Abstract:
A light sensor includes an element forming region having a light detection region. The element forming region excluding the light detection region is covered with a conductive film having a light shielding property, and the light detection region is covered with a conductive film having a light transmissive property. A method for preventing electromagnetic noise from entering a light sensor includes applying an electromagnetic shield conductive film to the light sensor and removing the electromagnetic shield conductive film at a region of the light sensor facing a light detection region.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to light sensors, and more specifically, to a light sensor having a function of shielding electromagnetic noise. 
         [0003]    2. Related Art 
         [0004]    A light sensor in which a photodiode (PD), an output amplification IC circuit, and the like are incorporated in a silicon substrate has been proposed. In such a light sensor, a photodiode capable of detecting light having a wavelength band of visible light to near-infrared light is used, and an optical filter is arranged at the upper part of the photodiode so that only the light of a specific wavelength band can be detected with the photodiode. 
         [0005]      FIG. 1  is a cross-sectional view schematically showing a conventional structure of such a light sensor.  FIGS. 2A to 2C ,  FIGS. 3A ,  3 B, and  FIGS. 4A ,  4 B are cross-sectional views describing manufacturing steps of the light sensor. Hereinafter, the manufacturing steps of the light sensor  11  will be sequentially described, and at the same time, the structure of the light sensor  11  will be described. 
         [0006]    First, a photodiode  13 , a ground contact region  14  (P +  diffusion layer), an IC circuit  15 , and the like are fabricated on a surface layer of a P-type silicon substrate  12  using a semiconductor manufacturing technique. As shown in  FIG. 2A , the upper surface of the silicon substrate  12  is covered with an insulating layer  16  made of SiO 2 . As shown in  FIG. 2B , a through-hole is opened in the insulating layer  16  at a position facing the upper surface of the contact region  14  and a conductive material such as metal or polysilicon is filled in the through-hole to form a conductive post  17 , which conductive post  17  is then subjected to thermal treatment to be Ohmic contacted to the contact region  14 . As shown in  FIG. 2C , a light shielding metal  18  made of a light shielding material such as AlSi is formed on the upper surface of the insulating layer  16 , where the upper side of the IC circuit  15  is shielded by the light shielding metal  18  and the light shielding metal  18  is conducted with the contact region  14  through the conductive post  17 . 
         [0007]    Furthermore, as shown in  FIG. 3A , a light receiving window  19  is opened in the light shielding metal  18  at above the photodiode  13  through the photolithography technique. As shown in  FIG. 3B , an insulating layer  20  made of SiO 2  is formed at the upper surface of the light shielding metal  18 . As shown in  FIG. 4A , an optical filter  21  is formed over the entire upper surface of the insulating layer  20  and, as shown in  FIG. 4B , with the optical filter  21  at the position facing the upper side of the photodiode  13  remaining, the optical filter  21  of other regions is removed through etching etc., to thereby fabricate the target light sensor  11 . 
         [0008]    Since the upper side of the IC circuit  15  is covered with the light shielding metal  18  for electromagnetic shield electrically grounded by connecting the contact region  14  to an earth line in such a light sensor  11 , the electromagnetic noise a from the outside can be shielded with the light shielding metal  18 , as shown in  FIG. 1 , and malfunction of the IC circuit  15  by the electromagnetic noise a can be prevented. The configuration of covering the upper side of the IC circuit with the light shielding metal is disclosed in Japanese Unexamined Patent Publication No. 11-40790. 
         [0009]    Since the light receiving window  19  is opened in the light shielding metal  18  facing the photodiode  13 , the incident light from the outside passes through the light receiving window  19  and enters the photodiode  13 , and is detected by the photodiode  13 . The photodiode  13  itself has sensitivity to light having a wavelength band of visible light to near-infrared light, but the optical filter  21  transmits only the light Lb (e.g., infrared light) having a specific wavelength of the incident light La and reflects light Lc of other wavelength bands, and thus only the light of the specific wavelength band Lb can be detected by the photodiode  13 . The configuration of limiting the wavelength of the light entering a light detection region using the optical filter is disclosed in Japanese Unexamined Patent Publication No. 7-170366. 
