Abstract:
A photedetector uses a semiconductor on insulator (SOI) substrate having an optical grating over the silicon semiconductor to change the direction of incident light to divert it into the silicon which functions as waveguide. The underlying insulator operates as one boundary of the waveguide and the overlying grating operates as the other. Photodetector electrodes are placed in the silicon, which puts them in close proximity to the carriers that are generated by the light entering the silicon waveguide.

Description:
RELATED APPLICATIONS 
     This application is related to application SC11378TP, entitled “An Opto-Coupling Device Structure and Method Therefor,” and assigned to the assignee hereof. 
     FIELD OF THE INVENTION 
     The invention relates to optical devices and more particularly to photodetectors made as an integrated circuit. 
     RELATED ART 
     A continuing object of integrated circuit manufacturing is to increase the speed of operation. One of the issues relating to using integrated circuits is the interconnect with the integrated circuit itself. The interconnect itself has and creates speed limitations. Some of these relate to the physical interconnect and others relate to distances that must be covered by the signal that is either received or transmitted by the integrated circuit. One of the techniques that is being studied to improve this is the use of light as opposed to an electrical signal for the source of information for the integrated circuit. The typical integrated circuit has a silicon substrate, which does provide the capability, albeit a not very good one, of being a photodetector. One of the reasons silicon is not considered a particularly good photodetector is that its absorption coefficient is low compared to some other materials such as germanium. 
     The technique for detecting light using silicon or germanium is to detect carriers generated by the incident light. The incident light must be at frequency that is absorbed by the material as opposed to frequency at which the light is passed. In silicon the frequency of the light that is absorbed has a wavelength less than 1.1 microns, whereas frequencies with a wavelength greater than 1.1 microns are passed. One standard frequency below the 1.1 micron wavelength is the standard for local area networks (LAN), which has a wavelength of 850 nanometers. The light that has a frequency that can be absorbed by the silicon, which generates holes and electrons as the light penetrates and is adsorbed by the silicon. These carriers are then collected to perform the detection of the incident light using biased doped regions in the silicon. The bias is sufficient to fully deplete the substrate or well regions. The incident light carries the information that is to be processed by the integrated circuit. 
     The efficiency of the detector is increased if more of these carriers, which are generated by the light, can be collected. One of the difficulties with silicon is that about 98% of the carriers are generated over about 20 microns of distance, i.e., the light penetrates into the silicon about 20 microns before it is substantially fully adsorbed. It is difficult to collect most of these carriers, the 98%, rapidly. The electric field provided by the biased doped regions attracts the carriers. As the distance between the doped regions and the carriers increases, the electric field diminishes. These carriers that are in the low electric field areas are too slow in reaching the doped regions where they can be detected. The result is a rate of detection which is not a fast enough to provide a significant improvement over that available by using normal electrical signals. 
     To have the requisite speed of detection, the collectors of the carriers must be in closer proximity to the generation of the carriers. A technique for improving the speed was to isolate many of the carriers that were generated relatively far from the doped regions using conventional SOI type substrates. Thus, the incident light would generate carriers at the surface and continue generating carriers but most of the carriers would be generated below the insulating layer which is part of an SOI substrate. This improves the speed because only the carriers that were generated close to the electrodes reached the doped regions, but most of the carriers were generated below the insulating layer so that the detection itself was difficult. Detection typically includes biasing doped regions to attract the holes or electrons that are the carriers that are generated by the incident light. The fact that these doped regions are biased inherently makes it difficult to detect very small amounts of charge. Thus, the more charge that is available for detection, the more effective the detection will be. 
