Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to photodiodes. In particular, it pertains to avalanche photodiodes with large surface areas made with planar technology. 
     2. Description of the Prior Art 
     Previous segmented photodetectors, such as quadrant detectors, were made of silicon. Silicon, an indirect gap semiconductor, has a relatively deep penetration depth. The depth varies for different wavelengths. The depth is 200 μm, for 1.06 μm light. Photocarriers are generated within a volume equal to the surface area times the penetration depth. Therefore, in multi-element silicon detectors the active areas have to be separated by 100-200 μm to limit cross-talk. In some infrared target seeker optics, it is necessary to be able to observe images smaller than the element separation of silicon detectors. 
     Thus, silicon photodetectors require a trade-off between the possibility of losing the image as it moves across the strip separating the quadrants or sectors of the photodetector and losing the image because of cross-talk causing a false tracking signal. 
     The penetration depth of 1.06 μm radiation in direct gap semiconductors, such as GaAs l-x  Sb x , is 1-5 μm. If the direct gap semiconductor material exists as an epitaxial layer on a substrate of wider gap energy, then the substrate material can serve as a filter. 
     SUMMARY OF THE INVENTION 
     A large area avalanche quadrant photodiode is made with planar technology. An epitaxially grown heterostructure on a semiconductor substrate serves as a self-filtering photodetector. The epitaxial layer side of the heterostructure is cycled through three processes each consisting of applying photoresist, exposing the photoresist through a mask, developing the photoresist, and removing the photoresist. The first mask defines an area to be coated with metallic conductor. The second mask defines the area that serves as the active photodetector area. The third mask provides areas for electrical contacts to the metallic conductor. 
     Between the second and third cycles described above, the epitaxial side of the photodetector is bombarded with protons. The proton bombardment is limited to areas surrounding the quadrants. The proton bombardment produces semi-insulating volumes of depth sufficient to reach into all regions of the structure in which photocarriers are generated. These semi-insulating volumes eliminate edge breakdown and prevent cross-talk between quadrants. 
     Ohmic contacts are sputtered on the substrate side of the heterostructure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a superposition of the areas defined by the three masks; 
     FIG. 2 shows a cross-section of the invention along line A--A of FIG. 1; 
     FIG. 3 is an enlarged view of the center area in FIG. 1; and 
     FIG. 4 shows a cross-section similar to FIG. 2 that is suitable for polychromatic light. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Layers are grown epitaxially on a semiconductor substrate. The substrate can be Sn doped GaAs with a carrier concentration, N D . N D  can be 10 18  cm -3 . The epitaxial layers can be GaAs l-x  Sb x  where x equals the mole fraction of GaSb. 
     In order to prevent cross-talk between quadrants, the depth of the volume of semi-insulating material separating the quadrants must be greater than the penetration depth of the light. The semi-insulating materials produced by proton bombardment usually do not exceed 10 μm in depth. Since the penetration depth of 1.06 μm light in GaAs l-x  Sb x  is 1-5 μm, while the corresponding value for Si is 200 μm, cross-talk between quadrants in Si detectors can not be prevented using proton bombardment. 
     The sample is first dipped in photoresist to coat both sides. It is then pulled from the photoresist. A pulling rate of approximately 1.75 cm/min is acceptable for a positive photoresist such as Shipley AZ1350J photoresist. Kodak 747 may also be used at this rate. 
     The sample is air dried for at least ten minutes. The sample is then baked in a circulating air oven at 100° C. for an additional ten minutes. Alternative drying and baking procedures can be used. The listed ones are exemplary. 
     The photoresist covering the epitaxial layer side of the sample is exposed through a Mask B, FIG. 1. The exposing light has a preselected wavelength λ and a preselected exposure time, t, for example 30 seconds. Mask B is shown divided into four quadrants. Any number of segments can be used. The circular shape is an arbitrary choice. As shown in FIG. 1, Mask B has radius R B , identified by quadrant areas 26, and the quadrants are separated by width D as shown in FIG. 3. 
     The exposed photoresist is then developed. Developing for 60 seconds in a 5:1 mixture of water and Shipley AZ 351 is one method of developing the photoresist. The developed photoresist is then post-baked for thirty minutes at 100° C. The sample is then etched. This can be done with a 1:1 solution of HCl:H 2  O. The sample can be dipped in the solution for one to two minutes and then rinsed with deionized water. 
