Patent Publication Number: US-2022216590-A1

Title: Antenna module, manufacturing method thereof, and collective board

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of International Patent Application No. PCT/JP2020/029223, filed Jul. 30, 2020, which claims priority to Japanese patent application JP 2019-177382, filed Sep. 27, 2019, the entire contents of each of which being incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an antenna module, a manufacturing method of the antenna module, and a collective board, and more particularly, to a structure that can reduce warpage in the manufacturing process of an antenna module formed by a multilayer substrate. 
     BACKGROUND ART 
     International Publication No. 2016-067969 (Patent Document 1) discloses an antenna module including radiating elements and a radio-frequency semiconductor device disposed in an integrated manner at a dielectric substrate having a multilayer structure. In the antenna module disclosed in Patent Document 1, transmission lines for suppling radio-frequency signals from the radio-frequency semiconductor device to the radiating elements are extended from the radio-frequency semiconductor device, passed in a dielectric layer between the mounting surface of the dielectric substrate having the radio-frequency semiconductor device and a ground electrode disposed in the dielectric substrate, further routed to portions under the radiating elements, and extended upwards to the radiating elements. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: International Publication No. 2016/067969 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the antenna modules such as the antenna module disclosed in International Publication No. 2016/067969, elements including feed lines for suppling a radio-frequency signal to radiating elements, and connection wires for connecting stubs and filters coupled to the feed lines, and other electronic components are usually formed in a dielectric layer (hereinafter also referred to as “wiring region”) below a ground electrode in a dielectric substrate, for the purpose of reducing unnecessary coupling with the radiating elements to achieve sufficient antenna characteristics. 
     In such a structure, the ratio (residual copper ratio) of a conductor (typically, copper) included in a dielectric layer on the radiating element side (hereinafter also referred to as “antenna region”) with respect to the ground electrode is lower than the residual copper ratio of the wiring region below the ground electrode. Resins and ceramics forming dielectrics are more easily deformed by residual stress or thermal stress than conductors used for wiring patterns. The dielectric layer of a relatively low residual copper ratio is thus more largely deformed than the dielectric layer of a relatively high residual copper ratio. As a result, in the case in which a dielectric substrate is formed by subjecting a stack of dielectric layers to a process such as pressing or heat pressing, when the residual copper ratio is unbalanced among stacked layers as in the antenna module described above, non-uniform deformation may cause warpage in the finished dielectric substrate. 
     The present disclosure has been made to address such a problem, and one object thereof is to reduce warpage of a dielectric substrate having a multilayer structure in an antenna module formed with the dielectric substrate. 
     SOLUTION TO PROBLEM 
     An antenna module according to a first aspect of the present disclosure includes a dielectric substrate formed by stacking a plurality of dielectric layers, a radiating element formed at the dielectric substrate, a ground electrode facing toward the radiating element, and peripheral electrodes. The peripheral electrodes are formed in a plurality of layers between the radiating element and the ground electrode at end portions of the dielectric substrate. The peripheral electrodes are electrically coupled to the ground electrode. 
     A collective board according to a second aspect of the present disclosure forms a dielectric layer used for an antenna module. The collective board includes a first region including a plurality of individual boards of the dielectric layer and a second region formed between the plurality of individual boards. Peripheral electrodes are formed in the second region. 
     A manufacturing method of an antenna module according to a third aspect of the present disclosure includes manufacturing collective boards that respectively correspond to the plurality of dielectric layers and each include a plurality of individual boards. Each collective board includes a first region including the plurality of individual boards and a second region formed between the plurality of individual boards and including peripheral electrodes. The manufacturing method further includes stacking the collective boards and forming the antenna module by removing the second region and dividing the first region. 
     Advantageous Effects 
     In the antenna module according to the present disclosure, peripheral electrodes are arranged in a plurality of layers between the radiating elements and the ground electrode at end portions of the dielectric substrate. These peripheral electrodes increase the residual copper ratio of a region (antenna region) between the radiating elements and the ground electrode. As a result, it is possible to reduce warpage of the finished dielectric substrate because the difference in the residual copper ratio between the antenna region and the wiring region provided below the ground electrode is decreased. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a communication device using an antenna module according to a first embodiment. 
