Patent Publication Number: US-6992297-B2

Title: Image sensor and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation Application of PCT Application No. PCT/JP02/01919, filed Mar. 1, 2002, which was not published under PCT Article 21(2) in English. 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-057346, filed Mar. 1, 2001, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image sensor, which detects radiation, and to a method of manufacturing image sensors. 
     2. Description of the Related Art 
     An image sensor, which detects radiation such as hard X-rays or γ-rays so as to generate image information, has been used in various technical fields. For example, a radiant field from a certain celestial body is detected, and thereby, it is possible to know the physical status of the celestial body and the spatial structure thereof. Further, X-rays are irradiated to a human body, and then, the transmitted wave is investigated, and thereby, tomography of the human body can be obtained. Besides, the image sensors have also been used in other fields, such as nuclear power (glass solidification check of radioactive waste, radiation monitor, etc.), non-destructive tests (semiconductor tester, etc.) and mineral surveying (mineral resources research). 
     The image sensor conventionally used for the above fields has the following structure, for example. 
       FIG. 1A  is a view showing a typical example of the above conventional image sensor  80 , and  FIG. 1B  is a cross-sectional view taken along the B—B direction of  FIG. 1A . As shown in  FIG. 1A  and  FIG. 1B , the image sensor  80  has a detecting element (Si element or Ge element)  81 , and an amplifying IC  84 . More specifically, the detecting element  81  detects incident radiation so as to generate an electric signal. The amplifying IC  84  is arranged on the same plane as the detecting element  81 , and amplifies the electric signal. In the image sensor  80 , an interconnection line  83  led out of the detecting element  81  is connected to the amplifying IC  84  by wire bonding. 
     In general, according to the above structure in which the detecting element and the amplifying IC are arranged on the same plane and mutually connected, it is difficult to greatly increase the number of the detecting elements  81  of the image sensor. The reason is because it is technically difficult to further lead many signal lines out of the peripheral region of the detecting element  81  by wire bonding. 
     The image sensor using the Si element detects only X-rays having a low energy from several KeV to tens of KeV; for this reason, the image sensor does not have the sensitivity required for practical use. 
     As typified by the CCD, the recent visible sensor technique has been specialized in slowly reading two-dimensional information under the condition that a signal to noise ratio is very good. However, in photons other than the visible light region, environmental noise is high and signals are weak; for this reason, high-speed operation and high-reduction of noise are required as not so compared with the case of detecting the visible light. As a result, there is a need of connecting a read circuit to each one of fine pixels, and the development of a high-speed parallel readable system has been required. 
     It has been known that silicon becomes transparent with respect to hard X-rays or γ-rays having a wavelength shorter than soft X-rays. Therefore, in order to obtain the large stopping power, the development of a new CdTe semiconductor must progress quickly. 
     The present invention has been made in view of the above circumstances. Accordingly, it is an object to provide a highly sensitive image sensor, which can readily make an electric connection between each sensor element and an amplifying IC even if it has many sensor elements, and to provide a method of manufacturing the image sensor. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to achieve the above object, the present invention has taken the following means. 
     According to a first aspect of the present invention, there is provided an image sensor which comprises: a sensor element array having a plurality of sensor elements arrayed in a two-dimensional matrix; an IC substrate laminating the sensor element array and provided with a plurality of ICs for amplifying an electric signal based on radiation incident on any of the plurality of the sensor elements; and a connection layer interposed between the sensor element array and the IC substrate, and electrically connecting each electrode of the sensor elements with each electrode of the ICs. 
     According to a second aspect of the present invention, in the image sensor of the first aspect, the connection layer has a plurality of stud bumps formed on the electrode of each IC; and a plurality of thin film layers formed at the distal end of each stud bump and electrically connected with the electrode of each sensor element. 
     According to a third aspect of the present invention, in the image sensor of the second aspect, each of the stud bumps is made of gold; and each of the thin film layers is made of indium. 
     According to a fourth aspect of the present invention, in the image sensor of the second aspect, the connection layer has an insulating layer which buries the stud bumps and the thin film layers. 
     According to a fifth aspect of the present invention, in the image sensor of the first aspect, the connection layer has a plurality of multi-layer bumps formed in a manner that at least two-layer stud bump is laminated on the electrode of each IC, and a plurality of thin film layers formed at the distal end of the multi-layer bump and electrically connected with the electrode of each sensor element. 
     According to a sixth aspect of the present invention, in the image sensor of the fifth aspect, each of the multi-layer bumps is made of gold; and each of the thin film layers is made of indium. 
