the effective focal length EFL of the non-axial parabolic mirror 12a. The support arm 14a is controlled so as to be parallel to the direction orthogonal to the sample 6, that is, the terahertz light L4 incident on the plane mirror 11a. The rotation angle of the support arm 14a, the horizontal axis position and tilt angle of the plane mirror 11a, and the rotation angle of the sample 6 are controlled by an actuator (not shown), such as a servo motor or a stepping motor, for controlling the respective operations. (Control mechanism).

At the time of reflected light measurement shown in FIG. 2, the rotation stage 13c on which the sample 6 is mounted is rotated 90 degrees from the position at the time of transmitted light measurement so that the terahertz light L4 is incident on the sample 6 with an incident angle. is there. The tilt angle of the plane mirror 11a, the position of the plane mirror 11a in the horizontal axis direction when the central portion of the sample 6 is the rotation center, the tilt angle, and the rotation position of the support arm 14a are determined by the incident light and the reflection at the non-axis parabolic mirror 12a. The angle formed with the light is 90 degrees, and the distance between the center point of the sample 6 and the reflection position at the non-axial parabolic mirror 12a is controlled to be equal to the effective focal length EFL of the non-axial parabolic mirror 12a. The FIG. 4 is an explanatory diagram for calculating the relationship between the position in the horizontal axis direction of the plane mirror 11a, the tilt angle, and the rotational position of the support arm 14a.

In FIG. 4, the distance in the horizontal axis direction from the reflection position of the terahertz light L4 on the plane mirror 11a to the center point of the sample 6 is x, and the distance in the vertical axis direction is h. When the incident angle of the terahertz light L4 to the sample 6 (inclination angle from the direction orthogonal to the surface of the sample 6) is θi, the inclination angle θm of the plane mirror 11a is equal to the angle A and the angle A ′ shown in FIG. Diagonal relationship). Since the angle A is (180−θi) / 2, the inclination angle θm of the plane mirror 11a can be expressed as (180−θi) / 2.

The distance x in the horizontal axis direction from the reflection position of the terahertz light L4 at the plane mirror 11a to the center point of the sample 6 is the extension direction of the sample 6 from the center point of the sample 6 (the incident direction of the terahertz light L4 to the plane mirror 11a). Can be expressed by (y−h / tan θi), where y is the distance to the intersection of the direction of the light reflected by the plane mirror 11a. Here, y can be calculated as EFL / cos (90−θi) from a right triangle including EFL, and therefore x can be calculated by (Expression 1).

X = EFL / cos (90−θi) −h / tanθi (Equation 1)

1 and 2, the transmitted light that has passed through the sample 6 or the reflected light that has been reflected by the sample 6 is guided to the photodetector 8 by the mirror group 10b that is arranged symmetrically with the sample 6 in between. The mirror group 10b includes a plane mirror 11b and a non-axis parabolic mirror 12b.

The plane mirror 11b is mounted on a rotation stage 13e that can rotate around the position irradiated with the terahertz light L4, and the rotation stage 13e can linearly move in a direction parallel to the terahertz light L4. For example, a moving rail (not shown) is provided in a direction parallel to the terahertz light L4, and the moving rail can be moved linearly.

The non-axial parabolic mirror 12b is connected to a support arm 14b whose center of rotation is the central portion of the sample 6 to be measured. The support arm 14b is attached to the rotary stage 13d, and can rotate and move with the effective focal length EFL of the non-axial parabolic mirror 12b as the rotation radius. The terahertz light L4 collimated at the center portion of the sample 6, that is, the rotation center of the non-axial parabolic mirror 12b, is transmitted through the sample 6 and guided to the non-axial parabolic mirror 12b.

