Patent Application
20150192457
The present invention relates to an element used for detecting a terahertz wave by utilizing an electro-optic effect, and particularly relates to a terahertz-wave detection element used in an observation apparatus utilizing the terahertz wave.
The terahertz wave is generally an electromagnetic wave of a frequency from 0.1 THz to 30 THz. The terahertz wave is expected to be developed from the basic science field such as physical property, electron spectroscopy, life science, chemistry, pharmaceutical science, and the like to the application field such as atmospheric environmental measurement, security, material inspection, food inspection, communication, and the like.
For example, there have been expected applications of the terahertz wave to an image diagnosis apparatus which non-destructively diagnoses (inspects) an object in order to utilize characteristics that photon energy is small and the frequency is higher than that of a microwave and a millimeter wave. Particularly, because its wavelength range includes an absorption wavelength peculiar to a constitutive substance of a biological cell, there have been expected applications of the terahertz wave to an apparatus that can inspect and observe the biological cell in real time. Conventionally, inspection and observation of the biological cell cannot be performed without dyeing with a pigment. Therefore, it has taken time and labor for the inspection and the observation. For example, there is already publicly known an apparatus that can observe by utilizing the terahertz wave, a cell sample which it is difficult to observe by visible light (refer to Patent Document 1, for example).
In the apparatus disclosed in Patent Document 1, an electro-optic single crystal is used as a detection element of the terahertz wave. Specifically, there is used a characteristic of an electro-optic single crystal that a refractive index changes in accordance with the intensity of an incident terahertz wave. The change in the refractive index can be detected as a change in a phase, polarization, and intensity (a light quantity) of light, when the light such as infrared light (referred to as detection light, probe light, and the like) is irradiated in superposition to an electro-optic single crystal to which the terahertz wave is being irradiated. In the apparatus disclosed in Patent Document 1, a terahertz wave having a spatial distribution generated in the intensity (spatially modulated intensity) due to transmission through a specimen is incident to the electro-optic crystal. A spatial distribution of a refractive index change generated in the electro-optic single crystal in accordance with the intensity distribution is read as a light quantity distribution of near-infrared light. By this arrangement, the specimen can be observed.
In an observation apparatus that performs observation based on this principle, in order to obtain high spatial resolution, it is required to thin the electro-optic crystal as much as possible such that the terahertz wave transmitted through the specimen does not spread due to the influence of diffraction. In Patent Document 1, there is also disclosed a terahertz-wave detection element in which the electro-optic crystal is supported by a reinforcing member, by having the electro-optic crystal itself formed extremely thin.
On the other hand, there is also already publicly known a terahertz electromagnetic wave detector that uses a ZnTe crystal of a thickness equal to or larger than 5 μm and equal to or smaller than 100 μm as the electro-optic crystal, in order to reduce the influence of a multiple reflection and expand a measurable terahertz band (refer to Patent Document 2, for example). According to a technique disclosed in Patent Document 2, a ZnTe crystal is also used in a supporting substrate that supports the electro-optic crystal, and both crystals are joined together by thermocompression.
Further, there is also already publicly known an adhered body having a lithium niobate single crystal or a lithium tantalate single crystal as the electro-optic crystal equal to or larger than 0.1 μand equal to or smaller than 10 μm and having a supporting substrate adhered thereto by a resin having a fluorene skeleton(refer to Patent Document 3, for example).
As described above, in order to obtain high spatial resolution in the observation apparatus using the terahertz wave, it is required to thin the electro-optic crystal used for detection. In order to realize this, conventionally, there has been utilized a terahertz-wave detection element formed by thinning an electro-optic crystal after the electro-optic crystal and a supporting substrate are joined by thermocompression disclosed in Patent Document 2 or by a method of resin adhesion disclosed in Patent Document 3
In describing in more in detail, the terahertz-wave detection element is generally formed in a relatively small size of about a few mm square to a few cm square in a planar view. Therefore, as described above, in order to improve manufacturing efficiency and secure accuracy of thinning the layer, the terahertz-wave detection element having the thin-layer electro-optic crystal is usually obtained by performing what is called a multi-piece forming as follows. The electro-optic crystal and the supporting substrate are respectively prepared as large-size mother substrates. Both mother substrates are joined together to obtain a joined body. The electro-optic crystal is thinned by mechanical polishing and the like. Finally, the joined body is cut into elements (chips) of desired sizes. Further, a film formation processing (a coating processing) in the case of providing a total reflection film and a reflection prevention film on the front and back surfaces of the detection element in order to improve detection efficiency is also usually performed to the mother substrates.
However, in the case of manufacturing the detection element by performing a resin adhesion, in some cases, a resin component is solved from the adhesion layer when cleaning after the polishing, and is attached to the surface of the substrates (the mother substrates). In the case of forming the total reflection film and the reflection prevention film in a later process, there is a problem in that the attachment of the resin extremely lowers the adhesive strength of the total reflection film and the reflection prevention film. As a result, a defect of peeling off and the like occur. There is also a problem in that, at the time of observing the biological cell, the resin is solved from the adhesion layer into the culture solution, and the biological cell is damaged or caused to die.
In the case of thermocompression, because the mother substrate of the electro-optic crystal and the mother substrate of the supporting substrate are joined directly, the trouble attributable to the adhesion layer occurring in the resin adhesion does not occur. However, in order to perform the joining, both mother substrates need to be heated to a few hundred degrees Celsius or above. Therefore, in some cases, by the stress (thermal stress) attributable to a thermal expansion difference, there occurs a warping in the obtained joined body, there occurs a crack in the electro-optic crystal, or the joined body itself is broken. Even when a crack has not occurred in the electro-optic crystal at the joining time, there occurs a crack in some cases in the electro-optic crystal at the time of falling down a temperature in forming the total reflection film and the reflection prevention film or in performing a temperature cycle test after completing the element. When the thermal stress internally exists in the electro-optic crystal substrate as an internal stress, an electro-optic constant of the electro-optic crystal changes from its original value. As a result, there can also occur a problem that the refractive index change of the terahertz wave becomes small and detection sensitivity and spatial resolution are deteriorated.
