Photodetector for use with an optical recording medium
BACKGROUND OF THE INVENTION 
     This invention relates to a photodetector for use in reproducing information recorded on an optical recording medium. 
     Upon recording of data from a music or image source on an optical disc recording medium, the data are converted into a digital signal having logic &#34;0&#34; and &#34;1&#34; levels. According to the logic level, pits are formed to provide continuous spiral recording track in the optical disc recording medium. An optical pickup device is used to reproduce the recorded data by radiating a light beam from a light source such as a semiconductor laser through an objective lens which focuses the light beam on the optical disc recording medium and sensing the light beam reflected from the optical recording medium to determine the presence of pits. 
     According to an astigmatic method, the optical pickup device employs a photodetector placed at a position intermediate between the two astigmatic points of the light beam which has reflected from the optical disc recording medium and passed through a cylindrical lens. The photodetector has a center photosensitive section juxtaposed in a spaced relationship between two side photosensitive sections. The center photosensitive section is divided by a cross-shaped electric and optical insulator into four portions each being formed by a independent photodiode. The shape and position of the reflected light spot formed on the center photosensitive section is dependent upon the distance between the optical disc recording medium and the objective lens. A circular light spot is formed on the center photosensitive section when the objective lens is positioned in focus with respect to the optical disc recording medium. An eliptical light spot is formed on the center photosensitive section when the objective lens is at a shorter or longer distance from the optical disc recording medium. Formation of a light spot on the center photosensitive section causes currents to be derived from the respective divided portions of the center photosensitive section. The derived currents are used to reproduce the recorded data and also to detect an focusing error for a focusing servo control. The side photosensitive sections, each of which is formed by a photodiode, detect respective side beams for a tracking servo control. 
     One problem occurs in eliminating jitter of the main of RF signal transmitted from such a conventional photodetector in order to improve the reading performance of the optical pickup device. Although attempts have been made to eliminate such jitter by increasing the machining accuracy with which the optical disc recording medium is produced, the results were not satisfactory in some applications. 
     SUMMARY OF THE INVENTION 
     Therefore, a main object of the invention is to provide an improved photodetector which is free from the jitter problem. 
     It is another object of the invention to provide a photodetector having an improved sensitivity particular to low-frequency components of a light spot formed on its center photosensitive section. 
     There is provided, in accordance with the invention, a photodetector including a center photosensitive section having a surface for receipt of light reflected from an optical recording medium in producing information recorded on the optical recording medium. The center photosensitive section includes a plurality of photosensitive elements separated by a space from one another. A non-reflective transparent film is provided on substantially the whole area of the surface of the center photosensitive section to cover the photosensitive elements and the space. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of this invention which are believed to be novel are set forth with particularity in the appended claims. The invention together with its further objects and advantages thereof, may be best understood, however, by reference to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a top plan view showing a typical example of photodetector included in an optical pickup device used in reproducing information recorded on an optical disc recording medium; 
     FIG. 2 is a fragmentary sectional view showing the structure of a prior art photodetector; 
     FIG. 3 is a graph showing the sensitivity of the center photosensitive section of the prior art photodetector with respect to a distance from its center; 
     FIG. 4 is a graph of frequency versus sensitivity provided by the center photosensitive section of the prior art photodetector; 
     FIG. 5 is a fragmentary sectional view showing the structure of the photodetector of the invention; 
     FIGS. 6(A) to 6(E) are fragmentary sections used to explain the sequence of processes for producing the photodetector of the invention; 
     FIG. 7 is a graph showing the sensitivity of the center photosensitive section of the photodetector of the invention with respect to a distance from its center; and 
     FIG. 8 is a graph of frequency versus sensitivity provided by the center photosensitive section of the photodetector of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is illustrated a typical example of photodetector included in an optical pickup device used in reproducing information recorded on an optical recording medium. The photodetector has a center photosensitive section P1 comprised of four photodiodes A, B, C and D separated at slight distance from one another. Two side photosensitive sections P2 and P3 are positioned on the opposite sides of the center photosensitive section P1. The side photosensitive section P2 is comprised of a photodiode E and the side photosensitive section P3 is comprised of a photodiode F. 
