Abstract:
According to the present invention, there is provided a semiconductor photosensor having:
       a first photo detector and a second photo detector formed in a surface portion of a semiconductor substrate;   a first resin layer formed on a light-receiving region of the first photo detector, and including a first spectral sensitivity characteristic;   a second resin layer formed on a light-receiving region of the second photo detector, and including a second spectral sensitivity characteristic; and   an operation circuit which performs a predetermined operation between a first output from the first photo detector and a second output from the second photo detector, and outputs a result of the operation,   wherein the first spectral sensitivity characteristic is a characteristic which removes a wavelength component in a short-wavelength region, and the second spectral sensitivity characteristic is a characteristic which removes a wavelength component in an infrared region.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims benefit of priority under 35 USC §119 from the Japanese Patent Application No. 2006-45639, filed on Feb. 22, 2006, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    A semiconductor photosensor which outputs a linear signal in accordance with the ambient illuminance is widely used. Especially in a cellphone, the semiconductor photosensor is used to control ON/OFF of the backlight of a liquid crystal display or LEDs in a keypad in accordance with the ambient illuminance. 
         [0003]    For example, the semiconductor photosensor is used as an illuminance sensor for turning off the backlight or the LEDs in the keypad when the environment is bright, and turning on the backlight or LEDs or performing brightness adjustment or the like when the environment is dark, thereby suppressing unnecessary power consumption. 
         [0004]    To meet these demands, the semiconductor photosensor as an illuminance sensor is required to have a spectral sensitivity characteristic substantially equal to the spectral sensitivity of a human eye. 
         [0005]    An illuminance sensor described in patent reference  1  (to be described later) is the conventional technique of obtaining the spectral sensitivity characteristic close to the visual sensitivity. This illuminance sensor includes a photodiode having an infrared transmitting filter and a photodiode having no infrared transmitting filter, and performs an operation between photocurrents generated from these photodiodes, thereby excluding infrared light from a detection wavelength band. In this manner, the sensitivity to infrared light can be excluded from the spectral sensitivity characteristic of the illuminance sensor. 
         [0006]    The spectral sensitivity characteristic is conventionally improved at long wavelengths as described above, but it is not improved at short wavelengths. This produces a difference from the visual sensitivity at short wavelengths. 
         [0007]    The reference disclosing the conventional semiconductor photosensor using an infrared transmitting filter is as follows. 
         [0008]    Japanese Patent Laid-Open No. 2004-214341 
       SUMMARY OF THE INVENTION 
       [0009]    According to one aspect of the present invention, there is provided a semiconductor photosensor having: a first photo detector and a second photo detector formed in a surface portion of a semiconductor substrate; a first resin layer formed on a light-receiving region of the first photo detector, and including a first spectral sensitivity characteristic; a second resin layer formed on a light-receiving region of the second photo detector, and including a second spectral sensitivity characteristic; and an operation circuit which performs a predetermined operation between a first output from the first photo detector and a second output from the second photo detector, and outputs a result of the operation, wherein the first spectral sensitivity characteristic is a characteristic which removes a wavelength component in a short-wavelength region, and the second spectral sensitivity characteristic is a characteristic which removes a wavelength component in an infrared region. 
