Abstract:
A semiconductor photoelectric conversion device includes: a number of photoelectric conversion elements formed in a principal surface of the semiconductor substrate; functional devices formed in the semiconductor substrate adjacent to photoelectric conversion elements; a light shielding film formed above the semiconductor substrate for shielding light above the functional devices and having a window above each photoelectric conversion element; and an effective wavelength shortening member disposed in the windows, and being made of transmissive material having a high refractive index, thereby shortening an effective wavelength of light passing through the windows.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    This application is based on Japanese Patent Application No. 2001-400665, filed on Dec. 28, 2001, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    A) Field of the Invention  
           [0003]    The present invention relates to a photoelectric conversion device, and more particularly to a semiconductor photoelectric conversion device suitable for miniaturization.  
           [0004]    B) Description of the Related Art  
           [0005]    Most of semiconductor photoelectric conversion devices use a pn junction diode as a photoelectric conversion element and read accumulated electrons to generate a signal representative of the amount of received light. When light becomes incident upon a pn junction diode, photoelectric conversion occurs to generate pairs of electron-hole and electrons are accumulated in an n-type region.  
           [0006]    In a CCD semiconductor photoelectric conversion device, accumulated electrons are transferred by charge transfer channel CCDs and amplified by an output amplifier to acquire image signals. In a MOS semiconductor photoelectric conversion device, accumulated charges are amplified by a MOS transistor and picked up via a wiring line. In both the cases, functional devices such as CCDs and transistors are disposed near at a pn junction diode or photoelectric conversion element.  
           [0007]    These functional devices include pn junction diodes which generate charges corresponding to the amount of incident light. It is desirable that these charges are eliminated because they add noises to the light reception signal obtained from the pn junction diode. In order to eliminate the charges, a light shielding film is disposed above a semiconductor substrate. The light shielding film has a light transmission window above each photoelectric conversion element and shields light which otherwise enters the peripheral functional devices.  
           [0008]    A micro lens is disposed above the light shielding film in order to make light passed through a taking lens efficiently enter each photoelectric conversion element. Incident light passed through the micro lens is converged and enters the widow of the light shielding film. If the micro lens does not exist, light is guided to the window in order not to allow the light to enter the light shielding film and become invalid incidence light to thereby improve photoelectric conversion efficiency.  
           [0009]    An example of prior art will be described with reference to the accompanying drawings.  
           [0010]    [0010]FIG. 10A is a schematic cross sectional view showing the structure of a CCD semiconductor photoelectric conversion device according to the prior art. A silicon substrate  10  has a p-type layer  2  on an n-type region  1 . The p-type layer  2  has an n-type region  3  constituting a photodiode and an n-type region  5  constituting a vertical charge transfer channel VCCD. On the n-type region  3 , a p + -type region  4  is formed to bury the photodiode. A p-type region  6  is formed on the bottom surface of the n-type region  5  to electrically separate the vertical charge transfer channel, and a p + -type region  7  functioning as a channel stopper is formed between adjacent columns.  
           [0011]    On the surface of a silicon substrate  10 , a silicon oxide film  11  thermally oxidized is formed. On the silicon oxide film  11 , a charge transfer electrode  12  of a so-called double polysilicon structure is formed to drive the vertical charge transfer channel. The upper space of the photodiode is made open in order to introduce light. After the surface of the charge transfer electrode  12  is covered with a silicon oxide layer  14 , a light shielding film  15  made of tungsten or the like is formed, the light shielding film having a window above each photodiode.  
           [0012]    An insulating layer  17  having a flat surface is formed covering the light shielding film  15 , the insulating layer being made of a boron—phosphorous—silicon oxide (borophosphosilicate glass, BPSG) or the like. Color filters  31  are formed on the insulating layer  17 . The color filter layer  31  is covered with a surface planarizing layer  32  such as resist, and thereafter micro lenses  33  are formed on the surface of the surface planarizing layer  32  by using resist material or the like.  
           [0013]    With these processes, a semiconductor photoelectric conversion device with color filters is formed. Color filters are omitted for a three-plate type photoelectric conversion device. Incidence light upon the photodiode is limited by the light shielding film, and only the light passed through the window of the light shielding film can enter the photodiode.  
           [0014]    [0014]FIG. 10B is a plan view showing an example of the layout of a semiconductor image pickup device. A number of photodiodes PD are disposed in rows and columns in a tetragonal matrix shape. A vertical charge transfer channel VCCD is formed adjacent to each photodiode column. A horizontal charge transfer channel HCCD is coupled to one ends of the vertical charge transfer channels VCCDs.  
