A semiconductor device, an image sensor device, and methods for producing the same

Abstract: A semiconductor device according to the present invention includes: a light-transmitting substrate having a first surface and a second surface; a circuit conductor layer formed on the first surface of the light-transmitting substrate; a semiconductor chip having electrodes formed on a surface thereof, the semiconductor device being mounted face-down on the first surface of the light-transmitting substrate; a photo-thermal cross-linkable insulating resin layer for fixing the semiconductor chip on the light-transmitting substrate; a plated metal layer formed on at least a portion of the circuit conductor layer, the electrodes of the semiconductor chip being connected to the circuit conductor layer through the plated metal layer; an alloy layer formed in an abutting portion between each plated metal layer and the circuit conductor layer; and a second alloy layer formed in an abutting portion between each electrode and each plated metal layer, the alloy layers being formed by melting and recoagulating the plated metal layer upon the circuit conductor layer.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a semiconductor device in which a semiconductor chip is mounted, and a method of fabrication thereof. 2. Description of the Related Art: There are conventionally known methods for mounting a semiconductor chip such as an LSI (Large Scale Integrated Circuit) on a wiring board. Examples of such methods are a wire-bonding method, a flip-chip-bonding method, a film-carrier-bonding method, and a beam-lead-bonding method. In a wire-bonding method, the semiconductor chip is fixed by means of an electrically conductive adhesive on a substrate having a circuit conductor layer formed thereon. Electrodes on the semiconductor chip are connected to the circuit conductor layer by means of metal wires such as those made of Au or Al. However, the bonding process in a wire-bonding method is troublesome using the metal wires. Moreover, it is necessary to form the electrodes of the semiconductor chip in a peripheral portion of the semiconductor chip, making it difficult to reduce the pitch between adjoining electrodes. Therefore, it is difficult to apply such a wire-bonding method to mounting of a semiconductor chip having a large number of electrodes. In order to solve such a problem, US-A-5,065,006 and US-A-5,138,145 each discloses a method for mounting an image sensor chip in an image sensor device, as a method for mounting a semiconductor chip through a wireless-bond-method. The methods for producing a conventional image sensor device, as are disclosed in the above patents, will be described with reference to Figures 8 and 9 . Figure 8 is a cross-sectional view showing a conventional image sensor device, in which an image sensor chip 16 is mounted face-down on a transparent substrate 11 . The image sensor device shown in Figure 8 is fabricated in the following manner: As is shown in Figure 8 , a photo-curable insulating resin 18 is applied on the transparent substrate 11 . The transparent substrate 11 has a circuit conductor layer 12 formed thereon. Thereafter, the image sensor chip 16 , which includes electrodes 15 and light-sensitive elements 19 such as photodiodes, is pressed face-down on the transparent substrate 11 . While a pressure is applied, the photo-curable insulating resin 18 is irradiated with ultra violet rays through the transparent substrate 11 , so that the photo-curable insulating resin is cured, and the image sensor chip 16 is fixed on the substrate. Finally, a resin such as that of a silicon type is applied thereon to form a passivation layer 17 . Thus, the image sensor device is fabricated. Figure 9 shows a step in the fabrication process of another conventional image sensor device. The image sensor device shown in Figure 9 is fabricated in the following manner: As is shown in Figure 9 , an image sensor chip 26 has electrodes 25 formed thereon. On each electrode 25 , another electrode (bump) 24 is formed. Each bump 24 has a ring tab formed at a tip portion thereof. The bumps 24 abut a circuit conductor layer 22 formed on a substrate 21 when the image sensor chip 26 is mounted face-down on the substrate 21 . By applying a pressure on the image sensor chip 26 , the ring tabs of the bumps 24 collapse, so that the circuit conductor layer 22 and the bumps 24 are connected to each other in a close contact. As is described above, in the first conventional method, the photo-curable insulating resin is cured by being irradiated with ultra violet rays through the transparent substrate. Therefore, a portion of the photo-curable insulating resin which is shaded by the circuit conductor layer formed on the transparent substrate may not be sufficiently irradiated with the ultra violet rays, and therefore may not be cured completely. In such cases, since the photo-curable insulating resin is not completely cured, the physical connection between the semiconductor chip and the substrate is insufficient, causing the device to have poorer physical endurance. Moreover, the semiconductor chip and the circuit conductor layer are likely to have poor electrical connection. Furthermore, in cases where the device is an image sensor device, since the insulating resin is not completely cured, optical characteristics thereof are likely to lack uniformity, causing various problems. For example, if the resin has portions having different refractive indices, light incident to the device is refracted and/or scattered thereby. As a result, the light-sensitive element tends to read images with errors, or the resolution of the read images are degraded. With respect to the second conventional method, since the ring tabs of the bumps are collapsed by