JP7620586B2 - Waveguide element - Google Patents

Description

The present invention relates to a waveguide element.

Waveguide elements are being developed as one type of element for guiding millimeter waves to terahertz waves. Waveguide elements are expected to be applied and developed in a wide range of fields, such as optical waveguides, next-generation high-speed communications, sensors, laser processing, and solar power generation. As an example of such a waveguide element, a technology has been proposed that uses a microstrip antenna including a transparent substrate having a thickness of 2 mm, an antenna conductor provided on the transparent substrate, and a transparent conductive film provided on the surface of the transparent substrate opposite to the antenna conductor (Patent Document 1).
However, such technology has a problem in that propagation loss increases significantly when millimeter waves to terahertz waves are guided.

International Publication No. 2019/107514

The main objective of the present invention is to provide a waveguide element that can ensure excellent low propagation loss performance even when guiding high-frequency electromagnetic waves with frequencies of 30 GHz or higher.

（式中、ｔは、無機材料基板の厚みを表す。λは、導波部材に導波される電磁波の波長を表す。εは、無機材料基板の比誘電率を表す。ａは、３以上の数値を表す。）
１つの実施形態においては、上記式（１）において、ａが６以上の数値を表す。
１つの実施形態においては、上記無機材料基板の３００ＧＨｚにおける比誘電率εと誘電正接（誘電体損失）ｔａｎδは、それぞれ３．５以上１２．０以下、０．００３以下である。
１つの実施形態においては、上記無機材料基板は、石英ガラス基板である。
１つの実施形態においては、上記導体層は、コプレーナ型電極である。
１つの実施形態においては、上記導波部材を伝搬する電磁波の周波数が３０ＧＨｚ以上５ＴＨｚ以下において、上記無機材料基板の厚みは、１０μｍ以上である。
１つの実施形態においては、上記導波素子は、上記無機材料基板において、上記導体層が形成される面とは反対側の面に接地電極を備えている。
１つの実施形態においては、上記導体層は、マイクロストリップ型電極であって、上記導波素子は、上記無機材料基板において、上記導体層が形成される面とは反対側の面に接地電極を備えている。
１つの実施形態においては、上記導波素子は、上記導波部材の下部に設けられ、上記導波部材を支持する支持基板を備えている。 A waveguide element according to an embodiment of the present invention includes a waveguide member capable of guiding electromagnetic waves having a frequency of 30 GHz to 20 THz. The waveguide member includes an inorganic material substrate and a conductor layer provided on the inorganic material substrate. The inorganic material substrate has a thickness t that satisfies the following formula (1):

(In the formula, t represents the thickness of the inorganic material substrate, λ represents the wavelength of the electromagnetic wave guided by the waveguide member, ε represents the relative dielectric constant of the inorganic material substrate, and a represents a numerical value of 3 or more.)
In one embodiment, in the above formula (1), a represents a numerical value of 6 or more.
In one embodiment, the inorganic material substrate has a relative dielectric constant ε and a dielectric loss tangent (dielectric loss) tan δ at 300 GHz of 3.5 to 12.0 and 0.003 or less, respectively.
In one embodiment, the inorganic material substrate is a quartz glass substrate.
In one embodiment, the conductor layer is a coplanar electrode.
In one embodiment, when the frequency of the electromagnetic wave propagating through the waveguide is 30 GHz or more and 5 THz or less, the inorganic material substrate has a thickness of 10 μm or more.
In one embodiment, the waveguide element includes a ground electrode on a surface of the inorganic material substrate opposite to a surface on which the conductor layer is formed.
In one embodiment, the conductor layer is a microstrip electrode, and the waveguide element includes a ground electrode on a surface of the inorganic material substrate opposite to a surface on which the conductor layer is formed.
In one embodiment, the waveguide element includes a support substrate provided below the waveguide member and supporting the waveguide member.

According to an embodiment of the present invention, it is possible to realize a waveguide element that can ensure excellent low propagation loss performance even when guiding high-frequency electromagnetic waves with frequencies of 30 GHz or more.

1 is a schematic perspective view of a waveguide element according to an embodiment of the present invention; 2 is a cross-sectional view of the waveguide element of FIG. 1 along line II-II'. FIG. 2 is a schematic perspective view of a waveguide element according to another embodiment of the present invention. 4 is a cross-sectional view of the waveguide element of FIG. 3 taken along line IV-IV'. FIG. 13 is a schematic perspective view of a waveguide element according to yet another embodiment of the present invention. 6 is a cross-sectional view of the waveguide element of FIG. 5 along the line VI-VI'. FIG. 13 is a schematic perspective view of a waveguide element according to yet another embodiment of the present invention. 8 is a cross-sectional view of the waveguide element of FIG. 7 taken along line VIII-VIII'.

