Abstract:
A polarization sensitive solid state image sensor includes an integrated photodetector and polarizing assembly. The polarizing assembly is formed directly on an upper dielectric surface of the photodetector as a metal grid. The integral form of the device improves polarization operation and reduces manufacturing costs.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to the field of solid state image sensors, and, more particularly, to a polarization sensitive solid state image sensor, and method of forming the same.  
       BACKGROUND OF THE INVENTION  
       [0002]     Current polarization sensitive solid state image sensors comprise a separately formed polarizing material which is mounted in the optical stack. However, the polarizing material is difficult to align accurately, and may also move during the sensor&#39;s operation, which can lead to derogation or loss of polarizing function.  
         [0003]     Moreover, in a solid state image sensor manufacturing process, the step of aligning the polarizing material adds to the time taken and costs incurred. Furthermore, using existing technology, it is difficult or impractical to produce an image sensor with two or more regions which are sensitive to different polarizations of light, for example an image sensor that comprises a first horizontally polarized image sensor and a second, vertically polarized image sensor. At present, manufacture of such an image sensor would require the use of separate materials, and subsequent individual realignment on the sensor. It would be desirable to allow for the practical manufacture of several pixels, each with their own particular polarization sensitivity, on the same device.  
       SUMMARY OF THE INVENTION  
       [0004]     According to a first aspect of the present invention, there is provided a polarization sensitive solid state image sensor comprising an integrated photodetector and polarizing assembly. The polarizing assembly can cover all or any portion of an image sensing surface of the image sensor. Preferably, the polarizing assembly comprises two or more regions which among them have two or more different polarizations.  
         [0005]     The polarizing assembly comprises a plurality of parallel metal lines. The metal lines have a pitch of a value less than a wavelength in the spectral range of visible light. The pitch may be much greater than 30 nm. Also, the parallel lines may be spaced apart by a distance equal to their width. The photodetector may comprise a charge amplifier. Optionally, the photodetector comprises a light to frequency converter. The polarizing assembly may be formed on a dielectric material which is itself formed on the photodetector.  
         [0006]     According to a second aspect of the present invention, there is provided a method of forming a polarization sensitive solid state image sensor comprising forming an integrated photodetector and polarizing assembly. The method comprises forming a photodetector having an upper dielectric layer and forming the polarizing assembly upon the dielectric layer. The plurality of parallel metal lines are formed by a lithographic or etching process.  
         [0007]     From further aspects, the present invention provides for an integrated circuit, an optical mouse, a digital camera, and a mobile telephone incorporating a digital camera, each of which comprises a polarization sensitive solid state image sensor according to the first aspect, which can be formed by a method according to the second aspect. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0009]      FIG. 1  is a schematic plan view illustrating an image sensor in accordance with a first embodiment of the invention;  
         [0010]      FIG. 2  is a schematic cross-sectional view taken across the line A-A′ of  FIG. 1 ;  
         [0011]      FIG. 3  is a schematic diagram illustrating an example of a charge amplifier for use with the present invention;  
         [0012]      FIG. 4  is a timing diagram illustrating the variation of an output voltage of the charge amplifier of  FIG. 3  over time;  
         [0013]      FIG. 5  is a schematic diagram illustrating an example of a light to frequency conversion circuit for use with the present invention; and  
         [0014]      FIG. 6  is a waveform diagram illustrating exemplary waveforms generated by the light to frequency conversion circuit of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     The present invention provides for a polarization sensitive solid state image sensor comprising an integrated photodetector and polarizing assembly. An embodiment of such a sensor is illustrated in  FIGS. 1 and 2 . Advances in CMOS technology have resulted in smaller dimensions for transistor gates, for example, 0.35 μm, 0.18 μm, or 0.13 μm. Along with this, the dimensions of metal interconnects have also reduced in size, for example, 0.5 μm, 0.32 μm, or 0.16 μm. The wavelength of light in air is between 0.45 μm and 0.65 μm. As the sizes of these metal structures is less than the wavelength of light, they are therefore suitable for diffractive-type optics. The short distance (1-3 μm, as seen in  FIG. 2 ) between the metal layers and the surface of the silicon is too short to allow interference, thus preventing the use of diffractive optics. However, this is not an issue for polarizers.  
         [0016]     A polarizing assembly  10  according to a first embodiment of the present invention is shown in  FIGS. 1 and 2 . An array of parallel metal lines  12  is provided having a width  14  of 0.16 μm, and are spaced apart by a spacing  16  of 0.16 μm. A pitch of the grid is defined as the summed width of the width  14  of one metal line  12  and the spacing  16  adjacent to that line. Thus, the pitch of the illustrated array is 0.32 μm.  
         [0017]     As seen in  FIG. 2 , the metal lines  12  are formed directly on a dielectric layer  18  of a silicon photodetector. Thus, the image sensor produced comprises an integrated photodetector and polarizing assembly. In this context, “integrated” is taken to mean that the polarizing assembly is immovably attached in a fixed spatial relationship to a photodetector. These metal patterns can be produced very accurately as part of a normal manufacturing process, and thus the cost of incorporating a polarization assembly is kept very low.  
