Abstract:
An image sensor includes a unit cell of four pixels. The unit cell includes four photosensitive regions that collect charge in response to light; four transfer transistors that respectively pass the charge from each of the four photosensitive regions to one common charge-to-voltage conversion mechanism; three control wires in which a first control wire controls two of the transfer transistors and a second control wire controls one of the transfer transistors and a third control wire controls one of the transfer transistors; an amplifier connected to the common charge-to-voltage conversion mechanism that outputs an output signal in response to a signal from the charge-to-voltage conversion mechanism; and a reset transistor connected to the common charge-to-voltage conversion mechanism for resetting the charge-to-voltage conversion mechanism to a predetermined signal level.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to image sensors and more particularly to such image sensors having improved openings between the metal wires covering the photodiode for increasing sensitivity. 
     BACKGROUND OF THE INVENTION 
     As pixel sizes are reduced to less than 1.7 μm to meet demand, there are tradeoffs. The primary disadvantages are reduced sensitivity because the pixel area is smaller, and the smaller opening between pixel wires blocks too much light. 
     For example, referring to  FIG. 1 , the pixel schematic from U.S. Patent Publication 2008/0062290 A1 is shown. The unit cell has four transfer gates  107 ,  108 ,  109 , and  110  that control the flow of charge between the floating diffusion and photodiodes  112 ,  113 ,  114 , and  111 , respectively. Transistor  106  resets the floating diffusion to the level of the power supply VDD. Transistor  105  buffers the floating diffusion voltage and the row select transistor  103  switches that voltage onto the output signal line. In this pixel schematic, there are a total of 6 horizontal wires and two vertical wires. If a 1.4 μm pixel size is desired, then the unit cell of 2.8 μm must contain 6 horizontal wires. If the typical metal  2  wire line and space requirement is 0.18 μm, then those 6 wires will occupy 1.80 μm out of 2.8 μm leaving only a 1.0 μm hole for two photodiodes in the pixel unit cell. A 0.5 μm opening for each photodiode is smaller than the wavelength of red light (650 nm). As a result, the pixel quantum efficiency will be very poor. 
     The opening in the metal wires could be made larger by using 3 or more layers of metal. Camera lenses for cell phones typically have light rays that may be incident at an angle of 25 degrees from normal. A tall stack of metal wires  3  or more layers high will block light incident at 25 degrees from reaching the photodiodes. 
     The present invention will address the problem of narrow openings between metal wires above photodiodes while at the same time permitting improved sensitivity by summing pixels in low resolution imaging modes. 
     SUMMARY OF THE INVENTION 
     An image sensor includes a unit cell of four pixels. The unit cell includes four photosensitive regions that collect charge in response to light; four transfer transistors that respectively pass the charge from each of the four photosensitive regions to one common charge-to-voltage conversion mechanism; three control wires in which a first control wire controls two of the transfer transistors and a second control wire controls one of the transfer transistors and a third control wire controls one of the transfer transistors; an amplifier connected to the common charge-to-voltage conversion mechanism that outputs an output signal in response to a signal from the charge-to-voltage conversion mechanism; and a reset transistor connected to the common charge-to-voltage conversion mechanism for resetting the charge-to-voltage conversion mechanism to a predetermined signal level. 
     It is an object of the present invention to increase the area of the opening above the photodiode while at the same time permitting improved sensitivity by summing pixels in low resolution imaging modes. 