（式中、ｔは、無機材料基板の厚みを表す。λは、導波部材に導波される電磁波の波長を表す。εは、無機材料基板の比誘電率を表す。ａは、３以上の数値を表す。）
無機材料基板の厚みが上記式（１）を満足すると、導波部材が上記した高周波数の電磁波を導波する場合であっても、スラブモードの誘起を抑制でき、導波部材が支持基板に支持される構成では基板共振の発生を抑制できる。
そのため、上記導波素子は、上記した高周波数の電磁波を導波する場合であっても、伝搬損失が増大することを抑制でき、優れた低伝搬損失性能を確保できる。
また、導波素子は小型化の開発が進められており、将来的には回路の集積化が見込まれるため、導波部材（線路構造）もそれに伴う小型化が求められると予想される。上記の導波素子では、導波部材（線路構造）が備える無機材料基板の薄板化が図られているので、優れた低伝搬損失性能を確保しながら、小型化の要望にも対応することができる。 Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these embodiments.
A. Overall Configuration of the Waveguide Element A-1. Overall Configuration of the Waveguide Element 100 FIG. 1 is a schematic perspective view of a waveguide element according to one embodiment of the present invention; FIG. 2 is a cross-sectional view of the waveguide element of FIG. 1 taken along line II-II'.
The illustrated waveguide element 100 includes a waveguide member 10. The waveguide member 10 is capable of guiding electromagnetic waves having a frequency of 30 GHz to 20 THz, in other words, millimeter waves to terahertz waves. Note that millimeter waves are typically electromagnetic waves having a frequency of about 30 GHz to 300 GHz, and terahertz waves are typically electromagnetic waves having a frequency of about 300 GHz to 20 THz.
The waveguide member 10 includes an inorganic material substrate 1 and a conductor layer 6 provided on the inorganic material substrate 1. The thickness of the inorganic material substrate 1 satisfies the following formula (1).

(In the formula, t represents the thickness of the inorganic material substrate, λ represents the wavelength of the electromagnetic wave guided by the waveguide member, ε represents the relative dielectric constant of the inorganic material substrate, and a represents a numerical value of 3 or more.)
When the thickness of the inorganic material substrate satisfies the above formula (1), the induction of a slab mode can be suppressed even when the waveguide member guides the above-mentioned high-frequency electromagnetic waves, and in a configuration in which the waveguide member is supported by a supporting substrate, the occurrence of substrate resonance can be suppressed.
Therefore, even when the director element guides the above-mentioned high-frequency electromagnetic waves, it is possible to suppress an increase in propagation loss, and it is possible to ensure excellent low propagation loss performance.
In addition, development of miniaturized waveguide elements is underway, and since circuit integration is expected in the future, it is expected that the waveguide member (line structure) will also be required to be miniaturized accordingly. In the above-mentioned waveguide element, the inorganic material substrate of the waveguide member (line structure) is made thinner, so it is possible to meet the demand for miniaturization while maintaining excellent low propagation loss performance.

In one embodiment, in the above formula (1), a represents a numerical value of 6 or more.
When the thickness of the inorganic material substrate satisfies the formula (1) in which a is a numerical value of 6 or more, it is possible to stably reduce the propagation loss when guiding the above-mentioned high-frequency electromagnetic waves.

The relative dielectric constant ε of the inorganic material substrate 1 at 300 GHz is typically 3.5 or more, and typically 12.0 or less, preferably 10.0 or less, and more preferably 5.0 or less.
The dielectric loss tangent (dielectric loss) tan δ at 300 GHz of the inorganic material substrate 1 is typically 0.0030 or less, preferably 0.0020 or less, and more preferably 0.0015 or less.
When the dielectric constant ε and the dielectric loss tangent (dielectric loss) tan δ of the inorganic material substrate are in the above range, the propagation loss can be more stably reduced when guiding the above-mentioned high-frequency electromagnetic waves (particularly electromagnetic waves of 300 GHz or more). The dielectric constant ε and the dielectric loss tangent (dielectric loss) tan δ can be measured by terahertz time domain spectroscopy. In this specification, when the measurement frequency is not mentioned for the dielectric constant and the dielectric loss tangent, the dielectric constant and the dielectric loss tangent at 300 GHz are meant.