         [0018]     As the width  14  of the metal lines  12  and the spacing  16  between adjacent lines is equal, the illustrated array will allow approximately 50% of incident radiation to pass therethrough. It will be appreciated that the pitch can be varied by choosing different values of one or both of the width  14  of the metal lines  12  or the spacing  16  of the metal lines  12 , to allow different proportions of incident light to pass through the array, as required.  
         [0019]     Components of incident radiation having an electric field parallel to the array structure, i.e. parallel to the longitudinal axis of the metal lines  12 , will pass relatively freely through the array. However, components of incident radiation having an electric field orthogonal to the array structure will collapse because the conducting properties of the metal lines  12  cause the electric field to collapse. It will be appreciated that any metal can be chosen, as long as it has a conductivity suitable to act effectively in an array as a polarizer of incident light.  
         [0020]     The efficiency of the polarizing assembly  10  is optimal if the pitch of the array is less than one tenth of the wavelength of incident radiation. For visible light, an optimal pitch is then 460 nm/(10N si )=31.5 nm, where N si  is the refractive index of silicon, having a typical value of 1.45.  
         [0021]     Current high-volume manufacturing techniques cannot yet produce a structure this fine. However, the inventors have found that, surprisingly, a significant polarizing effect occurs with structures produced using a pitch which is much greater than this optimal pitch. Accordingly, a pitch of much greater than 30 nm can be used to provide a polarizing effect which is useful for production of a polarization sensitive image sensor. In this context, “much greater” than a particular value means that the pitch must be at least twice that particular value. In the illustrated embodiment, the pitch is approximately ten times the particular value, i.e. over 300 nm. The minimum allowable pitch will be governed by the manufacturing process.  
         [0022]     The polarizing structure can formed on the surface of any type of silicon photodetector, but one of high sensitivity is preferred as the polarizer introduces a significant attenuation. For example, in the embodiment illustrated in  FIG. 2 , at least 50% of incident light is lost.  
         [0023]     A technique which is particularly suitable for this method is a light to frequency (LTF) converter. Examples of LTF converters are product numbers TSL235, TSL245 and similar devices available from Texas Advanced Optoelectronic Solutions. These include a photodiode and a current to frequency converter integrated on the same CMOS IC.  
         [0024]     LTF converters are particularly useful for the present application because they use a charge sensing technique. This permits the use of large area photodetectors, which collect more photons but have a large capacitance. More conventional readout techniques (e.g. 3 transistor) use a voltage sensing technique, so the large capacitance of the photodetector (the capacitance is proportional to its area) effectively cancels out the advantage of the greater number of photons collected by the larger area.  
         [0025]      FIG. 3  illustrates a charge sensing amplifier that uses an operational amplifier  20  with a feedback capacitor, Cfb  22 . The amplifier  20  has a very high input impedance and so no current flows into it. The output will change so that the inverting input remains at the same potential as the non-inverting input. In doing so, a current will flow through the feedback capacitor Cfb  22 . This will be of exactly the same magnitude (but opposite sign) to the photocurrent Ipd. Vout=−Ipd×Tint/Cfb (equation 1), and so the output voltage is independent of the pixel&#39;s capacitance.  
         [0026]     Because of this, the architecture is very suitable for large photodiodes. The disadvantage is that the charge amplifier  20  needs to be reset periodically as the output voltage will swing outside the operating range of the amplifier  20 , as shown in  FIG. 4 . This reset can be achieved as part of a system&#39;s AEC (automatic exposure control). A practical method for resetting the charge-amp&#39;s feedback capacitor  22  is shown in  FIG. 5 , the readout signals from which are illustrated in  FIG. 6 .  
         [0027]     As shown in Equation 1, the slope of the charge-amplifier&#39;s  20  output is proportional to the light. Hence the frequency of the light to frequency conversion is also proportional to the light. Fout=Ipd/(2 ×Cfb× ( Vthreshold−Vrt )) (equation 2).  
         [0028]     The above described architecture provides a number of advantages, as follows: output signal is independent of photodiode capacitance/size; Feedback capacitance can be chosen or designed for application; System is auto-exposing; Output is digital and therefore immune to noise; ADC is included in the architecture; Digital signal is easy to measure over large dynamic range (e.g.120 dB).  
         [0029]     Additionally, the above concepts can be applied in a manufacturing process to form a polarizing structure on selected portions of an image sensor. These portions may or may not cover the entire image sensing surface. Furthermore, the orientation of the lines that are formed can be predetermined and can be different for different portions of the image sensor, so that different polarizations can be detected. This polarization sensitive image sensor can be incorporated in a number of different products, which include but are not limited to, a chip or integrated circuit, an optical mouse, and a digital camera provided as a separate unit or as part of a mobile telephone or webcam.  
         [0030]     It will be appreciated that standard techniques may be used by the skilled artisan to implement the invention in these and other ways. Various improvements and modifications can be made to the above without departing from the scope of the invention. In particular, an image sensor made in accordance with the principles of the invention may be incorporated in a product together with a non-polarized detector to gather information about ambient light.