Patent Document 1: Japanese Patent Application Laid-Open Number 2010-156674
Patent Document 2: Japanese Patent Application Laid-Open Number 2003-270598
Patent Document 3: Japanese Patent Application Laid-Open Number 2002-337274
The invention of the present application has been made in view of the above problems. An object of the present invention is to provide a terahertz-wave detection element having high spatial resolution, suitably suppressing the occurrence of a warping in the element as a whole and a crack in the electro-optic crystal, and suitably securing joining strength between the electro-optic crystal and a supporting substrate.
In order to solve the above problems, according to a first aspect of the present invention, there is provided a terahertz-wave detection element capable of detecting a spatial intensity distribution that an incident terahertz wave has. The terahertz-wave detection element includes: an electro-optic crystal layer consisting of an electro-optic crystal in which a refractive index at an incident position of the terahertz wave changes in accordance with incident intensity of the terahertz wave at the incident position; and a supporting substrate that supports the electro-optic crystal layer. The terahertz-wave detection element is configured to detect the spatial-characteristics distribution which is generated in probe light irradiated the electro-optic crystal layer in superposition with the terahertz wave and which corresponds to a spatial distribution of a refractive index generated in the electro-optic crystal layer by incidence of the terahertz wave, thereby to detect the spatial intensity distribution of the incident terahertz wave. A joined part between the electro-optic crystal layer and the supporting substrate is an amorphous layer consisting of an oxide including an element of a substance constituting the electro-optic crystal and an element constituting the supporting substrate, and also having a thickness equal to or larger than 1 nm and equal to or smaller than 50 nm A thickness of the electro-optic crystal layer is equal to or larger than 1 μm and equal to or smaller than 30 μm .
According to a second aspect of the present invention, the terahertz-wave detection element according to the first aspect further includes a total reflection layer consisting of a first dielectric multilayer film, formed on a surface of the electro-optic crystal, and a reflection prevention layer consisting of a second dielectric multilayer film, formed on a surface of the supporting substrate.
According to a third aspect of the present invention, in a terahertz-wave detection element according to the first or second aspect, a coefficient of thermal expansion of the amorphous layer is an intermediate value between a coefficient of thermal expansion of the electro-optic crystal layer and a coefficient of thermal expansion of the supporting substrate.
According to a fourth aspect of the present invention, in a terahertz-wave detection element according to any of the first to third aspects, flatness of the supporting substrate is equal to or smaller than 20 μm, and parallelism is equal to or smaller than 3 μm.
According to a fifth aspect of the present invention, in a terahertz-wave detection element according to any of the first to fourth aspects, the spatial-characteristics distribution generated in the probe light is an intensity distribution of the probe light.
According to a sixth aspect of the present invention, in a terahertz-wave detection element according to any one of the first to fifth aspects, the joined part is formed by joining a first substrate consisting of the electro-optic crystal and a second substrate of a material same as that of the supporting substrate, at an ordinary temperature and under an ultrahigh vacuum.
According to a seventh aspect of the present invention, there is provided a method of manufacturing a terahertz-wave detection element capable of detecting a spatial intensity distribution that an incident terahertz wave has. The method includes: a joining step of joining a first substrate consisting of an electro-optic crystal layer in which a refractive index at an incident position of a terahertz wave changes in accordance with incident intensity of the terahertz wave at the incident position, and a second substrate that supports the electro-optic crystal layer; a polishing step of thinning the first substrate to a thickness equal to or larger than 1 μm and equal to or smaller than 30 μm by polishing the first substrate of a joined part obtained by the joining step; and a segmentation step of obtaining a large number of terahertz-wave detection elements by cutting a joined body after the polishing step into pieces of a predetermined element size. In the joining step, at an ordinary temperature and under an ultrahigh vacuum, a surface of the first substrate and a surface of the second substrate are sputtered and both surfaces are contacted to each other so as to form a joined part consisting of an amorphous oxide including an element of a substance constituting the first substrate and an element constituting the second substrate, in a thickness equal to or larger than 1 nm and equal to or smaller than 50 nm, thereby joining the first substrate and the second substrate.
According to an eighth aspect of the present invention, the method of manufacturing the terahertz-wave detection element according to the seventh aspect further includes: a total-reflection layer formation step of forming a total reflection layer consisting of a first dielectric multilayer film on a surface of the electro-optic crystal of the joined body after the polishing step; and a reflection-prevention layer formation step of forming a reflection prevention layer consisting of a second dielectric multilayer film on a surface of the second substrate of the joined body after the polishing step.
According to a ninth aspect of the present invention, there is provided an observation apparatus that includes a terahertz-wave detection element capable of detecting a spatial intensity distribution that an incident terahertz wave has. The terahertz-wave detection element includes: an electro-optic crystal layer consisting of an electro-optic crystal in which a refractive index at an incident position of the terahertz wave changes in accordance with incident intensity of the terahertz wave at the incident position; and a supporting substrate that supports the electro-optic crystal layer. The observation apparatus further includes: a terahertz-wave irradiation optical system that irradiates the terahertz wave toward the mounting surface on which the specimen is mounted; a probe-light irradiation optical system that irradiates the probe light from a side of the supporting substrate to the electro-optic crystal layer; and an observation optical system that observes an image of the probe light having a spatial characteristics distribution emitted from the electro-optic crystal layer in which a spatial distribution of the refractive index is generated by incidence of the terahertz wave. The terahertz-wave detection element is configured to detect the spatial-characteristics distribution which is generated in probe light irradiated the electro-optic crystal layer in superposition with the terahertz wave and which corresponds to a spatial distribution of a refractive index generated in the electro-optic crystal layer by incidence of the terahertz wave, thereby to detect the spatial intensity distribution of the incident terahertz wave. A joined part between the electro-optic crystal layer and the supporting substrate is an amorphous layer consisting of an oxide including an element of a substance constituting the electro-optic crystal and an element constituting the supporting substrate, and also having a thickness equal to or larger than 1 nm and equal to or smaller than 50 nm. A thickness of the electro-optic crystal layer is equal to or larger than 1 μm and equal to or smaller than 30 μnm.
