JP7602996B2 - Waveguide element and method for manufacturing the same - Google Patents

Description

The present invention relates to a waveguide element and a method for manufacturing a waveguide element.

Waveguide elements are being developed as one type of element that guides 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 grounded coplanar waveguide that is composed of a glass substrate with a thickness of 300 μm, a coplanar conductor provided on the glass substrate, and a ground electrode provided on the surface of the glass substrate opposite to the coplanar conductor (Patent Document 1).
When using such a waveguide element in various industrial products, mounting the waveguide element on a support substrate such as an IC substrate, a printed circuit board, etc. However, when a waveguide element is mounted on a support substrate and millimeter waves to terahertz waves (particularly electromagnetic waves of 300 GHz or higher) are guided, there is a problem that the propagation loss increases significantly.

Special Publication No. 2021-509767

The main objective of the present invention is to provide a waveguide element and a method for manufacturing the same that can sufficiently reduce propagation loss 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、導波素子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、上記導体層と上記第２接地電極は、マイクロストリップ型電極である。
１つの実施形態においては、上記導波素子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、上記第１ビアが配置される第１ビアホールであって、上記無機材料基板、上記第２接地電極および上記支持基板を貫通している第１ビアホールを複数有している。
１つの実施形態においては、上記第１ビアホールは、上記無機材料基板の表面（上面）方向から見て円形状を有し、上記第２接地電極に近づくにつれて小径となるテーパ形状を有している。
１つの実施形態においては、上記第１ビアホールは、上記無機材料基板の表面（上面）方向から見て円形状を有し、上記第２接地電極に近づくにつれて大径となるテーパ形状を有している。
本発明の別の局面による導波素子の製造方法は、上記した導波素子を製造する方法であって、上記無機材料基板、上記第２接地電極および上記支持基板をこの順に備える積層体であり、上記無機材料基板、上記第２接地電極および上記支持基板を一括して貫通する第１ビアホールと、上記無機材料基板を貫通し上記支持基板を貫通しない（上記無機材料基板のみ貫通する）第２ビアホールとを有する積層体を準備する工程と；上記第１ビアホール内に上記第１ビアを形成し、上記第２ビアホール内に上記第２ビアを形成し、上記支持基板の下部に上記第３接地電極を形成し、上記無機材料基板の上部に上記導体層を形成する工程と；を含んでいる。 The waveguide element according to the embodiment of the present invention can guide electromagnetic waves having a frequency of 30 GHz or more and 20 THz or less. The waveguide element includes an inorganic material substrate; a conductor layer provided on the upper part of the inorganic material substrate, the conductor layer including a signal electrode extending in a predetermined direction and a first ground electrode arranged at a distance from the signal wiring in a direction intersecting the predetermined direction; a support substrate located on the opposite side of the conductor layer with respect to the inorganic material substrate; a second ground electrode located between the inorganic material substrate and the support substrate; a third ground electrode located on the opposite side of the support substrate from the second ground electrode; a first via electrically connecting the first ground electrode and the third ground electrode and electrically connecting the second ground electrode; and a second via electrically connecting the first ground electrode and the second ground electrode. The waveguide element includes a plurality of the first vias, and the second via is arranged between adjacent first vias among the plurality of first vias. 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 director element, 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, the conductor layer and the second ground electrode are microstrip type electrodes.
In one embodiment, when the frequency of the electromagnetic wave propagating through the waveguide element 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 has a plurality of first via holes in which the first vias are arranged, the first via holes penetrating the inorganic material substrate, the second ground electrode and the support substrate.
In one embodiment, the first via hole has a circular shape when viewed from the surface (upper surface) direction of the inorganic material substrate, and has a tapered shape whose diameter decreases toward the second ground electrode.
In one embodiment, the first via hole has a circular shape when viewed from the surface (upper surface) direction of the inorganic material substrate, and has a tapered shape that becomes larger in diameter as it approaches the second ground electrode.
A method for manufacturing a waveguide element according to another aspect of the present invention is a method for manufacturing the above-mentioned waveguide element, comprising the steps of: preparing a laminate having the inorganic material substrate, the second ground electrode, and the support substrate in this order, the laminate having a first via hole penetrating the inorganic material substrate, the second ground electrode, and the support substrate collectively, and a second via hole penetrating the inorganic material substrate but not the support substrate (penetrating only the inorganic material substrate); forming the first via in the first via hole, forming the second via in the second via hole, forming the third ground electrode at the bottom of the support substrate, and forming the conductor layer on the top of the inorganic material substrate.

According to an embodiment of the present invention, it is possible to realize a waveguide element that can sufficiently reduce propagation loss even when guiding high-frequency electromagnetic waves with frequencies of 30 GHz or more. In addition, according to an embodiment of another aspect of the present invention, the above-mentioned waveguide element can be smoothly manufactured.

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'. 3 is a cross-sectional view of the waveguide element of FIG. 1 taken along line III-III'. 4 is a cross-sectional view of the waveguide element of FIG. 1 taken along line IV-IV'. FIG. IV-IV' cross-sectional view of a waveguide element according to another embodiment of the present invention. FIG. IV-IV' cross-sectional view of a waveguide element according to yet another embodiment of the present invention. 2 is a cross-sectional view of a waveguide element according to yet another embodiment of the present invention; FIG. 2 is a cross-sectional view of a waveguide element according to yet another embodiment of the present invention; FIG.

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 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'; FIG. 3 is a cross-sectional view of the waveguide element of FIG. 1 taken along line III-III'; and FIG. 4 is a cross-sectional view of the waveguide element of FIG. 1 taken along line IV-IV'.
The illustrated waveguide element 100 can guide electromagnetic waves having a frequency of 30 GHz or more and 20 THz or less, 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. In particular, the waveguide element 100 can guide electromagnetic waves having a frequency of 30 GHz to 2 THz or less (particularly electromagnetic waves having a frequency of 30 GHz to 1 THz or less) with excellent propagation loss.

