Abstract:
A trench photosensor for use in a CMOS imager having an improved charge capacity. The trench photosensor may be either a photogate or photodiode structure. The trench shape of the photosensor provides the photosensitive element with an increased surface area compared to a flat photosensor occupying a comparable area on a substrate. The trench photosensor also exhibits a higher charge capacity, improved dynamic range, and a better signal-to-noise ratio. Also disclosed are processes for forming the trench photosensor.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to improved semiconductor imaging devices and in particular to a silicon imaging device that can be fabricated using a standard CMOS process.  
         BACKGROUND OF THE INVENTION  
         [0002]    There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices and hybrid focal plane arrays. CCD technology is often employed for image acquisition and enjoys a number of advantages which makes it the incumbent technology, particularly for small size imaging applications. CCDs are capable of large formats with small pixel size and they employ low noise charge domain processing techniques. However, CCD imagers also suffer from a number of disadvantages. For example, they are susceptible to radiation damage, they exhibit destructive read-out over time, they require good light shielding to avoid image smear and they have a high power dissipation for large arrays. Additionally, while offering high performance, CCD arrays are difficult to integrate with CMOS processing in part due to a different processing technology and to their high capacitances, complicating the integration of on-chip drive and signal processing electronics with the CCD array. While there have been some attempts to integrate on-chip signal processing with CCD arrays, these attempts have not been entirely successful. CCDs also must transfer an image by line charge transfers from pixel to pixel, requiring that the entire array be read out into a memory before individual pixels or groups of pixels can be accessed and processed. This takes time. CCDs may also suffer from incomplete charge transfer from pixel to pixel which results in image smear.  
           [0003]    Because of the inherent limitations in CCD technology, there is an interest in CMOS imagers for possible use as low cost imaging devices. A filly compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital applications such as, for example, in cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detection systems, image stabilization systems and data compression systems for high-definition television.  
           [0004]    The advantages of CMOS imagers over CCD imagers are that CMOS imagers have a low voltage operation and low power consumption; CMOS imagers are compatible with integrated on-chip electronics (control logic and timing, image processing, and signal conditioning such as A/D conversion); CMOS imagers allow random access to the image data; and CMOS imagers have lower fabrication costs as compared with the conventional CCD because standard CMOS processing techniques can be used. Additionally, low power consumption is achieved for CMOS imagers because only one row of pixels at a time needs to be active during the readout and there is no charge transfer (and associated switching) from pixel to pixel during image acquisition. On-chip integration of electronics is particularly advantageous because of the potential to perform many signal conditioning functions in the digital domain (versus analog signal processing) as well as to achieve a reduction in system size and cost.  
           [0005]    A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including either a photogate, photoconductor or a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. A readout circuit is connected to each pixel cell and includes at least an output field effect transistor formed in the substrate and a charge transfer section formed on the substrate adjacent the photogate, photoconductor or photodiode having a sensing node, typically a floating diffusion node, connected to the gate of an output transistor. The imager may include at least one electronic device such as a transistor for transferring charge from the underlying portion of the substrate to the floating diffusion node and one device, also typically a transistor, for resetting the node to a predetermined charge level prior to charge transference.  
           [0006]    In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node accompanied by charge amplification; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. The charge at the floating diffusion node is typically converted to a pixel output voltage by a source follower output transistor. The photosensitive element of a CMOS imager pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate. For photodiodes, image lag can be eliminated by completely depleting the photodiode upon readout.  
           [0007]    CMOS imagers of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994), as well as U.S. Pat. No. 5,708,263 and U.S. Pat. No. 5,471,515, which are herein incorporated by reference.  
           [0008]    To provide context for the invention, an exemplary CMOS imaging circuit is described below with reference to FIG. 1. The circuit described below, for example, includes a photogate for accumulating photo-generated charge in an underlying portion of the substrate. It should be understood that the CMOS imager may include a photodiode or other image to charge converting device, in lieu of a photogate, as the initial accumulator for photo-generated charge.  