         FIG. 2  provides a plan view and a cutaway side view of a first example of the antenna module according to the first embodiment. 
         FIG. 3  is a cutaway side view of a second example of the antenna module according to the first embodiment. 
         FIG. 4  is a first drawing illustrating an antenna characteristic of the antenna module in  FIG. 2  and an antenna characteristic of the antenna module in  FIG. 3 . 
         FIG. 5  is a second drawing illustrating an antenna characteristic of the antenna module in  FIG. 2  and an antenna characteristic of the antenna module in  FIG. 3 . 
         FIG. 6  is a cutaway side view of an antenna module of a first modification. 
         FIG. 7  is a plan view of an antenna module of a second modification. 
         FIG. 8  illustrates a collective board according to a second embodiment. 
         FIG. 9  is an enlarged view of a portion including peripheral electrodes of the collective board in  FIG. 8 . 
         FIG. 10  illustrates a manufacturing process of an antenna module in the case of using the collective board according to the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Identical or corresponding portions in the drawings are assigned identical reference characters, and descriptions thereof are not repeated. 
     First Embodiment 
     Basic Configuration of Communication Device 
       FIG. 1  is an example of a block diagram of a communication device  10  using an antenna module  100  according to a first embodiment. Examples of the communication device  10  include portable terminals such as a mobile phone, a smartphone, and a tablet computer, and a personal computer having communication functionality. An example of frequency bands of radio waves used for the antenna module  100  according to the present embodiment is radio waves in millimeter-wave bands with center frequencies including 28 GHz, 39 GHz, and 60 GHz, but radio waves in frequency bands other than this example can also be used. 
     Referring to  FIG. 1 , the communication device  10  includes the antenna module  100  and a baseband integrated circuit (BBIC)  200  implementing a baseband-signal processing circuit. The antenna module  100  includes a radio-frequency integrated circuit (RFIC)  110 , which is an example of a feed circuit, and an antenna device  120 . In the communication device  10 , a signal is transferred from the BBIC  200  to the antenna module  100 , up-converted into a radio-frequency signal by the RFIC  110 , and emitted from the antenna device  120 . In the communication device  10 , a radio-frequency signal is received by the antenna device  120 , transferred to the RFIC  110  and down-converted into a signal, and processed by the BBIC  200 . 
     For ease of description,  FIG. 1  illustrates only configurations corresponding to four fed elements (radiating elements)  121  out of a plurality of fed elements  121  constituting the antenna device  120 . Configurations corresponding to the other fed elements  121  having the same configuration are omitted.  FIG. 1  illustrates an example in which the antenna device  120  is constituted by the plurality of fed elements  121  arranged in a two-dimensional array, but the fed elements  121  may be arranged in line as a one-dimensional array. The antenna device  120  may be constituted by only one fed element  121 . In the present embodiment, the fed element  121  is a patch antenna formed as a flat plate. 
     The RFIC  110  includes switches  111 A to  111 D,  113 A to  113 D, and  117 , power amplifiers  112 AT to  112 DT, low-noise amplifiers  112 AR to  112 DR, attenuators  114 A to  114 D, phase shifters  115 A to  115 D, a signal combiner and splitter  116 , a mixer  118 , and an amplifier circuit  119 . 
     When a radio-frequency signal is being transmitted, the switches  111 A to  111 D, and  113 A to  113 D are switched to establish connection to the power amplifiers  112 AT to  112 DT, and the switch  117  establishes connection to a transmit amplifier of the amplifier circuit  119 . When a radio-frequency signal is received, the switches  111 A to  111 D, and  113 A to  113 D are switched to establish connection to the low-noise amplifiers  112 AR to  112 DR, and the switch  117  establishes connection to a receive amplifier of the amplifier circuit  119 . 
     A signal transferred from the BBIC  200  is amplified by the amplifier circuit  119  and up-converted by the mixer  118 . The transmit signal, which is the up-converted radio-frequency signal, is split into four signals by the signal combiner and splitter  116 . The four signals are passed through four signal paths and separately supplied to the different fed elements  121 . At this time, by controlling the phase shifters  115 A to  115 D disposed on the signal paths with respect to phase, the directivity of the antenna device  120  can be controlled. 