At the time of transmitted light measurement shown in FIG. 1, the inclination angle of the plane mirror 11b is 135 (45) degrees symmetrical to the plane mirror 11a, and the distance between the central portion of the sample 6 and the plane mirror 11b is the non-axial parabolic mirror 12b. Is equal to the effective focal length EFL. The support arm 14b is controlled to be parallel to the direction orthogonal to the sample 6, that is, the terahertz light L4 emitted from the plane mirror 11b. The rotation angle of the support arm 14b, the horizontal axis position and the inclination angle of the plane mirror 11b are controlled by an actuator (not shown), such as a servo motor or a stepping motor, which controls the respective operations (control mechanism).

At the time of the reflected light measurement shown in FIG. 2, the inclination angle of the plane mirror 11b, the position in the horizontal axis direction of the plane mirror 11b when the central portion of the sample 6 is the rotation center, the inclination angle, and the rotation position of the support arm 14b are non-axial. The angle formed by the incident light and the reflected light at the parabolic mirror 12b is 90 degrees, and the distance between the center point of the sample 6 and the reflection position at the non-axial parabolic mirror 12b is the same as that of the non-axial parabolic mirror 12b. It is controlled to be equal to the effective focal length EFL. Since the mirror group 10b is arranged symmetrically with the mirror group 10a and the sample 6 in between, the inclination angle of the plane mirror 11b can be expressed by 180− (180−θi) / 2, and the terahertz light L4 from the plane mirror 11b can be expressed. The distance x to the reflection position can also be calculated by (Equation 1) described above with the central portion of the sample 6 as the rotation center.

That is, the position and inclination angle of the plane mirror 11b in the horizontal axis direction, and the rotation position of the support arm 14b are symmetric with respect to the plane mirror 11a and the sample 6 by the actuator (not shown) that controls the respective operations. Further, the plane mirror 11b is controlled so that the tilt angle of the plane mirror 11b is symmetric with the plane mirror 11a, and the rotational position of the support arm 14b is symmetric with the support arm 14a. The terahertz light L4 parallel to the terahertz light L4 can be reflected by the plane mirror 11b. Therefore, even when the incident angle to the sample 6 is changed, it is not necessary to perform complicated adjustment work such as optical axis alignment of reflected light each time by automatically controlling the operation of the actuator according to the method described above. The reflected light can be easily guided to the photodetector 8.

That is, the reflected light reflected by the sample 6 is reflected and guided to the plane mirror 11b so that the angle formed by the incident light and the reflected light in the non-axial parabolic mirror 12b of the mirror group 10b is 90 degrees. The terahertz light L4 guided by the plane mirror 11b to the non-axial parabolic mirror 7 disposed at a position substantially symmetrical to the non-axial parabolic mirror 5 and at least parallel to the terahertz light L4 is detected by the photodetector 8. Is incident on.

On the other hand, the pulsed light L3 dispersed by the beam splitter 2 is condensed by the condenser lens 3b via the optical delay line 9, and enters the photodetector 8. By doing in this way, the signal according to the electric field strength at the time of the terahertz light L4 reaching the photodetector 8 can be detected. Further, the pulse waveform of the terahertz light L4 can be acquired by delay sweeping the arrival time of the pulsed light L3 reaching the photodetector 8 to the photodetector 8 by the optical delay line 9.

As described above, according to the first embodiment, the positions and angles of the two pairs of

mirror groups

10a and 10b arranged at symmetrical positions with the sample 6 in between are changed in cooperation, thereby providing a terahertz light source element. Since the incident angle of the pulsed light (terahertz light) L4 emitted from 4 to the sample 6 can be changed, either transmitted light or reflected light is detected with the terahertz light source element 4 and the photodetector 8 fixed. It is possible to adjust the incident angle of the pulsed light (terahertz light) L4 to the sample 6 when the reflected light is detected. Therefore, it is not necessary to change the relative positions of the terahertz light source element 4 and the light detector 8 according to the sample 6, and the transmitted light and the reflected light can be detected by the single light detector 8. Further, it is not necessary to perform complicated adjustment work such as optical axis alignment every time the incident angle to the sample 6 is changed, and the measurer can easily measure the physical property value of the sample 6.