（式中、ｔは無機材料基板の厚みを表す。λは導波素子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。）
上記した信号電極を含む導体層を備える導波素子では、導波素子に入力される高周波数の電磁波は、無機材料基板中を伝搬する。
上記の構成によれば、無機材料基板の厚みが上記式（１）を満足するので、導波素子が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および／または基板共振の発生を抑制できる。しかし、無機材料基板の厚みが上記式（１）を満足すると、伝搬する電磁波が無機材料基板から支持基板に漏洩し、支持基板の誘電体損失による伝搬損失が大きくなるという新たな問題が生じ得る。
これに対して、上記の構成では、第２接地電極が無機材料基板と支持基板との間に配置され、第３接地電極が支持基板に対して第２接地電極と反対側に配置されているので、電磁波が支持基板に漏洩することを抑制することができる。そのため、スラブモードの誘起および／または基板共振の発生を抑制できつつ、電磁波の支持基板への漏洩を抑制できる。
また、第１ビアが第１接地電極と第２接地電極と第３接地電極とを電気的に接続しているので、グランドを強化でき、周囲の線路や素子による浮遊容量を抑制できる。また、基板に放熱機能を付加することができる。さらに、高次モードでの伝送を抑制することができる。
この点、第１接地電極と第２接地電極とを接続するビアと、第２接地電極と第３接地電極とを接続するビアとを別々に設けて、第１接地電極と第２接地電極と第３接地電極とを電気的に接続することも想定され得る。しかし、そのような構成では、第１接地電極および第２接地電極を接続するビアと、第２接地電極および第３接地電極を接続するビアとの相対的な位置精度を確保することが難しく、位置ずれが大きいと周波数特性においてリップルが顕著に発生する場合がある。さらに、そのような構成を有する導波素子の製造は、非常に煩雑である。
これに対して、上記の構成では、第１ビアが第１接地電極と第２接地電極と第３接地電極とを電気的に接続しているので、第１ビアにおいて、第１接地電極と第２接地電極との間に位置する部分と、第２接地電極と第３接地電極との間に位置する部分との相対的な位置精度を簡便に確保することができ、リップルの発生を抑制することができる。
さらに、第２ビアが互いに隣り合う第１ビアの間に配置されているので、無機材料基板における第１ビアおよび第２ビアのピッチを、支持基板における第１ビアのピッチよりも小さくすることができる。そのため、無機材料基板が上記のように薄厚化されていても、無機材料基板の強度を十分に確保することができる。
これらの結果、導波素子において、周波数が３０ＧＨｚ以上である高周波数の電磁波を導波する場合であっても、伝搬損失を十分に低減できる。
なお、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。上記の導波素子では、無機材料基板の薄板化が図られているので、優れた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。 The waveguide element 100 comprises an inorganic material substrate 1; a conductor layer 2 having a signal electrode 2a and first ground electrodes 2b, 2c; a support substrate 7; a second ground electrode 3; a third ground electrode 4; a first via 5; and a second via 6.
The conductor layer 2 is provided on the inorganic material substrate 1. The signal electrode 2a extends in a predetermined direction (waveguiding direction). The first ground electrodes 2b and 2c are arranged at intervals from the signal electrode 2a in a direction intersecting the predetermined direction in which the signal electrode 2a extends. The support substrate 7 is located on the opposite side of the inorganic material substrate 1 to the conductor layer 2. The second ground electrode 3 is located between the inorganic material substrate 1 and the support substrate 7. The third ground electrode 4 is located on the opposite side of the support substrate 7 to the second ground electrode 3. The first via 5 electrically connects the first ground electrodes 2b and 2c to the third ground electrode 4, and is also electrically connected to the second ground electrode 3. The director element 100 includes a plurality of the first vias 5. The second via 6 electrically connects the first ground electrodes 2b and 2c to the third ground electrode 3. The second via 6 is arranged between adjacent first vias 5 among the plurality of first vias 5. The thickness t 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 element, ε represents the relative dielectric constant of the inorganic material substrate, and a represents a value of 3 or more.)
In the director element having the conductor layer including the signal electrode described above, a high-frequency electromagnetic wave input to the director element propagates through the inorganic material substrate.
According to the above configuration, since the thickness of the inorganic material substrate satisfies the above formula (1), even when the director element guides a high-frequency electromagnetic wave, induction of a slab mode and/or occurrence of substrate resonance can be suppressed. However, if the thickness of the inorganic material substrate satisfies the above formula (1), a new problem may arise in that the propagating electromagnetic wave leaks from the inorganic material substrate to the supporting substrate, and the propagation loss due to the dielectric loss of the supporting substrate increases.
In contrast, in the above-mentioned configuration, the second ground electrode is disposed between the inorganic material substrate and the support substrate, and the third ground electrode is disposed on the opposite side of the support substrate to the second ground electrode, so that it is possible to suppress leakage of electromagnetic waves to the support substrate, thereby suppressing induction of slab modes and/or occurrence of substrate resonance while suppressing leakage of electromagnetic waves to the support substrate.
In addition, since the first via electrically connects the first ground electrode, the second ground electrode, and the third ground electrode, the ground can be strengthened and the stray capacitance due to the surrounding lines and elements can be suppressed. In addition, a heat dissipation function can be added to the substrate. Furthermore, transmission in higher modes can be suppressed.
In this regard, it may be possible to provide a via connecting the first ground electrode and the second ground electrode and a via connecting the second ground electrode and the third ground electrode separately, and electrically connect the first ground electrode, the second ground electrode, and the third ground electrode. However, in such a configuration, it is difficult to ensure the relative positional accuracy of the via connecting the first ground electrode and the second ground electrode and the via connecting the second ground electrode and the third ground electrode, and if the positional deviation is large, ripples may occur significantly in the frequency characteristics. Furthermore, the manufacture of a waveguide element having such a configuration is very complicated.
In contrast, in the above configuration, the first via electrically connects the first ground electrode, the second ground electrode, and the third ground electrode, so that the relative positional accuracy of the portion of the first via located between the first ground electrode and the second ground electrode and the portion located between the second ground electrode and the third ground electrode can be easily ensured, and the occurrence of ripples can be suppressed.
Furthermore, since the second vias are disposed between the adjacent first vias, the pitch of the first and second vias in the inorganic material substrate can be made smaller than the pitch of the first vias in the support substrate, so that the strength of the inorganic material substrate can be sufficiently ensured even if the inorganic material substrate is thinned as described above.
As a result, even when the director element guides high-frequency electromagnetic waves having frequencies of 30 GHz or more, the propagation loss can be sufficiently reduced.
Development of miniaturized waveguide elements is underway, and circuit integration is expected in the future. The above-mentioned waveguide elements use thin inorganic material substrates, so they can meet the demand for miniaturization while maintaining excellent 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 dielectric constant of the inorganic material substrate 1 at 100 GHz to 10 THz is, for example, 10.0 or less, preferably 3.7 or more and 10.0 or less, and more preferably 3.8 or more and 9.0 or less. When the frequency used is 300 GHz, the relative dielectric constant ε of the inorganic material substrate 1 is typically 3.5 or more and typically 12.0 or less, preferably 10.0 or less, and more preferably 5.0 or less. If the dielectric constant of the inorganic material substrate is in such a range, the delay of the propagating electromagnetic wave can be suppressed.
The dielectric loss tangent (tan δ) of the inorganic material substrate is preferably 0.01 or less, more preferably 0.008 or less, even more preferably 0.006 or less, and particularly preferably 0.004 or less, at the frequency used. When the frequency used is 300 GHz, the dielectric loss tangent tan δ of the inorganic material substrate 1 is preferably 0.0030 or less, more preferably 0.0020 or less, and even more preferably 0.0015 or less.
If the dielectric loss tangent is in this range, the propagation loss in the waveguide can be reduced. The smaller the dielectric loss tangent, the more preferable it is. The dielectric loss tangent can be, for example, 0.001 or more.
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 element 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.