     By contrast, radio-frequency signals received by the fed elements  121  are transferred through four different signal paths and combined together by the signal combiner and splitter  116 . The combined receive signal is down-converted by the mixer  118 , amplified by the amplifier circuit  119 , and transferred to the BBIC  200 . 
     The RFIC  110  may be formed as, for example, a one-chip integrated-circuit component having the circuit configuration described above. Alternatively, in the RFIC  110 , the particular devices (switches, power amplifier, low-noise amplifier, attenuator, and phase shifter) corresponding to each fed elements  121  may be formed as a one-chip integrated-circuit component for the fed element  121 . 
     Antenna Module Structure 
     Next, a structure of the antenna module according to the first embodiment will be described in detail with reference to  FIG. 2 .  FIG. 2  illustrates the antenna module  100  of a first example according to the first embodiment. In  FIG. 2 , a plan view of the antenna module  100  is provided on the upper side ( FIG. 2(A) ), and a cutaway side view is provided on the lower side ( FIG. 2(B) ). 
     The antenna module  100  includes, in addition to the fed elements  121  and the RFIC  110 , a dielectric substrate  130 , a feed line  140 , peripheral electrodes  150 , and ground electrodes GND 1  and GND 2 . In the following description, the normal direction (radiation direction of radio wave) of the dielectric substrate  130  is determined as the Z-axis direction, and a plane perpendicular to the Z-axis direction is defined by the X and Y axes. The side in the forward direction of the Z axis in the drawings may be referred to as upper side, and the side in the reverse direction may be referred to as lower side. 
     The dielectric substrate  130  may be, for example, a low temperature co-fired ceramics (LTCC) multilayer substrate, a multilayer resin substrate formed by stacking a plurality of layers made of a resin such as epoxy or polyimide, a multilayer resin substrate formed by stacking a plurality of resin layers made of a liquid crystal polymer (LCP) having a relatively low permittivity, a multilayer resin substrate formed by stacking a plurality of resin layers made of a fluorocarbon resin, or a multilayer ceramic substrate made of a ceramic other than LTCC. 
     The dielectric substrate  130  is formed in a substantially rectangular shape. The fed element  121  is disposed in a layer (upper layer) close to an upper surface  131  (surface in the forward direction of the Z axis). The fed element  121  may be exposed at a surface of the dielectric substrate  130  or disposed inside the dielectric substrate  130  as in the example in  FIG. 2 . It should be noted that, for ease of description, the embodiments of the present disclosure use the example in which only fed elements serving as radiating elements are used, but unfed elements and/or parasitic elements may be included in addition to fed elements. 
     In the example in  FIG. 2 , as illustrated in  FIG. 2(A) , the sides of the substantially square fed element  121  are tilted by 45° with respect to the sides of the dielectric substrate  130 . This placement is applied with the aim of expanding the frequency bandwidth for radiation of radio wave by leaving a space from the end of the fed element  121  to the end of the dielectric substrate  130  in the polarization direction of radio waves radiated by the fed element  121 . 
     The plate-like ground electrode GND 2  is disposed in a layer (lower layer) closer to a lower surface  132  (surface in the reverse direction of the Z axis) than to the fed element  121  in the dielectric substrate  130 . The ground electrode GND 2  faces toward the fed element  121 . The ground electrode GND 1  is disposed in a layer between the fed element  121  and the ground electrode GND 2 . 
     The layer between the ground electrodes GND 1  and GND 2  is used as a wiring region. A wiring patterns  170  forming elements such as feed lines for suppling a radio-frequency signal to radiating elements, and connection wires for connecting stubs and filters coupled to the feed lines, and other electronic components is disposed in the wiring region. As such, the wiring region is formed in a dielectric layer opposite to the fed element  121  with respect to the ground electrode GND 1 , and as a result, it is possible to reduce unnecessary coupling between the fed element  121  and the wiring patterns  170 . 
     The RFIC  110  is mounted on the lower surface  132  of the dielectric substrate  130  with the solder bumps  160  interposed between the RFIC  110  and the dielectric substrate  130 . The RFIC  110  may be coupled to the dielectric substrate  130  by a multi-pole connector instead of solder joints. 
