Abstract:
A photoresponsive device wherein a plurality of slots is formed in a continuous polycrystalline silicon film that forms a top electrode of the photoresponsive device structure. The resulting “picket fence-like” fingers are capable of generating a depletion region that extends beyond the physical extent of the fingers themselves. The shorter wavelengths of light can reach these unobstructed depletion areas of the substrate and be rapidly detected instead of being absorbed in the electrode. By appropriate spacing of the individual fingers, the individual depletion regions can be effectively merged into one continuous depletion region.

Description:
TECHNICAL FIELD 
     The present invention generally relates to the field of optoelectronic image sensing. More specifically, the present invention relates to a photodetector or other photoresponsive device. 
     BACKGROUND OF THE INVENTION 
     Single chip photoresponsive devices are useful for vehicular applications, including occupant detection, vehicle guidance and collision avoidance. Generally, in both automotive and non-automotive applications, photoresponsive devices have been implemented to provide information that is subsequently displayed to the operator of the vehicle or machine. In such applications, providing information in a manner compatible with the human visual system is important to optimize the transfer of information to the operator. For automotive systems, however, the need to display the information is of less importance than the need to electronically analyze the data. Thus, requirements for an automotive photoresponsive device diverge from those of mainstream commercial photoresponsive devices. 
     Most known imagers integrate active transistors directly into the imager chip. The degree of integration varies depending on both the application and upon the process technology. Imagers that can be formed using a conventional Complementary Metal Oxide Semiconductor (CMOS) process offer the greatest opportunity for peripheral electronics integration and cost reduction. 
     In a typical CMOS imager, light penetrates various transparent insulating films deposited upon the surface of the wafer for electrical and mechanical protection, and is absorbed in an active device, generally in the form of a photodiode or photocapacitor. It is important to note that the absorption coefficient of light in silicon is inversely proportional to the wavelength of the light. Thus, blue light, which has a shorter wavelength, is absorbed at relatively shallow depths, as compared to red light, which has a longer wavelength and consequently is absorbed more deeply in the silicon. 
     When a photon strikes a semiconductor it can promote an electron from the valence band to the conduction band creating an electron/hole pair. In order for the absorption of light to be detected in the photoresponsive device, the electron hole pairs that are produced must be separated before they can recombine. This separation is generally accomplished by the application of an electric field, either in the form of a photodiode structure or in the form of a photocapacitor or phototransistor structure. The electric field produces a depletion region in the affected semiconductor region and any free charges in this region are rapidly swept away. Significantly, positive charges are driven in one direction while negative charges are driven in the opposite direction. Consequently, electron/hole pairs are separated before they can recombine and can be detected by external means, and the presence of the original photon can be inferred. 
     However, the structures required to produce the electric field cover most of the substrate. Existing alternatives use a continuous polycrystalline silicon film that forms the top electrode of the photoresponsive device structure. Generally, a higher optical absorption coefficient for blue light means that a significant amount of the shorter (blue) wavelength light is absorbed in the top layer of silicon. Light absorbed in this region is not detected. Thus, photoresponsive devices typically exhibit poor performance for shorter wavelength light. 
     A complementary problem is typically observed for long wavelength red and near IR photons. These photons can penetrate relatively deeply into the active charge collection material. In fact, they can penetrate sufficiently deep that adjacent devices can collect the resulting electron/hole pairs, thus reducing the resolution of the image in the long wavelength regions of the optical band. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the invention to provide a photoresponsive device with improved sensitivity to light. A further object of the invention is to allow a photoresponsive device to be adjusted for more or less sensitivity to short (blue) wavelength light with respect to its&#39; sensitivity to long (red or near IR) wavelength light. 
     In one aspect of the invention, a plurality of slots is formed in a continuous polycrystalline silicon film that forms a top electrode of the photoresponsive device structure. The resulting “picket fence-like” fingers are capable of generating a depletion region that extends beyond the physical extent of the fingers themselves. The shorter wavelengths of light can reach these unobstructed depletion areas of the substrate and be rapidly detected instead of being absorbed in the electrode. By appropriate spacing of the individual fingers, the individual depletion regions associated with each finger can be effectively merged into one continuous depletion region. 
     With the digitated electrode described above, the electrode layer can be made thicker than existing photoresponsive devices and still allow light to reach the substrate. Also, the size of the aperture openings, finger widths and electrode thickness can be adjusted to ‘tune’ the photoresponsive device to the desired sensitivity to short or long wavelength light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a vehicle; 
     FIG. 2 is a top view of a photoresponsive element formed according to the present invention; 
     FIG. 3 is a cross sectional view along line  3 — 3  of FIG. 2; 
     FIG. 4 is a cross sectional view along line  4 — 4  of FIG. 2; and 
     FIG. 5 is a cross sectional view of a photodiode implementation of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a vehicle  10  may use an imaging system  11  for various applications such as occupant detection/protection, vehicle guidance, front/rear collision avoidance or other vision applications. Imaging system  11  consists of a sensor array  12  and a controller  14  that controls a vehicle function  16 . Imaging sensor array  12  is made up of a set of photoresponsive elements  18  that convert light to an electronic signal. Controller  14 , preferably microcontroller based, analyzes the electronic signals and provides the vehicle function  16  such as described above. 
