JP2006203775A - Driving method of solid state imaging element and imaging device and system using imaging element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To carry out high-speed electronic shutter operation with no distortion of an image caused by reading-out scanning time without adding a circuit for total electronic shutter operation. <P>SOLUTION: The imaging device has an imaging element having a light receiving element for generating an optical charge according to the receiving amount and accumulating it and a storage unit for accumulating the optical charge temporarily, a first scanning means for carrying out charge storage initiation scan of the imaging element, a second scanning means for carrying out storage finish scanning of the imaging element by transferring the optical charge of the receiving unit to the storage unit, and a third scanning means for reading out the signal of the storage unit. The storage initiation is performed by the first scanning means, and after transfer finish is performed by the second scanning means, the read-out is performed by the third scanning means. In this case, the first scanning means and the second scanning means scan in a nonlinear way. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a configuration and a driving method of a solid-state imaging device for realizing an accurate and high-speed shutter operation in an imaging apparatus and system using the solid-state imaging device.

  Conventionally, in an imaging apparatus, an exposure time of an imaging element is controlled using a shutter that is a mechanical light shielding member. In this case, in the exposure at a short time, it is necessary to operate the shutter at a very high speed, and mechanical accuracy and control become difficult. Some image pickup devices use a focal plane shutter with improved control accuracy, but a complicated mechanism and accuracy are required, which increases costs. In addition, there is a limit to mechanical shutters, and electronic shutter operations are indispensable for realizing higher-speed shutters.

  Hereinafter, an electronic shutter operation using an image sensor employing a conventional XY address type scanning method will be described.

  First, a scan for removing unnecessary charges accumulated in a pixel for each pixel or line, that is, a reset scan is executed, and then a signal charge is read for each pixel or line after a predetermined time has elapsed. It is known that it can be realized by doing. Such an electronic shutter is hereinafter referred to as a rolling electronic shutter.

  The structure and driving method of a conventional image sensor will be described with reference to FIGS. 1 and 3, and the rolling electronic shutter operation will be described.

  FIG. 1 shows the configuration of an image sensor that employs an XY address type scanning method. Reference numeral 101 denotes a unit pixel. Reference numeral 102 denotes a photodiode (hereinafter referred to as PD) that converts light into electric charge. Reference numeral 103 denotes a transfer switch that transfers charges generated in the PD by the transfer pulse φTX to an FD described later. Reference numeral 104 denotes a region for temporarily storing charges (hereinafter referred to as FD), and reference numeral 105 denotes an amplification MOS amplifier that functions as a source follower. Reference numeral 106 denotes a selection switch that selects a pixel by a selection pulse φSEL, and reference numeral 107 denotes a reset switch that removes charges accumulated in the FD by a reset pulse φRES. The FD 104, the amplification MOS amplifier 105, and a constant current source 109 described later constitute a floating diffusion amplifier, and the signal charge of the pixel selected by the selection switch 106 is converted into a voltage and output to the readout circuit 113 via the signal output line 108. The Reference numeral 109 denotes a constant current source serving as a load for the amplification MOS amplifier 105.

  Reference numeral 110 denotes a selection switch that selects an output signal from the readout circuit 113 and is driven by the horizontal scanning circuit 114. Reference numeral 112 denotes a vertical scanning circuit for selecting the switches 103, 106, and 107.

  The nth scanning line selected by the vertical scanning circuit 112 in each of φTX, φRES, and φSEL is defined as φTXn, φRESn, and φSELn.

  FIG. 3 shows a driving pulse and an operation sequence in the rolling electronic shutter operation. In FIG. 3, description is made with respect to n + 3 lines from n lines selected by the vertical scanning circuit 112.