applying a certain level of pressure or more thereto, the image sensor chip and/or the transparent substrate may be destroyed in some cases. Furthermore, the circuit conductor layer and the bumps (electrodes) may have poor electrical connection. The reasons are: the contact between the circuit conductor layer and the bumps may be insufficient because the contact is obtained through physical deformation of the bumps; it is difficult to substantially offset such factors as variations in the heights of the electrodes, bumps (electrodes), and the circuit conductor layer and warping of the image sensor chip and/or the transparent substrate. EP-A-0 388 011 discloses a semiconductor device including a substrate having a wiring layer formed on its major surface and the semiconductor element having an electrode formed on its major surface with the major surface of the semiconductor element oppositely mounted over the major surface of the semiconductor substrate to bond the wiring layer to the electrode, wherein the semiconductor device has first bumps formed on the electrode and second bumps located between the first bump and the wiring layer to obtain an electrical/mechanical bond between the first bumps and the wiring layer, wherein the second bumps contain a metal, such as indium. The bumps are heated to a temperature not exceeding the melting point of the second bumps. US-A-4,749,120 discloses a method of connecting a semiconductor device to a wiring board in which the electrical connection of metal bumps of a semiconductor device and wiring pattern of a wiring board is accomplished by pressure application between the bumps and the wiring pattern, and mechanical fixing of this semiconductor device on the wiring board is accomplished by stiffened resin. A part of the resin is stiffened by light application so that the semiconductor device and the wiring board are temporarely fixed and thereafter they are completely fixed by heat application. SUMMARY OF THE INVENTION Hereinafter the term "electrical connection" also includes the possibility that an alloy layer is formed between the respective parts. A semiconductor device according to the present invention comprises: a light-transmitting substrate having a first surface and a second surface; a circuit conductor layer formed on the first surface of the light-transmitting substrate; a semiconductor chip having electrodes formed on a surface thereof, the semiconductor device being mounted face-down on the first surface of the light-transmitting substrate; a photo-thermal cross-linkable or crosslinked insulating resin layer being disposed between the semiconductor chip and the light-transmitting substrate for fixing the semiconductor chip on the light-transmitting substrate; a plated metal layer formed on at least a portion of the circuit conductor layer, the electrodes of the semiconductor chip being connected to the circuit conductor layer through the plated metal layer; and an electrical connection formed in an abutting portion between the plated metal layer and each of the electrodes, wherein in the electrical connection the plated metal layer contacts each electrode along minute superficial rises and falls of the abutting portion of the electrode due to fusion and recoagulation of the plated metal layer. In one embodiment of the invention, the semiconduciar device further comprises a second electrical connection formed in an abutting portion between the circuit conductor layer and each plated metal layer. In another embodiment of the invention, the plated metal layer is made of solder. In still another embodiment of the invention, the semiconductor device further comprises a metal bump formed on each electrode, each metal bump being connected to the circuit conductor layer through each plated metal layer; and a second electrical connection formed in an abutting portion between each metal bump and each plated metal layer, the second electrical connection being formed by melting and recoagulating the plated metal layer. In still another embodiment of the invention, the metal bumps are made of Au. In still another embodiment of the invention, the total thickness of the light-transmitting substrate and the photo-thermal cross-linkable insulating resin layer is 150 µm or less. A method of producing a semiconductor device according to the present invention comprises the steps of: forming a circuit conductor layer on a first surface of a light-transmitting substrate having the first surface and a second surface, forming a plated metal layer on at least a portion of the circuit conductor layer: forming a photo-thermal cross-linkable insulating resin layer on the first surface of the light-transmitting substrate; disposing a semiconductor chip having electrodes on a surface thereof face-down so that the electrodes are in contact with the plated metal layer; conducting a first curing step for curing the photo-thermal cross-linkable insulating resin by radiating ultra violet rays through the light-transmitting substrate from the second surface of the light-transmitting substrate while pressing the semiconduotor chip against the light-transmitting substrate; and conducting a second curing step for further curing the photo-thermal cross-linkable insulating resin by heating the light-transmitting substrate with the semiconductor chip disposod thereon at a certain temperature for a period of time, the temperature being higher than the melting point of the plated metal layer and than the curing temperature of the photo-thermal cross-linkable resin, wherein the second curing step comprises the formation of an electrical connection in an abutting portion between the plated metal layer and each of the electrodes, wherein the electrical connection is formed by metling and recoagulating the