Patent Publication Number: US-2023135774-A1

Title: Interconnect substrate, method of manufacturing the same, and semiconductor apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims priority to Japanese Patent Application No. 2021-179520 filed on Nov. 2, 2021, with the Japanese Patent Office, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The disclosures herein relate to interconnect substrates, methods of making the same, and semiconductor apparatuses. 
     BACKGROUND 
     An interconnect substrate for mounting a semiconductor chip as known in the art constitutes part of a semiconductor apparatus on which the semiconductor chip is mounted (see, for example, Patent Document 1). When the semiconductor chip is designed to handle radio-frequency signals, a microstrip line or a stripline may be provided on the interconnect substrate in order to control the characteristic impedance. 
     The line width and space of the interconnect pattern constituting a stripline can be made smaller than the line width and space of the interconnect pattern constituting a microstrip line having the same characteristic impedance. Use of a stripline thus enables the increase of interconnect density in an interconnect substrate. 
     Reducing the line width and space of the interconnect pattern constituting a stripline ends up lowering the adhesion strength between the interconnect pattern and insulating layers. There is thus a risk that the interconnect pattern may be disconnected from the insulating layers. 
     Accordingly, it may be preferable to improve adhesion between an interconnect pattern constituting a stripline and insulating layers in an interconnect substrate having the stripline. 
     PRIOR ART DOCUMENT 
     [Patent Document 1] Japanese Laid-Open Patent Publication No. 2014-063801 
     SUMMARY 
     According to an aspect of the embodiment, an interconnect substrate includes a first interconnect layer, a first insulating layer covering a side surface and a lower surface of the first interconnect layer, a second insulating layer disposed on the first insulating layer and covering an upper surface of the first interconnect layer, a second interconnect layer formed on a lower surface of the first insulating layer, and a third interconnect layer formed on an upper surface of the second insulating layer, wherein the second interconnect layer includes a first ground plane, wherein the third interconnect layer includes a second ground plane, and wherein the first ground plane, the first insulating layer, the first interconnect layer, the second insulating layer, and the second ground plane constitute a stripline. 
     The object and advantages of the embodiment will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view illustrating an example of an interconnect substrate according to a first embodiment; 
         FIGS.  2 A through  2 C  are partial plan views illustrating the interconnect substrate according to the first embodiment; 
         FIGS.  3 A through  3 C  are drawings illustrating an example of a method of making the interconnect substrate of the first embodiment; 
         FIGS.  4 A through  4 C  are drawings illustrating the example of the method of making the interconnect substrate of the first embodiment; 
         FIGS.  5 A through  5 C  are drawings illustrating the example of the method of making the interconnect substrate of the first embodiment; 
         FIGS.  6 A and  6 B  are drawings illustrating the example of the method of making the interconnect substrate of the first embodiment; 
         FIG.  7    is a cross-sectional view illustrating an example of an interconnect substrate according to a first variation of the first embodiment; 
         FIGS.  8 A through  8 C  are drawings illustrating an example of a method of making the interconnect substrate of the first variation of the first embodiment; 
         FIGS.  9 A and  9 B  are drawings illustrating the example of the method of making the interconnect substrate of the first variation of the first embodiment; 
         FIGS.  10 A through  10 C  are drawings illustrating an example of a detailed method of forming an interconnect layer; 
         FIGS.  11 A and  11 B  are drawings illustrating the example of the detailed method of forming an interconnect layer; 
         FIGS.  12 A through  12 C  are drawings illustrating a further example of the detailed method of forming an interconnect layer; 
         FIGS.  13 A and  13 B  are drawings illustrating the further example of the detailed method of forming an interconnect layer; 
         FIGS.  14 A and  14 B  are drawings illustrating the further example of the detailed method of forming an interconnect layer; 
         FIG.  15    is a cross-sectional view illustrating an example of a semiconductor apparatus according to an example of practical application of the first embodiment; and 
         FIG.  16    is a cross-sectional view illustrating another example of the semiconductor apparatus according to an example of practical application of the first embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, embodiments of the present invention will be described with reference to the accompanying drawings. In the drawings, the same components are referred to by the same reference numerals, and a duplicate description thereof may be omitted. 
     First Embodiment 
     [Structure of Interconnect Substrate] 
       FIG.  1    is a cross-sectional view illustrating an example of an interconnect substrate according to a first embodiment.  FIGS.  2 A through  2 C  are partial plan views illustrating the interconnect substrate according to the first embodiment. Specifically,  FIG.  2 A  is a partial plan view of the portion A in  FIG.  1    viewed from the upper surface side of a first interconnect layer  11 .  FIG.  2 B  is a partial plan view of the portion B in  FIG.  1    viewed from the lower surface side of a second interconnect layer  14 .  FIG.  2 C  is a partial plan view of the portion C in  FIG.  1    viewed from the upper surface side of a third interconnect layer  15 . 
     Referring to  FIG.  1   , the interconnect substrate  1  includes a first interconnect layer  11 , a first insulating layer  12 , a second insulating layer  13 , a second interconnect layer  14 , a third interconnect layer  15 , a third insulating layer  16 , a fourth interconnect layer  17 , a solder resist layer  21 , and a solder resist layer  22 . 