  In the rolling electronic shutter operation, at time n31 to time t31, first, pulses are applied to φRESn and φTXn to turn on transfer switch 103 and reset switch 107, and are stored in PD102 and FD104 on the nth line. A reset operation for removing charges is performed. At time t32, the transfer switch 103 is turned off, and an accumulation operation for accumulating the photocharge generated in the PD is started. Next, at time t <b> 34, a pulse is applied to φTXn to turn on the transfer switch 103, and a transfer operation for transferring the photocharge accumulated in the PD 102 to the FD 104 is performed. Note that the reset switch 107 needs to be turned off prior to the transfer operation, and in this figure, the reset switch 107 is turned off simultaneously with the transfer switch 103 at time t32. Here, the accumulation time is from time t32 to time t35. After the transfer operation for the nth line is completed, a pulse is applied to φSELn and the selection switch 106 is turned on, whereby the charge held in the FD 104 is converted into a voltage and output to the reading circuit 113. The signals temporarily held by the readout circuit 113 are sequentially output from the time t36 by the horizontal scanning circuit 112. The time from the start of transfer at time t34 to the end of reading at time t37 is T3read, and the time from time t31 to time t33 is T3wait. Similarly, in other lines, the time from the start of transfer to the end of reading is T3read, and the time from the start of resetting of one line to the start of resetting of the next line is T3wait.

  From the upper sequence of FIG. 3, the rolling electronic shutter has a problem that the accumulation time is shifted by the time required for scanning the screen at the top and bottom of the screen. This is because the time T3wait from the reset and transfer scan of a line to the reset and transfer scan of the next line must be set to a time longer than T3read. This is because if the T3wait time is shorter than the T3read time, the information of the pixel for which the reading operation has not been completed enters the reading circuit and the information of the next line is lost. Therefore, if the readout scanning of the signal output cannot be performed at high speed, this deviation becomes large especially when the number of pixels is large.

Further, there is a MOS type image pickup device that performs a reset operation and a read operation collectively as described in JP-A-2003-17677. A sequence of each line in such an operation is shown in FIG. In FIG. 4, the reset operation of all lines is performed simultaneously from time t41 to time t42. The transfer operation from time t43 to time t44 is also performed at the same time. Such an electronic shutter is hereinafter referred to as a batch electronic shutter. When the collective electronic shutter is performed, the accumulation time is from t42 to t44 for all lines, and an electronic shutter in which the accumulation time does not deviate up and down the screen is obtained. However, since the reset operation and the transfer operation are selected at once and the read operation is scanned for each row, a circuit for realizing the collective selection operation is newly required. The cost will increase.
JP 2003-17677 A

  By the way, when a still image is taken with an electronic shutter, reading is often performed after light is shielded with a mechanical shutter after the accumulation with the electronic shutter is completed. This is because, even after the electronic shutter operation is performed by the image pickup device, the surface of the image pickup device is still exposed to light. Therefore, when a CCD is used as the image pickup device, the frame transfer type CCD or interline 2 among the CCDs is used. This is because, when a field readout type CCD is used, it is necessary to read out the charges in a state where light is blocked. In addition, in other CCDs as well, in order to pursue high image quality by actively preventing smears (a phenomenon in which charges overflowing from CCD pixels flow into the transfer section and light appears to pull up and down the screen) Similarly, it is necessary to read out the light while blocking the light to the CCD.

  In addition, when a MOS type image pickup device is used, if there is an image pickup device in which light leaks into the FD, photoelectrons are generated in the FD. Therefore, after transfer, the image must be shielded by a mechanical shutter.

  Here, when a collective electronic shutter is used using a mechanical focal plane shutter, the scanning time of the shutter is different between the mechanical shutter and the collective electronic shutter, so that uneven exposure occurs in the storage unit in the scanning direction of the mechanical shutter.

  Therefore, an object of the present invention is to provide a high-speed electronic shutter that does not distort an image due to the time required for readout scanning without adding a circuit for batch electronic shutter operation.

  Although it is ideal that the mechanical shutter should move linearly, the mechanical shutter has the property that the movement starts slowly and gradually becomes faster.

  When sensor driving similar to this movement is performed, the pulse may or may not overlap depending on the line, and the offset of the signal changes for each line, resulting in belt-like noise.

  If there is periodicity, the same offset occurs in all the lines, so there is no problem as an image.

  Since the periodicity of the pulse is lost, the overlap amount of the pulse changes and becomes a factor for generating a fixed pattern.

  Also, if the number of lines that are driven simultaneously is different, the driving capability becomes insufficient when the number of lines is large, and there is a possibility of occurrence of fixed pattern noise.