(Embodiment 2)
FIG. 5 is a schematic diagram showing a configuration of a light measurement apparatus according to Embodiment 2 of the present invention. FIG. 5 shows the configuration of the light measurement device at the time of reflected light measurement. The same reference numerals are given to the same configurations as those in the first embodiment, and detailed description thereof is omitted.

As shown in FIG. 5, the light measurement device according to the second embodiment splits the pulsed light L1 emitted from the pulsed laser 1 with the beam splitter 2, and guides one pulsed light L2 to the condenser lens 3a. The pulsed light L2 collected by the condensing lens 3a is converted into pulsed light of 0.1 × 10 ¹² to 100 × 10 ¹² hertz, that is, a terahertz frequency region through a terahertz light source element (terahertz light source) 4; The converted terahertz light L4 is guided to the mirror group 10a by the non-axis parabolic mirror 5.

The terahertz light L4 is incident on the polarizer 21, and only linearly polarized light L5 that oscillates in a direction inclined by 45 degrees from a plane perpendicular to the incident surface is extracted. Here, the “polarizer” is, for example, a wire grid in which fine metal wires are periodically arranged. The 45-degree linearly polarized light L5 is a vector sum of s-polarized light and p-polarized light having the same electric field amplitude intensity. When the terahertz light source element 4 includes a photoconductive antenna, linearly polarized terahertz light that vibrates in the direction in which a voltage is applied to the antenna gap is generated. Therefore, as shown in FIG. By tilting, 45-degree linearly polarized light can be obtained. However, since the terahertz light L4 generated at this time contains a slightly elliptically polarized component, only a completely linearly polarized component can be extracted by inserting the polarizer 21 whose angle is set to 45 °.

FIG. 6 is a schematic diagram showing a change in polarization of pulsed light in the light measurement device according to the second embodiment. FIG. 6A is a schematic diagram showing 45-degree linearly polarized light L5 extracted by the polarizer 21. FIG. In FIG. 6A, the arrow indicates the vibration direction of the electric field vector.

Referring back to FIG. 5, the mirror group 10a is composed of a plane mirror 11a and a non-axis parabolic mirror 12a. The plane mirror 11a is mounted on a rotation stage 13a that can rotate around the position irradiated with the terahertz light L4, and the rotation stage 13a can linearly move in a direction parallel to the terahertz light L4. For example, a moving rail (not shown) is provided in a direction parallel to the terahertz light L4, and the moving rail can be moved linearly.

The non-axial parabolic mirror 12a is connected to a support arm 14a whose center of rotation is the central portion of the sample 6 to be measured. The support arm 14a is attached to the rotary stage 13b, and can rotate and move with the effective focal length EFL of the non-axial parabolic mirror 12a as the rotation radius. The terahertz light L4 is collimated at the center portion of the sample 6, that is, the rotation center of the non-axial parabolic mirror 12a. For this purpose, the position and tilt angle of the plane mirror 11a are controlled so that the angle formed between the incident light and the reflected light in the non-axial parabolic mirror 12a is always 90 degrees. In addition, since the terahertz light L4 is collimated in the vicinity of the sample 6, it is possible to reliably measure the physical property value even when the light receiving area of the sample 6 is small.

The rotation position of the rotation stage 13c on which the sample 6 is mounted is the position at the time of reflected light measurement, that is, the rotation stage on which the sample 6 is mounted so that the 45-degree linearly polarized light L5 is incident on the sample 6 with an incident angle. 13c is rotated 90 degrees from the position at the time of transmitted light measurement. The tilt angle of the plane mirror 11a, the position of the plane mirror 11a in the horizontal axis direction when the central portion of the sample 6 is the rotation center, the tilt angle, and the rotation position of the support arm 14a are determined by the incident light and the reflection at the non-axis parabolic mirror 12a. The angle formed by the light is 90 degrees, and the distance between the center point of the sample 6 and the reflection position at the non-axial parabolic mirror 12a is controlled to be equal to the effective focal length EFL of the non-axial parabolic mirror 12a. The Specifically, similarly to the method disclosed in the first embodiment, the inclination angle of the plane mirror 11a can be expressed by (180−θi) / 2, and the sample is measured from the reflection position of the 45-degree linearly polarized light L5 on the plane mirror 11a. The distance x to the center point 6 can also be calculated by the above-described (Expression 1).