       SUMMARY 
       [0010]    However, since the light receiving window  19  is opened in the light shielding metal  18  at the position facing the photodiode  13  in the light sensor  11  having the structure shown in  FIG. 1 , the electromagnetic noise β may pass through the light receiving window  19 , and enter the inner side of the light shielding metal  18 . Thus, in places where there are great amount of electromagnetic noise such as inside a factory, malfunction may occur at the photodiode  13  and the contact region  14  due to the electromagnetic noise β that passed through the light receiving window  19 . Applications are limited as use is not suitable in places where there are great amount of electromagnetic noise such as inside the factory. 
         [0011]    The present invention has been devised to solve the problems described above, and an object thereof is to provide a light sensor including an electromagnetic shield conductive film, the electromagnetic shield conductive film being removed at a region facing a light detection region, where the electromagnetic noise is prevented from entering even at the region facing the light detection region. 
         [0012]    In accordance with one aspect of the present invention, in order to achieve the above object, there is provided a light sensor, wherein an element forming region excluding a light detection region is covered with a conductive film having a light shielding property, the light detection region being covered with a conductive film having a light transmissive property. 
         [0013]    According to the light sensor of the present invention, since the element forming region such as the IC circuit excluding the light detection region is covered with a conductive film having a light shielding property, light and electromagnetic noise do not reach the IC circuit and the like, and malfunction of the IC circuit and the like due to the light and the electromagnetic noise from the outside can be prevented. 
         [0014]    Furthermore, since the light detection region such as the photodiode and the phototransistor is covered with the conductive film having a light transmissive property, the light entering from the outside is transmitted through the conductive film having the light transmissive property and detected in the light detection region, but the electromagnetic noise is shielded by the conductive film having the light transmissive property so that the electromagnetic noise can be more reliably shielded, and malfunction of the light detection region, the IC circuit, and the like can be prevented. 
         [0015]    Therefore, according to the light sensor of the present invention, use can be made in places where there are great amount of electromagnetic noise such as inside the factory. 
         [0016]    In accordance with one aspect of the light sensor according to the present invention, the conductive film having the light transmissive property has optical filter characteristics. According to such aspect, only the light of a specific wavelength (band) can be selectively detected in the light detection region. 
         [0017]    In accordance with another aspect of the light sensor of the present invention, the conductive film having the light transmissive property electrically contacts the conductive film having the light shielding property. According to such aspect, the structure of the light sensor can be simplified since if one of the conductive film having the light transmissive property or the conductive film having the light shielding property is grounded, the other will also be grounded. 
         [0018]    The means for solving the problems in the present invention have characteristics appropriately combining the components described above, and the present invention is intended to enable numerous various by combining the components. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic cross-sectional view showing a structure of a light sensor of a conventional example; 
           [0020]      FIGS. 2A ,  2 B, and  2 C are cross-sectional views showing manufacturing steps of the light sensor of the conventional example; 
           [0021]      FIGS. 3A and 3B  are cross-sectional views showing manufacturing steps following  FIG. 2C ; 
           [0022]      FIGS. 4A and 4B  are cross-sectional views showing manufacturing steps following  FIG. 3B ; 
           [0023]      FIG. 5  is a schematic cross-sectional view showing a structure of a light sensor according to a first embodiment of the present invention; 
           [0024]      FIGS. 6A and 6B  are cross-sectional views showing manufacturing steps of the light sensor of the first embodiment; 
           [0025]      FIGS. 7A and 7B  are cross-sectional views showing manufacturing steps following  FIG. 6B ; 
           [0026]      FIG. 8  is a schematic cross-sectional view showing one example of a conductive optical filter; and 
           [0027]      FIG. 9  is a schematic cross-sectional view showing a structure of a light sensor according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
       First Embodiment 
       [0029]      FIG. 5  is a cross-sectional view schematically showing a structure of a light sensor  31  (light sensor chip) according to a first embodiment of the present invention. In the light sensor  31 , a photodiode  13  (light detection region), a ground contact region  14  (P +  diffusion layer), an IC circuit  15  including a MOS transistor, and the like are fabricated on a surface layer of a P-type silicon substrate  12 . The upper surface of the silicon substrate  12  is covered with an insulating layer  16  made of SiO 2 , a through-hole is opened in the insulating layer  16  at a position facing the upper surface of the contact region  14  and a conductive material such as metal or polysilicon is filled in the through-hole to form a conductive post  17 , which conductive post  17  is then Ohmic contacted to the contact region  14 . A light shielding metal  18  made of a light shielding material such as AlSi is formed on the upper surface of the insulating layer  16 , where the upper side of the IC circuit  15  is covered by the light shielding metal  18 . A light receiving window  19  is opened in the light shielding metal  18  at above the photodiode  13 . The light receiving window  19  is covered with an optical filter  21  at a position facing the upper side of the photodiode  13 , and an insulating layer  20  made of SiO 2  is formed at the upper surface of the light shielding metal  18 . 
