Transmission line

Abstract

Provided is a transmission line including: a substrate composed of a dielectric or a semi-insulating substrate and having a groove formed on a main surface; conductive layers formed on the surfaces of at least both side surfaces of the groove; a first dielectric embedded in the groove so as to be in contact with the conductive layers; a second dielectric embedded, on the first dielectric, in the groove so as to be in contact with the conductive layers; a signal line disposed between the first dielectric and the second dielectric; a ground conductor layer formed on the second dielectric and forming a microstrip line with the signal line; and a groove part formed on an upper surface of the second dielectric along the signal line. The characteristic impedance value of the microstrip line can be adjusted by adjusting the width and depth of the groove part.

Description

Transmission Line

[0001] The present disclosure relates to a transmission line with high isolation for microwave / millimeter wave integrated circuits.

[0002] With the increase in operating frequency of modules in recent years, consideration of electrical design such as characteristic impedance has become increasingly important for high-frequency transmission line substrates. For example, electromagnetic and electromagnetic wave behaviors such as reflection and crosstalk propagating through signal lines become prominent, so countermeasures for matching and reflection noise are required. In particular, parallel wiring in integrated circuits is composed of many adjacent wires, and the problem of radio wave interference, so-called crosstalk noise, has become prominent.

[0003] In addition, cost reduction has become an important factor along with the increase in operating frequency, and high-density integration is required for this purpose. However, the higher the integration density, the narrower the distance between lines, and the aforementioned problems such as crosstalk between transmission lines become more prominent.

[0004] For propagating high-speed signals on high-frequency transmission line substrates, microstrip lines, coplanar lines, grounded coplanar lines and the like are used as typical transmission line structures. For example, a microstrip line forms a transmission line by forming a ground conductor of a planar conductive layer on one main surface of a dielectric substrate, and forming a strip-shaped signal line on the other main surface. The characteristic impedance of these transmission lines is determined by the width and thickness of the signal line, the permittivity and thickness of the dielectric substrate, and the geometric dimensions of the distance between the signal line and the ground conductor.

[0005] It is also known that higher-order electromagnetic modes occur in microstrip lines when operating in an ultra-high frequency band. To avoid this, it is necessary to reduce the width of the strip line and the thickness of the substrate, especially to make the substrate thickness extremely thin, which causes a problem that the manufacturing yield and the reliability of the integrated circuit are significantly deteriorated.

[0006] To resolve the above-mentioned problems, a conventional example is a structure in which, as shown in Figure 1, the transmission line is formed on a dielectric material for radio wave confinement in which a ground wall is placed on the side so as to surround the transmission line, in addition to the ground conductor that is present in a normal microstrip line (Patent Document 1).

[0007] Figure 1 is a cross-sectional view of a conventional transmission line. Figure 1(a) is a diagram illustrating the structure of a conventional example, and Figure 1(b) is a diagram illustrating the structure with an increased signal line width, illustrating the problems of the conventional example. In Figure 1, an example is shown with two transmission lines, but since the two transmission lines have the same structure, only one transmission line will be given a reference numeral for explanation.

[0008] In Figures 1(a) and 1(b), the conventional transmission line 10 comprises two transmission lines consisting of a substrate 11 made of a dielectric or semi-insulating conductor, a ground conductor 12 formed on one main surface of the substrate 11, and a microstrip line 13 (hereinafter also referred to as the "signal line") formed on the other main surface of the dielectric substrate 11.

[0009] Grooves 14 are formed on the other main surface of the substrate 11, and metal films 15 are formed along the surface of each groove 14. A dielectric material 16 for electromagnetic wave confinement is embedded in the grooves 14, and the microstrip line 13 is positioned on the upper surface of the dielectric material 16.

[0010] This transmission line 10 has excellent electromagnetic wave confinement effect (high isolation) because the metal film 15 exists as a ground wall on the left and right sides 14a of the groove. In addition, the transmission line 10 makes it easier to avoid the generation of higher-order electromagnetic wave modes by controlling the thickness of the dielectric 16 for electromagnetic wave confinement, which makes it possible to make the distance from the ground wall at the bottom of the groove considerably smaller than the thickness of the substrate.