plated metal layer so that the plated metal layer contacts each electrode along the minute superficial rises and falls of the abutting portion of the electrode. The semiconductor chip can further comprise a metal bump formed on each electrode, and the second curing step can further comprise a step for forming an electrical connection in an abutting portion between each plated metal layer and the circuit conductor layer and for forming a second electrical connection in an abutting portion between each metal bump and each plated metal layer, the electrical connection being formed by melting and recoagulating the plated metal layer. In still another embodiment of he invention, the second curing step is conducted while the semiconductor chip is pressed against the light-transmitting substrate. In still another embodiment of the invention, the step for forming the circuit conductor layer includes: a step for forming a conductor layer on the first surface of the light-transmitting substrate; and a step for forming the circuit conductor layer by patterning the conductor layer by a photolithography method. In still another embodiment of the invention, the step for forming the circuit conductor layer is conducted by attaching a flexible printed wiring board to the first surface of the light-transmitting substrate. An image sensor device can be produced using the disclosure according to the present invention. The method of producing a semiconductor device according to the present invention may be used to manufacture an image sensor device. According to the present invention, electrodes (or bumps) formed on a semiconductor chip or an image sensor chip can have a firm electrical connection to a circuit conductor layer formed on a substrate, since plated metal layers are formed on the circuit conductor layer. Therefore, a reliable semiconductor device and a reliable image sensor device can be provided. Moreover, the semiconductor device and the image sensor device each can be firmly mounted on a light-transmitting substrate, since a photo-thermal cross-linkable insulating resin included therein is completely cured through first and second curing steps, greatly improving the mounting yield of the devices. Moreover, the semiconductor device and the image sensor device are prevented from deterioration, leading to improvement in the endurance thereof. Thus, the invention described herein makes possible the advantages of (1) providing a semiconductor device and an image sensor device having high reliability, in which an insulating resin is completely cured and electrodes of a semiconductor chip have a firm electrical connection with a circuit conductor layer formed on a substrate, and (2) providing methods for producing such a semiconductor device and an image sensor device. These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a view showing a front cross-section of a semiconductor device according to the present invention. Figure 2 is a view showing a front cross-section of an image sensor device according to the present invention. Figure 3 is a view showing a front cross-section of an image sensor device according to the present invention. Figure 4 is a top view showing an image sensor device according to the present invention. Figure 5 is a view showing a cross-section of the image sensor device shown in Figure 3 taken along Line A-A' . Figure 6 is a view showing a cross-section of the image sensor device shown in Figure 3 taken along Line B-B' . Figure 7 is a graph showing MTF values (4 lp/mm) of a direct-contact type image sensor device as a function of distances between the surface of a manuscript and a light-sensitive element of the image sensor device. Figure 8 is a view showing a front cross-section of a conventional image sensor device. Figure 9 is a view showing a step in the fabrication of a conventional image sensor device. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described by way of examples, with reference to the accompanying drawings. Figure 1 shows a cross-section of a semiconductor device according to the present invention. A light-transmitting substrate 1 is a glass substrate, but may alternatively be a transparent film made of polyarylate (PA), polyethersulfone (PES), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or the like. The light-transmitting substrate 1 has a first surface (top face) and a second surface (bottom face). On the first surface of the light-transmitting substrate 1 , a circuit conductor layer 2 made of a metal such as Cu or Al is formed. A semiconductor chip 6 having electrodes 5 formed on a surface thereof is mounted face-down on the first surface of the light-transmitting substrate 1 . Various devices may optionally be formed on the semiconductor chip 6 . On each electrode 5 of the semiconductor chip 6 , a metal bump 4 is formed. In the present example, the metal bumps 4 are made of Au. However, the metal bumps 4 are not essential components of a semiconductor device according to the present invention. Between the semiconductor chip 6 and the light-transmitting substrate 1 , a photo-thermal cross-linkable insulating resin layer 8 is formed so as to fix the semiconductor chip 6 on the light-transmitting substrate 1 . The photo-thermal cross-linkable insulating resin is an acrylate-type transparent resin in the present example, but may alternatively be a methacrylate-type resin, or the like. Generally speaking, a photo-thermal cross-linkable insulating resin is cured by being irradiated with light. Strictly speaking, however, a photo-thermal cross-linkable insulating resin includes those which are to be cured by being irradiated with light but are not to be cured by being heated. The photo-thermal cross-linkable insulating resins used in the present application can be