Abstract:
An imaging sensor includes a defect marker allowing an imaging device in which the imaging sensor is installed to determine which pixels in the CMOS sensor are defective. During manufacturing, the pixels in the imaging sensor are tested. Defect markers are used for defective pixels, preferably using a non-volatile marking technique. After the imaging sensor is installed in the imaging device, the imaging device reads the defect markers from the imaging sensor to determine the defective pixels. The defect markers are read by exposing the pixels in the imaging sensor to photons. Eventually, all pixels in the imaging sensor should show some exposure. Pixels that still read as unexposed are then defective pixels. The imaging device can then compensate for defective pixels: e.g., by interpolating the defective pixels from their neighbors.

Description:
FIELD 
     This invention pertains to an imaging sensor, and more particularly to an imaging sensor having defects. 
     BACKGROUND 
     Complementary Metal Oxide Semiconductor (CMOS) sensors are an alternative to Charge Coupled Devices (CCDs) in imaging devices. Unlike a CCD, the individual picture elements (pixels) in the CMOS sensor are separately addressable. This gives CMOS sensors an advantage over CCDs: a defective pixel does not make the entire CMOS sensor unusable. Today, CMOS sensors are used in all manners of imaging devices: for example, digital still and video cameras, optical scanners, facsimile machines, and robotics, to name just a few. 
     But detecting defects is an expensive process, employing special equipment and test patterns to identify defective pixels in the CMOS sensor. This testing is done after production of the CMOS sensor to determine if the defects make the CMOS sensor unusable. An imaging device utilizing a CMOS sensor may compensate for a few defective pixels by interpolating from neighboring pixels. But too many defects clustered together may result in the CMOS sensor to be discarded. 
     Once the CMOS sensor passes testing, the results of the testing are discarded. Imaging device manufacturers then retest the CMOS sensor to determine which pixels are defective and “compensated.” Because CMOS sensor testing is expensive and complicated, few facilities have the capability to do the testing. Further, if the imaging device is damaged, it is generally cheaper to replace the imaging device than to re-test the CMOS sensor after the imaging device has been repaired. 
     The present invention addresses at least in part these and other problems associated with the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram that shows an embodiment of a digital camera including an embodiment of a Complementary Metal Oxide Semiconductor (CMOS) sensor in accordance with the invention. 
     FIG. 2 shows typical defects that may occur in a CMOS sensor, such as the one of FIG.  1 . 
     FIG. 3 is a flowchart showing an embodiment of a method testing CMOS sensors in accordance with an embodiment of the invention. 
     FIG. 4 is a flowchart showing an embodiment of a method of testing digital cameras using CMOS sensors according to an embodiment in accordance with the invention. 
     FIG. 5 is a schematic diagram that shows an embodiment of testing equipment testing the CMOS sensor embodiment of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic diagram that shows a Complementary Metal Oxide Semiconductor (CMOS) sensor embodiment  105  with picture elements (pixels)  110  in accordance with the invention. Although the embodiment described herein utilizes a CMOS sensor, a person skilled in the art will recognize that the invention is applicable to other types imaging sensors. Pixels  110  generally comprise photo diodes that change their electrical characteristics in response to illumination, referred to here as photodiodes and are arranged in an array on a die. Although FIG. 1 shows the pixels arranged in a rectangular array on the die, a person skilled in the art will recognize that other arrangements are possible: for example, a hexagonal arrangement. Some pixels may be defective; other pixels may operate properly. In this context, a pixel is considered defective if its behavior significantly deviates from an expected response. For example, pixel  120 - 1  shows a pixel that is defective. Pixel  120 - 1  may have one of a number of defects as described herein; more detail can be found hereafter with reference to FIG.  2 . In comparison, pixel  120 - 2  is illustrated as an operational pixel. Provided the number of defective pixels in CMOS sensor embodiment  105  is relatively low enough and generally not too close together, CMOS sensor embodiment  105  will pass testing. Testing is known in the art and will not be described here. For example, although the invention is not limited in scope in this respect, a sensor may be limited to a specific number of defective pixels with a predetermined radius. 
     In an embodiment, defective pixels, like pixel  120 - 1  for example, are modified to return a known value when interrogated. The specifics of how defective pixels may be marked, called defect markers hereafter, may vary: for example, buried gate technology, such as is used in flash memory, or a metal fuse link may be used. A person skilled in the art will also recognize other ways defective pixels may be marked. It is desirable that the marking operate without applying power to the CMOS sensor, for convenience and usability. For example, in FIG. 1, interrogation line  125 - 1 , used to interrogate pixel  120 - 1  as to its value, has been severed by burning out a fuse in interrogation line  125 - 1 . Thus, in this example of CMOS sensor embodiment  105 , upon interrogation, defective pixel  120 - 1  will return a value as if it were unexposed to photons. In contrast, because interrogation line  125 - 2  is unbroken, operational pixel  125 - 1  may be read to provide a value related to the number of photons impinging upon it. 