         [0010]    According to one aspect of the present invention, there is provided a semiconductor photosensor having: a first photo detector and a second photo detector formed in a surface portion of a semiconductor substrate; a first resin layer formed on a light-receiving region of the first photo detector, and including a first spectral sensitivity characteristic; a second resin layer formed on a light-receiving region of the second photo detector, and including the first spectral sensitivity characteristic; a third resin layer formed on the light-receiving region of the second photo detector such that the second resin layer and the third resin layer are stacked, and including a second spectral sensitivity characteristic; and an operation circuit which performs a predetermined operation between a first output from the first photo detector and a second output from the second photo detector, and outputs a result of the operation, wherein the first spectral sensitivity characteristic is a characteristic which removes a wavelength component in a short-wavelength region, and the second spectral sensitivity characteristic is a characteristic which removes a wavelength component in an infrared region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a circuit diagram showing the arrangement of a semiconductor photosensor according to the first embodiment of the present invention; 
           [0012]      FIG. 2A  is a sectional view showing the longitudinal sectional structure of photodiode portions in the semiconductor photosensor according to the first embodiment;  FIG. 2B  is a sectional view showing a modification of  FIG. 2A ; 
           [0013]      FIG. 3A  is a graph showing an example of the transmittance characteristic of a short-wavelength cut filter in the semiconductor photosensor according to the first embodiment;  FIG. 3B  is a graph showing an example of the transmittance characteristic of an infrared transmitting filter in the semiconductor photosensor according to the first embodiment;  FIG. 3C  is a graph showing an example of the characteristic of a photodiode usable in the semiconductor photosensor according to the first embodiment when no filter is used;  FIG. 3D  is a graph showing an example of the characteristic obtained in a photodiode portion  1  using only a short-wavelength cut filter in the semiconductor photosensor according to the first embodiment;  FIG. 3E  is a graph showing an example of the characteristic obtained in a photodiode portion  2  using a short-wavelength cut filter and an infrared transmitting filter  11  in the semiconductor photosensor according to the first embodiment;  FIG. 3F  is a graph showing an example of the characteristic finally obtained by using the photodiode portions  1  and  2  in the semiconductor photosensor according to the first embodiment; 
           [0014]      FIG. 4A  is a plan view showing an example of a chip layout pattern in the semiconductor photosensor according to the first embodiment;  FIG. 4B  is a plan view showing another example of the chip layout pattern in the semiconductor photosensor according to the first embodiment; 
           [0015]      FIG. 5  is a circuit diagram showing the arrangement of a semiconductor photosensor according to a reference example; 
           [0016]      FIG. 6  is a graph showing an example of the spectral sensitivity characteristic of the semiconductor photosensor according to the reference example; 
           [0017]      FIG. 7  is a graph showing the light emission spectra of various light sources; 
           [0018]      FIG. 8  is a circuit diagram showing the arrangement of a semiconductor photosensor according to the second embodiment of the present invention; 
           [0019]      FIG. 9  is a circuit diagram showing the arrangement of a semiconductor photosensor according to the third embodiment of the present invention; 
           [0020]      FIG. 10A  is a sectional view showing the longitudinal sectional structure of photodiode portions in a semiconductor photosensor according to the fourth embodiment of the present invention;  FIG. 10B  is a sectional view showing a modification of  FIG. 10A ; and 
           [0021]      FIG. 11  is a sectional view showing the longitudinal sectional structure of photodiode portions in a semiconductor photosensor according to the fifth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Embodiments of the present invention will be described below with reference to the accompanying drawings. 
       (1) First Embodiment 
       [0023]      FIG. 1  shows the circuit-configuration of a semiconductor photosensor according to the first embodiment of the present invention. This semiconductor photosensor has photodiode portions  1  and  2 , amplifiers  3 ,  4 , and  5 , and an output circuit  6 . 
         [0024]      FIG. 2A  shows the longitudinal sectional structure of the photodiode portions  1  and  2 . 
         [0025]    The photodiode portion  1  is formed by an n-type epitaxial layer  8  formed on a p-type semiconductor substrate  7 , and a p-type diffusion layer  9  formed on the surface of the n-type epitaxial layer  8 . A short-wavelength cut filter  10   a  which removes light in a short-wavelength region is formed on top of the resultant structure via insulating films  204  and  205 . 
         [0026]    The photodiode portion  2  has the same photodiode structure as the photodiode portion  1 . In addition, an infrared transmitting filter  11  and short-wavelength cut filter  10   b  are stacked on top of the structure via the insulating films  204  and  205 . The infrared transmitting filter  11  and short-wavelength cut filter  10   b  can be stacked in this order or reversed order. In the first embodiment, the infrared transmitting filter  11  and short-wavelength cut filter  10   b  are stacked in this order on the insulating film  205 . 
         [0027]    In each of the photodiode portions  1  and  2 , the n-type epitaxial layer  8  and p-type diffusion layer  9  are isolated from the circumference by a p + -type element isolation region  201 , and a p + -type buried layer  202  and n + -type buried layer  203  are also formed. 