           [0009]    Reference is now made to FIG. 1 which shows a simplified circuit for a pixel of an exemplary CMOS imager using a photogate and having a pixel photodetector circuit  14  and a readout circuit  60 . It should be understood that while FIG. 1 shows the circuitry for operation of a single pixel, that in practical use there will be an M×N array of pixels arranged in rows and columns with the pixels of the array accessed using row and column select circuitry, as described in more detail below.  
           [0010]    The photodetector circuit  14  is shown in part as a cross-sectional view of a semiconductor substrate  16  typically a p-type silicon, having a surface well of p-type material  20 . An optional layer  18  of p-type material may be used if desired, but is not required. Substrate  16  may be formed of, for example, Si, SiGe, Ge, or GaAs. Typically the entire substrate  16  is p-type doped silicon substrate and may contain a surface p-well  20  (with layer  18  omitted), but many other options are possible, such as, for example p on p− substrates, p on p+ substrates, p-wells in n-type substrates or the like. The terms wafer or substrate used in the description includes any semiconductor-based structure having an exposed surface in which to form the circuit structure used in the invention. Wafer and substrate are to be understood as including silicon-on-insulator (SOI) technology, silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a wafer or substrate in the following description, previous process steps may have been utilized to form regions/junctions in the base semiconductor structure or foundation.  
           [0011]    An insulating layer  22  such as, for example, silicon dioxide is formed on the upper surface of p-well  20 . The p-type layer may be a p-well formed in substrate  16 . A photogate  24  thin enough to pass radiant energy or of a material which passes radiant energy is formed on the insulating layer  22 . The photogate  24  receives an applied control signal PG which causes the initial accumulation of pixel charges in n+ region  26 . The n+ type region  26 , adjacent one side of photogate  24 , is formed in the upper surface of p-well  20 . A transfer gate  28  is formed on insulating layer  22  between n+ type region  26  and a second n+ type region  30  formed in p-well  20 . The n+ regions  26  and  30  and transfer gate  28  form a charge transfer transistor  29  which is controlled by a transfer signal TX. The n+ region  30  is typically called a floating diffusion region. It is also a node for passing charge accumulated thereat to the gate of a source follower transistor  36  described below. A reset gate  32  is also formed on insulating layer  22  adjacent and between n+ type region  30  and another n+ region  34  which is also formed in p-well  20 . The reset gate  32  and n+ regions  30  and  34  form a reset transistor  31  which is controlled by a reset signal RST. The n+ type region  34  is coupled to voltage source V DD , e.g., 5 volts. The transfer and reset transistors  29 ,  31  are n-channel transistors as described in this implementation of a CMOS imager circuit in a p-well. It should be understood that it is possible to implement a CMOS imager in an n-well in which case each of the transistors would be p-channel transistors. It should also be noted that while FIG. 1 shows the use of a transfer gate  28  and associated transistor  29 , this structure provides advantages, but is not required.  
           [0012]    Photodetector circuit  14  also includes two additional n-channel transistors, source follower transistor  36  and row select transistor  38 . Transistors  36 ,  38  are coupled in series, source to drain, with the source of transistor  36  also coupled over lead  40  to voltage source V DD  and the drain of transistor  38  coupled to a lead  42 . The drain of row select transistor  38  is connected via conductor  42  to the drains of similar row select transistors for other pixels in a given pixel row. A load transistor  39  is also coupled between the drain of transistor  38  and a voltage source V SS , e.g. 0 volts. Transistor  39  is kept on by a signal V LN  applied to its gate.  
           [0013]    The imager includes a readout circuit  60  which includes a signal sample and hold (S/H) circuit including a S/H n-channel field effect transistor  62  and a signal storage capacitor  64  connected to the source follower transistor  36  through row transistor  38 . The other side of the capacitor  64  is connected to a source voltage V SS . The upper side of the capacitor  64  is also connected to the gate of a p-channel output transistor  66 . The drain of the output transistor  66  is connected through a column select transistor  68  to a signal sample output node V OUTS  and through a load transistor  70  to the voltage supply V DD . A signal called “signal sample and hold” (SHS) briefly turns on the S/H transistor  62  after the charge accumulated beneath the photogate electrode  24  has been transferred to the floating diffusion node  30  and from there to the source follower transistor  36  and through row select transistor  38  to line  42 , so that the capacitor  64  stores a voltage representing the amount of charge previously accumulated beneath the photogate electrode  24 .  