     The sample is now mounted on an electroplating apparatus. A conductive surface, such as palladium, is plated on the exposed GaAs l-x  Sb x . The area that was exposed through Mask B is now covered with palladium, area 26 in FIG. 1. After electroplating, the sample is rinsed in H 2  O. The unexposed photoresist is removed with acetone, followed by alcohol and deionized water rinses. 
     A layer of photoresist is now applied to the entire wafer. This layer can also be formed by dipping the sample in Shipley AZ 1350J photoresist and then withdrawing it at approximately 15.7 cm/min. A thick layer of photoresist, at least 4 μm thick, is obtained. The layer is dried and baked in a manner similar to that previously described. 
     The layer is now exposed through Mask A. Once again, a preselected wavelength and a preselected time period are used. As shown in FIG. 1, Mask A is not identical to Mask B. For this particular example, Mask A is the negative of Mask B and the radius, R A , is less than R B . FIG. 3 shows the detail of the center of Masks A and B. 
     The exposed photoresist layer is then developed as previously described. The development stage is again followed by baking for thirty minutes at 100° C. After these steps, quadrants of photoresist cover nearly all the area covered by palladium. The wafer is now bombarded with 400 Kev protons to produce semi-insulating material beneath all the area not covered by photoresist, and leaving palladium Schottky barriers 22. After bombardment the photoresist is removed. 
     The sample is again dipped into and pulled from the photoresist. Shipley AZ1350J or Kodak 747 photoresist can be used at a pulling rate of 2.5 cm/min. This layer is air-dried and baked as described previously. If Kodak 747 photoresist is used, the baking step is changed to 70° C. for twenty minutes. Different photoresists have different developing parameters. 
     The layer is then exposed through a Mask C to yield segments 20 of FIG. 1. There is one segment for each quadrant. Exposure at a selected wavelength for 20 seconds is usually adequate. 
     The exposed layer is then developed and baked as previously described. The exposed segments are then etched to allow for better electrical contact to the sample. Etching in 1:1, HCl:H 2  O for one minute is sufficient. Electrical contacts are then electroplated on the etched areas. Thick gold pads are electroplated at 55°-60° C. After electroplating, the sample is throughly rinsed with deionized water. The undeveloped photoresist is then removed by plasma etching in O 2  at 1.5 torr for twenty minutes. 
     Substrate 16, FIG. 2, is sputter etched, and ohmic contacts 24 are sputtered onto substrate 16 through a metal mask, not shown. Satisfactory ohmic contacts 24 are obtained using a 12% Ge-88% Au target. The edges of the sample are then removed to reduce leakage current. They are removed by cleaving. 
     The finished sample is then mounted substrate side down in a standard TO can, not shown, which has a hole in the center. The epoxy mounting provides electrical contact to ohmic contacts 24. Gold wires are then bonded to the gold segments 20 of FIG. 2. Either thermal or compression bonding can be used. 
     Incident light 10 passes through an opening in the TO can and strikes substrate 16. Light 10 is filtered by substrate 16 and lattice matching layers 19A and 19B. As many lattice matching layers as desired can be used. Lattice matching layers 19A and 19B have a wider energy gap than epitaxial layer 18, which is the active region absorbing the specific monochromatic wavelength. Upon absorption of light 10 in the active layer 18, photo carriers are generated. The carriers are collected by the palladium electrode. Layers 18 and 19 are not drawn to scale. Substrate 16 is thicker than the entire epitaxial stack, layers 18 and 19. A photocurrent is induced in the circuit comprised of the heterostructure, the palladium Schottky barrier 22, the ohmic contacts 24, and a current amplifier not shown. Dashed lines 28 define the proton bombarded volumes. A photocarrier generated in section 30 can not cross volumes 28 and appear in an other quadrant section 32. 
     FIG. 4 shows a modification of the device for polychromatic light. If polychromatic light is incident on the device, then absorption in lattice matching layers 19A and 19B may occur. As shown in FIG. 2, photocarriers in lattice matching layers 19A and 19B can drift laterally, this can result in cross-talk. To prevent this, proton bombarded volumes delineated by lines 28 must reach into substrate 16. Photocarriers generated by absorption in substrate 16 occur near the surface. The thickness of substrate 16 keeps them from reaching the epitaxial layers.

Technology Category: 4