     According to a seventh aspect of the present invention, in the image sensor of the fifth aspect, the connection layer has an insulating layer which buries the stud bumps and the thin film layers. 
     According to an eighth aspect of the present invention, in the image sensor of the first aspect, the sensor element is a CdTe element. 
     According to a ninth aspect of the present invention, there is provided a method of manufacturing an image sensor which comprises: forming a stud bump on each electrode pad of the predetermined number of IC chips provided in a first substrate; carrying out indium plating with respect to a second substrate; transferring the plated indium to the second substrate to the distal end of each stud bump so that a plurality of thin film layers can be formed; connecting each of the thin film layers with the electrode of the sensor element so that a sensor element array having a plurality of sensor elements arrayed like two-dimensional matrix is mounted to each IC of the first substrate by flip chip mounting; and injecting an insulating resin between the first substrate and the sensor element array, and thereafter, hardening the insulating resin. 
     According to a tenth aspect of the present invention, in the method of the ninth aspect, each of the stud bumps is made of gold, and each of the thin film layers is made of indium. 
     Therefore, according to the above structure, even if the image sensor has many sensor elements, it is possible to provide a high-sensitive image sensor, which can readily make electric connection between each sensor element and amplification IC, and to provide a method of manufacturing the image sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1A  and  FIG. 1B  are views to explain a conventional image sensor; 
         FIG. 2A  and  FIG. 2B  are views to schematically explain the structure of an image sensor  10 ; 
         FIG. 3A  to  FIG. 3C  are cross-sectional views taken along the line A—A of the image sensor shown in  FIG. 2A ; 
         FIG. 4A  and  FIG. 4B  are views to schematically explain the structure of an IC substrate included in the image sensor  10 ; 
         FIG. 5  is a flowchart schematically showing a method of manufacturing the image sensor  10 ; 
         FIG. 6  is a conceptual view to explain indium (In) transfer in step S 2 ; 
         FIG. 7  is a schematic view showing an indium (In) layer  131  formed at the distal end of an au stud bump  130  formed in the process of step S 2 ; and 
         FIG. 8  is a flowchart showing the embodiment of the method of manufacturing the image sensor  10 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments of the present invention will be described below with reference to the accompanying drawings. In the following description, the identical reference numerals are given to constituent components having substantially the same function and structure, and the overlapping explanation will be made in only necessary case. 
     The image sensor  10  of the embodiment is formed in a manner of mounting a sensor element array on the IC substrate mounted with the amplifying IC according to flip chip mounting (hereinafter, referred to as “FC mounting”) by the stud bump process described later. The sensor element array is formed by arranging a plurality of sensor elements made of CdTe (cadmium telluride) in a matrix. The structure of the sensor element array and the IC substrate will be described below with reference to  FIG. 1  to  FIG. 3 . 
       FIG. 2A  and  FIG. 2B  are views to schematically explain the structure of the image sensor  10 .  FIG. 2A  is a view showing an upper surface of the image sensor  10  (and an upper surface of the sensor element array  11 ).  FIG. 2B  is an enlarged view showing a portion surrounded by a circle shown in  FIG. 2A . 
       FIG. 3A  is a cross-sectional view taken along the line A—A of the image sensor  10  shown in  FIG. 2A . 
     As is illustrated in  FIG. 2A  and  FIG. 3A , the sensor element array  11  has a CdTe element  110 , a first electrode  111 , a second electrode  112 , and an active contact  113 . 
     The CdTe element  110  is a compound semiconductor consisting of Cd (cadmium) and Te (tellurium). The energy gap of the CdTe element  110  is about 1.47 eV under room temperature. 
     The first electrode  111  is arrayed like two-dimensional matrix on the radiation incident side of the CdTe element  110 , and is formed of Pt, for example. The second electrode  112  is arrayed on an electric signal fetch side of the CdTe element  110  facing the first electrode  111  via the CdTe element  110 , and is formed of Pt, for example. A predetermined voltage for detecting radiation is applied between the first and second electrodes  111  and  112 . 
     More specifically, the sensor element array  11  is formed in the following manner. The sensor element is formed in a manner of sandwiching the CdTe element  110  between the first and second Pt electrodes  111  and  112 , and thereafter, in a two-dimensional matrix. When radiation is incident on a depletion layer formed by applying the voltage between the first and second Pt electrodes  111  and  112 , many electrons and holes are generated along the track of radiation. Positive and negative charges are fetched as an electric signal, and thereby, the sensor array element  11  generates image information based on the incident radiation. 
