Abstract:
A photoelectric converter has a first semiconductor region having a first conductivity type, a pixel region for accumulating generated carriers, a second semiconductor region having a second conductivity type disposed within the first semiconductor region and inside the pixel region, an electrode region having the second conductivity type disposed on the second semiconductor region, and an oxide film disposed around the electrode region. A first aluminum wiring contacts the electrode region via a contact hole disposed in an intermediate insulating film for transforming a signal according to a quantity of the generated carriers accumulated in the pixel region. A second aluminum wiring encircles an outer periphery of the pixel region and is held at a predetermined constant potential. A transparent conductive film is disposed on the oxide film and inside the second semiconductor region and contacts the second aluminum wiring.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a photoelectric converter for receiving a reflection light from an original to which light is irradiated and then converting into an electric signal, and an image sensor integrated circuit (IC) in which a plurality of the photoelectric converters are arranged in a line or a matrix shape. More particularly, the present invention relates to an improvement of that pixel structure. 
   2. Description of the Related Art 
     FIG. 14  shows a plan view of a light receiving element of a photoelectric converter used in a conventional image sensor, and  FIG. 15  shows a sectional view of the A–A′ of  FIG. 14  (refer to JP 2000-311997 A). 
   In the present invention, in order to protect an image quality from being dropped by low frequency noise, such as power supply noise and the like, which is radiated from outside the photoelectric converter, a conductor  6 ′ serving as a shield is formed on an output wiring  5  of the light receiving element. 
   However, in the above-mentioned photoelectric converter, an output wiring  5  is an AL wiring. Thus, a conductor thereon needs to be constituted by the AL of a second layer, or another conductive film needs to be additionally formed. Thus, since the formation can not be implemented only by a typical AL one layer process, this results in the problem of an expensive cost. Also, a photo diode diffusion region  2  is smaller than an opening portion  1 . Hence, in the diffusion region  2 , optical carriers generated in the opening portion  1  can not be sufficiently caught, which results in the problem of a poor sensibility efficiency. So, if the diffusion region  2  is made larger, since any shield layer does not exist on the diffusion region, the external noise brings about the potential variation in the diffusion region, which results in the problem of the drop in an image quality. 
   SUMMARY OF THE INVENTION 
   To solve the above-mentioned conventional problems, according to an aspect of the present invention, there is provided a photoelectric converter, including: a first semiconductor region of a first conductive type which is formed within a pixel region; a second semiconductor region of a second conductive type which is formed within the first semiconductor region and in which optically generated carriers can be accumulated; and a conductor which is formed through an insulator on the second semiconductor region and held at a predetermined potential. Further, in the photoelectric converter, the conductor is made of poly-silicon. 
   According to another aspect of the present invention, there is provided a photoelectric converter, including: a first semiconductor region of a first conductive type which is formed within a pixel region; a second semiconductor region of a second conductive type which is formed within the first semiconductor region and in which optically generated carriers can be accumulated; a wiring for electrically connecting the second semiconductor region and a circuit element configured outside the pixel region; and a conductor which is formed through an insulator on the wiring and held at a predetermined potential. Further, in the photoelectric converter, the wiring is made of poly-silicon. 
   According to the photoelectric converter, since the portion on the photodiode diffusion region of the light receiving element is shielded by the poly-silicon of a constant potential, the influence of the external noise can be suppressed. Also, since the poly-silicon has a certain degree of transmittivity to visible light, the drop in the sensibility can be reduced. Also, the output wiring of the photodiode is made of the poly-silicon, and the portion thereon is shielded by the AL of the constant potential. Thus, the influence of the external noise can be suppressed. 