     In the present embodiment, for the sake of convenience, the side of the interconnect substrate  1  on which the solder resist layer  21  is situated in  FIG.  1    is referred to as an upper side or a first side, and the side on which the solder resist layer  22  is situated is referred to as a lower side or a second side. A surface (or face) of a given member that faces toward the upper side is referred to as a first surface (or first face) or an upper surface (or upper face), and a surface (or face) of the given member that faces toward the lower side is referred to as a second surface (or second face) or a lower surface (or lower face). It may be noted, however, that the interconnect substrate  1  may be used in an upside-down position, or may be placed at any angle. Further, a plan view refers to a view taken in the direction perpendicular to the first surface of the solder resist layer  21 , and a plane shape refers to the shape of an object as viewed in the direction perpendicular to the first surface of the solder resist layer  21 . When the interconnect substrate  1  is illustrated upside down relative to  FIG.  1   , the definition of the upper surface and the lower surface becomes opposite to the above-noted definition in accordance with the orientation of what is illustrated in the drawing. 
     The first interconnect layer  11  includes a plurality of interconnect patterns  11 W and a plurality of pads  11 P. The plane shape of the pads  11 P is, for example, a circle having a diameter of about 60 μm to 120 μm. Within this noted range, any diameter of the pads  11 P is smaller than the diameter of first signal pads  14 A and the diameter of second signal pads  15 A, which will be described later. Cu or the like, for example, may be used as the material of the interconnect patterns  11 W and the pads  11 P. The interconnect patterns  11 W and the pads  11 P may have a laminated structure comprised of a plurality of metal layers. The thicknesses of the interconnect patterns  11 W and the pads  11 P may be, for example, about 10 to 30 μm. 
     The first interconnect layer  11  has the highest density among the interconnect patterns of the interconnect substrate  1 . In other words, the minimum interval between adjacent interconnect patterns  11 W in the first interconnect layer  11  is narrower than the minimum interval between adjacent interconnect patterns in the other interconnect layers. The line width and space of the interconnect patterns  11 W may approximately be a line width of 8 to 10 μm and a space of 8 to 10 μm, for example. The line width represents the width of an interconnect line, and the space represents an interval between adjacent interconnect lines (i.e., interconnect interval). The notation “a line width of 8 pm and a space of 10 μm” represents that the width of an interconnect line is 8 μm and the interval between adjacent interconnect lines is 10 μm. 
     The first insulating layer  12  is formed so as to cover the side surface and the lower surface of the first interconnect layer  11 . The upper surface of the first insulating layer  12  may be flush with the upper surface of the first interconnect layer  11 , for example. The material of the first insulating layer  12  may be, for example, an insulating resin containing an epoxy-based resin or a polyimide-based resin as a main component. The thickness of the first insulating layer  12  may be, for example, about 20 to 30 μm. Here, the thickness of the first insulating layer  12  is a distance from the lower surface of the first interconnect layer  11  to the lower surface of the first insulating layer  12 . The same applies to the thicknesses of other insulating layers and the like. The first insulating layer  12  may contain a filler such as silica (SiO 2 ). The content of the filler in the first insulating layer  12  may be appropriately set according to the required coefficient of thermal expansion (CTE). 
     The second insulating layer  13  that covers the upper surface of the first interconnect layer  11  is laminated on the upper surface of the first insulating layer  12 . The material and thickness of the second insulating layer  13  may be, for example, substantially the same as those of the first insulating layer  12 . The second insulating layer  13  can contain a filler such as silica (SiO 2 ). The content of the filler in the second insulating layer  13  may be, for example, substantially the same as that in the first insulating layer  12 . 
     The second interconnect layer  14  is formed on the lower surface of the first insulating layer  12 . The second interconnect layer  14  includes a first ground plane  14 G, first signal pads  14 A, a first ground pad  14 B, and via interconnects  14 V. The first ground plane  14 G is formed in a substantially solid shape so as to cover most of the lower surface of the first insulating layer  12 . The first signal pads  14 A are disposed in openings  14   x  provided in the first ground plane  14 G. The plane shape of the first signal pads  14 A is, for example, a circle having a diameter of about 80 μm to 200 μm. The first ground pad  14 B forms part of the first ground plane  14 G. 
     Each of the first signal pads  14 A and the first ground pad  14 B is formed integrally with a corresponding one of the via interconnects  14 V filling the via holes  12   x  that extend through the first insulating layer  12  and expose the lower surface of the first interconnect layer  11 . The via holes  12   x  may be a truncated cone-shaped recess, with the size of the opening thereof on the third insulating layer  16  side being larger than the size of the bottom surface thereof formed by the lower surface of the pads  11 P. In this case, the area of the portion of each via interconnect  14 V connected to the lower surface of the pads  11 P is smaller than the area of the portion of each via interconnect  14 V connected to the upper surface of the first signal pads  14 A or the first ground pad  14 B. 
     The plurality of pads  11 P include pads  11 P electrically connected to the first signal pads  14 A through the respective via interconnects  14 V extending through the first insulating layer  12 , and also include a pad  11 P electrically connected to the first ground pad  14 B through a corresponding one of the via interconnects  14 V extending through the first insulating layer  12 . The material of the second interconnect layer  14  and the thickness of the ground plane and the pads may be substantially the same as those of the first interconnect layer  11 , for example. 
     The third interconnect layer  15  is formed on the upper surface of the second insulating layer  13 . The third interconnect layer  15  includes a second ground plane  15 G, second signal pads  15 A, a second ground pad  15 B, and via interconnects  15 V. The second ground plane  15 G is formed in a substantially solid shape so as to cover most of the upper surface of the second insulating layer  13 . The second signal pads  15 A are disposed in openings  15   x  provided in the second ground plane  15 G. The plane shape of the second signal pads  15 A is, for example, a circle having a diameter of about 80 μm to 200 μm. The second ground pad  15 B forms part of the second ground plane  15 G. 
