Abstract:
An imaging device formed as a CMOS semiconductor integrated circuit includes a nitrogen containing insulating material beneath a photogate. The nitrogen containing insulating material, preferably be one of a silicon nitride layer, an ONO layer, a nitrode/oxide layer and an oxide/nitrode layer. The nitrogen containing insulating layer provides an increased capacitance in the photogate region, higher breakdown voltage, a wider dynamic range and an improved signal to noise ratio. The invention also provides a method for fabricating a CMOS imager containing the nitrogen containing insulating layer.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to improved semiconductor imaging devices and in particular to a silicon imaging device which can be fabricated using a standard CMOS process. Particularly, the invention relates to CMOS imager having a nitride, a layered nitride and oxide film or a layered film of SiO 2 , SiN and SiO 2  (“ONO”) as an insulator between the semiconductor substrate and the photogate. The invention also provides a method for fabricating the CMOS imager. 
       DISCUSSION OF RELATED ART 
       [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 plan arrays. CCDs are often employed for image acquisition and enjoy a number of advantages which makes it the incumbent technology, particularly for small size imaging applications. CCDs are also 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 has been some attempts to integrate on-chip signal processing with the CCD array, 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 during charge transfer which also 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 fully 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 since standard CMOS processing techniques can be used. Additionally, low power consumption is achieved for 
         [0005]    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. 
         [0006]    A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including either a photogate 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 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. 
         [0007]    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 photo diodes, image lag can be eliminated by completely depleting the photodiode upon readout. 
         [0008]    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. 
         [0009]    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. 
         [0010]    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. 
         [0011]    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, and 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. 
         [0012]    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 VDD. 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. 
         [0013]    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 VDD 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 VSS. Transistor  39  is kept on by a signal VLN applied to its gate. 
         [0014]    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 VSS. 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 VOUTS and through a load transistor  70  to the voltage supply VDD. 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 . 
         [0015]    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 VSS. 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 VOUTR and through a load transistor  80  to the supply voltage VDD. 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. 
         [0016]    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 VOUTR and VOUTS of the readout circuit  60 . These voltages are then subtracted (VOUTS-VOUTR) 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. 
         [0017]      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 . 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 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. 
         [0018]      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 the reset signal RST is nominally set at 2.5V. As can be seen from the figure, the process is begun at time t o  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 VDD voltage present at n+ region  34  (less the voltage drop Vth of transistor  31 ). This resets the floating diffusion node  30  to a predetermined voltage (VDD-Vth). 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. 
         [0019]    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 pages 17-25 (1995) as well as other publications. These references are incorporated herein by reference. 
         [0020]    Prior CMOS imagers which use a silicon dioxide (SiO 2 ) insulator between the photogate and the charge accumulation area beneath the photogate suffer from low capacitance, which reduces the signal to noise ratio of the imager. Low capacitance reduces the charge storage capacity of the accumulated electrical charge within the pixel and decreases the dynamic range of CMOS imagers. Since the size of the pixel electrical signal is very small due to the collection of photons in the photo array, the signal to noise ratio of the pixel should be as high as possible. In order to increase capacitance, a thinner SiO 2  insulator is required. However, if the insulator layer is too thin it inhibits proper operation of the photogate and the various transistor gates of the imager by the creation of pinhole defects, gate to substrate leakage and low breakdown voltage of the insulator. 
       SUMMARY OF THE INVENTION 
       [0021]    The present invention provides an imaging device formed as a CMOS integrated circuit using a standard CMOS process. The imager circuit includes a floating diffusion region which is connected to a gate of a source follower transistor. In a preferred implementation, the CMOS imager includes a nitride, nitride/oxide or ONO insulating material between diffusion regions in the substrate and overlying gate area of the photogate. The nitrogen containing insulating material provides increased capacitance in the CMOS imager, better breakdown voltage than prior SiO 2  insulators, better breakdown voltage characteristics between the photogate and transfer gate for the case of a photo gate overlapping the transfer gate, and a wider dynamic range and an improved signal to noise ratio. The higher breakdown voltage also allows the imager photogate to operate at higher potentials with a further increase in charge storage capacity. 
