Abstract:
The present disclosure provides systems and methods for configuring and constructing a single photo detector or array of photo detectors with all fabrications circuitry on a single side and an architecture that enables the laser step to be the final step or a late step in the fabrication process. Both the anode and the cathode contacts of the diode are placed on a single side, while a layer of laser treated semiconductor is placed on the opposite side for enhanced cost-effectiveness, photon detection, and fill factor.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. provisional application No. 61/034,313 filed on Mar. 6, 2008, hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to laser treated semiconductor diode structure and design. In particular, the present disclosure relates to laser-treated semiconductor diodes with single side fabrication on bulk material. 
       BACKGROUND 
       [0003]    Current laser treated semiconductor diodes use a combination of top-side and back-side contact schemes. Combination designs require additional lithography steps following the laser step to place contacts on the top-side. The top side is generally the side of a photo-responsive semiconductor device that is exposed to a source of light or electromagnetic radiation of interest, for example in a sensor or detector device. The additional lithography following the laser step limits the number of fabrication houses that can produce laser treated semiconductor diodes, because many fabrication houses do not allow re-entry of partially processed material. In many cases, the company that performs the lithography is a different company than the company that performs the laser step and the steps are performed at separate locations. 
         [0004]    In addition, fill factor is an important parameter in area array image detector performance. Combination designs by their very nature require that contacts be placed on the top side of the detector, taking up space that could be used by the laser treated semiconductor layer for photon detection thereby reducing fill factor. 
       SUMMARY 
       [0005]    In some embodiments, some or all of the traditional fabrication steps are completed before the laser processing step, allowing the laser step to become the final or a late step in the process. By making the laser step the final or a late step in the process, it would be unnecessary to insert partially processed material into a fabrication house, thereby increasing the amount of potential fabrication houses available to produce laser treated semiconductor diodes. 
         [0006]    In some embodiments, vertically stacking the laser treated semiconductor above a silicon integrated circuit provides a greater fill factor, in some embodiments almost or at 100% fill factor, and enables the laser processing step to be the final step or a late step since all or substantially all electrical contacts are located on the back side of the device and no or little additional top-side structure is required. 
         [0007]    In some embodiments, the resulting arrangement of the electromagnetic field lines and potentials within such a device provides advantageous design and operation characteristics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates a cross sectional area of an exemplary embodiment of a laser treated semiconductor diode with back-side metal contact pads. 
           [0009]      FIG. 2  illustrates a back view of a laser treated semiconductor diode with back-side metal contact pads. 
           [0010]      FIG. 3  illustrates an exemplary array of laser treated semiconductor diodes with back-side metal contact pads. 
           [0011]      FIG. 4  illustrates a cross sectional area of an exemplary embodiment of a laser treated semiconductor diode with back-side metal contact pads including an exemplary circuit diagram approximation of the diode in operation. 
           [0012]      FIG. 5  illustrates an exemplary diagram of the field lines in a cross sectional area during operation of a laser treated semiconductor diode with back side metal contact pads. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates a cross sectional area of an exemplary embodiment of a laser treated semiconductor diode  100  with back-side metal contact pads  110  and  112 . In the exemplary embodiment, the laser treated semiconductor diode is an N-type diode, though persons skilled in the art will recognize that the diode  100  may be with other types (such as P-type, Schottky diodes, etc.). The exemplary laser treated semiconductor diode has a front  102  and a back  104  side. The exemplary laser treated semiconductor diode  100  contains a bulk layer of silicon  106  between the front  102  and the back  104  sides. The bulk layer  106  may be doped with N-type doping, depending on the doping of the laser treated layer  108 . In the exemplary embodiment, the front  102  side is covered by a laser treated semiconductor layer  108  and connected to the bulk layer  106 . The laser treated semiconductor layer  108  is photo active and has an increased sensitivity as compared to an undoped, untreated layer. The back  104  side may be connected to aluminum cathode  110  and anode  112  contacts and may be covered by a thin layer of SiO2 TEOS  114 . The back  104  side may also be doped using N-Type dopants  1116  under the cathode contact  10  and P-type dopants  118  under the anode contact  112 . In this embodiment, the laser treated semiconductor layer  108  on the front  102  side acts as a cathode, while the back  104  side P-type doped section  118  acts as an anode. Additionally, an embodiment is contemplated wherein the laser treated semiconductor diode  100  is devoid of ohmic contacts. 