     In FIG. 1, CMOS sensor embodiment  105  is installed in digital camera embodiment  130 . A person skilled in the art will recognize that CMOS sensor embodiment  105  may be installed in other imaging devices, for example, digital video cameras, optical scanners, facsimile machines, and robotic devices (e.g., robots that rely on visual input to perform their tasks). Digital camera embodiment  130  includes testing mechanism  135  and storage device  140 . Testing mechanism  135 , in this camera embodiment, reads the defect markers from CMOS sensor embodiment  105  and identifies the defective pixels of CMOS sensor embodiment  105  in a defect map stored in storage device  140 . Testing mechanism  135  may read the defect markers in CMOS sensor embodiment  105  while calibrating digital camera embodiment  130 , although the invention is not limited to reading the defect markers during instrument calibration. In this embodiment, storage device  140  comprises a programmable read-only memory (PROM), but a person skilled in the art will recognize that other storage devices can be used, such as non-volatile memory, for example. Further, volatile memory may also be used, although this may be more complex: i.e., accommodating due to the risk of loss of the stored information. Digital camera embodiment  130  may also include compensation unit  145 , which in this embodiment compensates for the defective pixels stored in the defect map in storage device  140 . For example, compensation unit  145  may identify pixels to be used in place of defective pixels. 
     FIG. 2 uses plots to illustrate typical defects that can occur in CMOS sensor embodiment  105 , although these defects are just examples. For example, graph  205  shows the performance graph of an operational pixel in CMOS sensor embodiment  105 . After normalization, the voltage stored by a pixel in CMOS sensor  105  will typically increase in a linear relationship with the number of photons to which the pixel has been exposed. Here, assume that the units are such that the slope of the line in graph  205  is one. Graph  210 , in contrast, shows the performance graph of a pixel, here, with an offset. In an offset defect, the pixel returns a voltage, although no photons are impinging. Graphs  215 - 1  and  215 - 2  show performance graphs of defective pixels with gain defects. In a gain defect, the voltage accumulates too quickly or too slowly relative to the number of photons to which the pixel has been exposed. Graph  220  shows the performance graph of a defective pixel with a “black” defect. In a “black” defect, the pixel provides a voltage as if it has not been exposed to photons, even when the pixel has been. Graph  225  shows the performance graph of a pixel with a “white” defect. In a “white” defect, the pixel provides a voltage as if it has been exposed to photons, even when the pixel has not. A person skilled in the art will also recognize other defects applicable to pixels. 
     Pixels suffering from these and other defects beyond a certain tolerance are not generally usable in capturing the image. The pixel will generally reflect a color or intensity that differs from a desired color or intensity. Pixels suffering from these defects, as examples, are considered defective. The testing procedures referred to above with reference to FIG. 1 therefore are typically employed to identify pixels suffering from these defects. 
     FIG. 3 is a flowchart showing an embodiment of a method of testing CMOS sensors in accordance with the invention. In FIG. 3, the pixels in the CMOS sensor are tested at block  305 . At block  310 , pixels with defects, such as, as one example, pixels whose performances deviate from a standard or expected capability by more than a threshold amount) are identified. At block  312 , defect markers are constructed from the identified defective pixels. If pixels are identified, then at block  320  the defective pixels are marked. This stores the defect markers in the CMOS sensor. As discussed above, in this embodiment the defective pixels are marked to provide a value as if unexposed, even if photons are impinging on the device. 
     FIG. 4 is a flowchart of an embodiment of a method of testing a CMOS sensor according to the one embodiment. At block  402 , the CMOS sensor is reset. At block  405 , the pixels of the CMOS sensor are exposed to light. At block  410 , the pixels, including the defective pixels, are interrogated for their values. By interrogating the defective pixels for their values, the imaging device can build a defect map of the CMOS sensor. In this embodiment, the defective pixels read as unexposed, even when exposed to light. Thus, after a sufficient exposure period, pixels still reading “black” are pixels marked as defective. At block  415 , the imaging device creates a defect map. Finally, at block  420 , the imaging device stores in its memory the defect map. 
     FIG. 5 shows an embodiment of testing equipment employed to mark pixels in the CMOS sensor embodiment of FIG. 1 as defective. In FIG. 5, CMOS sensor embodiment  105  is mounted on testing equipment  505 . Testing equipment  505  uses test patterns, such as test pattern  510 , to determine which pixels have defects, as discussed above with reference to FIGS. 1 and 2 as examples. Testing equipment embodiment  505  exposes CMOS sensor embodiment  105  to test pattern  510  for a sufficient period to properly test each pixel in CMOS sensor embodiment  105 . Testing equipment embodiment  505  also includes marking equipment to set a pixel, such as are defective. For example, testing equipment  505  includes fuse burner  515 , which can burn out a fuse, severing the interrogation line of a defective pixel. A person skilled in the art will also recognize other embodiments of testing equipment may be employed. 
     The previously described embodiment includes several advantages. Testing the CMOS sensor after installation in the digital camera or other imaging device will omit formal testing. The user simply has to expose the CMOS sensor to light to allow the imaging device to determine which pixels in the CMOS sensor are defective. Alternatively, because CMOS sensors respond to dark noise, with sufficient exposure to heat, the operational pixels in the CMOS sensor will eventually read as “white,” even with a dark image. 
     Furthermore, repairing broken imaging devices is feasible for the previously described embodiment. In the past, because of the costs of testing, broken imaging devices have been discarded rather than repaired. The previously described embodiment enables identifying defective pixels, making repair potentially feasible. 
     Having illustrated and described the principles of my invention, it should be readily apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. I claim all modifications coming within the spirit and scope of the accompanying claims.