Abstract:
A PIN photodiode having a substrate, a first type electrode layer disposed on the substrate, a first layer of intrinsic material disposed over a portion of the first-type electrode layer, a first type window layer disposed over said intrinsic layer. An island shaped region of intrinsic material is disposed over the window layer and a dielectric layer disposed over the island region and at least the peripheral portion of said island shaped region whereby an opening is formed in the island shaped region. A dopant is diffused through the opening so as to form a PN junction that extends into the first layer of intrinsic material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates generally to PIN photodiodes. More particularly, the invention relates to a PIN photodiode semiconductor structure diode which has high reliability, with extremely low dark current, and suitable to be used in high temperature, high humidity environments, and processes for fabrication of such devices. 
         [0003]    2. Description of the Related Art 
         [0004]    Fiber optic communications typically employ a modulated light source, such as a laser, a photodiode light detector, and an optical fiber interconnecting the laser and the photodiode. The laser is modulated to emit light pulses that are transmitted over an optical fiber and received at a remote unit that includes a photodiode to convert the optical signal into an electrical signal. In particular, PIN diodes are widely used the photodiodes in the optical receiver for high-speed fiber optics communication. A typical lateral PIN diode fabricated by Zn diffusion has an exposed PN junction on the top surface. Traditionally, the exposed PN junctions on the surface of the devices are covered by dielectric for passivation. However, the dielectric protection often degrades under various reliability test environments such as high temperature, high humidity test (HTHH), etc. One of the most commonly used methods is Zn diffusion, where Zn atoms are diffused through a window layer and forms an active PN junction just inside the absorbing layer. A typical Zn diffused PIN photodiode will also create a side PN junction on the surface of the window layer, which is also a semiconductor material. 
         [0005]    Patterned area Zinc (Zn) diffusion has been widely employed for fabricating InP based high-speed PIN photodiodes. The commonly used epitaxial structure of the photodiode before diffusion is the layer sequence n-InP, intrinsic InGaAs and n++InP  103  (or with an undoped InGaAs top layer). In the traditional process, a dielectric layer is first deposited on top of the InP or the InGaAs layer and diffusion opening formed. The p-type doping is achieved by diffusing the Zn atoms directly (or via InGaAs layer) into the InP layer  103  through the diffusion openings in the dielectric layer. The final structure after diffusion is p-InP  104 ; Intrinsic InGa  102  and n++InP  101  PIN, which is typical for a PIN photodiode. 
         [0006]    One of the drawbacks for the traditional process is that there is an exposed PN junction on the surface of the top InGaAs or InP layer. Without re-depositing a dielectric layer for high quality passivation, the surface recombination current is potentially high, which gives rise to inconsistent operating dark current after fabrication. Even with good initial leakage characteristics, significant degradation of operation dark current often exists after aging, especially under conditions of high temperatures operating live (HTOL) tests of high temperature/high humidity reverse biased tests (THRB), where dielectric protection is normally weak if attacked by corrosion. 
         [0007]    Prior to the present invention, there has not been suitable means for protecting the PN junction in a PIN diode fabricated by Zn diffusion without the use of an additional dielectric sealing layer in the PIN photodiode. 
       SUMMARY OF THE INVENTION 
     1. Objects of the Invention 
       [0008]    It is an object of the present invention to provide an improved semiconductor structure for a PIN photodiode with a zinc diffusion region. 
         [0009]    It is another object of the present invention to provide an improved PIN photodiode with a protected PN junction. 
         [0010]    It is also another object of the present invention to provide a diffusion island for PIN photodiodes. 
         [0011]    It is also an object of the present invention to provide a process to provide a protected PIN junction in a PIN photodiode and thereby provide consistent fabrication and reliability of such devices. 
       2. Features of the Invention 
       [0012]    Briefly, and in general terms, the present invention provides a PIN photodiode with the following epitaxial structure: a substrate, a first type electrode layer disposed on the substrate, an intrinsic layer disposed over the first type electrode layer, a first type window layer disposed over the intrinsic layer, and an intrinsic layer disposed over the first type window layer. 
         [0013]    More particularly, the present invention provides a PIN photodiode having a substrate, a first type electrode layer disposed on the substrate, a first layer of intrinsic material disposed over a portion of the first type electrode layer, a first type window layer disposed over the intrinsic layer, and an island shaped region of intrinsic material disposed over said window layer. 