Reflected light reflected by the sample 6 is reflected by the sample 6 to become elliptically polarized light L6. This is because the amplitude reflection coefficient ratio and phase of s-polarized light and p-polarized light change according to the complex dielectric constant of the sample 6. Therefore, the complex dielectric constant of the sample 6 can be derived by analyzing the elliptically polarized light L6. FIG. 6B is a schematic diagram showing elliptically polarized light L6.

Referring back to FIG. 5, the elliptically polarized light L6 is guided to the mirror group 10b arranged symmetrically with the sample 6 in between. The mirror group 10b includes a plane mirror 11b and a non-axis parabolic mirror 12b.

The plane mirror 11b is mounted on a rotation stage 13e that can rotate around a position irradiated with terahertz light, and the rotation stage 13e can linearly move in a direction parallel to the terahertz light L4. For example, a moving rail (not shown) is provided in a direction parallel to the terahertz light L4, and the moving rail can be linearly moved.

The non-axial parabolic mirror 12b is connected to a support arm 14b whose center of rotation is the central portion of the sample 6 to be measured. The support arm 14b is attached to the rotary stage 13d, and can rotate and move with the effective focal length EFL of the non-axial parabolic mirror 12b as the rotation radius. The 45-degree linearly polarized light L5 collimated at the center portion of the sample 6, that is, the rotation center of the non-axial parabolic mirror 12b, is reflected by the surface of the sample 6 and guided to the non-axial parabolic mirror 12b.

The tilt angle of the plane mirror 11b, the position of the plane mirror 11b in the horizontal axis direction when the center portion of the sample 6 is the center of rotation, the tilt angle, and the rotation position of the support arm 14b are determined by incident light and reflection at the non-axis parabolic mirror 12b. The angle formed by the light is 90 degrees, and the distance between the center point of the sample 6 and the reflection position at the non-axial parabolic mirror 12b is controlled to be equal to the effective focal length EFL of the non-axial parabolic mirror 12b. The Since the mirror group 10b is arranged symmetrically across the mirror group 10a and the sample 6, the inclination angle of the plane mirror 11b can be expressed by 180− (180−θi) / 2, and the elliptically polarized light L6 at the plane mirror 11b The distance x to the reflection position can also be calculated by (Equation 1) described above with the central portion of the sample 6 as the rotation center.

That is, the reflected light reflected by the sample 6 is reflected and guided to the plane mirror 11b so that the angle formed by the incident light and the reflected light in the non-axial parabolic mirror 12b of the mirror group 10b is 90 degrees. The elliptically polarized light L6 reflected by the plane mirror 11b is guided to the non-axial parabolic mirror 7 disposed at a position symmetrical to the non-axial parabolic mirror 5 via the polarizer 22 and the polarizer 23. .

The elliptically polarized light L6 is extracted as linearly polarized light L7 separated by the polarizer 22 into an s-polarized component and a p-polarized component. That is, by setting the angle of the polarizer 22 to 0 degree or 90 degrees, only s-polarized light or p-polarized light is extracted. FIG. 6C is a schematic diagram showing s-polarized light or p-polarized linearly polarized light L7. In FIG.6 (c), the arrow has shown the oscillating direction of the electric field vector, and has shown s polarized light or p polarized light.

The linearly polarized light L7 is incident on the photodetector 8 through the polarizer 23. The polarizer 23 is set at an angle of −45 degrees with respect to a plane perpendicular to the incident plane, whereby both the s-polarized component and the p-polarized component can be detected by the photodetector 8.

