Patent Publication Number: US-9905340-B2

Title: Resistive element and method for manufacturing the same

Description:
RELATED APPLICATIONS 
     This application is a 371 application of PCT/JP2014/080573 having an international filing date of Nov. 19, 2014, which claims priority to JP 2013-256325 filed Dec. 11, 2013. The contents of these applications are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a resistive element and a method for manufacturing the same. More specifically, the present invention relates to a technique of forming electrodes for a resistive element. 
     BACKGROUND ART 
     Conventionally, resistive elements connected to wires, which are formed on a printed circuit board and the like, by wire bonding have been used. 
     For example, Patent Literature 1 discloses a small-size chip resistor that can be bonded by wire bonding and a method for manufacturing the same. In the chip resistor described in Patent Literature 1, a resistor is formed across a first electrode and a second electrode that are formed spaced apart from each other on a chip substrate. Providing a wire on the first electrode can obtain an electrical connection. If a chip resistor is mounted using solder, there will be restrictions such that the chip resistor cannot be used in an environment at a temperature of greater than or equal to the melting point of the solder. However, using wire bonding can avoid such a problem. 
     In Patent Literature 1, electrodes are formed using metal glaze of silver(Ag)-palladium(Pd)-glass, for example, at opposite ends on an upper surface of a chip substrate made of an alumina sintered body, which is a substrate with an electrical insulating property, in the longitudinal direction thereof, and a resistor is formed using ruthenium oxide (RuO 2 ) between the electrodes. Finally, the electrodes are bonded by wire bonding (see FIG. 10 of Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP H09-162002 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     When electrical connections are obtained by bonding a resistor using wire bonding, how to increase the connection strength between the electrodes of the resistor and bonding wires is an issue. To this end, electrode layers that form each electrode including the surface thereof should be dense. However, the conventional electrodes have problems with the density. 
     It is an object of the present invention to provide a technique of forming electrodes for a resistor, which are adapted to obtain electrical connections through wire bonding, as dense, thick conductive films. 
     It is another object of the present invention to provide a resistor having dense electrodes that are suitable for connection of aluminum wires and the like thereto using wedge bonding. 
     Solution to Problem 
     According to an aspect of the present invention, there is provided a method for manufacturing a chip resistive element including a substrate, a resistor formed on the substrate, and electrodes connected to opposite ends of the resistor, the method including an electrode forming step of forming each electrode on the substrate. The electrode forming step includes a step of forming a first electrode layer on the substrate using a first electrode material containing silver, and a step of forming a second electrode layer on the first electrode layer using a second electrode material containing silver and palladium. The first electrode material contains a higher silver content than the second electrode material. 
     As the material of the first electrode layer, which is the lower layer, of the electrode has a higher silver content, diffusion of silver to the second electrode layer, which is the upper layer, becomes dominant when Ag mutually diffuses during heat treatment (baking) and the like. Thus, air bubbles and the like that are generated in the second electrode layer can be filled with the diffused silver, and thus, the electrode becomes dense. 
     In addition, palladium can prevent migration of silver to the resistor as well as prevent sulfuration of Ag. 
     The step of forming the first electrode layer includes a step of depositing a paste of a silver-platinum-based metal material and glass as the first electrode material on the substrate. The step of forming the second electrode layer includes a step of depositing a paste of a silver-palladium-based metal material and glass as the second electrode material on the first electrode layer and baking the paste. 
     The electrode is fused after the second electrode material is baked, and a resistor is formed thereafter. Thus, the electrode becomes dense, and the resistor does not contact the electrode. 
     The first electrode material contains greater than or equal to 95 wt % silver at a rate of metal components contained in the first electrode material, and the second electrode material contains less than or equal to 90 wt % silver at a rate of metal components contained in the second electrode material. 
     As the first electrode material contains greater than or equal to 95 wt % (95 to 99.5 wt %) silver at a rate of metal components (excluding glass components) contained in the first electrode material, and the second electrode material contains less than or equal to 90 wt % (70 to 90 wt %) silver at a rate of metal components contained in the second electrode material, mutual diffusion of silver is promoted, and a dense electrode is thus obtained. 
     The content of palladium is set in the range of 10 to 30 wt % to prevent sulfuration and migration of silver, while the content of platinum is set in the range of 0.5 to 5 wt % to increase the adhesion between the substrate and the electrode. 
