Abstract:
A photosensitive device is disclosed which comprises a semiconductor substrate, at least one reverse biased device, such as a P-N junction diode formed in the semiconductor substrate, and at least one photosensitive layer disposed above the semiconductor substrate and substantially covering the reverse biased device, the photosensitive layer releasing electrons and holes when struck by photons, wherein the photon generated electrons and holes in the photosensitive layer reach the reverse biased device and create a combination current therein when a light shines thereon.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to semiconductor image sensors and more particularly to an image sensor structure with enhanced photosensitivity. 
         [0002]    In the 1950s, researchers found that a fully charged reverse-biased P-N junction would discharge at a rate proportional to the light it received. This is because photons (light) can assist electrons and holes overcome the energy gap. These electron-hole pairs incur discharging current when they recombine after their lifetimes expire. As a result, the P-N junctions can be used as a solid-state image sensor to replace vacuum tube devices with photomultipliers to detect radiations. A CMOS image sensor, which comprises arrays of active MOS image sensor cells that are produced in a CMOS process, is one of the typical image sensing devices that utilize the photoconductive characteristics of the reverse-biased P-N junction structure. 
         [0003]      FIG. 1  illustrates a conventional  3 T CMOS image sensor cell  100  which comprises a P-N junction diode  110 , a reset NMOS transistor  120 , an amplifier NMOS transistor  130  and a row select NMOS transistor  140 . The P-N junction diode  110 , which serves as a photo-detector, and the reset NMOS transistor  120  are serially connect between a power supply VRST and a ground (GND). When the reset MOS transistor  120  is turned on by the RST signal, the P-N junction diode  110  is effectively connected to the VRST and reverse biased. When light shines on the P-N junction diode  110 , an additional combination current generated by photon created electron-hole pairs cause a voltage drop at node VC. The voltage drop is then amplified by the NMOS transistor  130 , which has a power supply VDD. However, the VDD is traditionally tied to the VRST. The row select NMOS transistor  140  is coupled between the amplifier NMOS transistor  130  and a column line (COL). A row line (ROW) is connected to a gate of the NMOS transistor  140 . Therefore, the row select NMOS transistor  140  is a switch that allows a signal row of an array the CMOS image sensor cells  100  to be read by a read-out circuit. The aforementioned combination current flowing through the P-N junction diode  110  is proportional to the intensity of the light, therefore the read-out voltage and/or current at the COL is also proportional to the intensity of the light. 
         [0004]      FIG. 2  is a cross-sectional view of such CMOS image sensor  100  forming an array of cells  200  in a semiconductor substrate  210 . The P-N junction diode  110  and NMOS transistors  120 ,  130  and  140  are formed in the substrate  210 . A passivation layer  220  is applied on the substrate  210 . Then a planarization layer  230  is processed on top of the passivation layer  220  to make the semiconductor surface flat, for subsequent applications of a color filter  240 , a spacer  250  and micro-lenses  260 . All these layers  220  through  260  merely pass the light to the substrate  210 , where the P-N junction diode  110  is the only device that has the photoconductive effect. Therefore, the conventional CMOS image sensor cell  200  has only mediocre optical sensitivity and signal-to-noise ratio. 
         [0005]    As such, what is needed is an improved image sensor cell structure that has enhanced photosensitivity. 
       SUMMARY 
       [0006]    The present invention discloses a CMOS image sensor with enhanced photosensitivity. In one embodiment of the present invention, the CMOS image sensor has a photosensitive device, which includes a semiconductor substrate, at least one reverse biased device, such as a P-N junction diode formed in the semiconductor substrate, and at least one photosensitive layer disposed above the semiconductor substrate and substantially covering the reverse biased device, the photosensitive layer releasing electrons and holes when struck by photons, wherein the photon generated electrons and holes in the photosensitive layer reach the reverse biased device and create a combination current therein when a light shines thereon. In another embodiment of the present invention, the photosensitive device further includes a transparent insulation layer interposed between the photosensitive layer and the semiconductor substrate. 
         [0007]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic diagram illustrating a conventional CMOS image sensor cell. 
           [0009]      FIG. 2  is a cross-sectional view of such conventional CMOS image sensor cells formed in a semiconductor substrate. 
           [0010]      FIGS. 3A and 3B  are cross-sectional views of photosensitive P-N junction diodes according to embodiments of the present invention. 
           [0011]      FIGS. 4A˜4D  are cross-sectional views of alternative photo sensitivity enhancing structures according to other embodiments of the present invention. 
           [0012]      FIGS. 5A through 5B  are cross-sectional views of CMOS image sensor cells with photosensitive layers applied according to embodiments of the present invention. 
       
    
    
     DESCRIPTION 
       [0013]    This invention describes a CMOS image sensor cell with a photoconductive layer for improving its sensitivity to a change of illumination. The following merely illustrates the various embodiments of the present invention for purposes of explaining the principles thereof. It is understood that those skilled in the art will be able to devise various equivalents that, although not explicitly described herein, embody the principles of this invention. 