         [0028]      FIG. 3A  shows an example of the transmittance characteristic of the short-wavelength cut filters  10   a  and  10   b . As shown in  FIG. 3A , the short-wavelength cut filters  10   a  and  10   b  have a spectral sensitivity characteristic by which the wavelength at the half width (the wavelength at which the transmittance is 50%) is 400 to 600 nm. 
         [0029]      FIG. 3B  shows an example of the transmittance characteristic of the infrared transmitting filter  11 . The infrared transmitting filter  11  has a spectral sensitivity characteristic by which the wavelength at the half width is 550 to 700 nm. 
         [0030]      FIG. 3C  shows a spectral sensitivity characteristic obtained when none of the filters  10   a ,  10   b , and  11  is used in the photodiode portions  1  and  2 . A photodiode formed on a semiconductor substrate made of silicon has a characteristic by which the transmittance peaks at 500 to 600 nm as shown in  FIG. 3C . 
         [0031]      FIG. 3D  shows a spectral sensitivity characteristic obtained in the photodiode portion  1  using only the short-wavelength cut filter  10   a , i.e., obtained when the short-wavelength cut filter  10   a  having the characteristic shown in  FIG. 3A  is used in a photodiode having the characteristic shown in  FIG. 3C . 
         [0032]      FIG. 3E  shows a spectral sensitivity characteristic obtained in the photodiode portion  2  using the short-wavelength cut filter  10   b  and infrared transmitting filter  11 , i.e., obtained when the short-wavelength cut filter  10   a  having the characteristic shown in  FIG. 3A  and the infrared transmitting filter  11  having the characteristic shown in  FIG. 3B  are used in a photodiode having the characteristic shown in  FIG. 3C . 
         [0033]      FIG. 3F  shows a spectral sensitivity characteristic finally obtained by the first embodiment by using the photodiode portions  1  and  2 . As shown in  FIG. 3F , a spectral sensitivity characteristic substantially equal to the spectral sensitivity of a human eye can be obtained by the first embodiment. 
         [0034]    The operation of the semiconductor photosensor according to the first embodiment which realizes the spectral sensitivity characteristic as described above will be explained below with reference to  FIG. 1 . 
         [0035]    When the photodiode portions  1  and  2  are irradiated with light having a predetermined illuminance, the photodiode portion  1  outputs a photocurrent from the visible light region to the infrared region transmitted through the short-wavelength cut filter  10   a.    
         [0036]    The photodiode portion  2  outputs a photocurrent in the infrared region corresponding to the spectral sensitivity characteristic of the infrared transmitting filter  11 . These output photocurrents are amplified at equal magnifications by the amplifier  3  having a current mirror configuration made up of transistors  3   a  and  3   b  having the same size. 
         [0037]    The outputs of the photodiode portion  1  and amplifier  3  are connected to the amplifier  4 . The amplifier  4  receives an electric current obtained by subtracting the output of the amplifier  3  from the output of the photodiode portion  1 , i.e., an electric current from which the infrared component is subtracted. After that, the amplifiers  4  and  5  and the output circuit  6  amplify the electric current to a necessary magnitude and output the amplified current outside. 
         [0038]      FIG. 4A  shows an example of a chip layout pattern in the first embodiment. Referring to  FIG. 4A , the photodiode portions  1  and  2  are arranged adjacent to each other. 
         [0039]    In a chip layout pattern shown in  FIG. 4B , the photodiode portions  1  and  2  are divided into a plurality of portions, and the divided photodiode portions  1  and  2  are alternately arranged adjacent to each other to form a checker pattern. The divided portions of the photodiode portion  1  are connected in parallel by an interconnecting layer not shown in  FIG. 1 , and the divided portions of the photodiode portion  2  are also connected in parallel by an interconnecting layer not shown in  FIG. 1 . 
         [0040]    In the first embodiment as described above, not only the infrared region but also the short-wavelength region is removed from the optical output from the photodiode. Therefore, a spectral sensitivity characteristic close to the visual sensitivity can be obtained. 
         [0041]      FIG. 2B  shows the structure of a modification of the first embodiment. This modification uses a short-wavelength cut filter  10   c  obtained by continuously forming the short-wavelength cut filters  10   a  and  10   b  in the first embodiment. This structure obviates the need for patterning for separating the short-wavelength cut filter  10   c  into  10   a  and  10   b . In addition, obliquely incident light which passes between the short-wavelength cut filters  10   a  and  10   b  in the first embodiment passes through the short-wavelength cut filter  10   c  in this modification. As a consequence, a more desirable characteristic can be obtained. 