           [0014]    The readout circuit  60  also includes a reset sample and hold (S/H) circuit including a S/H transistor  72  and a signal storage capacitor  74  connected through the S/H transistor  72  and through the row select transistor  38  to the source of the source follower transistor  36 . The other side of the capacitor  74  is connected to the source voltage V SS . The upper side of the capacitor  74  is also connected to the gate of a p-channel output transistor  76 . The drain of the output transistor  76  is connected through a p-channel column select transistor  78  to a reset sample output node V OUTR  and through a load transistor  80  to the supply voltage V DD . A signal called “reset sample and hold” (SHR) briefly turns on the S/H transistor  72  immediately after the reset signal RST has caused reset transistor  31  to turn on and reset the potential of the floating diffusion node  30 , so that the capacitor  74  stores the voltage to which the floating diffusion node  30  has been reset.  
           [0015]    The readout circuit  60  provides correlated sampling of the potential of the floating diffusion node  30 , first of the reset charge applied to node  30  by reset transistor  31  and then of the stored charge from the photogate  24 . The two samplings of the diffusion node  30  charges produce respective output voltages V OUTR  and V OUTS  of the readout circuit  60 . These voltages are then subtracted (V OUTS −V OUTR ) by subtractor  82  to provide an output signal terminal  81  which is an image signal independent of pixel to pixel variations caused by fabrication variations in the reset voltage transistor  31  which might cause pixel to pixel variations in the output signal.  
           [0016]    [0016]FIG. 2 illustrates a block diagram for a CMOS imager having a pixel array  200  with each pixel cell being constructed in the manner shown by element  14  of FIG. 1. FIG. 4 shows a 2×2 portion of pixel array  200 . Pixel array  200  comprises a plurality of pixels arranged in a predetermined number of columns and rows. The pixels of each row in array  200  are all turned on at the same time by a row select line, e.g., line  86 , and the pixels of each column are selectively output by a column select line, e.g., line  42 . A plurality of rows and column lines are provided for the entire array  200 . The row lines are selectively activated by the row driver  210  in response to row address decoder  220  and the column select lines are selectively activated by the column driver  260  in response to column address decoder  270 . Thus, a row and column address is provided for each pixel. The CMOS imager is operated by the control circuit  250  which controls address decoders  220 ,  270  for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry  210 ,  260  which apply driving voltage to the drive transistors of the selected row and column lines.  
           [0017]    [0017]FIG. 3 shows a simplified timing diagram for the signals used to transfer charge out of photodetector circuit  14  of the FIG. 1 CMOS imager. The photogate signal PG is nominally set to 5V and pulsed from 5V to 0V during integration. The reset signal RST is nominally set at 2.5V. As can be seen from the figure, the process is begun at time to by briefly pulsing reset voltage RST to 5V. The RST voltage, which is applied to the gate  32  of reset transistor  31 , causes transistor  31  to turn on and the floating diffusion node  30  to charge to the V DD  voltage present at n+ region  34  (less the voltage drop V TH  of transistor  31 ). This resets the floating diffusion node  30  to a predetermined voltage (V DD −V TH ). The charge on floating diffusion node  30  is applied to the gate of the source follower transistor  36  to control the current passing through transistor  38 , which has been turned on by a row select (ROW) signal, and load transistor  39 . This current is translated into a voltage on line  42  which is next sampled by providing a SHR signal to the S/H transistor  72  which charges capacitor  74  with the source follower transistor output voltage on line  42  representing the reset charge present at floating diffusion node  30 . The PG signal is next pulsed to 0 volts, causing charge to be collected in n+ region  26 . A transfer gate voltage TX, similar to the reset pulse RST, is then applied to transfer gate  28  of transistor  29  to cause the charge in n+ region  26  to transfer to floating diffusion node  30 . It should be understood that for the case of a photogate, the transfer gate voltage TX may be pulsed or held to a fixed DC potential. For the implementation of a photodiode with a transfer gate, the transfer gate voltage TX must be pulsed. The new output voltage on line  42  generated by source follower transistor  36  current is then sampled onto capacitor  64  by enabling the sample and hold switch  62  by signal SHS. The column select signal is next applied to transistors  68  and  70  and the respective charges stored in capacitors  64  and  74  are subtracted in subtractor  82  to provide a pixel output signal at terminal  81 . It should also be noted that CMOS imagers may dispense with the transfer gate  28  and associated transistor  29 , or retain these structures while biasing the transfer transistor  29  to an always “on” state.  