         [0014]      FIG. 2  illustrates a back view of a laser treated semiconductor diode  100  with back-side metal contact pads  110  and  112 . In the exemplary embodiment, aluminum contacts  110  and  112  are coupled to the back side  104 . The outer contact  110  is coupled to the N-type doping area of the back side  102  and acts as a cathode contact point for the diode  100 . The inner contact  112  is connected to the P-type doping area of the back  104  side and acts as an anode contact point for the diode  100 . Each of the bounded regions (e.g.,  112 ) in an array of such regions can act as a discrete pixel. For example, the regions  112  can each represent a pixel providing a color-sensing element in a color imager or a magnitude-sensing element in a monochromatic or gray-scale imager. 
         [0015]    In the exemplary embodiment, the anode contact  112  is electrically isolated from the cathode contact  110 . A diode using a back side contact configuration allows for single sided fabrication, reducing cost and reducing complexity of manufacture. Also, a diode using a back side contact configuration may be substantially fully fabricated before the laser step process is performed. Fabrication before the laser step removes the need to re-enter the material into the foundry after the laser step, eliminating the contamination risk typically associated with the re-entry of partially processed material and increasing the number of available fabrication partners. 
         [0016]      FIG. 3  illustrates an exemplary array of laser treated semiconductor diodes with back-side metal contact pads. In the exemplary embodiment, the cathode contacts  110  are all electrically connected and form a common-cathode configuration. In the exemplary embodiment, the cathode and anode connections are arranged in a grid pattern where the cathode contact  10  is configured in a square grid pattern and the anode contacts  112   a  and  112   b  are configured as individual square contacts within the cathode grid pattern  110 . An array using the above configuration may be vertically bonded to readout circuitry and create a fully functional imager. 
         [0017]      FIG. 4  illustrates a cross sectional area of an exemplary embodiment of a laser treated semiconductor diode with back-side metal contact pads including an exemplary circuit diagram approximation of the diode in operation. During operation, the cathode contact  110  may be positively biased in relation to the anode contact  112 . The bias voltage (“V-bias”) can be about 1 to 10 volts in some embodiments. In some embodiments the bias voltage can be about 3 to 5 volts. In some embodiments, the required bias voltage is substantially less than that in corresponding conventional devices. When the cathode is sufficiently positively biased, it will create an electric field that extends through the bulk layer  106  to the laser treated semiconductor layer  108  on the front  102  side, attracting the mobile electrons and depleting the laser treated semiconductor layer  108 . The anode contact  112  is held negative with respect to the cathode contact  110  and the laser treated semiconductor layer  108 . During operation, the bulk layer  106  may be modeled as a series resistance  402  between the cathode contact  110  and the laser treated semiconductor layer  108 . While the electric field generated between the anode and the cathode by their P-N junction  404  has a small relative volume, the field generated by the P-N junction  406  between the laser treated semiconductor layer  108  and the anode contact  112  has a larger, or even significantly larger volume. Therefore, electron hole/pairs generated by photon absorption at or near the laser treated semiconductor layer  108  are separated in this field. The electrons travel to the cathode contact  110  through the laser treated semiconductor layer  108 , while the holes travel to the anode contact  112  through the field established by the P-N junction  406  between the laser treated semiconductor layer  108  and the anode contact  112 . 
         [0018]    While the exemplary diode shown was a P-N junction type diode, many other diode types may be implemented as discussed above, including Schottky diodes and P-type implementations. The p-type implementation may be implemented by reversing the dopant type throughout the diode and reversing the bias applied to the diode during use. The P-type implementation will function similarly to the N-type implementation except that the electron and hole flow paths will reverse direction. 
         [0019]      FIG. 5  illustrates an exemplary diagram of the field lines  119 , in a cross sectional area during operation of a laser treated semiconductor diode with back-side metal contact pads. Each field line represents a contour of equal voltage, or representing lines of equipotential. A tighter spacing of field lines indicates a stronger electromagnetic field.