         [0042]    A semiconductor photosensor according to a reference example will be described below. 
         [0043]      FIG. 5  shows the arrangement of photodiodes and a signal processor for photocurrents in this semiconductor photosensor. The semiconductor photosensor comprises a photodiode  101  using an infrared transmitting filter, and a photodiode  102  using no infrared transmitting filter and having a fourfold area. The photodiode  101  outputs a photocurrent I 1 , and the photodiode  102  outputs a photocurrent I 2 . 
         [0044]    The photocurrent I 2  which is four times as large as the photocurrent I 1  from the photodiode  101  flows through an amplifier  103 . 
         [0045]    A signal processor  104  is connected to the output of the amplifier  103  and the anode of the photodiode  102 . The signal processor  104  subtracts the photocurrent I 1 , which is amplified by the amplifier  103 , of the photodiode  101 , i.e., an infrared component having passed through the infrared transmitting filter, from the photocurrent I 2  of the photodiode  102 , and outputs the operation result. 
         [0046]    This makes it possible to obtain a spectral sensitivity characteristic having substantially no sensitivity to infrared light, which is indicated by the dotted line in  FIG. 6 , like a spectral sensitivity characteristic indicated by the solid line in  FIG. 6 . 
         [0047]    Referring to  FIG. 6 , however, a hatched component at short wavelengths is not removed. This portion is the difference between the spectral sensitivity characteristic of the reference example and the visual sensitivity. 
         [0048]    Data based on experiments related to the ratio of the output when an incandescent lamp is lit to the output when a fluorescent lamp is lit (=the output when an incandescent lamp is lit/the output when a fluorescent lamp is lit) in the semiconductor photosensor according to the reference example using the infrared transmitting filter and in a semiconductor photosensor comprising photodiodes using no infrared transmitting filter will be described below. 
         [0049]    The ratio of the output when an incandescent lamp is lit to the output when a fluorescent lamp is lit (=the output when an incandescent lamp is lit/the output when a fluorescent lamp is lit) is 1.0 if a photosensor has a spectral sensitivity characteristic matching the visual sensitivity. Accordingly, to perform brightness adjustment or the like of a liquid crystal display by using a semiconductor photosensor, the photosensor is presumably required to have an output ratio of about 0.8 to 1.2. 
         [0050]    By contrast, a sensor using no infrared transmitting filter has an output ratio of 1.3 or more, and the sensor using the infrared transmitting filter according to the reference example has an output ratio of 0.8 to 0.9. In the reference example described above, therefore, the use of the infrared transmitting filter improves the ratio of the output when an incandescent lamp is lit to the output when a fluorescent lamp is lit (=the output when an incandescent lamp is lit/the output when a fluorescent lamp is lit), compared to a sensor using no infrared transmitting filter. 
         [0051]    Unfortunately, the output when a fluorescent lamp is lit increases. In addition, it is difficult to decrease the output ratio to 0.8 to 1.2 due to variation factors in the mass-production process and the like. 
         [0052]    When a photodiode is formed on a semiconductor substrate made of, e.g., silicon, silicon has sensitivity to light having a wavelength of 360 to 550 nm. Therefore, as in the hatched region shown in  FIG. 6 , a difference is produced in a short-wavelength region between the visual sensitivity and the spectral sensitivity characteristic of the semiconductor photosensor according to the reference example. 
         [0053]      FIG. 7  shows the light emission spectra of light sources generally used as illumination. In the hatched region shown in  FIG. 6 , i.e., in a wavelength band of 360 to 550 nm, the light emission intensity of an incandescent lamp is low, and that of a halogen lamp is also relatively low. 
         [0054]    On the other hand the light emission intensity of a fluorescent lamp having three wavelength bands or a white LED which is attracting attention as energy-saving illumination is high at 360 to 550 nm. In the semiconductor photosensor according to the reference example, therefore, a difference from the visual sensitivity is produced at short wavelengths, so the ratio of the output when an incandescent lamp is lit to the output when a fluorescent lamp is lit (=the output when an incandescent lamp is lit/the output when a fluorescent lamp is lit) is not 1.0. 
