JP2005175409A - Method and apparatus for terahertz electromagnetic-wave irradiation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus having high resolution capable of measuring the transmitting or reflecting intensity of a terahertz electromagnetic wave for obtaining the image of metallic parts inside an electronic component or a card. <P>SOLUTION: A high-frequency terahertz electromagnetic wave is generated by irradiating a crystal for frequency mixing with a beam of a solid-state Raman laser which is oscillated by exciting with a pump laser and a beam of the pump laser. The generated terahertz electromagnetic wave is converged into a fine beam by a tapered metal waveguide. The converged beam is transmitted or reflected by an object to be tested placed on an xy moving stage to obtain the image. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a terahertz electromagnetic wave generating apparatus and method.

  An IC chip and metal wiring are embedded in the information terminal card and various cards, but cannot be seen from the outside. In addition, various mounted electronic components are covered with a case or coating material, and it is possible to visually check the internal state of various specimens, such as whether the internal metal wiring is broken, checked for defects, whether it has been forged, or forged. It is difficult. In order to detect the state of the metal wiring or the like in these specimens, a method of fluoroscopy with an x-ray apparatus is generally used. But x-ray equipment is expensive, and, x-ray is not going to mean that used anywhere because it is harmful to the human body.

  Terahertz electromagnetic waves are relatively safe for the human body and pass through the card material, but are strongly reflected at the metal part embedded in the card and can be used for the above purpose. However, the wavelength of the terahertz wave is 300 μm in the air at 1 THz. On the other hand, since the metal wiring and magnetic metal in the card have a width of about 15 μm, a resolution of about 15 μm or at most about 30 μm is required to know the shape. Further, the antenna wiring in the card, the connection wiring of the electronic components, etc. have a width of about 100 μm. However, the diffraction limit is about ½ of the wavelength that can limit the electromagnetic wave beam. Therefore, such a fine metal wiring structure cannot be detected as a high-resolution image at a conventional terahertz wave frequency of about 1 THz. Further, since the conventional terahertz wave generator using the difference frequency method requires two laser light sources, it must be expensive.

  The present invention provides a high-resolution terahertz electromagnetic wave irradiation method and apparatus for obtaining an image of a metal wiring portion or the like with a high resolution of 30 μm or less, at least 100 μm or less, excluding such drawbacks. Another feature of the present invention is to provide an inexpensive terahertz electromagnetic wave irradiation method and apparatus that uses one pump laser without requiring two laser light sources.

  A simple near-infrared laser such as a YAG laser is used as pump light to form a semiconductor or dielectric crystal Raman laser, and the pump light and Raman-oscillated Stokes light are applied to the frequency mixing crystal inside or outside the Raman laser resonator. When introduced, a terahertz electromagnetic wave equal to the frequency difference between the two lights is generated by difference frequency mixing. The generated terahertz electromagnetic wave is guided to the specimen on the xy stage using a metal waveguide or the like, and the terahertz electromagnetic wave is irradiated. The tip of the metal waveguide is narrowed down to several times the wavelength, and a thin terahertz beam transmits or reflects the specimen. This is detected by one or a plurality of detectors. By moving the xy stage, a terahertz image of a metal wiring portion embedded in the specimen can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the terahertz electromagnetic wave irradiation method and apparatus which irradiate terahertz electromagnetic waves to test substances, such as an electronic component and cards, and detect the presence and shape of the metal in a test substance and the material which does not permeate | transmit terahertz electromagnetic waves can be obtained. .

  As for the terahertz wave generation method using a GaP semiconductor crystal as a solid for performing Raman oscillation and using GaAs as the solid for frequency mixing, the present inventor published a paper (IEEE.QE-19, p. 19) in 1983. 1251, 1983)). When a YAG laser is used as a pump light source and an optical phonon of a GaP crystal is excited in an optical resonator, Raman oscillation occurs. Since GaP vertical optical phonons (LO phonons) have particularly high Raman scattering efficiency and easily oscillate at a low threshold, they are optimal as solids for Raman lasers of the present invention.

  In the case of GaP, the LO phonon frequency has a constant value of 12 THz. A pump light wavelength of 1.064 mm corresponds to 282 THz. Accordingly, 270 THz Stokes light oscillates due to LO phonon Raman scattering. The pump light 282 THz and the Stokes light 270 THz are frequency-mixed when passing through a difference frequency mixing solid such as GaAs, GaSe, or CdSe disposed in the resonator, and generate an electromagnetic wave having a difference frequency of 12 THz. The LO phonon itself in the Raman laser GaP does not generate THz electromagnetic waves, but the mixing crystal enables generation of THz electromagnetic waves. This method is inexpensive because a terahertz wave can be generated by a single pump laser.