[0011] However, in the conventional structure, when the width of the signal line 13 is increased as shown in Figure 1(b), the distance between the signal line 13 and the ground walls located on the left and right sides 14a of the groove becomes shorter. As a result, the electromagnetic coupling between the signal line and the ground walls strengthens, causing the characteristic impedance of the transmission line 10' to become extremely low. For this reason, the signal line width was limited in the conventional structure.

[0012] If there are limitations on the signal line width, restrictions will be placed on the allowable current of the transmitted signal. Furthermore, since the characteristic impedance value cannot be adjusted by the signal line width, it becomes difficult to easily change the characteristic impedance, making it impractical.

[0013] Furthermore, the conditions that determine the characteristic impedance of a transmission line now include not only the conventional width and thickness of the signal line, the dielectric constant and thickness of the dielectric substrate, and the distance between the signal line and the ground conductor, but also the distance between the signal line and the ground walls on the left and right sides of the groove. This has led to a problem where characteristic impedance fluctuations due to manufacturing errors are likely to occur (tolerances have a large effect).

[0014] Japanese Patent Application Publication No. 08-125412

[0015] This disclosure aims to resolve the above-mentioned problems and, in part, provides a transmission line that has high isolation, relaxes the limitations on signal line width, and further reduces the influence of tolerances by facilitating adjustment of characteristic impedance values.

[0016] An embodiment of the present disclosure for solving the above problems is a transmission line comprising a substrate made of a dielectric or semi-insulating substrate and having grooves formed on its main surface; a conductive layer formed on the surface of at least both sides of the grooves; a first dielectric embedded in the groove in contact with the conductive layer; a second dielectric embedded on the first dielectric in contact with the conductive layer within the groove; a signal line disposed between the first dielectric and the second dielectric; a ground conductor layer formed on the second dielectric and forming the signal line and a microstrip line; and a groove formed on the upper surface of the second dielectric along the signal line.

[0017] In the transmission line structure according to the embodiment of this disclosure, the electromagnetic coupling between the conductive layer (ground wall) formed on the surface of both sides of the groove and the signal line is relaxed by adjusting the width and depth of the groove formed along the signal line on the upper surface of the second dielectric, thereby relaxing the constraints on the signal line width. Furthermore, this increases the degree of freedom in setting the characteristic impedance value compared to conventional microstrip lines.

[0018] Furthermore, since the groove is formed on the upper surface of the second dielectric along the signal line and directly above the signal line, the groove can be formed in the final step of the manufacturing process. This provides the excellent effect of being able to form a transmission line with predetermined high-frequency characteristics even if impedance fluctuations occur due to manufacturing tolerances.

[0019] Figure 1 is a cross-sectional view showing the schematic configuration of a conventional transmission line, where (a) is a diagram to show the structure of a conventional example, and Figure 1(b) is a diagram showing the structure with an increased signal line width to explain the problems of the conventional example. Figure 2 is a top view showing the schematic configuration of the transmission path according to this embodiment. Figure 3 is a cross-sectional view showing the schematic configuration of the transmission line according to this embodiment, and is a cross-sectional view taken along line III-III in Figure 2. Figure 4 is a perspective view showing the schematic configuration of the transmission path according to this embodiment, and is a perspective view taken along line III-III in Figure 2.

[0020] Embodiments of this disclosure will be described in detail below with reference to the drawings. Identical or similar reference numerals indicate identical or similar elements, and repeated descriptions may be omitted. The following description is illustrative, and embodiments with some configurations modified are possible without departing from the gist of this disclosure.

[0021] This embodiment is a transmission line comprising a substrate made of a dielectric or semi-insulating substrate, having grooves formed on its main surface; a conductive layer formed on the surface of at least both sides of the grooves; a first dielectric embedded in the groove in contact with the conductive layer; a second dielectric embedded on the first dielectric in contact with the conductive layer within the groove; a signal line disposed between the first dielectric and the second dielectric; a ground conductor layer formed on the second dielectric to form the signal line and a microstrip line; and a groove formed on the upper surface of the second dielectric along the signal line.