In one embodiment, the waveguide element comprises a coplanar line, i.e., the conductor layers of the waveguide element are coplanar electrodes.
In one embodiment, the conductor layer 2 is a coplanar electrode. When the conductor layer 2 is a coplanar electrode as shown in Fig. 2, the high-frequency electromagnetic wave is coupled with an electric field generated between the signal electrode 2a and the first ground electrodes 2b and 2c, and propagates through the inorganic material substrate 1.
When the conductor layer 2 is a coplanar electrode, the signal electrode 2a has a linear shape extending in a predetermined direction (waveguiding direction). The first ground electrode 2b is arranged so as to form a predetermined gap between the signal electrode 2a and the first ground electrode 2b in a direction intersecting (preferably perpendicular) with the longitudinal direction of the signal electrode 2a. The first 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 intersecting (preferably perpendicular) with the longitudinal direction of the signal electrode 2a, and is arranged so as to form a predetermined gap between the first ground electrode 2b and the signal electrode 2a. The gap extends in the longitudinal direction of the signal electrode 2a.
The width w (dimension in a direction perpendicular to the longitudinal direction) of the signal electrode 2a of the coplanar electrode is, for example, 2 μm or more, preferably 20 μm or more, for example, 200 μm or less, preferably 150 μm or less.
The width g of the gap (dimension in a direction intersecting the longitudinal direction) is, for example, 2 μm or more, preferably 5 μm or more, and is, for example, 100 μm or less, preferably 80 μm or less.

In one embodiment, the director element comprises a microstrip line, i.e., the conductor layer of the director element and the second ground electrode are microstrip type electrodes.
In one embodiment, the conductor layer 2 and the second ground electrode 3 are microstrip type electrodes. As shown in Fig. 8, in the case of the microstrip type electrodes, the above-mentioned high-frequency electromagnetic wave is coupled with an electric field generated between the signal electrode 2a and the second ground electrode 3 and propagates through the inorganic material substrate 1.
When the conductor layer 2 is a microstrip electrode, the signal electrode 2a has a flat belt shape extending in a predetermined direction (waveguiding direction). The second ground electrode 3 is arranged so as to form a predetermined gap (thickness of the inorganic material substrate) between the signal electrode 2a and the second ground electrode 3 in a direction intersecting (preferably perpendicular) the longitudinal direction of the signal electrode 2a. The gap extends in the longitudinal direction of the signal electrode 2a. On the other hand, the first ground electrodes 2b and 2c can also be installed in the same manner as the coplanar electrode, and the first ground electrodes 2b and 2c are located away from the signal electrode 2a in a direction intersecting (preferably perpendicular) the longitudinal direction of the signal electrode 2a by a distance greater than the width g of the gap of the coplanar electrode described above.
The width (dimension in a direction perpendicular to the longitudinal direction) w of the signal electrode 2a of the microstrip electrode is, for example, 100 μm or more, preferably 300 μm or more, for example, 800 μm or less, preferably 500 μm or less.

In the illustrated example, regardless of whether the conductor layer 2 is a coplanar electrode or a microstrip electrode, the signal electrode 2a extends across the entire waveguide element 100, but the longitudinal dimension of the signal electrode 2a can be any appropriate dimension as long as it is equal to or smaller than the dimension of the waveguide element in the waveguide direction. In addition, multiple signal electrodes may be provided in the waveguide element so as to be aligned in the waveguide direction.

In this specification, the term "waveguide element" encompasses 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.
The specific configuration of each component of the waveguide element will be described below in Sections B to H. A method for manufacturing the waveguide element will be described in Section I.

B. Inorganic Material Substrate As shown in Figures 1 and 2, the inorganic material substrate 1 has an upper surface on which the conductor layer 2 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 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.

The resistivity of the inorganic material substrate 1 is, for example, 100 kΩ·cm or more, preferably 300 kΩ·cm or more, more preferably 500 kΩ·cm or more, and even more preferably 700 kΩ·cm or more. If the resistivity is in this range, the electromagnetic waves can propagate through the material with low loss without affecting the electronic conduction. Although the details of this phenomenon are not clear, it can be assumed that if the resistivity is low, the electromagnetic waves bind with electrons and the energy of the electromagnetic waves is taken away by electronic conduction, resulting in loss. From this perspective, the higher the resistivity, the more preferable it is. The resistivity can be, for example, 3000 kΩ (3 MΩ)·cm or less.

The bending strength of the inorganic material substrate 1 is, for example, 50 MPa or more, and preferably 60 MPa or more. If the bending strength is in this range, the substrate is less likely to deform, so the pore diameter and pore period are stable, and a waveguide element with small changes in characteristics can be realized. The higher the bending strength, the more preferable it is. The bending strength can be, for example, 700 MPa or less. The bending strength can be measured in accordance with JIS standard R1601.