         [0014]    A dielectric layer is disposed over the island region and at least the peripheral portion of the island shaped region whereby an opening is formed in the island shaped region, and a dopant is diffused through the opening so as to form a PN junction that extends into the first layer of intrinsic material. 
         [0015]    In a second embodiment, the present invention provides a PIN photodiode having a substrate, a first type electrode layer disposed on the substrate, a first layer of intrinsic material disposed over a portion of the first type electrode layer, a first type window layer disposed over said intrinsic layer, a second type layer of intrinsic material disposed over the second type electrode layer. A dielectric layer is disposed over the second layer of intrinsic material having an opening, and a dopant is diffused through the opening, extending into the first layer of intrinsic material so as to form a PN junction. A metal electrode is also disposed over the second layer of intrinsic material around the opening in the dielectric layer. The top layer of intrinsic material is etched down so that the layer is depleted due to surface depletion. 
         [0016]    In another aspect, the present invention provides a processing method step for fabricating a PIN diode including (i) an etching followed by a diffusion step, and (ii) a diffusion step followed by an etching process. Both processes (i) and (ii) result in forming a passivated region in the last intrinsic layer away from the substrate over the PN junction, and a second type electrode layer is disposed over at least a portion of the intrinsic layer so as to form a PN junction. A peripheral passivation layer is disposed over the PN junction on the planar surface of the photodiode and generally surrounding the active region of the photodiode. 
         [0017]    Still, another aspect of the present invention is to provide a method of manufacturing a PIN photodiode comprising providing a substrate, forming a first type electrode layer on the substrate, forming an intrinsic layer on the first type electrode layer, forming a first type window layer on the intrinsic layer, forming a second type electrode layer disposed over at least a portion of the intrinsic layer so as to form a PN junction, and depositing a peripheral passivation layer disposed over the PN junction on the planar surface of the photodiode and generally surrounding the active region of the photodiode. 
         [0018]    The protection of the PN junction in the PIN diode as a result of the fabrication process of the present invention is a substantial improvement of the PN diode reliability and lifetime, particularly under extreme conditions such as those conditions simulated under testing at 85 degrees Centigrade and 85% humidity conditions. 
         [0019]    The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    These and other features and advantages of this invention will be better understood and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein: 
           [0021]      FIG. 1  is a cross-sectional view of the structure of the epitaxial stack of a lateral PIN photodiode construed in accordance with the prior art; 
           [0022]      FIG. 2A  is a cross-sectional view of a lateral PIN photodiode in accordance with the present invention during a first step of the fabrication process; 
           [0023]      FIG. 2B  is a top plain view of the lateral PIN photodiode of  FIG. 2A ; 
           [0024]      FIG. 3A  is a cross-sectional view of a lateral PIN photodiode during the second fabrication step in accordance with the present invention; 
           [0025]      FIG. 3B  is a top plan view of the lateral PIN photodiode of  FIG. 3A ; 
           [0026]      FIG. 3C  is an enlarged cross-sectional view of a portion of the lateral PIN photodiode of  FIG. 3A ; 
           [0027]      FIG. 4A  is a cross-sectional view of the lateral PIN photodiode during a subsequent fabrication step in accordance with the present invention; 
           [0028]      FIG. 4B  is a top plan view of the lateral PIN photodiode of  FIG. 4A ; 
           [0029]      FIG. 5A  is a cross-sectional view of the lateral PIN photodiode during a subsequent fabrication step in accordance with the present invention in which the circumferential edge of the p-contact metal is used to define the region of the diffusion island  104 A to be removed by etching, 
           [0030]      FIG. 5B  is a top plan view of the lateral PIN photodiode of  FIG. 5A ; 
           [0031]      FIG. 6  is a cross-sectional view of the structure of the epitaxial stack of a lateral PIN photodiode construed in accordance with the prior art; 
           [0032]      FIG. 7A  is a cross-sectional view of a lateral PIN photodiode in accordance with a second embodiment of the present invention during a first step of the fabrication process; 
           [0033]      FIG. 7B  is a top plan view of the lateral PIN photodiode of  FIG. 7A ; 
           [0034]      FIG. 8A  is a cross-sectional view of a lateral PIN photodiode of  FIG. 7A  during the second fabrication step in accordance with the present invention; 
           [0035]      FIG. 8B  is a top plan view of the lateral PIN photodiode of  FIG. 8A ; 
           [0036]      FIG. 9A  is a cross-sectional view of the lateral PIN photodiode of  FIG. 8A  during a subsequent fabrication step in accordance with the present invention; 
           [0037]      FIG. 9B  is a top plan view of the lateral PIN photodiode of  FIG. 9A ; 
           [0038]      FIG. 10A  is a cross-sectional view of the lateral PIN photodiode of  FIG. 9A  during a subsequent fabrication step in accordance with the present invention; 
           [0039]      FIG. 10B  is a top plan view of the lateral PIN photodiode of  FIG. 10A  during a subsequent processing step depicting the annular p-contact metal ring  210  overlying the circumferential edge position of the diffusion region  204 A; 
           [0040]      FIG. 11A  is a cross-section view of the lateral PIN photodiode of  FIG. 10A  during a subsequent processing step; and 
           [0041]      FIG. 11B  is a top plan view of the lateral PIN photodiode of  FIG. 11A . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0042]    Details of the present invention will now be described, including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of the exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of actual embodiments nor the relative dimension of the depicted elements, and are not drawn to scale. 