cured by not only light-irradiation but also heat treatment. The electrodes 5 of the semiconductor chip 6 are connected to the circuit conductor layer 2 by way of plated metal layers 3 formed in at least a portion of the circuit conductor layer 2 . More specifically, the plated metal layers 3 are melted on the circuit conductor layer 2 and thereafter are recoagulated thereon so as to form an alloy layer in an abutting portion between each plated metal layer 3 and the circuit conductor layer 2 . At the same time, the plated metal layers 3 are melted on the metal bumps 4 as well, so as to form an alloy layer in an abutting portion between each plated metal layer 3 and each metal bump 4 . The metal bumps 4 are thus connected to the circuit conductor layer 2 , by way of the alloy layers formed in the abutting portions between the metal bumps 4 and the plated metal layers 3 and by way of the alloy layers formed in the abutting portions between the plated metal layers 3 and the circuit conductor layer 2 . In cases where no metal bump is provided, an alloy layer is formed in an abutting portion between each electrode 5 and each plated metal layer 3 in a similar process to that mentioned above, while an alloy layer is similarly formed in the abutting portion between each plated metal layer 3 and the circuit conductor layer 2 . The plated metal layers 3 can be made of solder of an In/Ga group, an In/Sn group, or a Pb/Sn group in both cases. Hereinafter, a method for producing the above-mentioned semiconductor device will be described. First, the semiconductor chip 6 is fabricated in the following manner: By a semiconductor fabrication process, various device elements (not shown) and the electrodes 5 are formed on a single-crystal silicon substrate (silicon wafer). The metal bumps 4 are formed on the electrodes 5 by such methods as an electric plating method and a ball-bonding method. The metal bumps 4 are made of Au in the present example. Then, the silicon wafer is cut out by a fine dicing technique so as to form the semiconductor chip 6 . Next, the circuit conductor layer 2 is formed on the first surface of the light-transmitting substrate 1 . The light-transmitting substrate 1 is a glass substrate, but may alternatively be a transparent film made of polyarylate (PA), polyethersulfone (PES), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or the like. The circuit conductor layer 2 may be formed by depositing a layer of metal such as Cu or Al on the light-transmitting substrate 1 by a sputtering method, a vapor deposition method or the like and then patterning the deposited layer by a photolithography method. The circuit conductor layer 2 may alternatively be formed using a metal foil or the like. Moreover, the circuit conductor layer 2 may alternatively be formed by attaching a flexible printed wiring board to the first surface of the light-transmitting substrate 1 . Moreover, the plated metal layers 3 are formed in prescribed portions of the circuit conductor layer 2 by an electrolysis method, a non-electrolysis method, or other known methods. Then, the photo-thermal cross-linkable insulating resin layer 8 is formed by applying a prescribed amount of an acrylate-type transparent insulating resin to be cured by use of both light and heat to a prescribed portion of the first surface of the light-transmitting substrate 1 by a stamping method, a screen printing method, etc. The semiconductor chip 6 having the electrodes 5 and the like on a surface thereof is disposed face-down on the photo-thermal cross-linkable insulating resin layer 8 so that the electrodes 5 abut the plated metal layers 3 . Then, while the semiconductor chip 6 is pressed onto the light-transmitting substrate 1 , the photo-thermal cross-linkable insulating resin layer 8 is irradiated with ultra violet rays through the light-transmitting substrate 1 so as to be cured (First curing step). Herein, the ultra violet rays are radiated from the second surface of the light-transmitting substrate 1 onto the photo-thermal cross-linkable insulating resin layer 8 . At this first curing step, a pressure in the range of 0.5 kgf to 2.5 kgf is applied to the semiconductor chip 6 from above. While the above-mentioned pressure is applied to the semiconductor chip 6 , the photo-thermal cross-linkable insulating resin layer 8 is irradiated with ultra violet rays having an intensity in the range of 200 mW to 500 mW for 0.5 to 2 minutes. Then, the light-transmitting substrate 1 with the semiconductor chip 6 disposed thereon is placed in an electric furnace so as to be heated at a certain temperature for a while so that the photo-thermal cross-linkable insulating resin is further cured (Second curing step). The temperature should be higher than the melting point of the plated metal layers 3 and than the curing temperature of the photo-thermal cross-linkable insulating resin. In cases where eutectic solder is used as the plated metal layers 3 , the heat treatment is conducted at a temperature in the range of 185°C to 200°C for about 5 to 30 minutes. In the present example, the heat treatment is conducted at 190°C for 10 minutes. At the second curing step, the plated metal layers 3 are melted so that alloy layers are formed in abutting portions between the metal bumps 4 and the plated metal layers 3 , and in abutting portions between the plated metal layers 3 and the circuit conductor layer 2 . The following effects are obtained by melting the plated metal layers 3 at the above-mentioned second curing step: Since each plated metal layer 3 becomes liquid so as to have a configuration with minute superficial rises and falls corresponding to those of the abutting portion of each electrode 5 , the plated metal layers 3 and the electrodes 5 can contact in a larger area. Moreover, the