     Herein, the first electrode layer is formed to a thickness of greater than or equal to that of the second electrode layer, whereby diffusion of silver from the first electrode layer with a higher concentration of silver to the second electrode layer is promoted. 
     According to another aspect of the present invention, there is provided a chip resistive element including a substrate, a resistor formed on the substrate, and electrodes connected to opposite ends of the resistor. Each electrode contains silver, and the electrode includes a silver concentration sloped layer in which a concentration of silver in the electrode is sloped downward in a thickness direction from a substrate side to a side opposite to the substrate. 
     Further, the electrode includes a palladium-rich layer on the side opposite to the substrate, the palladium-rich layer having a high content of palladium as metal other than silver. 
     The concentration of silver in the silver concentration sloped layer is sloped in a range of 95 to 90 wt %. 
     The present specification incorporates the content described in the specification and/or the drawings of JP Patent Application No. 2013-256325 that claims the priority of the present application. 
     Advantageous Effects of Invention 
     Forming a dense electrode can reduce damage thereto that may occur during a wire bonding step, an inspection step, and the like, and can increase the contact strength. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are perspective views each showing an exemplary appearance/configuration of a chip resistive element in accordance with an embodiment of the present invention. 
         FIGS. 2A, 2B and 2C  are top views showing the steps of manufacturing the chip resistive element in accordance with this embodiment. 
         FIGS. 3A, 3B, 3C and 3D  are top views showing the steps of manufacturing the chip resistive element in accordance with this embodiment. 
         FIGS. 4A, 4B and 4C  are top views showing the steps of manufacturing the chip resistive element in accordance with this embodiment. 
         FIGS. 5A, 5B and 5C  are cross-sectional views showing the steps of manufacturing the chip resistive element in accordance with this embodiment. 
         FIGS. 6A and 6B  are cross-sectional views showing the steps of manufacturing the chip resistive element in accordance with this embodiment. 
         FIGS. 7A and 7B  are cross-sectional views showing the steps of manufacturing the chip resistive element in accordance with this embodiment. 
         FIG. 8  is a chart showing exemplary metal components, weight rates, and layer thicknesses of a first electrode material and a second electrode material that are the electrode materials of a first electrode layer and a second electrode layer, respectively, forming an electrode. 
         FIG. 9  is a chart showing an exemplary distribution of the Ag concentration after the second electrode layer (i.e., upper layer electrode) and the first electrode layer (i.e., base electrode) are baked. 
         FIG. 10  is a cross-sectional view schematically showing the electrode structure. 
         FIG. 11  is a perspective view schematically showing an exemplary mount structure that uses the chip resistor in accordance with this embodiment. 
         FIG. 12  shows a view in which a secondarily split chip is inspected. 
         FIG. 13  shows a view in which an electrode is bonded using wire bonding. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a resistive element and a method for manufacturing the resistive element in accordance with an embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
       FIGS. 1A and 1B  are perspective views each showing an exemplary appearance/configuration of a chip resistive element (hereinafter referred to as a “chip resistor”) in accordance with an embodiment of the present invention. As shown in  FIGS. 1A and 1B , a chip resistor  1  in accordance with this embodiment is formed using an alumina sintered body, which is a chip substrate  11  with an electrical insulating property, for example, and electrodes  15  are formed at opposite ends of, among side surfaces  11   a , end surfaces  11   b , an upper surface  11   c , and a lower surface  11   d  that are the exposed surfaces of the chip substrate  11 , the upper surface  11   c  in the longitudinal direction thereof, for example, and a resistor (not shown) is formed between the electrodes  15 , and then, a protective film  17  is formed over the resistor. Each electrode  15  has a probe trace  23  at a position touched with a probe for adjusting the resistance value during the step, for example. An end surface of the electrode  15  is flush with the end surface  11   b  of the chip substrate  11 . 
     As shown in  FIG. 1B , the lower surface  11   d  of the chip substrate  11  has a lower surface electrode (i.e., lower surface terminal)  13  formed thereon. The lower surface electrode  13  is provided so as to be connected via solder to a substrate or a lead frame. 
     Hereinafter, a method for manufacturing the chip resistor in accordance with this embodiment will be described in detail with reference to the drawings.  FIGS. 2A, 2B and 2C to 4A, 4B and 4C  are top views showing the manufacturing steps, and  FIGS. 5A, 5B and 5C to 7A and 7B  are cross-sectional views showing the manufacturing steps. 