     Prior to the description of the preferred embodiment of the invention, the prior art photodetector of FIG. 2 is briefly described in order to specifically point out the difficulties attendant thereon. 
     Referring to FIG. 2, the prior art photodetector includes a center photosensitive section P1 which is shown, in section along the lines II--II of FIG. 1, as comprising an N -  silicon semiconductor substrate (a) and an N -  semiconductor layer (b) placed on the surface of the N +  silicon semiconductor substrate (a). An SiO 2  field insulation film (c) having a thickness of 7,000 Å is formed on the surface of the N -  semiconductor layer (b) to provide electric insulation between the photodiodes A and B. The field insulation film (c) is formed with windows (d) by using an etching technique. These windows (d) are utilized in diffusing a small quantity of P-type impurity to form P +  diffusion regions e(A) and e(B) in the N -  semiconductor layer (b). The diffusion region e(A on the left as viewed in FIG. 2 constitutes a part of the photodiode A and the diffusion region e(B) on the right as viewed in FIG. 2 constitutes a part of the photodiode B. An SiO 2  film (f) having a thickness of 2,000 Å  is provided to cover over the whole area of each of the windows (d). A nitride film (g) having a thickness ranging from 3,000 to 4,000 Å is provided to cover the whole area of the surface of the device including the SiO 2  field insulation film (c) and the SiO 2  film (f). 
     It is to be noted that a nitride film having a thickness ranging from about 3,000 Å to about 4,000 Å is active with an SiO 2  film having a thickness of about 2,000 Å to form a non-reflective transparent film. Therefore, the nitride film (g) acts with the SiO 2  window cover films (f) to form non-reflective transparent films (h) so that the photodiodes A and B can receive the light incident on the non-reflective transparent cover films (h). On the other hand, the nitride film (g) acts with the SiO 2  insulation films (c) having a thickness of 7,000 Å to form reflective cover films (i) so as to provide optical insulation between the photodiodes A and B. As a result, the photodiodes A and B cannot receive the light incident on the reflective cover films (i). Similar electric and optical insulation is provided to separate the photodiode A from the photodiode D, the photodiode B from the photodiode C, and the photodiode C from the photodiode D. 
     One problem occurs in an attempt to increase the reading performance of the optical pickup device by eliminating jitter of the main of RF signal transmitted from such a prior art center photosensitive section P1. The term &#34;jitter&#34; means a deviation of the actual zero crossing point of the main or RF signal with respect to a predetermined zero cross point. Although it is possible to reduce the jitter to some extent by increasing the machining accuracy with which the optical disc recording medium is produced, this is not satisfactory in some applications. The inventors found that the optical characteristic of the center light receipt section is another factor contributing to the jitter problem. This factor will be described in greater detail with reference to FIGS. 3 and 4. 
     The light spot formed on the center photosensitive section P1 has low-frequency components concentrated around its center 0 and high-frequency components concentrated near its periphery. The high-frequency components concentrated near the periphery of the light spot contribute to the jitter problem. 
     As can be seen from FIG. 3 which is a graph showing the sensitivity of the center photosensitive section P1 with respect to a distance from its center 0, the center light receipt section has a relatively large blind region around its center 0 at which the sensitivity is zero. The blind region corresponds to the light spot portion on which low-frequency components are concentrated. For this reason, the center photosensitive section P1 has a low sensitivity for low-frequency components, as can be seen from FIG. 4 which is a graphic illustration of the sensitivity of the center photosensitive section P1 with respect to frequencies. As a result, the RF signal transmitted from the center photosensitive section P1 includes high-frequency components, which contribute to the jitter problem, at a high percentage. 