         [0055]    By contrast, in the semiconductor photosensor of the first embodiment described above, not only the infrared region but also the short-wavelength region is removed from the optical output of the photodiode. Consequently, a spectral sensitivity characteristic close to the visual sensitivity can be realized. 
       (2) Second Embodiment 
       [0056]      FIG. 8  shows the arrangement of a semiconductor photosensor according to the second embodiment of the present invention. This semiconductor photosensor comprises photodiode portions  1  and  2 , amplifiers  14 ,  15 , and  16 , and an output circuit  17 . A photocurrent from the photodiode portion  2  is amplified by the amplifier  14 , and a photocurrent from the photodiode portion  1  is amplified by the amplifier  15 . After that, like the amplifier  3  of the first embodiment, the amplifier  16  subtracts the photocurrent of the photodiode  1  from the photocurrent of the photodiode portion  2 . The output circuit  17  amplifies the obtained photocurrent, and outputs the amplified photocurrent outside. 
         [0057]    The photodiode portions  1  and  2  have the same structure as in the first embodiment, so an explanation thereof will be omitted. 
         [0058]    The second embodiment differs from the first embodiment in that the amplifiers  14  and  15  amplify the photocurrents from the photodiode portions  2  and  1  to such an extent that the influence of an electric current of photocarriers generated in the substrate by light incident from the side surface of the chip is negligible, or the influence of a diffusion electric current from the photodiode is negligible. 
         [0059]    The output photocurrents from the photodiode portions  2  and  1  are amplified to such an extent that the influence of an electric current of photocarriers generated in the substrate by light incident from the side surface of the chip is negligible, or the influence of a diffusion electric current from the photodiode is negligible. After that, these photocurrents undergo subtraction. Accordingly, the influence of the photocarriers generated in the semiconductor substrate can also be canceled by the subtraction. 
         [0060]    The output photocurrents from the photodiode portions  1  and  2  are as very small as a few nA. Therefore, the S/N ratio increases when subtraction is performed after these photocurrents are amplified. 
         [0061]    To equalize the influences of light components from the side surfaces of the chip, the layout is preferably given pairness such that the distance from the chip end in the photodiode portion  1  is equal to that in the photodiode portion  2 , i.e., the arrangements are equivalent in characteristics. 
         [0062]    Also, to equalize the influences which light components from the side surfaces of the chip have on circuit elements forming the amplifiers  14  and  15 , and equalize the influences of diffusion electric currents from the photodiode portions  1  and  2 , the arrangements of the amplifiers  14  and  15  are preferably symmetrical such that the distances from the photodiode portions  1  and  2  are equal, and the distances from the chip ends are equal. 
       (3) Third Embodiment 
       [0063]    A semiconductor photosensor according to the third embodiment of the present invention will be explained below with reference to  FIG. 9  showing the arrangement of the photosensor. This semiconductor photosensor comprises photodiode portions  1  and  2 , amplifiers  3 ,  18 , and  19 , a reference voltage generator  20 , a comparative voltage generator  21 , a comparator  22 , and a logic circuit  23 . 
         [0064]    As in the first embodiment, the amplifier  3  subtracts a photocurrent of the photodiode portion  1  from a photocurrent of the photodiode portion  2 , thereby matching the characteristic with the visual sensitivity. 
         [0065]    After that, the output from the amplifier  3  is amplified by the amplifiers  18  and  19 , and input to the comparator  22 . 
         [0066]    The reference voltage generator  20  generates a reference voltage such as a bandgap constant voltage. On the basis of the reference voltage generated by the reference voltage generator  20 , the comparative voltage generator  21  generates a comparative voltage. This comparative voltage is equivalent to a reference value for determining whether the ambient illuminance is one or the other of two stages, in order to turn on or off the backlight of a liquid crystal display or the like. 
         [0067]    The comparator  22  compares the comparative voltage output from the comparative voltage generator  21  with that output from the amplifier  19 , which has the value corresponding to the photocurrent, and outputs the comparison result. This output from the comparator  22  is input to the logic circuit  23 , and the logic circuit  23  outputs a logic signal “1” or “0”. 