     First, as shown in  FIGS. 2A and 5A , a large substrate made of alumina or the like is prepared for manufacturing a plurality of chip resistors. Regions of individual chip resistors (hereinafter referred to as “chip regions”) are defined on the lower surface side (which corresponds to the lower surface  11   d  side in  FIG. 1B ), and slits  31   a  and  31   b , which are arranged in two directions intersecting with each other and adapted to be used for splitting the substrate into individual chip resistors, are formed. The slits  31   a  and  31   b  are formed by performing a pattern pressing step, a laser irradiation step, and the like on a pre-baked substrate. In addition, slits are also formed on the front surface side of the substrate at the same positions. 
     After that, as also shown in  FIG. 5A , the lower surface terminal  13  is formed in each chip region. The lower surface terminal  13  is formed by patterning a paste containing an Ag—Pd-based metal material and glass using screen printing, for example, and baking the paste at 850° C. 
     Next, as shown in  FIGS. 2B and 5B , a first electrode (i.e., lower layer electrode)  15   a  is formed on the upper surface (i.e., upper surface  11   e  in  FIG. 1A ) side of the large substrate at a position to stride over the slit  31   a ′ on the upper surface side, for example. At this time, a paste containing an Ag—Pt-based metal material and glass is used as the first electrode material, for example, and is patterned using screen patterning, and is then subjected to drying treatment and baking at 850° C. A region where the first electrode  15   a  is formed is a region to be split in two at the slit  31   a ′. Accordingly, when the first electrode  15   a  is split in two along the slit  31   a ′, the end surface  11   b  of the chip substrate  11  becomes substantially flush with the end surface of the first electrode  15   a.    
     Next, as shown in  FIGS. 2C and 5C , a second electrode (i.e., upper layer electrode)  15   b  is formed at a position and a region overlapping the first electrode  15   a . At this time, a paste containing an Ag—Pd-based metal material and glass is used as the second electrode material, for example, and is patterned using screen printing, and is then subjected to drying treatment and baking at 850° C. 
     Accordingly, as shown in  FIG. 6A , the electrode  15 , which has been fused based on the first electrode  15   a  and the second electrode  15   b , is formed on the upper surface side of the chip substrate  11 . 
     The details related to the steps of forming the electrode will be described later. 
     As shown in  FIGS. 3A and 6B , a resistor is formed on the upper surface of the substrate having the electrodes  15  formed thereon, using a paste of a resistor material of RuO 2  and glass, for example. In order that the opposite ends of the resistor may contact the electrodes  15 , screen printing is performed, for example, so as to allow the resistor and the electrodes  15  to be laid one on top of the other and thus obtain an electrical connection therebetween. Next, drying and a baking step at 850° C. are performed to form a resistor  41 . Although the resistor  41  is formed by a serpentine pattern in the drawing to increase the withstand voltage, the resistor  41  may be of any shape. 
     As shown in  FIGS. 3B and 7A , a borosilicate glass paste is applied to the upper surface of the chip substrate  11  that has the electrodes  15  and the resistor  41  formed thereon, and screen printing is performed so as to cover the resistor  41 . Then, drying treatment and baking treatment at 600° C. are performed to form a primary protective film  43 . The primary protective film  43  also has a function of mitigating shocks to the resistor  41  that may occur due to laser trimming described below. 
     As shown in  FIGS. 3C and 7B , a slit  45  is formed in a part of the resistor  41  using a laser processing technique so as to adjust the resistance value of the resistor  41 . At this time, the resistance value of the resistor  41  can be adjusted by measuring the resistance between the electrodes  15  while touching the electrodes  15  with probes. 
     Next, as shown in  FIGS. 3D and 7B , a borosilicate glass paste is applied to the upper surface of the chip substrate  11  that has the electrodes  15  and the resistor  41  formed thereon and has an adjusted resistance value, and screen printing is performed so as to cover the slit  45 , which has been formed in the resistor  41  using laser, and then, drying treatment and baking treatment at 600° C. are performed to form a secondary protective film  47 . The secondary protective film  47  may also be formed using a resin-based material. However, using borosilicate glass can suppress adverse effects on bonding that may result from, if a resin-based protective agent is used, a spread of the resin components on the electrode surface during heat curing treatment. 