     Each of the second signal pads  15 A and the second ground pad  15 B is formed integrally with a corresponding one of the via interconnects  15 V filling the via holes  13   x  that extend through the second insulating layer  13  and expose the upper surface of the first interconnect layer  11 . The via holes  13   x  may be an inverted truncated cone-shaped recess, with the size of the opening thereof on the solder resist layer  21  side being larger than the size of the bottom surface thereof formed by the upper surface of the pads  11 P. In this case, the area of the portion of each via interconnect  15 V connected to the upper surface of the pads  11 P is smaller than the area of the portion of each via interconnect  15 V connected to the lower surface of the second signal pads  15 A or the second ground pad  15 B. 
     The plurality of pads  11 P include pads  11 P electrically connected to the second signal pads  15 A through the respective via interconnects  15 V extending through the second insulating layer  13 , and include a pad  11 P electrically connected to the second ground pad  15 B through a corresponding one of the via interconnects  15 V extending through the second insulating layer  13 . The material of the third interconnect layer  15  and the thickness of the ground plane and the pads may be substantially the same as those of the first interconnect layer  11 , for example. 
     The third insulating layer  16  is formed on the lower surface of the first insulating layer  12  so as to cover the second interconnect layer  14 . The material and thickness of the third insulating layer  16  may be, for example, substantially the same as those of the first insulating layer  12 . The third insulating layer  16  may contain a filler such as silica (SiO 2 ). The content of the filler in the third insulating layer  16  may be, for example, substantially the same as that in the first insulating layer  12 . 
     The fourth interconnect layer  17  is formed on the second side of the third insulating layer  16 . The fourth interconnect layer  17  includes via interconnects filling via holes  16   x  extending through the third insulating layer  16  and exposing the lower surface of the second interconnect layer  14 , pads formed on the lower surface of the third insulating layer  16 , and interconnect patterns. The pads constituting the fourth interconnect layer  17  are electrically connected to the first signal pads  14 A or the first ground pad  14 B through the via interconnects. The via holes  16   x  may be a truncated cone-shaped recess, with the size of the opening thereof on the solder resist layer  22  side being larger than the size of the bottom surface thereof formed by the lower surface of the first signal pads  14 A or the first ground pad  14 B. The material of the fourth interconnect layer  17  and the thickness of the pads and the interconnect patterns may be substantially the same as those of the first interconnect layer  11 , for example. 
     The solder resist layer  21  is an outermost layer on the first side of the interconnect substrate  1 , and is formed on the upper surface of the second insulating layer  13  so as to cover the third interconnect layer  15 . The solder resist layer  21  may be formed of, for example, a photosensitive resin such as an epoxy-based resin or an acrylic-based resin. The thickness of the solder resist layer  21  may be, for example, about 15 to 35 μm. 
     The solder resist layer  21  has openings  21   x . Part of the upper surface of the second signal pads  15 A or the second ground pad  15 B constituting the third interconnect layer  15  is exposed at the bottom of the openings  21   x.  The plane shape of the openings  21   x  may be, for example, a circle. The second signal pads  15 A and the second ground pad  15 B of the third interconnect layer  15  exposed in the openings  21   x  may be used for electrical connection with a semiconductor chip. That is, the solder resist layer  21  side is the side on which a semiconductor chip is mounted. 
     If necessary, a metal layer may be formed, or an anti-oxidation treatment such as an organic solderability preservative (i.e., OSP) treatment may be performed, on the upper surface of the second signal pads  15 A or the second ground pad  15 B exposed in the openings  21   x.  Examples of the metal layer include an Au layer, an Ni/Au layer (i.e., a metal layer in which an Ni layer and an Au layer are laminated in this order), and an Ni/Pd/Au layer (i.e., a metal layer in which an Ni layer, a Pd layer, and an Au layer are laminated in this order). 
     The solder resist layer  22  is the outermost layer on the second side of the interconnect substrate  1 , and is formed on the lower surface of the third insulating layer  16  so as to cover the fourth interconnect layer  17 . The material and thickness of the solder resist layer  22  may be, for example, substantially the same as those of the solder resist layer  21 . The solder resist layer  22  has openings  22   x . Part of the lower surface of the pads of the fourth interconnect layer  17  is exposed in the openings  22   x . The plane shape of the openings  22   x  may be, for example, a circle. The pads of the fourth interconnect layer  17  exposed in the openings  22   x  may be used for electrical connection with a mounting substrate (not illustrated) such as a motherboard. If necessary, the above-described metal layer may be formed, or an anti-oxidation treatment such as an OSP treatment may be performed, on the lower surface of the pads of the fourth interconnect layer  17  exposed in the openings  22   x.    
     As described above, the interconnect substrate  1  is configured such that the first interconnect layer  11  is interposed between the first ground plane  14 G and the second ground plane  15 G in the vertical direction, with the intervening insulating layers (i.e., the first insulating layer  12  and the second insulating layer  13 ). That is, the interconnect substrate  1  is configured such that the first ground plane  14 G, the first insulating layer  12 , the first interconnect layer  11 , the second insulating layer  13 , and the second ground plane  15 G constitute a stripline SL. With this arrangement, the characteristic impedance of the first interconnect layer  11  can be controlled, and electromagnetic waves leaking from the first interconnect layer  11  can be reduced. 
     The stripline SL is preferably disposed directly below the solder resist layer  21  on which a semiconductor chip is mounted. This arrangement ensures a short interconnect path connecting the semiconductor chip and the first interconnect layer  11 . As a result, a semiconductor apparatus made by placing the semiconductor chip on the interconnect substrate  1  will be provided with improved electrical characteristics. 
     Further, the interconnect substrate  1  is configured such that the first interconnect layer  11  constituting the stripline SL is an embedded interconnect whose side surface and lower surface are covered with the first insulating layer  12 . As a result, the adhesion between the first insulating layer  12  and the interconnect patterns  11 W or pads  11 P constituting the first interconnect layer  11  is improved, as compared with the conventional configuration using a non-embedded interconnect that has only the lower surface thereof in contact with an insulating layer and that has the side surface thereof exposed outside the insulating layer. 
