Abstract:
Disclosed herein is a photodiode cell, including: a first-type substrate; a second-type epitaxial layer disposed on the first-type substrate; heavily-doped second-type layers, each having a small depth, formed on the second-type epitaxial layer; and heavily-doped first-type layers, each having a narrow and shallow section, disposed on the second-type epitaxial layer and formed between the heavily-doped second-type layers, wherein the first-type and second-type have opposite doped states.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2009-0026412, filed Mar. 27, 2009, entitled “Photo Diode Cell structure of Photo Diode Integrated Circuit for Optical Pickup and method thereof”, which is hereby incorporated by reference in its entirety into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a photodiode cell structure of a photodiode integrated circuit for optical pickup and a method of manufacturing the same, and, more particularly, to a photodiode cell structure of a photodiode integrated circuit (hereinafter, referred to as ‘PDIC’) for optical pickup, in which highly-concentrated impurities are thinly doped between adjacent light-receiving regions and then floated, and thus the adjacent light-receiving regions are isolated from each other, thereby improving the optical efficiency of a photodetector, and a method of manufacturing the same. 
         [0004]    2. Description of the Related Art 
         [0005]    Generally, a photodetector is a transducer which detects light and then converts its strength into electrical signals. Examples of the photodetector include a photoelectric cell (silicon, selenium), a photoconductive device (cadmium sulfide, cadmium selenide), a photodiode, a phototransistor, a photomultiplier tube, a photoelectric tube (vacuum, gas sealing), and the like. Generally, the photodetector is fabricated using a direct transition semiconductor having excellent light conversion efficiency. 
         [0006]    The photodetector has various structures, and generally includes a PIN photodetector using P-N junction, a Schottky photodetector using Schottky junction, a MSM (metal semiconductor metal) photodetector, and the like. 
         [0007]    Meanwhile, recently, the importance of lasers and PDICs has been greatly highlighted in the optical data storage field. In order to replay and store optical data in a large-capacity storage medium, the wavelength of a laser must be shortened, and in order to detect the shortened wavelength thereof, the efficiency of a photodetector is required to be increased. 
         [0008]    Currently, a photodetector which can detect wavelengths of 650 nm and 780 nm has a PIN structure. In order to increase the efficiency of such a photodetector, it is required to change the cell of a conventional photodetector used in a PDIC to a high-efficiency cell. 
         [0009]      FIG. 1  is a plan view showing a conventional PIN-structured photodiode (PD) cell, and  FIGS. 2A and 2B  are sectional views of the photodiode (PD) cell taken vertically to a substrate along the line A-A′ of  FIG. 1 , respectively. 
         [0010]    In a conventional photodiode cell  100  of a photodiode integrated circuit for optical pickup, as shown in  FIGS. 1 ,  2 A and  2 B, second-type epitaxial layers  103   a  and  103   b  (for example, N-type epitaxial layers), which become intrinsic layers having a P-I-N structure, are formed on a first-type substrate  101   a  or  101   b  (for example, a P-type substrate) through an epitaxial growth method. Subsequently, the second-type expitaxial layers  103   a  and  103   b  are formed thereon with heavily-doped second type layers  105   a - 1 ˜ 4  and  105   b - 1 ˜ 4  (for example, N +  layers) which are multi-divided light-receiving regions. 
         [0011]    The heavily-doped second-type layers  105   a - 1 ˜ 4  and  105   b - 1 ˜ 4  formed in this way must be isolated from each other in order to increase their optical efficiency. For this, a method of completely isolating the heavily-doped second-type layers  105   a - 1 ˜ 4  from each other by physically forming a first-type well  107   a  (for example, P-well) and a first-type BUR  109   b  (For example, PBUR) between adjacent heavily-doped second-type layers  105   a - 1 ˜ 4 , for example, two adjacent heavily-doped second-type layers  105   a - 1  and  105   a - 2  such that they are connected to a substrate  101   a  has been used (refer to  FIG. 2A ). However, this method is problematic in that the width of the first-type well  107   a  is excessively large, so that effective light-receiving regions become small, thereby decreasing optical efficiency. 
         [0012]    Alternatively, a method of electrically isolating the heavily-doped second-type layers  105   b - 1 ˜ 4  from each other by forming a first-type well  107   b  between adjacent heavily-doped second-type layers  105   b - 1 ˜ 4 , for example, two adjacent heavily-doped second-type layers  105   b - 1  and  105   b - 2  and then grounding the first-type well  107   b  (refer to  FIG. 2B ) has been used. However, this method is also problematic in that the first-type well  107   b  is grounded and thus connected to the ground (GND), so that leakage current is generated, thereby causing noise. 
       SUMMARY OF THE INVENTION 
       [0013]    Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and the present invention provides a photodiode cell structure of a photodiode integrated circuit for optical pickup and a method of manufacturing the same, in which divided adjacent photodiode cells can be completely isolated from each other without using a physical isolation method using a well or an electrical isolation method using the application of voltage and grounding. 