The thickness of the inorganic material substrate 1 satisfying the above formula (1) is specifically 1 μm or more, preferably 2 μm or more, more preferably 10 μm or more, and even more preferably 20 μm or more, and is, for example, 1700 μm or less, preferably 500 μm or less, more preferably 200 μm or less, and even more preferably 100 μm or less. In addition, when the frequency of the electromagnetic wave propagating through the waveguide member is 30 GHz or more and 5 THz or less, the thickness of the inorganic material substrate 1 is preferably 10 μm or more.
If the thickness of the inorganic material substrate 1 falls below the lower limit, the thickness and width of the electrodes constituting the waveguide element will be reduced to about a few μm, resulting in increased propagation loss due to the skin effect and a significant decrease in the tolerance of line performance due to manufacturing variations.
When the thickness of the inorganic material substrate 1 is equal to or less than the upper limit, induction of slab mode and occurrence of substrate resonance are suppressed, and a waveguide element with small propagation loss over a wide frequency range (i.e., broadband) can be realized.

The illustrated waveguide 10 constitutes a coplanar line. That is, the conductor layer 6 of the waveguide 10 is a coplanar electrode 2.
The coplanar electrode 2 is provided on the upper surface of the inorganic material substrate 1. The coplanar electrode 2 is composed of a signal electrode 2a, a first ground electrode 2b, and a second ground electrode 2c. The signal electrode 2a has a linear shape extending in a predetermined direction. The width w of the signal electrode 2a is, for example, 2 μm or more, preferably 20 μm or more, for example, 200 μm or less, preferably 150 μm or less. The first ground electrode 2b is disposed at a distance from the signal electrode 2a in a direction perpendicular to the longitudinal direction of the signal electrode 2a. The second ground electrode 2c is located on the opposite side of the first ground electrode 2b with respect to the signal electrode 2a in a direction perpendicular to the longitudinal direction of the signal electrode 2a, and is disposed at a distance from the signal electrode 2a. As a result, a gap (slit) extending in the longitudinal direction of the signal electrode 2a is formed between the signal electrode 2a and the ground electrodes 2b and 2c. The width g of the gap (slit) is, for example, 2 μm or more, preferably 5 μm or more, and for example, 100 μm or less, preferably 80 μm or less.

In such a coplanar line, when a voltage is applied to the coplanar electrode 2, an electric field is generated between the signal electrode 2a and the ground electrodes 2b, 2c. When the high-frequency electromagnetic wave described above is input to the waveguide element 100, it couples with the electric field generated between the signal electrode 2a and the ground electrodes 2b, 2c and propagates through the inorganic material substrate 1.

The illustrated waveguide element 100 further includes a support substrate 20 that is provided below the waveguide member 10 and supports the waveguide member 10 .
When a waveguide element is provided with a supporting substrate, the mechanical strength of the waveguide element can be increased. However, when the waveguide member guides the above-mentioned high-frequency electromagnetic waves, substrate resonance may occur in the thickness including the substrate, resulting in increased propagation loss.
However, in the above configuration, since the thickness of the inorganic material substrate satisfies the above formula (1) and the dielectric constants of the support substrate and the waveguide are different, the occurrence of substrate resonance can be suppressed even when the waveguide guides the above-mentioned high-frequency electromagnetic waves. Therefore, even when the above-mentioned director element includes a support substrate and guides the above-mentioned high-frequency electromagnetic waves, an increase in propagation loss can be suppressed.
From this viewpoint, the greater the difference in dielectric constant between the waveguide and the support substrate, the better, and the dielectric constant of the support substrate should be smaller than that of the waveguide. If the dielectric constant of the support substrate is larger than that of the waveguide, a layer with a smaller dielectric constant may be provided between the waveguide and the support substrate.
Furthermore, in order to completely suppress substrate resonance, a grounded coplanar line or a microstrip line can be adopted as described below.

The illustrated waveguide element 100 includes a support substrate that supports the waveguide member, but the waveguide element of the present invention does not need to include a support substrate. In other words, the waveguide element may consist of only a waveguide member. The same is true for the waveguide elements 101 and 102 described below.

A-2. Overall Configuration of the Director Element 101 Fig. 3 is a schematic perspective view of a director element according to another embodiment of the present invention; Fig. 4 is a cross-sectional view of the director element of Fig. 3 taken along the line IV-IV'.
In the illustrated waveguide element 101, the waveguide member 11 forms a grounded coplanar line and includes a coplanar electrode 2, an inorganic material substrate 1, and a ground electrode 3. The ground electrode 3 is provided on the surface of the inorganic material substrate 1 opposite to the surface on which the coplanar electrode 2 (conductor layer 6) is formed. In the illustrated waveguide element 101, the ground electrode 3 is located between the inorganic material substrate 1 and the support substrate 20.
When the waveguide member 11 is provided with the ground electrode 3, the electric field generated between the signal electrode 2a and the ground electrodes 2b, 2c is prevented from leaking from the inorganic material substrate 1 to the supporting substrate 20, thereby suppressing the reduction in propagation loss due to substrate resonance and the generation of stray capacitance.
The width w of the signal electrode 2a is, for example, 2 μm or more, preferably 20 μm or more, for example, 300 μm or less, preferably 250 μm or less. The width g of the gap (slit) is, for example, 2 μm or more, preferably 5 μm or more, for example, 200 μm or less, preferably 150 μm or less.