     In the interconnect substrate  1 , the area of the portion of each via interconnect  14 V connected to the lower surface of the pads  11 P is smaller than the area of the portion of each via interconnect  14 V connected to the upper surface of the first signal pads  14 A or the first ground pad  14 B. The area of the portion of each via interconnect  15 V connected to the upper surface of the pads  11 P is smaller than the area of the portion of each via interconnect  15 V connected to the lower surface of the second signal pads  15 A or the second ground pad  15 B. As a result, the area of the lower surface of each pad  11 P can be made smaller than the area of the upper surface of each first signal pad  14 A, and the area of the upper surface of each pad  11 P can be made smaller than the area of the lower surface of each second signal pad  15 A, thereby making it possible to increase the density of the first interconnect layer  11 . 
     [Method of Making Interconnect Substrate] 
     In the following, a method of making the interconnect substrate according to the first embodiment will be described.  FIGS.  3 A to  6 B  are drawings illustrating a manufacturing process of the interconnect substrate according to the first embodiment. The example of the process illustrated here is directed to the configuration in which a layer structure is formed only on the first side of a support, but may as well be directed to the configuration in which a layer structure is formed on both the first side and the second side of a support. Broken lines C in each drawing indicate positions to be cut when the interconnect substrate is divided into individual pieces. A region located between the adjacent broken lines C in a cross-sectional view is made into a single peace, which constitutes one interconnect substrate. 
     In the step illustrated in  FIG.  3 A , a support  300  is prepared. The support  300  is structured, for example, such that a carrier-attached copper foil  304  is laminated on the first side of a core substrate  301 . The core substrate  301  is, for example, a resin substrate having a thickness of about 0.7 mm, and may include a reinforcing member such as glass fibers. The carrier-attached copper foil  304  is structured such that a thin foil  304   a  made of, for example, Cu and having a thickness of about 1.5 to 5 μm is detachably attached to a thick foil (carrier foil)  304   b  made of, for example, Cu and having a thickness of about 10 to 50 μm via a release layer (not shown). The thick foil  304   b  is provided as a support to facilitate handling of the thin foil  304   a.    
     It may be noted that the above-described structure of the support  300  is only a non-limiting example. For example, the support  300  may be configured such that a laminate made by sticking together a plurality of prepregs may be used instead of the core substrate  301 . Further, the support  300  may be structured such that the carrier-attached copper foil  304  is disposed on the first side of a glass substrate, a metal substrate, or the like via a release layer. 
     In the step illustrated in  FIG.  3 B , the first interconnect layer  11  including the interconnect patterns  11 W and the pads  11 P is formed on the support  300 . To be specific, a resist layer having openings at the positions where the first interconnect layer  11  is to be formed is formed on the upper surface of the carrier-attached copper foil  304  (i.e., the upper surface of the thin foil  304   a ) by using a dry film resist or the like. Then, the first interconnect layer  11  is formed as an electrolytic plating layer on the upper surface of the carrier-attached copper foil  304  exposed in the openings by an electrolytic plating method using the carrier-attached copper foil  304 , which is a metal layer, as a power feeding layer. The material and thickness of the first interconnect layer  11  are as described above. Subsequently, the resist layer is removed by peeling. 
     In the step illustrated in  FIG.  3 C , the first insulating layer  12  is formed on the support  300  to cover the surface of the first interconnect layer  11  that is not in contact with the support  300 . Specifically, the first insulating layer  12  covering the first interconnect layer  11  is formed on the upper surface of the carrier-attached copper foil  304 . First, for example, a semi-cured insulating resin film containing a thermosetting resin as a main component is prepared. Then, the insulating resin film is laminated on the upper surface of the carrier-attached copper foil  304 , and is cured by application of heat and pressure to form the first insulating layer  12 . Alternatively, instead of laminating the insulating resin film, an insulating resin liquid or paste may be applied and then cured to form the first insulating layer  12 . The material and the thickness of the first insulating layer  12  are as described above. 
     In the step illustrated in  FIG.  4 A , the via holes  12   x  that extend through the first insulating layer  12  and expose the upper surface of the first interconnect layer  11  are formed in the first insulating layer  12 . The via holes  12   x  may be formed by, for example, a laser processing method using a CO 2  laser, a YAG laser, an excimer laser, or the like. After the via holes  12   x  are formed, a desmear treatment is preferably performed to remove resin residues adhering to the surface of the first interconnect layer  11  exposed at the bottom of the via holes  12   x.    
     In the step illustrated in  FIG.  4 B , the second interconnect layer  14  is formed on the first insulating layer  12 . As was previously described, the second interconnect layer  14  includes the first ground plane  14 G, the first signal pads  14 A, the first ground pad  14 B, and the via interconnects  14 V. The material and thickness of the second interconnect layer  14  are as described above. A method of forming the second interconnect layer  14  will be described later in detail. 
     In the steps illustrated in  FIGS.  4 C and  5 A , the support  300  illustrated in  FIG.  4 B  is removed. In order to remove the support  300 , the core substrate  301  and the thick foil  304   b  are mechanically peeled off from the thin foil  304   a  as illustrated in  FIG.  4 C . Then, as illustrated in  FIG.  5 A , the thin foil  304   a  is removed by wet etching using, for example, a ferric chloride aqueous solution, a cupric chloride aqueous solution, an ammonium persulfate aqueous solution, or the like. 