     Thus there is seen a need for a photodetector in a semiconductor that can be fast enough and reliable enough to detect signal information from light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not by limitation in the accompanying figures, in which like references indicate similar elements, and in which: 
     FIG. 1 is a cross section of a portion of an integrated circuit at a stage in processing according to an embodiment of the invention; 
     FIG. 2 is a cross section of the integrated circuit of FIG. 1 at a subsequent stage in processing; 
     FIG. 3 is a top view of the integrated circuit of FIG. 2; 
     FIG. 4 is a cross section of integrated circuit of FIG. 2 at a subsequent stage in processing; 
     FIG. 5 is a top view of the integrated circuit of FIG. 4; 
     FIG. 6 is a cross section of the integrated circuit of FIG. 4 at a subsequent stage in processing; 
     FIG. 7 is a cross section of a integrated circuit of FIG. 6 at a subsequent stage in processing; 
     FIG. 8 is a cross section of an integrated circuit at a stage in processing according to an alternative embodiment; 
     FIG. 9 is a cross section of an integrated circuit of FIG. 8 at a subsequent stage in processing; 
     FIG. 10 is a cross section of an integrated circuit of FIG. 9 at subsequent stage in processing; 
     FIG. 11 is a top view of an arrangement of grating features according to an embodiment of the invention; 
     FIG. 12 is a cross section of the semiconductor substrate having a photodetector and processing circuitry according to a preferred embodiment of the present invention; and 
     FIG. 13 is a cross section of a portion of the detector shown in FIG.  12 . 
    
    
     Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention. 
     DETAILED DESCRIPTION 
     A photodetector is made using a semiconductor as an absorber and detector. At the surface of a substrate that has the semiconductor is a grating that is used to redirect the light. Below the silicon is an insulating layer so that the silicon operates as a waveguide. The incident light is redirected in the direction of the silicon. The detector is located in the silicon in the form of doped regions. Because the light is redirected so that it is contained within the silicon area, all the carriers that are generated are in close proximity to the doped regions, which operate as collectors of the carriers. This provides for an efficient photodetector that is both fast and provides for relative easy detection. 
     Shown in FIG. 1 is a portion of an integrated circuit  10  comprising a insulating region  12 , a semiconductor region  14 , a patterned photoresist  16 , an N-doped region  18 , an N-doped region  20 , and an N-doped region  22 . Pattern photoresist  16  results from patterning a photoresist layer which was deposited over semiconductor region  14 . Typically an oxide layer, not shown, would be between the photoresist and the silicon. After patterning this photoresist layer, photoresist  16  is provided. An N+ implant then occurs to form N+ regions  18 ,  20  and  22 . Insulating region  12  and semiconductor region  14  comprise a silicon on insulator (SOI) substrate, which is readily available in the industry. 
     Shown in FIG. 2 is integrated circuit  10  after pattern photoresist  16  has been deposited and another photoresist layer has been deposited and patterned to result in patterned photoresist layer  24 . Pattern photoresist layer  14  provides as a mask for a P-type implant, which results in P-doped regions  26  and  28 . P-doped regions  26  and  28  are interleaved between N-doped regions  18 ,  20  and  22 . 
     Shown in FIG. 3 is a top view of an additional portion of integrated circuit  10  showing the interleaving of N and P doped regions. This shows N-doped regions having a common connection at the bottom and P-doped regions having a common connection at the top. This portion of integrated circuit  10  shows the target area of incident light. 
     Shown in FIG. 4 is after additional processing of integrated circuit  10 . A nitride layer  30  is deposited over semiconductor region  14 . A photoresist layer is then deposited over nitride layer  30  and patterned to form a pattern photoresist layer comprising photoresist pillars  32 ,  34 ,  36 ,  38 ,  40 ,  42 ,  44 ,  46 ,  48 ,  50  and  52 . These pillars  32 - 52  are substantially cylindrical in shape, which is reasonably easy to achieve. Shown in FIG. 5 is a portion of integrated circuit  10 , as a top view at the stage in processing shown in FIG. 4., which shows the pillars in a matrix fashion. 