           [0018]    The operation of the charge collection of the CMOS imager is known in the art and is described in several publications such as Mendis et al., “Progress in CMOS Active Pixel Image Sensors,” SPIE Vol. 2172, pp. 19-29 (1994); Mendis et al., “CMOS Active Pixel Image Sensors for Highly Integrated Imaging Systems,” IEEE Journal of Solid State Circuits, Vol. 32(2) (1997); and Eric R. Fossum, “CMOS Image Sensors: Electronic Camera on a Chip,” IEDM Vol. 95, pp. 17-25 (1995) as well as other publications. These references are incorporated herein by reference.  
           [0019]    Prior CMOS pixel photosensors suffer dynamic range and charge capacity limitations, and undesirably low signal-to-noise ratios. Attempts to increase charge capacity and improve signal-to-noise ratios have typically focused on using photogate photosensors instead of photodiodes, adding transfer gate stacks to enhance charge transfer, and increasing the size of the photosensor. These methods add process complexity, may limit the use of advantageous features such as silicided gates, and may result in increased pixel cell sizes, thereby reducing pixel array densities.  
           [0020]    There is needed, therefore, an improved pixel photosensor for use in an imager that exhibits improved dynamic range, a better signal-to-noise ratio, and improved charge capacity for longer integration times. A method of fabricating a pixel photosensor exhibiting these improvements is also needed.  
         SUMMARY OF THE INVENTION  
         [0021]    The present invention provides a trench photosensor formed in a doped semiconductor substrate for use in a pixel sensor cell. The trench photosensor comprises a doped region on the sides and bottom of a trench, with a conductive layer formed over the doped region. For a photogate-type photosensor, a dielectric layer is preferably formed on the trench sides and bottom prior to forming the conductive layer. Also provided are methods for forming the trench photosensor of the present invention.  
           [0022]    Additional advantages and features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a representative circuit of a CMOS imager.  
         [0024]    [0024]FIG. 2 is a block diagram of a CMOS pixel sensor chip.  
         [0025]    [0025]FIG. 3 is a representative timing diagram for the CMOS imager.  
         [0026]    [0026]FIG. 4 is a representative pixel layout showing a 2×2 pixel layout.  
         [0027]    [0027]FIG. 5 is a cross-sectional view of a pixel sensor cell according to one embodiment of the present invention.  
         [0028]    [0028]FIG. 6 is a cross-sectional view of a semiconductor wafer undergoing the process of a preferred embodiment of the invention.  
         [0029]    [0029]FIG. 7 shows the wafer of FIG. 6 at a processing step subsequent to that shown in FIG. 6.  
         [0030]    [0030]FIG. 8 shows the wafer of FIG. 6 at a processing step subsequent to that shown in FIG. 7.  
         [0031]    [0031]FIG. 9 shows the wafer of FIG. 6 at a processing step subsequent to that shown in FIG. 8.  
         [0032]    [0032]FIG. 10 shows the wafer of FIG. 6 at a processing step subsequent to that shown in FIG. 9.  
         [0033]    [0033]FIG. 11 shows the wafer of FIG. 6 at a processing step subsequent to that shown in FIG. 10.  
         [0034]    [0034]FIG. 12 shows the wafer of FIG. 6 undergoing an alternative process according to an embodiment of the present invention.  
         [0035]    [0035]FIG. 13 shows the wafer of FIG. 12 at a processing step subsequent to that shown in FIG. 12.  