[0022] Figure 2 is a top view showing a schematic configuration of the transmission path in the embodiment of this model, Figure 3 is a cross-sectional view taken along line III-III in Figure 2, and Figure 4 is a perspective view taken along line III-III in Figure 2.

[0023] Figures 2 to 4 show a transmission line 20 that is an embodiment of the present disclosure. Similar to the conventional example shown in Figure 1, the transmission line 20 of this embodiment illustrates a case in which two identical transmission lines are provided. Since the two transmission lines in Figures 2 to 4 have the same structure, only one will be given a reference numeral and described, and the description of the other will be omitted.

[0024] The transmission line 20 has grooves 22 formed on the main surface of a dielectric substrate 21 made of benzocyclobutene (BCB), and signal lines 23 are arranged within the grooves 22. In the embodiment shown in Figures 2 to 4, two transmission lines 20 are shown side by side, but there may be two or more transmission lines. Also, in this embodiment, the signal lines 23 of each transmission line are described as a single line, but there may be two differential lines.

[0025] In this embodiment, the substrate constituting the transmission line is a dielectric substrate such as benzocyclobutene. However, in addition to benzocyclobutene, fluororesins such as polytetrafluoroethylene and polychlorotrifluoroethylene may be used as the material for the dielectric substrate, or epoxy resins, polyimide resins, alumina, quartz, etc. may also be used. Furthermore, ceramic materials such as sapphire may be used. In addition, a semiconductor substrate made of materials such as GaAs and Si can be used instead of the dielectric substrate. The signal line 23 arranged in the groove 22 is formed of Au, but the signal line 23 can be made of any material with good conductivity, and in addition to Au, for example, Cu, Al, Ag, etc. may be used.

[0026] A conductive layer 24 made of a conductive material is formed on the main surface of the substrate 21 along each groove 22, and the conductive layer 24 is also formed on the left and right side surfaces 22a of the grooves 22. The conductive layer 24 formed on these side surfaces 22a functions as a ground wall (hereinafter, the conductive layer 24 formed on the side surfaces 22a will be referred to as the ground wall 24a). The conductive layer 24 may also be formed on the bottom surface of the grooves 22, but in that case, it is desirable to make the thickness of the second dielectric greater than that of the first dielectric.

[0027] In this embodiment, the conductive layer 24 is formed of a metal film such as Au, but any material with good conductivity may be used. In addition to Au, for example, Cu, Al, Ag, etc. may be used.

[0028] A first dielectric 25 is embedded in the groove 22, and the first dielectric 25 is in contact with the left and right ground walls 24a. A second dielectric 26 is embedded on each of the first dielectrics 25 in the groove 22, and the second dielectrics 26 are also in contact with the left and right ground walls 24a.

[0029] In this embodiment, the dielectric constant ε0 of the dielectric substrate 21, the dielectric constant ε1 of the first dielectric 25, and the dielectric constant ε2 of the second dielectric 26 are all different. However, their dielectric constants may be the same, or any two of them may be the same. Furthermore, if their dielectric constants are the same, the same dielectric material may be used, or different dielectric materials may be used.

[0030] If the dielectric constants of the two dielectric layers embedded in the groove 22 are different, the degree of freedom in adjusting the characteristic impedance can be increased compared to when materials with the same dielectric constant are used.

[0031] As materials for the first and second dielectrics, fluororesins such as benzocyclobutene, polytetrafluoroethylene, and polychlorotrifluoroethylene may be used, or epoxy resins, polyimide resins, alumina, quartz, etc. may be used.

[0032] The signal lines 23, which are placed within each groove 22, are positioned between the first dielectric 25 and the second dielectric 26. In this embodiment, the signal lines 23 are positioned on the upper surface of the first dielectric 25, but the signal lines may also be positioned so as to be embedded in the first dielectric 25.

