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Coundrver RESEN K Prise DIGITAL 10-BIT MAGING REDIBLUE PROCESSING DATAOUT US 20090 160979A1 (19) United States (12) Patent Application Publication (10) Pub. No.: US 2009/0160979 A1 Xu et al. (43) Pub. Date: Jun. 25, 2009 (54) METHODS AND APPARATUSES FOR (22) Filed: Dec. 20, 2007 DOUBLESIDED DARK REFERENCE PIXEL ROW-WISE DARK LEVEL Publication Classification NON-UNIFORMITY COMPENSATION IN IMAGE SIGNALS (51) Int. Cl. H04N 9/64 (2006.01) (75) Inventors: Chen Xu, Alhambra, CA (US); Yaowu Mo, Arcadia, CA (US); Mei (52) U.S. Cl. ................................. 348/243: 348/E05.081 Yan, Pasadena, CA (US) (57) ABSTRACT Correspondence Address: DCKSTEIN SHAPRO LLP Methods and apparatuses for row-wise dark level non-unifor 1825. EYE STREET NW mity compensation of imaging sensor pixel signals. A column Washington, DC 20006-5403 (US) dependent dark reference value is determined as one of a linear and parabolic function of signal values from two areas (73) Assignee: MICRON TECHNOLOGY, INC. of dark reference pixels and a column location and then used for dark level non-uniformity compensation of signal values (21) Appl. No.: 12/004,035 from imaging pixels. 842 CONTROLREGISTERS O TIMING AND CONTROL 809 808 810 804 7 COUNDECODERAE counDRVER RESEN K prise DIGITAL 10-BIT MAGING REDIBLUE PROCESSING DATAOUT SENSOR CHANNEL 806 81 SENSORCORE ROWDECODER 812 ROWDRIVER Patent Application Publication Jun. 25, 2009 Sheet 1 of 12 US 2009/O160979 A1 O 3000 COLUMN NUMBER V(col) FIG. 1 Patent Application Publication Jun. 25, 2009 Sheet 2 of 12 US 2009/O160979 A1 100 12 2 3 10 N -- 19 FIG.2 Patent Application Publication Jun. 25, 2009 Sheet 3 of 12 US 2009/O160979 A1 READ OUT SIGNALS OF DARK REFERENCEPEXELS 1000 NAREA 16 FORAROWn DETERMINEADARKREFERENCE WALUEASAFUNCTION 1010 OF THE READ OUT DARK REFERENCEPIXELSIGNALS FROMAREA 16 READ OUT IMAGING 1020 PXELSIGNALS FOR ROWn DETERMINEDARKCURRENT COMPENSATED IMAGINGPXEL 1030 SIGNALSASA FUNCTION OF THE DETERMINED DARKREFERENCEVALUE FIG. 3 Patent Application Publication Jun. 25, 2009 Sheet 4 of 12 US 2009/O160979 A1 O 3000 COLUMNNUMBER V(col) FIG. 4 Patent Application Publication Jun. 25, 2009 Sheet 5 of 12 US 2009/O160979 A1 200 Patent Application Publication Jun. 25, 2009 Sheet 6 of 12 US 2009/O160979 A1 READ OUT SIGNALS OFDARK REFERENCEPIXELS 2000 INAREA26 FORAROWn READ OUT SIGNALS OF DARK REFERENCEPIXELS 2010 INAREA 36 FORAROW in DETERMINEADARKREFERENCE WALUEASAFUNCTION 2020 OF THE READ OUTDARK REFERENCE PIXELSIGNALS FROMAREA26 AND 36 READ OUTMAGING 2030 PXELSIGNALSFOR ROW in DETERMINEDARKCURRENT COMPENSATED IMAGINGPXE 2040 SIGNALSASA FUNCTION OF THE DETERMINEDDARKREFERENCEVALUE F.G. 6 Patent Application Publication Jun. 25, 2009 Sheet 7 of 12 US 2009/0160979 A1 3000 COLUMN NUMBER Patent Application Publication Jun. 25, 2009 Sheet 8 of 12 US 2009/O160979 A1 3000 COLUMNNUMBER W(CO) FIG. 8 Patent Application Publication Jun. 25, 2009 Sheet 9 of 12 US 2009/O160979 A1 3000 COLUMN NUMBER W(CO) FIG. 9 Patent Application Publication Jun. 25, 2009 Sheet 10 of 12 US 2009/O160979 A1 076 976 S ***===,=*!---??,«»«) - - - se - a se - - - - - - - - 1.- 908 006 BHOOHOSNES Patent Application Publication Jun. 25, 2009 Sheet 11 of 12 US 2009/O160979 A1 078 Patent Application Publication Jun. 25, 2009 Sheet 12 of 12 US 2009/O160979 A1 006 <!=>[,], US 2009/01 60979 A1 Jun. 25, 2009 METHODS AND APPARATUSES FOR value as the column number increases. The increasing pixel DOUBLESIDED DARK REFERENCE PIXEL signal value is due to row-wise dark level non-uniformity ROW-WISE DARK LEVEL noise and will appear as a horizontal shading across the array. NON-UNIFORMITY COMPENSATION IN This shading across the array might not be significant in lower IMAGE SIGNALS resolution imaging sensors having fewer columns of pixels or imaging devices with lower pixel clock frequencies. How FIELD OF THE INVENTION ever, the shading across the array is more pronounced in higher resolution imaging sensors (e.g., greater than 1750 0001. The embodiments described herein relate generally columns) and imaging devices with high pixel clock fre to imaging devices and, more specifically, to methods and quency (e.g., greater than 75 MHz). Accordingly, improved apparatuses for row-wise dark level non-uniformity compen row-wise dark level non-uniformity compensation methods sation in image signals from imaging sensors employed in and apparatuses are needed. Such devices. BRIEF DESCRIPTION OF THE DRAWINGS BACKGROUND OF THE INVENTION 0006 FIG. 1 shows a graph of the pixel signal value versus 0002 Solid State imaging devices, including charge column number for a row that has not been dark level non coupled devices (CCD), complementary metal oxide semi uniformity compensated. conductor (CMOS) imaging devices, and others, have been 0007 FIG. 2 illustrates a top view of a CMOS pixel array used in photo imaging applications. A Solid state imaging with dark columns located