Multilayer substrate and antenna module

Abstract

A multilayer substrate according to an embodiment of the present technology is provided with a plurality of conductor layers and an interlayer connection section. The plurality of conductor layers has a line conductor layer having a coplanar line and a pair of adjacent conductor layers neighboring the line conductor layer. The interlayer connection section has a plurality of columnar conductors connecting the plurality of conductor layers to each other. The multilayer substrate is such that, in a frequency band used by the coplanar line, a path of a return current is configured such that transmission loss due to the return current flowing through the columnar conductors between the coplanar line and the pair of adjacent conductor layers is prevented.

Description

Multilayer Substrate and Antenna Module

[0001] The present technology relates to a multilayer substrate provided with a coplanar line and an antenna module.

[0002] As a signal line for transmitting a high-frequency signal, a coplanar line in which a ground conductor is provided so as to sandwich a signal line in the same conductor layer is known. For example, Patent Document 1 describes a millimeter-wave band communication module including a matching circuit composed of a coplanar line formed on the surface of a silicon substrate. Patent Document 2 also describes a coplanar line provided on the surface of a silicon substrate and provided with a recess between the signal line and the ground conductor to suppress attenuation due to electromagnetic wave leakage.

[0003] Japanese Patent Application Laid-Open No. 2000-68714 Japanese Patent Application Laid-Open No. 2010-81487

[0004] In recent years, a method of providing a coplanar line on a multilayer substrate for wiring a communication module or the like has also been developed. Generally, a multilayer substrate is provided with a protective film such as a solder resist or a gold plating for protecting the surface. However, depending on the configuration of the multilayer substrate, it is conceivable that the protective film or the internal conductor may affect the transmission characteristics of the coplanar line.

[0005] In view of the above circumstances, an object of the present technology is to provide a multilayer substrate and an antenna module that exhibit highly reliable and excellent transmission characteristics.

[0006] To achieve the above object, a multilayer substrate according to one embodiment of the present technology includes a plurality of conductor layers and an interlayer connection portion. The plurality of conductor layers include a line conductor layer having a coplanar line and a pair of adjacent conductor layers adjacent to the line conductor layer. The interlayer connection portion has a plurality of columnar conductors that connect the plurality of conductor layers to each other.In the use band of the coplanar line, the path of the return current is configured so that transmission loss due to the return current flowing between the coplanar line and the pair of adjacent conductor layers through the columnar conductor is suppressed.

[0007] In this multilayer substrate, a line conductor layer having a coplanar line is provided between a pair of adjacent conductor layers, and each conductor layer is connected via columnar conductors. Furthermore, the return current path is configured so that transmission loss due to return current flowing through the columnar conductors is suppressed within the bandwidth of the coplanar line. As a result, since the line conductor layer is an inner layer, even if a protective film is provided, for example, it has little impact on the transmission characteristics of the coplanar line. In addition, transmission loss due to return current is suppressed, and degradation of transmission characteristics is avoided. This makes it possible to achieve highly reliable and excellent transmission characteristics.

[0008] One of the pair of adjacent conductor layers may have a first conductor portion facing the coplanar line. In this case, the line conductor layer may be arranged adjacent to the one adjacent conductor layer and may have a signal line forming the coplanar line and a second conductor portion surrounding the signal line. The interlayer connection portion may also have a plurality of first columnar conductors connecting the line conductor layer and the other adjacent conductor layer of the pair of adjacent conductor layers. The plurality of first columnar conductors may also be arranged at a predetermined distance or more from the signal line.

[0009] The predetermined distance may be set such that the return current flowing between the signal line and the other adjacent conductor layer via the first columnar conductor is substantially zero.

[0010] The multilayer substrate may further include a core material disposed between the line conductor layer and the other adjacent conductor layer. In this case, the plurality of first columnar conductors may be through vias penetrating the core material.

[0011] The predetermined distance may be greater than or equal to the thickness of the core material.

[0012] The interlayer connection section has a plurality of second columnar conductors that connect the line conductor layer and the one adjacent conductor layer.

[0013] The plurality of second columnar conductors may include a plurality of columnar conductors for the line arranged to surround the signal line. In this case, the plurality of first columnar conductors may be arranged so as not to overlap with the plurality of columnar conductors for the line.

[0014] The aforementioned adjacent conductor layer may be the outermost conductor layer. In this case, the multilayer substrate may further include a protective film provided on the outside of the aforementioned adjacent conductor layer.

[0015] The protective film may be at least one of a gold film layer with a nickel underlayer, or a solder resist.

[0016] The aforementioned adjacent conductor layer may have a connecting pad. In this case, the plurality of second columnar conductors may include connecting columnar conductors that connect the connecting pad to the signal line.

[0017] One of the adjacent conductor layers may have an RF element mounted on it that outputs or detects a high-frequency signal. In this case, the connection pad may be connected to the connection terminal of the RF element.

[0018] The columnar conductor for connection may have the connection pad connected to the signal line directly below the connection terminal.

[0019] The interlayer distance between the line conductor layer and at least one of the pair of adjacent conductor layers may be set to suppress transmission loss due to the return current.

[0020] The multilayer substrate may further include an antenna section having a power supply section connected to the signal line of the coplanar line.

[0021] The aforementioned coplanar line may also be a transmission line for millimeter-wave RF signals.

[0022] An antenna module according to one embodiment of this technology comprises a plurality of conductor layers, an interlayer connection section, and an antenna element. The plurality of conductor layers include a line conductor layer having a coplanar line and a pair of adjacent conductor layers adjacent to the line conductor layer. The interlayer connection section has a plurality of columnar conductors connecting the plurality of conductor layers to each other. The antenna section has a power supply section connected to the signal line of the coplanar line. The multilayer substrate is configured such that the path of the return current is such that transmission loss due to the return current flowing through the columnar conductors between the coplanar line and the pair of adjacent conductor layers is suppressed.

[0023] This is a perspective view showing an example of a multilayer substrate according to the first embodiment of this technology. This is a schematic diagram for explaining the layer structure of the multilayer substrate. This is a schematic diagram showing the pattern of a coplanar transmission line. This is a perspective view of a multilayer substrate with signal lines on the surface layer side. This is a graph showing another example of the transmission characteristics of the multilayer substrate shown in Figure 4. This is a graph showing another example of the transmission characteristics of the multilayer substrate shown in Figure 4. This is a perspective view of a multilayer substrate with signal lines on the inner layer side. This is a graph showing an example of the transmission characteristics of the multilayer substrate shown in Figure 7. This is a schematic diagram explaining the path of the return current between the signal line and the ground conductor. This is a perspective view for explaining an example of the arrangement of inner columnar conductors in the multilayer substrate of this embodiment. This is a graph showing an example of the transmission characteristics of the multilayer substrate of this embodiment. This is a graph showing the measurement results of the transmission characteristics of the multilayer substrate of this embodiment. This is a plan view showing an example of the surface layer configuration of an antenna module according to the second embodiment. This is a plan view showing an example of the inner layer configuration of the antenna module shown in Figure 13.

[0024] The embodiments of this technology will be described below with reference to the drawings.

[0025] <First Embodiment> [Multilayer Substrate Configuration] Figure 1 is a perspective view showing an example of a multilayer substrate according to the first embodiment of this technology. Figure 2 is a schematic diagram illustrating the layer structure of the multilayer substrate. Figure 3 is a schematic diagram showing the pattern of a coplanar transmission line.

[0026] The multilayer substrate 100 is a laminated substrate on which a coplanar transmission line 10 is formed. The coplanar transmission line 10 is configured as a transmission line for transmitting, for example, a high-frequency signal (RF signal), and the multilayer substrate 100 is applied to a substrate on which an RF element that outputs or detects an RF signal is mounted. The substrate on which the RF element is mounted becomes an RF module that performs object detection or communication using electromagnetic waves corresponding to the RF signal.

[0027] In this embodiment, the coplanar line 10 is configured as a transmission line for millimeter-wave RF signals. In this case, the multilayer substrate 100 becomes, for example, a millimeter-wave module that performs object detection or communication using millimeter waves. The frequency of the RF signal transmitted by the coplanar line 10 is not limited, and this technology can also be applied to substrates that transmit frequency bands other than millimeter waves.

[0028] Figures 1 and 2 illustrate an example of a multilayer substrate 100 on which a coplanar transmission line 10 is formed. The multilayer substrate 100 has a plurality of conductor layers 12, a plurality of insulating layers 13, interlayer connection portions 14, and a protective film 15. Figure 1A shows the multilayer substrate 100 as viewed from the side of the uppermost conductor layer 12 (first conductor layer L1) among the plurality of conductor layers 12, and the protective film 15 is omitted. Figure 1B shows the state in which the first conductor layer L1 has been removed and the conductor layer 12 (second conductor layer L2) located in the lower layer is exposed.

[0029] The multilayer substrate 100 has a longitudinal shape that extends in one direction as a whole. The coplanar lines 10 are formed along the longitudinal direction. In the following, the planes parallel to each layer in the multilayer substrate 100 are referred to as the XY planes, the direction along the coplanar lines 10 (longitudinal direction) is referred to as the X direction, and the direction perpendicular to the X direction in each layer is referred to as the Y direction. Furthermore, the direction perpendicular to the XY planes in the multilayer substrate 100, i.e., the stacking direction of the multilayer substrate 100, is referred to as the Z direction.