  In order to solve the above-described problems, the present invention provides a light receiving unit that generates and accumulates photocharges according to the amount of received light, and an image pickup device having a storage unit that temporarily stores the light charges of the light receiving unit, and the charge of the image pickup device A first scanning unit that performs accumulation start scanning; a second scanning unit that performs accumulation end scanning of the image sensor by transferring photocharges of the light receiving unit to the accumulation unit; and a third unit that reads out signals from the accumulation unit. In the imaging apparatus having the scanning means, the first scanning means sequentially starts to accumulate rows or columns, and the third scanning means sequentially reads out the rows or columns that have been transferred by the second scanning means. Features.

  In the imaging apparatus, the first scanning unit and the second scanning unit can simultaneously scan a plurality of rows or a plurality of columns.

  In the imaging apparatus, the imaging apparatus further comprises a mechanical shutter that scans the mechanical shutter in the same direction as the first scanning unit and the second scanning unit after completion of reading.

Further, in the above imaging apparatus, in the case of the imaging apparatus nonlinear, characterized in that the scanning time of the first scanning means and the second scanning means is combined with the scanning time of the mechanical shutter, the periodicity of the pulse is lost. Even in such a case, the occurrence of a fixed pattern is eliminated by avoiding the overlap of pulses.

  In addition, by adjusting the pulse width according to the number of lines that are driven simultaneously, variation in characteristics due to drive capability is improved.

  A high-speed electronic shutter operation without increasing the chip area is realized, and light is shielded by a mechanical shutter after transfer, thereby obtaining a more accurate exposure.

  At this time, even with a non-linear mechanical shutter, the timing of the first scanning means and the second scanning means can be adjusted to be non-linear so that still images can be taken without exposure unevenness after transfer. By doing so, no band-like noise is generated.

  In addition, the drive load differs due to the difference in the number of lines that are driven simultaneously, resulting in a difference in the waveform at the actual gate of the pulse, but by adjusting the pulse width, variations in characteristics and fixed pattern noise are reduced. It can be eliminated.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows an image sensor. 101 represents a unit pixel. Reference numeral 102 denotes a photodiode (hereinafter referred to as PD) that converts light into electric charge. Reference numeral 104 denotes an accumulation region (hereinafter referred to as FD) for temporarily accumulating charges, and reference numeral 103 denotes a transfer switch for transferring charges generated in the PD to the FD by a transfer pulse φTX. Reference numeral 105 denotes an amplification MOS amplifier that functions as a source follower. Reference numeral 107 denotes a reset switch that resets the FD by a reset pulse φRES. Reference numeral 106 denotes a selection switch that selects a pixel by a selection pulse φSEL, and the selected pixel signal is transferred to the readout circuit 113 through the signal output line 108. The readout circuit 113 has a function of holding the transferred pixel signal. Reference numeral 109 denotes a load current source. Reference numeral 110 denotes a selection switch for selecting an output signal held by the readout circuit, which is sequentially selected by the horizontal scanning circuit 114. Reference numeral 112 denotes a vertical scanning circuit for selecting the switches 103, 106, and 107. The pixel signal selected by the horizontal scanning circuit 114 is amplified by the output amplifier 111 and output. In each of φTX, φRES, and φSEL, the nth scan line is set to φTXn, φRESn, and φSELn. Here, for the sake of simplicity, an image sensor in which the unit pixel is composed of 4 × 4 rows is shown, but the number of pixels is not limited to 4 × 4.

  FIG. 5 shows a driving method and an operation sequence in the embodiment of the present invention. First, by applying a pulse to φRESn and φTXn from time t51 to t52, switches 107 and 103 are turned on, and a reset operation is performed to reset PD102 and FD104 on the nth line. From time t52, an accumulation operation in which photoelectric charges are generated in the PD on the n-th line starts, and the reset operation up to the final line is completed at t53. From time t54 to t55, a pulse is applied to φTXn to turn on the switch 103, and a transfer operation for transferring the photocharge accumulated in the PD 102 to the FD 104 is performed. The accumulation time from the time t52 to the time t54 is the nth line accumulation time. The transfer operation from the (N + 1) th line is sequentially performed at time after t55, and transfer of all lines is completed at time t57. Here, after the transfer operation for the Nth line is completed, a pulse is applied to φSELn at time t56 and the switch 106 is turned on, whereby the charge held at 104 is converted into a voltage by 105 and read out to the reading circuit 113. The signals temporarily held in the reading circuit 113 are sequentially output from the time 15 by the horizontal scanning circuit 112. Reading of the (n + 1) th line is performed after time t58.