     Referring to FIG. 5, there is illustrated one embodiment of a photodetector made in accordance with the invention. The photodetector is shown as comprising an N +  silicon semiconductor substrate 1 and an N -  semiconductor layer 2 placed on the surface of the N +  silicon semiconductor substrate 1. The photodetector also comprises N+ diffusions 3 formed in the N -  semiconductor layer 2 to separate the center photosensitive section P1 from the side photosensitive sections P2 and P3. A small quantity of P-type impurity is diffused to from P +  diffusions 4(A), 4(E) and 4(F) in the N -  semiconductor layer 2. The P -  diffusion region 4(A) and 4(B) are separated with a gap 5 from each other. The P +  diffusion 4(A) constitutes a part of the photodiode A, the P +  diffusion 4(B) constitutes a part of the photodiode B, the P +  diffusion 4(E) constitutes a part of the photodiode E, and the P +  diffusion 4(F) constitutes a part of the photodiode F. An SiO 2  film 6 is provided on the semiconductor layer 2 to cover the whole area of the surface of the semiconductor layer 2. The SiO 2  film 6 has a thickness of 2,000 Å . A nitride film 7 is provided on the SiO 2  film 6 to over the whole area of the surface of the SiO 2  film 6. The nitride film 7 has a thickness ranging from about 3,000 Å to about 4,000 Å . The nitride film 7 acts with the SiO 2  film 6 to form a non-reflective transparent cover film over the whole area of the surface of the photodetector including the photodiodes A, B, C and D (see FIG. 1) of the center photosensitive section P1 and the photodiodes E and F (see FIG. 1) of the side photosensitive sections P2 and P3. 
     Referring to FIGS. 6 (A) to 6(E), there is illustrated the sequence of processes for producing the photodetector of the invention. 
     First of all, an N -  semiconductor layer 2 is formed on an N +  semiconductor substrate 1. A heating oxidation treatment is applied to form an SiO 2  field insulation film 8 over the whole area of the surface of the N -  semiconductor layer 2. This process is followed by application of an etching technique to form windows 9 in the field insulation film 8 at positions corresponding to the photodiodes, as shown in FIG. 6(A). The windows 9 are utilized in diffusing a small quantity of P-type impurity to form P +  diffusions 4(A) and 4(B) in the N -  semiconductor layer 2. The windows 9 are covered with SiO 2  films 10, as shown in FIG. 6(B). Following this process, both of the field insulation film 8 and the SiO 2  films 10 are removed from the semiconductor layer 2, as shown in FIG. 6(C). Thereafter, a heating oxidation treatment is applied to form an SiO 2  film 6 having a thickness 2,000 Å on the surface of the semiconductor layer 2, as shown in FIG. 6(D). This process is followed by application of a CVD technique to form a nitride film 7 having a thickness ranging from 3,000 Å to 4,000 Å on the SiO 2  film 6, as shown in FIG. 6(E). It is to be noted that the N +  diffusions 9 may be formed before or after formation of the P +  diffusions 4(A) and 4(B) substantially in the same manner as the diffusions 4 are formed in the N -  semiconductor layer 2. 
     As can be seen from the foregoing, the photodetector includes a center photosensitive section having four photodiodes separated by a space from one another and a non-reflective transparent film provided on the whole area of the surface of the center photosensitive section to cover the photodiodes and the space. Any of the photodiodes can receive the light incident on the non-reflective transparent film at a position just above the space. It is, therefore, apparent that the center photosensitive section has an increased sensitivity around its center. 
     FIG. 7 shows the sensitivity of the center photosensitive section P1 with respect to a distance from the center 0 (see FIG. 1) of the center photosensitive section P1. As can be seen from a comparison between FIGS. 3 and 7, the photodetector of the invention includes a center photosensitive section which has a blind region very much smaller than the prior art photodetector. 
     FIG. 8 is a curve of frequency versus sensitivity provided by the photodetector of the invention. As can be seen from a consideration of FIG. 8, there is no sensitive drop for low-frequency components. The result is that the main or RF signal transmitted from the center photosensitive section includes high-frequency components, which contribute to the jitter problem, at a percentage very much smaller than provided with the prior art photodetector. 
     It has been proven in practice that the jitter standard margin increases to an extent resulting in a 3% or more yield increase upon application of the invention to an optical pickup photodetector. 
     Although the non-reflective transparent cover film is shown and described as provided on the whole area of the surface of the photodetector, it is to be noted the non-reflective transparent cover film may be provided only on the photosensitive sections P1, P2 and P3. 
     While this invention has been described in conjunction with a specific embodiment thereof, it is evident that may alternatives, modifications and variations will be apparent to those skilled in the art. For example, the non-reflective transparent cover film is not limited to the double-layer structure of the SiO 2  film and the nitride film as long as it is non-reflective and transparent. Accordingly, it is intended to embrace all alternatives, modifications and variations that fall within the scope of the appended claims.