According to the first to eighth aspects of the present invention, there can be realized a terahertz-wave detection element with crack free and high spatial resolution.
According to the ninth aspect of the present invention, it is possible to realize an observation apparatus capable of observing a biological sample in high spatial resolution and in real time.
<Configuration of Terahertz-Wave Detection Element>
As shown in
The terahertz-wave detection element 10 is mainly used in the observation apparatus 1000 that performs inspection and observation of a biological cell, as shown in
For an electro-optic crystal that forms the electro-optic crystal layer 1, there can be exemplified a lithium niobate (LN) crystal, a lithium tantalate (LT) crystal, a ZnTe crystal, a GaAs crystal, a GaP crystal, a KTP (KTiOPO4) crystal, and a DAST (4-dimethylamino-N-methyl-4-stilbazolium tosylate) crystal, for example. Among the above, LN and LT may be in a stoichiometry composition, or may be doped with MgO and the like in advance for the purpose of reducing an optical damage. Further, in the case of using LN and LT, it is preferable that an x-cut plate or a y-cut plate having a z axis existing in an in-plane direction is used so that r33 of a large electro-optic effect can be utilized as an electro-optic constant.
A thickness of the electro-optic crystal layer 1 is preferably equal to or smaller than 30 μm. When a thickness of the electro-optic crystal layer 1 is set larger than 30 μm, a crack can occur in the electro-optic crystal layer 1 in a manufacturing process of the terahertz-wave detection element 10, and therefore, this size is not preferable. A thickness of the electro-optic crystal layer 1 is preferably equal to or smaller than 10 μm. When a thickness of the electro-optic crystal layer 1 is equal to or smaller than 10 μm, it is possible to set the spatial resolution to equal to or smaller than 20 μm when the terahertz-wave detection element 10 is used in the observation apparatus 1000. When the spatial resolution is equal to or smaller than 20 μm, the biological sample can be satisfactorily observed.
The smaller a thickness of the electro-optic crystal layer 1 is, the higher the spatial resolution becomes. However, from the viewpoint of processing accuracy and from the viewpoint of detection accuracy of the probe light PB, it is preferable that the electro-optic crystal layer 1 has a thickness equal to or larger than 1 μm.
The supporting substrate 2 is a substrate that supports the electro-optic crystal layer 1 having a small thickness as described above. The supporting substrate 2 may be configured by any of amorphous, a single crystal, and a polycrystal, but is preferably not an electro-optic crystal. In addition, it is preferable that an azimuth of the supporting substrate 2 is determined so that susceptibility to an electric field in a substrate horizontal direction is small. In view of the above points, for the supporting substrate 2, it is suitable to use a glass substrate, a quartz substrate, an alumina substrate, a magnesium oxide substrate, and the like. Regarding a thickness of the supporting substrate 2, there is no particular limit so far as a certain level of strength and handleability can be secured. However, it is suitable to use the supporting substrate 2 of a thickness of about a few hundred μm to a few mm From the viewpoint of preventing a scattering of the probe light PB, it is preferable that a surface roughness of the supporting substrate 2 is equal to or smaller than ⅕ of the wavelength of the probe light PB.
The amorphous layer 3 is a joining layer between the electro-optic crystal layer 1 and the supporting substrate 2. The amorphous layer 3 consists of an amorphous oxide including both constituent elements of the electro-optic crystal layer 1 and the supporting substrate 2. The amorphous layer 3 has a coefficient of thermal expansion of an intermediate value of a coefficient of thermal expansion of the electro-optic crystal layer 1 and a coefficient of thermal expansion of the supporting substrate 2. The amorphous layer 3 having this composition and the characteristic is formed by directly joining the electro-optic crystal layer 1 and the supporting substrate 2. The joining of the electro-optic crystal layer 1 and the supporting substrate 2 will be described later.
When the electro-optic crystal layer 1 consists of an LN crystal of which a z axis exists in the in-plane direction, a coefficient of thermal expansion in the z axis direction is 5 ppm/° C., and a coefficient of thermal expansion in the y axis direction is 16 ppm/° C., and when the supporting substrate 2 is a TEMPAX glass substrate of which a coefficient of thermal expansion is 3 ppm/° C., a coefficient of thermal expansion of the amorphous layer 3 becomes about 9 ppm/° C.
With respect to holding a joined state of the electro-optic crystal layer 1 and the supporting substrate 2, it is preferable that the amorphous layer 3 has a thickness equal to or larger than 1 nm However, with respect to no occurrence of a peeling off in a manufacturing process of the terahertz-wave detection element 10 described later, it is more preferable that the amorphous layer 3 has a thickness equal to or larger than 2 nm.
If the amorphous layer 3 is too thick, loss of the probe light PB due to scattering and absorption becomes conspicuous. Therefore, a thickness of the amorphous layer 3 is preferably equal to or smaller than 50 nm, and more preferably, equal to or smaller than 10 nm.
The electro-optic crystal layer 1, the supporting substrate 2, and the resin layer 3 that joins the electro-optic crystal layer 1 and the supporting substrate 2 described above together are basic components of the terahertz-wave detection element 10.
However, the terahertz-wave detection element 10 according to the present embodiment includes a reflection prevention layer 4 and a total reflection layer 5, for the purpose of improving observation performance when used in the observation apparatus 1000.