[0030] First, the layer structure of the multilayer substrate 100 will be described with reference to Figure 2. Each of the multiple conductor layers 12 is made of a conductor material and forms wiring and conductor patterns. Typically, copper is used as the conductor material, but other metals may also be used. As shown in Figure 2, the multiple conductor layers 12 have a first conductor layer L1, a second conductor layer L2, a third conductor layer L3, and a fourth conductor layer L4. The first conductor layer L1, the second conductor layer L2, the third conductor layer L3, and the fourth conductor layer L4 are stacked in this order.

[0031] In the following, the side of the multilayer substrate 100 on which the first conductive layer L1 is provided will be referred to as the upper side, and the side on which the fourth conductive layer L4 is provided will be referred to as the lower side. Here, the terms "upper side" and "lower side" are used to explain the relative positional relationship in the multilayer substrate, but this does not limit the orientation in which the multilayer substrate 100 is used.

[0032] The multiple conductor layers 12 include a line conductor layer having a coplanar line 10 and a pair of adjacent conductor layers adjacent to the line conductor layer. In this embodiment, the second conductor layer L2 is the line conductor layer. The first conductor layer L1 and the third conductor layer L3 form a pair of adjacent conductor layers. Therefore, as shown in Figure 1, in the multilayer substrate 100, the line conductor layer (L2) on which the coplanar line 10 is provided is sandwiched between the upper and lower adjacent conductor layers (L1 and L3). Thus, the coplanar line 10 can be said to be a transmission line formed in the inner conductor layer of the multilayer substrate 100 and incorporated into the inner layer.

[0033] Returning to Figure 2, each of the multiple insulating layers 13 is composed of an insulating material (such as a core material or prepreg) and is placed between each of the conductor layers 12. Typically, insulating dielectric materials are used for the insulating layers 13. Examples of such materials include insulating organic materials such as epoxy resins and fluororesins such as polytetrafluoroethylene, and insulating inorganic materials such as ceramics. Each insulating layer 13 may be composed of the same dielectric material, or each layer may be composed of a different dielectric material. The dielectric constant of the insulating layer 13 is arbitrarily set according to the frequency of the RF signal passing through the coplanar transmission line 10.

[0034] The multiple insulating layers 13 include an inner insulating layer M1, an upper insulating layer M2, and a lower insulating layer M3. The inner insulating layer M1 is provided between the second conductor layer L2 and the third conductor layer L3. The upper insulating layer M2 is provided between the second conductor layer L2 and the first conductor layer L1. The lower insulating layer M3 is provided between the third conductor layer L3 and the fourth conductor layer L4.

[0035] The interlayer connection section 14 has a plurality of columnar conductors 16 that connect a plurality of conductor layers 12 to each other. The columnar conductors 16 are columnar conductors that penetrate the insulating layer provided between each conductor layer 12 and electrically connect each conductor layer 12. The columnar conductors 16 may be hollow structures with a metal film formed on the inner circumference of the through-hole, or they may be configured as metal columns with conductors such as metal plating or metal plugs filled inside.

[0036] As shown in Figure 2, the interlayer connection section 14 has a plurality of inner columnar conductors V1, a plurality of upper columnar conductors V2, and a plurality of lower columnar conductors V3. The inner columnar conductors V1 penetrate the inner insulating layer M1 and connect the second conductor layer L2 and the third conductor layer L3. The upper columnar conductors V2 penetrate the upper insulating layer M2 and connect the second conductor layer L2 and the first conductor layer L1. The lower columnar conductors V3 penetrate the lower insulating layer M3 and connect the third conductor layer L3 and the fourth conductor layer L4.

[0037] The inner columnar conductor V1, the upper columnar conductor V2, and the lower columnar conductor V3 can basically be placed at any position, but they may also be formed individually in the same planar position, or they may be formed integrally as a single through via.

[0038] Each part of the multilayer substrate 100 will be described in detail with reference to Figures 1 to 3.

[0039] As shown in Figure 1A, the first conductor layer L1 is the outermost conductor layer 12 located on the upper side of the multilayer substrate 100. In this embodiment, the first conductor layer L1 corresponds to one of a pair of adjacent conductor layers. The first conductor layer L1 has a first conductor portion 20 and two connecting pads 21.

[0040] The first conductor section 20 faces the coplanar line 10. Specifically, the first conductor section 20 is a conductor pattern formed so as to overlap with the region constituting the coplanar line 10 in a plan view from the Z direction. The first conductor section 20 is typically used as a ground conductor connected to the ground potential (such as the ground potential used in RF modules, etc.) and exhibits a shielding function that shields the coplanar line 10. Therefore, by providing the first conductor section 20, it is possible to prevent external noise from entering the coplanar line 10 and to block noise radiated from the coplanar line 10.

[0041] In this embodiment, a solid film with two openings 22 formed therein for providing each connection pad 21 is used as the first conductor portion 20. However, it is not limited to this, and the first conductor portion 20 can be configured in any shape as long as it covers the area of ​​the coplanar line 10, which will be described later.

[0042] The connection pads 21 connect to the signal line 25 of the coplanar line 10 and function as input or output terminals of the signal line 25. In Figure 1A, two connection pads 21 are provided, assuming that terminals for outputting or detecting RF signals are provided on the first conductor layer L1. The connection pads 21 are formed inside the opening 22 so as not to short-circuit with the first conductor portion 20.

[0043] The connection pads 21 shown in Figure 1A are linear conductor patterns formed to be approximately the same width as the signal lines 25. Here, vias (connection columnar conductors 31, described later) for connecting to the signal lines 25 are provided at the opposite ends of each connection pad 21. In this way, the multilayer substrate 100 is configured such that a portion of the signal lines 25 is provided on the upper layer, the first conductor layer L1. Note that the signal lines 25 and the connection pads 21 do not necessarily have to be directly connected, as they may be connected via internal wiring. Also, since one end of the signal line 25 may be connected to an antenna section or the like, the connection pads 21 do not necessarily have to be provided at both ends of the signal line 25.

[0044] As described above, the second conductor layer L2 is the conductor layer 12 (line conductor layer) on which the coplanar line 10 is formed. The second conductor layer L2 is disposed adjacent to the first conductor layer L1 (one of the adjacent conductor layers). As shown in FIG. 1B, the second conductor layer L2 has a signal line 25 and a second conductor portion 26 surrounding the signal line 25. Both the signal line 25 and the second conductor portion 26 are conductor patterns forming the coplanar line 10 The second conductor portion 26 is used as a ground conductor connected to the ground potential, similar to the first conductor portion 20. Thereby, the coplanar line 10 is configured, and it becomes possible to efficiently transmit RF signals.

[0045] FIG. 3 schematically shows a typical planar shape of the coplanar line 10. The signal line 25 is a line-shaped conductor pattern formed with a predetermined line width W. The second conductor portion 26 is a conductor pattern formed along the signal line 25 with a predetermined interval (gap G) so as to surround the signal line 25. Therefore, it can also be said that a slit portion 27 for arranging the signal line 25 and forming the gap G on both sides of the signal line 25 is formed in the second conductor portion 26. The width of the slit portion 27 is W + 2G.

[0046] In the present embodiment, the sizes of the respective parts of the coplanar line 10 are set so as to transmit a millimeter-wave signal in the 60 GHz band. In this case, the line width W is set to, for example, 0.11 mm, and the gap G is set to 0. That is, the specific values of the line width W and the gap G of the coplanar line 10 are not limited to this, and for example, the values of the line width W and the gap G may be set so that an RF signal of a desired frequency can be transmitted.

[0047] In the example shown in FIGS. 1 and 3, a straight-type coplanar line 10 in which the signal line is formed linearly is shown. Of course, the shape of the coplanar line 10 is not limited, and for example, a curved line may be formed (see FIGS. etc.).

[0048] Thus, the coplanar line 10 is formed by the signal line 25 and the second conductor portion 26. In the present disclosure, for example, a predetermined range including the second conductor portion 26 centered on the signal line 25 is defined as the configuration range of the coplanar line 10. The configuration range is a range set so as to obtain desired transmission characteristics, and basically, portions other than the gap G inside the configuration range are composed of conductors (the signal line 25 and the second conductor portion 26). For example, a range expanded outward by the length of the gap G from the range overlapping with the slit portion 27 (a range having a width of W + 4G centered on the signal line 25) can be regarded as the configuration range. Note that a wider range may be set as the configuration range.

[0049] Further, the second conductor portion 26 is a solid film formed with the slit portion 27 here, but may be formed in any pattern as long as a conductor layer is formed on the portion excluding the slit portion 27 inside the configuration range of the coplanar line 10. Also, the first conductor portion 20 described above may be formed in any pattern as long as at least the range overlapping with the configuration range is covered with a conductor.