         [0022]    The above and other advantages and features of the invention will be more clearly understood from the following detailed description which is provided in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a representative circuit of a CMOS imager. 
           [0024]      FIG. 2  is a block diagram of a CMOS active pixel sensor chip. 
           [0025]      FIG. 3  is a representative timing diagram for the CMOS imager. 
           [0026]      FIG. 4  illustrates a partially cut away side view of a portion of a semiconductor CMOS imager wafer in an interim stage of processing. 
           [0027]      FIG. 5  illustrates a partially cut away side view of a portion of a semiconductor CMOS imager wafer subsequent to  FIG. 4 . 
           [0028]      FIG. 6  illustrates a partially cut away side view of a portion of a semiconductor CMOS imager wafer subsequent to  FIG. 5 . 
           [0029]      FIG. 7  illustrates a partially cut away side view of a portion of a semiconductor CMOS imager wafer subsequent to  FIG. 6 . 
           [0030]      FIG. 8  illustrates a partially cut away side view of a portion of a semiconductor CMOS imager wafer subsequent to  FIG. 7 . 
           [0031]      FIG. 9  illustrates a partially cut away side view of a portion of a semiconductor CMOS imager wafer subsequent to  FIG. 8 . 
           [0032]      FIG. 10  illustrates a partially cut away side view of a portion of a semiconductor CMOS imager wafer subsequent to  FIG. 9 . 
           [0033]      FIG. 11  is an illustration of a computer system having a CMOS imager according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    The present invention will now be described with reference to the figures. While the invention is described with respect to an imager pixel using n-channel transistors in a p-well, it should be understood that the present invention may also be used with p-channel transistors in an n-well. Referring now to  FIG. 4 . This figure shows a portion of a CMOS imager  300  at one point in the fabrication process. The substrate  310  has field oxide regions  341  formed to surround and isolate the cells which may be formed by thermal oxidation. 
         [0035]    A first insulating layer  315  is grown over a surface of substrate  310 . 
         [0036]    Preferably the first insulating layer  315  is a silicon dioxide layer which may be formed, i.e., grown, by conventional methods. Preferably the first insulating layer  315  is grown by thermal oxidation of silicon and the first insulating layer  315  has a thickness of from about 30 to about 500 angstroms. A transfer gate stack  320  and a reset gate stack  325  are deposited and patterned over the first insulating layer  315  as shown in  FIG. 5 . The gate stacks  320 ,  325  include a doped polysilicon layer on insulating layer  315 , or a doped polysilicon layer with a silicide layer on top of the polysilicon on the insulating layer  315  or a doped polysilicon/silicide/insulator on the insulating layer  315 , or the gate stacks  320 ,  325  may be formed of any material known to form a gate electrode. 
         [0037]    An insulating layer  343  is formed, i.e., grown, on the edges of the exposed polysilicon gate stacks  320 ,  325  and anisotropically etched to remove the insulator deposited on the horizontal surfaces as shown in  FIG. 6 . Typically the insulating layer  343  is a deposited oxide or nitride. 
         [0038]    As shown in  FIG. 7 , the first insulating layer  315  is partly or wholly removed from the substrate by etching such that it only remains under gate stacks  320  and  325 . The doped regions  312  and  313  are then formed in the substrate  310  as shown in  FIG. 7 . Any suitable doping process may be used, such as, for example, ion implantation. A resist and mask (not shown) may be used to shield other areas that are not to be doped. Doped region  311  may also formed in this step. The region  311  may be formed in a similar manner to doped regions  312  and  313 . The doped region  311  may be formed with the same mask used for forming doped regions  312  and  313  or doped region  311  may be formed using a separate mask in an additional step. After the doped regions  311 ,  312  and  313  are formed, thermal processing is used to fully form regions  311 ,  312  and  313  under portions of the gate stacks  320  and  325  as shown in  FIG. 7 . 
