Abstract:
Optical color sensor using diffractive elements. Semiconductor fabrication processes are used to form diffraction gratings as part of a photosensor. In a first embodiment, photosensors such as photodiodes are formed on a substrate, and diffraction gratings of fixed spacing are formed using the metallization layers common to semiconductor fabrication techniques. In a second embodiment, a linear photodiode array is formed on a substrate, and a diffraction grating with changing spacing is formed in the metal layers, providing a continuous color sensor. Other metal layers commonly used in semiconductor processing techniques may be used to provide apertures as needed.

Description:
TECHNICAL FIELD  
       [0001]     Embodiments in accordance with the invention relate generally to electrical means for sensing optical color of incident light.  
       BACKGROUND  
       [0002]     Sensing the spectral content of incident light is a common problem. A commonly used solution to this problem is to use a plurality of silicon photodiodes combined with a plurality of filters which selectively pass light of predetermined wavelengths.  
         [0003]     This solution has a number of problems. The performance of such a sensor is limited by the accuracy of the light transmission characteristics of the filter. The selectivity of such a sensor is limited by the availability of filtering materials. The filter materials attenuate light, and different colored filters attenuate light differently, requiring additional calibration. The long-term stability of such a sensor is also dependent on the long-term stability of the sensor materials used.  
       SUMMARY  
       [0004]     In accordance with the invention, photodiodes or other light-sensitive elements are fabricated with diffraction gratings. A first embodiment uses a photosensor with an integrated single frequency grating. A second embodiment uses a linear photosensor array and an integrated diffraction grating covering a range of frequencies. The diffraction gratings are formed using metallization layers common to semiconductor fabrication. Additional metal layers may be used to form apertures as required.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The invention will best be understood by reference to the following detailed description of embodiments in accordance with the invention when read in conjunction with the accompanying drawings, wherein:  
         [0006]      FIG. 1  shows a first optical sensor according to the invention,  
         [0007]      FIG. 2  shows a first optical sensor with processing electronics, and  
         [0008]      FIG. 3  shows a second optical sensor according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0009]     The invention relates to sensing the spectral content of incident light. The following description is presented to enable one skilled in the art to make and use the invention, and is provided in the context of a patent application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments. Thus, the invention is not intended to be limited to the embodiments show but is to be accorded the widest scope consistent with the appended claims and with the principles and features described herein.  
         [0010]      FIG. 1  shows a first sensor according to the present invention. Substrate  100  has photosensors  110 ,  112 ,  114  fabricated using fabrication techniques known to the semiconductor and integrated circuit arts such as photolithography. Note that there may be intervening layers between substrate  100  and photosensors  110 ,  112 ,  114 . Photosensors  110 ,  112 ,  114  may be photodiodes, phototransistors, or other light-sensitive device, fabricated from semiconductor materials such as silicon, silicon-germanium, or like materials. Dielectric layer  120  also passes wavelengths of interest. Again, there may be additional layers between the layer  120  and the layer containing photosensors  110 ,  112 ,  114 . Materials such as silicon dioxide (S i O 2 ), insulating materials, or other materials known to the art may be used for layer  120 . Diffraction gratings  130 ,  132 ,  134  are formed on top of dielectric layer  120 . Diffraction gratings  130 ,  132 ,  134  are formed of a material opaque to the wavelengths of interest, such as metal.  
         [0011]      FIG. 1  shows a simplified representation of the present invention, with only key layers represented. Photosensors  110 ,  112 ,  114  may be fabricated at any layer in the semiconductor device. Diffraction gratings  130 ,  132 ,  134  are formed above the photosensors, with any number of intervening layers  120 , as long as those intervening layers pass light in the wavelength range of interest.  
         [0012]     The spatial distribution of light from a diffraction grating is controlled solely by the relationship of the wavelength of incident light compared with the physical dimensions of the grating. The grating, in conjunction with the spatial arrangement of the photodetector, directs light of desired wavelengths onto the photodetector. Note that the incident light reaching gratings  130 ,  132 ,  134  and photosensors  110 ,  112 ,  114  should be collimated. This collimation may be achieved through traditional optical means, such as slits, lenses, and the like. Because gratings  130 ,  132 ,  134  are manufactured with integrated circuit lithographic techniques, their optical properties are highly accurate and repeatable.  