The linearly polarized light L7 is reflected by the non-axial parabolic mirror 7 and enters the photodetector 8. On the other hand, the pulsed light L3 dispersed by the beam splitter 2 is also condensed by the condenser lens 3b via the optical delay line 9 and is incident on the photodetector 8. By doing in this way, the signal according to the electric field strength when the linearly polarized light L7 reaches the photodetector 8 can be detected. Further, the pulse waveform of the terahertz light L4 can be acquired by delay sweeping the arrival time of the pulsed light L3 to the photodetector 8 by the optical delay line 9.

The time waveforms of s-polarized light and p-polarized light are acquired by attaching the polarizer 22 to the rotary stage and switching the rotation angle to 0 degree or 90 degrees. That is, when the acquisition of the time waveform of s-polarized light or p-polarized light is completed, the rotation angle of the polarizer 22 is switched to acquire the time waveform of s-polarized light or p-polarized light.

Since the complex reflection coefficients r _s and r _p of s-polarized light and p-polarized light can be obtained by performing Fourier transform on the acquired time waveform, the order of s-polarized light and p-polarized light that can be measured by the ellipsometry method. Using the phase difference Δ and the amplitude reflection coefficient ratio tan Ψ of s-polarized light and p-polarized light, the ratio ρ of the complex reflection coefficients r _s and r _p in the complex plane can be defined as shown in (Equation 2).

In (Equation 2), the absolute value of the phase [delta] _p and the amplitude reflection coefficient of the s-polarized light of the phase [delta] _s, p-polarized light _{_{| r s |, | r p}} | when the use, the complex reflection coefficient r _s, r _p is (formula 3), and the phase difference Δ between the s-polarized light and the p-polarized light and the amplitude reflection coefficient ratio tan Ψ between the s-polarized light and the p-polarized light can be expressed as (Equation 4).

Here, when the so-called Fresnel equation is applied to the ratio ρ of the complex reflection coefficients r _s and r _p in the complex plane in consideration of only reflection at the interface between air and the sample 6, the complex permittivity ε _{cn of the} sample 6 is obtained. Can be expressed by (Equation 5), and is a complex plane calculated by (Equation 2) from the measured phase difference Δ between s-polarized light and p-polarized light and the amplitude reflection coefficient ratio tan Ψ between s-polarized light and p-polarized light. The complex permittivity ε _cn (= ε′−iε ″) of the sample 6 can be calculated based on the ratio ρ of the complex reflection coefficients r _s and r _p at.

FIG. 7 is a graph showing the result of measuring the amplitude reflection coefficient ratio tan Ψ of a p-type Si substrate having a resistivity of 1.1 to 1.4 Ω · cm as the sample 6 in the optical measurement device according to the second embodiment. FIG. 8 shows the result of measuring the phase difference Δ of a p-type Si substrate having a resistivity of 1.1 to 1.4 Ω · cm as the sample 6 by the light measurement apparatus according to the second embodiment. It is a graph. When the complex permittivity ε _cn of Si is (12−i · 0.5), the Brewster angle is approximately 73 degrees. Therefore, when the measurement in FIGS. 7 and 8 is performed, the terahertz light L4 is incident on the sample 6. The positions and rotation angles of the

mirror groups

10a and 10b were adjusted so that the angle was 70 degrees.

Note that the incident angle of the terahertz light L4 on the sample 6 is adjusted in the vicinity of the Brewster angle of the sample 6 because the complex reflection coefficients r _s and r _p of the s-polarized light and the p-polarized light of the sample 6 are adjusted in the vicinity of the Brewster angle. Because the ratio ρ is maximized, the complex permittivity ε _cn of the sample 6 can be obtained with high accuracy based on (Equation 5).