   Since the above-mentioned structure can be constituted by the AL one layer process of a typical CMOS, the cost can be suppressed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a plan view of a light receiving element of a photoelectric converter according to an embodiment of the present invention; 
       FIG. 2  is a sectional view taken along line A–A′ of  FIG. 1 ; 
       FIG. 3  is a schematic view of an image sensor IC according to the embodiment of the present invention; 
       FIG. 4  is a schematic view of a close contact image sensor according to the embodiment of the present invention; 
       FIG. 5  is a schematic circuit diagram of the photoelectric converter according to the embodiment of the present invention; 
       FIG. 6  is a timing chart of a signal processing circuit and the photoelectric converter according to the embodiment of the present invention; 
       FIG. 7  is an entire configuration view of the photoelectric converter according to the embodiment of the present invention; 
       FIG. 8  is a block diagram of the signal processing circuit according to the embodiment of the present invention; 
       FIG. 9  is a circuit diagram of a sample holding circuit according to the embodiment of the present invention; 
       FIG. 10  is a circuit diagram of a buffer circuit according to the embodiment of the present invention; 
       FIG. 11  is a circuit diagram of an amplifying circuit according to the embodiment of the present invention; 
       FIG. 12  is a circuit diagram of a subtracter according to the embodiment of the present invention; 
       FIG. 13  is a circuit diagram of a clamping circuit according to the embodiment of the present invention; 
       FIG. 14  is a plan view of a light receiving element of a conventional photoelectric converter; and 
       FIG. 15  is a sectional view taken along the line A–A′ of  FIG. 14 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiment 
     FIG. 2  is a sectional view taken along the line A–A′ of  FIG. 1 . 
   The light receiving element in the present invention has a P-type semiconductor substrate region  60  as a first conductive type of a first semiconductor region within a pixel region  51 ; an N-type semiconductor region  52  which is provided in the P-type semiconductor substrate region  60  as a second conductive type of a second semiconductor region in which optically generated carriers are accumulated; and a poly-silicon  56  serving as a conductor which is formed through a LOCOS oxide film  59  serving as an insulator on the N-type semiconductor region  52  and held at a predetermined potential. The poly-silicon  56  is electrically connected to an AL wiring  55  (hereinafter “AL  55 ”), which also functions as a light shielding device of a VSS potential, through a contact  63 . The N-type semiconductor region  52  as a photodiode diffusion region may be constituted by an N-well. The output of the photodiode is inputted through an N+ region  54  constituting an electrode and through an AL wiring (photodiode output terminal)  53  and a poly-silicon wiring  62  to a circuit  58  for resetting the photodiode and to a source follower circuit  57 . The circuit  58  is composed of NMOS transistors and the like. The circuit  57  is composed of PMOS transistors and the like. 
   The output wiring of the photodiode is connected through a contact  61  from the AL to the poly-silicon wiring  62 . The AL  55  of the VSS potential exists on the poly-silicon wiring  62 . 
   The semiconductor region  52  of the light receiving element is at the floating state in the case of the accumulation. Thus, when the region receives the radiation noise from outside, its potential is varied. 
   However, as mentioned above, the conductive film  56  of the constant potential exists on the photodiode semiconductor region  52 . Hence, the radiation noise coming from above the photodiode can be shielded. According to  FIG. 1 , although the conductive film  56  does not perfectly cover on the photodiode diffusion region  52 , the shielding effect against the noise can be sufficiently obtained. 
   Also, when the output wiring of the photodiode receives the radiation noise from the outside, its potential is varied similarly to the diffusion region  52  of the light receiving element. However, as shown in  FIG. 1 , the AL  55  which is the conductive film of the constant potential exists on the photodiode output wiring  62 . Thus, it is possible to shield the radiation noise coming from above the photodiode. Since the conductive film of the constant potential does not exist on the photodiode output terminal  53 , the influence of the radiation noise is received. However, this influence can be reduced by making the output terminal  53  as short as possible. 
   The manufacturing method will be described below with reference to  FIG. 2 . The N-well serving as the substrate of the PMOS transistor is formed on the P-type substrate, and simultaneously with the formation, the N-type semiconductor region  52  of the photodiode is formed. This diffusion region may be formed apart from the N-well. Next, the LOCOS oxide film  59  is formed. Next, the poly-silicon conductive film  56  and the poly-silicon wiring  62  are formed simultaneously with the gate region of the transistor. Next, an electrode  54  of the photodiode is formed simultaneously with the source drain regions of the NMOS. Next, an intermediate insulation film  64  is formed, and a contact hole is formed. Next, simultaneously with the formation of the AL wiring, the AL  55  is formed. Then, a passivation film  65  is formed. 
   In the above-mentioned explanation, the P-type substrate may be replaced by the N-type substrate, and the diffusion region  52  and the photodiode electrode  54  may be replaced by the P-type semiconductors. 
   As mentioned above, according to this embodiment, the light receiving element can be manufactured without adding any step to the typical AL one layer CMOS process. 