         [0036]    [0036]FIG. 14 is an illustration of a computer system having a CMOS imager according to the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0037]    In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention.  
         [0038]    The terms “wafer” and “substrate” are to be understood as including silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a “wafer” or “substrate” in the following description, previous process steps may have been utilized to form regions or junctions in the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, germanium, or gallium arsenide.  
         [0039]    The term “pixel” refers to a picture element unit cell containing a photosensor and transistors for converting electromagnetic radiation to an electrical signal. For purposes of illustration, a representative pixel is illustrated in the figures and description herein, and typically fabrication of all pixels in an imager will proceed simultaneously in a similar fashion. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.  
         [0040]    The structure of the pixel cell  14  of the first embodiment is shown in more detail in FIG. 5. The pixel cell  14  may be formed in a substrate  16  having a doped layer or well  20  of a first conductivity type, which for exemplary purposes is treated as a p-type substrate. The doped layer  20  is provided with three doped regions  26 ,  30 , and  34 , which are doped to a second conductivity type, which for exemplary purposes is treated as n-type. The first doped region  26  is the photosite, and it underlies a conductive layer  102  of material transparent to radiant energy, such as polysilicon. The photosite  26  and the conductive layer  102  together form a photosensor  24 . An insulating layer  100  of silicon dioxide, silicon nitride, ON (oxide-nitride), NO (nitride-oxide), ONO (oxide-nitride-oxide) or other suitable material is formed between the conductive layer  102  and the photosite  26 . If a deposited insulating layer  100  is used (as opposed to a grown layer), it may extend over a pixel-isolating field oxide region  114  on the opposite side of the photosensor  24  from the transfer gate  28 . The second doped region  30  is the floating diffusion region, sometimes also referred to as a floating diffusion node, and it serves as the source for the reset transistor  31 . The third doped region  34  is the drain of the reset transistor  31 , and is also connected to voltage source V DD .  
         [0041]    The trench photosensor  24  is manufactured through a process described as follows, and illustrated by FIGS. 6 through 11. Referring now to FIG. 6, a substrate  16 , which may be any of the types of substrates described above, is doped to form a doped substrate layer or well  20  of a first conductivity type, which for exemplary purposes will be described as p-type. A field oxide layer  114  is formed around the cell  14  at this time, and is shown in FIG. 5 as residing on a side of the photosite  26  opposite the transfer gate  28  and adjacent to the third doped region  34 . The field oxide layer  114  may be formed by any known technique such as thermal oxidation of the underlying silicon in a LOCOS process or by etching trenches and filling them with oxide in an STI process.  
         [0042]    Next, the reset transistor gate stack  32  and an optional transfer gate stack  28  are formed. These include a silicon dioxide or silicon nitride insulator  106  on the doped layer  20 , and a conductive layer  108  of doped polysilicon, tungsten, or other suitable material over the insulating layer  106 . An insulating cap layer  110  of, for example, silicon dioxide, silicon nitride, ON, NO, or ONO may be formed if desired; also a more conductive layer such as a silicide layer (not shown) may be used between the conductive layer  108  and the cap  110 . Insulating sidewalls  112  are also formed on the sides of the gate stacks  28 ,  32 . These sidewalls  112  may be formed of, for example, silicon dioxide, silicon nitride, ON, NO or ONO.  
         [0043]    As shown in FIG. 7, the next step is to form a trench in the doped layer  20 . A resist and mask (not shown) are applied, and photolithographic techniques are used to define the area to be etched-out. A directional etching process such as Reactive Ion Etching (RIE), or etching with a preferential amsotropic etchant is used to etch into the doped layer  20  to a sufficient depth, e.g., about 0.05 to 10 μm, to form a trench  104 . The deeper the trench  104 , the higher the charge storage capacitance of the imager. The resist and mask are removed, leaving a structure that appears as shown in FIG. 7.  