         [0039]    A nitrogen containing insulating layer  330  is then deposited onto the substrate as illustrated in  FIG. 8 . The nitrogen containing insulating layer  330  is preferably a silicon nitride layer, a nitride oxide (NO) layer, an oxide/nitride (ON) layer or an ONO (oxide-nitride-oxide) layer deposited by a combination of CVD and thermal oxidation. For example, the first oxide layer may be thermally grown or deposited by CVD. The nitride layer is then typically deposited by CVD. The final step in forming the ONO layer is to thermally oxidize the nitride layer to form the final oxide layer. However, it should be understood that any deposition or growth method may be used or any combination of growth and deposition. Preferably the nitrogen containing insulating layer has a thickness of from about 20 to about 500 angstroms, more preferably from about 30 to about 100 angstroms. 
         [0040]    A second conductive layer  350  is next deposited over the substrate  310  and the nitrogen containing insulating layer  330 . Preferably the second conductive layer  350  is formed of a doped polysilicon, however any partially transparent conductive material such as indium-tin-oxide (ITO), tin oxide, indium oxide or doped hydrogenated amorphous silicon may be used. The second conductive layer  350  is patterned and etched so that the second conductive layer  350  remains over the photogate  360  as shown in  FIG. 9 . The deposited nitride, ON, NO, ONO film provides better isolation between gate  320  and conductor  350  than a grown or deposited oxide layer. During the etching of layer  350  or in a subsequent etch, the nitrogen containing layer  330  is removed wherever it is not covered and protected by the  350  conductor as shown in  FIG. 10 . 
         [0041]    The CMOS imager  300  is processed from the device shown in  FIG. 10  to an operable CMOS imager by conventional processing methods to form contacts and wiring to connect gate lines and other connections in the pixel cell. For example, the entire surface may be covered with a passivation layer of, e.g., silicon dioxide, BPSG, PSG, BSG or the like which may be planarized, typically by CMP and etched to provide contact holes, which are then metallized to provide contacts to the gates and active area diffusions of the device. Conventional multiple layers of conductors and insulators may also be used to interconnect the structures in the manner shown in  FIG. 1 . 
         [0042]    The nitrogen containing layer  330  increases the capacitance between the photogate  350  and underlying n-type region  311 . This improves the charge storage and signal acquisition and dynamic range of the CMOS imager sensor. The increased capacitance also provides a better signal to noise ratio, a better ability of the imager to see bright scenes and a wider dynamic range. 
         [0043]    A typical processor based system which includes a CMOS imager device according to the present invention is illustrated generally at  400  in  FIG. 11 . The illustrated system is exemplary of a device having digital circuits which include CMOS imager devices. Other types of processor systems which include the same or similar systems of  FIG. 11  include 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. 
         [0044]    Referring to  FIG. 11 , a processor based system, such as a computer system, generally comprises a central processing unit (CPU)  444  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 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 an integrated circuit which includes a nitrogen containing insulating layer, as previously described with respect to  FIGS. 4-7 . 
         [0045]    The above description and accompanying drawings are only illustrative of preferred embodiments which can achieve the features and advantages of the present invention. For example, the CMOS imager array can be formed on a single chip together with the logic or the logic and array may be formed on separate IC chips. Further, the invention has been described with reference to n-regions and an n-doped channel in a p-well, it should be understood that the present invention includes p-regions and a p-doped channel in an n-well. Additionally, the CMOS imager is described having a transfer gate and a photogate that overlaps the transfer gate; however, the photogate need not overlap the transfer gate. Furthermore, the invention may also be used in CMOS imagers where no transfer gate is employed. 
         [0046]    The invention is not to be considered as being limited to the embodiments shown and described in detail herein as many modifications can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not limited by the forgoing descriptions, but is only limited by the scope of the following claims.