Abstract:
The image sensor has a plurality of pixels arranged in rows and columns to form a pixel array, each pixel column having a column bitline. The image sensor includes a column readout for each column bitline and at least one additional column readout. The additional column readout may be selectively connected to one of the column bitlines. In one embodiment, a defective column readout can be isolated and the additional column readout used as a replacement.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of image sensing, and particularly to an image sensor with readout amplifier redundancy. 
     BACKGROUND OF THE INVENTION 
     There is an increasing tendency for larger number of pixels on image sensors used in devices such as digital still cameras, mobile phone cameras and optical pointing devices, for example. Consequently, sensors have either increased in size or pixels have been manufactured smaller, or often both. The use of finer geometry technologies increase the chance of defects occurring. Typically, the defectivity of an image sensor is proportional to the area. 
     Defects are caused during manufacture and are usually caused by dust particles obstructing the photolithography process. Resulting defects may be open circuit or short circuit connections. If a defect occurs within a pixel during manufacture, a defective pixel is usually the result. The defective pixel can be either ignored by the user or, if the defective pixel can be identified, corrected for by interpolating between neighboring pixels. If the defect occurs on a connection that is common to either a row or column of the pixel array, then a series of pixels in the row or column may be defective, rather than a single pixel. In some cases, the entire row or column can be defective. Defects that disrupt the operation of more than one pixel are far more noticeable to a user and much harder to compensate for. 
     Typically, as shown in  FIG. 1 , a pixel array  100  has a matrix of pixels  102 . Each column of pixels  102  in the pixel array  100  is connected by a common column bitline  104 . Each row of pixels  102  is connected by a common row select  106 . When the row select  106  is activated (set to “high”) by row drivers  108 , the pixels  110  in that row are enabled for readout and the values of the pixels  102  are read out in parallel on to the column bitline  104  to readout amplifiers  110 . 
     A horizontal scan  112  sends an “enable signal” along enable connection  114  to each readout amplifier  110  in turn. The amplified pixel value can then be read on an output bus  116 . If the readout amplifier  110  is defective, the entire column of pixels cannot be readout correctly and, as such, a noticeable error occurs in the image. 
     U.S. Pat. No. 6,741,754 Hamilton, “Correcting for defects in a digital image taken by an image sensor caused by pre-existing defects in two pixels in adjacent columns of an image sensor”, discloses a method for correcting for defects in a digital image taken by an image sensor when there are pre-existing defects in two pixels in adjacent columns of the image sensor which causes two adjacent lines of pixels in the digital image to have corrupted data. 
     U.S. Pat. No. 5,436,659, “Method and apparatus for determining defective pixel location”, attempts to use digital timing techniques to identify defective pixels and store their locations for correction by an appropriate technique, such as substituting a neighboring pixel value. U.S. Pat. No. 5,291,293, “Electronic imaging device with defect correction”, utilizes redundant sensor elements for defect compensation by using a plurality of arrays and pixels in one sensor used to correct info on the other sensor. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an image sensor having a plurality of pixels arranged in rows and columns to form a pixel array, each pixel column having a column bitline. The image sensor comprises a column readout for each column bitline and at least one additional column readout, wherein the at least one additional column readout may be selectively connected to one of the column bitlines. 
     Preferably, the at least one additional column readout may be selectively connected to one or more column bitlines by a first switch. Preferably, each column readout may be selectively disconnected from the respective column bitline by a second switch. Switches, in this context, include one-time on or off switches, such as fuses, as well as other types of switches which may be alternated between on and off. 
     In one embodiment, the first switch and the second switch are fuse links. In an alternative embodiment, the image sensor further comprises a configuration storage unit and the first switch is a first electronic device operable by the configuration storage unit. Preferably, the second switch is a second electronic device operable by the configuration storage unit. 
     Preferably, the configuration storage unit is connected by a first connection to one of the first or second electronic devices and the other of the first or second electronic devices is connected to an inverter which is also connected to the first connection. Preferably the first and second electronic devices are transistors and further preferably the transistors are PMOS or NMOS Field Effect Transistors (FETs). 
     According to a second aspect of the present invention there is provided an optical pointing device comprising an image sensor according to the first aspect of the invention. Preferably, the optical pointing device is an optical mouse. 
     According to a third aspect of the present invention there is provided a mobile device comprising an image sensor according to the first aspect of the invention. Preferably, the mobile device is one or more of the following: a mobile cellular telephone; a camera; a portable computer; a Personal Digital Assistant (PDA) or Palm device; or a Web Cam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram illustrating a prior art image sensor having an array of pixels and readout circuitry; 
         FIG. 2  is a schematic diagram illustrating an image sensor, according to one embodiment of the present invention, having an array of pixels, an additional readout device and a first configuration of fusable links. 