   Furthermore, the conductive film  56  need not be made of poly-silicon, and may be a different type of conductive film which is transparent For example, the conductive film such as ITO and the like may be formed on or under the passivation film  65 . Also, the potential of the conductive film  56  may not be VSS, and it may be fixed to the constant potential. 
     FIG. 3  is a schematic view of the image sensor IC in this embodiment. This image sensor IC  41  is provided with a signal processing circuit  42 , a photoelectric converter  43 , a reference voltage circuit  44  and a signal output terminal  47 . The common signal line of the photoelectric converter  43  is inputted to the signal processing circuit  42 , and the output of the signal processing circuit  42  is connected to the signal output terminal  47 . 
     FIG. 4  is a schematic view of a close contact image sensor provided with the image sensors IC  41  of  FIG. 3 . This close contact image sensor is composed of the three image sensors IC  41 . The signal output terminals  47  of all of the image sensors IC  41  are externally connected and outputted from a VOUT 2  terminal to outside. 
     FIG. 8  is a block diagram of the signal processing circuit  42  in this embodiment of the present invention. The signal inputted to an input terminal VIN is inputted to a sample holding circuit  21  and a buffer amplifier  23 . The output of the sample holding circuit  21  is inputted to a buffer amplifier  22 . The output of the buffer amplifier  22  and the output of the buffer amplifier  23  are inputted to a subtracter  24 , and the output of the subtracter  24  is inputted to a clamping circuit  25 . Reference voltages of the subtracter  24  and the clamping circuit  25  can be made common and are connected to a VREF terminal. The output of the clamping circuit  25  is inputted to a buffer amplifier  26 . By the way, the buffer amplifier  26  may be replaced by an amplifying circuit. Moreover, the reference voltage of this amplifying circuit may be common to the VREF terminal. The output of the buffer amplifier  26  is inputted to a sample holding circuit  27 . The output of the sample holding circuit  27  is inputted to a buffer amplifier  28 . The output of the buffer amplifier  28  is inputted to a transmission gate  29 . The output of the transmission gate  29  is inputted to an output terminal VOUT 2 . By the way, the transmission gate  29  may be unnecessary depending on the usage field. 
     FIG. 9  is a circuit diagram of the sample holding circuit according to the embodiment of the present invention, and it can be used for the sample holding circuit  21  and the sample holding circuit  27 . The sample holding circuit is composed of a transmission gate  30 , a dummy switch  31  and a capacitor C 1 . In this sample holding circuit, in order to cancel out the noises of a ΦSH pulse and a ΦSHX pulse which is the inversion thereof, the sizes of the NMOS and PMOS transistors of the transmission gate  30  are set to be equal and the gate areas of the NMOS and PMOS transistors of the dummy switch  31  are set to be half the gate area of the transistor of the transmission gate. 
     FIG. 10  is a circuit diagram of the buffer amplifier according to the embodiment of the present invention, and it is constituted by an operating amplifier  32 . This circuit can be used for the buffer amplifiers  22 ,  23 ,  26  and  28 . By the way, the buffer amplifier may be a source follower amplifier. 
     FIG. 11  is a circuit diagram of the amplifying circuit according to the embodiment of the present invention, and it is composed of the operating amplifier  32  and resistors. If this circuit is used instead of the buffer amplifier  26 , the amplification factor of the signal processing circuit can be increased. Also, the reference voltage VREF of this amplifying circuit may be common to the VREF terminal in  FIG. 1 . 
     FIG. 12  is a circuit diagram of the subtracter according to the embodiment of the present invention, and it is composed of the operating amplifier  32  and resistors. This circuit multiplies the voltage, in which the voltage INM is subtracted from the voltage INP, by the gain times determined from the ratio between the resistors, and then outputs it with the voltage of VREF as a reference. If the terminals inputted to the INP and the INM are made reverse, the output can be inverted with the voltage of VREF as the reference. 
     FIG. 13  is a circuit diagram of the clamping circuit according to the embodiment of the present invention, and it can be used for the clamping circuit  25 . The clamping circuit is composed of the transmission gate  30 , the dummy switch  31  and a capacitor  33 . In this clamping circuit, in order to cancel the noises of a ΦCLAMP pulse and a ΦCLAMPX pulse which is the inversion thereof, the sizes of the NMOS and PMOS transistors of the transmission gate  30  are set to be equal, and the gate areas of the NMOS and PMOS transistors of the dummy switch  31  are set to be half the gate area of the transistor of the transmission gate. 