  In FIG. 4, the anteroposterior relationship between times t53 and t54 and the anteroposterior relationship between t56 and t57 may be reversed.

  Since the time from the start of scanning of one line to the start of scanning of the next line can be determined by the above driving method without depending on the readout time, image distortion due to the rolling electronic shutter can be reduced.

  Based on FIG. 2, an embodiment in the case where the above-described driving method of the imaging element is applied to a digital camera which is an imaging apparatus will be described in detail.

  In FIG. 2, reference numeral 201 denotes a lens unit that forms an optical image of a subject on the image sensor 203, and zoom control, focus control, aperture control, and the like are performed by the lens driving device 202. A shutter unit 203 has a shutter curtain corresponding to the rear curtain of a focal plane type shutter used in a so-called single-lens reflex camera, and is controlled by a shutter control unit 204. Reference numeral 205 denotes an image pickup device for capturing an object imaged by the lens unit 201 as an image signal, and 206 denotes an A / D conversion for performing amplification of an image signal output from the solid-state image pickup device 204 or analog-digital conversion, and A / This is an imaging signal processing circuit that performs various corrections on the image data after D conversion and compresses the data. Reference numeral 207 denotes a solid-state imaging device 205, a timing generation circuit that is a driving unit that outputs various timing signals to the imaging signal processing circuit 206, 209 a control circuit that controls various operations and the entire imaging apparatus, and 208 temporarily stores image data. Memory for storing, 210 an interface for recording or reading on a recording medium, 211 a removable recording medium such as a semiconductor memory for recording or reading image data, and 212 communicating with an external computer or the like Interface.

  Next, the operation of the digital camera at the time of shooting in the above configuration will be described.

  When the main power supply is turned on, the power supply for the control system is turned on, and the power supply for the image pickup system circuit such as the image pickup signal processing circuit 204 is turned on.

  Then, when a release button (not shown) is pressed, in order to control the exposure amount, the control circuit 206 performs photometry with the photometry device 213 to determine the brightness, and the lens driving device determines the aperture of the optical system according to the result. To control.

  Next, based on the signal output from the distance measuring device 214, a high frequency component is extracted and the distance to the subject is calculated by the control circuit 206. Thereafter, the lens driving device 202 drives the lens unit to determine whether or not it is in focus. When it is determined that the lens unit is not in focus, the lens unit is driven again to perform distance measurement.

  Then, after the in-focus state is confirmed, the photographing operation starts.

  A sequence of photographing operations will be described with reference to FIG. First, resetting of the image sensor is started. When the reset is started, scanning of the reset operation is sequentially performed in the line direction of the image sensor. Thereafter, transfer is started after a certain accumulation time T61. When the transfer is started, the charge transfer operation is sequentially performed in the line direction of the image sensor. Thereafter, reading of charges is started, but a mechanical shutter is scanned to prevent the image sensor from being exposed during the reading of charges. By making the scanning direction of the mechanical shutter the same as the scanning direction of the image sensor, the difference on the image sensor in the post-accumulation exposure time T62 can be reduced. This mechanical shutter is not provided to define the accumulation time, but the accumulation time is defined by the image sensor. Since the reset scanning and the transfer operation are electronic shutters in the image sensor, the time T61 is not significantly different between the upper and lower sides of the image sensor, and a highly accurate shutter operation is possible. Since it is used to shield the image sensor after the charge transfer, it may be a conventional single-lens reflex camera type, so-called front and rear curtain two-blade type, or a single-blade type with only the rear curtain. Good. In the single blade type, the thickness of the shutter unit can be reduced as compared with the conventional two-blade type focal plane shutter, which is advantageous in terms of space.

  Furthermore, since the scanning time of the mechanical shutter varies when the image sensor is scanned, this difference can be further reduced by making the scanning time of the mechanical shutter substantially the same as the scanning time of the image sensor.