The thermal expansion coefficient (linear expansion coefficient) of the inorganic material substrate 1 is, for example, 10×10 ⁻⁶ /K or less, and preferably 8×10 ⁻⁶ /K or less. If the thermal expansion coefficient is in this range, thermal deformation (typically, warpage) of the substrate can be effectively suppressed. The thermal expansion coefficient can be measured in accordance with JIS standard R1618.

As described above, the dielectric loss tangent tan δ of the inorganic material substrate 1 is preferably as small as possible. One method for reducing the dielectric loss tangent (tan δ) of the inorganic material substrate 1 in the 300 GHz band is to reduce the concentration of OH groups contained in the inorganic material substrate. When the waveguide element 100 guides electromagnetic waves with frequencies of 250 GHz to 350 GHz, the concentration of OH groups in the inorganic material substrate is, for example, 100 wtppm or less, preferably 15 wtppm or less, and more preferably 10 wtppm or less. The concentration of OH groups in the inorganic material substrate may typically be 0 wtppm or more. The concentration of OH groups can be measured by FTIR (Fourier transform infrared spectroscopy), Raman scattering spectroscopy, or the Karl Fischer method.

The dielectric loss of the inorganic material substrate 1 can be evaluated by the FQ value. The FQ value is calculated by multiplying the inverse of the dielectric tangent (tan δ) by the frequency of the electromagnetic wave guided by the director element 100.
When the OH group concentration of the inorganic material substrate 1 is 100 wtppm or less, if the frequency of the electromagnetic waves is 150 GHz or more and less than 250 GHz, the FQ value is preferably 45,000 GHz or more, and if the frequency of the electromagnetic waves is 250 GHz or more and less than 350 GHz, the FQ value is preferably 75,000 GHz or more.
Furthermore, when the OH group concentration of the inorganic material substrate 1 is 15 wtppm or less, if the frequency of the electromagnetic wave is 150 GHz or more and less than 250 GHz, the FQ value is preferably 75,000 GHz or more, and if the frequency of the electromagnetic wave is 250 GHz or more and less than 350 GHz, the FQ value is preferably 105,000 GHz or more.
Furthermore, when the OH group concentration of the inorganic material substrate 1 is 10 wtppm or less, if the frequency of the electromagnetic waves is 150 GHz or more and less than 250 GHz, the FQ value is preferably 150,000 GHz or more and typically 270,000 GHz or less, and if the frequency of the electromagnetic waves is 250 GHz or more and less than 350 GHz, the FQ value is preferably 250,000 GHz or more and typically 390,000 GHz or less.

The porosity of the inorganic material substrate 1 is, for example, 0.5 ppm to 3000 ppm, preferably 0.5 ppm to 1000 ppm, more preferably 0.5 ppm to 100 ppm, for pores with a size of 1 μm or more. If the porosity is in this range, densification is possible, and further, due to the synergistic effect with the effect of setting the pore size within a predetermined range, a stable waveguide element can be realized in terms of both mechanical strength and long-term reliability. Furthermore, since the particle size can be made small, there is an advantage that the shape of the via hole described later can be made uniform without variation. Note that if the porosity exceeds 3000 ppm, the propagation loss in the waveguide may increase. It is difficult to make the porosity less than 0.5 ppm using technology that uses an inorganic material substrate.

The size of the pores refers to the diameter if the pores are approximately spherical, the diameter when viewed in a planar view if the pores are approximately cylindrical, and the diameter of a circle inscribed in the pores if the pores have any other shape. The presence or absence of pores can be confirmed, for example, by optical CT (Computed Tomography) or a transmittance meter. The size of the pores can be measured, for example, by a scanning electron microscope (SEM).

C. Conductive Layer The conductor layer 2 is located on the opposite side of the inorganic material substrate 1 to the second ground electrode 3, and is provided on the surface of the inorganic material substrate 1. The conductor layer 2 is typically in direct contact with the inorganic material substrate 1.
The conductor layer 2 is typically made of a metal. Examples of metals include chromium (Cr), nickel (Ni), copper (Cu), gold (Au), silver (Ag), palladium (Pd), and titanium (Ti). The metals can be used alone or in combination. The conductor layer 2 may be a single layer, or may be formed by laminating two or more layers. The conductor layer 2 is formed on the inorganic material substrate 1 by, for example, plating, sputtering, vapor deposition, or printing.
The thickness of the conductor layer 2 is, for example, 1 μm or more, preferably 4 μm or more, and, for example, 20 μm or less, preferably 10 μm or less.

D. Second Ground Electrode In one embodiment, the second ground electrode 3 is provided on the surface of the inorganic material substrate 1 opposite to the conductor layer 2. The second ground electrode 3 is disposed at a distance from the signal electrode 2a in the thickness direction of the inorganic material substrate 1. The second ground electrode 3 is typically in direct contact with the inorganic material substrate 1. The second ground electrode 3 can be made of the same metal as the conductor layer 2. From the viewpoint of bonding the inorganic material substrate 1 and the support substrate 7, the second ground electrode 3 needs to ensure an adhesion strength that makes it easy to flatten the bonding surface, and the metal of the second ground electrode 3 may be different from the metal of the conductor layer 2. The thickness range of the second ground electrode 3 is the same as the thickness range of the conductor layer 2.
The second ground electrode 3 is typically formed on the inorganic material substrate 1 by sputtering or plating.

E. Support Substrate The support substrate 7 can provide the waveguide element with excellent mechanical strength. This allows the thickness t 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 7. Specific examples of materials constituting the support substrate 7 include indium phosphide (InP), silicon (Si), glass, sialon (Si ₃ N ₄ -Al ₂ O ₃ ), mullite (3Al ₂ O ₃ ·2SiO ₂ , 2Al ₂ O ₃ ·3SiO ₂ ), aluminum nitride (AlN), magnesium oxide (MgO), aluminum oxide (Al ₂ O ₃ ), spinel (MgAl ₂ O ₄ ), sapphire, quartz, quartz, gallium nitride (GaN), silicon carbide (SiC), silicon nitride (Si ₃ N ₄ ), and gallium oxide (Ga ₂ O ₃ ).
The support substrate 7 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 examples of materials constituting the support substrate 7 from this viewpoint 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 7 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 7 is within a range of 50% to 150% of the linear expansion coefficient of the material constituting the inorganic material substrate 1.
In addition, it is preferable that the dielectric loss tangent of the material constituting the support substrate 7 is small. In the case of a coplanar line, when the thickness of the waveguide element is small, the propagating electromagnetic waves may seep into the support substrate, and the propagation loss can be suppressed by reducing the dielectric loss tangent. From this viewpoint, it is preferable that the dielectric loss tangent of the support substrate 7 is 0.07 or less.