7 and 8, the ground electrodes 2b and 2c may be electrically connected to the ground electrode 3. When the ground electrodes 2b and 2c are electrically connected to the ground electrode 3, the ground can be strengthened and stray capacitance due to surrounding lines and elements can be suppressed.
In the illustrated example, a plurality of via holes are formed in the inorganic material substrate 1, and the first ground electrode 2b and the ground electrode 3, and the second ground electrode 2c and the ground electrode 3 are short-circuited by the vias 5 located in each via hole. The vias 5 short-circuiting the first ground electrode 2b and the ground electrode 3, and the vias 5 short-circuiting the second ground electrode 2c and the ground electrode 3 are arranged at intervals in a direction intersecting the longitudinal direction of the signal electrode 2a. The vias 5 are typically conductive films. The arrangement of the plurality of via holes is not particularly limited, but in the illustrated example, the plurality of via holes are lined up in the longitudinal direction of the signal electrode 2a.

A-3. Overall Configuration of the Waveguide Element 102 Fig. 5 is a schematic perspective view of a waveguide element according to yet another embodiment of the present invention; Fig. 6 is a cross-sectional view of the waveguide element of Fig. 5 taken along line VI-VI'.
In the illustrated example of the waveguide element 102 , the waveguide member 12 constitutes a microstrip line and includes a microstrip electrode 4 as a conductor layer 6 , an inorganic material substrate 1 , and a ground electrode 3 .
The microstrip electrode 4 has a flat belt shape extending in a predetermined direction. The width w of the microstrip electrode 4 is, for example, 100 μm or more, preferably 300 μm or more, for example, 800 μm or less, preferably 500 μm or less.
The ground electrode 3 is provided on the surface of the inorganic material substrate 1 opposite to the surface on which the microstrip electrode 4 (the conductor layer 6) is formed. In the illustrated example of the waveguide element 102, the ground electrode 3 is located between the inorganic material substrate 1 and the supporting substrate 20.

In such a microstrip line, when a voltage is applied to the microstrip electrode 4 and the ground electrode 3, an electric field is generated between the microstrip electrode 4 and the ground electrode 3. When the high-frequency electromagnetic wave described above is input to the waveguide element 102, it couples with the electric field generated between the microstrip electrode 4 and the ground electrode 3 and propagates through the inorganic material substrate 1.

In this specification, the term "waveguide element" includes both a wafer on which at least one waveguide element is formed (waveguide element wafer) and a chip obtained by cutting the waveguide element wafer.

B. Inorganic Material Substrate The inorganic material substrate 1 has an upper surface on which the conductor layer 6 is provided, and a lower surface located within the composite substrate.
The inorganic material substrate 1 is made of an inorganic material. Any suitable material can be used as the inorganic material as long as the effect of the embodiment of the present invention can be obtained. Representative examples of such materials include single crystal quartz (relative dielectric constant 4.5, dielectric loss tangent 0.0013), amorphous quartz (quartz glass, relative dielectric constant 3.8, dielectric loss tangent 0.0010), spinel (relative dielectric constant 8.3, dielectric loss tangent 0.0020), AlN (relative dielectric constant 8.5, dielectric loss tangent 0.0015), sapphire (relative dielectric constant 9.4, dielectric loss tangent 0.0030), SiC (relative dielectric constant 9.8, dielectric loss tangent 0.0022), magnesium oxide (relative dielectric constant 10.0, dielectric loss tangent 0.0012), and silicon (relative dielectric constant 11.7, dielectric loss tangent 0.0016). The inorganic material substrate 1 is preferably a quartz glass substrate made of amorphous quartz.
If the inorganic material substrate 1 is a quartz glass substrate, the increase in propagation loss can be more stably suppressed even when guiding the above-mentioned high-frequency electromagnetic waves. Furthermore, since the dielectric constant is larger than that of resin-based substrates, the substrate size can be made smaller, and since the dielectric constant is relatively small among inorganic materials, it is advantageous in terms of reducing delay.