     The RFIC  110  supplies a radio-frequency signal to a feed point SP 1  of the fed element  121  through the feed line  140 . The feed line  140  is extended upwards from the RFIC  110  through the ground electrode GND 2  and routed in the wiring region. The feed line  140  is further extended upwards from a portion under the fed element  121  through the ground electrode GND 1  and consequently coupled to the feed point SP 1  of the fed element  121 . 
     In the example in  FIG. 2 , the feed point SP 1  of the fed element  121  is offset from the center of the fed element  121  by a given distance in both the forward direction of the X axis and the forward direction of the Y axis. Because the feed point SP 1  is provided at this position, the fed element  121  radiates a radio wave polarized in the direction tilted by 45° from the forward direction of the X axis to the forward direction of the Y axis. 
     The peripheral electrodes  150  are formed in a plurality of dielectric layers between the fed element  121  and the ground electrode GND 1  at end portions of the dielectric substrate  130 . When viewed in a plan view in the normal direction (forward direction of the Z axis) of the dielectric substrate  130 , the peripheral electrodes  150  are disposed along the sides of the rectangular dielectric substrate  130  in the antenna module  100 . The peripheral electrodes  150  disposed along the sides are symmetrically positioned with respect to the fed element  121 . 
     When the dielectric substrate  130  is viewed in a plan view, the peripheral electrodes  150  disposed along one side of the dielectric substrate  130  coincide with each other in the stacking direction. This means that the peripheral electrodes  150  form deemed conductive walls along the sides of the dielectric substrate  130 . It is preferable that the peripheral electrodes  150  be formed as meshes having a plurality of cavities as illustrated in  FIG. 9  described later. Because the peripheral electrodes  150  have cavities, when the dielectric substrate  130  is formed by pressure-bonding a plurality of dielectric layers with each other, adjacent dielectrics are joined through the cavities, resulting in increased close contact between the dielectric layers of the dielectric substrate  130 . 
     In  FIG. 2 , the conductors forming, for example, the fed element elements, electrodes, and vias are made of a metal mainly containing aluminum (Al), copper (Cu), gold (Au), silver (Ag), or an alloy thereof. 
     Of a patch antenna formed by a dielectric substrate having the multilayer structure described above, antenna characteristics are affected by the structure of an antenna region between a ground electrode and a radiating element facing toward the antenna region. For example, when a device or wire establishing coupling with the radiating element is disposed in the antenna region, losses may increase, or the frequency band for radiation of radio wave may narrow. 
     For this reason, elements such as connection wires for connecting stubs and filters coupled to the feed lines, and other electronic components are usually formed in a dielectric layer (wiring region) below a ground electrode in a dielectric substrate, for the purpose of reducing unnecessary coupling with the radiating elements to achieve sufficient antenna characteristics. 
     In such a structure, the residual copper ratio of the antenna region on the radiating element side with respect to the ground electrode of the dielectric substrate is lower than the residual copper ratio of the wiring region below the ground electrode. Resins and ceramics forming dielectrics are more easily deformed by residual stress or thermal stress than conductors used for wiring patterns. The dielectric layer of a relatively low residual copper ratio is thus more largely deformed than the dielectric layer of a relatively high residual copper ratio. As a result, in the case in which a dielectric substrate is formed by subjecting a stack of dielectric layers to a process such as pressing or heat pressing, when the residual copper ratio is unbalanced among stacked layers as in the antenna module described above, non-uniform deformation may cause warpage in the finished dielectric substrate. 
     In the antenna module  100  according to the first embodiment, as described above, the conductive walls of the peripheral electrodes  150  are formed at end portions of the dielectric substrate  130 . As compared to the structure without the peripheral electrodes  150 , this structure can increase the residual copper ratio of the antenna region between the fed element  121  and the ground electrode GND 1 . As such, the difference between the residual copper ratio of the wiring region below the ground electrode GND 1  of the dielectric substrate  130  and the residual copper ratio of the antenna region can be decreased, and thus, it is possible to reduce warpage of the finished dielectric substrate  130 . 
     When the area of the ground electrode is not large enough for the radiating element, some lines of electric force between the radiating element and the ground electrode may be directed behind the ground electrode. The directivity accordingly appears on the back side, and as a result, the gain in the desired direction may decrease, or the frequency bandwidth may narrow. 