[0050] The third conductor layer L3 is a conductor layer disposed on the opposite side of the first conductor layer L1 (one adjacent conductor layer) adjacent to the second conductor layer L2 (line conductor layer). In the present embodiment, the third conductor layer L3 corresponds to the other adjacent conductor layer, and when centered on the core material described later, it becomes a conductor layer provided in the inner layer on the lower side of the multilayer substrate 100. Since the third conductor layer L3 sandwiches the core material described later, the distance from the first conductor layer L1 to the second conductor layer L2 is larger than that of the first conductor layer L1.

[0051] The planar configuration of the third conductor layer L3 is not limited, but in many cases, the portion overlapping with the coplanar line 10 is generally formed as a ground conductor. In the example shown in FIG. 1, the third conductor layer L3 is configured as a solid film covering the entire surface of the substrate.

[0052] The fourth conductor layer L4 is the outermost conductor layer 12 on the lower side of the multilayer substrate 100. The conductor pattern and the like of the fourth conductor layer L4 are not particularly limited, and for example, any wiring pattern, terminal pattern, or the like required on the lower layer side of the multilayer substrate 100 may be appropriately formed.

[0053] The multiple conductor layers 12 (L1 to L4) are typically composed of copper films. The thickness of each conductor layer 12 is set to approximately 40 μm. Note that the thickness of each conductor layer 12 may be set individually. Alternatively, the thickness of each conductor layer 12 may be set to create a symmetrical structure on the upper and lower sides of the multilayer substrate 100. In this case, the thicknesses of the inner conductor layers 12 (L2 and L3) are set to the same value, and the thicknesses of the outer conductor layers 12 (L1 and L4) are set to the same value. In addition, the thickness of each conductor layer 12 can be appropriately set depending on the application of the multilayer substrate 100.

[0054] Next, we will describe the inner insulating layer M1, the upper insulating layer M2, and the lower insulating layer M3 that make up the multiple insulating layers 13.

[0055] The inner insulating layer M1 is a core material positioned between the second conductor layer L2 (line conductor layer) and the third conductor layer L3 (the other adjacent conductor layer). The core material is thicker than the other insulating layers (upper insulating layer M2 and lower insulating layer M3). This makes it easier to ensure the rigidity of the multilayer substrate 100 and reduces the manufacturing cost of the multilayer substrate 100, compared to, for example, the case where the inner insulating layer M1 is constructed by laminating relatively thin insulating layers. The thickness of the inner insulating layer M1 is set to, for example, about 1.3 mm.

[0056] The upper insulating layer M2 and the lower insulating layer M3 are constructed using, for example, a prepreg material. In this case, the upper insulating layer M2 and the lower insulating layer M3 are laminated together with a plurality of conductor layers L1 to L4 on both sides of the inner insulating layer M1, which is the core material, using a build-up method. The thickness of the upper insulating layer M2 and the lower insulating layer M3 is set to, for example, about 50 μm.

[0057] Next, we will describe the multiple inner columnar conductors V1, the multiple upper columnar conductors V2, and the multiple lower columnar conductors V3 that constitute the interlayer connection section 14.

[0058] As described above, the multiple inner columnar conductors V1 connect the second conductor layer L1 (line conductor layer) and the third conductor layer L3 (the other adjacent conductor layer). In this embodiment, the multiple inner columnar conductors V1 correspond to the multiple first columnar conductors. As described above, the inner insulating layer M1 between the second conductor layer L1 and the third conductor layer L3 is the core material. Therefore, the multiple inner columnar conductors V1 are through vias that penetrate the core material. As shown in Figures 1 and 2, the inner columnar conductors V1 are vias with a larger length in the Z direction compared to the other columnar conductors 16 (V2 and V3). In the example shown in Figure 1, the multiple inner columnar conductors V1 are arranged to surround the outer edge of the multilayer substrate 100. The specific arrangement of the inner columnar conductors V1 will be described in detail later.

[0059] Multiple upper columnar conductors V2 connect the second conductor layer L2 (line conductor layer) and the first conductor layer L1 (one adjacent conductor layer). Figure 1A shows the pattern of the upper columnar conductors V2 as seen through the first conductor layer L1. Figure 1B shows the upper columnar conductors V2 exposed. The multiple upper columnar conductors V2 include multiple line columnar conductors 30, two connecting columnar conductors 31, and multiple linking columnar conductors 32.

[0060] Multiple columnar conductors 30 for the track are arranged to surround the signal line 25 of the coplanar track 10 provided in the second conductor layer L2. The columnar conductors 30 for the track connect the first conductor section 20 and the second conductor section 26 around the signal line 25 and surround the signal line 25 along the XY plane, thereby forming a conductor wall with respect to the signal line 25. This sufficiently enhances the shielding effect with respect to the signal line 25.

[0061] Here, the columnar conductors 30 for the railway line are configured as columnar conductors 16 with a relatively small diameter. Furthermore, each columnar conductor 30 for the railway line is arranged to double-surround the slit portion 27 of the second conductor layer L1 and the opening 22 of the first conductor layer L1. This makes it possible to further improve the shielding effect. Note that the arrangement pattern of the columnar conductors 30 for the railway line is not limited, and for example, a pattern in which the signal line 25 is triple-surrounded, or a pattern in which the columnar conductors 30 for the railway line are arranged alternately may be used.

[0062] The connecting columnar conductor 31 connects the connecting pad 21 provided on the first conductor layer L1 to the signal line 25. Here, connecting columnar conductors 31 are provided at both ends of the signal line 25, and each connecting columnar conductor 31 is connected to the connecting pad 21 on its upper side.

[0063] The multiple connecting columnar conductors 32 are columnar conductors 16 connected to the multiple inner columnar conductors V1, and their diameters are set to be the same size as the inner columnar conductors V1. In this way, the multiple connecting columnar conductors 32 constitute vias that connect the first conductor layer L1, the second conductor layer L2, and the third conductor layer L3. The positions where the connecting columnar conductors 32 (inner columnar conductors V1) are provided are outside the area enclosed by the columnar conductors 30 for the track.

[0064] Multiple lower columnar conductors V3 connect the third conductor layer L3 (the other adjacent conductor layer) and the fourth conductor layer L4. In Figure 1, the lower columnar conductors V3 are not visible, but for example, similar to the upper columnar conductors V2, connecting columnar conductors connected to multiple inner columnar conductors V1 are provided. In addition, the lower columnar conductors V3 may be arranged in any pattern according to the wiring on the underside of the multilayer substrate 100.

[0065] The protective film 15 is a film for protecting the surface of the conductive layers 12 (first conductive layer L1 and fourth conductive layer L4) arranged on the outside of the multilayer substrate 100. In this embodiment, the protective film 15 consists of an upper protective film P1 and a lower protective film P2. The upper protective film P1 is a protective film 15 provided on the outside of the first conductive layer L1 (one adjacent conductive layer). The lower protective film P2 is a protective film 15 provided on the outside of the fourth conductive layer L4. By providing the protective film 15, oxidation and sulfidation of the copper conductive layers can be avoided, and deterioration of the properties of the multilayer substrate 100 can be suppressed. As the protective film 15, a gold film layer with a nickel underlayer or solder resist can be used. As the protective film 15, either a gold film layer or solder resist may be used, or both may be used.

[0066] The gold film layer typically uses an extremely thin gold plating called a gold flash. A gold flash is a gold plating with a thickness of less than 0.25 μm, corresponding to, for example, Class 0.25, and in recent years, even thinner films (0.1 μm or less, etc.) are often used. When performing such gold plating, nickel plating is applied as an undercoat. Therefore, when a protective film 15 consisting of a gold film layer is provided, the surface of the first conductor layer L1 will have a copper film, a nickel plating film, and a gold plating film stacked in this order from the inside out. The thickness of the nickel plating is, for example, about 3 μm, and the thickness of the gold plating is about 0.025 μm.

[0067] The gold film layer, along with the nickel plating layer, functions as an electric current path in conjunction with the first conductor layer L1. For this reason, the gold film layer is formed in the same pattern as the conductor pattern of the first conductor layer L1, for example, so as not to short-circuit it.

[0068] Solder resist is a resin material that forms a protective layer for protecting wiring. A solder resist commonly used for protecting wiring boards and the like can be used as appropriate. For example, the solder resist is formed over the entire surface of the first conductor layer L1, excluding the connection terminals (in this case, the connection pads 21). The thickness of the solder resist is, for example, about 20 μm.

[0069] As explained above, in the multilayer substrate 100, the first conductor layer L1 and the third conductor layer L3 are arranged on both sides of the second conductor layer L2 on which the coplanar line 10 is formed. The conductor layers L1 and L2 are connected as appropriate via columnar conductors. In this configuration, it is assumed that a return current will be generated between the coplanar line 10 and the conductor layers L1 and L3 above and below it. Here, the return current is, for example, the current that returns from the surrounding ground when an RF signal is transmitted. In particular, a return current path is generated between the signal line 25 and the first conductor layer L1 and the third conductor layer L3 via the columnar conductor 16 (upper columnar conductor V2 and inner columnar conductor V1) (see Figure 9).

