Abstract:
An imager device that has an isolation structure such that pinned photodiode characteristics are maintained without increasing doping levels. The invention provides an isolation structure to maintain pinned photodiode characteristics without increasing doping levels around the photodiode. By creating a substrate region surrounding the charge-collection region of the photodiode, the photodiode may be electrically isolated from the bulk substrate. This region fixes the depletion region so that it does not migrate toward the surface of the substrate or the STI region. By doing so, the region prevents charge from being depleted from the substrate and the accumulation region, reducing dark current.

Description:
FIELD OF THE INVENTION 
   The invention relates to the field of semiconductor devices and, in particular, to a pixel cell using a high-k dielectric film to create a strong accumulation region for providing isolation and optimizing characteristics of the cell. 
   BACKGROUND OF THE INVENTION 
   A CMOS imager circuit includes a focal plane array of pixel cells, each cell includes a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for producing a photo-generated charge in a doped region of the substrate. A readout circuit is provided for each pixel cell and includes at least a source follower transistor and a row select transistor for coupling the source follower transistor to a column output line. The pixel cell also typically has a floating diffusion node, connected to the gate of the source follower transistor. Charge generated by the photosensor is sent to the floating diffusion region. The imager may also include a transistor for transferring charge from the photosensor to the floating diffusion node and another transistor for resetting the floating diffusion region node to a predetermined charge level prior to charge transference. Each pixel cell is isolated from other pixel cells in the array by a field oxide region (STI), which surrounds it and separates the doped regions of the substrate within that pixel cell from the doped regions of the substrate within neighboring pixel cells. 
   In a CMOS imager, the active elements of a pixel cell, for example a four transistor pixel, perform the necessary functions of (1) photon to charge conversion; (2) transfer of charge to the floating diffusion node; (3) resetting the floating diffusion node to a known state before the transfer of charge to it; (4) selection of a pixel cell for readout; and (5) output and amplification of a signal representing a reset voltage and a pixel signal voltage based on the photo converted charges. The charge at the floating diffusion node is converted to a pixel output voltage by a source follower output transistor. 
     FIG. 1  illustrates a block diagram of a CMOS imager device  100  having a pixel array  110  with each pixel cell being constructed as described above. Pixel array  110  comprises a plurality of pixels arranged in a predetermined number of columns and rows (not shown). The pixels of each row in array  110  are all turned on at the same time by a row select line, and the pixels of each column are selectively output by respective column select lines. A plurality of row and column lines are provided for the entire array  110 . The row lines are selectively activated by the row driver  145  in response to row address decoder  155  and the column select lines are selectively activated by the column driver  160  in response to column address decoder  170 . Thus, a row and column address is provided for each pixel. 
   The CMOS imager is operated by a control circuit  150 , which controls decoders  155 ,  170  for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry  145 ,  160 , which apply driving voltage to the drive transistors of the selected row and column lines. The pixel column signals, which typically include a pixel reset signal Vrst and a pixel image signal Vsig for each pixel are read by sample and hold circuitry  161 ,  162  associated with the column device  160 . A differential signal Vrst−Vsig is produced for each pixel, which is amplified and digitized by analog-to-digital converter  175 . The analog to digital converter  175  converts the analog pixel signals to digital signals which, are fed to an image processor  180  to form a digital image. 
   Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524, and 6,333,205, assigned to Micron Technology, Inc. The disclosures of each of the forgoing are hereby incorporated by reference herein in their entirety. 
   A schematic diagram of an exemplary CMOS four-transistor (4T) pixel cell  10  is illustrated in  FIG. 2 . The four transistors include a reset transistor  34 , source follower transistor  36 , row select transistor  38  and a transfer gate  32 . A photosensor  40  converts incident light into an electrical charge. A floating diffusion region  50  receives the charge from the photosensor  40  through the transfer gate  32  (when activated by a transfer gate control signal TG) and is connected to the reset transistor  34  and the gate of the source follower transistor  36 . The source follower transistor  36  outputs a signal proportional to the charge accumulated in the floating diffusion region  50  when the row select transistor  38  is turned on. The reset transistor  34  resets the floating diffusion region  50  (when activated by a reset control signal RST) to a known potential prior to transfer of charge from the photosensor  40 . The photosensor  40  may be a photodiode, photogate, or photoconductor. If a photodiode is employed, the photodiode may be formed below a surface of the substrate and may be a buried PNP photodiode, buried NPN photodiode, a buried PN photodiode, or a buried NP photodiode, among others. 