         [0068]    In the third embodiment, after the photocurrent from the photodiode portion  1  is subtracted from the photocurrent obtained from the photodiode portion  2 , the difference is compared with the predetermined comparative voltage. Accordingly, an output to be used to control ON/OFF of, e.g., the backlight of a liquid crystal display can be obtained. This output is given a spectral sensitivity characteristic close to the visual sensitivity by removing the infrared region and short-wavelength region from the optical output of the photodiode. 
       (4) Fourth Embodiment 
       [0069]      FIG. 10A  shows the sectional structure of photodiodes in a semiconductor photosensor according to the fourth embodiment of the present invention. In the first embodiment described previously, the infrared transmitting filter  11  and short-wavelength cut filter  10   b  are stacked in this order on top of the photodiode portion  2  via the insulating films  204  and  205 . 
         [0070]    In the fourth embodiment, however, a short-wavelength cut filter  10   b  and infrared transmitting filter  11  are stacked in this order on an insulating film  205 . 
         [0071]    In a photodiode portion  2  using this filter stacking order of the fourth embodiment, the short-wavelength cut filter  10   b  is formed close to an epitaxial layer  8  in which a photodiode is formed, as in a photodiode portion  1 . Unlike in the first embodiment, therefore, light obliquely incident from above the photodiode portion  2  passes through the short-wavelength cut filter  10   b  under the same conditions as light obliquely incident from above the photodiode portion  1 , and then enters the photodiode portion  1 . 
         [0072]    Accordingly, a short-wavelength cut filter  10   a  in the photodiode portion  1  and the short-wavelength cut filter  10   b  in the photodiode portion  2  are arranged in equivalent positions, so an operation can be performed at high accuracy when a photocurrent from the photodiode portion  1  is subtracted from a photocurrent from the photodiode portion  2 . 
         [0073]    In the photodiode portion  2  of the semiconductor photosensor of each of the second and third embodiments, the filters can be stacked in the order described in either the first or fourth embodiment. 
         [0074]      FIG. 10B  shows the structure of a modification of the fourth embodiment. In this modification, a short-wavelength cut filter  10   d  is formed by continuously forming the short-wavelength cut filters  10   a  and  10   b  in the fourth embodiment. This structure obviates the need for patterning for separating the short-wavelength cut filter  10   d  into  10   a  and  10   b . In addition, obliquely incident light which passes between the short-wavelength cut filters  10   a  and  10   b  in the fourth embodiment passes through the short-wavelength cut filter  10   d  in this modification. As a consequence, a more desirable characteristic can be obtained. 
       (5) Fifth Embodiment 
       [0075]      FIG. 11  shows the sectional structure of photodiodes in a semiconductor photosensor according to the fifth embodiment of the present invention. The fifth embodiment differs from the first and fourth embodiments in that no short-wavelength cut filter  10  is formed in a photodiode portion  2 , and only an infrared transmitting film  11  is formed on an insulating film  205 . 
         [0076]    When a short-wavelength cut filter  10  having a very high transmittance in a wavelength band of 550 nm or more as shown in  FIG. 3A  is used, it is possible to form the short-wavelength cut filter  10  on only a photodiode portion  1 , and form only the infrared transmitting filter  11  on the photodiode portion  2 . 
         [0077]    The photodiode portions  1  and  2  in each of the second and third embodiments may also have the photodiode structures of the fifth embodiment. 
         [0078]    In the first to fifth embodiments as described above, in a semiconductor photosensor using photodiodes having a spectral sensitivity characteristic from the visible light region to the infrared region, an operation is performed between a photocurrent output from the photodiode portion  1  on which only the short-wavelength cut filter  10   a  is formed, and a photocurrent output from the photodiode portion  2  on which the short-wavelength cut filter  10   b  and infrared transmitting filter  11  are formed, thereby removing the short-wavelength component and infrared component. Accordingly, a semiconductor photosensor having a spectral sensitivity characteristic close to the visual sensitivity can be implemented. 
         [0079]    Note that each embodiment described above is merely an example and does not limit the present invention. Therefore, these embodiments can be variously modified within the technical scope of the present invention. For example, the photo detector is not limited to a photodiode, and can be any element which generates an electrical signal corresponding to a received light amount.