         FIG. 3  is a schematic diagram illustrating an image sensor, according to another embodiment of the present invention, having an array of pixels, an additional readout device and a second configuration of fusable links. 
         FIG. 4  is a schematic diagram illustrating an image sensor, according to another embodiment of the present invention, having an array of pixels, an additional readout device, a readout selection unit and a first configuration of a transistor switching unit; and 
         FIG. 5  is a schematic diagram illustrating an image sensor, according to another embodiment of the present invention, having an array of pixels, an additional readout device, a readout selection unit and a second configuration of a transistor switching unit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Typically, two types of issues cause pixel column faults: problems connecting pixels in the readout (“bitline”); and defective column readout. “Column readout” is used as a generic term to describe pixel readout circuitry, which is usually on a per column basis. The column readout may be a simple voltage or charge amplifier; with modern technology, it usually incorporates an analog-digital converter (ADC). The column readout usually also has other processing, such as CDS (correlated double sampling) or other fixed-pattern noise or offset reduction methods. In the context of this invention, “column readout” includes any circuitry connected to a column bitline for reading pixel values. 
     With prior-art sensors, if there is a defective column readout circuit, either the whole sensor is declared defective, or the system forced to use some digital correction, which is never 100% transparent and requires post-processing. Post-processing requires additional circuitry, which ultimately increases cost and power usage. 
     With “low end” digital cameras, the cost of the image sensor is at a premium and so the sensor is designed to be a small as possible. The size of a pixel is often limited by the size of the column circuitry, so in this case there is no possibility to make the column narrower. Typically, the width of the column circuitry is in the region of 2 μm-5 μm. 
     In devices such as those found in the “professional consumer” market, including digital SLR (single lens reflex) cameras and also professional cameras, the image sensors are large (for example ⅓ or ½ or ⅔ the 35 mm format or larger). Pixels are also large to enable them to have a “large full well” and store a large number of electrons, thereby reducing photon shot noise. Photon shot noise is the most significant source of noise when the sensor is used on an illuminated scene. As dictated by quantum mechanics, the photons on the sensor follow a Poisson distribution. The Poisson distribution gives the variation in number (i.e. the noise in the signal) proportional to the square-root of the mean. Hence, the pixel&#39;s signal to noise ratio, which is the mean divided by the noise, is now proportional to the mean of the number of photons. Therefore, a less noisy pixel has a large capacity to store photons. 
     Increasing the full well results in pixels greater than the minimum column readout circuitry, usually a column amplifier, pitch. As these sensors are also large, they are expensive and yield enhancement techniques are especially desirable. Typical pixel width are 5 μm-30 μm. Column readout circuitry, and specifically the readout amplifier, due to its complexity and demanding specification, is unfortunately an area which has a high probability of defectivity. 
     The following embodiments are described with reference to readout amplifiers as the column readout circuitry. It should be appreciated that additional, and alternative, readout circuitry components may be present, such as those described previously. With prior-art sensors, a defective readout amplifier results in a defective column and hence a large number of defective pixels. Even more problematic, is that the defective pixels are grouped together, which produces a more noticeable error in a final image. 
     Referring to  FIG. 2 , in an embodiment of the invention, a pixel array  200  has a matrix of pixels  202 . Each column of pixels  202  in the pixel array  200  is connected by a common column bitline  204 . Each row of pixels  202  is connected by a common row select  206 . When the row select  206  is activated (set to “high”) by row drivers (not shown), the pixels  210  in that row are enabled for readout and the values of the pixels  202  are read out in parallel on to the column bitline  204  to readout amplifiers  210 . 
     A horizontal scan  212  sends an “enable signal” along a series of enable connections  214  to each readout amplifier  210  in turn. The amplified pixel value can then be read on an output bus  216 . An additional readout amplifier  218  is connected to the bitlines  204  by first fuse links  220   a ,  220   b ,  220   c.  Furthermore, the enable connections  214  on each column have second fuse links  222   a ,  222   b ,  222   c  between the readout amplifiers  210  and the horizontal scan  212 . The additional readout amplifier  218  has an additional enable connection  224  connected to the enable connections  214  by third fuse links  226   a ,  226   b ,  226   c  between the second fuse links  222   a ,  222   b ,  222   c  and the horizontal scan  212 . The additional readout amplifier  218  is a “spare”, which can be used if one of the other readout amplifiers  210  is defective. 
