Photodetector and device employing the photodetector for converting an optical signal into an electrical signal

Abstract

An anti-reflective coating having a composite layer of silicon nitride and silicon dioxide may be formed over the entire photosensitive region of the photodetector to minimize the amount of reflection. The composite layer comprises a silicon nitride layer and a dielectric layer contiguous to the silicon nitride layer. The anti-reflective coating may be formed in a CMOS process for fabricating the PN junction in the photodiode and CMOS devices for amplifying the photodetector signal, where the polysilicon gate layer is used as a etch stop. The P+ or N+ material in the PN junction of the photodiode has a distributed design where two portions of the region are separated by a distance in the range of Xd to 2Xd, where Xd is the one-sided junction depletion width, to enhance the electric field and to reduce the distance traveled by the carriers for enhancing bandwidth. A heavily doped region of the opposite type may be added between the two portions to further enhance the electric field. A mask is used to shield a portion of the substrate in which the photodetector region has been or is to be formed when other portions of the substrate region are implanted with a dopant to adjust at least one of the threshold voltages of the other portions. The mask prevents the photodetector region from being affected by such implant.

What is claimed is:  
     
         1 . A photodetector comprising: 
 a semiconductor substrate having a photosensitive region on a surface of the substrate; and    an anti-reflective filter over the entire region, said filter comprising a first layer of silicon nitride and a second dielectric layer contiguous with the first layer.    
     
     
         2 . The photodetector of    claim 1   , said second layer in contact with the region.  
     
     
         3 . The photodetector of    claim 2   , said region comprising a PN junction, said second layer in contact with the junction to reduce leakage current.  
     
     
         4 . The photodetector of    claim 2   , said region comprising a PN junction.  
     
     
         5 . The photodetector of    claim 4   , said region comprising: 
 a first region of semiconductor material of a first type; and    a second region of semiconductor material of a second type forming the PN junction with the first region;    said photodetector further comprising means for applying a reverse bias voltage across the junction;    wherein the first region has at least two portions that are spaced apart by not more than twice the one-sided junction depletion width at said reverse bias voltage.    
     
     
         6 . The photodetector of    claim 1   , said first and second layer having thicknesses that are substantially optimized for minimizing the reflection of light at wavelengths of 650 and 790 nm.  
     
     
         7 . The photodetector of    claim 6   , said first layer having a thickness in the range of 7000±500 Angstroms, said second layer having a thickness in the range of 2400 to 2850 Angstroms.  
     
     
         8 . The photodetector of    claim 1   , said coating further including a transparent package layer over said silicon nitride layer, said package layer having an index of refraction in the range of about 1.52 to 1.57.  
     
     
         9 . The photodetector of    claim 1   , said second layer comprising silicon dioxide.  
     
     
         10 . A method for making a photodetector comprising: 
 providing a structure having a photosensitive region on or in a semiconductor substrate, a masking layer over said region and a material over said masking layer;    removing from said structure the material over said masking layer using said masking layer as a mask;    removing said masking layer; and    forming a first layer of silicon nitride and a second layer of dielectric material contiguous with the first layer over said region, the second layer having a predetermined thickness.    
     
     
         11 . The method of    claim 10   , wherein said forming forms the first and second layers over substantially the entire photosensitive region.  
     
     
         12 . The method of    claim 10   , wherein said providing provides a structure having a polysilicon layer over the substrate, a portion of said polysilicon layer being the masking layer.  
     
     
         13 . The method of    claim 10   , wherein said providing provides a CMOS structure having a polysilicon gate layer over the substrate, and includes forming said masking layer together with the polysilicon gate layer.  
     
     
         14 . The method of    claim 10   , wherein said providing provides a CMOS structure having a polysilicon gate layer and a capacitor over the substrate, said capacitor comprising a first and a second polysilicon layer, and wherein said providing includes forming said masking layer together with the first or the second polysilicon layer or together with the polysilicon gate layer.  
     
     
         15 . The method of    claim 14   , wherein the polysilicon gate layer and said first polysilicon layer are formed together, and said masking layer and said second polysilicon layer are formed together.  
     
     
         16 . The method of    claim 10   , said region comprising a PN junction, wherein said forming forms the second layer in contact with the junction to reduce leakage current.  
     
     
         17 . The method of    claim 10   , wherein said forming forms said first and second layer so that said first layer has a thickness in the range of 7000±500 Angstroms, and said second layer has a thickness in the range of 2400 to 2850 Angstroms.  
     