In the second embodiment, as in the first embodiment, the incident angle to the sample 6 can be adjusted by changing the position and the rotation angle of the

mirror groups

10a and 10b, so that the terahertz light source element 4 and The photodetector 8 can be fixed, and complicated adjustment work such as optical axis alignment becomes unnecessary when changing the incident angle. Therefore, even when the sample 6 is frequently replaced, a troublesome operation does not occur for the measurer, and the measurement result can be acquired in a relatively short time.

As is apparent from FIGS. 7 and 8, by using the optical measurement apparatus according to the second embodiment, the amplitude reflection coefficient ratio tan Ψ and the phase difference Δ of the p-type Si substrate are changed according to the frequency in the terahertz band. It can be seen that the changing state is clearly measured.

FIG. 9 shows the result of calculating the complex permittivity ε _cn based on the amplitude reflection coefficient ratio tan Ψ and the phase difference Δ of the p-type Si substrate measured by the optical measurement apparatus according to the second embodiment and the theoretical calculation described later. It is the graph which compared the result. In FIG. 9, the real part ε ′ and the imaginary part ε ″ when the complex dielectric constant ε _cn is expressed as ε _cn = ε′−iε ″ are calculated as a function of frequency.

9, the real part 91 calculated based on (Expression 5) indicated by ■ is substantially coincident with the real part 92 (solid line) obtained as a result of theoretical calculation based on the Drude model described later. Further, the imaginary part 93 obtained by calculation based on (Equation 5) indicated by a circle is substantially coincident with the imaginary part 94 (solid line) obtained by theoretical calculation based on the Drude model described later. It is clear that the tendency that the value of the imaginary part ε ″ increases in the region can be measured with high accuracy.

Further, in the case of a sample that can ignore the absorption due to phonon scattering in the terahertz frequency region, such as Si, the dielectric function when free carrier scattering is added is expressed by (Drude) model as shown in (Expression 6). Can be represented. Note that ω represents a frequency.

Here, σ _cn (ω) represents the complex electrical conductivity, and can be represented by the sum of the real part σ ′ (ω) and the imaginary part σ ″ (ω). Complex dielectric obtained by (Expression 5) When the real part σ ′ (ω) and the imaginary part σ ″ (ω) are respectively expressed by complex permittivity using the rate ε _cn (= ε′−iε ″), the following equation (7) is obtained. In Equation 7), the complex permittivity in vacuum is ε ₀ , and the complex permittivity in the high frequency limit is ε (∞).

The resistivity Ρ can be obtained as the reciprocal of the real part σ ′ (ω) of the complex electrical conductivity σ _cn (ω). FIG. 10 is a graph showing an example in which the complex electrical conductivity σ _cn (ω) is calculated by the optical measurement device according to the second embodiment of the present invention. As the sample 6, the p-type Si substrate described above is used.

In FIG. 10, ◆ mark real sigma 'the (omega) of the complex electrical conductivity σ _cn (ω), ● mark imaginary part of the complex electrical conductivity σ _cn (ω) σ "the (omega), respectively As shown in Fig. 10, since the carrier cannot follow the speed of the electric field change as the frequency becomes higher, the complex electric conductivity σ _cn (ω) On the other hand, the resistivity 算出 calculated as the reciprocal of the real part σ ′ (ω) of the complex electrical conductivity σ _cn (ω) is low-frequency side (near ω is 0). 1.0 to 1.1 Ω · cm, which is substantially equal to the resistivity Ρ of the p-type Si substrate used.

Further, the carrier concentration N, the carrier scattering time τ, and the mobility μ can be calculated based on the complex dielectric constant ε _cn calculated by (Equation 5). By applying the Drude equation to the complex permittivity ε _cn (= ε′−iε ″) and using the plasma frequency ω _p and the carrier scattering time τ, the complex permittivity ε _cn is expressed as (Equation 8). In (Equation 8), the complex permittivity in vacuum is ε ₀ , and the complex permittivity in the high frequency limit is ε (∞).

The plasma frequency ω _p can be expressed by (Equation 9) where N is the carrier concentration, m ^{* is} the effective mass, and e is the charge amount of electrons.