  When the photographing operation is completed, the image signal output from the solid-state imaging device 204 is subjected to processing such as amplification and A / D conversion by the photographing signal processing circuit 206 and written to the memory by the control circuit 209.

  Thereafter, the data stored in the memory 208 is recorded on a removable recording medium 211 such as a semiconductor memory through the recording medium control I / F unit 210 under the control of the control circuit 209.

  Further, the image may be processed by directly inputting to a computer or the like through the external I / F unit 213.

  In addition, when the image pickup device has a large amount of light from the subject whose PD is saturated at time T62, the charge generated in the PD leaks into the FD. This can be prevented by using an image sensor having a function of discarding surplus charges.

  In the imaging device, the vertical scanning circuit that shifts by one stage shown in the timing chart of FIG. 7 is used as the vertical scanning circuit, but by using a scanning circuit that can perform the scanning operation shown in the timing chart of FIG. Furthermore, an electronic shutter with high speed and less distortion may be realized.

  FIG. 9 shows a timing chart of a vertical running circuit corresponding to a mechanical shutter that operates nonlinearly.

  FIG. 10 shows a photographing sequence when a non-linear mechanical shutter is used, and is the same as the operation of FIG.

  9 differs from FIG. 8 in that the number of lines driven simultaneously is not constant but gradually increases.

  Here, it is important that the pulses match or do not overlap even if the number of driven lines is different. If they overlap, another scanning pulse may come in as noise during scanning. When driving linearly, the same noise is applied to all lines and canceled, but in the case of non-linear driving, they remain without being canceled.

  Further, since the number of simultaneously driven lines is different, the pulse time is lengthened in order to suppress the occurrence of a fixed pattern due to a load difference due to insufficient drive capability.

  In FIG. 9, the time of the uniform pulse is lengthened, but the time may be varied according to the number of simultaneously driven pulses.

  Further, the first and second scanning means have been described so far, but when the first and second are overlapped in time, the first and second pulses are not overlapped. Needless to say.

An image sensor used in the image pickup apparatus according to the embodiment of the present invention. The imaging device of embodiment of this invention. The timing chart of the conventional drive method. The timing chart of the conventional drive method. A timing chart of a driving method. Timing chart of shooting operation. 6 is a timing chart of a scanning circuit for sequential scanning. The timing chart of the scanning circuit of a block division. The timing chart of a non-linear scanning circuit. Non-linear shooting operation timing chart.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Unit pixel 102 Photodiode 103 Transfer switch 104 Storage area 105 Amplification MOS amplifier 106 Selection switch 107 Reset switch 108 Signal output line 109 Load current source 110 Horizontal selection switch 111 Output amplifier 112 Vertical scanning circuit 113 Reading circuit 114 Horizontal scanning circuit 201 Lens Unit 202 Lens driving device 203 Shutter 204 Shutter driving device 205 Imaging device 206 Imaging signal processing circuit 207 Timing generation unit 208 Memory unit 209 Overall control / calculation unit 210 Recording medium control I / F unit 211 Recording medium 212 External I / F unit 213 Photometry device 214 Distance measurement device

Claims (5)

An image sensor having a light receiving unit that generates and accumulates photocharge according to the amount of received light and a storage unit that temporarily stores the photocharge of the light receiving unit, and a first scanning unit that performs a charge accumulation start scan of the image sensor;
A second scanning means for performing an accumulation end scan of the image pickup device by transferring the photocharge of the light receiving unit to the accumulation unit;
A third scanning unit for reading out signals from the storage unit;
In the apparatus in which accumulation is started by the first scanning unit and transfer is completed by the second scanning unit, and then reading is performed by the third scanning unit, the first scanning unit and the second scanning unit scan non-linearly. It is characterized by.   2. The imaging apparatus according to claim 1, wherein the first scanning unit and the second scanning unit can simultaneously scan a plurality of rows or a plurality of columns.   2. The imaging apparatus according to claim 1, further comprising a mechanical shutter that scans in the same direction as the second scanning unit.   3. The imaging apparatus according to claim 2, wherein the non-linear scanning of the first scanning unit and the second scanning unit is controlled so that the pulses do not interfere with each other.   According to a second aspect of the present invention, the pulse width is adjusted when there are many lines for simultaneous scanning.

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