     The active contact  113  is formed in the first and second electrodes, and is a pad for flip-chip-mounting the sensor array element  11  on the IC substrate  15  by the stud bump process described later. The electric signal detected by each sensor element is inputted to the IC from the active contact  113  via a stud bump described later, thereafter, is subjected to predetermined signal processing such as amplification. 
       FIG. 3  is a view to schematically explain the structure of the IC substrate  15  included in the image sensor  10 .  FIG. 4A  is a view showing an upper surface of the IC substrate  15 , and  FIG. 4B  is an enlarged view showing a portion surrounded by a circle shown in  FIG. 4A . 
     As shown in  FIG. 4A , in the IC substrate  15 , a flip chip bump  150  (hereinafter, referred to as “FC pad”) included in each of plural X-ray resistant ICs (not shown) is arrayed like two-dimensional matrix. The position of the FC pad  150  corresponds to that of the above active contact  113 , and FC mounting is carried out based on both positions. A stud bump described later is formed on the FC pad  150 . 
     Next, a connecting layer  13  formed between the image sensor  10  and the IC substrate  15  will be described below with reference to  FIG. 3B  and  FIG. 3C . 
       FIG. 3B  and  FIG. 3C  are views to explain FC mounting of the image sensor  10  to the IC substrate  15 .  FIG. 3B  is an enlarged view showing part of the connecting layer  13  shown in  FIG. 3A . 
     As seen from  FIG. 3B  and  FIG. 3C , the connecting layer  13  has a stud bump  130 , an indium layer  131  formed at the distal end of the stud bump  130 , and an insulating layer  132 . 
     The stud bump  130  is formed on the FC pad  150  on each IC substrate  15 , and is made of gold. The stud bump  130  is formed in a manner that a projection-shaped bump is laminated on the FC pad  150  in one or two-stage or more. The stud bump  130  performs the function of absorbing the current application between the sensor element and the IC, and a connection error in FC mounting described later. Therefore, it is preferable that the material used for the stud bump is a relatively soft metal having excellent conductivity. 
     The indium layer (In layer)  131  is a thin film layer, which is formed at the distal end of the stud bump  130 . The indium layer  131  is formed into a tapered shape so as to have a predetermined height in the manufacturing stage (see  FIG. 7 ). In FC mounting, the indium layer  131  is pressed and welded by the second electrode and the active contact  113 , and thus, has a shape shown in  FIG. 3C . Further, the indium layer  131  performs the function of applying a current between the sensor element and the IC, and providing a predetermined height required for FC mounting described later. Since the CdTe element is used in the embodiment, it is preferable that a solder having a melting point of 100° C. or less is used. For example, bismuth may be used in addition to indium. 
     The insulating layer  132  is a resin layer formed in underfill, and is formed of an epoxy resin, for example. Further, the insulating layer  132  performs the function of burying the stud bump  130  and the indium layer  131  so that they can be both electrically insulated and reinforced. The insulating layer  132  prevents the thermal stress generated by the difference in thermal expansion coefficient between the sensor element array  11  and the IC substrate  15  from concentrating on the stud bump  130  and the indium layer  131 . 
     One of the features of the above image sensor  10  is that the sensor element array  11  and the IC substrate  15  are mounted in the laminated form by the connection layer  13 , in particular, the stud bump  130  and the In layer  131 . More specifically, in the conventional image sensor, the IC and the sensor element arrayed on the same plane are connected by wire bonding; a so-called “two-dimensional mounting mode” has been employed. On the contrary, in the image sensor  10 , the sensor element array  11  is laminated on the IC substrate  15  by the stud bump  130  and the In layer  131 ; a so-called “three-dimensional mounting mode” is employed. 
     In the image sensor made employing the above three-dimensional mounting, IC connection is made from the lower portion of each sensor element. Therefore, even if the number of sensor elements increases, it is possible to easily extract signals. As a result, it is possible to generate an image having a great number of pixels as compared with the conventional case. Further, the three-dimensional mounting is employed, and thereby, it is possible to down-size the image sensor. 
     (Method of Manufacturing the Image Sensor) 
     Next, the method of manufacturing the image sensor  10  will be described below with reference to  FIG. 5 . 
       FIG. 5  is a flowchart schematically showing the method of manufacturing the image sensor  10 . In  FIG. 5 , first, the IC substrate  15  having the plurality of ICs arrayed in a predetermined form is prepared. A stub bump (hereinafter, referred to as “Au stud bump”) using gold as the base material is molded on each flip chip pad  150  (step S 1 ). As the need arises, there is the case where the au stub bump is formed in plural stages (i.e., the Au stud bump is laminated). 