The reflection prevention layer 4 is formed on a surface of the supporting substrate 2 at the opposite side of the amorphous layer 3. The reflection prevention layer 4 is provided to prevent reflection on the surface of the supporting substrate 2, of the probe light PB incident to the terahertz-wave detection element 10 from the supporting substrate 2 side.
Specifically, the reflection prevention layer 4 is formed on the main surface of the supporting substrate 2, as a dielectric multilayer film formed by repeatedly and alternately stacking a first-unit reflection prevention layer 4a and a second-unit reflection prevention layer 4b made of dielectrics of mutually different compositions by about several dozen layers in total. As dielectrics that can be used to form the reflection prevention layer 4, there can be exemplified silicon oxide, tantalum oxide, titanium oxide, magnesium fluoride, zirconia, aluminum oxide, hafnium oxide, niobium oxide, and zinc sulfide.
For example, by a deposition method, it is suitable to provide the reflection prevention layer 4 in a total thickness of about 0.2 μm to 0.5 μm, by forming the first-unit reflection prevention layer 4a consisting of Ta2O5 and the second-unit reflection prevention layer 4b consisting of SiO2 respectively in a thickness of several dozens of nm to a hundred and several dozens of nm Accordingly, the reflection prevention layer 4 of reflectance equal to or lower than 0.1% can be provided.
The total reflection layer 5 is formed on the surface of the electro-optic crystal layer 1, at the opposite side of the amorphous layer 3. The total reflection layer 5 is provided to make a total reflection of the probe light PB incident to the terahertz-wave detection element 10 from the supporting substrate 2 side.
Specifically, the total reflection layer 5 is formed on the main surface of the electro-optic crystal layer 1, as a dielectric multilayer film formed by stacking repeatedly and alternately the first-unit total reflection layer 5a and the second-unit total reflection layer 5b consisting of dielectrics of mutually different compositions. The total reflection layer 5 has a coefficient of thermal expansion smaller than the coefficient of thermal expansion of the electro-optic crystal layer 1. In the present embodiment, the coefficient of thermal expansion of the total reflection layer 5 as a multi-layer film of the first-unit total reflection layer 5a and the second-unit total reflection layer 5b of mutually different compositions is assumed to be an effective (average) value as a layer in total. As the dielectrics that can be used to form the total reflection layer 5, there can be exemplified silicon oxide, tantalum oxide, titanium oxide, magnesium fluoride, zirconia, aluminum oxide, hafnium oxide, niobium oxide, and zinc sulfide.
For example, by a deposition method, it is suitable to provide the total reflection layer 5 in a total thickness of about a few μm, by forming the first-unit total reflection layer 5a consisting of SiO2 and the second-unit total reflection layer 5b consisting of Ta2O5 respectively in a thickness of a submicron order, respectively. Accordingly, it is possible to provide the total reflection layer 5 of reflectance equal to or higher than 99% and a coefficient of thermal expansion smaller than a coefficient of thermal expansion in the z axis direction of the LN crystal.
When the terahertz-wave detection element 10 includes the reflection prevention layer 4 and the total reflection layer 5, the loss of the incident probe light PB is reduced, and therefore, the quality of an observation image in the case of using the terahertz-wave detection element 10 in the observation apparatus 1000 improves.
<Observation by Observation Apparatus>
Next, there will be described a configuration of the observation apparatus 1000, and an observation mode of the specimen S using the observation apparatus 1000. The observation apparatus 1000 is an apparatus capable of performing the observation of the specimen S based on the EO sampling method. As shown in
The terahertz-wave irradiation optical system OS1 mainly includes a terahertz-wave generation source 101 and a parabolic mirror 102. The terahertz-wave generation source 101 is configured to generate a terahertz wave TH by irradiating a femtosecond titanium laser beam of a wavelength 800 nm to a terahertz-wave conversion element.
The probe-light irradiation optical system OS2 mainly includes a probe-light light source 103, a first intermediate lens 104, a non-polarization beam splitter 105, and an objective lens 106. For the probe light PB, there is used a femtosecond titanium laser beam which is the same as that used in the terahertz-wave generation source 101. Therefore, by branching the femtosecond titanium laser beam emitted from the probe-light light source 103 into two directions in the middle, one femtosecond titanium laser beam may be used as the probe light PB, and the other femtosecond titanium laser beam may be utilized to generate the terahertz wave TH in the terahertz-wave generation source 101. In this case, the probe-light light source 103 may be configured as a system capable of measuring by what is called a THz-TDS (Terahertz Time Domain Spectroscopy) method, where the terahertz light is detected by sampling with a light delay unit.
The observation optical system OS3 mainly includes a second intermediate lens 107, a ¼ wavelength plate 108, a polarizer 109, and an imaging device 110 consisting of a CCD, for example.
In the observation apparatus 1000 having the above configuration, in the state that the specimen S is mounted on the mounting surface 10s of the terahertz-wave detection element 10, the terahertz wave TH emitted from the terahertz-wave generation source 101 as shown by an arrow AR101, and reflected and converged by the parabolic mirror 102, is irradiated to the specimen S. As described above, because the terahertz-wave detection element 10 includes the total reflection layer 5, actually, the surface of the total reflection layer 5 becomes the mounting surface 10s.
The terahertz wave TH irradiated to the specimen S is absorbed in accordance with a spatial distribution (a two-dimensional distribution) of a cellular component, a thickness, and the like in the specimen S, and intensity of the terahertz wave TH is spatially (two-dimensionally) modulated. Then, the modulated terahertz wave TH is incident to the electro-optic crystal layer 1 of the terahertz-wave detection element 10. After that, in the electro-optic crystal layer 1, there is generated by a Pockels effect a distribution in levels of a refractive index change by a double refraction, in accordance with an intensity distribution generated in the incident terahertz wave TH. In other words, the refractive index at the incident position of the terahertz wave TH varies in accordance with incident intensity of the terahertz wave TH at its incident position. As a result, the distribution of the refractive index change (the distribution of the refractive index eventually) reflects spatial information of the specimen S.