         [0043]    Referring to  FIG. 1 , there is shown a fragmentary, cross-sectional view of a semi-conductor structure or the epitaxial stack representing the initial layers of a lateral PIN photodiode during the fabrication process of the present invention that is depicted with generic first and second type electrodes. In particular, the photodiode  100  includes an n+ contact layer  101  and an intrinsic absorbing layer  102  disposed on the n+ contact layer. An n-type window layer  103  is disposed on the absorbing layer  102 , and an intrinsic layer  103  is disposed on the layer  103 . The n+ and n− regions  101  and  103  normally are doped to high carrier concentrations while the intrinsic region  102  typically has a small, residual n type carrier concentration. 
         [0044]      FIG. 2A  is a cross-sectional view of a lateral PIN photodiode in accordance with the present invention during a first step of the fabrication process. A circular diffusion island  104 A is formed on the surface of the semiconductor structure of  FIG. 1 . A diffusion mask  105  is applied over the entire surface of the wafer surrounding the diffusion island  104 A and concentrically overlapping the entire circumferential edge of the island. 
         [0045]      FIG. 2B  is a top plain view of the lateral PIN photodiode of  FIG. 2A , depicting the diffusion mask  105  overlapping the circumferential edge of the diffusion island  104 A. 
         [0046]      FIG. 3A  is a cross-sectional view of a lateral PIN photodiode during the second fabrication step in accordance with the present invention after AN diffusion. A p-type window layer  103 A is formed in the n-type window layer  103  below the diffusion island  104 A. A pn junction is formed both on the side  107  of the p-type layer  103 A, and on the bottom  106  of the layer. 
         [0047]      FIG. 3B  is a top plan view of the lateral PIN photodiode of  FIG. 3A  showing the location of the side pn junction  107  being located intermediate the edge of the diffusion island and the edge of the diffusion mask. 
         [0048]      FIG. 3C  is an enlarged cross-sectional view of a portion of the lateral PIN photodiode of  FIG. 3A  depicting the region between the diffusion island  104 A and the diffusion mask  105  in greater detail. In particular, the portion  109  of the diffusion island which is p-type is depicted, adjourning the depleted region  108  in the island  104 A and forming a side pn junction there between. 
         [0049]      FIG. 4A  is a cross-sectional view of the lateral PIN photodiode during a subsequent fabrication step in accordance with the present invention in which a p-contact metal ring region is lithographically defined and a contact metal is deposited to form the p-type contact to the device. 
         [0050]      FIG. 4B  is a top plan view of the lateral PIN photodiode of  FIG. 4A  depicting the annular p contact metal ring overlying the circumferential edge portion of the diffusion island  104 A. 
         [0051]      FIG. 5A  is a cross-sectional view of the lateral PIN photodiode during a subsequent fabrication step in accordance with the present invention in which the circumferential edge of the p-contact metal is used to define the region of the diffusion island  104 A to be removed by etching; 
         [0052]      FIG. 5B  is a top plan view of the lateral PIN photodiode of  FIG. 5A  showing the exposed p-type window layer  103 A after the diffusion island  104 A has been removed. 