         [0014]    An aspect of the present invention provides a photodiode cell, including: a first-type substrate; a second-type epitaxial layer disposed on the first-type substrate; heavily-doped second-type layers, each having a small depth, formed on the second-type epitaxial layer; and heavily-doped first-type layers, each having a narrow and shallow section, disposed on the second-type epitaxial layer and formed between the heavily-doped second-type layers, wherein the first-type and second-type have opposite doped states. 
         [0015]    In the photodiode cell, the first type may be a P-type, and the second-type may be an N-type. 
         [0016]    Further, the first-type substrate may have an impurity concentration of 10 16  cm −3  or more, and the second-type epitaxial layer may have an impurity concentration of 10 14  cm −3  or less. 
         [0017]    Further, each of the heavily-doped first-type layers may have a width of 1 μm or less, and the depths of the heavily-doped first-type layers may be equal to or greater than those of the heavily-doped second-type layers. 
         [0018]    Further, the heavily-doped first-type layers may be floated. 
         [0019]    Another aspect of the present invention provides a method of manufacturing a photodiode cell, including: forming a second-type epitaxial layer on a first-type substrate; forming heavily-doped second-type layers, each having a small depth, on the second-type epitaxial layer; and forming heavily-doped first-type layers, each having a narrow and shallow section, between the heavily-doped second-type layers. 
         [0020]    In the forming of the heavily-doped first-type layers, each of the heavily-doped first-type layers may have a width of 1 μm or less. 
         [0021]    Further, the method of manufacturing a photodiode cell may further include: floating the heavily-doped first-type layers after the forming of the heavily-doped first-type layers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0023]      FIG. 1  is a plan view showing a conventional PIN-structured photodiode (PD) cell; 
           [0024]      FIG. 2A  is a sectional view of the photodiode (PD) cell taken along the line A-A′ of  FIG. 1 ; 
           [0025]      FIG. 2B  is another sectional view of the photodiode (PD) cell taken along the line A-A′ of  FIG. 1 ; 
           [0026]      FIG. 3  is a plan view showing a four-divided photodiode cell structure of a photodiode integrated circuit for optical pickup according to an embodiment of the present invention; and 
           [0027]      FIG. 4  is a sectional view of the four-divided photodiode cell structure taken along the line B-B′ of  FIG. 3 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    Various objects, advantages and features of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings. 
         [0029]    Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Throughout the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant descriptions thereof are omitted. 
         [0030]    Generally, a photodiode cell used for optical pickup has a multi-divided structure, for example, an 8-divided or 12-divided structure. For the convenience of description, in the present invention, four-divided photodiode cells are used, and only two adjacent photodiode cells are shown in a sectional view taken vertically to a substrate. 
         [0031]    Hereinafter, a photodiode cell structure of a photodiode integrated circuit for optical pickup and a method of manufacturing the same according to an embodiment of the present invention will be described. 
         [0032]      FIG. 3  is a plan view showing a four-divided photodiode cell structure of a photodiode integrated circuit for optical pickup according to an embodiment of the present invention, and  FIG. 4  is a sectional view of the four-divided photodiode cell structure taken along the line B-B′ of  FIG. 3 . 
         [0033]    Referring to  FIGS. 3 and 4 , a photodiode cell  200  according to an embodiment of the present invention includes a first-type substrate  201 , a second-type epitaxial layer  203 , heavily-doped second-type layers  205 , and heavily-doped first-type layers  207 . In this regard, the doped states of the first-type and second-type are opposite to each other (for example, if the first type is a P-type, the second-type is an N-type). 
         [0034]    As shown in  FIG. 4 , the photodiode cell  200  according to an embodiment of the present invention further includes a first-type well  209  and a first-type BUR  211 , which are disposed at the edge thereof. 
         [0035]    The first-type substrate  201  may be a silicon-based substrate, for example, a P-type silicon substrate or an N-type silicon substrate. In this embodiment, a P-type silicon substrate, represented by ‘P-sub’ in the drawings, is used as the first-type substrate  201 . 
         [0036]    Further, the concentration of impurities doped on the first-type substrate  201  may be 10 16  cm −3  or more. When the concentration thereof is less than 10 16  cm −3 , the resistance of the first-type substrate is increased, thus deteriorating the frequency characteristics of the photodiode cell  200 . 
         [0037]    The second-type epitaxial layer  203  is made of a silicon-based material, and may be formed by epitaxially growing the silicon-based material on the first-type substrate  201  using chemical vapor deposition (CVD). 
         [0038]    This second-type epitaxial layer  203  is a light-absorbing layer, and the light absorbed therein can be converted into electrical signals. Specifically, heavily-doped second-type layers  205 - 1 ˜ 4  are formed on the second-type epitaxial layer  203  through an ion implantation process to form a P-I-N, and thus the second-type epitaxial layer  203  can absorb light having wavelengths of 650 nm and 780 nm, and the light absorbed therein can be converted into electrical signals. 