           [0015]    [0015]FIG. 10C is a plan view showing another example of the layout of a semiconductor image pickup device. A number of photodiodes PD are disposed shifted by a half pitch in the row and column directions, and has a so-called honeycomb layout. VCCD extends in a zigzag way between pixels of the honeycomb structure.  
           [0016]    [0016]FIG. 11 is a schematic cross sectional view showing the structure of another CCD semiconductor photoelectric conversion device according to the prior art. As compared to the structure shown in FIG. 10A, a low refractive index insulating layer  18  and a high refractive index insulating layer  19  are inserted between the light shielding film  15  and insulating layer  17 . The high refractive index insulating layer  19  has a downward convex shape to provide a lens function.  
           [0017]    [0017]FIGS. 12A, 12B,  12 C and  12 D show the results of optical path calculations of the structures shown in FIGS. 10A and 11. FIGS. 12A and 12B show the calculation results of the structure shown in FIG. 10A when incidence light enters at an incidence angle of 0 and 10 degrees. FIGS. 12C and 12D show the calculation results of the structure shown in FIG. 11 when incidence light enters at an incidence angle of 0 and 10 degrees. In the structure shown in FIG. 10A, as the incidence angle becomes oblique, the incidence light enters not only the window of the light shielding film but also other regions so that the efficiency lowers.  
           [0018]    If each pixel of a photoelectric conversion device is made small by proportionally reducing the size of constituent elements, it is expected that the sensitivity proportional to a pixel size is obtained without lowering the sensitivity per unit area. However, there is the phenomenon that as the opening size of a light shielding film is reduced, the sensitivity lowers at a higher rate than the reduction rate of the pixel size. This phenomenon that the sensitivity lowers at a higher rate than the reduction rate of a light reception area becomes conspicuous as the incidence angle becomes large.  
         SUMMARY OF THE INVENTION  
         [0019]    An object of this invention is to provide a semiconductor photoelectric conversion device suitable for reducing a pixel size.  
           [0020]    Another object of the invention is to prevent the sensitivity of a semiconductor photoelectric conversion device from being lowered as the pixel size is made small.  
           [0021]    According to one aspect of the present invention, there is provided a semiconductor photoelectric conversion device, comprising: a semiconductor substrate having a principal surface; a number of photoelectric conversion elements formed in the principal surface of the semiconductor substrate; functional devices formed in the principal surface of the semiconductor substrate adjacent to a plurality of the photoelectric conversion elements; a light shielding film formed above the semiconductor substrate, the light shielding film shielding light above the functional devices and having windows for opening an upper space of a predetermined region of each of the photoelectric conversion elements; and an effective wavelength shortening member disposed in at least some of the windows above the semiconductor substrate, the effective wavelength shortening member being made of transmissive material having a refractive index higher than a refractive index of silicon oxide and shortening an effective wavelength of light passing through the windows.  
           [0022]    As above, an effective wavelength of light in a size-reduced opening of a semiconductor photosensitive device is shortened to improve the light reception efficiency of a photodiode. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIGS. 1A and 1B are schematic cross sectional views of a semiconductor photoelectric conversion device according to an embodiment of the invention.  
         [0024]    [0024]FIG. 2 is a schematic cross sectional view of a semiconductor photoelectric conversion device according to another embodiment of the invention.  
         [0025]    [0025]FIG. 3 is a schematic cross sectional view of a semiconductor photoelectric conversion device according to still another embodiment of the invention.  
         [0026]    [0026]FIG. 4 is a schematic cross sectional view of a semiconductor photoelectric conversion device according to another embodiment of the invention.  
         [0027]    [0027]FIG. 5A is a schematic cross sectional view of a semiconductor photoelectric conversion device according to an embodiment of the invention, and FIG. 5B is a diagram showing how light travels at an interface between a high refractive index transmissive material layer and a transmissive material layer.  
         [0028]    [0028]FIG. 6 is a schematic cross sectional view of a semiconductor photoelectric conversion device according to an improved modification of the embodiment shown in FIGS. 5A and 5B.  
         [0029]    [0029]FIG. 7 is a schematic cross sectional view of a semiconductor photoelectric conversion device according to an improved modification of the embodiment shown in FIG. 2.  
         [0030]    [0030]FIG. 8 is a schematic cross sectional view of a semiconductor photoelectric conversion device according to another embodiment of the invention.  