     In the step illustrated in  FIG.  5 B , the second insulating layer  13  is formed on the first side of the first insulating layer  12  so as to cover the surface of the first interconnect layer  11  that was in contact with the support  300 . Further, the third insulating layer  16  is formed on the second side of the first insulating layer  12  so as to cover the second interconnect layer  14 . Specifically, for example, two semi-cured insulating resin films containing a thermosetting resin as a main component are prepared. Then, the insulating resin films are laminated on the first surface and the second surface of the first insulating layer  12 , and are cured by application of heat and pressure to form the second insulating layer  13  and the third insulating layer  16 . Alternatively, instead of laminating the insulating resin films, an insulating resin liquid or paste may be applied and then cured to form the second insulating layer  13  and the third insulating layer  16 . The material and thickness of the second insulating layer  13  and the third insulating layer  16  may be, for example, substantially the same as those of the first insulating layer  12 . It may be noted that what is illustrated in  FIG.  5 B  is upside down relative to  FIG.  5 A . The same applies to  FIGS.  5    C through  6 B described below. 
     In the step illustrated in  FIG.  5 C , the via holes  13   x  that extend through the second insulating layer  13  and expose the upper surface of the first interconnect layer  11  are formed in the second insulating layer  13 . Further, the via holes  16   x  that extend through the third insulating layer  16  and expose the lower surface of the second interconnect layer  14  are formed in the third insulating layer  16 . The via holes  13   x  and  16   x  may be formed by, for example, a laser processing method using a CO 2  laser or the like. After the formation of the via holes  13   x  and the via holes  16   x,  a desmear treatment is preferably performed according to need so as to remove resin residues adhering to the surface of the first interconnect layer  11  exposed at the bottom of the via holes  13   x  and the surface of the second interconnect layer  14  exposed at the bottom of the via holes  16   x.    
     In the step illustrated in  FIG.  6 A , the third interconnect layer  15  is formed on the second insulating layer  13 . The third interconnect layer  15  includes via interconnects  15 V filling the via holes  13   x,  and includes the second ground plane  15 G, the second signal pads  15 A, and the second ground pad  15 B formed on the upper surface of the second insulating layer  13 . The second ground plane  15 G is formed in a substantially solid shape so as to cover most of the upper surface of the second insulating layer  13 . The second signal pads  15 A are disposed in the openings  15   x  provided in the second ground plane  15 G. The second ground pad  15 B forms part of the second ground plane  15 G. The second signal pads  15 A and the second ground pad  15 B are electrically connected to the pads  11 P through the via interconnects  15 V. The material of the third interconnect layer  15  and the thickness of the second ground plane  15 G, the second signal pads  15 A, and the second ground pad  15 B may be substantially the same as those of the first interconnect layer  11 , for example. 
     Further, the fourth interconnect layer  17  is formed on the second side of the third insulating layer  16 . The fourth interconnect layer  17  includes via interconnects filling the via holes  16   x,  and includes interconnect patterns and pads formed on the lower surface of the third insulating layer  16 . The material of the fourth interconnect layer  17  and the thickness of the interconnect patterns and the pads may be substantially the same as those of the first interconnect layer  11 , for example. The fourth interconnect layer  17  is electrically connected to the second interconnect layer  14  exposed at the bottom of the via holes  16   x.    
     In the step illustrated in  FIG.  6 B , the solder resist layer  21  is formed on the upper surface of the second insulating layer  13  so as to cover the third interconnect layer  15 . Further, the solder resist layer  22  is formed on the lower surface of the third insulating layer  16  so as to cover the fourth interconnect layer  17 . The solder resist layer  21  may be formed by, for example, a screen-printing method, a roll coating method, a spin coating method, or the like applying a photosensitive epoxy-based insulating resin or acrylic insulating resin in a liquid or paste form to the upper surface of the second insulating layer  13  such as to cover the third interconnect layer  15 . Alternatively, a photosensitive epoxy-based insulating resin or acrylic insulating resin in a film form, for example, may be laminated on the upper surface of the second insulating layer  13  such as to cover the third interconnect layer  15 . The method of forming the solder resist layer  22  is substantially the same as that of the solder resist layer  21 . 
     The solder resist layers  21  and  22  are exposed to light and developed. The openings  21   x  are thus formed (by photolithography) in the solder resist layer  21  to expose part of the upper surface of the third interconnect layer  15 . Further, the openings  22   x  are formed (by photolithography) in the solder resist layer  22  to expose part of the lower surface of the fourth interconnect layer  17 . The openings  21   x  and  22   x  may alternatively be formed by a laser processing method or a blasting process. In such a case, there is no need to use a photosensitive material for the solder resist layers  21  and  22 . The plane shape of each of the openings  21   x  and  22   x  may be, for example, a circle. The diameters of the openings  21   x  and  22   x  may be selected in accordance with what is connected thereto (e.g., a semiconductor chip, a motherboard, or the like). 
     In the step described above, the metal layer as previously described may be formed on the upper surface of the third interconnect layer  15  exposed at the bottom of the openings  21   x  and the lower surface of the fourth interconnect layer  17  exposed at the bottom of the openings  22   x  by, for example, an electroless plating method or the like. Instead of forming the metal layer, an anti-oxidation treatment such as an OSP treatment may alternatively be performed. After the step illustrated in  FIG.  6 B , cuts are made along the broken lines C to obtain an individual interconnect substrate  1 . 
     As described above, the method of making the interconnect substrate  1  is configured such that the first interconnect layer  11  constituting the stripline SL is an embedded interconnect whose side surface and lower surface are covered with the first insulating layer  12 . In conventional methods for making an interconnect substrate, an embedded interconnect is arranged on the outermost layer of an interconnect substrate, which makes it possible to form a microstrip line, but does not enable the formation of a stripline. In contrast, the method of making the interconnect substrate  1  enables an embedded interconnect to form a stripline because an insulating layer and an interconnect layer are additionally formed after the support is removed to expose the upper surface of the embedded interconnect. This arrangement improves the adhesion between the first insulating layer  12  and the first interconnect layer  11  constituted by the interconnect patterns  11 W and the pads  11 P, thereby enabling the realization of a highly reliable stripline SL. 