at one side of the pixel array. device circuit includes a focal plane array of pixel cells or 0008 FIG. 3 illustrates a flowchart of a row-wise dark pixels as an imaging sensor, each cell including a photosen level non-uniformity compensation method based on a pixel Sor, which may be a photogate, photoconductor, a photo array with one dark reference pixel area. diode, or other photosensor having a doped region for accu 0009 FIG. 4 shows a graph of the pixel signal value versus mulating photo-generated charge. For CMOS imaging column number for a row that has been dark level non-uni devices, each pixel has a charge storage region, formed on or formity compensated when the method of FIG. 3 is imple in the Substrate, which is connected to the gate of an output mented. transistor that is part of a readout circuit. The charge storage (0010 FIG. 5 illustrates a top view of a CMOS pixel array region may be constructed as a floating diffusion region. In with dark columns located at two sides of the pixel array. some CMOS imaging devices, each pixel may further include 0011 FIG. 6 illustrates a flowchart of a row-wise dark at least one electronic device Such as a transistor for transfer level non-uniformity compensation method based on a pixel ring charge from the photosensor to the storage region and array with two dark reference pixel areas. one device, also typically a transistor, for resetting the storage 0012 FIG. 7 shows a graph of dark level non-uniformity region to a predetermined charge level prior to charge trans versus column number when the method of FIG. 6 is imple ference. mented with a dark level non-uniformity reference value 0003. In a CMOS imaging device, the active elements of a determined by a constant function. pixel perform the necessary functions of: (1) photon to charge 0013 FIG. 8 shows a graph of dark level non-uniformity conversion; (2) accumulation of image charge; (3) resetting versus column number when the method of FIG. 6 is imple the storage region to a known state; (4) storage of charge in mented with a column dependent dark level non-uniformity the storage region; (5) selection of a pixel for readout, and (6) reference value determined by a linear function. output and amplification of a signal representing pixel charge. 0014 FIG. 9 shows a graph of dark level non-uniformity Photo charge may be amplified when it moves from the initial versus column number when the method of FIG. 6 is imple charge accumulation region to the storage region. The charge mented with a column dependent dark level non-uniformity at the storage region is typically converted to a pixel output reference value determined by a parabolic function. Voltage by a source follower output transistor. 0015 FIG. 10A illustrates a block diagram of system-on 0004 CMOS imaging devices of the type discussed above a-chip imaging device constructed in accordance with an are generally known as discussed, for example, in U.S. Pat. embodiment. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310, 0016 FIG. 10B illustrates an example of a sensor core 366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524, and used in the FIG. 10A device. U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc. 0017 FIG. 11 shows a system embodiment incorporating 0005 Ideally, the digital images created by a CMOS imag at least one imaging device. ing device are exact duplications of the light image projected upon the device pixel array. That is, for a flat-field image, all DETAILED DESCRIPTION OF THE INVENTION of the imaging pixel signals should have the same signal value. However, various noise sources can affect individual 0018. In the following detailed description, reference is pixel outputs and thus distort the resulting digital image. As made to the accompanying drawings which form a part CMOS pixel arrays increase in size to obtain higher resolu hereof, and in which is shown by way of illustration specific tion, the physical non-uniformity of the arrays becomes more embodiments that may be practiced. These embodiments are prominent. One issue occurring in higher resolution imaging described in sufficient detail to enable those of ordinary skill sensors, such as, for example, eight or more megapixel sen in the art to make and use them, and it is to be understood that sors, is row-wise dark level non-uniformity that increases structural, logical, or procedural changes may be made to the across the pixel array as the column number increases, caus specific embodiments disclosed. ing a horizontal shading across the array. For example, FIG. 1 0019 Row-wise dark level non-uniformity has two com represents imaging pixel signal values of a row n of a flat-field ponents, amplitude and phase. When row-wise dark level image and shows an exponentially increasing pixel signal non-uniformity compensation is applied, if a pixel signal US 2009/01 60979 A1 Jun.
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