         [0053]      FIG. 6  is a cross-sectional view of the structure of the epitaxial stack of a lateral PIN photodiode construed in accordance with the prior art and used as the initial structure in the process according to the second embodiment of present invention. Similar to  FIG. 1 , there is shown a fragmentary, cross-sectional view of a semiconductor structure or epitaxial stack with generic first and second type electrodes. In particular, the photodiode structure includes an n+ contact layer  201  and an intrinsic absorbing layer  202  disposed on the n+ contact layer. An n-type window layer  203  is disposed on the absorbing layer  202 , and an intrinsic layer  203  is disposed on the layer  203 . The n+ and n− regions  201  and  203  respectively are normally doped to high carrier concentrations, while the intrinsic region  202  typically has a small, residual n-type carrier concentration. 
         [0054]      FIG. 7A  is a cross-sectional view of a lateral PIN photodiode in accordance with a second embodiment of the present invention representing a first step of the fabrication process. A diffusion mask  205  with a circular aperture  204 A is applied over the surface of the semiconductor structure. Zn atoms are then diffused into the surface not covered by the mark. 
         [0055]      FIG. 7B  is a top plan view of the lateral PIN photodiode of  FIG. 7A  showing the circular diffusion area  204 A defined by the diffusion mask  205  through which the Zn atoms are to be diffused. 
         [0056]      FIG. 8A  is a cross-sectional view of a lateral PIN photodiode of  FIG. 7A  representing the second fabrication step in accordance with the present invention after zn diffusion, and removal of the diffusion mask. A p-type window layer  203 A is formed in the n-type window layer  203  below the diffusion aperture  204 A. A PN junction is formed both on the side  207  of the p-type layer  203 A, and on the bottom  206  of the layer in the intrinsic absorbing layer  206 . 
         [0057]      FIG. 8B  is a top plan view of the lateral PIN photodiode of  FIG. 8A  depicting the diffused area below the diffusion aperture  204 A. 
         [0058]    In the steps that follow, a ring shaped mask is used to form a metal ring around the diffusion aperture  204 A to make electrical contact to the p-type top layer  204 A after the diffusion. A predetermined portion  204 B of the intrinsic layer  204  outside of the ring is then etched down to a extent so that the portion  204 B is surface depleted. The portion  204 B serves as a passivation ledge covering the side PN junction  207  in the window layer under the intrinsic layer  204 . 
         [0059]      FIG. 9A  is a cross-sectional view of the lateral PIN photodiode of  FIG. 8A  depicting a subsequent fabrication step in accordance with the present invention in which the diffusion mask  205  is removed, leaving the intrinsic layer  204  as the top surface of the semiconductor structure. 
         [0060]      FIG. 9B  is a top plan view of the lateral PIN photodiode of  FIG. 9A  showing the top surface of the intrinsic layer  204 , with a circular portion  204 A of the intrinsic layer  204  which has been diffused. 
         [0061]      FIG. 10A  is a cross-sectional view of the lateral PIN photodiode of  FIG. 9A  depicting a subsequent fabrication step in accordance with the present invention in which a p-contact metal ring region is lithographically defined and a contact metal  210  is deposited to form the p-type contact to the device. 
         [0062]      FIG. 10B  is a top plan view of the lateral PIN photodiode of  FIG. 10A  depicting the annular p-contact metal ring  210  overlying the diffused portion  204  but interior of the circumferential edge of the diffusion region  204 A; 
         [0063]      FIG. 11A  is a cross-section view of the lateral PIN photodiode of  FIG. 10A  during a subsequent fabrication step in accordance with the present invention in which the outer circumferential edge of the p-contact metal ring  210  is used to define a ledge, and a portion of the intrinsic region  204  and  204 A is subsequentially removed by etching, in particular leaving a thin surface depletion ledge  204 B extending circumferentially around the p-metal contact ring  210 . 
         [0064]      FIG. 11B  is a top plan view of the lateral PIN photodiode of  FIG. 11A  showing the p-metal contact ring  210  and the top surface of the surface depletion ledge after etching a portion of the layer  204 . The side PN junction  207  is thereby protected without the use of a dielectric layer. 
         [0065]    It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above. 
         [0066]    While the invention has been illustrated and described as embodied in a device and method for making a PIN photodiode with a zinc diffusion region, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
         [0067]    Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various application without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should, and are intended to, be comprehensive within the meaning and range of equivalence of the following claims.