On the other hand, in the observation apparatus 1000, the probe light PB emitted from the probe-light source 103 as a parallel light is converted into a non-parallel light by the first intermediate lens 104. After that, as shown by an arrow AR2 and an arrow AR3, the non-parallel light passes through the non-polarization beam splitter 105 and the objective lens 106, and is incident as a parallel light from the supporting substrate 2 side (from the reflection prevention layer 4 side) to the terahertz-wave detection element 10. For the probe light PB, there is used the probe light PB of a wavelength band 800 nm Since the terahertz-wave detection element 10 according to the present embodiment includes the reflection prevention layer 4 on the surface of the supporting substrate 2, the probe light PB is incident to the electro-optic crystal layer 1 substantially without receiving loss.
The probe light PB incident to the electro-optic crystal layer 1 is totally reflected by the total reflection layer 5 comprised in the terahertz-wave detection element 10, while it is refracted in accordance with the refractive index distribution generated in the electro-optic crystal layer 1 according to the intensity distribution of the terahertz wave TH as described above. Then, as shown by an arrow AR4, the probe light PB is emitted toward the objective lens 106 and the non-polarization beam splitter 105. The probe light PB emitted from the terahertz-wave detection element 10 in this manner has a spatial distribution of intensity (a light quantity) reflecting the spatial distribution of the refractive index (a refractive index change).
The probe light PB emitted from the terahertz-wave detection element 10 and incident to the non-polarization beam splitter 105 is reflected by a half-mirror 105m comprised in the non-polarization beam splitter 105. Then, as shown by an arrow AR5, the probe light PB is converted into a parallel light by the second intermediate lens 107, sequentially passes through the ¼ wavelength plate 108 and the polarizer 109, and is incident to the imaging device 110.
As described above, the probe light PB incident to the imaging device 110 has the intensity distribution reflecting the refractive index distribution generated in the electro-optic crystal layer 1 in the terahertz-wave detection element 10. The refractive index distribution has been generated by the incidence to the electro-optic crystal layer 1, of the terahertz wave TH which has transmitted through the specimen S. As a result, in the observation apparatus 1000, an image formed in the imaging device 110 represents a distribution in the spatial (two-dimensional) state of the specimen S. Accordingly, in the observation apparatus 1000, the specimen S can be observed in real time by observing the image formed in the imaging device 110.
<Manufacturing Method of Terahertz-Wave Detection Element>
Next, a manufacturing method of the terahertz-wave detection element 10 having the above configuration according to the present embodiment will be described in detail.
As described above, the terahertz-wave detection element 10 according to the present embodiment is based on the configuration having the electro-optic crystal layer 1 and the supporting substrate 2 joined together by the amorphous layer 3. Because the planar size of the terahertz-wave detection element 10 is at most about a few mm square, it is difficult and inefficient to perform joining by preparing the electro-optic crystal layer 1 and the supporting substrate 2 of this planar size. Therefore, in the present embodiment, the terahertz-wave detection element 10 is manufactured by what is called a multi-piece forming
First, as shown in
The first mother substrate 1M is a substrate having the same composition and the same crystal state as those of the electro-optic crystal layer 1, and also having a large thickness. Concerning the thickness of the first mother substrate 1M, a value of the thickness is required to be such a degree that a certain level of strength and handleability can be secured. On the other hand, when a difference in the thickness between the first mother substrate 1M and the electro-optic crystal layer 1 finally configuring the terahertz-wave detection element 10 is too large, excessive time is required in the polishing process described later. Therefore, it is suitable to use the first mother substrate 1M of a thickness of about a few hundred μm to a few mm, for example.
The second mother substrate 2M is a substrate having the same composition, the same crystal state, and the thickness as those of the supporting substrate 2. However, for the second mother substrate 2M, it is preferable to use the second mother substrate 2M of flatness equal to or smaller than 25 μm and parallelism equal to or smaller than 3 μm, and more preferably, flatness equal to or smaller than 15 μm and parallelism equal to or smaller than 2 μm. When these requirements are satisfied, warping and unevenness in the terahertz-wave detection element 10 can be suppressed. Therefore, the terahertz-wave detection element 10 of excellent spatial resolution can be obtained. Specifically, when the second mother substrate 2M has flatness equal to or smaller than 25 μm and parallelism equal to or smaller than 3 μm, it is possible to realize spatial resolution equal to or smaller than 20 μm. When the second mother substrate 2M has flatness equal to or smaller than 15 nm and parallelism equal to or smaller than 2 μm, it is possible to realize more excellent spatial resolution equal to or smaller than 10 μm.
Unless particularly specified, in the present description, flatness and parallelism are expressed as values converted for a substrate (or a joined body) of 4-inch diameter.
After the first mother substrate 1M and the second mother substrate 2M have been prepared, both mother substrates are joined together. In the present embodiment, it is characteristic that this joining is directly performed at an ordinary temperature without using an adhesive and joining agent (Step S2). In the present embodiment, the joining in this manner will be referred to as an ordinary-temperature direct joining More specifically, in the present embodiment, as the method of the ordinary-temperature direct joining, a surface activation method will be employed. However, as the method of the ordinary-temperature direct joining, an atomic diffusion joining method may be used.
Even after each mother substrate has been cleaned by the organic solvent as described above, an oxide is formed on a top surface of each mother substrate, or further, a water molecule and an organic molecule are absorbed on the oxide layer, so long as each mother substrate is exposed to the atmosphere. Even if the mother substrates in such surface state are contacted and joined together, large joining force cannot be expected in this way. Therefore, in the present embodiment, the sputter etching is performed under the ultrahigh vacuum to remove adsorption molecules such as O atoms constituting the oxide on the top surface of each mother substrate and H atoms and C atoms forming an adsorbate on the oxide.