         [0031]    [0031]FIG. 9 is a schematic plan view showing the layout of a MOS photoelectric conversion device.  
         [0032]    [0032]FIGS. 10A, 10B and  10 C are a cross sectional view and schematic plan views showing the structure of a conventional semiconductor photoelectric conversion device.  
         [0033]    [0033]FIG. 11 is a schematic cross sectional view showing the structure of a conventional semiconductor photoelectric conversion device.  
         [0034]    [0034]FIGS. 12A to  12 D are graphs showing the results of optical path calculations when vertical incidence light and oblique incidence light are applied to the structures shown in FIG. 10A and FIG. 11. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    Embodiments of the invention will be described with reference to the accompanying drawings, by taking as an example a CCD semiconductor image pickup device. Like elements to those of a conventional device are represented by identical reference numerals and the description thereof is simplified where appropriate.  
         [0036]    [0036]FIG. 1A is a cross sectional view showing the structure of a semiconductor photoelectric conversion device according to an embodiment of the invention. A silicon substrate  10  has a p-type layer  2  on an n-type region  1 , the p-type layer  2  being formed by epitaxial growth or ion implantation. The p-type layer  2  has therein an n-type region  3  constituting a photodiode and an n-type region  5  constituting a vertical charge transfer channel VCCD, respectively formed by ion implantation or the like.  
         [0037]    A p + -type region  4  burying the photodiode is being formed on the n-type region  3  by ion implantation or the like. A p-type region  6  is being formed on the bottom surface of the n-type region  5  by ion implantation or the like in order to electrically separate the vertical charge transfer channel VCCD. A p + -type region  7  functioning as a channel stopper is being formed between adjacent columns.  
         [0038]    The silicon substrate  10  has a silicon oxide film  11  formed on the surface thereof by thermal oxidation. A charge transfer electrode  12  of a so-called double polysilicon structure is being formed on the silicon oxide film  11  to drive the vertical charge transfer channel. The upper space of the photodiode is made open to introduce light. After the surface of the charge transfer electrode  12  is covered with a silicon oxide layer  14 , a light shielding film  15  made of tungsten or the like and having a window above the photodiode is formed.  
         [0039]    A transmissive material layer  21  having a higher refractive index than silicon oxide, e.g., silicon nitride having a refractive index of about 2, is formed burying the low step structure formed by the light shielding film  15 . The high refractive index transmissive material layer  21  has a higher refractive index than silicon oxide frequently used as transmissive material, and has a function of enlarging the effective window size defined by the light shielding film  15  as will be described in the following. A transmissive insulating layer  17  having a flat surface and made of a boron-phosphorous-silicon oxide (borophosphosilicate glass (BPSG)) film or the like covers the silicon substrate  10 . The layout of photodiodes may be a square matrix shape shown in FIG. 10B or a honeycomb shape shown in FIG. 10C. In this structure of the CCD semiconductor image pickup device, of the materials constituting the optical path, the high refractive index transmissive insulating layer  21  has a higher refractive index than other materials. In addition to silicon nitride, the high refractive index transmissive insulating layer  21  may be made of carbon material of the diamond structure, tantalum oxide or the like. If silicon nitride is used, it is preferably used in an amorphous phase.  
         [0040]    A light flux can be converged into a point in terms of geometrical optics and can pass through a hole however small it is. However, since light is a wave, it spreads also in the plane perpendicular to the traveling direction. In terms of wave optics, there is a limit of the diameter of an opening through which light can pass. The opening is generally filled with transmissive insulating material such as silicon oxide. If the opening has a diameter of at most threefold of the wavelength of light in vacuum, it becomes necessary to consider the degree of light kicked at the edge of the opening. Generally, the light intensity is greatly attenuated when the light passes through the opening having a diameter similar to the wavelength of light in vacuum. Even if the diameter of an opening is about twofold of the wavelength of light in vacuum, attenuation cannot be neglected.  
         [0041]    [0041]FIG. 1B shows a distribution of refractive indices near the light shielding film shown in FIG. 1A. Light having a wavelength λ in vacuum has a wavelength λ(n)=λ/n in medium having a refractive index n. The silicon oxide transmissive insulating layers  11 ,  17  have a refractive index of about 1.5. The high refractive index transmissive material layer  21  has a refractive index of about 2.  