C. Conductor Layer and Ground Electrode The conductor layer 6 is typically made of a metal. Examples of metals include chromium (Cr), nickel (Ni), and copper (Cu). The metals can be used alone or in combination. The conductor layer 6 may be a single layer, or may be formed by laminating two or more layers. The conductor layer 6 is formed on the inorganic material substrate 1 by, for example, sputtering.
The thickness of the conductor layer 6 is, for example, 1 μm or more, preferably 4 μm or more, and, for example, 20 μm or less, preferably 10 μm or less.
Moreover, the ground electrode 3 is made of the same metal as the conductor layer 6 , and the range of the thickness of the ground electrode 3 is the same as the range of the thickness of the conductor layer 6 .

D. Support Substrate The support substrate 20 has an upper surface located within the composite substrate and a lower surface exposed to the outside. The support substrate 20 is provided to increase the strength of the composite substrate, and this allows the thickness of the inorganic material substrate to be thinned so as to satisfy the above formula (1). Any appropriate configuration can be adopted as the support substrate 20. Specific examples of materials constituting the support substrate 20 include indium phosphide (InP), silicon ( _Si ), glass, sialon ( _Si3N4 - _Al2O3 ), mullite ( _3Al2O3.2SiO2 , _2Al2O3.3SiO2 ) _, aluminum nitride ( _AlN ), magnesium _oxide ( _MgO ), _aluminum oxide ( _Al2O3 ), spinel ( _MgAl2O4 ), sapphire, quartz, crystal _, gallium nitride ₍ GaN) _, silicon carbide (SiC), silicon nitride ( _{Si3N4 )} , and gallium oxide ( _Ga2O3 ).
The support substrate 20 is preferably made of at least one material selected from the group consisting of indium phosphide, silicon, aluminum nitride, silicon carbide, and silicon nitride, and more preferably made of silicon.
When active elements such as an oscillator or a receiver are mounted on the waveguide element 100, the inorganic material substrate may heat up, which may deteriorate the characteristics of other active elements and mounted components. To prevent this, a material with high thermal conductivity may be used for the support substrate. In this case, the thermal conductivity is preferably 150 W/Km or more, and from this viewpoint, examples of the support substrate 20 include silicon (Si), aluminum nitride (AlN), gallium nitride (GaN), silicon carbide (SiC), and silicon nitride (Si ₃ N ₄ ).
The linear expansion coefficient of the material constituting the support substrate 20 is preferably as close as possible to the linear expansion coefficient of the material constituting the inorganic material substrate 1. With such a configuration, thermal deformation (typically, warping) of the composite substrate can be suppressed. Preferably, the linear expansion coefficient of the material constituting the support substrate 20 is within a range of 50% to 150% of the linear expansion coefficient of the material constituting the inorganic material substrate 1.
In addition, in a coplanar line, it is preferable that the dielectric loss tangent of the material constituting the support substrate 20 is small. In the case of a coplanar line, if the thickness of the waveguide member is small, the propagating electromagnetic waves may seep into the support substrate, and by making the dielectric loss tangent small, the propagation loss can be suppressed. From this viewpoint, it is preferable that the dielectric loss tangent is 0.07 or less.

The support substrate 20 typically supports the waveguide members (waveguide members 10, 11, 12) by directly bonding to the waveguide members (waveguide members 10, 11, 12). In this specification, "direct bonding" means that two layers or substrates are bonded together without the use of an adhesive. The form of direct bonding can be appropriately set depending on the configuration of the layers or substrates to be bonded together.
By integrating them by direct bonding, peeling in the waveguide element can be effectively suppressed, and as a result, damage (e.g., cracks) to the inorganic material substrate caused by such peeling can be effectively suppressed.
Although not shown, the waveguide elements (respectively, the waveguide elements 100, 101, and 102) may further include a joint provided between the waveguide member (the waveguide member 10, 11, and 12) and the supporting substrate 20 to join the waveguide member 10 and the supporting substrate 20.
Specifically, in the waveguide element 100 shown in Figures 1 and 2, the joint may be located between the inorganic material substrate 1 and the support substrate 20, and they may be integrated together. In the waveguide element 101 shown in Figures 3 and 4, the joint may be located between the ground electrode 3 and the support substrate 20, and they may be integrated together. In the waveguide element 102 shown in Figures 5 and 6, the joint may be located between the ground electrode 3 and the support substrate 20, and they may be integrated together.

The bonding portion may be a single layer, or may be a laminate of two or more layers. Examples of the bonding portion include a _SiO2 layer, an amorphous silicon layer, and a tantalum oxide layer. The thickness of the bonding portion is, for example, 0.1 μm or more and 3 μm or less.