[0070] In this disclosure, each part of the multilayer substrate 100 is configured according to the characteristics of such return current. Specifically, in the multilayer substrate 100, the return current path is configured such that, in the operating bandwidth of the coplanar line 10, transmission loss due to the return current flowing between the coplanar line 10 and the first conductor layer L1 and the third conductor layer L3 (a pair of adjacent conductor layers) via the columnar conductor 16 is suppressed.

[0071] Here, the return current path is determined, for example, by the arrangement of the columnar conductors 16 and the length of the columnar conductors 16 (the thickness of each insulating layer 13). Hereafter, the return current path will be referred to as the return path. Transmission loss corresponds, for example, to the attenuation of the RF signal transmitted through the coplanar line 10. By suppressing transmission loss due to the return current, it is possible to maintain a high level of transmission efficiency. This makes it possible to exhibit excellent transmission characteristics.

[0072] In this embodiment, to realize such a return path, multiple inner columnar conductors V1 are arranged at a predetermined distance or greater from the signal line 25. That is, the inner columnar conductors V1 arranged between the second conductor layer L2 and the third conductor layer L3 are not arranged closer to the signal line 25 than a predetermined distance. Here, the distance from the signal line 25 means, for example, the distance between the center line of the signal line 25 and the outermost point. This makes it possible to realize a coplanar line 10 with excellent transmission characteristics even when a protective film or the like is provided. This point will be explained in detail later with reference to Figures 10 to 12, etc.

[0073] [Multilayer substrate with signal lines on the surface side] Figure 4 is a perspective view of a multilayer substrate with signal lines on the surface side. The multilayer substrate 110 has the same laminated structure as the multilayer substrate 100 shown in Figure 1. In Figure 4A, the first conductor layer L1 is visible, and in Figure 4B, the second conductor layer L2 is visible.

[0074] As shown in Figure 4A, the multilayer substrate 110 has a coplanar line 10 arranged on the first conductor layer L1. That is, the multilayer substrate 110 is provided with a coplanar line 10 with the upper surface layer (L1) serving as the signal line 25. In this case, as shown in Figure 4B, the second conductor layer L2 of the multilayer substrate 110 is a solid film-like conductor that shields the coplanar line 10. The line width W and gap G of the coplanar line 10 were set to the same values ​​as the pattern shown in Figure 3.

[0075] Furthermore, in the multilayer substrate 100 shown in Figure 1, within the area where multiple columnar conductors 30 for lines (small columnar conductors 16 surrounding the signal line 25) are provided, a columnar conductor 16 (an upper columnar conductor V2 and an inner columnar conductor V1 connected) is provided that connects the first conductor layer L1 through the second conductor layer L2 to the third conductor layer L3. This columnar conductor 16 can be considered as a connecting columnar conductor 32 as the upper columnar conductor V2. Therefore, in the multilayer substrate 110, the inner columnar conductor V1 is positioned closer to the signal line 25 compared to the multilayer substrate 100.

[0076] A transmission line with a configuration like that of the multilayer substrate 110 was used, for example, as a transmission line to connect the terminals of an IC chip (RF element) to the antenna section in a millimeter-wave module.

[0077] Figure 5 is a graph showing another example of the transmission characteristics of the multilayer substrate shown in Figure 4. Figure 5 shows the simulation results of the transmission characteristics of a multilayer substrate 110 with a coplanar transmission line 10 on the surface. Here, a model was used in which a protective film 15 made of solder resist was attached to the coplanar transmission line 10. The horizontal axis of the graph is frequency [GHz], and the vertical axis is the S-parameter expressed in dB. Here, data for the bandwidth from 50 GHz to 68 GHz is plotted.

[0078] S21 is a parameter generally called the pass-through characteristic, and it plots the power flowing from the first terminal to the second terminal for each frequency, with the two ends of the signal line 25 designated as the first terminal and the second terminal, respectively. For example, in Figure 5, the terminal visible on the near side is the first terminal, and the terminal visible on the far side is the second terminal.

[0079] S11 and S22 are parameters generally called reflection characteristics (return loss characteristics). S11 is a plot of the signal detected at the first terminal when a signal is input to the first terminal. Similarly, S22 is a plot of the signal detected at the second terminal when a signal is input to the second terminal.

[0080] For the coplanar transmission line 10, a higher S21 characteristic, for example, allows for more efficient transmission of RF signals with less loss. When the solder resist protective film 15 was applied, the S21 characteristic was below -6 dB at 50 GHz, but it decreased as the frequency increased, reaching below -8 dB in the graph.

[0081] Solder resist is often used as a protective film to protect the pattern on the substrate surface from degradation due to oxidation and sulfidation. On the other hand, solder resist is a material with a relatively large parameter called the dielectric loss tangent, Tanδ. The dielectric loss tangent is a numerical value that represents the degree of electrical energy loss within a dielectric material.

[0082] A large dielectric loss tangent Tanδ can lead to a deterioration in transmission characteristics such as transmission performance. For example, the energy loss in a transmission line (transmission line loss A [dB]) is expressed by the following equation: A [dB] = 0.90551 × f [GHz] × tanδ × sqrt(εr) ... (1) In equation (1), f is the frequency in gigahertz units, and sqrt(εr) is the square root of the relative permittivity εr of the material.

[0083] For example, the dielectric constant εr is 3.12 for prepreg, 3.87 for core material, and 3.5 for solder resist. Similarly, the dielectric loss tangent Tanδ is 0.016 for prepreg, 0.014 for core material, and 0.030 for solder resist. This shows that solder resist has a dielectric loss tangent Tanδ that is about twice as high as other dielectric materials, and equation (1) indicates that transmission line losses are relatively large.

[0084] One way to avoid such degradation of transmission characteristics is to not cover the substrate surface with solder resist, but instead to perform gold plating. The gold film layer formed by gold plating has the effect of preventing degradation of the substrate surface due to oxidation and sulfidation, similar to solder resist.

[0085] Figure 6 is a graph showing an example of the transmission characteristics of the multilayer substrate shown in Figure 4. Figure 6 uses a model in which a coplanar transmission line 10 is provided on the surface layer of a multilayer substrate 110, and a protective film 15 made by gold plating is provided on the coplanar transmission line 10. With the gold-plated coplanar transmission line 10, for example, the S21 characteristic is -5 dB or higher at 50 GHz, and it can be seen that the S21 value is higher and the signal can be transmitted more efficiently compared to the case in Figure 5 where solder resist is used as the protective film 15. Thus, when the coplanar transmission line 10 is provided on the surface layer, the transmission characteristics are improved by using a gold film layer as the protective film 15 instead of solder resist.

[0086] On the other hand, applying a gold film layer (gold plating / gold flash treatment) increases the price of the substrate due to the high cost of gold. Furthermore, when a gold film layer is applied, the transmission lines are exposed, making them susceptible to scratches. If conductive foreign matter adheres to them, the transmission lines could short-circuit, significantly degrading their performance.

[0087] In addition, nickel plating is generally used as the base plating in gold plating / gold flash plating. However, nickel is a metal with lower conductivity than gold or copper. Furthermore, at high frequencies such as millimeter waves, the so-called skin effect causes current to flow on the surface of the conductor. In this case, the current flows not through the copper film that constitutes the first conductor layer L1, but through the nickel film formed on its surface, which can be a cause of transmission line loss.

[0088] One way to mitigate the skin effect is to increase the thickness of the gold plating on the surface to prevent transmission line loss, but this method further increases costs. Other options include using exposed unplated copper or using tin plating instead of gold plating. However, if unplated copper is exposed, the line (signal line, etc.) may deteriorate due to oxidation or sulfidation, potentially leading to disconnection. Furthermore, tin plating is generally difficult to process finely, making it unsuitable for high-density patterns. In addition, there are concerns that metal crystals (whiskers) that form on the tin-plated surface may cause short circuits, making it difficult to guarantee long-term reliability.

[0089] From this perspective, the inventors considered a configuration in which the coplanar track 10 is placed in the inner layer without being placed in the surface layer.

[0090] [Multilayer substrate with signal lines on the inner layer side] Figure 7 is a perspective view of a multilayer substrate with signal lines on the inner layer side. The multilayer substrate 111 has the same laminated structure as the multilayer substrate 100 shown in Figure 1 and the multilayer substrate 110 shown in Figure 4. In Figure 7A, the first conductor layer L1 is visible, and in Figure 7B, the second conductor layer L2 is visible. The multilayer substrate 111 is a multilayer substrate 110 shown in Figure 4 with the coplanar line 10 as an inner layer.

[0091] As shown in Figure 7B, the coplanar line 10 is provided on the second conductor layer L2, which is the layer below the first conductor layer L1, which is the surface layer of the multilayer substrate 111. In this case, as shown in Figure 7A, a solid film-like conductor that shields the coplanar line 10 is arranged as the surface layer (first conductor layer L1) of the multilayer substrate 111. Thus, the multilayer substrate 111 has a configuration similar to that of the multilayer substrate 110 in Figure 4, but with the first conductor layer L1 and the second conductor layer L2 swapped. The line width W and gap G of the coplanar line 10 are set to the same values ​​as the pattern shown in Figure 3.