     In the antenna module according to the first embodiment, the adjacent peripheral electrodes  150  in the stacking direction can be capacitive-coupled to each other. Additionally, the lowest peripheral electrodes  150  can also be capacitive-coupled to the ground electrode GND 1 . This means that the conductive walls formed by the peripheral electrodes  150  can be deemed as the equivalent of the structure formed by extending the end portions of the ground electrode GND 1  toward the upper surface of the dielectric substrate  130 . The conductive walls can thus strengthen the degree of coupling between the fed element  121  and the ground electrode GND 1 . This structure can reduce the likelihood that some lines of electric force between the radiating element and the ground electrode are directed behind the ground electrode. As described above, when the area of the dielectric substrate  130  is not large enough for the fed element  121  for the purpose of downsizing the device, the leakage of lines of electric force out of the dielectric substrate  130  is reduced by providing the peripheral electrodes  150  described above to strengthen the degree of coupling between the fed element  121  and the ground electrode GND 1 . As a result, antenna characteristics can be improved. 
     Second Example 
       FIG. 3  is a cutaway side view of an antenna module  100 A of a second example according to the first embodiment. The arrangement of the peripheral electrodes  150  in the stacking direction in the antenna module  100 A is different from the antenna module  100  illustrated in  FIG. 2 . Other structures of the antenna module  100 A are the same as the antenna module  100 , and redundant descriptions of the same elements are not repeated. 
     More specifically, referring to  FIG. 3 , as the dielectric layer including the peripheral electrode  150  approaches the ground electrode GND 1 , the peripheral electrode  150  approaches the middle of the dielectric substrate  130  in the antenna module  100 A. In other words, when viewed in a plan view in the normal direction of the dielectric substrate  130 , as the peripheral electrode  150  approaches the ground electrode GND 1 , the peripheral electrode  150  approaches the fed element  121 . 
     This structure also strengthens the degree of coupling between the fed element  121  and the ground electrode GND 1 , resulting in improved antenna characteristics. Moreover, the dielectric surrounded by the fed element  121 , the ground electrode GND 1 , and the conductive walls of the peripheral electrodes  150  is smaller than the structure of the antenna module  100  illustrated in  FIG. 2 , and thus, the electrostatic capacity between the fed element  121  and the ground electrode GND 1  is decreased. Thus, it is possible to expand the frequency bandwidth for radiation of radio wave. 
     Antenna Characteristics 
     The following describes antenna characteristics of the antenna modules  100  and  100 A according to the first embodiment with reference to  FIGS. 4 and 5 . As a comparative example, an antenna module  100  # without the peripheral electrodes  150  is described with reference to  FIGS. 4 and 5 . Except for the peripheral electrodes  150 , the structure of the antenna module  100  # of the comparative example is the same as the antenna modules  100  and  100 A, and the description thereof is not repeated. 
       FIG. 4  illustrates the result of a simulation about return loss with respect to the antenna module  100  # of the comparative example, the antenna module  100  of the first example, and the antenna module  100 A of the second example. In graphs in  FIG. 4 , the horizontal axis indicates frequency, and the vertical axis indicates return loss. In this simulation, the target pass band is 24 to 30 GHz, and the return loss range in specifications is the range of 10 dB or less. 
     Referring to  FIG. 4 , the return loss of the antenna module  100  # of the comparative example exceeds the limit in the specifications over the target pass band except frequencies close to 30 GHZ. By contrast, the return loss of the antenna module  100  of the first example is within the specification range over the entire target pass band. As such, the antenna characteristic of the antenna module  100  of the first example is improved as compared to the comparative example. 
     The return loss of the antenna module  100 A of the second example is decreased more than the antenna module  100  of the first example, and additionally, the specification of return loss is satisfied over an expanded frequency band. 
       FIG. 5  indicates peak gain of each antenna module. In  FIG. 5 , the horizontal axis indicates angle relative to the normal direction of the fed element  121 , and the vertical axis indicates peak gain. In  FIG. 5 , a solid line LN 10  indicates the case of the antenna module  100 A of the second example, a dashed line LN 11  indicates the case of the antenna module  100  of the first example, and a dot-dash line LN 12  indicates the case of the antenna module  100  # of the comparative example. 