     Further, as will be described later in &lt;Detail of Method of Making Interconnect Layer&gt;, a step of etching a seed layer by using an electrolytic plating layer as a mask is required when using a semi-additive method or a modified semi-additive method to form an interconnect layer. In this step, the side surface of the electrolytic plating layer is also etched at the time of etching the seed layer, so that the intervals of interconnects in the interconnect layer are widened, which makes it difficult to provide a narrow line width and space. In contrast, the side surface of the first interconnect layer  11 , which serve as embedded interconnects, is covered with the first insulating layer  12  in the step of etching and removing the thin foil  304   a  constituting the support  300 , so that the intervals of the interconnects are not widened at the time of etching and removing the thin foil  304   a,  which enables the provision of a narrow line width and space. For example, the line width and space that can be formed by the semi-additive method or the modified semi-additive method are about 11 to 13 μm and 11 to 13 μm, respectively. In the first interconnect layer  11  serving as embedded interconnects, on the other hand, a line width of about 8 to 10 μm and a space of about 8 to 10 μm can be achieved. That is, the stripline SL in the interconnect substrate  1  can be formed as fine interconnects that are the interconnect patterns  11 W having a pitch of 20 μm or less. Further, the miniaturization of interconnects makes it possible to reduce the size of the interconnect substrate  1 . 
     First Variation of First Embodiment 
     A first variation of the first embodiment is directed to an example of an interconnect substrate having five interconnect layers. In the first variation of the first embodiment, a description of the same components as those of the previously described embodiment may be omitted. 
       FIG.  7    is a cross-sectional view illustrating an example of an interconnect substrate according to the first variation of the first embodiment. Referring to  FIG.  7   , an interconnect substrate  1 A includes a fourth insulating layer  18  and a fifth interconnect layer  19  between the fourth interconnect layer  17  and the solder resist layer  22  of the interconnect substrate  1 . 
     The fourth insulating layer  18  is formed on the second surface of the third insulating layer  16  such as to cover the fourth interconnect layer  17 . The material and thickness of the fourth insulating layer  18  may be, for example, substantially the same as those of the first insulating layer  12 . The fourth insulating layer  18  may contain a filler such as silica (SiO 2 ). The content of the filler in the fourth insulating layer  18  may be, for example, substantially the same as that in the first insulating layer  12 . 
     The fifth interconnect layer  19  is formed on the second side of the fourth insulating layer  18 . The fifth interconnect layer  19  includes via interconnects filling via holes  18   x  extending through the fourth insulating layer  18  and exposing the lower surface of the fourth interconnect layer  17 , pads formed on the lower surface of the fourth insulating layer  18 , and interconnect patterns. The pads of the fifth interconnect layer  19  are electrically connected to the pads of the fourth interconnect layer  17  through the via interconnects. The via holes  18   x  may be a truncated cone-shaped recess, with the size of the opening thereof on the solder resist layer  22  side being larger than the size of the bottom surface thereof formed by the lower surface of the pads constituting the fourth interconnect layer  17 . The material and the thickness of the interconnect patterns of the fifth interconnect layer  19  may be substantially the same as those of the first interconnect layer  11 , for example. 
     The solder resist layer  22  is the outermost layer on the second side of the interconnect substrate  1 A, and is formed on the lower surface of the fourth insulating layer  18  such as to cover the fifth interconnect layer  19 . The solder resist layer  22  has the openings  22   x,  and part of the lower surface of the pads of the fifth interconnect layer  19  is exposed in the openings  22   x.  The plane shape of the openings  22   x  may be, for example, a circle. The pads of the fifth interconnect layer  19  exposed in the openings  22   x  may be used for electrical connection to a mounting substrate (not illustrated) such as a motherboard. According to need, the metal layer as previously described may be formed, or an anti-oxidation treatment such as an OSP treatment may be performed, on the lower surface of the pads of the fifth interconnect layer  19  exposed in the openings  22   x.    
     In order to manufacture the interconnect substrate  1 A, after the step of  FIG.  4 B  of the first embodiment, the third insulating layer  16  is formed on the first side of the first insulating layer  12  as illustrated in  FIG.  8 A  in the same manner as in the step illustrated in  FIG.  5 B . Thereafter, in the step illustrated in  FIG.  8 B , the via holes  16   x  are formed in the third insulating layer  16  such as to extend through the third insulating layer  16  and expose the upper surface of the second interconnect layer  14  similarly to the step illustrated in  FIG.  5 C . After the via holes  16   x  are formed, a desmear treatment is preferably performed according to need so as to remove resin residues adhering to the surface of the second interconnect layer  14  exposed at the bottom of the via holes  16   x.  In the step illustrated in  FIG.  8 C , the fourth interconnect layer  17  is formed on the first side of the third insulating layer  16  in the same manner as in the step illustrated in  FIG.  6 A . 
     In the step illustrated in  FIG.  9 A , the support  300  illustrated in  FIG.  8 C  is removed in the same manner as in the steps illustrated in  FIGS.  4 C and  5 A . In the step illustrated in  FIG.  9 B , similarly to the step illustrated in  FIG.  5 B , the second insulating layer  13  is formed on the first side of the first insulating layer  12  such as to cover the surface of the first interconnect layer  11  that was in contact with the support  300 , and, then, the third interconnect layer  15  is further formed. Moreover, the fourth insulating layer  18  is formed on the second side of the third insulating layer  16  such as to cover the fourth interconnect layer  17 , and, then, the fifth interconnect layer  19  is further formed. Thereafter, a step similar to that illustrated in  FIG.  6 B  is performed to complete the interconnect substrate  1 A. It should be noted that what is illustrated in  FIG.  9 B  is upside down relative to  FIG.  9 A . 