     These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
     ADVANTAGEOUS EFFECT OF THE INVENTION 
     The present invention has the advantage of increasing the area of the opening covering the photodiode for improved sensitivity of the photodiode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a prior art image sensor; 
         FIG. 2  illustrates the area and shape of the unit cell configuration of the present invention; 
         FIG. 3  illustrates the unit cell pixel configuration of the present invention having rectangular shaped pixels versus conventional square pixels; 
         FIG. 4  illustrates a Bayer color filter arrangement of the prior art; 
         FIG. 5  illustrates a pan-chromatic color filter arrangement of the prior art using pan-chromatic filters; 
         FIG. 6  illustrates another pan-chromatic color filter arrangement of the prior art using pan-chromatic filters; 
         FIG. 7  is a schematic diagram of a unit cell of the present invention; 
         FIG. 8  is a layout of the unit cell of  FIG. 7 ; 
         FIG. 9  is alternative embodiment a unit cell of the present invention; 
         FIG. 10  is a layout of the schematic of  FIG. 9 ; 
         FIG. 11  illustrates the metal wiring overlaid on top of  FIG. 10 ; 
         FIG. 12  shows horizontal cross-section B-B of  FIG. 11 ; 
         FIG. 13  shows vertical cross-section A-A of  FIG. 11 ; 
         FIG. 14  is a top view of an image sensor of the present invention; and 
         FIG. 15  shows a block diagram of an image capture device having the image sensor of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will first disclose the structure of a pixel with increased area of the opening between wires covering the photodiode. Referring to  FIG. 2 , the typical pixel has equal width, W, and height, H. So a 1.4 μm pixel is 1.4 μm square. There is no fundamental requirement that pixels must be square. The pixel aspect ratio can be changed so it has a height greater than width. This aspect ratio spaces out the horizontal wires. It reduces the space between vertical wires, but there are fewer vertical wires than horizontal wires. Therefore, there is a net increase in the opening for light between metal wires with a rectangular shaped pixel. 
     With rectangular pixels in the image sensor, the camera will use image processing to transform the rectangular pixels of the image sensor into one with square pixels for display. It is desirable to make the aspect ratio of the pixel be the ratio of two perfect squares such as 16:9 or 25:16 and other similar aspect ratios. Referring to  FIG. 3 , this makes the image processing algorithm for generating square pixels easier. The left side of  FIG. 3  is an array of square pixels with 5 rows and 4 columns. If the rectangular pixel has an aspect ration of 25 by 16, two perfect squares, then an array of rectangular pixels having 4 rows and 5 columns has exactly the same number of pixels and same area as the array of square pixels. Therefore both rectangular and square array layouts are equal in pixel count. The rectangular array has less vertical resolution, but it also has greater horizontal resolution than the square array. 
     The generalized case would be a rectangular pixel having a width 
               W   =       n     n   +   1       ⁢   A       ,         
and a height
 
               H   =         n   +   1     n     ⁢   A       ,         
where A is the desired area of the pixel and n is an integer. The aspect ratio of the pixel would be
 
               H   W     =         (       n   +   1     n     )     2     .           
The width and height of the rectangular pixel is not limited to those values, but those values allow for implementation of simple image processing algorithms. Consider the 1.4 μm pixel as an example. For an aspect ratio of 25:16 (n=4) the width is 1.12 μm and height is 1.75 μm. For the case of six 0.18 μm horizontal metal wires, this adds 0.35 μm onto the 0.5 μm metal opening to make it 0.85 μm. This is a significant improvement.
 
     Further improvement can be obtained by realizing that the row select transistor  103  of  FIG. 1  can be eliminated by clocking the VDD power supply and using the reset transistor  106  as a row selector. This eliminates the RS horizontal wire. 
     It is noted the present invention includes color filter layouts that can eliminate another horizontal wire.  FIG. 4  shows the traditional Bayer color filter pattern used by a majority of color image sensors. Pixel summing is difficult with this pattern and each pixel can receive only one color. Color filter patterns using red, green, blue, and pan-chromatic pixels were described in U.S. Patent Publication 2007/0024879 A1 and reproduced in  FIGS. 5 and 6 . For both patterns in  FIGS. 5 and 6 , the pixel unit cell would contain two pan-chromatic pixels and two color pixels. The pan-chromatic pixels form a high-resolution luminance component of the final image that is superior to the green color channel of the Bayer pattern. The pan-chromatic pixel patterns leverage the fact that the human eye is more sensitive to luminance resolution than it is to color resolution. The pixel architecture can also leverage this fact by summing together the two color pixels within the 4-pixel unit cell. There is a noise advantage to summing the color pixels in the charge domain on the pixel floating diffusion instead of in the digital domain after each of the 2 color pixels have been digitized. 