     Referring to  FIG. 5 , it can be seen that the peak gain of the antenna module  100  according to the first embodiment and the peak gain of the antenna module  100 A according to the first embodiment at an angle of 0° is higher by approximately 1 dBi than the comparative example. When the antenna module  100  is compared to the antenna module  100 A, the peak gain of the antenna module  100 A is higher by approximately 0.1 dB than the antenna module  100 . 
     Concerning the radiation of radio wave in the range exceeding ±90°, that is, the radiation behind the back side of the antenna module, the gain of the antenna module  100  according to the first embodiment and the gain of the antenna module  100 A according to the first embodiment are lower than the comparative example. This means that the radiation of radio wave in unnecessary directions (back surface) is reduced. 
     As described above, antenna characteristics of the antenna module formed by a dielectric substrate having a multilayer structure can be improved by forming conductive walls of peripheral electrodes at end portions of the dielectric substrate. With this structure, the antenna module can achieve desired specifications when the dielectric substrate is not large enough for the radiating element. 
     First Modification 
     In the descriptions of the antenna modules  100  and  100 A of the first and second examples, the peripheral electrodes are capacitive-coupled to each other, and the peripheral electrodes and the ground electrode are capacitive-coupled to each other. The peripheral electrodes may be, however, directly coupled to the ground electrode. 
       FIG. 6  is a cutaway side view of an antenna module  100 B according to a first modification. Referring to  FIG. 6 , in the antenna module  100 B, the adjacent peripheral electrodes  150  in the stacking direction are coupled to each other by vias  155 . The lowest peripheral electrodes  150  are also coupled to the ground electrode GND 1  by the vias  155 . This means that in the antenna module  100 B the peripheral electrodes  150  effectively serve as the ground electrode GND 1 . Consequently, the fed element  121  and the peripheral electrodes  150  are more easily coupled to each other, and thus, antenna characteristics can be further improved. 
     Dielectrics such as resins and ceramics used for the dielectric substrate  130  are usually easy to cause static electricity. Hence, in the manufacturing process of the antenna module, the dielectric substrate  130  may be transported in the state in which the dielectric substrates charged with static electricity are stacked. Static electricity in the dielectric can be reduced by providing peripheral electrodes coupled to the ground electrode in a plurality of layers of the dielectric substrate  130  as the antenna module  100  of the first modification. As such, it is possible to reduce the likelihood of occurrence of faults during transportation of the dielectric substrate. 
     It is preferable that in the antenna module  100 B the vias  155  formed in adjacent dielectric layers in the stacking direction do not overlap when viewed in a plan view in the normal direction of the dielectric substrate  130 . When pressurized, the compressibility of the conductive material (typically, copper) forming the via  155  is lower than the compressibility of the dielectric material. Hence, if the vias  155  of the individual layers are all disposed at the same position when viewed in a plan view in the normal direction of the dielectric substrate  130 , when the dielectric substrate  130  is pressed to bond dielectric layers by pressure bonding, the thickness of the portion of the via  155  is reduced more than other dielectric portions. This may cause variations in thickness in the dielectric substrate  130 . In this regard, by disposing at different positions the vias  155  of adjacent dielectric layers in the stacking direction, it is possible to improve the precision of thickness of the finished dielectric substrate  130 . 
     The peripheral electrodes may be coupled to each other by capacitive coupling as in  FIG. 2  and vias as in  FIG. 6  in a mixed manner. This means that in the present embodiment the expression “electrically coupled” includes both direct coupling using vias and capacitive coupling. The peripheral electrodes are not necessarily disposed at regular intervals in the stacking direction. For example, some peripheral electrodes may be disposed at an interval wider than other peripheral electrodes. 
     Second Modification 
     The first embodiment and the first modification have described the example of an antenna module including one fed element serving as a radiating element, but the antenna module may be an array antenna including a plurality of radiating elements. 
       FIG. 7  is a plan view of an antenna module  100 C of a second modification. In the example of the antenna module  100 C, four fed elements  121  are arranged in line in the long-side direction (X-axis direction in  FIG. 7 ) of the rectangular dielectric substrate  130 , forming a structure of one-dimensional array. In the antenna module  100 C the sides of each fed elements  121  are parallel to the sides of the dielectric substrate  130 , but the fed elements may be tilted with respect to the sides of the dielectric substrate  130  as in the first embodiment. The antenna module may be an array antenna including the fed elements  121  arranged in a two-dimensional array. 