     On the other hand, a stainless substrate is plated with indium (In) so as to form the transfer destination for indium (In) transfer to the distal end of the Au stud bump (step S 1 ′). 
     Sequentially, the indium (In) is transferred to the distal end of the Au stud bump (step S 2 ). The transfer in step S 2  is carried out in the following manner using an FC bonder. 
       FIG. 6  is a conceptual view to explain the indium (In) transfer in step S 2 . As is illustrated in  FIG. 6 , the IC substrate  15  with the Au stud bump  130  provided on the head side of the FC bonder (not shown) is pressed against a stainless substrate  16  plated with In. The above pressing operation is carried out under predetermined head temperature control, predetermined head speed and predetermined load control, and further, there is the case where the pressing operation is repeated plural times. After a predetermined pressing time elapses, when the head is pulled up under predetermined head temperature control, predetermined head speed and predetermined load control, the In transfer to the distal end of the Au stud bump  130  is completed. 
       FIG. 7  is a view schematically showing the In layer  131  formed at the distal end of the Au stud bump  130  formed in the process of step S 2 . As is illustrated in  FIG. 7 , the In layer  131  formed in step S 2  has a tapered shape and a predetermined height. Also, the shape of the Au stud bump  130  does not crush as shown in  FIG. 3C . 
     Sequentially, FC mounting of the sensor element array  11  to the IC substrate  15  is carried out (step S 3 ). More specifically, first, flattening is carried out with respect to the In layer  131  formed in step S 2 . This is to prevent an extra pressure from being applied to one point on the second electrode of the sensor element array  11  (in other words, pressure is uniformly applied to each second electrode  112 ). After the above flattening, the pressing/heating by the FC bonder destroys/melts an oxide film on the surface of each In layer  131  so that the In layer  131  can be welded to the corresponding second electrode  112 , and thereby, FC welding is achieved between the sensor element array  11  and the IC substrate  15 . In this case, no gap is generated between the second electrode  112  and the In layer  131  by the above press. Therefore, it is possible to prevent oxidation between the In layer and the second electrode in heating (i.e., the oxide film on the surface of each In layer  131  is destroyed/melted so that the In layer  131  can be welded to Pt on the electrode surface of the CdTe chip). 
     Finally, the insulating layer  132  is formed (step S 4 ). More specifically, the underfill seals the gap (FC gap) between the sensor element array  11  and the IC substrate  15 , which have been FC-connected in the process of step S 3 . Thereafter, the underfill is heated and hardened for a predetermined time so that the insulating layer  132  can be formed. 
     The above steps describe formation of the image sensor  10 . 
     (Embodiment of the Manufacturing Method) 
     The embodiment of the manufacturing method will be described below in detail with reference to  FIG. 8 . 
       FIG. 8  is a flowchart showing the embodiment of the method of manufacturing the image sensor  10 . In  FIG. 8 , first, the IC substrate  15  having the plurality of ICs arranged in a predetermined form is prepared. A stub bump (hereinafter, referred to as “Au stud bump”) using gold as the base material is molded on each flip chip pad  150  (step S 1 ). 
     On the other hand, the stainless substrate is plated with indium (In) to a thickness of 20 μm, and thus, the transfer destination for In transfer to the distal end of the Au stud bump is formed (step S 1 ′). 
     Sequentially, the indium (In) is transferred to the distal end of the Au stud bump (step S 2 ). The transfer in step S 2  is carried out in the following manner using an FC bonder. 
     More specifically, the IC substrate with the stud bump  150  is set to the head side of the FC bonder. On the other hand, the stainless substrate plated with indium (In) is set to the table side. In this case, the table temperature is kept at 50° C., for example. After the above setup is completed, the head is pulled down, and then, the distal end of each Au stud bump  130  can be pressed against the In plating layer of the stainless substrate two times. In this case, the first-time press is carried out under the following conditions. That is, the head is pressed at loading: 2.64 [kgf], pressing time: 11.0 sec., head temperature: 250° C., and head pull-down speed 0.2 [mm/s], and thereafter, pulled up by the height of 5 mm at the pull-up speed of 0.5 [mm/s] while maintaining the head temperature. Thereafter, the In plating layer is left to cool for 40 seconds, and then, the second-pressing is carried out under the following conditions. That is, the head is pressed at loading: 2.64 [kgf], pressing time: 5.0 sec., head temperature: 250° C., and head pull-down speed 0.2 [mm/s], and thereafter, pulled up by the height of 5 mm at the pull-up speed of 0.08 [mm/s] while maintaining the head temperature. The In transfer to the distal end of the Au stud bump  130  is then completed. 