alloy layers are more easily formed in the above-mentioned abutting portions, thus further securing the connection between the plated metal layers 3 and the electrodes 5 . Moreover, if organic dusts are present on surfaces of the plated metal layers 3 and the electrodes 5 , such organic dusts can be removed. At the first curing step, a portion of the photo-thermal cross-linkable insulating resin which is in the shade of the circuit conductor layer 2 formed on the light-transmitting substrate 1 remains uncured. That portion of the photo-thermal cross-linkable insulating resin is, however, completely cured during the second curing step, so that the mounting of the semiconductor chip 6 is completed. The heat treatment at the second curing step is conducted while a pressure of 0 to 1.0 kgf is applied to the semiconductor chip 6 from above so that the semiconductor chip 6 is pressed against the light-transmitting substrate 1 . After the mounting of the semiconductor chip 6 is completed, a resin such as that made of silicon is applied by means of a dispenser, etc. on a resultant complex of the semiconductor chip 6 and the light-transmitting substrate 1 so as to form a resin coating 7 . Thus, the semiconductor device has been fabricated. As has been described, by conducting curing of the photo-thermal cross-linkable insulating resin through the first and second curing steps, the portion of the photo-thermal cross-linkable insulating resin which remains uncured at the first curing step is also cured. Moreover, the semiconductor chip 6 is electrically connected to the circuit conductor layer 2 with more stability because alloy layers are formed in the abutting portions between the plated metal layers 3 and the metal bumps 4 and in the abutting portions between the plated metal layers 3 and the circuit conductor layer 2 . Accordingly, the mounting of the semiconductor chip 6 becomes more secure. Hereinafter, an image sensor device according to the present invention will be described with referrence to Figures 2 , 3 , 4 , 5 , and 6 . Figure 2 shows a cross-section of the image sensor device. Figure 4 is a top view of the image sensor device. Figures 5 and 6 show cross-sections of the image sensor device taken along, respectively, Line A-A' and Line B-B' in Figure 3 , which also shows the front cross-section of the image sensor device. A light-transmitting substrate 21 is a transparent film made of polyarylate (PA), polyethersulfone (PES), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or the like, but may alternatively be a glass substrate as in the case of the above-described semiconductor device. In cases of a contact type image sensor device, the thickness T s of the light-transmitting substrate 21 should be in the range of 0.5 mm to 2.0 mm. In cases of a direct-contact type image sensor device, T s should be in the range of 10 µm to 200 µm, and more preferably, in the range of 10 µm to 75µm. On a first surface of the light-transmitting substrate 21 , a circuit conductor layer 22 made of a metal such as Cu or Al, or a precious metal such as Au or Ag-Pt is formed. The thickness T o of the circuit conductor layer 22 should be in the range of 3 µm to 35 µm. As is shown in Figures 4 and 6 , an image sensor chip 26 has electrodes 25 , light-sensitive elements 29 , and various other device elements (not shown) formed on a surface thereof. The light-sensitive elements 29 may be phototransistors, photodiodes, etc., forming a light-sensitive element array. The other device elements are access circuits including CCDs (Charge Coupled Devices), MOS (Metal Oxide Semi-Conductor)-ICs (Integrated Circuits), or bipolar ICs. On each electrode 25 , a metal bump 24 is formed. The metal bumps 24 are made of Au. The thickness T b of each metal bump 24 should be in the range of 3 µm to 60 µm. However, the metal bumps 24 are not essential components of an image sensor device according to the present invention. The image sensor chip 26 is mounted face-down on the first surface of the light-transmitting substrate 21 . Between the semiconductor chip 26 and the light-transmitting substrate 21 , a photo-thermal cross-linkable insulating resin layer 28 is formed so as to fix the image sensor chip 26 on the light-transmitting substrate 21 . The photo-thermal cross-linkable insulating resin is an acrylate-type transparent resin in the present example, but may alternatively be a methacrylate-type resin, or the like. The electrodes 25 of the image sensor chip 26 are connected to the circuit conductor layer 22 by way of plated metal layers 23 formed in at least a portion of the circuit conductor layer 22 . More specifically, the plated metal layers 23 are melted on the circuit conductor layer 22 and thereafter are recoagulated thereon so as to form an alloy layer in an abutting portion between each plated metal layer 23 and the circuit conductor layer 22 . At the same time, the plated metal layers 23 are melted on the metal bumps 24 as well, so as to form an alloy layer in an abutting portion between each plated metal layer 23 and each metal bump 24 . The metal bumps 24 are thus connected to the circuit conductor layer 22 , by way of the alloy layers formed in the abutting portions between the metal bumps 24 and the plated metal layers 23 and by way of the alloy layers formed in the abutting portions between the plated metal layers 23 and the circuit conductor layer 22 . In cases where no metal bump is provided, an alloy layer is formed in an abutting portion between each electrode 25 and each plated metal layer 23 in a similar process to that mentioned above, while an alloy layer is similarly formed in the abutting portion between each plated metal layer 23 and the circuit conductor layer 22 . The plated metal layers 23 can be made of solder of an