       FIG. 7  shows the schematic for an embodiment of the present invention. This is designed to work with the color filter pattern shown in  FIG. 5 . This schematic is of a unit cell  204  having two transfer gates  208  and  210  that are controlled by the same control wire TG 2 . The transfer gates  208  and  210  transfer charge from the photosensitive regions, preferably photodiodes,  211  and  213  that collect charge in response to incident light. Photodiodes  213  and  211  are under either red, green, or blue color filters, and their charges are summed together on the charge-to-voltage conversion mechanism, preferably a floating diffusion,  203  when TG 2  is activated. Photodiodes  212  and  214  are under pan-chromatic filters and are independently transferred to the floating diffusion  203  when transfer gates  207  or  209  are activated. Transistor  206  resets the floating diffusion  203  to the level of the power supply voltage VDD. Transistor  206  also serves as a row select transistor. The amplifier transistor  205  buffers the floating diffusion  203  voltage onto the output signal wire Out. This unit cell pixel  204  has a design with only four wires in the horizontal direction. Using the previous example of a rectangular 1.4 μm pixel with a 1.75 μm height and 0.18 μm horizontal metal wires, the opening between pairs of metal wires is now 1.21 μm. That more than doubles the prior art metal opening of 0.5 μm. Therefore, the quantum efficiency of the invention is much higher than the prior art. 
       FIG. 8  shows the silicon layout of the unit cell  204  having four pixels. The four transfer gates  207  through  210  are symmetrically arranged about the floating diffusion  203 . The transistors  205  and  206  are placed on the other side of the photodiodes. This pixel  204  layout provides regularly spaced photodiodes in the horizontal and vertical directions with their optical centers marked by X. The regularly spaced photodiodes are important for maintaining optical symmetry for light incident at an angle. 
       FIG. 9  shows the schematic for an alternative embodiment of the present invention that is designed to work with the color filter pattern shown in  FIG. 6 . This schematic is of a unit cell  304  having two transfer gates  309  and  310  controlled by the same control wire TG 3 . The transfer gates  309  and  3   10  transfer charge from the photodiodes  311  and  314  that collect charge in response to incident light. Photodiodes  311  and  314  are under either red, green, or blue color filters and their charges are summed together on the floating diffusion  303  when TG 2  is activated. Photodiodes  312  and  313  are under pan-chromatic filters and are independently transferred to the floating diffusion  303  when transfer gates  307  or  308  are activated. Transistor  306  resets the floating diffusion  303  to the level of the power supply voltage VDD. Transistor  306  also serves as a row select transistor. Amplifier transistor  305  buffers the floating diffusion  303  voltage onto the output signal wire Out. This pixel  304  has a design with only four wires in the horizontal direction. Using the previous example of a rectangular 1.4 μm pixel with a 1.75 μm height and 0.18 μm horizontal metal wires, the opening between pairs of metal wires is now 1.21 μm. That more than doubles the prior art metal opening of 0.5 μm. Therefore, the quantum efficiency of the invention is much higher than the prior art. 
       FIG. 10  shows the silicon layout of the unit cell  304  of  FIG. 9  which also include  4  pixels. The four transfer gates  307  through  310  are symmetrically arranged about the floating diffusion  303 . The transistors  305  and  306  are placed on the other side of the photodiodes. This pixel  304  layout provides regularly spaced photodiodes in the horizontal and vertical directions with their optical centers marked by X. The regularly spaced photodiodes are important for maintaining optical symmetry for light incident at an angle. Because the pan-chromatic pixels are all in one row, a cylindrical micro-lens can be used to focus light on the photodiodes. 
       FIG. 11  shows the metal wiring overlaid on top of  FIG. 10 . The four horizontal wires TG 1 , TG 2 , TG 3 , and RG are arranged in pairs of two overtop of the boundary between rows of photodiodes. This provides the maximum opening between wires for passage of light to the photodiodes. Vertical wires Out and VDD are fabricated on a different level of metallization. 
       FIG. 12  shows horizontal cross-section B-B of  FIG. 11 . Of particular advantage of this pixel layout is the horizontal cross-section B-B only has one layer of metal for minimal obstruction of incoming light. The wires alternate between one floating diffusion  315  wire between photodiodes and two wires Out and VDD between the next pair of photodiodes. This does introduce an optical left night asymmetry that can be mitigated by making the floating diffusion wire  315  wider. 