         [0030]    In the present embodiment, the photodiode  13  is fabricated for the light detection region, but may be a phototransistor. Since the photodiode  13  has sensitivity to light having a wide wavelength band of visible light to near-infrared light, a conductive optical filter  32  is arranged so that only the light of specific wavelength, that is, the light having a transmissive wavelength (band) of the conductive optical filter  32  is detected by the light sensor  31 . The specific wavelength is a light emitting wavelength of an LED used as a pair with the light sensor  31 , where the wavelength is λ=870 nm in the case of an infrared LED. 
         [0031]    The contact region  14  is connected to the earth line when the light sensor  31  is mounted on a circuit board and the like, whereby the light shielding metal  18  is also grounded by way of the conductive post  17  and the contact region  14 . The light shielding metal  18  is opaque and does not transmit light. Thus, the light shielding metal  18  covering the IC circuit  15  acts as an electromagnetic shield film or a light shielding film, so that the electromagnetic noise a and the incident light from the outside can be shielded by the light shielding metal  18  as shown in  FIG. 5 , and malfunction of the IC circuit  15  by the electromagnetic noise a and the incident light can be prevented. 
         [0032]    The conductive optical filter  32  has both a function as a band-pass filter for transmitting light of a specific wavelength (e.g., infrared light wavelength) and a function as a conductive film. The conductive optical filter  32  is formed so as to contact and electrically conduct with the light shielding metal  18 , and is maintained at the same potential (i.e., ground potential) as the light shielding metal  18 . 
         [0033]    Since the photodiode  13  is covered with such a conductive optical filter  32 , the light sensor  31  can detect only the light of a specific wavelength, that is, the light of the transmissive wavelength band of the conductive optical filter  32 . In other words, only the light Lb (e.g., infrared light) having a specific wavelength band of the incident light La transmits through the conductive optical filter  32 , and the light Lc of other wavelength bands is reflected by the conductive optical filter  32 , and thus only the light of the specific wavelength band Lb is detected by the photodiode  13 . 
         [0034]    Furthermore, the electromagnetic noise β entered to the light receiving window  19  from the outside is shielded by the conductive optical filter  32 . Thus, the electromagnetic noises α, β can be shielded over the entire surface by the light shielding metal  18  and the conductive optical filter  32 , and the malfunction of the photodiode  13 , the IC circuit  15 , and the like can be more reliably prevented. Thus, according to such a light sensor  31 , use can be made in places where there are great amount of electromagnetic noise such as inside the factory. 
         [0035]    The manufacturing steps of the light sensor  31  will now be described with  FIGS. 6A ,  6 B, and  FIGS. 7A ,  7 B. As shown in  FIG. 6A , the step of forming the insulating layer  16 , the conductive post  17 , and the light shielding metal  18  on the upper surface of the silicon substrate  12  fabricated with the photodiode  13 , the contact region  14 , the IC circuit  15 , and the like, until opening the light receiving window  19  in the light shielding metal  18  is as described in the conventional example, and thus the description up to this step will not be given. 
         [0036]    After opening the light receiving window  19  in correspondence to the photodiode  13  as in  FIG. 6A , the conductive optical filter  32  is formed over the entire upper surface of the light shielding metal  18 , as shown in  FIG. 6B . Thereafter, as shown in  FIG. 7A , leaving the conductive optical filter  32  at the position facing the upper side of the photodiode  13 , the conductive optical filter  32  of other regions is removed through etching and the like. Furthermore, as shown in  FIG. 7B , the insulating film  20  made of SiO 2  is formed on the upper surface of the light shielding metal  18  and the conductive optical filter  32  to thereby fabricate the target light sensor  31 . 