   In a conventional CMOS imager pixel with a buried photodiode, the photodiode converts incident light to an electrical charge. The photodiode accumulates this charge throughout the sampling period. At the end of the sampling period, the transfer gate closes (i.e., is activated) and the charge is drained from the photodiode through the transfer gate. 
   A buried photodiode has a shallow implant of a first conductivity (referred to herein as an accumulation region) above a deeper implant of another conductivity (referred to herein as a charge-collection region) in a substrate lightly doped with the first conductivity type. A depletion region exists at the interface between the accumulation region and the charge collection region. For example, in a p-type substrate, a shallow low-dose p-type implant is applied over an n-type photosensitive region. This also produces a dual-junction sandwich that alters the visible light spectral response (the sensitivity to optical radiation of different wavelengths) of the pixel. The upper junction is optimized for responding to lower wavelengths while the lower junction is more sensitive to the longer wavelengths. 
   However, the top surface of the photodiode is electrically connected to the bulk substrate via a portion of the accumulation region above the charge-collection region and a portion of the accumulation region between the field oxide region (referred to hereinafter as a STI region) and the charge-collection region. The depletion of charge from the substrate and accumulation region causes excessive leakage and creates a false signal, commonly known as “dark current.” Dark current is a current that is created without photoconversion of light. Dark current may be reduced by preventing depletion of the accumulation region. 
   Attempts to overcome a depletion of the accumulation region have involved increasing the doping level near the STI sidewall. However, increasing the doping level near the STI sidewall creates excess leakage, which is quite significant in the overall dark current leakage level in a pinned photodiode. 
   Therefore, it is desirable to create an isolation structure where pinned photodiode characteristics are maintained without increased doping levels. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention provides an isolation structure to maintain pinned photodiode characteristics without increasing doping levels around the photodiode. By creating a substrate region surrounding the charge-collection region of the photodiode, the photodiode may be electrically isolated from the bulk substrate. This region fixes the depletion region so that it does not migrate toward the surface of the substrate or the STI region. By doing so, the region prevents charge from being depleted from the substrate and the accumulation region, reducing dark current. 
   The region is achieved by depositing a high-k dielectric material on the surface of the substrate above the photodiode and on the sidewalls of the STI trench. The high-k dielectric material induces excess charge on the surface of the substrate above the photodiode and in the sidewalls of the STI region adjacent to the photodiode. 
   Aluminum oxide is one high-k dielectric material (a material with a dielectric constant greater than that of silicon dioxide) that induces an excess negative charge, as noted in Manchanda et al., “Si-Doped Aluminates for High Temperature Metal-Gate CMOS: Zr—Al—Si—O, A Novel Gate Dielectric for Low Power Applications,” IEEE IEDM Technical Digest (2000) pp.23–26; Lee et al., “Effect of Polysilicon Gate on the Flatband Voltage Shift and Mobility Degradation for ALD-Al 2 O 3  Gate Dielectric,” IEEE IEDM Technical Digest (2000) pp. 645–648; and Buchanan et al., “80 nm poly-silicon gated n-FETs with ultra-thin Al 2 O 3  gate dielectric for ULSI applications,” IEEE IEDM Technical Digest (2000) pp. 223–226. 