         [0044]    While the gate stacks may be formed after the trench is etched, for exemplary purposes and for convenience etching of the trench is described as occurring subsequent to gate stack formation. The order of these preliminary process steps may be varied as is required or convenient for a particular process flow, for example, if a photogate sensor which overlaps the transfer gate is desired, the gate stacks must be formed before the photogate, but if a non-overlapping photogate is desired, the gate stacks are preferably formed after photogate formation. Similarly, fabrication of a photodiode photosensor is greatly simplified if the gate stacks are fabricated before the trench is etched.  
         [0045]    In the next step of the process, doped regions are formed in the doped substrate layer  20  by any suitable doping process, such as ion implantation. A resist and mask (not shown) are used to shield areas of the layer  20  that are not to be doped. Three doped regions are formed in this step: the photosite  26 , which is formed in the sides and bottom of the trench  104 ; the floating diffusion region  30 ; and a drain region  34 .  
         [0046]    The ion implantation of doped region  26  is preferably performed as a series of angled implants, typically four, to assure a more uniformly doped trench sidewall. FIG. 8 illustrates a resist layer  120  which covers all of the surface of the substrate layer  20  except the trench  104  to be doped. The implants are performed at implantation angles θ 1  that are greater than the critical angle θ C , where each implant is orthogonal to the last implant performed. The value of θ C  is calculated according to the equation tan θ C =[(t+d)/(w)], where t is the thickness of the resist  120 , d is the depth of the trench  104 , and w is the width of the trench  104 . The dose of each implant is between 1×10 12  ions/cm 2  and 1×10 16  ions/cm 2 , preferably between 1×10 13  ions/cm 2  and 1×10 15  ions/cm 2 , and most preferably about 5×10 13  ions/cm 2 .  
         [0047]    After formation of the first doped region  26 , the resist  120  and mask are stripped, and a second resist and mask (not shown) are applied. Standard ion implantation is then performed to dope the second and third doped regions  30 ,  34 . As shown in FIG. 9, the doped regions  26 ,  30 ,  34  are doped to a second conductivity type, which for exemplary purposes will be considered to be n-type. The doping level of the doped regions  26 ,  30 ,  34  may vary but should be of comparable or greater strength than the doping level of the doped layer  20 . Doped region  26  may be variably doped, such as either n+ or n− for an n-channel device. Doped region  34  should be strongly doped, i.e., for an n-channel device, the doped region  34  will be doped as n+. Doped region  30  is typically strongly doped (n+), and would not be lightly doped (n−) unless a buried contact is also used. If desired, multiple masks and resists may be used to dope regions  30 ,  34  to different levels.  
         [0048]    Referring now to FIG. 10, an insulating layer  100  may now be formed on the sides and bottom of the trench  104  by chemical vapor deposition, thermal oxidation or other suitable means. The insulating layer  100  may be of silicon dioxide, silicon nitride, NO, ON, ONO, or other suitable material, and it has a thickness of approximately 20 to 500 Angstroms for a photogate photosensor. If a photodiode is formed instead of a photogate, the insulating layer  100  would typically be at least 30 Angstroms thick, and may, with the addition of further insulating and passivating layers on the device, be approximately 5 microns thick.  
         [0049]    As shown in FIG. 11, the final step in the process of the present invention is to form the photogate  24 . The photogate  24  has a thin conductive layer  102  that is at least partially transparent to electromagnetic radiation of the wavelengths desired to be sensed. The conductive layer  102  is of a first conductivity type, and may be doped polysilicon, indium tin oxide, tin oxide, or other suitable material. The thickness of the conductive layer  102  may be any suitable thickness, e.g., approximately 200 to 4000 Angstroms. If the conductive material is a silicon material, then the conductive layer  102  will be formed by CVD or other suitable means, and if the conductive material is a metal compound, CVD, evaporation or sputtering are preferred means of forming the conductive layer  102 . The conductive layer  102  is formed to cover substantial portions of the insulating layer  100 , and may extend at least partially over the field oxide layer  114  and a portion of the transfer gate  28 . The photosensor  24  at this stage is shown in FIG. 11.  