The thickness of the support substrate 7 is, for example, 50 μm or more, preferably 100 μm or more, more preferably 150 μm or more, and, for example, 3000 μm or less, preferably 2000 μm or less, more preferably 300 μm or less. In one embodiment, the thickness of the support substrate 7 is greater than the thickness of the inorganic material substrate. More specifically, the thickness of the support substrate 7 is, for example, 1.1 times or more, preferably 1.5 times or more, and, for example, 30 times or less, preferably 10 times or less, and more preferably 5 times or less, relative to the thickness of the inorganic material substrate. If the thickness of the support substrate is equal to or greater than the lower limit, the mechanical strength of the waveguide element can be stably improved. If the thickness of the support substrate is equal to or less than the upper limit, the slab mode propagation can be suppressed, the waveguide element can be made thinner (the mechanical strength of the waveguide element can be maintained), and the substrate resonance can be suppressed.

The supporting substrate 7 supports the conductor layer 2, the inorganic material substrate 1, and the second ground electrode 3. More specifically, the supporting substrate 7 may be directly bonded to the inorganic material substrate 1 only via the second ground electrode 3, or may be directly bonded to the inorganic material substrate 1 via the second ground electrode 3 and a bonding portion (not shown).
In this specification, the term "direct bonding" means that two layers or substrates are bonded to each other 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 to each other.
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. Details of direct bonding will be described in Section I below.
When the supporting substrate 7 is joined to the inorganic material substrate 1 only via the second ground electrode 3, the second ground electrode 3 functions as a joining portion joining the inorganic material substrate 1 and the supporting substrate 7, and the supporting substrate 7 is in direct contact with the second ground electrode 3.
When the support substrate 7 is joined to the inorganic material substrate 1 via the second ground electrode 3 and the joint, the joint is provided between the second ground electrode 3 and the support substrate 7. The joint may be a single layer, or may be a laminate of two or more layers. Examples of the joint include a _SiO2 layer, an amorphous silicon layer, and a tantalum oxide layer. In addition, from the viewpoint of ensuring adhesion strength and preventing migration, a metal film of Ti, Cr, Ni, Pt, or Pd may be formed as an intermediate layer between the inorganic material substrate and the second ground electrode or between the support substrate and the second ground electrode. The thickness of the joint is, for example, 0.01 μm or more and 3 μm or less.

F. Third Ground Electrode In one embodiment, the third ground electrode 4 is provided on the surface of the support substrate 7 opposite to the second ground electrode 3. The third ground electrode 4 is disposed at a distance from the second ground electrode 3 in the thickness direction of the inorganic material substrate 1. The third ground electrode 4 is typically in direct contact with the support substrate 7. The third ground electrode 4 is made of the same metal as the conductor layer 2, and the thickness range of the third ground electrode 4 is the same as the thickness range of the conductor layer 2. The third ground electrode 4 is formed on the support substrate 7 by, for example, sputtering or plating. The third ground electrode 4 does not necessarily have to be formed on the entire surface of the support substrate 7 opposite to the second ground electrode.

G. First Vias In the director element 100, the first vias 5 are provided on both sides of the signal electrode 2a in a direction intersecting (preferably perpendicular to) the longitudinal direction of the signal electrode 2a. In the following, the first vias that electrically connect the first ground electrode 2b and the third ground electrode 4 may be referred to as the first via 5a, and the first vias that electrically connect the first ground electrode 2c and the third ground electrode 4 may be referred to as the first via 5b, and may be distinguished from each other.
2, the first via 5a is in contact with the first ground electrode 2b and the third ground electrode 4, and extends continuously between the first ground electrode 2b and the third ground electrode 4. The first via 5b is in contact with the first ground electrode 2c and the third ground electrode 4, and extends continuously between the first ground electrode 2c and the third ground electrode 4. Each of the first vias 5a and 5b penetrates the second ground electrode 3 and is in contact with the second ground electrode 3. Note that the director element may include only one of the first vias 5a and 5b.

The first via 5 is typically a conductive film. The first via 5 is made of a conductive material, typically made of the same metal as the conductive layer 2. The shape of the first via 5 corresponds to the shape of the first via hole 8 in which it is arranged. That is, the waveguide element 100 has a plurality of first via holes 8 corresponding to a plurality of first vias 5. In the illustrated example, the first via hole in which the first via 5a is arranged may be referred to as the first via hole 8a, and the first via hole in which the first via 5b is arranged may be referred to as the first via hole 8b, and may be distinguished from each other. The first via hole 8 penetrates the inorganic material substrate 1, the second ground electrode 3, and the support substrate 7. The first via hole 8 typically has a circular shape when viewed from the surface (upper surface) direction of the inorganic material substrate 1. When the first via hole has a circular shape, the inner diameter of the first via hole is, for example, 10 μm or more, preferably 20 μm or more, and, for example, 200 μm or less, preferably 100 μm or less, more preferably 80 μm or less.
2, the first via hole 8 has a circular shape when viewed from the surface (upper surface) direction of the inorganic material substrate 1, and penetrates the inorganic material substrate 1, the second ground electrode 3, and the support substrate 7 linearly along the surface (upper surface) direction of the inorganic material substrate 1. When the first via hole is circular and linear, the first via 5 has a columnar or cylindrical shape extending along the surface (upper surface) direction of the inorganic material substrate 1. In this case, the range of the outer diameter of the first via 5 is the same as the range of the inner diameter of the first via hole described above.
5, the first via hole 8 may have a circular shape when viewed from the surface (top) direction of the inorganic material substrate 1, and may have a tapered shape with a smaller diameter toward the second ground electrode 3. Although not shown, the first via hole 8 may have a circular shape when viewed from the surface (top) direction of the inorganic material substrate 1, and may have a tapered shape with a larger diameter toward the second ground electrode 3.
When the first via hole has a tapered shape, it is possible to provide features such as facilitating the formation of a conductive layer in the first via and facilitating the securing of strength of the substrate. In addition, the first via may be formed such that a conductive material is embedded in the first via hole.
When the first via hole is circular and tapered, the structure of the first via 5 is not particularly limited, but the first via 5 preferably has an hourglass shape in which the contact portion with the second ground electrode 3 has a small diameter and the diameter increases as it moves away from the second ground electrode 3. In other words, the first via 5 preferably has a shape in which the apexes of two cones are connected to each other. In this case, the maximum outer diameter of the first via 5 is within the above range. In one embodiment, the outer diameter of one end of the first via 5 that contacts the first ground electrodes 2b and 2c is smaller than the outer diameter of the other end of the first via 5 that contacts the third ground electrode 4. In the first via 5, the taper angle on the conductor layer 2 side with respect to the second ground electrode is smaller than the taper angle on the third ground electrode side with respect to the second ground electrode.
In the illustrated example, the first ground electrode and the third ground electrode are each formed to block the first via hole, but the configuration of the first ground electrode and the third ground electrode is not limited to this. Each of the first ground electrode and the third ground electrode only needs to be electrically connected to the first via, and may be open without blocking the first via hole.