[0092] Furthermore, as shown in Figure 7B, the multilayer substrate 111 is provided with columnar conductors 16 (connecting columnar conductors 32) that connect the first conductor layer L1 to the third conductor layer L3, passing through the second conductor layer L2, in the same arrangement as the multilayer substrate 111 in which the coplanar line 10 is provided on the surface layer. In other words, when the coplanar line 10 (signal line 25) is made an inner layer, the arrangement of each columnar conductor 16 in the multilayer substrate 111 is not changed. Consequently, in the multilayer substrate 111 as well, the inner columnar conductor V1 is positioned closer to the signal line 25 compared to the multilayer substrate 100 in Figure 1.

[0093] In the multilayer substrate 111, a protective film 15 may or may not be provided. In either case, since the coplanar line 10 is in the inner layer, the influence on the transmission characteristics due to the type of protective film 15 provided on the surface layer or the presence or absence of the protective film 15 is sufficiently small.

[0094] Figure 8 is a graph showing an example of the transmission characteristics of the multilayer substrate shown in Figure 7. Figure 8 shows the simulation results of the transmission characteristics of the multilayer substrate 111 with a coplanar transmission line 10 as an inner layer. From the graph, the reflection characteristics S11 and S22 are -10 dB, which is lower than the results in Figures 5 and 6. This means that the intensity of reflected waves is low, indicating excellent transmission characteristics.

[0095] On the other hand, regarding the pass-through characteristics shown in S21, a notch is observed where the power locally decreases around 59 GHz. As a result, a large loss occurs when the RF signal passes through the vicinity of the notch, reducing the transmission efficiency. The inventors considered the reason for the occurrence of such a notch as follows.

[0096] Figure 9 is a schematic diagram illustrating the path of the return current between the signal line and the ground conductor. Figure 9 shows the structure in the Z direction around the signal line 25, and schematically illustrates a cross-section viewed from the Y direction, which is cut along the XZ plane so that the signal line 25 is visible.

[0097] Figure 9A shows a multilayer substrate 110 with the coplanar transmission line 10 shown in Figure 4 as the upper layer, and Figure 9B shows a cross-section of a multilayer substrate 111 with the coplanar transmission line 10 as the inner layer as shown in Figure 7. The shaded area corresponds to the signal line 25, and on the far side of the paper from the signal line 25, peripheral conductors (corresponding to the second conductor portion 26 in Figure 1) are arranged with a gap G along the signal line 25.

[0098] Generally, when transmitting high-frequency signals such as RF signals, a return current 35 is generated around the transmission line. This is a phenomenon in which the current that flows through the signal line returns from the surrounding ground, and the current that returns from the ground is called the return current 35. The path through which the return current 35 flows, that is, the route through which the signal returns from the line via the ground, is also called the return path 36, and it is formed according to the surrounding structure of the signal line.

[0099] As shown in Figure 9A, in the multilayer substrate 110, an RF signal is applied to a signal line 25 provided in the first conductor layer L1. In this case, a return path 36a is formed between the first conductor layer L1 and the second conductor layer L2 located below it, through which a return current 35 flows. This return path 36a is a path that passes through the upper columnar conductor V2 connecting the first conductor layer L1 and the second conductor layer L2.

[0100] Furthermore, the signal line 25 is shielded from the third conductor layer L3 by the second conductor layer L2. Therefore, as shown in Figure 9A, even though the second conductor layer L2 and the third conductor layer L3 are connected by the inner columnar conductor V1, no return path 36 is formed between the first conductor layer L1 and the third conductor layer L3.

[0101] Thus, in a multilayer substrate 110 in which the coplanar line 10 is provided on the upper layer, the signal line 25 is located in the first conductor layer L1 and is completed as a transmission line by the ground conductor formed in the second conductor layer L2, and the flow of the return current 35 is also completed within these two layers.

[0102] On the other hand, as shown in Figure 9B, in a multilayer substrate 111 in which the coplanar line 10 is incorporated as an inner layer, an RF signal is applied to a signal line 25 provided in the second conductor layer L2. In this case, a return path 36b is formed between the second conductor layer L2 and the first conductor layer L1 located above it, through which a return current 35 flows. Furthermore, a return path 36c is also formed between the second conductor layer L2 and the third conductor layer L3 located below it, through which a return current 35 flows. The return path 36b is a path that passes through the upper columnar conductor V2 connecting the first conductor layer L1 and the second conductor layer L2, similar to the return path 36a in Figure 9A, for example. The return path 36c is a path that passes through the inner columnar conductor V1 connecting the second conductor layer L2 and the third conductor layer L3.

[0103] Thus, in the multilayer substrate 111 in which the coplanar line 10 is incorporated as an inner layer, the signal line 25 is provided in the second conductor layer L2, causing the return path 36 to split into a path between it and the first conductor layer L1 (return path 36b) and a path between it and the third conductor layer L3 (return path 36b). Furthermore, in the case of a four-layer substrate such as the multilayer substrate 111, which has four conductor layers stacked on top of each other, a core material (inner insulating layer M1) is placed between the second conductor layer L2 and the third conductor layer L3. The thickness of the core material is considerably larger than the prepreg material (upper insulating layer M2) between the first conductor layer L1 and the second conductor layer L2. For this reason, the lower return path 36c, which passes through a relatively long ground via (inner columnar conductor V1), has a significantly longer path length than the upper return path 36b. Thus, it is thought that the return current flowing through the relatively long return path 36c affects the RF signal flowing through the signal line 25, causing the notch shown in Figure 8 to occur.

[0104] [Arrangement of Inner Columnar Conductor and Transmission Characteristics] In order to suppress the influence of the return current 35 flowing through the return path 36c, the inventor focused on the position in which the inner columnar conductor V1 is provided. For example, if the return path 36c is not located near the signal line 25, the return current 35 will concentrate in the path to the upper first conductor layer L1 (return path 36b), and it is thought that the return current 35 flowing through the return path 36c itself will be suppressed. Therefore, as explained with reference to Figure 1, the inventor considered a configuration in which the inner columnar conductor V1 is provided at a predetermined distance or more away from the signal line 25.

[0105] Figure 10 is a perspective view illustrating an example of the arrangement of the inner columnar conductor V1 in the multilayer substrate of this embodiment. Figure 10A is a perspective view of the multilayer substrate 111 shown in Figure 7, viewed from below where the fourth conductor layer L4 is located. Figure 10B is a perspective view of the multilayer substrate 120, obtained by removing a portion of the inner columnar conductor V1 from the multilayer substrate 111 shown in Figure 7, viewed from below.

[0106] The inventors have constructed a model (multilayer substrate 120) in which the inner columnar conductor V1 close to the signal line 25 is removed, as an example of a configuration in which the inner columnar conductor V1 is separated by a predetermined distance or more. Specifically, the multilayer substrate 120 shown in Figure 10B is a model in which the ground vias (inner columnar conductor V1) of the second conductor layer L2 and the third conductor layer L3, which are provided adjacent to the signal line 25, are removed from the multilayer substrate 111 shown in Figure 10A. The multilayer substrate 120 configured in this way is substantially the same model as the multilayer substrate 100 according to this embodiment. The multilayer substrate 120 is an example of a multilayer substrate according to this technology.

[0107] As shown in Figure 10B, no inner columnar conductors V1 are provided around the signal line 25, resulting in a state where, for example, the immediate return path 36c of the signal line 25 formed in Figure 10A is eliminated. Although a path through an inner columnar conductor V1 located away from the signal line 25 is also conceivable, this path is significantly farther from the signal line 25, so almost no return current 35 flows through such a path, and its impact on the transmission characteristics is considered negligible. With this configuration, the return current 35 associated with the transmission of the RF signal hardly flows to the third conductor layer L3, but is concentrated in the ground conductor of the first conductor layer L1. As a result, the transmission line can be completed using only the first conductor layer L1 and the second conductor layer L2.

[0108] As shown in Figure 10B, in the area where the inner columnar conductor V1 has been removed, the upper columnar conductor V2 connecting the first conductor layer L1 and the second conductor layer L2 remains. These form a return path 36 through which the upper return current 35 passes, and also function as the columnar conductor 30 for the line as described with reference to Figure 1.

[0109] In this configuration, where the inner columnar conductors V1 are separated from the signal lines 25, each inner columnar conductor V1 is arranged so as not to overlap with the multiple line columnar conductors 30. For example, even in the multilayer substrate 100 shown in Figure 1, no inner columnar conductors V1 are provided directly beneath the multiple line columnar conductors 30. This prevents situations where the lower return path 36 connects to the upper return path 36, and makes it possible to reliably suppress the return current 35 directed downwards.

[0110] Figure 11 is a graph showing an example of the transmission characteristics of the multilayer substrate of this embodiment. Figure 11 shows the simulation results of the transmission characteristics of the multilayer substrate 120 shown in Figure 10B. For example, S11 and S22, which represent the reflection characteristics, are below -10 dB, similar to the simulation results for the multilayer substrate 111 shown in Figure 7, indicating that reflected waves are sufficiently suppressed.