On the other hand, according to the Drude-Lorenz equation, the plasma frequency ω _p , the carrier scattering time τ, and the complex electrical conductivity σ _cn (ω) are in the relationship of (Equation 10).

Since the frequency ω approaches 0 as much as possible, that is, in direct current, the real part σ ′ (0) of the complex electrical conductivity σ _cn (ω) has a denominator of (Equation 10) as 1, as shown in (Equation 11). Can be represented.

Since the resistivity で is the reciprocal of the real part σ ′ (ω) of the complex electrical conductivity σ _cn (ω), σ ′ (0) = 1 / Ρ is substituted into (Equation 11) and the carrier scattering time. When τ is calculated, (Equation 12) is obtained.

By substituting (Equation 12) obtained in this way into (Equation 8) and expressing the complex dielectric constant ε _cn as a function of the plasma frequency ω _p , (Equation 13) is obtained.

By _obtaining the value of the plasma frequency ω _p when the complex permittivity ε _cn calculated by (Equation 13) and the measured complex permittivity ε _cn match using a fitting process or the like, the carrier concentration N , Carrier scattering time τ can be obtained. For example, the real part 92 and the imaginary part 94 (solid line) of the complex permittivity ε _cn in FIG. 9 indicate the real part ε ′ and the imaginary part ε ″ derived by the fitting process using the above-described method. Is used, the carrier concentration N is 1.3 × 10 ¹⁶ cm ⁻³ and the carrier scattering time τ is 1 × 10 ⁻¹³ sec.

The mobility μ can be calculated from (Equation 14) based on the carrier scattering time τ.

In the p-type Si substrate used in the second embodiment, the mobility μ can be a value of 436 cm ² V ⁻¹ sec ⁻¹ . It was confirmed that the physical property values of the above-described p-type Si substrate were generally consistent with general values obtained by a conventional semiconductor physical property measurement method such as hole measurement.

As described above, according to the second embodiment, the incident angle of the pulsed light emitted from the terahertz light source element 4 to the sample 6 is changed by changing the positions and angles of the two pairs of

mirror groups

10a and 10b. Therefore, the incident angle when detecting reflected light can be freely adjusted. Therefore, it is not necessary to change the relative positions of the terahertz light source element 4 and the light detector 8 according to the sample 6, the reflected light can be detected by the single light detector 8, and the incident angle to the sample 6 is changed. Each time, it is not necessary to perform complicated adjustment work such as optical axis alignment.

Further, by using a so-called ellipsometry method that calculates a physical property value based on the amplitude reflection coefficient ratio tan Ψ and the phase difference Δ of the s-polarized light and the p-polarized light of the sample 6, it is not necessary to perform a reference measurement each time measurement is performed. It becomes possible to maintain high measurement accuracy. In particular, by adjusting the incident angle in the vicinity of the Brewster angle of the sample 6, it is possible to measure at an incident angle at which the ratio of the amplitude reflection coefficient of the s-polarized light and the p-polarized light of the sample 6 is maximized. Measurement can be performed.

Further, the pulsed light emitted from the terahertz light source element 4 incident on the sample 6 is 45-degree linearly polarized light having the same intensity of s-polarized light and p-polarized light, and the s-polarized light and p of the reflected light reflected by the sample. The amplitude reflection coefficient ratio tan Ψ and the phase difference Δ of the polarized light are obtained based on a signal detected when the angle of the polarizer is set to 0 degree or 90 degrees. As a result, a slight change in polarization that changes depending on the complex dielectric constant of the sample 6 can be detected, and measurement can be performed with high accuracy. Further, since the rotation angle of the polarizer is fixed at the set angle during the measurement, it is possible to reduce the mechanical error as compared with the conventional measurement method in which the polarizer is continuously rotated.