     Shown in FIG. 6 is integrated circuit  10  at a stage in processing after a partial etch of photoresist pillars  32 - 48  and nitride layer  30 . This shows that pillars  32 - 52  are not only being reduced in height but also being reduced in diameter. The etch of the nitride is directional but the photoresist is nonetheless etched laterally as well so that the photoresist pillars, whose sidewalls are exposed, are reduced in diameter to expose more and more of the nitride. But the nitride that has received the maximum exposure is that area which is between the original pillars. As the pillars decline in diameter additional nitride is exposed. Thus, there is a slope formed in the nitride toward the photoresist. The etch of the nitride between photoresist pillars continues and the result is shown in FIG.  7 . This shows hemispherical shaped nitride features  54 ,  56 ,  58 ,  60 ,  62 ,  64 ,  66 ,  68 ,  70 ,  72  and  74 . These nitride features are positioned to have a periodicity chosen for the frequency that is desired to be passed between semiconductor region  14  and an area above nitride features  54 - 74 . Nitride features  54 - 74  comprise an optocoupling grating. 
     Semiconductor region  14  operates as a waveguide with nitride features  54 - 74  operating as the opto-coupling diffraction grating. The silicon of semiconductor region  14  has a higher index of refraction than the silicon oxide of insulating layer  12 . Another insulating layer may also be used which has a lower index of refraction than that of the semiconductor layer above it. The nitride features also have a lower index of refraction than the semiconductor layer  14 . 
     There is a dimension of periodicity of the nitride features that will result in light traveling laterally in patterned semiconductor region  14  which will also pass through the diffraction grating of nitride features  54 - 74 . The typical angle of incident of light with respect to the opt-coupling grating is 80 degrees plus or minus 5. Thus, it is near vertical but not quite. The periodicity of nitride features  54 - 74  is selected based upon the frequency of the incident light at nominally 80 degrees. A typical and standard frequency for local area networks (LAN) is light with a wavelength of about 850 nanometers (nm) for the nominal angle of 80 degrees for the light entering the opto-coupling diffraction grating. The period using nitride for the grating is about 290 nanometers (nm). At this wavelength of 850 nm light is absorbed by silicon so the intended use is as a photodetector with the benefit of very good efficiency. The doped regions  18 - 22  and  26 - 28  are used to collect the photo-generated carriers. 
     This diffraction grating can also be used at for a wavelength of 1310 nm, which is the standard for metropolitan area network, but would be used as a transmitter with silicon as the waveguide as is the case shown in FIG. 7 for which semiconductor region  14  is described as being silicon. Semiconductor region  14  may, however, be a different composition that would make it sensitive to 1310 nm radiation. One way to do this is to alloy the silicon with germanium. 
     An advantage of the hemispherical shape is that the efficiency of coupling is not significantly affected by the polarization of the incident light. Thus, non-polarized light will pass very well through the opto-coupling grating in which the individual features are hemispherical. If instead of using a matrix of small features, a number of fingers that are in parallel with each other are used, the bending is effective based upon the periodicity of the fingers but the coupling is only good for the light that is polarized in the direction of the fingers. The light which has polarization aligned perpendicular to the features is substantially blocked. 
     An alternative to the square matrix shown for example, in FIG. 5, is to have each photoresist pillar be of equal distance from the others. This would be six pillars equidistant from any other pillar, as shown in FIG.  10 . This could be called a hexagonal pattern because it would be six pillars equidistant from any other pillar. This may be the most effective for passing non-polarized light and is mostly usefully implemented if the diffraction features, such as pillars nitride pillars  54 - 74 , are round. Shown in FIG. 11 is hexagonal pattern  92  comprised of round diffraction features  96 ,  98 ,  100 ,  102 ,  104 , and  106 , which surround diffraction feature  94  with a radius that is equal to the desired period for the grating to achieve the desired bending for the particular frequency. 
     The purpose of N-doped regions and P-doped regions such as  18 - 22  and  26 - 28  is to collect electrons for the case of N-doped regions and collect holes for the P+ regions. Semiconductor region  14  is doped very lightly to P−, a typical starting material for an integrated circuit but even lower doping levels may be even more advantageous. The use of an etchant, which directionally attacks the nitride  30  while simultaneously etching the sidewall of the photoresist pillar is used to advantage to obtain the round shape. This is a desirable shape which provides for a matrix such as shown in FIG. 5 or for the hexagonal approach of FIG. 10, which provides equal distance for all of the ultimate nitride features which make up the grating which then can maximize the coupling. The hexagonal pattern provides for the optimal symmetry, which provide the desired periodicity. The distance between two neighboring features plus the diameter of one of the features is the measure of the period. 