In/Ga group, an In/Sn group, or a Pb/Sn group in both cases. The thickness T m of each plated metal layer 23 should be in the range of 2 µm to 20 µm. In cases where the above-mentioned image sensor device is used as a direct-contact type image sensor device, the distance T between the array of the light-sensitive elements 29 and a second surface (i.e. back side) of the light-transmitting substrate 21 should be 150 µm or less, and preferably 100 µm or less. Herein, the distance T is defined as a sum total of the thicknesses T s , T o , T b , and T m . The distance T should be within the abovementioned range because the resolution of a direct-contact type image sensor device greatly depends on the distance T between the light-sensitive elements 29 and the surface of a manuscript (i.e. the second surface of the light-transmitting substrate 21 ) to be read by the image sensor device. Figure 7 describes this relationship between the resolution of a direct-contact type image sensor device and the distance T. In Figure 7 , MTF (Modulation-Transfer-Function) values are taken as the axis of ordinates. An MTF value indicates the resolution degree of an image sensor device, etc. In terms of practicality, an MTF value of 30% to 40% or more is required at 4 lp/mm, thus limiting the distance T to be within the above-mentioned range. Hereinafter, a.method for producing the above-mentioned image sensor device will be described. First, the image sensor chip 26 is fabricated in the following manner: By a semiconductor-fabricating process, phototransistors serving as the light-sensitive elements 29 , a driving circuit (not shown) and the electrodes 25 are formed on a single-crystal silicon substrate (silicon wafer). The metal bumps 24 are formed on the electrodes 25 by such methods as an electric plating method and a ball-bonding method. The metal bumps 24 are made of Au in the present example. Then, the silicon wafer is cut out by a fine dicing technique so as to form the image sensor chip 26 . Next, the circuit conductor layer 22 is formed on the first surface of the light-transmitting substrate 21 . The light-transmitting substrate 21 is a transparent film made of polyarylate (PA), polyethersulfone (PES), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or the like, but may alternatively be a glass substrate. The circuit conductor layer 22 may be formed by depositing a layer of metal such as Cu or Al on the light-transmitting substrate 21 by a sputtering method, a vapor deposition method or the like and then patterning the deposited layer by a photolithography method. The circuit conductor layer 22 may alternatively be formed using a metal foil or the like. Moreover, the circuit conductor layer 22 may alternatively be made of a precious metal such as Au or Ag-Pt. Moreover, the circuit conductor layer 22 may alternatively be formed by attaching a flexible printed wiring board to the first surface of the light-transmitting substrate 21 . Furthermore, the circuit conductor layer 22 may alternatively be formed by a thick-film printing method. Moreover, the plated metal layers 23 are formed in prescribed portions of the circuit conductor layer 22 by an electrolysis method, a non-electrolysis method, or other known methods. Then, the photo-thermal cross-linkable insulating resin layer 28 is formed by applying a prescribed amount of an acrylate-type transparent insulating resin which is to be cured by use of light and heat to a prescribed portion of the first surface of the light-transmitting substrate 21 by a stamping method, a screen printing method, etc. The image sensor chip 26 having the electrodes 25 , the light-sensitive elements 29 , and the like on a surface thereof is disposed face-down on the photo-thermal cross-linkable insulating resin layer 28 so that the electrodes 25 abut the plated metal layers 23 . Then, while the image sensor chip 26 is pressed onto the light-transmitting substrate 21 , the photo-thermal cross-linkable insulating resin layer 28 is irradiated with ultra violet rays through the light-transmitting substrate 21 so as to be cured (First curing step). Herein, the ultra violet rays are radiated from the second surface of the light-transmitting substrate 21 onto the photo-thermal cross-linkable insulating resin layer 28 . The image sensor chip used in the present example has 15 electrodes. A pressure in the range of 0.5 kgf to 2.5 kgf is applied to the image sensor chip 26 from above. While the above-mentioned pressure is applied to the image sensor chip 26 , the photo-thermal cross-linkable insulating resin layer 28 is irradiated with ultra violet rays having an intensity in the range of 100 mW to 1500 mW for 20 to 100 seconds. In view of mass-production effects, it is preferable that the first curing step be completed in 20 to 40 seconds. Then, the light-transmitting substrate 21 with the image sensor chip 26 disposed thereon is placed in an electric furnace so as to be heated at a certain temperature for a while so that the photo-thermal cross-linkable insulating resin is further cured (Second curing step). The temperature should be higher than the melting point of the plated metal layers 23 and than the curing temperature of the photo-thermal cross-linkable insulating resin. In cases where eutectic solder is used as the plated metal layers 23 , the heat treatment is conducted at a temperature in the range of 185°C to 200°C for about 5 to 30 minutes. In the present example, the heat treatment is conducted at 190°C for 10 minutes. At the second curing step, the plated metal layers 23 are melted so that alloy layers are formed in abutting portions between the metal bumps 24 and the plated metal layers 23 , and in abutting portions between the plated metal layers 23 and the circuit conductor layer 22 . The