         [0037]    The conductive optical filter  32  has a structure in which a transparent conductive film (metal thin film) is formed on both surfaces or one surface of the optical filter. One example of a configuration of such a conductive optical filter  32  is shown in  FIG. 8 . In such a conductive optical filter  32 , a multilayer film filter is configured by stacking an SiO 2  film  34  (insulating body) and a TiO 2  film  35  (insulating body) by two or more layers, and a Ti film  33  (metal) is formed on both surfaces as a transparent conductive film. Since the outer surface of the conductive optical filter  32  is configured by the Ti film  33 , the Ti film  33  and the light shielding metal  18  contact and the conductive optical filter  32  is grounded when the conductive optical filter  32  is formed on the light shielding metal  18 . 
         [0038]    The multilayer film filter is configured by stacking a plurality of materials having different index of refraction. In addition to SiO 2  and TiO 2  described above, metal oxide such as MgO, CrO 2 , Al 2 O 3 , CeO 2 , HfO 2 , Nb 2 O 5 , SnO 2 , Ta 2 O 5 , Y 2 O 3 , and ZrO 2 , and nitride such as SiN and TiN can be used for the multilayer film material. Furthermore, metals such as Al, Ag, Cu, Mo, and Cr can be used other than Ti for the material of the upper and lower transparent conductive films. 
         [0039]    Such multilayer film materials and transparent conductive materials are formed during the production process of the light sensor  31 , and thus need to have resistance with respect to temperature etc. used in the production process. For instance, since heat of about 400° is generally added in the process temperature in the formation of the light shielding metal, the organic thin film is inappropriate. The film forming method may be vapor deposition, sputtering, MBE, ion plating and the like. In the step of patterning so as to leave the film only at the upper side of the light receiving window  19 , a pattern is formed with a resist on the necessary region, and then the portion exposed from the resist is removed with dry etching and ion milling to fabricate the conductive optical filter  32  of a predetermined shape. 
         [0040]    The number of usage materials of the multilayer film filter is determined by the desired optical characteristics. In the example shown in  FIG. 8 , the multilayer filter is configured by repeating two types of layers of SiO 2 /TiO 2 , but may be two or more types of layers as long as the structure is a multilayer structure including thin film layers of different index of refraction, and three types of layers may be repeated as in a layer/b layer/c layer. The number of layers of the multilayer filter can be appropriately determined according to the characteristics such as wide and narrow of the transmissive wavelength band of the conductive optical filter  32 . 
       Second Embodiment 
       [0041]      FIG. 9  is a cross-sectional view schematically showing a light sensor  36  according to a second embodiment of the present invention. In this embodiment, the light receiving window  19  of the light shielding metal  18  is covered with a transparent conductive film  37 , and an optical filter  38  is formed on the insulating layer  20  so as to face the light receiving window  19 . The transparent conductive film  37  is preferably a transparent conductive film having a wide light transmissive wavelength band, and may be an ITO (Indium Tin Oxide), ZnO transparent conductive film, In 2 O 3 —ZnO transparent conductive film, Ga added ZnO film and the like. The optical filter  38  may be a multilayer film including two or more thin films, and is, for example, a band-pass filter having a transmissive wavelength band in the infrared region etc. 
         [0042]    Since the function of the electromagnetic shield and the function of the optical filter in the light detection region are separated to the transparent conductive film  37  and the optical filter  38  in the light sensor  36  of the embodiment, the restriction of the transparent conductive film  37  and the optical filter  38  becomes small and the degree of freedom in selecting the materials thereof becomes high. 
         [0043]    Furthermore, the optical filter  38  can be attached to the light sensor  36  afterwards since the optical filter  38  is arranged on the outer surface of the insulating layer  20 . Thus, in the assembly manufacturing company and the like, the optical filter  38  suited for application can be attached on the upper surface of the light sensor  36  for use at the stage before incorporating the light sensor  36  to the device, whereby the versatility of the light sensor  36  improves. 
         [0044]    Although not illustrated, an embodiment in which the optical filter  38  is removed from the light sensor  36  of the second embodiment may be adopted. In other words, the optical filter  38  may not be provided, and the light receiving window  19  may be covered with merely the transparent conductive film  37 . 
       Third Embodiment 
       [0045]    Although not illustrated, the optical characteristics of the conductive optical filter  32  of the first embodiment and the optical filter  38  of the second embodiment may be transmitting light of three wavelength bands. For instance, assuming red light (λ=around 650 nm), green light (λ=around 550 nm), and blue light (λ=around 450 nm), application can be made to a color imaging element such as a CCD.