  Since LO phonons have a higher frequency than TO phonons and polariton mode phonons, they have shorter wavelengths and therefore allow higher resolution. In the GaP crystal, the LO phonon frequency is 12 THz, so the wavelength is 25 μm, and the resolution determined by the diffraction limit is calculated as 13 μm, which meets the object of the present invention.

  Since the generated terahertz electromagnetic wave beam diameter is 1 mm to several mm, a tapered metal waveguide or a tapered waveguide made of silicon is used to reduce the diameter to several tens of μm or less. The output end of the tapered metal wave tube is brought close to a sample such as a card on a movable stage, and transmission is detected by a terahertz wave detector close to the opposite side of the card.

  In the microwave region, a tapered metal waveguide having a circular or rectangular cross section or a taper formed only in one direction of the rectangular cross section is known. In the present invention, a terahertz wave output is provided with such a structure. A cross section having a very small size from 100 μm to about 30 μm is used. A high-frequency terahertz electromagnetic wave generated from a terahertz wave generating unit in which a mixing crystal is arranged inside or outside of a Raman laser is irradiated close to a thin flat plate such as a card arranged on a movable xy stage, and is opposite to the card surface. The detector or the detector waveguide is brought close to the detector. By moving the stage minutely, it is possible to obtain a terahertz image such as a metal wiring in the specimen with high resolution.

  FIG. 1 shows a typical configuration of a terahertz electromagnetic wave irradiation apparatus of the present invention. The Raman laser of the terahertz electromagnetic wave generating section causes LO phonon Raman scattering by the GaP single crystal 1. The mirrors 2 and 3 constitute a resonator for the Stokes light. The pump light from the YAG laser introduced through the polarizer 4 Raman-excites the GaP crystal. The pump light and the Raman-oscillated Stokes light are mixed with the frequency mixing by the frequency mixing GaAs, CdSe, or GaSe crystal 5 arranged inside the resonator, and the generated 12 THz terahertz wave is passed through the converging mirrors 6 and 7. Then, the light is incident on a metal waveguide 8 having a circular cross section with a diameter of 1 mm.

  The metal waveguide is typically 20 cm in length and guided to the vicinity of the specimen 9 with a uniform cross section, and is narrowed down to a thin beam by a tapered portion near the specimen. The tapered portion of the metal waveguide is limited to a length of 50 mm, an incident side diameter of 1 mm, and an outlet side diameter of 50 μm. The detector 11 has one pyroelectric detector arranged on the back side of the sample. An image of a metal part in the sample can be obtained by measuring the transmitted terahertz intensity with the detector 11 while moving the stage 10 on which the sample is placed in the x and y directions. In this case, the terahertz intensity at the irradiation exit cross-section is not uniform, and the intensity is greater at the center. By moving it, a resolution of 15 μm can be obtained. In other words, since the edge of the metal wiring is sharper than 15 μm, when the opening moves with respect to the metal edge, the right end, the center, and the left end of the diameter have different transmission intensities, and a resolution of at least about 1/3 of the opening diameter can be sufficiently obtained. It is.

  When the required resolution is about 30 μm, the outlet side diameter of the tapered waveguide can be increased to 100 μm, so that the multiple reflection loss in the waveguide is reduced and the transmission intensity of the terahertz wave can be increased. Solids other than GaP can be used for the Raman laser, but the frequency of the solid optical phonon is a minimum of 5 THz. At this time, since the wavelength in the air of the terahertz wave is 60 μm, the limit of the resolution that can be obtained is 30 μm.

  In the first embodiment, the stage is moved at a pitch of 15 μm in each of the x direction and the y direction. Therefore, scanning on the plane requires a measurement time proportional to the number of spots. In the present embodiment, the same image can be obtained with a measurement time of 1/10 or less that of the first embodiment. The terahertz wave generation unit is the same as that of the first embodiment, but the waveguide is rectangular, and the tapered portion 12 has a slit shape with a length of 50 mm, an incident side of 1 mm × 1 mm, and an output side of 500 μm × 50 μm as shown in FIG. Squeezed. The detector 13 has 10 50 μm × 50 μm pyroelectric detectors DTGS arranged on an array. Thus, a terahertz image of a band-like region having a width of 500 μm is obtained by moving the stage once in the x direction.