       FIG. 13  shows vertical cross-section A-A of  FIG. 11 . This cross-section shows the TG 1 , TG 2 , TG 3 , and RG wires on a second layer of metal. The longer side of the rectangular pixel is arranges along this cross-section to maximize the opening between the second level metal wires. 
     The first embodiment of the invention has a similar wiring arrangement as shown in  FIGS. 11 ,  12 , and  13 .) 
     Both embodiments of the invention can be fabricated as NMOS pixels where electrons are the charge carriers, or as PMOS pixels where holes are the charge carriers. 
       FIG. 14  is a top view of the image sensor  320  of the present invention having a plurality of pixels  321  that arranged in a two dimensional array. The pixels  321  are electronically grouped in the unit cells  204  and  304 . Each unit cell  204  and  304  contains four pixels. For the present invention unit cell is defined as four pixels. 
       FIG. 15  is a block diagram of an imaging system that can be used with an image sensor that incorporates the pixel structure in accordance with the invention. Imaging system  1200  includes digital camera phone  1202  and computing device  1204 . Digital camera phone  1202  is an example of an image capture device that can use an image sensor incorporating the present invention. Other types of image capture devices can also be used with the present invention, such as, for example, digital still cameras and digital video camcorders. 
     Digital camera phone  1202  is a portable, handheld, battery-operated device in an embodiment in accordance with the invention. Digital camera phone  1202  produces digital images that are stored in memory  1206 , which can be, for example, an internal Flash EPROM memory or a removable memory card. Other types of digital image storage media, such as magnetic hard drives, magnetic tape, or optical disks, can alternatively be used to implement memory  1206 . 
     Digital camera phone  1202  uses lens  1208  to focus light from a scene (not shown) onto image sensor array  320  of imaging integrated circuit  1212 . Image sensor array  320  provides color image information using the Bayer color filter pattern in an embodiment in accordance with the invention. Image sensor array  320  is controlled by timing generator  1214 , which also controls flash  1216  in order to illuminate the scene when the ambient illumination is low. 
     The analog output signals output from the image sensor array  320  are amplified and converted to digital data by analog-to-digital (A/D) converter circuit  1218 . The digital data are stored in buffer memory  1220  and subsequently processed by digital processor  1222 . Digital processor  1222  is controlled by the firmware stored in firmware memory  1224 , which can be flash EPROM memory. Digital processor  1222  includes real-time clock  1226 , which keeps the date and time even when digital camera phone  1202  and digital processor  1222  are in a low power state. The processed digital image files are stored in memory  1206 . Memory  1206  can also store other types of data, such as, for example, music files (e.g. MP3 files), ring tones, phone numbers, calendars, and to-do lists. 
     In one embodiment in accordance with the invention, digital camera phone  1202  captures still images. Digital processor  1222  performs color interpolation followed by color and tone correction, in order to produce rendered sRGB image data. The rendered sRGB image data are then compressed and stored as an image file in memory  1206 . By way of example only, the image data can be compressed pursuant to the JPEG format, which uses the known “Exif” image format. This format includes an Exif application segment that stores particular image metadata using various TIFF tags. Separate TIFF tags can be used, for example, to store the date and time the picture was captured, the lens f/number and other camera settings, and to store image captions. 
     Digital processor  1222  produces different image sizes that are selected by the user in an embodiment in accordance with the invention. One such size is the low-resolution “thumbnail” size image. Generating thumbnail-size images is described in commonly assigned U.S. Pat. No. 5,164,831, entitled “Electronic Still Camera Providing Multi-Format Storage Of Full And Reduced Resolution Images” to Kuchta, et al. The thumbnail image is stored in RAM memory  1228  and supplied to display  1230 , which can be, for example, an active matrix LCD or organic light emitting diode (OLED). Generating thumbnail size images allows the captured images to be reviewed quickly on color display  1230 . 