     FIG. 5  is a schematic circuit diagram of the photoelectric converter  43  according to the embodiment of the present invention. The members within the frame of a photoelectric converting block An shown in  FIG. 5  are installed correspondingly to the number of the pixels, and a channel selection switch  107  of each block is connected to a common signal line  111 . Note that the photoelectric converting block An indicates the photoelectric converting block of an n-th bit.  FIG. 7  shows the entire configuration view of the photoelectric converter  43 . 
   This circuit is provided with: a photodiode  101  serving as a photoelectric converting unit; transferring switches serving as charge transferring units  114 ,  115 ,  116  and  117 ; a reset switch  102  serving as a reset unit; an amplifying unit  103 ; a capacitor  113  for holding an optical signal; a capacitor  112  for holding a reference signal serving as the reference of the photoelectric converting unit; a MOS transistor  106  for constituting a MOS source follower serving as a signal reading out unit; the channel selection switch  107  serving as a channel selecting unit; the common signal line  111 ; and a current source  108 . 
     FIG. 1  shows the plan view indicating the photodiode  101 , the reset switch  102 , and a part of the amplifying unit  103  in  FIG. 5 . The diffusion region  52  in  FIG. 1  corresponds to the photodiode  101  of  FIG. 5 , the circuit  58  corresponds to the reset switch  102 , and the circuit  57  corresponds to the part of the amplifying unit  103 , respectively. 
   One terminal of the reset switch  102  is connected to a Vreset terminal, and the Vreset terminals of all of the photoelectric converters  101  are common, as shown in  FIG. 3 . The amplifying unit  103  is composed of MOS source follower, voltage follower amplifiers and the like, and an amplifier enabling terminal  110  for selecting an operational state may be installed. 
   The output terminal VOUT of this photoelectric converter is inputted to the input terminal VIN of the signal processing circuit  42 . The photoelectric converter and the signal processing circuit can be formed on one semiconductor substrate.  FIG. 6  is a timing chart of the photoelectric converter  43  and the signal processing circuit  42  according to the embodiment of the present invention. The operations of the photoelectric converter  43  will be described below with reference to this timing chart. Symbols ΦR, ΦRIN, ΦSIN and ΦSEL in  FIG. 6  are simultaneously operated in all of the bits. Symbols ΦSO, ΦRO and ΦSCH are represented together with (n), because their operation timings are different depending on the bits. 
   At first, the operation of the photoelectric converting block of the n-th bit is explained. The optical signal obtained after the accumulation of the charges generated by the light entering the photodiode  1  by turning on the transferring switch  15  through the pulse at a position S 1  of the ΦSIN is read out to a capacitor  13 . Next, when a reset switch  2  is turned on through the pulse of a position R 2  of the ΦR, an output terminal Vdi of the photodiode  1  is fixed to the reference voltage Vreset. Then, when the reset switch  2  is turned off, the voltage of Vdi has the value in which an off noise is added to the Vreset. 
   Next, immediately after the reset switch  2  is turned off, the transferring switch  14  is turned on through the pulse of a position R 2  of the ΦRIN, and the reference voltage after the photodiode  1  is reset is read out to a capacitance  12 . After that, the optical charges are accumulated in the photodiode  1 , and the potential Vdi is varied correspondingly to the amount of the optical charges. This accumulation period is the period from the end of the period R 2  of the ΦR to the end of the period S 2  of the ΦSIN of a next cycle. Thus, the accumulation period becomes the period TS 2  of  FIG. 6 , and it is the same period with regard to all of the bits. If the photodiode receives the radiation noise from the outside in this period, the potential of the photodiode is varied. However, due to the structure of the present invention, the radiation noise is shielded, which makes the variation in the potential of the photodiode caused by the radiation noise small. 
   The operations for reading out the reference signal and the optical signal will be described below. In the accumulation period TS 2  of  FIG. 6 , when a channel selection switch  7  is opened through the pulse ΦSCH(n) and the transferring switch  17  is simultaneously opened through the pulse ΦSO(n), the optical signal held in the capacitor  13  is read out to the common signal line  11 . This period is the portion S 1  of the ΦSCH (n). This optical signal is the signal accumulated in the period TS 1 , and the reset voltage reset through the pulse of a position R 1  of the ΦR is defined as the reference. 