   Because of these properties, when using, for example, aluminum oxide (or other high-k dielectric material) to line or fill the STI regions and cover the surface of the substrate overlying a PNP photodiode, there will be an excess negative charge in the aluminum oxide layers that induces and maintains a hole-accumulation region between the aluminum oxide layer and the charge-collection region of the photodiode. Similarly, when using a high-k dielectric material in the STI trenches and on the surface of the substrate overlying an NPN photodiode, there will be an excess positive charge in the high-k dielectric material that induces and maintains an electron-rich accumulation region. By maintaining the accumulation region, the depletion region between the accumulation region and the charge-collection region is prevented from migrating toward the STI regions and the substrate surface, thereby electrically isolating the photodiode and decreasing charge leakage from the bulk substrate into the photodiode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the present invention will be apparent from the following detailed description and drawings which include various embodiments of the invention, in which: 
       FIG. 1  is a block diagram of an imaging device; 
       FIG. 2  is a schematic diagram of a four-transistor (4T) pixel; 
       FIG. 3  a cross-section of an exemplary pixel cell of the present invention at an initial stage of fabrication; 
       FIG. 4  is an illustration of the pixel cell of  FIG. 3  at a subsequent stage of fabrication; 
       FIG. 5  is an illustration of the pixel cell of  FIG. 4  at a subsequent stage of fabrication; 
       FIG. 6  is an illustration of the pixel cell of  FIG. 5  at a subsequent stage of fabrication; 
       FIG. 7  is an illustration of the pixel cell of  FIG. 6  at a subsequent stage of fabrication; 
       FIG. 8  is an illustration of the pixel cell of  FIG. 7  at a subsequent stage of fabrication; 
       FIG. 9  is an illustration of the pixel cell of  FIG. 8  at a subsequent stage of fabrication; 
       FIG. 10   a  is an illustration of the pixel cell of  FIG. 9  at a subsequent stage of fabrication; 
       FIG. 10   b  is an illustration of the pixel cell of  FIG. 7  at an alternative subsequent stage of fabrication; 
       FIG. 10   c  is an illustration of the pixel cell of  FIG. 7  at an alternative subsequent stage of fabrication; 
       FIG. 11   a  is an illustration of the pixel cell of  FIG. 10   a  at a subsequent stage of fabrication; 
       FIG. 11   b  is an illustration of the pixel cell of  FIG. 10   a  at an alternative subsequent stage of fabrication; 
       FIG. 12  is an illustration of the pixel cell of  FIG. 11   a  at a subsequent stage of fabrication; 
       FIG. 13  is an illustration of the pixel cell of  FIG. 12  at a subsequent stage of fabrication; 
       FIG. 14  is an illustration of the pixel cell of  FIG. 13  at a subsequent stage of fabrication; 
       FIG. 15  is an illustration of the pixel cell of  FIG. 14  at a subsequent stage of fabrication; 
       FIG. 16  is an illustration of the pixel cell of  FIG. 15  at a subsequent stage of fabrication; 
       FIG. 17  is an illustration of the pixel cell of  FIG. 16  at a final stage of fabrication; and 
       FIG. 18  shows a processor system incorporating at least one imager device constructed in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   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. Additionally, processing steps described and their progression are exemplary of preferred embodiments of the invention. However, the sequence of steps is not limited to that set forth herein and may be changed as is known in the art, with the exception of steps necessarily occurring in a certain order. 
   The term “substrate” is to be understood as a semiconductor-based material including silicon, 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 “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. 
   The term “pixel” refers to a photo-element unit cell containing a photosensor and transistors for converting light 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 not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
   Referring now to the drawings, where like elements are designated by like reference numerals,  FIG. 3  shows a pixel cell  10  an initial stage of processing in accordance with the invention. A substrate  15  with a first conductivity is provided. For the purposes of illustration, the first conductivity type is p-type. A silicon dioxide layer  22  is grown over the surface of the substrate  15 . 
   Referring to  FIG. 4 , a nitride hard mask layer  23  is deposited over the silicon dioxide layer  22 . The nitride hard mask layer  23  protects the active area underneath during subsequent processing steps. The nitride hard mask layer  23  is approximately 200 Å to 1000 Å in thickness. 
   Referring to  FIG. 5 , a photoresist mask  20  is formed and patterned with an opening  21 . The nitride hard mask layer and the substrate then undergo an etching process to create a trench  24 , as shown in  FIG. 6 . In this illustration, only one trench  24  is shown. However, it should be appreciated that more than one opening and trench would be formed in a pixel cell array. Trenches of about 1,000 Å to about 4,000 Å in depth, preferably about 1,500 Å to about 3,000 Å, with a width of about 500 Å to about 10,000 Å, preferably about 1,000 Å to about 3,000 Å, are typically desired. The trench  24  is formed in the substrate  15  by anisotropic etching. The photoresist mask  20  is subsequently removed, as shown in  FIG. 6 . 
   Referring to  FIG. 7 , a layer of silicon dioxide  22  is grown on the sidewalls of the trench  24  over the surface of the substrate  15 . The silicon dioxide layer  22  in the trench  24  is optional and acts as a dielectric in the final structure. When the silicon dioxide layer  22  is grown as part of the final pixel cell  10 , the layer  22  enhances the dielectric effects of the final structure. However, layer  22  may be removed before the next stage of fabrication. Growing a layer of silicon dioxide over a silicon substrate heals the defects in the surface of the silicon substrate, even if the silicon dioxide is subsequently removed. For the purposes of illustration, the embodiment without removing the silicon dioxide layer  22  shall be discussed herein. 