Direct bonding can be achieved, for example, by the following procedure. In a high vacuum chamber (for example, about 1×10 ⁻⁶ Pa), a neutralizing beam is irradiated onto each bonding surface of the components (layers or substrates) to be bonded. This activates each bonding surface. Next, in a vacuum atmosphere, the activated bonding surfaces are brought into contact with each other and bonded at room temperature. The load during this bonding can be, for example, 100 N to 20,000 N. In one embodiment, when performing surface activation using a neutralizing beam, an inert gas is introduced into the chamber, and a high voltage is applied from a DC power source to an electrode placed in the chamber. With this configuration, electrons are moved by an electric field generated between the electrode (positive electrode) and the chamber (negative electrode), and a beam of atoms and ions is generated by the inert gas. Of the beams that reach the grid, the ion beam is neutralized by the grid, and a beam of neutral atoms is emitted from the high-speed atom beam source. The atomic species that constitute the beam is preferably an inert gas element (for example, argon (Ar), nitrogen (N)). The voltage during activation by beam irradiation is, for example, 0.5 kV to 2.0 kV, and the current is, for example, 50 mA to 200 mA. Note that the direct bonding method is not limited to this, and surface activation methods using FAB (Fast Atom Beam) or an ion gun, atomic diffusion methods, plasma bonding methods, etc. can also be applied.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

Example 1
1-1. Fabrication of a Waveguide Element (Coplanar Waveguide Line) A waveguide element shown in FIGS. 1 and 2 was fabricated.

A 0.5 mm thick quartz glass wafer (quartz glass substrate, inorganic material substrate) was prepared, and a 0.2 μm thick amorphous silicon film was formed on the quartz glass wafer by sputtering. After film formation, the amorphous silicon film was polished and flattened. The arithmetic mean roughness of the surface of the amorphous silicon film over a 10 μm square was measured using an atomic force microscope and found to be 0.2 nm.

A silicon wafer (support substrate) with a thickness of 525 μm was also prepared. Using an atomic force microscope, the arithmetic mean roughness of the surface of the silicon wafer, 10 μm square, was measured and found to be 0.2 nm.

The amorphous silicon surface of the quartz glass wafer and the silicon wafer were directly bonded as follows. First, the quartz glass wafer and the silicon wafer were put into a vacuum chamber, and in a vacuum of the 10 ⁻⁶ Pa range, both bonding surfaces (the amorphous silicon surface of the quartz glass wafer and the surface of the silicon wafer) were irradiated with a high-speed Ar neutral atom beam (accelerating voltage 1 kV, Ar flow rate 60 sccm) for 70 seconds. After irradiation, the quartz glass wafer and the silicon wafer were left to cool for 10 minutes, and then the bonding surfaces of the quartz glass wafer and the silicon wafer (the surface beam irradiated surfaces of the quartz glass wafer and the silicon wafer) were brought into contact with each other, and the quartz glass wafer and the silicon wafer were bonded by applying pressure of 4.90 kN for 2 minutes. After bonding, the quartz glass wafer was polished until the thickness was 150 μm to form a composite wafer. In the obtained quartz glass/silicon composite substrate, no defects such as peeling were observed at the bonding interface.

Next, a resist was applied to the surface (polished surface) of the quartz glass wafer opposite to the silicon wafer, and the resist was patterned by photolithography to expose the portion where the coplanar electrode pattern was to be formed. After that, a 50 nm thick Cr film and a 100 nm thick Ni film were formed by sputtering on the upper surface of the quartz glass wafer exposed from the resist to form a base electrode. Furthermore, a copper film was formed by electrolytic plating on the base electrode to form a coplanar electrode pattern. The length of the signal electrode in the waveguiding direction was 10 mm.
In this manner, a waveguide element including a waveguide member having a coplanar electrode and an inorganic material substrate, and a supporting substrate was obtained.

1-2. Calculation of Propagation Loss To measure the propagation loss of the director element, three director elements with signal electrodes having lengths of 30 mm, 40 mm, and 50 mm were fabricated in the same manner as described above.
Next, an RF signal generator was coupled to the input side of the waveguide member by a probe, and an electromagnetic wave was coupled to an RF signal receiver installed on the output side of the waveguide member by a probe.
Next, a voltage was applied to the RF signal generator, causing the RF signal generator to transmit electromagnetic waves at the frequencies shown in Table 1. This caused the electromagnetic waves to propagate through the coplanar line (waveguide member). The RF signal receiver measured the RF power of the electromagnetic waves output from the coplanar line. From the measurement results of three waveguide elements with different signal electrode lengths, the propagation loss (dB/cm) was calculated and evaluated according to the following criteria. The results are shown in Table 1.
◎: Less than 0.5 dB/cm ◯: 0.5 dB/cm or more and less than 1 dB/cm △: 1 dB/cm or more and less than 2 dB/cm ×: 2 dB/cm or more

Example 2
2-1. Fabrication of a Waveguide Element (Coplanar Line with Ground) A waveguide element shown in FIGS. 3 and 4 was fabricated.