         [0013]     In an embodiment such as that shown in  FIG. 1 , gratings  130 ,  132 , and  134  could be designed to pass red, green, and blue light respectively. Other embodiments of the invention could provide one photosensor—grating pair sensing a single wavelength range, two photosensor—grating pairs sensing a pair of wavelengths, such as red and blue, or more than three photosensor—grating pairs, as an example sensing red, blue, green, cyan, and magenta wavelengths. Single-wavelength sensors may be fabricated responsive to particular wavelengths of interest, such those produced by lasers.  
         [0014]     An additional metal layer, or other opaque layer, may be used to provide an aperture. This aperture may be located between grating  130  and photosensor  120 . The aperture  150  may be supported on an additional dielectric layer  140 , between the grating and the light source. Such an aperture may act as a collimating element. Additionally, such an aperture may be used to insure that only certain areas of the device are illuminated, or to compensate for the difference in response of the photosensors at different wavelengths.  
         [0015]     Additionally, gratings may be formed on more than one layer of metallization separated by intervening dielectric layers to further define the relationship between spatial distribution of the incident light and the wavelength. Moreover, the grating need not be active solely in one-dimension. For example, a two-dimensional spatial distribution as a function of wavelength is achievable using grating elements with active components which are substantially orthogonal to each other.  
         [0016]     As standard integrated circuit techniques are used, additional circuitry can easily be included with the photosensors. This is shown in  FIG. 2 , where transimpedance amplifiers are included on the same substrate. Photodiode  110  is fabricated with grating  130  to be responsive to a particular wavelength of incident light. Amplifier  140  in conjunction with resistors  150  and  160  form a transimpedance amplifier which converts the photocurrent from photodiode  110  into a voltage output  170 . A second wavelength is sensed by photodiode  112  coupled with grating  132 . Amplifier  142  in conjunction with resistors  152  and  162  form a transimpedance amplifier which converts the photocurrent from photodiode  112  to voltage  172 . This embodiment may be fabricated with one or a plurality of wavelength sensors on a single die.  
         [0017]     A second embodiment of the invention is shown in  FIG. 3 . In this embodiment, an N-element photodiode array is coupled with a grating optionally having varying element spacing, providing a sensor which provides a continuous spectral response defined by the spacing of the diffraction grating elements. N-element photodiode sensor array  110  is formed above substrate  100 . Layer  120 , which passes to the range of wavelengths of interest, supports diffraction grating  130 .  
         [0018]     In an embodiment in which the spacing of grating elements  130  is uniform, a varying frequency response is obtained in photodiode array  110  due to the operation of grating  130 . Spatial distribution of light as a function of wavelength is dependent on the spacing between grating elements. Uniform grating spacing produces a spatial distribution which is logarithmic vs. wavelength.  
         [0019]     In an embodiment where grating  130  is nonuniform, the spacing between elements  132 ,  134 , and  136 ,  138 , changes. As an example, if the spacing between elements  132  and  134  is larger than the spacing between elements  136  and  138 , grating  130  in the region of elements  132 ,  134  will pass longer wavelengths than in the region of elements  136 ,  138 . Non-uniform spacing of grating elements adds the ability to engineer the distribution of light vs. wavelength, for example, to produce a linear distribution with respect to wavelength. It should be noted that this embodiment may take the form of a one or two dimensional array depending on the nature of the grating structure.  
         [0020]     As with the previous embodiment, an additional metallization or other opaque layer (not shown) can be used to form an aperture of appropriate dimensions to act as a collimating device, shutter or other light regulating mechanism.  
         [0021]     Other processing elements may also be integrated onto substrate  100 , for example, to process the output of photodiode sensor array  110  or to control the spectral output of the incident light source, thereby forming a closed-loop control system.  
         [0022]     The foregoing detailed description of the present invention is provided for the purpose of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Accordingly the scope of the present invention is defined by the appended claims.