   Referring to  FIG. 8 , the substrate  15  is subjected to a conformal deposition process to deposit a high-k dielectric material such as, e.g., a thin aluminum oxide liner layer  26  over the silicon dioxide layer  22  on the surface of the substrate  15  and over the walls of the trench  24 . Although aluminum oxide is the material used in the present embodiment, any material with a high-k dielectric constant that induces excess negative charge is suitable for this embodiment. Materials such as aluminum nitride, silicon-rich aluminum oxides, and others are also suitable. Methods such as chemical vapor deposition, atomic layer deposition, plasma vapor deposition, or other suitable techniques may be employed in forming the aluminum oxide layer  26 . The aluminum oxide layer  26  is deposited to a thickness within the range of 30 Å and 500 Å, and preferably about 50 Å. 
   Referring to  FIG. 9 , an insulating layer  28  of dielectric material is deposited over the aluminum oxide layer  26 , filling the trench  24 . The insulating layer  28  may consist of an insulating material such as silicon dioxide, silicon nitride, oxide-nitride, nitride-oxide, oxide-nitride-oxide, or other suitable insulating material. This material is deposited within the trench  24  by chemical vapor deposition, low pressure chemical vapor deposition, or other suitable techniques. As shown in  FIG. 10   a , the substrate  15  is then planarized, removing excess insulating layer  28 , aluminum oxide layer  26 , and silicon dioxide layer  22  above the surface of the substrate  15 . Chemical mechanical polishing or RIE dry etching processes may be employed to achieve the resulting lined STI region  30 . 
   Alternatively,  FIG. 10   b  illustrates an embodiment where the aluminum oxide layer  26  entirely fills the trench  24  instead of using an insulating layer  28 , such that the resulting STI region  30 , uses aluminum oxide as the insulating material. By eliminating the step of depositing a separate dielectric layer  28 , this alternative offers additional processing simplicity. A further alternative, where the silicon dioxide layer  22  is removed prior to deposition of aluminum oxide layer  26 , is shown in  FIG. 10   c . For the purposes of illustration, the embodiment having an STI region  30  with a silicon dioxide layer  22 , an aluminum oxide layer  26 , and insulating layer  28  ( FIG. 10   a ) shall be discussed herein. However, the following steps may also be performed on the  FIG. 10   b  and  10   c  embodiments. 
   Referring to  FIGS. 11   a  and  11   b , the desired gate stacks, such as the stacks for transfer gate  32 , are layered, masked, and etched over the surface of the substrate  15 . A photoresist  31  is formed over the substrate  15  and patterned to partially overlap the gate stack of the transfer gate  32 , as shown in  FIG. 11   a . A dopant implant  201  of a first conductivity type (i.e., p-type) is performed on the substrate  15 , forming a p-type well  25  region having p-type ions (e.g., boron) beneath the active area of the pixel  10 . Alternatively, as shown in  FIG. 11   b , the dopant implant  201  may be performed without photoresist  31  and can form a blanket p-well  25  in the substrate  15 . For discussion purposes only, the  FIG. 11   a  embodiment having a p-well region is used to describe the following stages. 
   Next, as shown in  FIG. 12 , the substrate  15  is masked with photoresist  41 , leaving the portion of the substrate  15  where the photodiode is to be located exposed, and an angled ion implant  202  of a second conductivity type (i.e., n-type) is performed. This implant  202  can be performed by implanting appropriate n-type ions (e.g., arsenic, antimony, phosphorous, etc.) at an energy of about 10 KeV to about 400 KeV at an implant dosage of about 3×10 11  ions/cm 2  to about 1×10 15  ions/cm 2 , preferably 1×10 12  ions/cm 2  to about 1×10 14  ions/cm 2 . This implant  202  forms an n-type region  42 , which is the buried layer, or the charge-collection region, of the photodiode  40 . 
   Referring to  FIG. 13 , a dopant implant  203  of a first conductivity type (i.e., p-type) is performed to form a p-type region  43  over the n-type region  42 . The p-type region  43  serves as the accumulation layer of the photodiode  40 , pinning the potential of the photodiode  40  to a constant value when it is fully depleted. The photoresist  41  may then be removed. 