[0111] Furthermore, the graph of S21, which represents the transmission characteristics, does not show the notches shown in Figure 7, indicating that the transmission characteristics have improved compared to the multilayer substrate 111. This is thought to be because the influence of the long ground via (inner columnar conductor V1) provided on the underside of the coplanar line 10 is eliminated, preventing the return current 35 from escaping to the lower layer, and as a result the notches are eliminated. This makes it possible to avoid situations where the transmission efficiency decreases locally, even when the coplanar line 10 is made into an inner layer, and to achieve excellent transmission characteristics.

[0112] The multilayer substrate 120 shown in Figure 10B has the inner columnar conductors V1 surrounding the signal line 25 removed, thereby increasing the distance to the inner columnar conductor V1 closest to the signal line 25. However, the allowable spacing (predetermined distance) between the signal line 25 and the inner columnar conductor V1 can be set as appropriate, although this is not the only option.

[0113] In the following, a predetermined distance will be denoted as D0, and the spacing between the signal line 25 and the inner columnar conductor V1 will be denoted as D. The spacing D is the distance from the center position of the signal line 25 to the center position of the inner columnar conductor V1. Alternatively, for example, the shortest distance between the outer edge of the signal line 25 and the outer edge of the inner columnar conductor V1 may be defined as the spacing D. The predetermined distance D0 can be set appropriately according to the definition of the spacing.

[0114] The predetermined distance D0 is set such that the return current 35 flowing between the signal line 25 and the third conductor layer L3 (the other adjacent conductor layer) via the inner columnar conductor V1 is substantially zero. Here, substantially zero return current 35 means that the downward return current 35 is low, for example, to a degree where its effect on the pass characteristics is negligible. Therefore, for example, if the pass characteristics in the operating bandwidth are at or above the desired level, the return current 35 that affects the pass characteristics is considered to be substantially zero.

[0115] In this case, the predetermined distance D0 is the distance at which the inner columnar conductor V1 is separated until, for example, the return current 35 generated below the signal line 25 decreases to a negligible level.

[0116] Furthermore, if the inner columnar conductor V1 is a via that penetrates the core material, it is preferable that the predetermined distance D0 is greater than or equal to the thickness of the core material. The thickness of the core material is the thickness of the inner insulating layer M1 and the interlayer distance between the second conductor layer L2 and the third conductor layer L3. In this way, by not placing the inner columnar conductor V1 closer than the thickness of the core material when viewed from the signal line 25, it is possible to sufficiently reduce the return current to the third conductor layer L3 and suppress the influence on the transmission characteristics. In addition, the method of setting the predetermined distance D0 is not limited, and the predetermined distance D0 may be appropriately set using, for example, simulation, so that the desired transmission characteristics can be obtained.

[0117] Figure 12 is a graph showing the measurement results of the transmission characteristics of the multilayer substrate of this embodiment. Figure 12 shows the transmission characteristics measured using the multilayer substrate 100 shown in Figure 1 and the transmission characteristics measured using the multilayer substrate 112 for comparison. The multilayer substrate 112 is a substrate in which a coplanar line 10 is provided on the surface layer (first conductor layer L1) and a protective film 15 formed by gold flash treatment is provided. The basic design, including the shape of the coplanar line 10, is the same for both the multilayer substrate 100 and the multilayer substrate 112.

[0118] Figure 12 shows the measurement results for S21 and S11 in the multilayer substrate 100 as a coarse dotted line graph (inner layer S21) and a thick solid line graph (inner layer S11). Similarly, the measurement results for S21 and S11 in the multilayer substrate 112 are shown as a fine dotted line graph (surface layer S21) and a thin solid line graph (surface layer S11).

[0119] From Figure 12, the pass characteristics in the multilayer substrate 100 shown by the graph of the inner layer S21 are higher than those in the multilayer substrate 112 shown by the graph of the surface layer S21, indicating that the multilayer substrate 100 has better pass characteristics. In other words, placing the coplanar transmission line 10 in the inner layer improved the pass characteristics compared to placing the coplanar transmission line 10 on the surface and applying a gold flash treatment.

[0120] As mentioned above, in gold flash treatment (or gold plating), nickel plating is used as an undercoat. Furthermore, when transmitting RF signals, the higher the frequency, the more current flows on the surface of the transmission path due to the skin effect. As a result, in the multilayer substrate 112 with coplanar lines 10 on the surface, current flows not through the copper that constitutes the conductor layer 12, but through the gold plating layer and nickel plating layer that constitute the protective film 15, and it is thought that transmission loss occurred due to the resistive component of the nickel plating layer.

[0121] In contrast, in the multilayer substrate 100 according to this embodiment, since the coplanar line 10 is integrated into the inner layer, transmission loss due to the protective film 15 is almost eliminated. Therefore, in the multilayer substrate 100, a protective film 15 such as gold flash or solder resist can be appropriately provided on the surface of the first conductor layer L1 as needed. Furthermore, as described above, since the lower ground via (inner columnar conductor V1) of the coplanar line 10 is separated from the signal line 25 by a certain distance, localized transmission loss (notch) caused by the return current 35 does not occur. In this way, by integrating the signal line 25 of the coplanar line 10 into the inner layer, it is possible not only to reduce manufacturing costs and solve long-term reliability problems, but also to improve transmission characteristics.

[0122] As described above, in the multilayer substrate 100 according to this embodiment, a line conductor layer having a coplanar line is provided between a pair of adjacent conductor layers, and each conductor layer is connected via a columnar conductor. Furthermore, the path of the return current is configured so that transmission loss due to the return current flowing through the columnar conductor is suppressed within the bandwidth of the coplanar line. As a result, since the line conductor layer is an inner layer, even if a protective film is provided, for example, it has little effect on the transmission characteristics of the coplanar line. In addition, transmission loss due to the return current is suppressed, and deterioration of the transmission characteristics is avoided. This makes it possible to exhibit highly reliable and excellent transmission characteristics.

[0123] <Second Embodiment> A second embodiment of the antenna module according to this technology will be described. In the following description, parts that are similar to the configuration and operation of the multilayer substrate 100 described in the above embodiment will be omitted or simplified.

[0124] Figure 13 is a plan view showing an example of the surface layer configuration of an antenna module according to the second embodiment. Figure 14 is a plan view showing an example of the inner layer configuration of the antenna module shown in Figure 13. The antenna module 200 is a thin circuit module equipped with an antenna element 41 that radiates in a direction horizontal to the substrate surface. The antenna module 200 is configured, for example, as a millimeter-wave radar module. A millimeter-wave radar module is a module that detects people and obstacles using electromagnetic waves in the millimeter-wave band, and is beginning to become popular for automotive and home appliance applications. In addition to millimeter-wave radar modules, this technology may also be applied to other applications such as communication modules.

[0125] In Figures 13 and 14, the X, Y, and Z directions represent three mutually orthogonal axis directions, corresponding to the length direction (front-to-back direction), width direction (left-to-right direction), and thickness direction (height direction) of the antenna module 200, respectively.

[0126] The antenna module 200 is composed of a multilayer substrate 130 having a thickness direction in the Z direction. The multilayer substrate 130 is a substrate having a cross-sectional structure described with reference to, for example, Figure 2, and is composed of four conductive layers (L1 to L4) stacked on top of each other. The multilayer substrate 130 as a whole is composed of a plate material that is elongated in the Y direction.

[0127] A millimeter-wave radar IC 40, which outputs or detects millimeter-wave RF signals (millimeter-wave signals), is mounted on the first conductor layer L1 (one adjacent conductor layer), which forms the upper surface layer of the antenna module 200. In this embodiment, the millimeter-wave radar IC 40 is an example of an RF element that outputs or detects high-frequency signals.

[0128] The antenna module 200 comprises a plurality of antenna elements 41 and a plurality of transmission units 47, and is configured as a transmitting and receiving antenna for transmitting and receiving millimeter-wave electromagnetic waves. The plurality of antenna elements 41 are arranged in the Y direction with their antenna apertures facing the front side (right side in the figure) of the multilayer substrate 130. A transmission unit 47 is connected between each antenna element 41 and the millimeter-wave radar IC 40, and RF signals are transmitted via the transmission unit 47.

[0129] The multiple antenna elements 41 include a first transmitting antenna element 41a and a second transmitting antenna element 41b, which are transmitting antennas, and a first receiving antenna element 41c, a second receiving antenna element 41d, and a third receiving antenna element 41e, which are receiving antennas. The basic structure of each antenna element 41 is the same.

[0130] In the antenna module 200, the first receiving antenna element 41c, the second receiving antenna element 41d, the third receiving antenna element 41e, the first transmitting antenna element 41a, and the second transmitting antenna element 41b are arranged in order from one end in the longitudinal direction (the upper side in the figure). Each transmitting antenna constitutes a transmitting antenna array, and each receiving antenna constitutes a receiving antenna array.

[0131] Each antenna element 41 is an antenna configured within a multilayer substrate 130 and is constructed using a post-waveguide 42. Here, the post-waveguide 42 is a waveguide composed of a pair of conductor layers 12 and a post wall in which a plurality of columnar conductors 16 (posts) connecting the conductor layers 12 are arranged in a line. The post-waveguide 42 is constructed, for example, by arranging a pair of post walls at a predetermined interval in the region sandwiched between the pair of conductor layers 12.