Needless to say,

Embodiments

1 and 2 described above can be modified without departing from the spirit of the present invention. For example, the configuration of the

mirror groups

10a and 10b is particularly limited as long as the

mirror groups

10a and 10b are arranged at symmetrical positions with the sample 6 interposed therebetween, have at least one curved mirror, and can be accommodated in a housing. Is not to be done.

DESCRIPTION OF SYMBOLS 1 Pulse laser 2 Beam splitter 3a Condensing lens 4 Terahertz light source element (terahertz light source)
6

Sample

5, 7 Non-axis parabolic mirror 8 Optical detector 9

Optical delay line

10a,

10b Mirror group

11a,

11b Plane mirror

12a, 12b Non-axis parabolic mirror (curved mirror)
13a, 13b, 13c, 13d, 13e

Rotating stage

14a, 14b Support arm

Claims

A terahertz light source that emits pulsed light in the terahertz frequency range;
A photodetector for detecting the pulsed light;
Consists of at least two or more mirrors that guide pulsed light emitted from the terahertz light source to a sample to be measured and guide transmitted light transmitted through the sample or reflected light reflected by the sample to the photodetector. In addition, two pairs of mirror groups arranged at symmetrical positions across the sample,
A control mechanism for controlling the operation of changing the incident angle of the pulsed light emitted from the terahertz light source to the sample by changing the position and angle of the two pairs of mirrors. measuring device.
2. The optical measurement apparatus according to claim 1, wherein the control mechanism is configured to change the position and angle of the two pairs of mirror groups by a support arm having a central portion of the sample as a rotation center.
The control mechanism is configured to change the position and angle of two pairs of mirror groups so that the photodetector detects transmitted light or reflected light. The light measuring device according to claim 1 or 2.
4. The light according to claim 1, wherein the mirror group includes at least one curved mirror, and collimates pulsed light emitted from the terahertz light source in the vicinity of the sample. measuring device.
The optical measurement apparatus according to claim 3 or 4, further comprising a polarizer on an optical path through which pulsed light emitted from the terahertz light source propagates.
6. The light according to claim 5, wherein when the reflected light is switched to be detected by the control mechanism, the physical property value is calculated based on an amplitude reflection coefficient ratio and a phase difference between the s-polarized light and the p-polarized light of the sample. measuring device.
The optical measurement apparatus according to claim 6, wherein the control mechanism controls an operation so that an incident angle of the pulsed light emitted from the terahertz light source to the sample substantially coincides with a Brewster angle of the sample. .
The pulsed light emitted from the terahertz light source that is incident on the sample is 45-degree linearly polarized light having the same light intensity of s-polarized light and p-polarized light,
The amplitude reflection coefficient ratio and the phase difference between the s-polarized light and the p-polarized light reflected by the sample are acquired based on a signal detected when the angle of the polarizer is set to 0 degree or 90 degrees. The light measuring device according to claim 6 or 7, wherein the light measuring device is provided.
The terahertz light source includes a photoconductive antenna that generates pulsed light in a terahertz frequency region using an optical rectification method,
9. The optical measurement apparatus according to claim 6, wherein the photoconductive antenna is arranged so that the polarization of the generated pulsed light is inclined by 45 degrees.
A terahertz light source that emits pulsed light in the terahertz frequency range;
A photodetector for detecting the pulsed light;
Consists of at least two or more mirrors that guide pulsed light emitted from the terahertz light source to a sample to be measured and guide transmitted light transmitted through the sample or reflected light reflected by the sample to the photodetector. And two pairs of mirror groups arranged at symmetrical positions with the sample in between,
An optical measurement method comprising: controlling an operation so as to change an incident angle of the pulsed light emitted from the terahertz light source to the sample by changing a position and an angle of the two pairs of mirror groups.
11. The optical measurement method according to claim 10, wherein the position and angle of the two pairs of mirror groups are changed by a support arm having a central portion of the sample as a rotation center.
12. The light according to claim 10, wherein the light detector switches between detecting transmitted light and reflected light by changing the position and angle of the two pairs of mirror groups. Measuring method.