     As an alternative to the hemispherical grating features, such as nitride features  54 - 74  which protrude above the semiconductor region  14 , the grating features may also be cavities in the semiconductor region. Shown in FIG. 8 is a portion of an integrated circuit  80  having a semiconductor region  84 , which in the present embodiment is silicon, and insulating region  82 , a patterned nitride layer  86  with openings  88  and  90  in the nitride. Also, this is achieved by applying photoresist, patterning the photoresist, and then etching the nitride according to the pattern in the photoresist. This leaves openings  88  and  90 . Openings  88  and  90  are then roughed by an ion bombardment. This lowers the level of silicon in openings  88  and  90  slightly as well as roughening the surface of the silicon at openings  88  and  90 . After the ion bombardment of openings  88  and  90 , an etch comprised of potassium hydroxide (KOH) in liquid form is performed. With the crystal structure of silicon, a wet KOH etch is anisotropic. This etch is along the 111 plane of the silicon, substrate  84 . The result is a pyramid shape removed from the silicon in openings  88  and  90 . 
     Shown in FIG. 9 is a stage in the processing using the wet KOH etch to etch at the angle shown and is along the 111 plane of silicon region  84 . The etch continues with the result shown in FIG. 10, which is a pyramid-shaped cavity in semiconductor region  82 . This is an advantageous process because it is highly repeatable. The etching will essentially stop once the pyramid is formed. The etch rate is extremely slow into the 111 plane but rapid along it. Thus, what is left is the silicon aligned in the 111 direction. These pyramids thus can replace the nitride features that protrude above the silicon surface and instead be cavities within the silicon region. The pyramid-shaped features, which are surrounded by air, form a layer with a lower average index of refraction than silicon. Thus the silicon, substrate  84 , is effective as a waveguide because it has a lower index of refraction both above and below it. 
     The period of these pyramid shapes can be achieved as desired. The period in this case is the distance between openings  88  and  90  plus the length of one of these openings shown in FIG.  9 . These pyramids shapes can be aligned in the matrix shown in FIG. 5 for the photoresist pillars. If integrated circuit  10  is used as a photodetector, the doped regions would be conveniently placed to optimize the collection of carriers generated by the incident light. 
     Shown in FIG. 12 is an integrated circuit  120  comprising a grating  122 , a detector  124 , an insulator  126 , gates and interconnect  128 , and sources and drains  130 . Integrated circuit  120  utilizes a conventional semiconductor-on-insulator (SOI) substrate in which the semiconductor is preferably silicon and insulator  126  is below the silicon. The silicon is the active region of integrated circuit  120  and is where detector  124  and drains and sources  130  are located. The silicon corresponds to semiconductor region  14  shown in FIG.  2 . The regions doped regions  18 - 22  and  26 - 28  are relatively highly doped compared to the rest of semiconductor region  14  which is lightly doped to less than or equal to about 10 14  atoms/cm 2 . The A relatively thick layer of silicon underlies insulator  126  primarily to provide physical support. Detector  124  is formed in the silicon above insulator  126 . Grating  122  is formed above the silicon substrate surface although, as an alternative, the grating may be formed as part of the silicon itself. Gates and interconnect  128  are formed above the silicon surface which in this case is coincident with top of detector  124 . Sources and drains  130  are formed in the silicon surface. The combination of gates and interconnect  128  and sources and drains  130  form a processing circuitry  132 , which utilizes information collected by detector  124 . In operation, incoming light  134  strikes grating  122  and generates in the silicon carriers that are detected by detector  124 . After detection by detector  124 , processing circuitry  132  processes this detected signal in a manner according to a chosen design. 
     Shown in FIG. 13 is grating  122 , detector  124 , and insulator  126  showing the action of incident light on grating  122 . This shows that the incident light is striking the grating area, that it enters detector area  124 , and that it stays in the area of detector  124 . Area between grating  122  and insulator  126  is a waveguide so that the light that enters this waveguide remains there. Thus, detector  124  is in close proximity to the carriers that are created by incident light entering the silicon. Because of grating  122 , the incident light is redirected so that it is contained within the waveguide. This results in all the carriers being generated in the waveguide. The detector is also located in the waveguide so that the detector is in close proximity to the areas where the carriers are generated. This results in short distances for the carriers so that are in the relatively strong electric field region of the doped regions that make up the detector. Thus, there is no speed problem in the collection portion of operation. 