     In the above In transfer, if the affinity between indium (In) and the stainless substrate is worse, the indium (In) of the stainless substrate is pressed against the Au stud bump so that the indium (In) can be heated/melted, and thereby, it is possible to effectively transfer the indium (In) to the au stud bump. 
     Sequentially, FC mounting of the sensor element array  11  to the IC substrate  15  is carried out (step S 3 ). 
     More specifically, flattening is carried out with respect to the In layer  131  formed in step S 2 . After the flattening, the IC substrate  15  is set to the head side of the FC bonder; on the other hand, the sensor element array  11  is set to the table side. After the IC substrate  15  and sensor element array  11  are positioned, the head is pulled down under the following conditions, that is, at loading: 10.00 [kgf], head temperature: 175° C., pressing time: 10.0 sec., and head pull-down speed 0.08 [mm/s]. Thereafter, the head is pressed against the stud bump  130  and the active contact  113  of the In layer  131  under the following conditions. After the above press is completed, the head position is held for 1.0 second while maintaining the head temperature at 175° C. Thereafter, the head is pulled up by the height of 5 mm at the pull-up speed of 0.17 [mm/s] while keeping the head temperature at 175° C., and thereby, the sensor element array  11  and the IC substrate  15  can be FC-welded together. In step S 3 , the table temperature is kept at 80° C. 
     Finally, the insulating layer  132  is formed (step S 4 ). First, on the hot plate, the sensor element array  11  is heated at a temperature from 60° C. to 80° C., and an epoxy resin is kept at a temperature from 25° C. to 40° C. The epoxy resin is injected into the FC gap formed in the process of step S 3 . In this case, care must be taken to prevent the sensor element array comprising CdTe and the FC welding portion separating. The heating temperature of the sensor element array  11  must be set so as not to exceed 80° C. If the temperature exceeds 80° C., the epoxy resin is set faster; as a result, injection is not smoothly achieved. The injection processing is carried out for about 3 minutes. 
     After the injection processing is completed, heating and hardening are carried out under the condition that the heating temperature is 125° C. and the heating time is two hours, and thereby, the insulating layer  132  is formed. 
     The above steps describe formation of the image sensor  10 . 
     One of the features of the above manufacturing method is the Au stud bump  130  (plural-stage Au stud bump  130 , as the case may be,) and the In layer  131  having a predetermined height, formed at the distal end of the Au stud bump  130 . More specifically, the height of the In layer  131  further increases in addition to the height of the au stud bump  130 . By doing so, it is possible to obtain a larger pressing width when pressure-welding the sensor element array  11  to the IC substrate  15  in FC welding. Further, the Au stud bump  130  absorbs the error in the height between bumps connected in FC welding. Therefore, according to the manufacturing method, it is possible to readily realize an image sensor having a great number of pixels. 
     Further, according to above manufacturing method, no special manufacturing apparatus is newly required, and it is possible to manufacture the image sensor  10  by applying presently available facilities. Therefore, the image sensor can be provided at low cost as compared with the conventional case. 
     Further, since the image sensor  10  uses the CdTe element, it is possible to provide a high-performance image sensor as compared with the conventional case. 
     The present invention has been described based on the embodiments. In the scope of the concept of the present invention, it should be also understood that various changes and modifications will readily occur to those skilled in the art, and therefore, these changes and modifications belong to the scope of the present invention. For example, the following various modifications may be made without departing from the scope of the general inventive concept. 
     In the above embodiment, the stud bump process has been described while giving the image sensor  10  using the CdTe element as one example. In this case, the stud bump process is applicable to the method of manufacturing the image sensors using the base materials other than CdTe as the sensor element; for example, Si, Ge, CdZnTe or HgCdTe (mercury cadmium telluride). 
     The present invention is not limited to the above embodiment, and various modifications may be made without departing from the scope of the general inventive concept in the working stage of the invention. Each embodiment may be carried out based on proper combinations as much as possible; in this case, combined effects are obtained. Further, the above embodiment includes various-step inventions, and the plurality of constituent requirements disclosed is properly combined, and thereby, various inventions can be extracted. For example, even if some constituent requirements are deleted from all constituent requirements disclosed in the embodiment, it is possible to solve the problem described in the column of the problem that the invention is to solve. When at least one of the effects described in the column of the effects of the invention is obtained, the construction in which the constituent requirements is deleted can be extracted as an invention. 
     According to the above structure, even if the image sensor has many sensor elements, it is possible to realize a highly sensitive image sensor, which can readily make electric connection between each sensor element and amplification IC, and to provide a method of manufacturing the image sensor.