[0033] A groove 27 is formed on the upper surface of the second dielectric 26 along the signal line 23. A ground conductor layer 28 is formed on the upper surface of the second dielectric 26 including the groove 27. In this transmission line 20, a microstrip line is formed by the ground conductor 28a, which is made up of the ground conductor layer formed on the bottom surface of the groove 27 of the second dielectric 26, and the signal line 23.

[0034] Therefore, the depth and width of the groove 27 formed on the upper surface of the second dielectric 26 determine the distance between the signal line 23 and the ground conductor 28a, as well as the magnitude of electromagnetic coupling. In this embodiment, the depth and width of the groove 27 are set such that the characteristic impedance with respect to the transmission path 20 is adjusted to a predetermined value.

[0035] In this embodiment, the ground conductor layer 28 is also formed of a metal film such as Au, similar to the conductor layer 24. However, any material with good conductivity is acceptable, and in addition to Au, for example, Cu, Al, Ag, etc., may be used. In this embodiment, a conductive layer 29 is provided on the main surface (bottom surface) of the dielectric substrate 21 opposite to the main surface on which the signal lines are formed. However, the conductive layer 29 may or may not be present.

[0036] As described above, according to the embodiment of this disclosure, by adjusting the width and depth of the groove 27, the electromagnetic coupling between the ground conductor 28a and the signal line 23 can be strengthened, thereby easing the electromagnetic coupling between the signal line 23 and the ground wall 24a, and thus the constraints on the signal line width can be eased. Therefore, the transmission path of this embodiment can have a greater degree of freedom in setting the characteristic impedance value compared to conventional ones.

[0037] Furthermore, in the transmission line structure according to this embodiment, the distance between the signal line and the ground conductor and the width of the ground conductor 28a, which constitute the signal line and the microstrip line, can be adjusted by adjusting the width and depth of the groove formed on the second dielectric at the end of the transmission line manufacturing process.

[0038] Therefore, even if impedance fluctuations occur due to manufacturing tolerances during the manufacturing process of the transmission line according to this embodiment, the characteristic impedance can be adjusted afterward, making it easy to form a transmission line with the desired high-frequency characteristics.

[0039] In the above embodiment, we described an example where each transmission line has one signal line (single line), but a differential line with two signal lines is also acceptable.

[0040] Furthermore, although the above embodiment was described using two transmission lines, there may be two or more transmission lines.

[0041] According to this disclosure, it is possible to provide a transmission line applicable to high-density mounting that has high isolation, achieving both improved wiring density and reduced crosstalk noise between wirings, while also relaxing the limitations on signal line width, allowing for adjustment of characteristic impedance values, and enabling smaller tolerances.

[0042] 20...Transmission line 21...Substrate 22...Groove 22a...Side surface 23...Signal line 24...Conductor layer 24a...Ground wall 25...First dielectric 26...Second dielectric 27...Groove section 28...Ground conductor layer 28a...Ground conductor

Claims

1. A transmission line comprising: a substrate made of a dielectric or semi-insulating substrate and having grooves formed on its main surface; a conductive layer formed on the surface of at least both sides of the grooves; a first dielectric embedded in the groove in contact with the conductive layer; a second dielectric embedded on the first dielectric in the groove in contact with the conductive layer; a signal line disposed between the first dielectric and the second dielectric; a ground conductor layer formed on the second dielectric and forming a microstrip line with the signal line; and a groove formed on the upper surface of the second dielectric along the signal line.

2. The transmission line according to claim 1, characterized in that the signal line is a single line.

3. The transmission line according to claim 1, characterized in that the signal line is a differential line.

4. The transmission line according to claim 1, characterized in that the conductive layer and / or the ground conductor layer are formed of a metallic material.

5. The transmission line according to claim 1, characterized in that the dielectric constant of the first dielectric and the dielectric constant of the second dielectric are different.

6. The transmission line according to claim 1, characterized in that the conductivity of the first dielectric and the conductivity of the second dielectric are the same.

7. The transmission line according to any one of claims 1 to 6, characterized in that the characteristic impedance value of the microstrip line can be adjusted by adjusting the depth and width of the groove.

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