         [0042]    The wavelength λ2 at the refractive index of 2 is about ¾ of the wavelengths λ1, λ3 at the refractive index of 1.5. The wavelength λ2 of light in the high refractive index transmissive material layer  21  is therefore shorter than the wavelengths λ1, λ3 of lights in the transmissive insulating layers  17 ,  11 . Even if the diameter of the opening of the light shielding film  14  becomes small, a ratio of the opening diameter to the wavelength can be maintained high. In other words, the high refractive index transmissive material layer functions to effectively enlarge the opening diameter. In this manner, it becomes possible to prevent the transmission factor from being lowered by a reduced opening diameter. Semiconductor such as silicon semiconductor has a high refractive index of 3 or higher so that a wavelength λ4 in the semiconductor substrate  10  becomes shorter.  
         [0043]    The high refractive index transmissive material layer  21  has a high refractive index so that the wavelength of light in the opening of the light shielding film  14  is effectively made short. If there is another medium having a different refractive index near the transmissive insulating film  17 , the effective refractive index is an average of these refractive indices. It is possible to make large the ratio of the opening diameter to the wavelength and mitigate the limit to be caused by this ratio. From this viewpoint, the high refractive index transmissive material layer  21  has a function of shortening an effective wavelength. The effects of shortening the effective wavelength become distinctive if the opening diameter is set to at most threefold of the wavelength of light in vacuum, particularly to twofold or shorter.  
         [0044]    [0044]FIG. 2 is a schematic cross sectional view of a semiconductor photoelectric conversion device according to another embodiment of the invention. In FIG. 2, like elements to those of the device shown in FIG. 1 are represented by using identical reference numerals and the description thereof is omitted. A high refractive index transmissive material layer  21  is made thicker than the high refractive index transmissive material layer shown in FIG. 1, and covers the whole surface of the silicon substrate  10 . A color filter layer  31  of organic resin is formed on a transmissive insulating film  17 , and a planarizing layer  32  of organic resin is formed on the color filter layer  31 . A resist pattern is formed on the planarizing layer  32  and fluidized to form micro lenses  33 .  
         [0045]    Light LA, LB and LC incident upon the micro lens  33  is focused by the refractive function of the micro lens and converged into the opening of the light shielding film  15 . The effective wavelength of light in the opening is shortened by the high refractive index insulating layer  21 , and the limit by the ratio of the opening diameter to the wavelength can be mitigated.  
         [0046]    [0046]FIG. 3 is a cross sectional view of a semiconductor photoelectric conversion device according to still another embodiment of the invention. As compared to the structure shown in FIG. 2, the shape of the upper region of a high refractive index insulating layer  21  is different. The high refractive index insulating layer  21  has an upward convex spherical surface  22  which is protruded into a relatively low refractive index insulating film  17 . In this manner, a convex lens (inner lens) is formed so that incidence light is focused. The micro lens  33  and the inner lens of the high refractive index material layer  21  allow incidence light to enter the photodiode more reliably.  
         [0047]    In a semiconductor photoelectric conversion device using primary red (R), green (G) and blue (B) color filters, each photoelectric conversion element receives light in one of R, G and B wavelength ranges. R has the longest wavelength range, G has the middle wavelength range and B has the shortest wavelength range. The relation between the opening diameter and wavelength is most severe in R, gentle in G and most gentle in B. The same limit does not therefore exist in the whole wavelength range.  
         [0048]    [0048]FIG. 4 is a cross sectional view of a semiconductor photoelectric conversion device according to another embodiment of the invention. As compared to the structure shown in FIG. 2, under the color filter of blue (B), not a high refractive index transmissive material layer  21  but a relatively low refractive index transmissive insulating film  17   b  of BPSG or the like is buried. A transmissive insulating film  17  includes a region  17   a  higher than the high refractive index transmissive material layer  21  and the buried region  17   b  in the opening of the light shielding film under the blue filter. The limit by the ratio of the opening diameter to the wavelength is gentle in the blue wavelength range because of a short wavelength, and the incidence light is less limited even if the high refractive index transmissive material layer is not buried. The high refractive index transmissive material layer may be omitted not only in the blue wavelength range but also in the green wavelength range. In the longest red wavelength range, it is preferable to form the high refractive index transmissive material layer  21  to shorten the effective wavelength.  