Next to the sputter etching, in the vacuum chamber in the ultrahigh vacuum state, the cleaned mother substrates are contacted together and are joined together (Step S2c).
The surfaces of the first mother substrate 1M and the second mother substrate 2M are in an active state with large joining force with other atoms by the sputter etching. Therefore, by contacting both mother substrates together, the joining between the atoms of both mother substrates is promoted without heating. That is, strong joining can be obtained at the ordinary temperature. Accordingly, a joined body 10M of the first mother substrate 1M and the second mother substrate 2M can be obtained.
In the joined body 10M, a joining interface portion becomes an amorphous oxide layer 3M containing constituent elements of both the first mother substrate 1M and the second mother substrate 2M. The amorphous oxide layer 3M has a coefficient of thermal expansion of an intermediate value between the coefficient of thermal expansion of the electro-optic crystal layer 1 and the coefficient of thermal expansion of the supporting substrate 2. By controlling a thickness of the amorphous oxide layer 3M by differentiating irradiation conditions of the Ar ion beam in the sputter etching prior to the joining, the joining strength of the first mother substrate 1M and the second mother substrate 2M can be adjusted.
After performing the process described later, the joined body 10M is finally cut into a large number of terahertz-wave detection elements 10. Then, portions originated from the first mother substrate 1M, the second mother substrate 2M, and the adhesion layer 3M respectively become the electro-optic crystal layer 1, the supporting substrate 2, and the amorphous layer 3 of the terahertz-wave detection element 10. For convenience sake, after the joined body 10M has been obtained, the first mother substrate 1M will be simply referred to as the electro-optic crystal layer 1, the second mother substrate 2M will be simply referred to as the supporting substrate 2, and the adhesion layer 3M will be simply referred to as the amorphous layer 3.
Next, the joined body 10M is extracted from the vacuum chamber, and then, the obtained electro-optic crystal layer 1 is polished by a publicly known sheet processing method, until the electro-optic crystal layer 1 has a thickness equal to the preferable thickness of the electro-optic crystal layer 1 in the above-described element state (Step S3).
After ending the polishing, an dielectric multilayer film serving as the total reflection layer 5 is formed on the polished electro-optic crystal layer 1 by a deposition method (Step S4). Next, on the supporting substrate 2, a dielectric multilayer film serving as the reflection prevention layer 4 is formed by the deposition method (Step S5). These dielectric multilayer films will be also referred to as the total reflection layer 5 and the reflection prevention layer 4, respectively, for convenience sake.
Finally, the joined body 10M formed up to the reflection prevention layer 4 is cut into predetermined element sizes on the surface along a joining direction by a publicly known method such as dicing to have a desired planar size. As a result, a large number of the terahertz-wave detection element 10 are obtained (step S6).
<Effects of Ordinary-Temperature Direct Joining>
In the above ordinary-temperature direct joining, because the first mother substrate 1M and the second mother substrate 2M are not heated, at the time of forming the joined body 10M, there occurs no warping due to a difference in the coefficient of thermal expansion between both mother substrates, and no crack occurs in the electro-optic crystal layer 1. However, in the case of forming the dielectric multilayer films as the total reflection layer 5 and the reflection prevention layer 4 by evaporation as described above, although the joined body 10M itself is not directly heated, a temperature of the joined body 10M becomes equal to or higher than 100° C. Therefore, it is concerned that, due to the difference in the coefficient of thermal expansion with the adjacent amorphous layer 3 and the total reflection layer 5, tensile stress acts in the electro-optic crystal layer 1, and that there may be a risk of the occurrence of warping in the joined body 10M and a crack in the electro-optic crystal layer 1.
However, in the present embodiment, the amorphous layer 3 is formed to have a coefficient of thermal expansion of an intermediate value between the coefficient of thermal expansion of the electro-optic crystal layer 1 and the coefficient of thermal expansion of the supporting substrate 2. Therefore, at the time of forming the total reflection layer 5 and the reflection prevention layer 4, even when a tensile stress acts in the electro-optic crystal layer 1 due to the difference in the coefficient of thermal expansion with the total reflection layer 5 and the amorphous layer 3, a magnitude of the tensile stress is suppressed to equal to or smaller than a threshold value (fracture strength) at which a crack occurs in the electro-optic crystal layer 1. Accordingly, the terahertz-wave detection element 10 with crack free can be obtained.
Further, when there is used the second mother substrate 2M having flatness equal to or smaller than 25 μm and parallelism equal to or smaller than 3 μm, preferably, flatness equal to or smaller than 15 μm and parallelism equal to or smaller than 2 μm, thermal expansion and thermal compression of the joined body 10M at the time of forming the dielectric multilayer film can be suppressed. Accordingly, in the terahertz-wave detection element 10 obtained by cutting the joined body 10M, flatness can be suppressed to equal to or smaller than 25 μm and parallelism equal to or smaller than 3 μm. Accordingly, in the terahertz-wave detection element 10, excellent spatial resolution that is equal to or smaller than 20 μm is realized.
As described above, according to the present embodiment, the ordinary-temperature direct joining is performed to join the electro-optic crystal substrate and the support mother substrate, and the terahertz-wave detection element is manufactured such that the thickness of the amorphous layer 3 as the joining layer becomes equal to or larger than 1 nm and equal to or smaller than 50 nm. Consequently, joining strength of the electro-optic crystal substrate and the support substrate is suitably secured, and a terahertz-wave detection element with crack free and high spatial resolution can be realized.
By applying the terahertz-wave detection element to the observation apparatus, the observation apparatus capable of observing a biological sample in high spatial resolution and in real time can be realized.
In the present example, a thickness of the electro-optic crystal layer 1 was differentiated to five levels and a thickness of the amorphous layer 3 was differentiated to seven levels to manufacture the joined body 10M under 35 manufacturing conditions in total. Presence of a defect such as a crack occurrence in the electro-optic crystal layer 1 was evaluated. Under each manufacturing condition, ten samples were manufactured.