     Referring now to FIGS. 2,  3  and  4  photoresponsive element  18  is illustrated. Photoresponsive element  18  is preferably fabricated using conventional CMOS techniques. One skilled in the art would recognize that photoresponsive elements  18  might also be formed using other techniques. 
     As will be further described below, photoresponsive element  18  may be implemented as a photocapacitor or a photodiode. For simplicity, the present invention will be described in relation to a photocapacitive implementation. 
     The photocapacitor embodiment ( 18 ) preferably has three layers. A substrate  20 , an insulator layer  22  and an electrode layer  24  make up these layers. Insulator layer  22  and electrode layer  24  may be consecutively layered on substrate  20 . As will be further described below, depending on the desired characteristics of the photoresponsive element to be fabricated, the presence, doping level, doping type and thickness of each layer may be modified. This particular implementation uses bulk silicon, which also forms the active layers, for conversion of light to electrical energy. However, this invention is also applicable to applications that use additional photon to electron conversion layers (not illustrated) that perform the light-to-electron conversion. 
     Substrate  20  is preferably a silicon wafer with a doping concentration between 1×10 14  and 1×10 16  cm −3 . Substrate  20  may have n or p type doping. In the present invention, the silicon wafer preferably is doped with boron to a concentration of 7×10 14  cm −3 . 
     Substrate  20  also has a substrate contact  34  for the induction of a bias voltage. Substrate contact  34  can be made of any conductive metal, such as aluminum. 
     Insulator layer  22  is preferably a high quality (i.e. gate insulator quality) silicon dioxide layer, between 4 and 100 nm in thickness. However, other suitable materials such as silicon nitride or other materials having intentionally high intrinsic bulk or surface charge concentrations to manipulate the extent of a depletion region  40  in the underlying substrate  20  may be employed. In the present example, insulator layer  22  is 40nm thick silicon dioxide. 
     Electrode layer  24  is preferably a heavily doped polycrystalline silicon film. However, amorphous silicon films and other compositions of conducting films that incorporate silicon or another semiconductor element may be used. Electrode layer  24  may have n or p type doping. In the present invention, electrode layer  24  preferably is polycrystalline silicon 300 nm in thickness doped with phosphorus to provide a resistivity of 11 ohms per square cm. As will be further described below, the thickness of electrode layer  24  may be modified to provide suitable characteristics. 
     Electrode layer  24  has an electrode layer contact  36  for the induction of a bias voltage. Electrode layer contact  36  can be made of any conductive metal, and preferably aluminum. 
     Electrode layer  24  has one or more slots, slits or elongated apertures  26  (hereinafter referred to as “slots”) with a width  30  to allow photons  38  to pass through electrode layer  24  and reach an unobstructed absorption region  42  in substrate  20 . Electrode layer  24  also has one or more fingers  28  defined by slots  26  with a land width  32  for generating depletion region  40  in substrate  20 . As will be further described below, slot width  30  and finger width  32  may be modified to provide suitable characteristics. While slots  26  are depicted in FIG. 2 as elongated rectangles, any various shapes of small openings through electrode layer  24  may be used. 
     A bias voltage is applied to substrate contact  34  and electrode layer contact  36 . The resulting electric field forms depletion regions  40  in substrate  20  that extend beyond the edges of fingers  28 . 
     Slot width  30  is chosen so that the depletion region  40  remains relatively continuous through substrate  20 . Some photocollection may also occur within approximately a diffusion length of the edge of the depletion region  40 , which could allow extension of slot width  30 , for some applications. In the preferred embodiment, slot width  30  may be twice the distance that depletion regions  40  extend beyond fingers  28  and land width  32  may be as small as possible. In the present example, slot  30  is 1.9 micrometers in width while land  32  is 1.2 micrometers in width. 
     This device may also be realized with a photodiode embodiment, as depicted in FIG.  5 . The main difference being the device  18 ′ shown in FIG.  5  and that described above with reference to FIGS. 2-4, is the lack of insulation layer  22  and the means for inferring incident light. 
     In operation, some short wavelength photons  38  pass through one or more slots  26  in electrode layer  24  of photoresponsive element  18  and to depletion region  40  in substrate  20  thereby avoiding absorption in electrode layer  24 . This allows the short wavelength photons  38  to promote electrons from a silicon valence band to a conduction band in depletion region  40 , which generates electron-hole pairs. The voltage bias then sweeps the electrons in one direction and the holes in the opposite direction. These free electrons generate an electronic signal that can then be detected. 
     Similarly, some long wavelength photons  38  pass through the electrode layer  24  of photoresponsive element  18 . This allows the long wavelength photons  38  to promote electrons from silicon valence bands to the conduction bands in depletion region  40 , which generates electron-hole pairs. The voltage bias then sweeps the electrons in one direction and the holes in the opposite direction, which generates an electronic signal. 
     The photoresponsive devices  18  and  18 ′ can also be adjusted to “tune” it to a desired sensitivity to short or long wavelength light. This can be accomplished by changing or adjusting the sizes and/or shapes of the slots or the land areas, and by changing the thickness of the land areas and electrode layer  24 .