[0132] The antenna element 41 is an antenna that transmits or receives millimeter-wave electromagnetic waves and has a feeding section 43 connected to the signal line 25. The feeding section 43 is a conductor formed along the stacking direction (Z direction) of the multilayer substrate 130, for example, and is composed of columnar conductors 16. The feeding section 43 is also insulated from the conductor patterns of each conductor layer 12 except for the signal line 25. For example, the first conductor layer L1 shown in Figure 13 has an opening that surrounds the feeding section 43 and is electrically isolated from the feeding section 43.

[0133] The antenna element 41 has a structure in which a feed point 43 is provided at one end of the post-waveguide 42 described above, and an open end is provided at the other end of the same post-waveguide 42. For example, electromagnetic waves corresponding to the RF signal input to the feed point 43 propagate within the post-waveguide 42 and are radiated from the open end. Alternatively, electromagnetic waves incident from the open end of the post-waveguide 42 propagate within the post-waveguide 42 and are converted into an RF signal at the feed point 43.

[0134] In the antenna module 200, which has four conductor layers 12, the outermost conductor layers 12 on the upper and lower sides (the first conductor layer L1 and the fourth conductor layer L4) and the posts connecting each conductor layer 12 (a columnar conductor 16 to which the upper columnar conductor V2, the inner columnar conductor V1, and the lower columnar conductor V3 are connected) constitute the post waveguide 42 of the antenna element 41.

[0135] Furthermore, a convex dielectric waveguide 44 is formed at the tip of each antenna element 41, in which a core material (inner insulating layer M1) not covered by the conductor layer 12 protrudes convexly. By providing the dielectric waveguide 44, the radiation efficiency of electromagnetic waves can be improved, and isolation can be improved by reducing the component reflected at the interface at the tip. In addition, between the third receiving antenna element 41e and the first transmitting antenna element 41a, a recessed region 45 is formed by setting back the four conductor layers 12 relative to the open end of each post-waveguide, in order to achieve isolation between the transmitting and receiving sides.

[0136] Each of the multiple transmission units 47 is a transmission line having a coplanar line 10, and connects the input / output terminals of the millimeter-wave radar IC 40 to the power supply units 43 of each antenna element 41 to transmit RF signals. The multiple transmission units 47 include a first transmitting transmission unit 47a and a second transmitting transmission unit 47b, which are transmission lines for transmission, and a first receiving transmission unit 47c, a second receiving transmission unit 47d, and a third receiving transmission unit 47e, which are transmission lines for reception.

[0137] The first transmitting unit 47a and the second transmitting unit 47b are connected to the first transmitting antenna element 41a and the second transmitting antenna element 41b, respectively. The first receiving unit 47c, the second receiving unit 47d, and the third receiving unit 47e are connected to the first receiving antenna element 41c, the second receiving antenna element 41d, and the third receiving antenna element 41e, respectively. Note that the basic structure of each transmission unit 47 is the same whether it is for transmitting or receiving.

[0138] The transmission unit 47 includes a connecting pad 21, a connecting columnar conductor 31, a first conductor section 20, a signal line 25, a second conductor section 26, and a columnar conductor 30 for the line. Of these, the signal line 25 and the second conductor section 26 are provided in the second conductor layer L2 and constitute the coplanar line 10. Therefore, the transmission unit 47 can be said to be a transmission line equipped with an inner-layered coplanar line 10.

[0139] As shown in Figure 13, the connection pad 21 is provided on the first conductive layer L1 on which the millimeter-wave radar IC 40 is mounted, and is connected to the connection terminals of the millimeter-wave radar IC 40. The connection terminals are output terminals or input terminals of RF signals provided on the millimeter-wave radar IC 40 (RF element). In this embodiment, a circular connection pad 21 is provided corresponding to each connection terminal.

[0140] The connecting columnar conductor 31 is an upper columnar conductor V2 that connects the first conductor layer L1 and the second conductor layer L2, and connects the connecting pad 21 to the signal line 25 directly below the connecting terminal. In this embodiment, one end of the signal line 25 is positioned directly below the connecting pad 21, and the connecting pad 21 and the signal line 25 are connected via the connecting columnar conductor 31. This makes it possible to connect the connecting terminal and the signal line 25 at the shortest possible distance, thereby suppressing RF signal transmission loss and noise caused by the RF signal.

[0141] The first conductor portion 20 is a conductor pattern formed in the first conductor layer L1 that faces the coplanar line 10 (signal line 25). As shown in Figure 13, in this embodiment, the first conductor portion 20 is composed of a solid film-like conductor formed in the first conductor layer L1. The first conductor portion 20 is a ground conductor connected to the earth potential and shields the coplanar line 10 from above.

[0142] As shown in Figure 14, the signal line 25 is a line-shaped pattern formed on the second conductor layer L2. The signal line 25 is configured to connect the connection terminal (connection pad 21) of the millimeter-wave radar IC 40 to the power supply section 43 of the antenna element 41 corresponding to the terminal.

[0143] Here, curved signal lines 25 are used. The length of each signal line 25 is adjusted in the transmitting transmission unit 47 (first transmitting transmission unit 47a and second transmitting transmission unit 47b) or the receiving transmission unit 47 (first receiving transmission unit 47c, second receiving transmission unit 47d, and third receiving transmission unit 47e) so that the phase of the RF signal does not shift.

[0144] The second conductor portion 26 is a conductor pattern formed in the second conductor layer L2 with a predetermined gap G to surround the signal line 25. As a result, a coplanar line 10 is formed in the second conductor layer L2. As shown in Figure 14, in this embodiment, the second conductor portion 26 is formed by a solid film-like conductor formed in the second conductor layer L2.

[0145] The columnar conductors 30 for the transmission line are upper columnar conductors V2 that connect the first conductor layer L1 and the second conductor layer L2, and are arranged to surround the signal line 25. As shown in Figure 14, a large number of columnar conductors 30 for the transmission line are arranged along the signal line 25. The columnar conductors 30 for the transmission line have a smaller diameter than, for example, the inner columnar conductors V1 that penetrate the core material, and are instead arranged at a higher density. The columnar conductors 30 for the transmission line serve as a path for the upper return current 35 and also have the effect of shielding the signal line 25.

[0146] Furthermore, the antenna module 200 is provided with an inner columnar conductor V1 that penetrates the core material (inner insulating layer M1) and connects the second conductor layer L2 and the third conductor layer L3. As described above, in Figure 14, the columnar conductor 16, which has a larger diameter than the columnar conductor 30 for the transmission line, is the inner columnar conductor V1. As shown in Figure 13, an upper columnar conductor V2 with a similar diameter is provided above the inner columnar conductor V1.

[0147] In the multilayer substrate 130 constituting the antenna module 200, similar to the multilayer substrate 100 described in the above embodiment, a plurality of inner columnar conductors V1 are arranged at a predetermined distance or more from the signal line 25. Here, as shown in Figures 13 and 14, no inner columnar conductors V1 are arranged within the region surrounded by the columnar conductors 30 for the transmission line. This makes it possible to substantially reduce the current traveling between the lower side (third conductor layer L3 side) of the signal line 25 (coplanar transmission line 10) and the upper side (first conductor layer L1 side) of the return current 35 associated with RF signal transmission, and concentrate the return current 35 on the upper side (first conductor layer L1 side). As a result, it is possible to avoid situations where the transmission efficiency decreases locally and to achieve excellent transmission characteristics.

[0148] <Other Embodiments> This technology is not limited to the embodiments described above, and various other embodiments can be realized.

[0149] In the above embodiment, in the multilayer substrate, a ground via (inner columnar conductor V1) between the second conductor layer L2 and the third conductor layer L3 is positioned away from the signal line 25 to suppress a localized degradation of transmission characteristics due to the return current 35 (notch countermeasure). However, the invention is not limited to this, and other methods may be employed to adjust the path of the return current to avoid the influence of the return current on the transmission characteristics.

[0150] For example, the current path of the return current 35 flowing through the third conductor layer L3 can be shortened by reducing the interlayer distance (thickness of the inner insulating layer M1, which is the core material) between the second conductor layer L2 and the third conductor layer L3. In this case, shortening the path of the lower return current 35 prevents situations such as the upper and lower return currents 35 canceling each other out, thereby suppressing a decrease in transmission characteristics. Conversely, one could also consider increasing the interlayer distance between the second conductor layer L2 and the first conductor layer L1 to prevent the upper and lower return currents 35 from canceling each other out. Alternatively, the interlayer distance can be adjusted on both the upper and lower sides of the second conductor layer L2.

[0151] Thus, the interlayer distance between the second conductor layer L2 (line conductor layer) and at least one of the first conductor layer L1 or the third conductor layer L2 (a pair of adjacent conductor layers) may be set so as to suppress transmission loss due to the return current 35. In this case, it is also possible to place the inner columnar conductor V1 without separating it from the signal line 25. This makes it possible to achieve excellent transmission characteristics and increase design flexibility in a configuration in which the coplanar line 10 is made into an inner layer.