Specifically, in each manufacturing condition, first, for the first mother substrate 1M, there was prepared an MgO 5 mol % doped x-cut plate LN single crystal substrate in a 4-inch diameter and in a 500 μm thickness. For the second mother substrate 2M, there was prepared a TEMPAX glass in a 4-inch diameter and in a 500 μm thickness.
Flatness of the second mother substrate 2M was within 3 μm as a result of measurement by a Fujinon interferometer. Parallelism was within 1 μm as a result of measurement by a micrometer.
Both mother substrates were cleaned by the organic cleaning method, and were respectively mounted at predetermined positions in the vacuum chamber of the ordinary-temperature direct joining apparatus. After the inside of the vacuum chamber was set to an ultrahigh vacuum state of about 10−6 Pa, sputter etching by Ar ion was performed to the surfaces (surfaces to be joined) of the first mother substrate 1M and the second mother substrate 2M to activate the surfaces. Next, in the vacuum chamber, the activated surfaces of both mother substrates were contacted together and both mother substrates were joined together.
In the sputter etching and in the subsequent joining, by changing the irradiation condition of the Ar ion, a thickness of the amorphous oxide layer 3M (a thickness of the amorphous layer 3) was changed to seven levels of 0.1 nm, 0.5 nm, 1 nm, 3 nm, 10 nm, 50 nm, and 60 nm. A relationship between the irradiation condition of the Ar ion and the thickness of the amorphous oxide layer 3M was specified in advance in preliminary experiments.
After the joined body 10M was obtained, the electro-optic crystal layer 1 was ground and polished by a publicly known sheet processing method. More specifically, for the joined body 10M of the same thickness of the amorphous layer 3, the thickness of the electro-optic crystal layer 1 was changed to five levels of 1 μm, 3 μm, 10 μm, 30 μm, and 35 μm. The results of the measurement of flatness and parallelism of the polished electro-optic crystal layer 1 were within 3 μm and 1 μm, respectively.
Thereafter, on the main surface of the electro-optic crystal layer 1 of each joined body 10M, by the deposition method, an SiO2 layer as the first-unit total reflection layer 5a and a Ta2O5 layer as the second-unit total reflection layer 5b were alternately formed such that 25 layers were formed in total. Accordingly, the total reflection layer 5 as the dielectric multilayer film was obtained. A total thickness of the total reflection layer 5 was about 3 μm, and thicknesses of the SiO2 layer and the Ta2O5 layer were 137 nm and 97 nm, respectively. As a result of evaluating the reflection characteristic, reflectance around 800 nm was equal to or higher than 99% in the range of 200 nm.
Further, on the main surface of the supporting substrate 2 of each joined body 10M, by the deposition method, a Ta2O5 layer as the first-unit reflection prevention layer 4a and an SiO2 layer as the second-unit reflection prevention layer 4b were alternately formed by 4 layers in total. As a result, the reflection prevention layer 4 as the dielectric multilayer film was obtained. More specifically, the reflection prevention layer 4 in a total thickness of 0.3 μm was formed by sequentially forming a Ta2O5 layer in a thickness 31 nm, an SiO2 layer in a thickness 40 nm, a Ta2O5 layer in a thickness 93 nm, and an SiO2 layer in a thickness 125 nm from a near side of the supporting substrate 2.
After forming the reflection prevention layer 4, the occurrence status of a peeling off and a crack in the electro-optic crystal layer 1 in each joined body 10M was observed visually and by an optical microscope.
Table 1 shows a list of thickness conditions of the amorphous layer 3 and the electro-optic crystal layer 1, and an evaluation result of the joined body 10M.
As is clear from Table 1, peeling off occurred in all samples (No. 1-1 to 1-5) for the joined body 10M in which the thickness of the amorphous layer 3 was 0.1 nm For the joined body 10M in which the thickness of the amorphous layer 3 was 0.5 nm, in samples (No. 2-1 and 2-2) in which the thickness of the electro-optic crystal layer 1 was 3 μm or smaller, each one peeling off occurred, in the middle of the manufacturing of a sample, specifically, in the polishing process. This result means that in the samples of these manufacturing conditions, the joining strength of the joined body 10M was weak.
For the joined body 10M in which the thickness of the amorphous layer 3 was equal to or larger than 0.5 nm and the thickness of the electro-optic crystal layer 1 was 35 μm (No. 2-5, 3-5, 4-5, 5-5, 6-5, and 7-5), there were samples in which a crack occurred in the electro-optic crystal layer 1. This is considered that, because the thickness of the electro-optic crystal layer 1 was large, the value of the tensile stress acting in the electro-optic crystal layer 1, due to the difference in the coefficient of thermal expansion between the electro-optic crystal layer 1 and the amorphous layer 3, exceeded the threshold value of the fracture strength.
On the other hand, for the joined body 10M in which the thickness of the amorphous layer 3 was equal to or larger than 1 nm and the thickness of the electro-optic crystal layer 1 was equal to or larger than 1 μm and equal to or smaller than 30 μm (No. 3-1 to 3-4, 4-1 to 4-4, 5-1 to 5-4, 6-1 to 6-4, and 7-1 to 7-4), no crack was confirmed in the electro-optic crystal layer 1 in any of the samples.
For samples in which there was no peeling off, flatness was measured by a thickness gage (manufactured by Heidenhain) of contact type. As a result, flatness was within 2 μm in all samples.
The terahertz-wave detection element 10 was manufactured for each of five kinds of samples (No. 3-2, 4-2, 5-2, 6-2, and 7-2) out of samples of the joined body 10M obtained in Example 1, in which the thickness of the electro-optic crystal layer 1 was 3 μm and the thickness of the amorphous layer 3 was equal to or larger than 1 nm Then, spatial resolution was evaluated by using the observation apparatus 1000.