     
         18 . The method of    claim 10   , wherein said forming forms the second layer so that the second layer includes a silicon dioxide material.  
     
     
         19 . A photodiode comprising: 
 a first region of semiconductor material of a first type;    a second region of semiconductor material of a second type forming a PN junction with the first region;    means for applying a reverse bias voltage across the junction;    wherein the first region has at least two portions that are spaced apart by not more than twice the one-sided junction depletion width at said reverse bias voltage.    
     
     
         20 . The photodiode of    claim 19   , wherein the at least two portions are spaced apart by not less than the one-sided junction depletion width at said reverse bias voltage.  
     
     
         21 . The photodiode of    claim 19   , wherein the at least two portions are spaced apart by not more than 15 microns.  
     
     
         22 . The photodiode of    claim 19   , said photodiode further comprising a third region between the two portions of the first region, said third region comprising a heavily doped semiconductor material of a second type.  
     
     
         23 . The photodiode of    claim 22   , said two portions including N+ material and said third region comprising P+ material.  
     
     
         24 . The photodiode of    claim 21   , said two portions including P+ material and said third region comprising N+ material.  
     
     
         25 . The photodiode of    claim 19   , further comprising an anti-reflective filter over the entire first and second regions, said filter comprising a first layer of silicon nitride and a second dielectric layer contiguous with the first layer.  
     
     
         26 . The photodiode of    claim 19   , wherein said first and second regions form a surface junction.  
     
     
         27 . The photodiode of    claim 19   , wherein said first and second regions form a buried junction.  
     
     
         28 . A device for reading recorded information from a surface of a disk, comprising: 
 means for supplying radiation to the surface; and    a photodiode for sensing radiation supplied by the supplying means and modified by the disk;    said photodiode including:    a first region of semiconductor material of a first type;    a second region of semiconductor material of a second type forming a PN junction with the first region;    means for applying a reverse bias voltage across the junction;    wherein the first region has at least two portions that are spaced apart by a spacing in the range of about 5 to 15 microns.    
     
     
         29 . The photodiode of    claim 28   , said photodiode further comprising a third region between the two portions of the first region, said third region comprising a heavily doped semiconductor material of a second type.  
     
     
         30 . The photodiode of    claim 29   , said two portions including N+ material and said third region comprising P+ material.  
     
     
         31 . The photodiode of    claim 29   , said two portions including P+ material and said third region comprising N+ material.  
     
     
         32 . The photodiode of    claim 28   , further comprising an anti-reflective filter over the entire first and second regions, said filter comprising a first layer of silicon nitride and a second dielectric layer contiguous with the first layer.  
     
     
         33 . A device for converting an optical signal into an electrical signal, comprising: 
 means for supplying light to the surface; and    a semiconductor substrate having a photodetector region that provides an electrical signal in response to a light signal, and a circuit region for processing the electrical signal, said circuit region comprising only CMOS devices.    
     
     
         34 . The device of    claim 33   , further comprising an anti-reflective filter over the entire photodetector region, said filter comprising a first layer of silicon nitride and a second dielectric layer contiguous with the first layer.  
     
     
         35 . The photodiode of    claim 34   , said circuit region having a polysilicon gate layer, said filter having been formed using as a mask a polysilicon layer over the photodetector region, said polysilicon layer formed together with the polysilicon gate layer.  
     
     
         36 . The device of    claim 33   , further comprising a capacitor in the circuit region, said circuit region having a polysilicon gate layer, wherein said capacitor comprises a first and a second polysilicon layer, said first polysilicon layer formed together with the polysilicon gate layer, said filter having been formed using as a mask a third polysilicon layer over the photodetector region, wherein said second and third polysilicon layers are formed in the same processing step.  
     
     
         37 . A method for making a photodetector comprising: 
 processing a semiconductor substrate to provide a photodetector region that provides an electrical signal in response to a light signal, and a circuit region for processing the electrical signal, said circuit region comprising only CMOS devices, said processing including:    providing a mask to shield a portion of the substrate in which the photodetector region has been or is to be formed; and    implanting said circuit region with a dopant to adjust at least one of its threshold voltages, but not said portion of the substrate because of the mask.    
     
     
         38 . The method of    claim 37   , wherein said implanting is performed prior to formation of the photodetector region and during formation of the CMOS devices.