     During a programming phase, typically performed during a manufacturing test phase of the image sensor fabrication, the image sensor  100  is illuminated and images are acquired. These images are then analyzed to determine if there are defective columns, which may highlight a defective readout amplifier. If a defective column is located, then it can be bypassed by modifying the first, second and third fuse links. There are various ways of achieving this, one requires making fuse links with machines such as “Focused Ion Beams”, but this tends to be rather slow and expensive. A more common technology is fusable links. These are fabricated as short-circuits during manufacture and can be programmed open by passing a high current which “blows the fuse”. Another alternative would be to use short metal or polysilicon traces which are then cut by a high powered laser. 
     If, during the test phase, all the columns on the image sensor are found to be operational, then the links are set as shown in Table 1. 
                                   TABLE 1                   Link settings where readout amplifiers       are not defective                LINK   OPEN/SHORT                       220a   OPEN           220b   OPEN           220c   OPEN           222a   SHORT           222b   SHORT           222c   SHORT           226a   OPEN           226b   OPEN           226c   OPEN                        
If, for example, readout amplifier  210   b  is defective, then the fuse links would be set as in Table 2.
 
                                   TABLE 2                   Link settings where column B readout amplifier       210b is defective                LINK   OPEN/SHORT                       220a   OPEN           220b   SHORT           220c   OPEN           222a   SHORT           222b   OPEN           222c   SHORT           226a   OPEN           226b   SHORT           226c   OPEN                        
With this pattern of link settings, column bitline  204   b  is disconnected from readout amplifier  210   b  and connected to the additional readout amplifier  218 . The readout sequence is maintained as the enable connection  214   b  is also disconnected from the readout amplifier  210   b  and connected to the additional readout amplifier  218 .
 
     An alternative embodiment of the invention is shown in  FIG. 3 . Once again, a pixel array  300  has a matrix of pixels  302 . Each column of pixels  302  in the pixel array  300  is connected by a common column bitline  304   a ,  304   b,    304   c.  Each row of pixels  302  is connected by a common row select  306 . The values of the pixels  302  are read out in parallel on to the column bitline  304   a ,  304   b ,  304   c  and, consequently, to readout amplifiers  310   a ,  310   b ,  310   c.  A horizontal scan  312  sends an “enable signal” along a series of enable connections  314   a ,  314   b ,  314   c  to each readout amplifier  310   a ,  310   b ,  310   c  in turn. The amplified pixel value can then be read on an output bus  316 . An additional readout amplifier  318  is connected to the bitlines  304   a,    304   b ,  304   c  by first fuse links  320   a ,  320   b ,  320   c.    
     In this embodiment of the invention, there is an additional enable connection  328  from the horizontal scan  312 . Consequently, each readout amplifier  310   a ,  310   b ,  310   c  (including the additional readout amplifier  318 ) has its own enable signal and are therefore independent. Furthermore, the second and third fuse links of  FIG. 2  are not required. In this case, the address sequence of the readout amplifiers must be generated to suit the defectivity. 
     If there are no defects in the readout amplifiers  310   a ,  310   b ,  310   c , then the horizontal scan  312  enables the readout amplifiers in the following order:  310   a ,  310   b  then  310   c.  Fuse links  320   a ,  320   b  and  320   c  would be open. If there is a defect in readout amplifier  310   b , then the horizontal scan  312  enables the readout amplifiers in the following order:  310   a ,  318  then  310   c.  Fuse links  320   a  and  320   c  would be open. 
     One method of enabling the horizontal scan  312  to modify the order in which it enables readout amplifiers is to have a RAM-based look-up table (LUT) incorporated into an address generator. Address generator circuitry is normal component in an image sensor and therefore would only require slight modification. The address of defective columns would be stored in the RAM. The address generator would increment as normal, but if it reached a value stored in the LUT, then the address of the additional readout amplifier would be used. If the number of columns in the image sensor were a power-of-two, the additional readout amplifier address would have a constant value for the lower number of bits, simplifying the implementation of the counter. Other methods of enabling the horizontal scan  312  to modify the order in which it enables readout would be apparent to those skilled in the art. 
     If the fuse links in the above embodiments are “one-time” fuse links, when testing for defective readout amplifiers, all the fuse links must be kept as short circuit during the sensor test. In the example in  FIG. 2  and  FIG. 3 , keeping the fuse links as short circuit effectively puts three columns in parallel, degrading system performance by reducing the resolution of the system. In any case, the performance of the column amplifiers can still be assessed and the appropriate links blown. 
     An alternative embodiment is shown in  FIG. 4 . As before, a pixel array  400  has a matrix of pixels  402 . Each column of pixels  402  in the pixel array  400  is connected by a common column bitline  404   a ,  404   b ,  404   c.  Each row of pixels  402  is connected by a common row select  406 . The values of the pixels  402  are read out in parallel on to the column bitline  404   a ,  404   b ,  404   c  and, consequently, to readout amplifiers  410   a ,  410   b ,  410   c.  A horizontal scan  412  sends an “enable signal” along a series of enable connections  414   a ,  414   b ,  414   c  to each readout amplifier  410   a ,  410   b ,  410   c  in turn. The amplified pixel value can then be read on an output bus  416 . 