     Also, with all the incident light being contained within range of the detectors, the efficiency is very high. This provides a benefit of ease of detection of the information containing in the incoming light. Grating  122  can be chosen from any of those described in the formation of a grating described for FIGS. 1-11. For example, grating  122  may be the nitride features  54 - 74  shown in FIG.  7 . In the alternative they may be inverted pyramids such as  88  and  90 , which are actually formed in the silicon. Detector  124  may be like that shown in FIGS. 2,  3 ,  4 ,  6 , and  7 . 
     The desired angle of incident light  134  is chosen to optimize the efficiency of transmitting the light into the waveguide created between grating  122  and insulator  126 . If 90-degree incident light is utilized then the light entering waveguide would also be 90 degrees in both directions shown in FIG.  13  and actually radially in all directions. This may be preferable. It may also be preferable to have the light come in on one portion of the grating at an angle so that it only goes into the waveguide in one direction or at least not in all directions. The grating area and the detector area in most cases would be generally the same size. The incoming light will have a spot size as well. It&#39;s desirable for efficiency for the grating and the detector to be larger than the spot size of the incident light. Thus it may be desirable for the spot to be received on one side of the grating and angled to the other side of the grating so that all the light is directed towards the side of the grating away from where the spot is received. In the alternative, especially if the light is received at 90 degrees, the beam spot would be desirably located in the middle of the grating. 
     Gates and interconnect  128  are depicted as a block above the silicon portion of the substrate. This is depicting a typical configuration of an integrated circuit made on silicon. The transistors are a combination of the gates that are above the silicon and sources and drains are in the silicon. The combination of sources and drains and gates and interconnects are the tools by which integrated circuits are typically made. These integrated circuits can be quite simple or they can be extremely complex such as a microcomputer or microprocessor. They can have a variety of functions as well such as memory, digital to analog converters, and amplifiers. This is shown to indicate the planned integration of a photodetector with normal integrated circuit structures. The information retrieved by detector  124  may be transmitted to processing circuitry  132  by a source and drain type of interconnect or it may be achieved by an above substrate interconnect such as metal or polysilicon. 
     The thicknesses of the insulator and detector and the height of the grating are chosen in relation to the frequency of the incident light. The spacing of the grating features is also chosen in relation to the frequency. In the present example, the expected frequency corresponds to a wavelength of 850 nm. The thickness of insulator  126  is chosen to be an odd multiple of a quarter optical wavelength of the light with respect to insulator  126 . Thus, the index of refraction must be taken into account. In this case insulator  126  is preferably silicon oxide, which has an index of refraction of 1.45. Thus the thickness of insulator  126  is preferably about one fourth of 850 nm divided by 1.45, which is about 146 nm, or odd multiples of this number. 
     Similarly, the sum of the thicknesses of the waveguide and the grating, which is shown in FIG. 12 as detector  124  and grating  122 , is one half of the optical wavelength or even multiples of this number. The average index of refraction of the silicon waveguide and the average index of refraction of the grating must be taken into account. In the case of the grating, the index of refraction of air must be averaged with the material that forms the grating feature. This averaging must take into account the square relationship required for averaging indexes of refraction. The waveguide is silicon with an index of refraction of 3.62. For a simple example where the volume of features is the same as the volume of air in the grating and the features are silicon, the average index of refraction equals the square root of the quantity of the square of 3.62 plus the square of 1 all divided by 2. Thus the index of refraction is the square root of 13.1 plus 1 divided by two, which equals the square root of 7.05, which equals 2.65. Thus, the thickness of the waveguide times  3 . 62  plus the height of the grating times 2.65 equals one half of 850 nm. A benefit of this approach is that the thickness of the waveguide and the height of the grating can be varied so long as this condition is met. 
     In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.