         [0039]    In this regard, the second-type epitaxial layer  203  is formed based on silicon carbide (SiC) or diamond having a similar lattice constant to silicon (Si), which is a group IV element, or silicon crystal. At the time of the epitaxial growth using the chemical vapor deposition (CVD), when a group III element (for example, boron) is added as a dopant, a P-type epitaxial layer can be obtained, and when a group V element (for example, phosphorus) is added as a dopant, an N-type epitaxial layer can be obtained. 
         [0040]    In this case, the second-type epitaxial layer  203  may have a lower impurity concentration than that of the first-type substrate  201  and may have higher resistivity than that of the first-type substrate  201 . Here, the impurity concentration of the second-type epitaxial layer  203  may be 10 14  cm −3  or less. When the impurity concentration thereof is more than 10 14  cm −3 , the frequency characteristics of the photodiode cell  200  to the light having wavelengths of 650 nm and 780 nm can be deteriorated. In this embodiment, an N-type epitaxial layer, represented by ‘N-epi’ in the drawings, is used as the second-type epitaxial layer  203 . 
         [0041]    As shown in  FIG. 3 , the heavily-doped second-type layers  205 - 1 ˜ 4  are formed such that four-divided light-receiving regions are disposed in the second-type epitaxial layer  203 , and are thinly formed on the second-type epitaxial layer  203  using a highly-concentrated group III or V element through an ion implantation process. In this embodiment, heavily-doped N-type layers, represented by ‘N + ’ in the drawings, are used as the heavily-doped second-type layers  205 - 1 ˜ 4 . 
         [0042]    The heavily-doped second-type layers  205 - 1 ˜ 4  formed in this way become light-receiving regions. However, the adjacent heavily-doped second-type layers  205 - 1 ˜ 4  must be completely isolated from each other in order to increase their optical efficiency. 
         [0043]    When the adjacent heavily-doped second-type layers  205 - 1 ˜ 4  are not completely isolated from each other, resistance is generated therebetween, thus badly influencing the output terminal of a photodetector for optical pickup. In other words, it means that the optical efficiency of the photodiode cell used in the photodetector for optical pickup is decreased. 
         [0044]    Therefore, in the photodiode cell of a photodiode integrated circuit for optical pickup according to an embodiment of the present invention, as shown in  FIG. 4 , for example, a heavily-doped first-type layer  207  is narrowly and thinly formed between the two adjacent heavily-doped second-type layers  205 - 1  and  205 - 2  in order to isolate them from each other, thus increasing the resistance between the adjacent heavily-doped second-type layers  205 - 1 ˜ 4 . In this case, the width of the heavily-doped first-type layer  207  may be about 1 μm or less, and the depth thereof may be equal to or greater than that of the adjacent heavily-doped second-type layers  205 - 1 ˜ 4 . In this case, the width of the heavily-doped first-type layer  207  corresponds to about ¼ of that of the conventional first-type well (for example, P-well). In this embodiment, heavily-doped P-type layers, represented by ‘P+’ in drawings, are used as the heavily-doped first-type layers  207 . 
         [0045]    When the heavily-doped first-type layer  207  is formed in this way and then floated, since the heavily-doped first-type layer  207  and the second-type epitaxial layer  203  are joined in opposite types, a depletion layer is formed by the movement of pairs of electrons-holes occurring in a C region. 
         [0046]    Accordingly, even when a voltage (for example, a reverse bias when the heavily-doped first-type layer  207  is a P-type layer, and a forward bias when the heavily-doped first-type layer  207  is an N-type layer) is not applied to the heavily-doped first-type layer  207  or the heavily-doped first-type layer  207  is not grounded, the depletion layer is formed even on the surface of the first-type substrate  201 , and thus the adjacent two heavily-doped second-type layers  205 - 1  and  205 - 2  are completely isolated from each other. 
         [0047]    The above-mentioned photodiode cell according to the present invention can be applied to 4×-speed PDICs for blue-ray. 
         [0048]    As described above, according to the present invention, since highly-concentrated impurities having an opposite type to those of light-receiving regions are narrowly and thinly doped between the light-receiving regions in order to isolate the adjacent light-receiving regions from each other, the distances between the light-receiving regions is decreased, so that the size of the photodetector using this photodiode cell can be decreased, with the result that effective light-receiving regions are more enlarged, thereby improving the optical efficiency of the photodiode cell. 
         [0049]    Further, according to the present invention, dark current can be decreased by floating the layers narrowly and thinly doped with highly-concentrated impurities having an opposite type to those of light-receiving regions in order to isolate the adjacent light-receiving regions from each other instead of applying an voltage to the layers or grounding the layers, and thus the noise characteristics of the photodiode cell can also be improved. 
         [0050]    Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.