         [0014]      FIGS. 3A and 3B  are cross-sectional views of P-N junction diodes  300  and  350  covered by photo sensitive thin films  330  and  360  according to embodiments of the present invention. Here the photo sensitive or photo conductive means extra carriers are generated when light is radiated into the film  330  and  360 . Referring to  FIG. 3A , the P-N junction diode comprises an N-type region  310  disposed inside a P-type semiconductor substrate (Psub)  320 . An N+ region  312  serves as a pick-up for the N-type region  310 . A P+region  322  serves as a pick-up for the Psub  320 . If the P-N junction is equivalent to the diode  110  of  FIG. 1 , then the Psub  320  is coupled to a ground (GND) through the P+ region  322 , and the N-type region  310  is coupled to node VC through the N+ region  312 . When light (photon) shines on the P-N junction  300 , electrons and holes will be generated in the N-type region  310 , and then combined in a photon collection region  315 . A combination current will then be the CMOS sensor cell  100  shown in  FIG. 1 . 
         [0015]    Referring to  FIG. 3A , through a thin oxide layer  335 , the N-type region  310  is coated by the photo sensitive thin film  330 , which serves two purposes. A first purpose is to convert incoming invisible lights, such as X-ray or UV, into visible light, as CMOS image sensors are typically designed to sense only visible lights. When the invisible lights shine on the photo sensitive thin film  330 , electrons in the thin film  330  are incited into higher energy levels by the incoming photons and then emit other photons after falling back into the ground states. In case of converting X-ray, the thin film  330  is made of phosphor. When the incoming X-ray light (photon) strikes the phosphor layer  330 , it will be converted into a visible wavelength, so that the P-N junction diode  300  can sense the incoming X-ray light. One having ordinary skills in the art would choose other materials for the thin film  330  when other incoming lights need to be converted. 
         [0016]    A second purpose of the photo sensitive thin film  330  is to generate electron-hole pairs when light shines on it. These generated electrons and holes will then tunnel through the thin oxide  335  and induce more combinations of electrons and holes in the photon collection region  315 . According to the embodiments of the present invention, the thin film  330  is made of a semiconductor material, such as phosphor, it is therefore transparent to light. Meanwhile the thin oxide  335  is also transparent. Both the photo sensitive thin film  330  and the P-N junction itself can generate electron-hole pairs when shined by light, therefore current generation efficiency of the P-N junction diode  300  is improved by adding the photo sensitive thin film  330 . Besides, as described in above paragraph, the thin film  330  may convert photons otherwise invisible to the P-N junction diode  300  to visible wavelengths, there will be more photons in the incoming light to generate electron-hole pairs, and the light sensitivity of the P-N junction diode  300  is further enhanced. 
         [0017]    Here the thickness of the thin oxide  335  can be adjusted to control the tunneling rate for the electron-hole pairs generated by the photoconductive layer  330 . For example, the thickness of the thin oxide  335  is preferably thinner than 100 angstroms. One having skills in the art may also recognize other dielectric materials may also be used in place of the thin oxide  335 . 
         [0018]    As shown in  FIG. 3A , in order to assist the generated electrons to tunnel through the thin oxide  335 , a bias voltage VP may be applied to the thin film  330 . Besides, in a semiconductor manufacturing process, the photo sensitive thin film  300  may be selectively coated with lithograph patterning to cater to various needs. 
         [0019]      FIG. 3B  shows another embodiment of the present invention with a photo sensitive thin film  360  overlays the N-type region  310 . The overlay alters the silicon surface potential, thus allows more carries to be stored in the depletion regions of the P-N junction discharging. 
         [0020]    The photo sensitive materials used for the thin film  330  or  360  are usually photoconductive semiconductor slab that generates carriers either by band-to-band transitions (intrinsic) or by transitions involving forbidden-gap energy levels (extrinsic). The photoconductive material may be selected from the group consisted of CdS, PbS, InSb, HgCdTe, GaAs, nickel-doped germanium (Ge—Ni) and phosphorus-doped silicon (Si—P). They can be pure material or in PN junction form with or without bias. 
         [0021]    CdS is commonly used as light-sensitive material in discrete devices for wavelength near 0.5 um. The resistance between two terminals of CdS film changes drastically when light shines on the surface. Whereas at 10 um, an HgCdTe photoconductor is preferred. In the wavelength from 100 to 400 um, a GaAs photoconductor is a better choice because of its higher detectivity. 