   As shown in  FIG. 14 , other conventional steps of masking and doping are performed to obtain a floating diffusion region  50  between the transfer gate  32  and the reset transistor  34  and a source/drain region  55  adjacent to the reset transistor  34 . 
   Referring to  FIG. 15 , a layer of silicon dioxide  45  is grown on the surface of the substrate  15  and etched back such that it remains only over the STI region  30  and photodiode  40 . As with silicon dioxide layer  22 , layer  45  may be optionally removed. By forming a silicon dioxide layer over a silicon substrate and subsequently removing it, the silicon dioxide heals defects in the surface of the substrate, providing a more uniform surface. Therefore, while the embodiment having the silicon dioxide layer  45  removed would not provide the same dielectric properties as an embodiment keeping the layer  45 , it would still have the benefits of providing a substrate surface which is essentially free of defects. For the purposes of illustration, the embodiment without removing silicon dioxide layer  45  shall be discussed herein. 
   Referring to  FIG. 16 , a thin aluminum oxide film  46  is selectively deposited over silicon dioxide layer  45  on the surface of the substrate  15 , over the STI region  30  and photodiode  40 , by methods such as chemical vapor deposition, atomic layer deposition, plasma vapor deposition, or other suitable techniques. The aluminum oxide film  46  may be deposited to a thickness within the range of about 30 Å to about 500 Å, preferably about 50 Å. A photo step may remove any excess aluminum oxide film from surfaces other than the tops of the STI region  30  and the photodiode  40 . Although the aluminum oxide film  46  is described as being deposited after the formation of the photodiode  40 , it may also be deposited before or after the formation of the photodiode  40 , depending on desired sequence of processing. 
   The high-k dielectric films (e.g., aluminum oxide, aluminum nitride, silicon-rich aluminum oxides) create a very shallow accumulation layer over the photodiode  40  and near the sidewalls of the STI region  30 . Aluminum oxide induces an excess negative charge in the sidewalls of the STI region  30  and above the photodiode  40 . Negative charge in these regions results in hole accumulation in the p-type regions of the substrate. This electrically disconnects the photodiode  40  from the bulk substrate  15 , preventing depletion of the substrate  15  and excessive leakage. 
   A spacer insulating layer  33  ( FIG. 17 ) is deposited over the pixel cell  10  and other conventional processing steps, such as conductive line formation to pixel cell  10 , may follow. 
   Although the above embodiments have been described with reference to the formation of n-channel devices, it must be understood that the invention is not limited to this embodiment. Accordingly, the invention has equal applicability to p-channel devices formed within an n-type substrate  15 . In such an embodiment the conductivity types of all structures changes accordingly. For example, in  FIG. 17 , aluminum oxide layers  26 ,  46  are replaced with layers  26 ,  46 , having excess positive charge (e.g., silicon nitride), to induce electron-rich accumulation regions in the n-type region  43  and the n-type substrate  15  surrounding the p-type region  42  of the photodiode  40 . 
     FIG. 18  shows a system  300 , a typical processor-based system modified to include an imager device  100 , as in  FIG. 1 , employing pixels of the present invention. Examples of processor-based systems, which may employ the imager device  100 , include, without limitation, computer systems, camera systems, scanners, machine vision systems, vehicle navigation systems, video telephones, surveillance systems, auto focus systems, star tracker systems, motion detection systems, image stabilization systems, and others. 
   System  300  includes a central processing unit (CPU)  302  that communicates with various devices over a bus  304 . Some of the devices connected to the bus  304  provide communication into and out of the system  300 , illustratively including an input/output (I/O) device  306  and imager device  100 . Other devices connected to the bus  304  provide memory, illustratively including a random access memory system (RAM)  310 , hard drive  312 , and one or more peripheral memory devices such as a floppy disk drive  314  and compact disk (CD) drive  316 . The imager device  100  may be combined with a processor, such as a CPU, digital signal processor, or microprocessor, in a single integrated circuit. The imager device  100  may be a CCD imager or CMOS imager constructed in accordance with any of the illustrated embodiments. 
   The above description and drawings are only to be considered illustrative of exemplary embodiments which achieve the features and advantages of the invention. Modification of, and substitutions to, specific process and conditions and structures can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.