4 and 5, in the director element 100, the multiple first vias 5a are arranged at intervals in the longitudinal direction of the signal electrode 2a. The direction in which the multiple first vias 5a are arranged is not limited to the longitudinal direction of the signal electrode 2a. As shown in FIG. 7, the multiple first vias 5a may be arranged at intervals in a direction intersecting (preferably perpendicular) the longitudinal direction of the signal electrode 2a. In other words, the director element may have multiple rows of the first vias 5a arranged in the longitudinal direction of the signal electrode 2a in a direction intersecting (preferably perpendicular) the longitudinal direction of the signal electrode 2a.
The pitch P1 of the multiple first vias 5a (the distance between the centers of adjacent first vias 5a) is, for example, 40 μm or more, preferably 60 μm or more, and, for example, 600 μm or less, preferably 400 μm or less, more preferably 200 μm or less.
The waveguide element 100 may also include a plurality of first vias 5b, similar to the first via 5a.

H. Second Via As shown in FIG. 3, in the director element 100, the second vias 6 are provided on both sides of the signal electrode 2a in a direction intersecting (preferably perpendicular to) the longitudinal direction of the signal electrode 2a. In the following, the second vias electrically connecting the first ground electrode 2b and the second ground electrode 3 may be referred to as the second via 6a, and the second vias electrically connecting the first ground electrode 2c and the third ground electrode 4 may be referred to as the second via 6b, and may be distinguished from each other. The second via 6a is in contact with the first ground electrode 2b and the second ground electrode 3, and is not in contact with the third ground electrode 4. The second via 6b is in contact with the first ground electrode 2c and the second ground electrode 3, and is not in contact with the third ground electrode 4. The director element may include only one of the second vias 6a and 6b.

The second via 6 is typically a conductive film. The second via 6 is made of a conductive material, typically the same metal as the first via 5. The shape of the second via 6 corresponds to the shape of the second via hole 9 in which it is arranged. That is, the waveguide element 100 has a second via hole 9 corresponding to the second via 6. In the illustrated example, the second via hole in which the second via 6a is arranged may be referred to as the second via hole 9a, and the second via hole in which the second via 6b is arranged may be referred to as the second via hole 9b, to be distinguished from each other.
The second via hole 9 penetrates at least the inorganic material substrate 1, but does not penetrate the support substrate 7. The second via hole 9 typically has a circular shape when viewed from the surface (upper surface) direction of the inorganic material substrate 1. When the second via hole has a circular shape, the range of the inner diameter of the second via hole is, for example, the same as the range of the inner diameter of the first via hole described above.
In Fig. 3, the second via hole 9 has a circular shape when viewed from the surface (upper surface) direction of the inorganic material substrate 1, and penetrates the inorganic material substrate 1 linearly along the surface (upper surface) direction of the inorganic material substrate 1. The second via hole 9 shown in Fig. 3 and Fig. 4 does not penetrate the second ground electrode 3. When the second via hole 9 is circular and linear, the second via 6 has a columnar or cylindrical shape extending along the thickness direction of the inorganic material substrate 1. In this case, the range of the outer diameter of the second via 6 is the same as the range of the inner diameter of the second via hole.
As shown in FIG. 5, the second via hole 9 may have a conical shape that tapers away from the conductor layer 2. The second via hole 9 shown in FIG. 5 penetrates the inorganic material substrate 1 and the second ground electrode 3, and its tip reaches the support substrate 7. When the second via hole 9 is conical, the structure of the second via 6 is not particularly limited, but the second via 6 is preferably conical in shape similar to the second via hole 9. In this case, the maximum outer diameter of the second via 6 is within the range of the inner diameter of the second via hole. In addition, the apex of the second via 6 (the end of the second via 6 opposite to the conductor layer 2) may reach the support substrate 7.
In the illustrated example, the first ground electrode is formed so as to block the second via hole, but the configuration of the first ground electrode is not limited to this. The first ground electrode only needs to be electrically connected to the second via, and may leave the second via hole open without blocking it.

4 to 7, the second vias 6 are disposed between adjacent first vias 5 among a plurality of first vias 5 arranged in a predetermined direction. The second vias 6 are typically located at the center of the interval between the adjacent first vias 5.
The illustrated waveguide element 100 has a plurality of second vias 6 (a plurality of second vias 6a and a plurality of second vias 6b). In the waveguide element 100 shown in Figs. 4 to 6, the second vias 6 are arranged between adjacent first vias 5 (first vias 5a or first vias 5b) arranged in the longitudinal direction of the signal electrode 2a. In the waveguide element 100 shown in Fig. 7, the second vias 6 are arranged between adjacent first vias 5 (first vias 5a or first vias 5b) arranged in a direction intersecting (preferably perpendicular to) the longitudinal direction of the signal electrode 2a.
Also, the second vias 6 can be arranged at any appropriate position between the first vias 5 adjacent to each other. The second vias 6 may be arranged every n first vias 5 in the direction in which the multiple first vias are arranged. n is, for example, 1 to 5, and preferably 1 or 2. More preferably, the first vias 5 and the second vias 6 are arranged alternately. Also, the multiple second vias 6 may all be arranged between the first vias 5 adjacent to each other as shown in FIG. 4 and FIG. 5, and may include a second via 6 that is not arranged between the first vias 5 as long as at least one second via 6 is arranged between the first vias 5 adjacent to each other as shown in FIG. 6.