     Next, as shown in  FIG. 4A , a tertiary protective film  51  is formed over the secondary protective film  47  using a borosilicate glass paste, from the state shown in  FIG. 3D . Accordingly, a number of resistive elements can be formed on the large substrate. It is also possible to use the chip substrate  11  without the slits  31   a  and  31   b  formed thereon up to the step of  FIG. 4A , and thereafter form the slits  31   a  and  31   b  while cutting the electrode  15  using laser scribing and splitting the substrate into individual resistive elements through dicing. 
     Next, as shown in  FIG. 4B , the large substrate is split along the slit  31   a  ( FIG. 2A ). As the electrode  15  has a relatively high Pd content, there is an advantage in that the electrode  15  can be easily split along the slit  31   a . If the Pd content in the electrode  15  is low, cracks will be easily generated in the electrode, and thus, the shapes of the split planes may easily vary, while if the Pd content in the electrode  15  is relatively high, cracks will be hardly generated in the electrode, and thus, the shapes of the split planes will hardly vary. 
     Next, as shown in  FIG. 4C , secondary splitting is performed along the slit  31   b  ( FIG. 2A ), whereby the chip resistor  1  can be created. 
     Hereinafter, the details of the steps of forming the electrode will be described. The electrode  15  is formed by stacking the second electrode layer (i.e., upper layer electrode)  15   b  made of the second electrode material over the first electrode layer (i.e., base electrode)  15   a  made of the first electrode material. It should be noted that as the electrode layers are fused in the baking step, the resulting electrode in its completed state is not in two layers. 
       FIG. 8  is a chart showing exemplary metal components, weight rates, and layer thicknesses of the first electrode material and the second electrode material that are the electrode materials of the first electrode layer (i.e., base electrode)  15   a  and the second electrode layer (i.e., upper layer electrode)  15   b , respectively, forming the electrode  15 . As shown in  FIG. 8 , the first electrode material contains Ag and Pt, for example, and the composition rates are Ag: 95 to 99.5 wt % (greater than or equal to 95 wt %) and Pt: 0.5 to 5 wt %. The layer thickness is 5 to 12 μm, and is greater than or equal to the layer thickness of the second electrode layer  15   b  that is the upper layer. The second electrode material contains Ag and Pd, for example, and the composition rates are Ag: 70 to 90 wt % (less than or equal to 90 wt %) and Pd: 10 to 30 wt %. The layer thickness is 5 to 12 μm, and is less than or equal to that of the first electrode layer  15   a  that is the lower layer. The above example is a preferred example, but even when the proportion of Pd is less than 10 wt %, for example, a proportion of 2 to 10 wt %, predetermined effects are obtained. 
     When the electrode paste is baked, the vehicle and the solvent may vaporize and the glass components may move, for example, which can result in a non-dense surface state of the electrode. Then, the fixing strength of bonding may not be obtained. 
     Thus, in this embodiment, an Ag—Pt paste is first printed and baked to form the first electrode layer  15   a  (i.e., base electrode) and then, an Ag—Pd paste is printed and baked to form the second electrode layer  15   b  (i.e., upper layer electrode). In the step of baking the second electrode layer  15   b  (i.e., upper layer electrode) and the first electrode layer  15   a  (i.e., base electrode), the Ag components contained in the two layers mutually diffuse. As the Ag components diffuse from the first electrode layer  15   a  (i.e., base electrode) to the second electrode layer  15   b  (i.e., upper layer electrode), the dense electrode layer  15  is obtained. 
       FIG. 9  is a chart showing an exemplary distribution of the Ag concentration after the second electrode layer  15   b  (i.e., upper layer electrode) and the first electrode layer  15   a  (i.e., base electrode) are baked. As the Ag concentration of the first electrode material is 99% and the Ag concentration of the second electrode material is 80%, a distribution of the Ag concentration is determined based on the concentration gradient of Ag in the mutual diffusion of Ag during baking. For example, as shown in  FIG. 9 , when the first electrode material with a higher Ag concentration and the second electrode material with a lower Ag concentration than the first electrode material are stacked and baked, Ag moves as a whole from the first electrode material to the second electrode material based on the concentration gradient of Ag in the mutual diffusion step of Ag. Thus, in the electrode  15 , the Ag concentration in a region of from the substrate side to the electrode upper surface tends to be sloped downward from the high concentration region toward the low concentration region in the depth direction. This is estimated to be due to the reason that as the first electrode material and the second electrode material containing glass components are deposited and baked at a temperature at which Ag diffuses, voids, such as air bubbles, which are likely to be generated when an electrode paste containing glass components is deposited thick and baked, are filled with the mutually diffused Ag, and thus, a dense Ag-based electrode can be finally formed. 