         [0050]    For the pixel cell  14  of the first embodiment, the photosensor  24  is essentially complete at this stage, and conventional processing methods may then be used to form contacts and wiring to connect gate lines and other connections in the pixel cell  14 . For example, the entire surface may then be covered with a passivation layer of, e.g., silicon dioxide, BSG, PSG, or BPSG, which is CMP planarized and etched to provide contact holes, which are then metallized to provide contacts to the photogate, reset gate, and transfer gate. Conventional multiple layers of conductors and insulators may also be used to interconnect the structures in the manner shown in FIG. 1.  
         [0051]    An alternative embodiment of the process is illustrated by FIG. 6 and FIGS. 12 and 13. As shown in FIG. 6, this process also begins with a substrate  16  having a doped layer or well  20  of a first conductivity type, e.g., p-type, on which the transfer gate  28  and the reset transistor gate  32  have been formed. Referring now to FIG. 12, the next step in the alternative process is to form doped regions and a deep doped well  116  in the doped layer  20 . A resist and mask (not shown) are used to expose only the areas to be doped, and a suitable doping process, such as ion implantation, is used to form a deep well  116  of a second conductivity type, e.g., n-type, in the doped layer  20 . The doped regions  30 ,  34  may also be formed at this time by ion implantation or other suitable means.  
         [0052]    As shown in FIG. 13, the next step is to form a trench in the well  116 . A resist and mask (not shown) are applied, and photolithographic techniques are used to define the area to be etched-out. A directional etching process such as Reactive Ion Etching (RIE), or etching with a preferential anisotropic etchant is used to etch into the well  116  to a sufficient depth, e.g., about 0.05 to 10 μm to form a trench  104 . The depth of the trench should be sufficient to form the photosensor  24  of the present invention therein. The resist and mask are removed, leaving a structure that appears as shown in FIG. 13. The photosensor  24  is then further formed according to the process described above in conjunction with reference to FIGS. 10 and 11. Pixel arrays having the photosensors of the present invention, and described with reference to FIGS.  5 - 13 , may be further processed as known in the art to arrive at CMOS imagers having the functions and features of those discussed with reference to FIGS.  1 - 4 .  
         [0053]    A typical processor based system which includes a CMOS imager device according to the present invention is illustrated generally at  400  in FIG. 14. A processor based system is exemplary of a system having digital circuits which could include CMOS imager devices. Without being limiting, such a system could include a computer system, camera system, scanner, machine vision system, vehicle navigation system, video telephone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system and data compression system for high-definition television, all of which can utilize the present invention.  
         [0054]    A processor system, such as a computer system, for example generally comprises a central processing unit (CPU)  444 , e.g., a microprocessor, that communicates with an input/output (I/O) device  446  over a bus  452 . The CMOS imager  442  also communicates with the system over bus  452 . The computer system  400  also includes random access memory (RAM)  448 , and, in the case of a computer system may include peripheral devices such as a floppy disk drive  454  and a compact disk (CD) ROM drive  456  which also communicate with CPU  444  over the bus  452 . CMOS imager  442  is preferably constructed as an integrated circuit which includes pixels containing a photosensor such as a photogate or photodiode formed in a trench, as previously described with respect to FIGS. 5 through 13. The CMOS imager  442  may be combined with a processor, such as a CPU, digital signal processor or microprocessor, with or without memory storage, in a single integrated circuit, or maybe on a different chip than the processor.  
         [0055]    As can be seen by the embodiments described herein, the present invention encompasses a photosensor such as a photogate or photodiode formed in a trench. The trench photosensor has an improved charge capacity due to the increase in surface area of the trench photosensor compared to conventional flat photosensors. In addition, the trench photosensor occupies a smaller area than a flat photosensor, thus allowing the size of the pixel cell to be decreased.  
         [0056]    It should again be noted that although the invention has been described with specific reference to CMOS imaging circuits having a photogate and a floating diffusion region, the invention has broader applicability and may be used in any CMOS imaging apparatus. Similarly, the process described above is but one method of many that could be used. The above description and drawings illustrate preferred embodiments which achieve the objects, features and advantages of the present invention. It is not intended that the present invention be limited to the illustrated embodiments. Any modification of the present invention which comes within the spirit and scope of the following claims should be considered part of the present invention.