The pitch P2 between adjacent first vias 5 and second vias 6 (the distance between the centers of adjacent first vias 5a and second vias 6a) is substantially 1/2 of the pitch P1 (the distance between the centers of adjacent first vias 5a), and is, for example, 25 μm or more, preferably 60 μm or more, and for example, 600 μm or less, preferably 400 μm or less, and more preferably 200 μm or less.
In this way, by arranging the second vias 6 between the adjacent first vias 5, the pitch P2 of the first vias 5 and the second vias 6 in the inorganic material substrate 1 can be made smaller than the pitch P1 of the first vias 5 in the support substrate 7. Therefore, even if the inorganic material substrate is thinned as described above, the strength of the inorganic material substrate can be sufficiently ensured.
5, if the first via hole 8 has a tapered shape that increases in diameter as it moves away from the second ground electrode 3, and the thickness of the support substrate 7 is greater than that of the inorganic material substrate 1, the outer diameter of the other end of the first via 5 that contacts the third ground electrode 4 may be larger than the outer diameter of the one end of the first via 5 that contacts the first ground electrodes 2b and 2c. In this case, if the pitch of the multiple first vias 5 is narrowed to the pitch P2 described above without providing the second vias 6, the other ends of the first vias 5 may interfere with each other. In contrast, in the waveguide element 100, the second vias 6 are arranged between the adjacent first vias 5, so that interference between the first vias 5 can be suppressed.

I. Method for Manufacturing a Director Element Next, a method for manufacturing the director element 100 will be described with reference to Figures 1 to 3. In one embodiment, the method for manufacturing the director element 100 includes the steps of: preparing a laminate 11 including an inorganic material substrate 1, a second ground electrode 3, and a support substrate 7 in this order, the laminate 11 having a first via hole 8 penetrating the inorganic material substrate 1, the second ground electrode 3, and the support substrate 7 together, and a second via hole 9 penetrating the inorganic material substrate 1 but not the support substrate 7; forming a first via 5 in the first via hole 8, forming a second via 6 in the second via hole 9, forming a third ground electrode 4 at the bottom of the support substrate 7, and forming a conductor layer 2 at the top of the inorganic material substrate 1;

When the inorganic material substrate 1 and the support substrate 7 are bonded via the second ground electrode 3, in order to prepare such a laminate 11, first, the metal constituting the second ground electrode 3 is sputtered on the surface of the inorganic material substrate 1 to form a first metal thin film as a first bonding portion. Furthermore, the metal constituting the second ground electrode 3 is sputtered on the surface of the support substrate 7 to form a second metal thin film as a second bonding portion. In addition, in terms of ensuring adhesion strength and preventing migration, a metal film of Ti, Cr, Ni, Pt, or Pd may be formed as an intermediate layer in the formation of the first metal thin film and the second metal thin film.
Next, in a high vacuum chamber (for example, about 1×10 ⁻⁶ Pa), a neutralizing beam is irradiated onto each of the bonding surfaces 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.
In this manner, the first metal thin film as the first bonding portion and the second metal thin film as the second bonding portion are directly bonded and integrated to form the second ground electrode 3. This results in a laminate 11 having a configuration of inorganic material substrate 1/second ground electrode 3/support substrate 7.

Also, when the inorganic material substrate 1 and the support substrate 7 are joined via the second ground electrode 3 and a joint, in order to prepare such a laminate 11, the above-mentioned second ground electrode 3 is formed on the surface of the above-mentioned inorganic material substrate 1 by sputtering. Next, the above-mentioned joint is formed on the second ground electrode 3 (more specifically, a Cr thin film having a thickness of 0.02 μm and an amorphous silicon layer having a thickness of 0.1 μm are formed in that order). After the film formation, a flattening process is performed, for example, by CMP polishing. Also, if necessary, a joint is formed on the support substrate in the same manner as described above.
Next, the inorganic material substrate and the support substrate are directly bonded in the same manner as above, thereby obtaining a laminate 11 having a structure of inorganic material substrate 1/second ground electrode 3/joint portion/support substrate 7.

Thereafter, for example, by laser processing (more specifically, processing using a laser having a wavelength of 515 nm and a pulse width of 10 ps), a first via hole 8 that penetrates the inorganic material substrate 1, the second ground electrode 3, and the support substrate 7 all at once, and a second via hole 9 that penetrates the inorganic material substrate 1 but not the support substrate 7 are formed.
Next, a base metal thin film (more specifically, a Ti thin film having a thickness of 0.15 μm and a palladium thin film having a thickness of 0.04 μm are sequentially formed) is deposited on the sidewall portions in the first via hole 8 and the second via hole 9, the front surface of the inorganic material substrate 1, and the rear surface of the support substrate 7, for example, by an ICP (inductively coupled plasma) sputtering device, and then a surface metal thin film (for example, a copper thin film having a thickness of 1.5 μm) is formed, for example, by plating (more specifically, electrolytic plating).
Thereafter, a resist is applied to the surface of the inorganic material substrate 1, and the resist is patterned by photolithography so as to expose the portions that form the gaps in the conductor layer 2 and mask the other portions. Thereafter, the conductor layer 2 (coplanar electrodes or microstrip electrodes) is formed by, for example, wet etching (more specifically, ferric chloride water).
As a result, a conductor layer is formed on the upper part of the inorganic material substrate 1, a first via 5 is formed in the first via hole 8, a second via 6 is formed in the second via hole 9, and a third ground electrode 4 is formed on the lower part of the support substrate 7. Therefore, compared with the case where the conductor layer, the first via, the second via, and the third ground electrode are formed separately, the waveguide element can be manufactured smoothly. Then, the resist is removed. In the above embodiment, after the first via 5, the second via 6, and the third ground electrode 4 are formed, the conductor layer 2 is formed by etching, but the present invention is not limited to this. The first via 5, the second via 6, the third ground electrode 4, and the conductor layer 2 can also be formed simultaneously.
In this manner, the waveguide element 100 can be manufactured.
In the above, the step of forming the first via hole and the second via hole after forming the laminate and preparing the laminate having the first via hole and the second via hole has been described in detail, but the step of preparing the laminate is not limited to this. After forming the second via hole in advance in the inorganic material substrate, holes can be formed in each of the inorganic material substrate, the second ground electrode, and the support substrate, and the inorganic material substrate, the second ground electrode, and the support substrate can be bonded so that the holes communicate with each other to form the first via hole.