     As described above, an interconnect substrate having the stripline SL is not limited to a structure having four interconnect layers such as the interconnect substrate  1 . For example, a structure having five interconnect layers such as the interconnect substrate  1 A will suffice. As long as there are three or more interconnect layers, a structure having a different number of interconnect layers than four or five layers will also suffice. 
     &lt;Detail of Method of Forming Interconnect Layer&gt; 
       FIGS.  10 A through  10 C  and  FIGS.  11 A and  11 B  are drawings illustrating an example of a detailed method of forming an interconnect layer. The following description will be directed to an example in which the second interconnect layer  14  is formed by a semi-additive method. 
     After the step illustrated in  FIG.  4 A  of the first embodiment, in the step illustrated in  FIG.  10 A , a seed layer  14   s  is formed on the surface of the first insulating layer  12  including the inner walls of the via holes  12   x  and the surface of the first interconnect layer  11  exposed in the via holes  12   x . The seed layer  14   s  may be formed of, for example, copper or the like by electroless plating or sputtering. 
     In the step illustrated in  FIG.  10 B , a resist layer  400  is formed on the seed layer  14   s  using a photosensitive dry film resist or the like. The resist layer  400  is then exposed to light and developed to form openings  400   x  whose shape matches with the shape of the second interconnect layer  14 . The seed layer  14   s  is exposed in the openings  400   x.    
     In the step illustrated in  FIG.  10 C , an electrolytic plating layer  14   p  is deposited on the seed layer  14   s  exposed in the openings  400   x  of the resist layer  400  by electrolytic plating in which power is supplied through the seed layer  14   s.  Copper or the like may be used as the material of the electrolytic plating layer  14   p.  In the step illustrated in  FIG.  11 A , the resist layer  400  is removed. 
     In the step illustrated in  FIG.  11 B , etching is performed using the electrolytic plating layer  14   p  as a mask to remove the seed layer  14   s  exposed through the electrolytic plating layer  14   p , thereby obtaining the second interconnect layer  14  including the seed layer  14   s  and the electrolytic plating layer  14   p.  As illustrated in  FIG.  1   , the structure of the second interconnect layer  14  includes the first ground plane  14 G, the first signal pads  14 A, the first ground pad  14 B, and the via interconnects  14 V. In the case of the seed layer  14   s  being formed of Cu, the seed layer  14   s  may be removed by wet etching using, for example, a ferric chloride aqueous solution, a cupric chloride aqueous solution, an ammonium persulfate aqueous solution, or the like. At this time, the upper surface and side surface of the electrolytic plating layer  14   p  are also slightly removed. 
       FIGS.  12 A through  14 B  are drawings illustrating another example of a detailed method of forming the interconnect layer. The following description will be directed to an example in which the second interconnect layer  14  is formed by a modified semi-additive method. 
     In the step illustrated in  FIG.  12 A , the first interconnect layer  11  is formed on the support  300  in substantially the same manner as illustrated in  FIGS.  3 A and  3 B  of the first embodiment. In the step illustrated in  FIG.  12 B , the first insulating layer  12  in a semi-cured state is formed on the upper surface of the carrier-attached copper foil  304  such as to cover the first interconnect layer  11 , and a metal foil  14   m  is disposed on the upper surface of the first insulating layer  12 , followed by curing the first insulating layer  12 , in substantially the same manner as illustrated in the step illustrated in  FIG.  3 C . The first insulating layer  12  is preferably cured by heat while pressing the metal foil  14   m  against the first insulating layer  12 . This arrangement ensures the adhesion strength between the metal foil  14   m  and the first insulating layer  12 , thereby preventing the metal foil  14   m  from detaching from the first insulating layer  12 . Cu or a Cu alloy, for example, may be used as the material of the metal foil  14   m . The metal foil  14   m  may be, for example, about 5 to 10 μm in thickness. 
     In the step illustrated in  FIG.  12 C , the first insulating layer  12  is irradiated with a laser beam through the metal foil  14   m  to form the via holes  12   x  that extend through the metal foil  14   m  and the first insulating layer  12  and that expose the upper surface of the first interconnect layer  11 . The via holes  12   x  may be formed by, for example, a laser processing method using a CO 2  laser, a YAG laser, an excimer laser, or the like. After the via holes  12   x  are formed, a desmear treatment is preferably performed to remove resin residues adhering to the surface of the first interconnect layer  11  exposed at the bottom of the via holes  12   x.    
     In the step illustrated in  FIG.  13 A , a seed layer  14   s  is formed on the surface of the metal foil  14   m,  the surface of the first insulating layer  12  constituting the inner walls of the via holes  12   x , and the surface of the first interconnect layer  11  exposed in the via holes  12   x.  The seed layer  14   s  may be formed of, for example, copper or the like by electroless plating or sputtering. 
     In the step illustrated in  FIG.  13 B , a resist layer  410  is formed on the seed layer  14   s  by use of a photosensitive dry resist film or the like. The resist layer  410  is then exposed to light and developed to form openings  410   x  whose shape matches with the shape of the second interconnect layer  14 . The seed layer  14   s  is exposed in the openings  410   x.    
     In the step illustrated in  FIG.  14 A , an electrolytic plating layer  14   p  is deposited on the seed layer  14   s  exposed in the openings  410   x  of the resist layer  410  by electrolytic plating in which power is supplied through the metal foil  14   m  and the seed layer  14   s.  Copper or the like, for example, may be used as the material of the electrolytic plating layer  14   p.    