[0152] In the above embodiment, a configuration was described in which a coplanar line is provided on the second conductor layer (L2) from the top in a four-layer multilayer substrate. The line conductor layer on which the coplanar line 10 is provided can be any conductor layer that is an inner layer in the multilayer substrate, that is, any conductor layer that is not a surface layer. For example, in a four-layer multilayer substrate, the third conductor layer (L3) may be used as the line conductor layer. Furthermore, even in a multilayer substrate with four or more conductor layers, an inner conductor layer can be appropriately used as a line conductor layer. In this case, the conductor layers adjacent to the upper and lower sides of the line conductor layer become a pair of adjacent conductor layers. By configuring the return current path between these adjacent conductor layers using the method described above, it is possible to avoid the influence of the return current on the transmission characteristics.

[0153] It is also possible to combine at least two of the feature features of the present technology described above. In other words, the various feature features described in each embodiment may be combined arbitrarily, regardless of the specific embodiment. Furthermore, the various effects described above are merely examples and not limiting, and other effects may also be exhibited.

[0154] In this disclosure, "same," "equal," "orthogonal," etc., are concepts that include "substantially the same," "substantially equal," "substantially orthogonal," etc. For example, states that fall within a predetermined range (e.g., a range of ±10%) based on "exactly the same," "exactly equal," "exactly orthogonal," etc.

[0155] Furthermore, this technology can also adopt the following configuration: (1) A multilayer substrate comprising a line conductor layer having a coplanar line, a plurality of conductor layers having a pair of adjacent conductor layers adjacent to the line conductor layer, and an interlayer connection portion having a plurality of columnar conductors connecting the plurality of conductor layers to each other, wherein the path of the return current is configured such that transmission loss due to the return current flowing between the coplanar line and the pair of adjacent conductor layers via the columnar conductors is suppressed in the operating bandwidth of the coplanar line. (2) A multilayer substrate according to (1), wherein one of the pair of adjacent conductor layers has a first conductor portion facing a coplanar line, the line conductor layer is arranged adjacent to the one adjacent conductor layer and has a signal line forming the coplanar line and a second conductor portion surrounding the signal line, the interlayer connection portion has a plurality of first columnar conductors connecting the line conductor layer and the other adjacent conductor layer of the pair of adjacent conductor layers, and the plurality of first columnar conductors are arranged at a distance of a predetermined distance or more from the signal line. (3) A multilayer substrate according to (2), wherein the predetermined distance is set such that the return current flowing between the signal line and the other adjacent conductor layer via the first columnar conductor is substantially zero. (4) A multilayer substrate according to (2) or (3), further comprising a core material disposed between the line conductor layer and the other adjacent conductor layer, wherein the plurality of first columnar conductors are through vias penetrating the core material. (5) A multilayer substrate according to (4), wherein the predetermined distance is greater than or equal to the thickness of the core material. (6) A multilayer substrate according to at least one of (2) to (5), wherein the interlayer connection portion has a plurality of second columnar conductors connecting the line conductor layer and the one adjacent conductor layer. (7) A multilayer substrate according to (6), wherein the plurality of second columnar conductors include a plurality of line columnar conductors arranged to surround the signal line, and the plurality of first columnar conductors are arranged so as not to overlap with the plurality of line columnar conductors.(8) A multilayer substrate according to at least one of (2) to (7), wherein the one adjacent conductor layer is the outermost conductor layer, and further comprises a protective film provided on the outside of the one adjacent conductor layer. (9) A multilayer substrate according to (8), wherein the protective film is at least one of a gold film layer having a nickel underlayer or a solder resist. (10) A multilayer substrate according to at least one of (2) to (9), wherein the one adjacent conductor layer has a connection pad, and the plurality of second columnar conductors include connecting columnar conductors that connect the connection pad to the signal line. (11) A multilayer substrate according to (10), wherein the one adjacent conductor layer mounts an RF element that outputs or detects a high-frequency signal, and the connection pad is connected to the connection terminal of the RF element. (12) A multilayer substrate according to (11), wherein the connecting columnar conductor connects the connection pad to the signal line directly below the connection terminal. (13) A multilayer substrate according to at least one of (1) to (12), wherein the interlayer distance between the line conductor layer and at least one of the pair of adjacent conductor layers is set such that transmission loss due to the return current is suppressed. (14) A multilayer substrate according to at least one of (1) to (13), further comprising an antenna section having a power supply section connected to the signal line of the coplanar line. (15) A multilayer substrate according to at least one of (1) to (14), wherein the coplanar line is a transmission line for millimeter-wave RF signals. (16) An antenna module comprising: a line conductor layer having a coplanar line; a plurality of conductor layers having a pair of adjacent conductor layers adjacent to the line conductor layer; an interlayer connection portion having a plurality of columnar conductors connecting the plurality of conductor layers to each other; and an antenna element having a power supply portion connected to the signal line of the coplanar line, wherein the path of the return current is configured such that transmission loss due to return current flowing through the columnar conductors between the coplanar line and the pair of adjacent conductor layers is suppressed.

[0156] L1...First conductor layer L2...Second conductor layer L3...Third conductor layer V1...Inner columnar conductor 10...Coplanar line 12...Conductor layer 14...Interlayer connection 15...Protective film 16...Columnar conductor 20...First conductor section 21...Connection pad 25...Signal line 26...Second conductor section 35...Return current 36, 36a, 36b, 36c...Return path 40...Millimeter wave radar IC 41...Antenna element 100, 120, 130...Multilayer substrate 200...Antenna module

Claims

1. A multilayer substrate comprising: a line conductor layer having a coplanar line; a plurality of conductor layers having a pair of adjacent conductor layers adjacent to the line conductor layer; and an interlayer connection portion having a plurality of columnar conductors connecting the plurality of conductor layers to each other, wherein the path for the return current is configured such that transmission loss due to return current flowing between the coplanar line and the pair of adjacent conductor layers via the columnar conductors is suppressed in the operating bandwidth of the coplanar line.

2. A multilayer substrate according to claim 1, wherein one of the pair of adjacent conductor layers has a first conductor portion facing a coplanar line, the line conductor layer is arranged adjacent to the one adjacent conductor layer and has a signal line forming the coplanar line and a second conductor portion surrounding the signal line, the interlayer connection portion has a plurality of first columnar conductors connecting the line conductor layer and the other adjacent conductor layer of the pair of adjacent conductor layers, and the plurality of first columnar conductors are arranged at a predetermined distance or more from the signal line.

3. A multilayer substrate according to claim 2, wherein the predetermined distance is set such that the return current flowing between the signal line and the other adjacent conductor layer via the first columnar conductor is substantially zero.

4. A multilayer substrate according to claim 2, further comprising a core material disposed between the line conductor layer and the other adjacent conductor layer, wherein the plurality of first columnar conductors are through vias penetrating the core material.

5. A multilayer substrate according to claim 4, wherein the predetermined distance is equal to or greater than the thickness of the core material.

6. A multilayer substrate according to claim 2, wherein the interlayer connection portion has a plurality of second columnar conductors connecting the line conductor layer and one adjacent conductor layer.

7. A multilayer substrate according to claim 6, wherein the plurality of second columnar conductors include a plurality of line columnar conductors arranged to surround the signal line, and the plurality of first columnar conductors are arranged so as not to overlap with the plurality of line columnar conductors.

8. A multilayer substrate according to claim 2, wherein the one adjacent conductor layer is the outermost conductor layer, and further comprises a protective film provided on the outside of the one adjacent conductor layer.

9. A multilayer substrate according to claim 8, wherein the protective film is at least one of a gold film layer having a nickel underlayer or a solder resist.

10. A multilayer substrate according to claim 2, wherein one adjacent conductor layer has a connection pad, and the plurality of second columnar conductors include connecting columnar conductors that connect the connection pad to the signal line.

11. A multilayer substrate according to claim 10, wherein one adjacent conductor layer mounts an RF element for outputting or detecting a high-frequency signal, and the connection pad is connected to the connection terminal of the RF element.

12. A multilayer substrate according to claim 11, wherein the connecting columnar conductor connects the connecting pad to the signal line directly below the connecting terminal.

13. A multilayer substrate according to claim 1, wherein the interlayer distance between the line conductor layer and at least one of the pair of adjacent conductor layers is set such that transmission loss due to the return current is suppressed.

14. A multilayer substrate according to claim 1, further comprising an antenna portion having a power supply portion connected to the signal line of the coplanar line.

15. A multilayer substrate according to claim 1, wherein the coplanar line is a transmission line for millimeter-wave RF signals.

16. An antenna module comprising: a line conductor layer having a coplanar line; a plurality of conductor layers having a pair of adjacent conductor layers adjacent to the line conductor layer; an antenna element having an interlayer connection portion having a plurality of columnar conductors connecting the plurality of conductor layers to each other; and a power supply portion connected to the signal line of the coplanar line, wherein the path of the return current is configured such that transmission loss due to return current flowing through the columnar conductors between the coplanar line and the pair of adjacent conductor layers is suppressed.

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