A 0.5 mm thick quartz glass wafer (quartz glass substrate, inorganic material substrate) was prepared, and a 50 nm thick Cr film and a 100 nm thick Ni film were formed on the quartz glass wafer by sputtering to form a base electrode. Furthermore, a copper film was formed on the base electrode by electrolytic plating to form a ground electrode. Next, a 0.2 μm thick amorphous silicon film was formed on the ground electrode by sputtering. After film formation, the amorphous silicon film was polished and flattened. Here, the arithmetic mean roughness of the surface of the amorphous silicon film over a square of 10 μm was measured using an atomic force microscope, and was found to be 0.2 nm.

A silicon wafer (support substrate) with a thickness of 525 μm was also prepared. Using an atomic force microscope, the arithmetic mean roughness of the surface of the silicon wafer, 10 μm square, was measured and found to be 0.2 nm.

Thereafter, the amorphous silicon surface formed on the ground electrode was directly bonded to the silicon wafer. The direct bonding was performed in the same manner as in Example 1. In the obtained quartz glass/ground electrode/silicon composite substrate, no defects such as peeling were observed at the bonding interface.
The quartz glass wafer was then polished to a thickness of 150 μm.

Next, a coplanar electrode pattern was formed on the surface (polished surface) of the quartz glass wafer opposite to the silicon wafer in the same manner as in Example 1. The length of the signal electrode in the waveguiding direction was 10 mm.
In this manner, a waveguide element was obtained that included a waveguide member including a coplanar electrode, an inorganic material substrate, and a ground electrode, and a supporting substrate.

2-2. Calculation of Propagation Loss In order to measure the propagation loss of the director element, three director elements with signal electrode lengths of 30 mm, 40 mm, and 50 mm were fabricated in the same manner as described above. Then, the RF power of the electromagnetic wave output from the coplanar line was measured by an RF signal receiver in the same manner as in Example 1. The propagation loss of the director element of Example 2 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

Example 3
3-1. Fabrication of a Waveguide Element (Microstrip Line) A waveguide element shown in FIGS. 5 and 6 was fabricated.

In the same manner as in Example 2, a quartz glass/ground electrode/silicon composite substrate was obtained.
Next, a resist was applied to the surface (polished surface) of the quartz glass wafer opposite to the silicon wafer, and patterned by photolithography to expose the portion where the microstrip electrode was to be formed. After that, a 50 nm thick Cr film and a 100 nm thick Ni film were formed by sputtering on the upper surface of the quartz glass wafer exposed from the resist to form a base electrode. Furthermore, a copper film was formed on the base electrode by electrolytic plating to form a microstrip electrode. The length of the microstrip electrode in the waveguiding direction was 10 mm.
In this manner, a waveguide element was obtained that included a waveguide member having a microstrip electrode and an inorganic material substrate, and a supporting substrate.

3-2. Calculation of Propagation Loss In order to measure the propagation loss of the director element, three director elements with microstrip electrodes of lengths of 30 mm, 40 mm, and 50 mm were fabricated in the same manner as described above. Then, the RF power of the electromagnetic waves output from the coplanar line was measured by an RF signal receiver in the same manner as in Example 1. The propagation loss of the director element of Example 3 was evaluated in the same manner as in Example 1. The results are shown in Table 1.

<Examples 4 to 6>
A waveguide element was fabricated in the same manner as in each of Examples 1 to 3, except that the thickness of the polished quartz glass wafer (inorganic material substrate) was changed to the value shown in Table 1.
For the obtained waveguide element, the propagation loss was calculated and evaluated in the same manner as in Example 1. The results are shown in Table 1.

Example 7
A waveguide element was fabricated in the same manner as in Example 1, except that the quartz glass wafer used as the inorganic material substrate was changed to a single crystal silicon wafer, and the thickness of the polished silicon wafer was changed to the value shown in Table 1.
For the obtained waveguide element, the propagation loss was calculated and evaluated in the same manner as in Example 1. The results are shown in Table 1.