     At the end portions of the short sides of the dielectric substrate  130 , the peripheral electrodes  150  are disposed in a layer between the fed element  121  and the ground electrode GND 1  in the direction (Y-axis direction) in which the short sides are extended. Also, at the end portions of the long sides of the dielectric substrate  130 , peripheral electrodes  151  are disposed in the direction (X-axis direction) in which the long sides are extended. About the antenna module  100 C, the example in which the plurality of peripheral electrodes  151  are arranged with spaces therebetween along the X axis is described, but one peripheral electrode may be extended over one long side, which is similar to the peripheral electrode  150  extended along the Y axis. 
     Also in such an array antenna, peripheral electrodes are arranged in a layer (antenna region) between the fed element  121  and the ground electrode GND 1 , and as a result, the residual copper ratio of the antenna region is increased. As such, it is possible to reduce warpage of the dielectric substrate  130 . Moreover, peripheral electrodes are arranged at end portions of the substrate at which it is difficult to leave sufficient areas of dielectric, and thus, the degree of coupling between the fed element  121  and the ground electrode GND 1  is increased, resulting in improved antenna characteristics. 
     In the structure as in  FIG. 7 , when the length of the peripheral electrode  151  disposed along the long side of the dielectric substrate  130  is shorter than the length of the peripheral electrode  150  disposed along the short side, it is possible to reduce warpage locally caused in the dielectric substrate  130  under the effect of the peripheral electrode  151  in the long-side direction. 
     To reduce warpage of the dielectric substrate  130 , the peripheral electrode  151  disposed along one long side of the dielectric substrate  130  may be formed to have a length different from the length of the peripheral electrode  151  disposed along the other long side. Alternatively, to reduce warpage of the dielectric substrate  130 , the peripheral electrode  151  disposed along one long side may differ from the peripheral electrode  151  disposed along the other long side with respect to the number of electrodes along the side and/or the number of electrodes in the thickness direction. As described above, by controlling the number and/or length of the peripheral electrodes  151  disposed along each of the two long sides, it is possible to reduce warpage especially when the distance from the fed element  121  to the end portion of the dielectric substrate  130  differs between the different long sides. In this case, the peripheral electrodes  151  may be arranged along only one side. 
     Second Embodiment 
     Collective Board Structure 
     As described in the first embodiment and modifications, the antenna module has the structure including a stack of dielectric layers. In a typical manufacturing process, a dielectric substrate is formed as follows: collective boards of different kinds of dielectric layers are individually formed by arranging in a matrix a plurality of individual boards of the same kind of dielectric layer; a stack of the collective boards are bonded together by heat pressing; the individual boards are cut off by, for example, a dicer. 
     The first embodiment has described the example in which peripheral electrodes are formed in an individual board. A second embodiment describes an example in which peripheral electrodes are formed around an individual board in a collective board, instead of arranging peripheral electrodes in an individual board. 
       FIG. 8  illustrates a collective board  300  according to the second embodiment. The collective board  300  is basically made of a dielectric plate and conductive members formed on a surface of the dielectric. The conductive members form, for example, the fed elements  121 , the ground electrodes GND 1  and GND 2 , the wiring patterns  170 , and vias, which have been described with reference to drawings including  FIG. 2 . 
     The collective board  300  has a structure including a plurality of individual boards  310  arranged in a matrix as a two-dimensional array. The individual boards  310  correspond to dielectric layers forming the dielectric substrate  130  illustrated in  FIG. 2 . The individual boards  310  in one collective board  300  have the same kind of dielectric layer. The conductive members are formed at the individual boards  310  at corresponding positions in the stacking direction. 
     Peripheral electrodes  350  are disposed between adjacent individual boards  310  and at outer side portions of the collective board  300 . This means that the peripheral electrodes  350  are shaped into a grid, and the individual boards  310  are formed inside the grid. 
       FIG. 9  provides an enlarged view of a portion including the peripheral electrodes  350  of the collective board  300 . As illustrated in the enlarged view in  FIG. 9(B) , a plurality of cavities  351  are formed as a mesh in the peripheral electrodes  350 . As described above, the dielectric substrate  130  is formed by stacking a plurality of kinds of the collective boards  300 , pressure-bonding the stack of the collective boards  300 , and dividing the stack of the collective boards  300  into the individual boards  310  by cutting. Because the cavities  351  are formed in the peripheral electrodes  350 , the dielectric materials are connected with each other through the cavities  351  during pressure bonding. This can strengthen the degree of close contact between dielectric layers. 