In the case of specifying the spatial resolution by using the spatial-resolution evaluation pattern PT, first, there is arranged in the observation apparatus 1000 the terahertz-wave detection element 10 in which a plurality of spatial-resolution evaluation patterns PT of different lines and spaces (line widths and line intervals) are formed on the main surface at the electro-optic crystal layer 1 side. Then, the spatial-resolution evaluation pattern PT is observed.
In an intensity curve line C in the line pattern arrangement direction obtained from each spatial-resolution evaluation pattern PT, a difference value between maximum intensity Imax and minimum intensity Imin is defined as ΔI. Then, there is obtained a distance Δx between a position x1 of I1=Imin+0.9ΔI (=Imax-0.1 ΔI) and a position x2 of I2=Imin+0.1 ΔI (=Imax-0.9 ΔI). Then, a minimum value of Δx obtained from each spatial-resolution evaluation pattern PT is prescribed as the spatial resolution of the terahertz-wave detection element 10.
In the case of the present example, on the respective main surfaces of the above-described five kinds of samples of the joined body 10M at their electro-optic crystal layer 1 side, four levels of spatial-resolution evaluation patterns PT (refer to
As shown in Table 2, in the samples of No. 3-2, 4-2, 5-2, and 6-2 in which the thickness of the amorphous layer 3 was equal to or smaller than 50 nm, excellent spatial resolution that is equal to or smaller than 20 μm was obtained. What is more, in the samples of No. 3-2, 4-2, and 5-2, in which the thickness of the amorphous layer 3 was equal to or smaller than 10 nm, extremely excellent spatial resolution that is equal to or smaller than 3 μm was obtained.
The joined body 10M was manufactured and evaluated using 35 manufacturing conditions under conditions and in procedures similar to those in Example 1, except that z-cut plate quartz of 4-inch diameter and 500 μm thickness was used for the second mother substrate 2M.
Flatness of the second mother substrate 2M was within 3 μm, and parallelism was within 1 μm. The results of the measurement of flatness and parallelism of the polished electro-optic crystal layer 1 were within 3 μm and 1 μm, respectively.
Table 3 shows a list of thickness conditions of the amorphous layer 3 and the electro-optic crystal layer 1 in the joined body 10M obtained in the present example, and an evaluation result of the joined body 10M.
As is clear from Table 3, also in the present example, peeling off occurred in all samples (No. 8-1 to 8-5) for the joined body 10M in which the thickness of the amorphous layer 3 was 0.1 nm, as with Example 1. For the joined body 10M in which the thickness of the amorphous layer 3 was 0.5 nm, in samples (No. 9-1 and 9-2) in which the thickness of the electro-optic crystal layer 1 was 3 μm or smaller, each one peeling off occurred, in the middle of the manufacturing of a sample, specifically, in the polishing process. This result means that in the samples of these manufacturing conditions, the joining strength of the joined body 10M was weak.
For the joined body 10M in which the thickness of the amorphous layer 3 was equal to or larger than 0.5 nm and the thickness of the electro-optic crystal layer 1 was 35 μm (No. 9-5, 10-5, 11-5, 12-5, 13-5, and 14-5), there were samples in which a crack occurred in the electro-optic crystal layer 1. On the other hand, for the joined body 10M in which the thickness of the amorphous layer 3 was equal to or larger than 1 nm and the thickness of the electro-optic crystal layer 1 was equal to or larger than 1 μm and equal to or smaller than 30 μm (No. 10-1 to 10-4, 11-1 to 11-4, 12-1 to 12-4, 13-1 to 13-4, and 14-1 to 14-4), no crack was confirmed in the electro-optic crystal layer 1 in any of the samples.
For samples in which there was no peeling off, flatness was measured by a thickness gage (manufactured by Heidenhain) of contact type. As a result, flatness was within 2 μm in all samples.
The terahertz-wave detection element 10 was manufactured, as with Example 2, for each of five kinds of samples (No. 10-2, 11-2, 12-2, 13-2, and 14-2) out of samples of the joined body 10M obtained in Example 1, in which the thickness of the electro-optic crystal layer 1 was 3 μm and the thickness of the amorphous layer 3 was equal to or larger than 1 nm Then, spatial resolution was evaluated by using the observation apparatus 1000. Table 4 shows obtained values of spatial resolution.
As shown in Table 4, in the samples of No. 10-2, 11-2, 12-2, and 13-2 in which the thickness of the amorphous layer 3 was equal to or smaller than 50 nm, excellent spatial resolution that is equal to or smaller than 20 μm was obtained. What is more, in the samples of No. 10-2, 11-2, and 12-2, in which the thickness of the amorphous layer 3 was equal to or smaller than 10 nm, extremely excellent spatial resolution that is equal to or smaller than 3 μm was obtained.
Results of both examples indicate the following. In the case that the first mother substrate 1M and the second mother substrate 2M are directly joined together at an ordinary temperature to form the joined body 10M such that the thickness of the amorphous layer 3 as the joining layer is equal to or larger than 1 nm, and furthermore, the thickness of the electro-optic crystal layer 1 is set equal to or larger than 1 μm and equal to or smaller than 30 μm, it is possible to obtain the terahertz-wave detection element 10 including the total reflection layer 5 and the reflection prevention layer 4 as well as with crack free and small warping.
Results of both examples indicate that the terahertz-wave detection element 10 with crack free and high spatial resolution can be realized, by manufacturing the terahertz-wave detection element 10 such that the thickness of the amorphous layer 3 becomes equal to or larger than 1 nm and equal to or smaller than 50 nm, through the direct joining at an ordinary temperature of the first mother substrate 1M and the second mother substrate 2M.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-209329 | Sep 2012 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2013/075272 | Sep 2013 | US |
| Child | 14665090 | US |