     In this embodiment, an additional readout amplifier  418  is connected to the bitlines  404   a ,  404   b ,  404   c  by a switch. In this case, the switches are field effect transistors (FETs)  430   a ,  430   b ,  430   c  operated as switches. The FETs  430   a ,  430   b ,  430   c  are controlled by a configuration storage unit  432  connected to their gate. Typically, the configuration storage unit  432  is a static RAM, which is uploaded to the image sensor during power-on. As RAM can be written to several times, it is possible to set the initial configuration to disable the additional readout amplifier  418  and enable the readout amplifiers  410   a ,  410   b ,  410   c.  If a defective readout amplifier were detected, then the RAM contents would be changed to disable the defective readout amplifier by allowing the respective FET to conduct and thereby enable the additional readout amplifier  418 . Although the FETs  430   a ,  430   b ,  430   c  shown in  FIG. 4  are NMOS FETs, a PMOS FET, or other suitable device may be used. For example, if there is a wide voltage swing, then a CMOS transmission gate (NMOS in parallel with PMOS, their gates connected to opposite polarity signals) is the most effective, but also most area consuming. 
     If a defect is found and the additional readout amplifier  418  is used, then the input to the defective column amplifier is connected in parallel to the additional readout amplifier  418 . If the readout amplifier has a high input impedance then this is not a problem. However, a readout amplifier&#39;s input usually incorporates an active load to facilitate the operation of the pixel&#39;s source-follower. In this case, having two inputs in parallel will effectively double the current through the pixel&#39;s output, leading to mismatch and other undesirable effects. 
     An alternative embodiment is shown in  FIG. 5 . As before, a pixel array  500  has a matrix of pixels  502 . Each column of pixels  502  in the pixel array  500  is connected by a common column bitline  504   a ,  504   b ,  504   c.  Each row of pixels  502  is connected by a common row select  506 . The values of the pixels  502  are read out in parallel on to the column bitline  504   a ,  504   b ,  504   c.  A horizontal scan  512  sends an “enable signal” along a series of enable connections  514   a,    514   b ,  514   c  to readout amplifiers  510   a ,  510   b ,  510   c  in turn. The amplified pixel value can then be read on an output bus  516 . An additional readout amplifier  518  is connected to the bitlines  504   a ,  504   b ,  504   c  by first switches, which in this example are first FETs  530   a ,  530   b ,  530   c.    
     Second switches, which in this example are second FETs  534   a ,  534   b ,  534   c , connects the readout amplifiers  510   a ,  510   b ,  510   c  and the column bitlines  504   a ,  504   b ,  504   c.  A configuration storage unit  532  is connected to each gate of the first and second FETs enabling control of the connection between drain and source. If a defective readout amplifier is detected, then the readout amplifier can be isolated from the bitline avoiding loading the pixel with both the defective amplifier and the additional readout amplifier. 
     The configuration storage unit  532  may be increased in size, from that of the configuration storage means  432 , to accommodate the extra storage required to accommodate the first and second FETs. In this configuration, when a first FET is switched “on”, to connect a bitline with the additional readout amplifier  518 , the second FET is switched “off”, to disconnect the readout amplifier with the bitline. For example, if readout amplifier  510   a  is defective, first FET  530   a  is switched “on” and second FET  534   a  is switched off. Furthermore, it holds true that each second FET  534   a ,  534   b ,  534   c  will typically be switched as opposite to each first FET  530   a ,  530   b ,  504   c.    
     As such, the configuration storage unit  532  only requires to store the configuration of one set of FETs. A simple inverter can then be used to connect the configuration storage unit  532  with the other set of FETs. For example, the configuration storage unit  532  is connected to the gate of first FET  530   a  and stores that the first FET  530   a  should be “off”, or open, as the readout amplifier  510   a  is not defective. The gate of second FET  534   a  can then be connected to an inverter (not shown), which is then connected to the connection between the gate of the first FET  530   a  and the configuration storage unit  532 . 
     In the embodiments described herein, three columns of pixels are shown in each case with one additional readout amplifier providing redundancy if a readout amplifier is defective. It should be appreciated that the invention allows for any combination of number of additional readout amplifiers to columns of pixels. Typically, this is dictated by the available space in the pixel array and relates to the width of the pixel in relation to the width of the readout amplifier. The present invention would also cover the situation where additional readout amplifiers where provided external to the pixel array. 
     Improvements and modifications may be incorporated without departing from the scope of the present invention.