  A terahertz image can be obtained with a resolution of 50 μm × 50 μm or less over the entire surface by sequentially moving this by 500 μm in the y direction and repeating this. When the stage is moved at a pitch of 15 μm in the x direction, a resolution of 15 μm is obtained in the x direction as described in the first embodiment. In order to obtain a resolution of 15 μm in the y direction, the DTGS detector is composed of a 10 μm cross section 50 μm × 50 μm, so instead of sweeping the stage once in the x direction, move the y direction by 15 μm. If it is swept three times, a resolution of about 15 μm can be obtained in the y direction.

  When a specimen card or the like is made of a material that strongly absorbs electromagnetic waves at a frequency near 12 THz, transmission measurement is impossible. In this case, the reflection method is used. FIG. 3 shows an example thereof, and the terahertz wave generation unit is not shown because it is the same as that of the first embodiment. A silicon thin plate is disposed at a 45 degree angle as a reflected wave beam splitter 15 in a part of the terahertz wave waveguide 14. Reflected terahertz electromagnetic waves 17 strongly reflected on the sample surface at the metal portion of the sample card 16 are reflected by the beam splitter 15 and guided to the terahertz wave detector 19 by the condensing silicon lens 18. The dimensions of the tapered waveguide on the incident side and the outlet side may be designed based on the same principle as in the first embodiment even in the case of reflection.

When the frequency mixing crystal is disposed inside the Raman laser resonator as in the first embodiment, an absorption loss of 10% or less is desired when it is disposed inside the resonator. Despite the high difference frequency generation efficiency of the crystal for mixing, the transmittance for near infrared light is not sufficiently high, that is, GaSe, ZnGeP ₂ , CdSe, or DAST (4-dimethylamino-N-methyl-4-stilbazolium tosylate ) Or the like may increase the threshold of Raman oscillation when placed inside the resonator.

  Therefore, as shown in FIG. 4, the difference frequency mixing crystal 22 is arranged outside the Raman laser resonator, and the resonator output light 20 of the Raman laser is introduced into the frequency mixing crystal. On the other hand, part of the pump light is split by a beam splitter to become pump light 21 and is introduced into the mixing crystal 22. Although the intensity of Stokes light extracted outside the resonator is lower than that inside the Raman laser resonator, a high-frequency terahertz wave output can be obtained because the difference frequency generation efficiency of the mixing crystal is high.

  1 is a diagram illustrating a configuration of Example 1. FIG.   It is a figure which shows the taper part and detector of a metal waveguide in Example 2. FIG.   It is a figure which shows the taper part and detector of a metal waveguide in Example 3. FIG.   FIG. 10 is a diagram showing a configuration of Example 4.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... GaP crystal 2, 3 ... Resonator mirror 4 ... Polarizing beam splitter 5 ... Crystal for frequency mixing 6, 7 ... Terahertz electromagnetic wave condensing mirror 8 ... Metal waveguide 9 ... Sample 10 ... Stage 11 ... Detector 8 ... Metal Waveguide taper portion 9 ... Array detector 14 ... Metal waveguide 15 ... Reflected wave beam splitter 16 ... Sample 17 ... Reflected terahertz beam 18 ... Silicon lens 19 ... Detector 20 ... Raman laser output light 21 ... Divided Pump light 22 ... Crystal for difference frequency mixing 23 ... Terahertz electromagnetic wave

Claims (4)

  A terahertz electromagnetic wave having a high frequency equal to or higher than 5 THz equal to the difference between the pump light source frequency and the Raman oscillation frequency by a Raman laser excited by a pump light source and oscillated by a solid Raman effect and a frequency mixing solid disposed inside or outside the Raman laser A terahertz electromagnetic wave generating part for generating the terahertz electromagnetic wave, and a beam forming part for narrowing the terahertz electromagnetic wave beam to several times the wavelength of the terahertz wave in at least one direction of the beam cross section or less. A terahertz electromagnetic wave irradiation method and apparatus for irradiating a specimen such as a card and detecting the presence and shape of a metal or a material that does not transmit terahertz electromagnetic waves in the specimen.   The terahertz electromagnetic wave irradiation method and apparatus according to claim 1, wherein the beam forming unit is a tapered metal waveguide disposed close to the specimen.   2. The terahertz electromagnetic wave irradiation method and apparatus according to claim 1, wherein the Raman laser solid is a GaP crystal and the terahertz wave frequency is 12 THz. 3. The terahertz electromagnetic wave irradiation method and apparatus according to claim 1, wherein the frequency mixing solid is any one of GaAs, GaSe, CdSe, ZnGeP ₂ , and DAST.

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