following effects, which are similar to those in the case of the earlier-mentioned semiconductor device, are obtained by melting the plated metal layers 23 at the above-mentioned second curing step: Since each plated metal layer 23 becomes liquid so as to have a configuration with minute superficial rises and falls corresponding to those of the abutting portion of each electrode 25 , the plated metal layers 23 and the electrodes 25 can contact in a larger area. Moreover, the alloy layers are more easily formed in the above-mentioned abutting portions, thus further securing the connection between the plated metal layers 23 and the electrodes 25 . Moreover, if organic dusts are present on surfaces of the plated metal layers 23 and the electrodes 25 , such organic dusts can be removed. At the first curing step, a portion of the photo-thermal cross-linkable insulating resin which is in the shade of the circuit conductor layer 22 formed on the light-transmitting substrate 21 remains uncured. That portion of the photo-thermal cross-linkable insulating resin is, however, completely cured during the second curing step, so that the mounting of the image sensor chip 26 is completed. The heat treatment at the second curing step is conducted while a pressure of 0 to 1.0 kgf is applied to the image sensor chip 26 from above so that the image sensor chip 26 is pressed against the light-transmitting substrate 21 . After the mounting of the image sensor chip 26 is completed, a resin such as that made of silicon is applied by means of a dispenser, etc. on a resultant complex of the image sensor chip 26 and the light-transmitting substrate 21 so as to form a resin coating 27 . Thus, the image sensor device has been fabricated. As is described above, a portion of the photo-thermal cross-linkable insulating resin remains uncured at the first curing step, since the curing is conducted only by irradiation with light (ultra violet rays). Therefore, if the second curing step is not conducted, mounting defects occur in an environmental test and the like to be conducted later on. Table 1 shows results of an environmental test with respect to a case where only the first curing step (i.e. curing by light-irradiation) is conducted, and to a case where both the first and second curing steps (i.e. curing by light-irradiation and curing by heat treatment) are conducted. As is seen from Table 1, no defects are found when the photo-thermal cross-linkable insulating resin is cured by both light-irradiation and heat treatment, in either of a case where the image sensor device is kept in a thermo-hygro static state (temperature: 85°C, humidity: 85%) or a case where heat shocks (between -30°C and +70°C) are given to the image sensor device. On the other hand, about 40% to 60% of all the image sensor devices that are subjected to the test are found defective when the photo-thermal cross-linkable insulating resin is cured only by light-irradiation. In other words, the mounting yield of the image sensor devices can be improved by conducting two steps of curing, that is, curing by light-irradiation and curing by heat treatment. Moreover, as is seen from Table 1, the image sensor devices are prevented from deterioration, leading to improvement in the endurance thereof. As has been described, according to the present invention, electrodes (or bumps) formed on a semiconductor chip or an image sensor chip can have a firm electrical connection to a circuit conductor layer formed on a substrate, since plated metal layers are formed on the circuit conductor layer. Therefore, a reliable semiconductor device and a reliable image sensor device can be provided. Moreover, the semiconductor device and the image sensor device each can be firmly mounted on a light-transmitting substrate, since a photo-thermal cross-linkable insulating resin included therein is completely cured through first and second curing steps, greatly improving the mounting yield of the devices. Moreover, the semiconductor device and the image sensor device are prevented from deterioration, leading to improvement in the endurance thereof.

Claims: A semiconductor device comprising: a) a light-transmitting substrate (1; 21) having a first surface and a second surface; b) a circuit conductor layer (2; 22) formed on the first surface of the light-transmitting substrate (1; 21); c) a semiconductor chip (6; 26) having electrodes (5; 25) formed on a surface thereof, the semiconductor device being mounted face-down on the first surface of the light-transmitting substrate (1; 21); and d) a plated metal layer (3; 23) formed on at least a portion of the circuit conductor layer (2; 22), the electrodes (5; 25) of the semiconductor chip (6; 26) being connected to the circuit conductor layer (2; 22) through the plated metal layer (3; 23); characterized by the following features: e) a photo-thermal crosslinked insulating resin layer (8; 28) is disposed between the semiconductor chip (6; 26) and the light-transmitting substrate (1; 21) for fixing the semiconductor chip (6; 26) on the light-transmitting substrate (1; 21); and f) an electrical connection formed in an abutting portion between the plated metal layer (3; 23) and each of the electrodes (5; 25), wherein in the electrical connection the plated metal layer (3; 23) contacts each electrode (5; 25) along minute superficial rises and falls of the abut ting portion of the electrode (5; 25) due to fusion and recoagulation of the plated metal layer (3; 23). A semiconductor device according to claim 1, further comprising a second electrical connection formed in an abutting portion between the circuit conductor layer (2) and each plated metal layer (3). A semiconductor device according to one of claims 1 or 2 , characterized in that the plated metal layer (3) is