The present invention will be specifically explained below with reference to the following reference examples, but the present invention is not limited to these examples.

<Reference Example 1>
1-1. Fabrication of a waveguide element (coplanar line) 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. Here, 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 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.

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 coplanar electrode, 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 above.
Next, an RF signal generator was coupled to the input side of the director element by a probe, and an electromagnetic wave was coupled to an RF signal receiver by installing a probe on the output side of the director element.
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 element). 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

<Reference Example 2>
2-1. Preparation of a waveguide element (coplanar line with ground) 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 second ground electrode. Next, a 0.2 μm thick amorphous silicon film was formed on the second ground electrode by sputtering. After the film formation, the amorphous silicon film was polished and flattened. Here, the arithmetic mean roughness of the surface of the amorphous silicon film over 10 μm square 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 Reference Example 1. In the obtained quartz glass/second 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 Reference Example 1. The length of the signal electrode in the waveguiding direction was 10 mm.
In this manner, a waveguide element including the coplanar electrode, the inorganic material substrate, the second ground electrode, and the supporting substrate was obtained.

2-2. Calculation of Propagation Loss In addition, 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, as in Reference Example 1, the RF power of the electromagnetic wave output from the coplanar line was measured by an RF signal receiver. The propagation loss of the director element of Reference Example 2 was evaluated in the same manner as Reference Example 1. The results are shown in Table 1.

<Reference Example 3>
3-1. Fabrication of a Waveguide Element (Microstrip Line) In the same manner as in Example 2, a quartz glass/second 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 including a microstrip electrode, an inorganic material substrate, and a supporting substrate was obtained.

3-2. Calculation of Propagation Loss In order to measure the propagation loss of the director element, three director elements with microstrip electrodes of 30 mm, 40 mm, and 50 mm in length 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 Reference Example 1. The propagation loss of the director element of Reference Example 3 was evaluated in the same manner as in Reference Example 1. The results are shown in Table 1.

<Reference Examples 4 to 6>
Waveguide elements were fabricated in the same manner as in each of Reference 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 Reference Example 1. The results are shown in Table 1.

<Reference Example 7>
A waveguide element was fabricated in the same manner as in Reference 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 Reference Example 1. The results are shown in Table 1.

<Reference Example 8>
A waveguide element was fabricated in the same manner as in Reference 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 Reference Example 1. The results are shown in Table 1.

<Reference Example 9>
A waveguide element was fabricated in the same manner as in Reference 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 AlN wafer after polishing 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 Reference Example 1. The results are shown in Table 1.

<Reference 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 Reference Example 1. The results are shown in Table 1.

<Reference 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 Reference Example 1. The results are shown in Table 1.

<Reference Example 15>
A waveguide element was fabricated in the same manner as in Example 2, except that the thickness of the polished quartz glass wafer was changed to 300 μm.
For the obtained waveguide element, the propagation loss was calculated and evaluated in the same manner as in Reference Example 1. The results are shown in Table 1.

<Reference Example 16>
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 Reference 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 Reference 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), the propagation loss when guiding high-frequency electromagnetic waves exceeding 30 GHz is relatively small.

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 conductive layer 2a signal electrode 2b, 2c first ground electrode 3 second ground electrode 4 third ground electrode 5 via 8 via hole 11 laminate

Claims (12)

A waveguide element capable of guiding an electromagnetic wave having a frequency of 30 GHz or more and 20 THz or less,
An inorganic material substrate;
a conductor layer provided on an upper portion of the inorganic material substrate, the conductor layer including a signal electrode extending in a predetermined direction and a first ground electrode disposed at an interval from the signal electrode in a direction intersecting the predetermined direction;
a support substrate located on the opposite side of the inorganic material substrate from the conductor layer;
a second ground electrode located between the inorganic material substrate and the support substrate;
a third ground electrode located on the opposite side of the support substrate from the second ground electrode;
a first via that electrically connects the first ground electrode and the third ground electrode and is also electrically connected to the second ground electrode;
a second via electrically connecting the first ground electrode and the second ground electrode;
the waveguide element includes a plurality of the first vias,
the second via is disposed between adjacent first vias among the plurality of first vias,
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 element; ε represents the relative dielectric constant of the inorganic material substrate; and a represents a 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 tanδ 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 any one of claims 1 to 4, wherein the conductor layer and the second ground electrode are microstrip type electrodes. The waveguide element according to claim 5 or 6, wherein the thickness of the inorganic material substrate is 10 μm or more when the frequency of the electromagnetic waves propagating through the waveguide element is 30 GHz or more and 5 THz or less. The waveguide element according to any one of claims 1 to 7, comprising a plurality of first via holes in which the first vias are disposed, the first via holes penetrating the inorganic material substrate, the second ground electrode, and the support substrate. The waveguide element according to claim 8, wherein the first via hole has a circular shape when viewed from the surface (top) direction of the inorganic material substrate, and has a tapered shape that becomes smaller in diameter as it approaches the second ground electrode. The waveguide element according to claim 8, wherein the first via hole has a circular shape when viewed from the surface (top) direction of the inorganic material substrate, and has a tapered shape that becomes larger in diameter as it approaches the second ground electrode. A waveguide element according to any one of claims 8 to 10, comprising a second via hole in which the second via is disposed, the second via hole penetrating the inorganic material substrate and not penetrating the support substrate. A method for manufacturing a waveguide element according to claim 11, comprising the steps of:
preparing a laminate including the inorganic material substrate, the second ground electrode, and the support substrate in this order, the laminate having a first via hole penetrating the inorganic material substrate, the second ground electrode, and the support substrate collectively, and a second via hole penetrating the inorganic material substrate but not the support substrate;
forming the first via in the first via hole, forming the second via in the second via hole, forming the third ground electrode on the lower part of the support substrate, and forming the conductor layer on the upper part of the inorganic material substrate.

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