         [0049]    [0049]FIG. 5A is a cross sectional view of a semiconductor photoelectric conversion device according to an embodiment of the invention. On a light shielding film  15 , a silicon oxide film  16  is formed surrounding an opening. The silicon oxide film  16  has a sidewall slanted relative to a surface insulating layer  11  in the opening. A high refractive index transmissive material layer  21  is filled in the opening defined by the silicon oxide film  16  and covers the surface of the silicon oxide film  16 . Similar to the structure shown in FIG. 3, the high refractive index transmissive material layer  21  has an upward convex lens shape. The other structures are similar to those shown in FIG. 3. The high refractive index region  21  and low refractive index region  16  surrounding the high refractive region have the structure analogous to that of a core and a clad of an optical fiber.  
         [0050]    [0050]FIG. 5B is a diagram illustrating how light travels at an interface between the high refractive index transmissive material layer  21  of silicon nitride or the like and the transmissive material layer  16  of silicon oxide or the like. Silicon nitride has a refractive index of about 2 and silicon oxide has a refractive index of about 1.5. At the interface between these different refractive index regions, a critical angle of total reflection is about 48.6 degrees. Light having an incidence angle larger than the critical angle is totally reflected. A light beam LD travels from the high refractive index transmissive material layer  21  toward the transmissive material layer  16  and is totally reflected at the interface.  
         [0051]    In the structure shown, if the angle θ1 between vertical incidence light and the slanted surface is 90−48.6=41.4 or smaller, the vertical incidence light is totally reflected. In order to satisfy this condition, the angle θ2 between the interface and the substrate surface is set to 48.6 degrees or larger.  
         [0052]    [0052]FIG. 6 is a schematic cross sectional view of a semiconductor photoelectric conversion device according to an improved modification of the embodiment shown in FIG. 5A. There is a high possibility that light traveling toward the interface between the high refractive index transmissive material layer  21  and silicon oxide layer  16  and totally reflected at the interface is also totally reflected at the interface between the surface insulating layer  11  and high refractive index transmissive material layer  21 . In order to avoid this, between the high refractive index transmissive material layer  21  and surface insulating layer  11 , an antireflection film  18  is formed which has a refractive index intermediate between silicon nitride and silicon oxide, e.g., silicon oxynitride. As the antireflection film having an intermediate refractive index is formed at the interface between two media, the critical angle of total reflection becomes large and the reflection factor lowers. It is therefore possible for incidence light to enter the photodiode more efficiently.  
         [0053]    [0053]FIG. 7 is a schematic cross sectional view of a semiconductor photoelectric conversion device according to an improved modification of the embodiment shown in FIG. 2. In this embodiment, an antireflection film  18  is formed in the opening. This antireflection film  18  reduces the amount of light reflected at the surface insulating layer  11  so that more light can enter the photodiode.  
         [0054]    Total reflection occurs when light travels from a high refractive index medium into a low refractive index medium.  
         [0055]    [0055]FIG. 8 is a cross sectional view of a semiconductor photoelectric conversion device according to another embodiment of the invention. In this embodiment, the surface insulating layer  11  in contact with the high refractive index transmissive material layer  21  shown in the structure of FIG. 7 is removed. Namely, an opening is formed through the surface insulating layer  11  and a high refractive index transmissive material layer  21   b  is filled in this opening. The high refractive index transmissive material layer  21  includes the region  21   a  similar to the embodiment shown in FIG. 7 and the region  21   b  in contact with the substrate surface. The antireflection film in the structure shown in FIG. 7 is not used. Of silicon nitride and semiconductor substrate in direct contact, the semiconductor substrate has a higher refractive index. Therefore, total reflection of light traveling from the silicon nitride layer does not occur. It is possible for incidence light to enter efficiently into a photodiode. Also, a dielectric layer  16  having a lower refractive index than that of the high refractive index layer  21 , similar to the layer  16  in FIG. 6, is provided.  
         [0056]    A semiconductor photoelectric conversion device is not limited to a CCD type.  
         [0057]    [0057]FIG. 9 is a schematic plan view showing the layout of a MOS photoelectric conversion device. In a MOS photoelectric conversion device, a MOS transistor circuit MOS for reading and amplifying charges is formed adjacent to a photodiode PD. An output signal is read from the MOS transistor circuit MOS to a bus line BL.  
         [0058]    Also in a MOS photoelectric conversion device, a light shielding film having openings corresponding to photodiodes is formed on the surface of a semiconductor substrate and covers the peripheral area of photodiodes. The structures similar to the above-described embodiments can be applied to each opening of the light shielding film.  
         [0059]    The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. It is apparent to those skilled in the art that various modifications, improvements, combinations and the like can be made.