made of solder. A semiconductor device according to one of claims 1 to 3, characterized in that a metal bump (4) is on each electrode (5), each metal bump (4) being connected to the circuit conductor layer (2) through each plated metal layer (3); and a second electrical connection formed in an abutting portion between each metal bump (4) and each plated metal layer (3), the second electrical connection being formed by melting and recoagulating the plated metal layer (3). A semiconductor device according to claim 4, characterized in that the metal bumps (4) are made of Au. A semiconductor device according to one of claims 1 to 5, characterized in that the semiconductor chip (6) is formed by an image chip (26) having electrodes (25) and at least one light-sensitive element (29) formed on a surface thereof. A semiconductor device according to one of claims 1 to 6, characterized in that the sum total of the thickness of the light-transmitting substrate (1) and the thickness of the photo-thermal cross-linkable insulating resin layer (7) is 150 µm or less. A semiconductor device according to one of claims 1 to 7, characterized in that a sum total of the thickness of the light-transmitting substrate (1) and the thickness of the photo-thermal cross-linkable insulating resin layer (7) is 100 µm or less. A method of producing a semiconductor device comprising the steps of: a) forming a circuit conductor layer (2; 22) on a first surface of a light-transmitting substrate (1; 21) having a first surface and a second surface; b) forming a plated metal layer (3; 23) on at least a portion of the circuit conductor layer (2; 22); c) forming a photo-thermal cross-linkable insulating resin layer (7; 27) on the first surface of the light-transmitting substrate (1; 21); d) disposing a semiconductor chip (6; 26) having electrodes (5; 25) on a surface thereof face-down so that the electrodes (5; 25) are in contact with the plated metal layer (3; 23); e) conducting a first curing step for curing the photo-thermal cross-linkable insulating resin by radiating ultra violet rays through the light-transmitting substrate (1; 21) from the second surface of the light-transmitting substrate (1; 21) while pressing the semiconductor chip (6; 26) against the light-transmitting substrate (1;21); and f) conducting a second curing step for further curing the photo-thermal cross-linkable insulating resin by heating the light-transmitting substrate (1; 21) with the semiconductor chip (6; 26) disposed thereon at a certain temperature for a period of time, the temperature being higher than the melting point of the plated metal layer (3; 23) and than the curing temperature of the photo-thermal cross-linkable resin, characterized in that g) the second curing step comprises the formation of an electrical connection in an abutting portion between the plated metal layer (3; 23) and each of the electrodes (5; 25), wherein the electrical connection is formed by metling and recoagulating the plated metal layer (3; 23) so that the plated metal layer (3; 23) contacts each electrode (5; 25) along the minute superficial rises and falls of the abutting portion of the electrode (5; 25). A method according to claim 9, characterized in that the second curing step further comprises a step for forming a second electrical connection in an abutting portion between each plated metal layer (3) and the circuit conductor layer (2), the electrical connection being formed by melting and recoagulating the plated metal layer (3). A method according to one of claims 9 and 10, characterized in that a metal bump (4) is formed on each electrode (5), and that the second curing step further comprises a step for forming a first electrical connection in an abutting portion between each plated metal layer (3) and each metal bump (4) and for forming a second electrical connection in an abutting portion between the circuit conductor layer (2) and each plated metal layer (3), the electrical connections being formed by melting and recoagulating the plated metal layer (3). A method according to one of claims 9 to 11, characterized in that the second curing step is conducted while the semiconductor chip (6) is pressed against the light-transmitting substrate (1). A method according to one of claims 9 to 12, characterized in that the step for forming the circuit conductor layer includes: a step for forming a conductor layer on the first surface of the light-transmitting substrate (1); and a step for forming the circuit conductor layer (2) by patterning the conductor layer by a photolithography method. A method according to one of claims 9 to 13, characterized in that the step for forming the circuit conductor layer (2) is conducted by attaching a flexible printed wiring board to the first surface of the light-transmitting substrate (1). A method according to one of claims 9 to 14, characterized in that the semiconductor chip (6) is formed by an image sensor chip (26) having electrodes (25) and at least one light-sensitive element (29) formed on a surface thereof. A method according to one of claims 9 to 15, characterized in that the sum total of the thickness of the light-transmitting substrate (1) and the thickness of the photo-thermal cross-linkable insulating resin layer (7) is 150 µm or less. A method according to one of claims 9 to 16, characterized in that the step for forming the circuit conductor layer (2) includes a step for forming the circuit conductor layer on the first surface of the light-transmitting substrate (1) by a thick-film printing method.

Inventors: FUJIWARA SHINJI, NAKAGAWA MASAHIRO, NAKAMURA TETSURO, TANAKA EIICHIRO, FUJIWARA, SHINJI, NAKAGAWA, MASAHIRO, NAKAMURA, TETSURO, TANAKA, EIICHIRO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: H01L31
IPC codes: H01L21, H01L31