     In another embodiment in accordance with the invention, digital camera phone  1202  also produces and stores video clips. A video clip is produced by summing multiple pixels of image sensor array  320  together (e.g. summing pixels of the same color within each 4 column×4 row area of the image sensor array  320 ) to create a lower resolution video image frame. The video image frames are read from image sensor array  320  at regular intervals, for example, using a 15 frame per second readout rate. 
     Audio codec  1232  is connected to digital processor  1222  and receives an audio signal from microphone (Mic)  1234 . Audio codec  1232  also provides an audio signal to speaker  1236 . These components are used both for telephone conversations and to record and playback an audio track, along with a video sequence or still image. 
     Speaker  1236  is also used to inform the user of an incoming phone call in an embodiment in accordance with the invention. This can be done using a standard ring tone stored in firmware memory  1224 , or by using a custom ring-tone downloaded from mobile phone network  1238  and stored in memory  1206 . In addition, a vibration device (not shown) can be used to provide a silent (e.g. non-audible) notification of an incoming phone call. 
     Digital processor  1222  is connected to wireless modern  1240 , which enables digital camera phone  1202  to transmit and receive information via radio frequency (RF) channel  1242 . Wireless modem  1240  communicates with mobile phone network  1238  using another RF link (not shown), such as a 3GSM network. Mobile phone network  1238  communicates with photo service provider  1244 , which stores digital images uploaded from digital camera phone  1202 . Other devices, including computing device  1204 , access these images via the Internet  1246 . Mobile phone network  1238  also connects to a standard telephone network (not shown) in order to provide normal telephone service in an embodiment in accordance with the invention. 
     A graphical user interface (not shown) is displayed on display  1230  and controlled by user controls  1248 . User controls  1248  include dedicated push buttons (e.g. a telephone keypad) to dial a phone number, a control to set the mode (e.g. “phone” mode, “calendar” mode” “camera” mode), a joystick controller that includes 4-way control (up, down, left, right) and a push-button center “OK” or “select” switch, in embodiments in accordance with the invention. 
     Dock  1250  recharges the batteries (not shown) in digital camera phone  1202 . Dock  1250  connects digital camera phone  1202  to computing device  1204  via dock interface  1252 . Dock interface  1252  is implemented as wired interface, such as a USB interface, in an embodiment in accordance with the invention. Alternatively, in other embodiments in accordance with the invention, dock interface  1252  is implemented as a wireless interface, such as a Bluetooth or an IEEE 802.11b wireless interface. Dock interface  1252  is used to download images from memory  1206  to computing device  1204 . Dock interface  1252  is also used to transfer calendar information from computing device  1204  to memory  1206  in digital camera phone  1202 . 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
     
         
           107  transfer gate 
           108  transfer gate 
           109  transfer gate 
           110  transfer gate 
           111  photodiodes 
           112  photodiodes 
           113  photodiodes 
           114  photodiodes 
           203  floating diffusion 
           204  unit cell 
           205  amplifier 
           206  reset transistor 
           207  transfer gate 
           208  transfer gate 
           209  transfer gate 
           210  transfer gate 
           211  photodiodes 
           212  photodiodes 
           213  photodiodes 
           214  photodiodes 
           303  floating diffusion 
           304  unit cell 
           305  amplifier 
           306  reset transistor 
           307  transfer gate 
           308  transfer gate 
           309  transfer gate 
           310  transfer gate 
           311  photodiodes 
           312  photodiodes PARTS LIST (con&#39;t) 
           313  photodiodes 
           314  photodiodes 
           320  image sensor 
           321  pixels 
           1200  imaging system 
           1202  digital camera phone 
           1204  computing device 
           1206  memory 
           1208  lens 
           1212  imaging integrated circuit 
           1214  timing generator 
           1216  flash 
           1218  A/D converter circuit 
           1220  buffer memory 
           1222  digital processor 
           1224  firmware memory 
           1226  clock 
           1228  RAM memory 
           1230  color display 
           1232  audio codec 
           1234  microphone 
           1236  speaker 
           1238  mobile phone network 
           1240  wireless modem 
           1242  RF Channel 
           1244  photo service provider 
           1246  Internet 
           1248  user controls 
           1250  dock 
           1252  dock interface