     When the collective boards  300  are cut to separate the individual boards  310 , the peripheral electrodes  350  are removed. This means that, unlike the case of the first embodiment, the peripheral electrodes  350  are not left in the individual boards  310  forming the dielectric layers of the dielectric substrate  130 . However, because the peripheral electrodes  350  are formed in the collective boards corresponding to the dielectric layers forming the antenna region between the fed element  121  and the ground electrode GND 1 , the residual copper ratio of the dielectric layers forming the antenna region is increased when the stack of the collective boards  300  are pressure bonded together. As a result, it is possible to reduce warpage of the collective board  300  after pressure bonding, and accordingly, it is also possible to mitigate warpage of the individual boards  310  separated by cutting. 
     Manufacturing Process of Antenna Module 
       FIG. 10  illustrates a manufacturing process of an antenna module using the collective board  300  according to the second embodiment. 
     Referring to  FIG. 10(A) , firstly, collective boards  301  to  307  corresponding to dielectric layers are prepared to form the dielectric substrate  130 . The collective board is formed by etching a desired shape in a copper foil fixed to one surface of a dielectric sheet. Vias are also formed to penetrate the dielectric sheet as needed. In each collective board, first regions AR 1  each including an individual board and second regions AR 2  formed between adjacent individual boards or formed at outer side portions with respect to the individual boards are formed. The peripheral electrodes  350  are formed in the second regions AR 2  formed at the outer side portions with respect to the individual boards. 
     The fed element  121  is formed in the first region AR 1  of the collective board  301 . The peripheral electrode  350  is formed in the second regions AR 2 . The collective boards  302  and  303  correspond to dielectric layers of the antenna region. In each of the first region AR 1  of the collective board  302  and the first region AR 1  of the collective board  303 , a via  340  forming a portion of the feed line  140  and an electrode pad  330  connected to the via  340  are formed. 
     The collective boards  304  and  306  correspond to dielectric layers respectively forming the ground electrodes GND 1  and GND 2 . In the collective boards  304  and  306 , the peripheral electrodes are formed in the second regions AR 2  in the manner in which the peripheral electrodes are combined with the ground electrodes. 
     The collective board  305  is disposed between the collective boards  304  and  306 . The collective board  305  corresponds to a dielectric layer forming a wiring layer. In the description of the example in  FIG. 10 , for ease of description, the collective board  305  solely corresponds to the wiring layer, but a plurality of collective boards may form the wiring layer. Wiring patterns forming, for example, connection wires connecting filters, stubs, and devices, and the via  340  and the electrode pad  330  forming a portion of the feed line  140  are formed in the first region AR 1  of the collective board  305 . The peripheral electrode  350  is formed in the second region AR 2  of the collective board  305 . 
     The collective board  307  corresponds to a dielectric layer for mounting devices such as the RFIC  110 . The via  340  and the electrode pad  330  for establishing electrical connection with external devices are formed in the first region AR 1  of the collective board  307 . 
     After all the collective boards  301  to  307  are prepared to form the dielectric substrate  130 , the collective boards  301  to  307  are stacked ( FIG. 10(B) ), and the collective boards are pressure bonded together by heat pressing ( FIG. 10(C) ). 
     Subsequently, the dielectric substrate  130  formed by pressure bonding the collective boards are cut by, for example, a dicer at the boundaries between the first regions AR 1  and the second regions AR 2 , which are indicated by dashed lines in the drawing, and the second regions AR 2  are then removed. As such, the antenna module  100 D is formed ( FIG. 10(D) ). 
     Because the antenna module is formed in accordance with the manufacturing process described above, in the pressure bonding process for collective boards, the residual copper ratio of the antenna region between the fed element and the ground electrode is increased by using the peripheral electrodes. It is thus possible to reduce warpage of the dielectric substrate when the pressure bonding process finishes. 
     The embodiments disclosed herein should be considered as examples in all respects and not construed in a limiting sense. The scope of the present disclosure is indicated by the claims instead of the above description of the embodiments, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.