       FIG. 10  is a cross-sectional view schematically showing the structure of the electrode  15 . That is, the electrode  15  is mainly formed of Ag, and has sequentially formed from the substrate side a Pt-containing layer  15 - 3  containing Pt, an Ag concentration sloped layer  15 - 1  containing Ag such that the concentration of Ag is sloped downward from the substrate side, and a Pd-rich layer  15 - 2  having a relatively high Pd content. The Ag concentration in the Ag concentration sloped layer  15 - 1  is sloped in the range of 95 to 90 wt %. 
     Herein, as Pd is distributed on the upper side of the electrode, it is possible to suppress migration of Ag to the side of RuO 2  that forms a resistor and thus suppress generation of silver sulfide with an insulating property due to sulfuration of Ag. Pt is distributed on the lower side of the electrode  15 , that is, on the substrate  11  side. Thus, Pt serves to secure the adhesion strength between the electrode  15  and the substrate. It should be noted that the glass components are distributed on the substrate side, and contribute to increasing the adhesion strength between the electrode  15  and the substrate  11 . 
     The secondarily split chip is shipped after subjected to inspection and packaging. It should be noted that a Ni film, a Ni—Au film, a Ni—Pd—Au film, or the like may also be formed on the electrode surface using Ni plating (electroplating). Herein, as shown in  FIG. 12 , if probes  61 , which are used for inspection, are allowed to touch the corners of the electrodes  15  of the chip resistor  1  that are formed dense as described above, it is possible to reduce damage and the like to the chip resistor, in particular, to the electrodes  15  while the resistance value of the chip resistor  1  is inspected. 
     (Step after the Chip Resistor is Formed) 
       FIG. 11  is a perspective view schematically showing an exemplary mount structure that uses the chip resistor  1  in accordance with this embodiment. As shown in  FIG. 11 , the chip resistor  1  whose electrodes  15  are exposed at the opposite ends thereof is electrically connected to circuits having bonding pads  81  and the like using bonding wires  71 . It should be noted that in the step of adjusting the resistance value in  FIG. 3C , the probes are preferably placed at positions off the centers of the electrodes  15  so that the probe traces  23  for measuring the resistance value are not located at the positions where the bonding wires  71  are connected to the electrodes  15  during the step of adjusting the resistance value in  FIG. 3C . 
       FIG. 13  shows a view in which the electrode  15  is bonded using wire bonding. The bonding wire  71  of Al that protrudes from an end of a hole of the probe  91  for wire bonding is pressed against the surface of the electrode  15 , and is subjected to ultrasonic bonding, thermocompression bonding, or the like, so that the tip end portion of the bonding wire  71  of Al is bonded to the surface of the electrode  15 . In such a case, if the electrode  15  to be bonded is formed dense, the electrode  15  can be suitable as the electrode for connection of the bonding wire of Al to the chip resistor  1  using wedge bonding, in particular. It should be noted that in addition to the bonding wire of Al, a wire of Au and the like may also be used. Further, ball bonding and the like may also be used. 
     Using the aforementioned electrode forming technique can form a dense electrode, and thus can reduce damage to the electrode in the wire bonding step, the inspection step, and the like as well as increase the contact strength. 
     In addition, as the electrode is formed in two layers, it is possible to obtain thick electrode layers while preventing cracking and the like, and reduce the resistance values of the electrode layers. Therefore, it is possible to reduce variations of the potential distributions of the electrode layers. 
     In addition, the electrode surface is formed dense, and the electrode is configured to be connected at the electrode surface to a resistor. Therefore, it is possible to reduce the contact resistance between the resistor and the electrode and increase the pulse resistance. Further, as the electrode can be formed thick, the resistor layer can also be formed thick. Therefore, the pulse resistance of the resistor layer can be increased. 
     In the aforementioned embodiments, configurations and the like shown in the attached drawings are not limited thereto, and can be changed as appropriate within the range that the advantageous effects of the present invention can be exerted. Further, such configurations can be changed as appropriate without departing from the scope of the object of the present invention. 
     Each feature of the present invention can be freely selected, and an invention that includes the freely selected feature also falls within the scope of the present invention. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to resistors. 
     All publications, patents, and patent applications that are cited in this specification are all incorporated by reference into this specification.