   Next, when the transferring switch  16  is opened through the pulse of the ΦRO(n), the reference signal held in the capacitor  12  is read out to the common signal line  11 . This reference signal is the signal reset by the pulse of the position R 2  of the ΦR. When the signal processing circuit at a later stage extracts the difference between the optical signal and the reference signal, the voltage difference resulting from the light can be extracted. 
   Next, after the ΦSCH(n) is turned off, when the channel selection switch  7  of a next bit is turned on through the ΦSCH(n+1) and the transferring switch  17  of the next bit is opened through the pulse of the ΦSO (n+1), the operation for reading out the optical signal of the next bit is started. The other pulses of the (n+1)-th bit are all delayed by the on period of the ΦSCH from the pulse of the n-th bit. 
   As mentioned above, outputted from the VOUT terminal are the optical signal of the n-th bit, the reference signal of the n-th bit, the optical signal of the (n+1)-th bit and the reference signal of the (n+1)-th bit in the stated order. Hereinafter, for the convenience, the output period of the optical signal is referred to as a former period, and the output period of the reference signal is referred to as a later period. 
   The operation of the signal processing circuit  42  will be described below. The output of the above-mentioned OUT terminal is inputted to the VIN terminal. A sample holding pulse ΦSH 1  is turned on after the optical signal begins to be outputted, and it is turned off before the optical signal is ended. Consequently, the optical signal is sample-held. The signal of the VIN and the signal after being sample-held are inputted to the subtracter. In the former period, the same optical signal is inputted to the subtracter. In the later period, the sample-held optical signal and the reference signal are inputted to the subtracter. Thus, the output of the subtracter becomes at a VREF level in the former period, and it becomes at the level in which the VREF level is added to the level where the difference between the optical signal and the reference signal is multiplied by the gain times, in the later period. Also, the offsets of the buffer amplifiers  22  and  23  and the subtracter  24  are overlapped with the output in the former period, and the offsets of the buffer amplifiers  22  and  23  and the subtracter  24  and the offset of the sample holding circuit  21  are overlapped with the output in the later period. 
   A clank pulse ΦCLAMP is added such that it is turned on before the ΦSH 1  is turned on and it is turned off before the ΦSH 1  is turned off. Consequently, the output of the clanking circuit  25  is clamped to the VREF level in the former period, and it becomes at the level in which the VREF level is added to the level where the former output is subtracted from the later output of the subtracter, in the later period. As a result, the offsets of the buffer amplifiers  22  and  23  and the subtracter  24  are not overlapped with the output in the later period of the clamping circuit. Also, the offset of the sample holding circuit  21  is small because the noises of the ΦSH pulse and the ΦSHX pulse that is the inversion thereof are cancelled out in the circuit. From the above-mentioned description, the output in the later period of the clamping circuit becomes at the level to which the level where the difference between the optical signal and the reference signal is multiplied by the gain times is added, with the VREF level as the reference. 
   A sample holding pulse ΦSH 2  is turned on before and after the reference signal begins to be outputted, and it is turned off before the reference signal is ended. Consequently, the output in the later period of the output after being clamped is sampled, and then held in the former period of a next bit. Thus, the output level can be maintained for a long period. 
   In this embodiment, while the photodiode is carrying out the accumulation operation in the period TS 2 , it is possible to read out the optical signal accumulated in the period of the previous accumulation period TS 1 . Thus, it is possible to read in the color image data by switching on the LEDs of three colors of RGB in turn. For example, it is possible to read in the red component by switching on the red LED in the period TS 1 , read in the green component by switching on the green LED in the period TS 2 , and read in the blue component by switching on the blue LED in the next period of the TS 2 . In this case, the red optical signal is read out within the period TS 2 . 
   In the explanation of the image sensor in the present invention as mentioned above, the signal processing circuit  42  need not be built in the IC. 
   Although the above-mentioned explanations have been mainly performed with regard to the linear image sensor IC, the configurations of  FIGS. 1 and 3  can be also applied to an area image sensor IC. 
   In the above-mentioned explanations, the present invention is not limited to the above-mentioned respective embodiments, and it can be variously modified and executed without departing from the spirit and scope of the invention. 
   The present invention can be used for a linear image sensor IC applied to an image reading apparatus, such as a facsimile and an image scanner, and a close contact image sensor IC in which a plurality of image sensors IC are mounted. Also, the present invention can be applied to an area image sensor IC.