Example 8
A waveguide element was fabricated in the same manner as in Example 1, except that the quartz glass wafer used as the inorganic material substrate was changed to a sapphire wafer, and the thickness of the polished sapphire wafer was changed to the value shown in Table 1.
For the obtained waveguide element, the propagation loss was calculated and evaluated in the same manner as in Example 1. The results are shown in Table 1.

<Example 9>
A waveguide element was fabricated in the same manner as in Example 1, except that the quartz glass wafer used as the inorganic material substrate was changed to a polycrystalline AlN wafer, and the thickness of the polished AlN wafer was changed to the value shown in Table 1.
For the obtained waveguide element, the propagation loss was calculated and evaluated in the same manner as in Example 1. The results are shown in Table 1.

Example 10
A waveguide element was fabricated in the same manner as in Example 1, except that the thickness of the polished quartz glass wafer (inorganic material substrate) was changed to the value shown in Table 1.
For the obtained waveguide element, the propagation loss was calculated and evaluated in the same manner as in Example 1. The results are shown in Table 1.

<Examples 11 to 14>
A waveguide element was fabricated in the same manner as in Example 3, except that the thickness of the polished quartz glass wafer (inorganic material substrate) was changed to the value shown in Table 1.
For the obtained waveguide element, the propagation loss was calculated and evaluated in the same manner as in Example 1. The results are shown in Table 1.

<Comparative Example 1>
A quartz glass wafer (quartz glass plate, inorganic material substrate) having a thickness of 2100 μm was prepared, and a waveguide element was fabricated in the same manner as in Example 3, except that the thickness of the polished quartz glass wafer was changed to 2000 μm.
For the obtained waveguide element, the propagation loss was calculated and evaluated in the same manner as in Example 1. The results are shown in Table 1.

As is clear from Table 1, when the thickness of the inorganic material substrate satisfies the above formula (1), even when guiding high-frequency electromagnetic waves exceeding 30 GHz, the propagation loss is small and excellent low propagation loss performance can be ensured.

The waveguide element according to the embodiment of the present invention can be used in a wide range of fields such as waveguides, next-generation high-speed communications, sensors, laser processing, and solar power generation, and can be particularly suitable for use as a waveguide for millimeter waves to terahertz waves. Such a waveguide element can be used, for example, in an antenna, a bandpass filter, a coupler, a delay line (phase shifter), or an isolator.

REFERENCE SIGNS LIST 1 inorganic material substrate 2 coplanar electrode 3 ground electrode 4 microstrip electrode 10 waveguide member 11 waveguide member 12 waveguide member 100 waveguide element 101 waveguide element 102 waveguide element

Claims (8)

A waveguide element including a waveguide member capable of guiding an electromagnetic wave having a frequency of 30 GHz or more and 20 THz or less,
The waveguide member includes an inorganic material substrate and a conductor layer provided on an upper portion of the inorganic material substrate,
The conductor layer is
A center electrode extending in a predetermined direction;
a first ground electrode disposed at a distance from the center electrode in a direction perpendicular to a longitudinal direction of the center electrode;
a second ground electrode located on an opposite side of the first ground electrode with respect to the center electrode, the second ground electrode being spaced apart from the center electrode;
The waveguide element is
a support substrate provided under the waveguide member, the support substrate supporting the waveguide member;
a third ground electrode located on the opposite side of the conductor layer with respect to the inorganic material substrate;
a first via that shorts the first ground electrode and the third ground electrode;
a second via that shorts the second ground electrode and the third ground electrode;
A waveguide element, wherein the thickness t of the inorganic material substrate satisfies the following formula (1):

(In the formula, t represents the thickness of the inorganic material substrate, λ represents the wavelength of the electromagnetic wave guided by the waveguide member, ε represents the relative dielectric constant of the inorganic material substrate, and a represents a numerical value of 3 or more.) The waveguide element according to claim 1, wherein in formula (1), a is a value of 6 or more. The waveguide element according to claim 1 or 2, wherein the relative dielectric constant ε and the dielectric loss tangent (dielectric loss) tan δ of the inorganic material substrate at 300 GHz are 3.5 or more and 12.0 or less, and 0.003 or less, respectively. The waveguide element according to claim 3, wherein the inorganic material substrate is a quartz glass substrate. A waveguide element according to any one of claims 1 to 4, wherein the conductor layer is a coplanar electrode. The waveguide element according to claim 5, wherein the thickness of the inorganic material substrate is 10 μm or more when the frequency of the electromagnetic waves propagating through the waveguide member is 30 GHz or more and 5 THz or less. A waveguide element according to any one of claims 1 to 4, wherein the conductor layer is a microstrip electrode. 8. The waveguide element according to claim 1, wherein the inorganic material substrate has a thickness of 100 [mu]m or less.

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