     In the step illustrated in  FIG.  14 B , after the resist layer  410  illustrated in  FIG.  14 A  is removed, etching is performed using the electrolytic plating layer  14   p  as a mask to remove the metal foil  14   m  and the seed layer  14   s  exposed through the electrolytic plating layer  14   p,  thereby obtaining the second interconnect layer  14  including the metal foil  14   m,  the seed layer  14   s,  and the electrolytic plating layer  14   p.  As illustrated in  FIG.  1   , the structure of the second interconnect layer  14  includes the first ground plane  14 G, the first signal pads  14 A, the first ground pad  14 B, and the via interconnects  14 V. In the case of the metal foil  14   m  and the seed layer  14   s  being made of copper, the metal foil  14   m  and the seed layer  14   s  may be removed by wet etching using, for example, a ferric chloride aqueous solution, a cupric chloride aqueous solution, an ammonium persulfate aqueous solution, or the like. At this time, the upper surface and side surface of the electrolytic plating layer  14   p  are also slightly removed. 
     As described above, an interconnect layer may be formed by a semi-additive method or a modified semi-additive method. 
     In the modified semi-additive method, a metal foil is laminated on an insulating layer, and a seed layer is formed on a surface of the metal foil. Metal foils having various surface roughness are available, and a metal foil having a small surface roughness can be selected. Use of a metal foil having a small surface roughness enables the formation of an interconnect layer having small surface irregularities suitable for radio-frequency transmission. It may be noted that the interconnect layer includes interconnect patterns, pads, and a ground plane. 
     In contrast, the semi-additive method is such that a seed layer is directly formed on the surface of an insulating layer. That is, the semi-additive method does not require a step of laminating a metal foil on an insulating layer, which serves to simplify the interconnect substrate manufacturing process. However, since the surface of an insulating layer often has a larger surface roughness than the surface of a metal foil, use of an insulating layer having a large surface roughness ends up providing an interconnect layer having relatively large surface irregularities. Even in the semi-additive method, an insulating layer having a small surface roughness may be selected for use, which enables the formation of an interconnect layer having small surface irregularities suitable for radio-frequency transmission. 
     Example of Practical Application of First Embodiment 
     An example of practical application of the first embodiment, which will be described below, is directed to a semiconductor apparatus in which a semiconductor chip is mounted on an interconnect substrate. In this example of practical application of the first embodiment, a description of the same components as those of the previously described embodiment may be omitted. 
       FIG.  15    is a cross-sectional view illustrating an example of a semiconductor apparatus according to the example of practical application of the first embodiment. Referring to  FIG.  15   , a semiconductor apparatus  100  includes the interconnect substrate  1  illustrated in  FIG.  1   , a semiconductor chip  110 , electrode posts  120 , bumps  130 , and an underfill resin  140 . 
     In the semiconductor chip  110 , a semiconductor integrated circuit (not shown) or the like is formed on a thin semiconductor substrate (not shown) made of silicon or the like, for example. The electrode posts  120  electrically connected to the semiconductor integrated circuit (not shown) are formed on the semiconductor substrate (not shown). 
     The bumps  130  are formed between the electrode posts  120  of the semiconductor chip  110  and the pads of the third interconnect layer  15  of the interconnect substrate  1 . Each of the bumps  130  covers, and electrically connects, the lower surface and at least part of the side surface of a corresponding one of the electrode posts  120  and the upper surface of a corresponding one of the pads belonging to the third interconnect layer  15  of the interconnect substrate  1 . The electrode posts  120  are copper posts, for example. The bumps  130  are solder bumps, for example. The material of the solder bumps may be, for example, an alloy containing Pb, an alloy of Sn and Cu, an alloy of Sn and Ag, an alloy of Sn, Ag, and Cu, or the like. The underfill resin  140  fills the gap between the semiconductor chip  110  and the upper surface of the interconnect substrate  1 . 
     In the manner described above, a semiconductor chip is mounted on the interconnect substrate of the first embodiment to provide a semiconductor apparatus. As in the case of a semiconductor apparatus  100 A illustrated in  FIG.  16   , the interconnect substrate  1 A may be used in place of the interconnect substrate  1 . In any of these semiconductor apparatuses, the stripline SL is provided in the interconnect substrate, which serves to control the characteristic impedance of the first interconnect layer  11  and to reduce electromagnetic waves leaking from the first interconnect layer  11 , thereby enabling high-speed signal transmission. 
     According to at least one embodiment, adhesion between an interconnect pattern constituting a stripline and insulating layers is improved in an interconnect substrate having the stripline. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 
     One aspect of the subject-matter described herein is set out non-exclusively in the following numbered clauses. 
     [Clause 1] A method of making an interconnect substrate, comprising: 
     forming a first interconnect layer on a support; 
     forming on the support a first insulating layer covering a surface of the first interconnect layer, the surface being not in contact with the support; 
     forming a second interconnect layer on the first insulating layer; 
     removing the support; 
     forming a second insulating layer covering a surface of the first interconnect layer that was in contact with the support; and 
     forming a third interconnect layer on the second insulating layer, 
     wherein the second interconnect layer includes a first ground plane, 
     wherein the third interconnect layer includes a second ground plane, and 
     wherein the first ground plane, the first insulating layer, the first interconnect layer, the second insulating layer, and the second ground plane constitute a stripline. 
     [Clause 2] The method of making an interconnect substrate as recited in clause 1, wherein the forming the first interconnect layer on the support includes: 
     forming a resist layer having openings on an outermost metal layer of the support; 
     forming the first interconnect layer in the openings by electrolytic plating using the metal layer as a power feeding layer; and 
     removing the resist layer.