         [0022]    One of the mechanisms to achieve multiple carrier generation is through photoconductivity. When light shines on a photoconductive material, such as Cds, amorphous silicon a-Si:H, etc., electron-hole pairs will be generated accordingly. The photoconductivity σph is determined by the product of the free-carrier lifetime τ and free-carrier mobility μ: 
         [0000]      σ ph=q·μ·τ·f   (Eq. 1) 
         [0000]    where f is an average optical generation rate, which is the number of carriers generated by the photons absorbed per second and per unit volume. The μ·τ product depends on the property of photoconductive material. In general, the τ product depends on the position of Fermi level Ef to the bandgap. The further the Fermi level is away from the midgap and closer to the conduction band edge Ec, the larger the μ·τ product. The mobility μ was found independently of Fermi level, the photoconductivity σph is therefore proportional to the recombination lifetime τ. 
         [0023]      FIGS. 4A through 4D  are cross-sectional views of alternative photo sensitivity enhancing structures according to other embodiments of the present invention.  FIG. 4A  illustrates a structure  400  in which addition photon generated electron-hole pairs come from a P-N junction formed by two layers  410  and  415  on top of an insulation layer  403  over a semiconductor substrate  410 . If layer  410  is an N-type, then layer  415  is a P-type, or vice versa. Photon generates electron-hole pairs in a depletion region at an interface of layers  410  and  415 . Traditional P-N junction is built in the semiconductor substrate  410  under the layers  410  and  415 . 
         [0024]      FIG. 4B  illustrates a P-Intrinsic-N (P-I-N) structure  420  which is slightly different from the structure  400  in that a thin intrinsic layer  425  is deposited between two doped layers  430  and  435 . If layer  430  is an N-type, then layer  435  is a P-type, or vice versa. Here, the depletion region is the intrinsic layer  425 , the thickness of which can be tailored to optimize the photo sensitivity. 
         [0025]      FIG. 4C  illustrates a heterojunction structure  450  deposed on top of the insulator  403  over the semiconductor substrate  410  for generating additional electron-hole pairs. As an example, the heterojunction structure  450  comprises an intrinsic semiconductor layer  455  being sandwiched between two semiconductor blocking layers  460  and  465  of different material composition, such as GaAs or GaAsInP. These layers  455 ,  460  and  465  have non-equal band gaps. The intrinsic semiconductor layer  455  generates electron-hole pairs when light shines on the structure  450 . These electrons and holes may tunnel through the thin insulation layer  403  and reach the semiconductor substrate  410 . 
         [0026]      FIG. 4D  illustrates a Schottkey barrier formed by a photoconductor layer  475 , e.g., a-Si:H, being sandwiched between two layers  480  and  485  biased as electrodes. The electrode layers  480  and  485  provide a Schottkey barrier lowering effect for electron-hole pairs being generated in the photoconductor layer  475  by light. {If this is a Schottkey barrier. Just say so.} Then these electrons and holes may tunnel through the thin insulation layer  403 . In all the cases of  FIG. 4A through 4D , the insulation layer  403  has to be thin enough to allow carriers to tunnel through and reach the semiconductor substrate  410  where the conventional P-N junction is formed therein. As a result of these tunneled-in carriers, a combination current in the photosensitive structure  400 ,  420 ,  450  or  470  will be larger than in a case that has only the conventional P-N junction. 
         [0027]      FIGS. 5A and 5B  are cross-sectional views of CMOS image sensor cells  500  and  550  with photosensitive layers  505  and  555  applied, respectively, according to embodiments of the present invention. The CMOS image sensor cells  500  and  550  are very similar to the CMOS image sensor cells  200  as shown in  FIG. 2 , where a conventional P-N junction diode (not shown) is formed in the substrate  210 . A passivation layer  220  is applied on the substrate  210 . Then a planarization layer  230  is processed on top of the passivation layer  220  to make the semiconductor surface flat for subsequent applications of a color filter  240 , a spacer  250  and micro-lenses  260 . All these layers  220  through  260  are transparent to light. Referring to  FIG. 5A , the photosensitive layer  505  is disposed between the color filter  240  and the spacer  250 . Photon generated carriers will have to travel through the color filter  240 , the planarization layer  230  and the passivation layer  220  to reach the semiconductor substrate. Referring to  FIG. 5B , the photosensitive layer  555  is disposed between the planarization layer  230  and the color filter  240 . Photon generated carriers will only have to travel through the planarization layer  230  and the passivation layer  220  to reach the semiconductor substrate  210 . The additional carriers that reach the semiconductor substrate  210  will increase the combination current therein and hence the photosensitivity of the CMOS image sensor cells  500  and  550 . Thicknesses of the passivation layers  220  and the planarization layer  230  may be used to adjust the sensitivity level of the CMOS image sensor cells  500  and  550 . 
         [0028]    Although the P-N junction diode is used for collecting the photon generated carriers in above embodiments of the present invention, one having skills in the art would appreciate other types of semiconductor devices may also serve that purpose as long as the device is reverse biased with little or no current of itself, yet, photon generated electrons and holes may combine therein and create a combination current with a magnitude proportional to an incoming photon density. 
         [0029]    The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
         [0030]    Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.