Patent Application
20080309808
1. Field of the Invention
The present invention relates to a method of driving a CCD (Charge Coupled Device) solid-state image pickup device which performs multiplication driving on signal charges, and also to an image pickup apparatus.
2. Background Art
A vertical charge transfer path (VCCD) 11 is disposed adjacent to each pixel column. A horizontal charge transfer path (HCCD) 13 is disposed along end portions of the vertical charge transfer paths 11. An amplifier 14 which outputs a voltage signal corresponding to the signal charge amount as a taken-image signal is disposed in an output end portion of the horizontal charge transfer path 13.
The charge packet expanding and contracting operations in which the charge packet 17 expands to under electrodes V2, V3 in state T1, contracts under the electrode V3 in next state T2, expands to under electrodes V3, V4 in next state T3, contracts to under the electrode V4 in next state T4, and so on are repeatedly performed, thereby transferring the signal charge 16 to the horizontal charge transfer path 13. A driving pulse for expanding and contracting a charge packet, i.e., a vertical transfer pulse is formed by, for example, voltages of 0 V and −8 V.
In the example, the charge packet expands and contracts in the sequence of one electrode→two electrodes→one electrode→. . . . Alternatively, the operations may be performed in various modes such as three electrodes→four electrodes→three electrodes→. . . .
In recent CCD solid-state image pickup devices, element miniaturization is advancing, and the saturated charge amount of each pixel is being reduced. When highly sensitive imaging is performed on a dark scene by such a device, the signal charge amount stored in each pixel is very small. Therefore, signal amplification is required. However, the floating diffusion amplifier (FDA) 14 and subsequent stage circuits which are disposed in the output stage of the CCD solid-state image pickup device are susceptible to noises. Even when signal amplification is performed in the output stage, it is impossible to obtain an amplification output of a high S/N ratio.
Therefore, signal amplification is preferably performed not in the output stage of the CCD solid-state image pickup device, but in the upstream side of a signal charge transfer path. In a prior art technique disclosed in JP-A-2002-290836 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) below, signal amplification is performed on the vertical charge transfer path 11 which is close to the pixel 12 generating a signal charge. The signal amplification is performed by using the impact ionization phenomenon.
In next state T4, when the barrier under the electrode V5 between the charge packets 19, 20 is lowered, signal charges in the charge packet 19 drain into the deep potential well 21, and the amount of electrons is multiplied by the avalanche effect.
Even in the case where the multiplication factor of one electron multiplication is very small such as “1.01”, when electron multiplication is repeated 100 times, for example, the total multiplication factor is 2.7 times. In state T5, therefore, the multiplied signal charge is moved into the charge packet 19 which is formed under electrodes V6, V7, and the change of states of T3→T4→T5→T3→ . . . is again repeated, whereby a desired multiplication factor is obtained.
In the case where electron multiplication is performed, a deep potential well must be formed under the electrode V3 which has been described with reference to
Therefore, a CCD solid-state image pickup device in which electron multiplication is performed must be produced by a material having a high physical resistance so that, even when a high voltage is repeatedly applied to a specific electrode, the specific electrode is not electrostatically broken. When semiconductors and electrodes are configured by a material or structure which is highly resistant, however, there arises a problem in that the production cost is increased.
In the case where electron multiplication is performed, a deep potential well must be formed under the electrode V3 which has been described with reference to
In the case where the deep potential well 15 is formed in order to perform electron multiplication, and electrons are moved into the potential well 15, when the movement control is not suitably conducted, electrons which jump over the place where the potential well 15 is formed are generated. The electrons are not subjected to electron multiplication, thereby producing a problem in that realization of highly accurate electron multiplication is inhibited by the electros.
Although the accuracy of the technique for producing a semiconductor device has been enhanced, it is difficult to cause the electron multiplication factor which is obtained by repeating electron multiplication 100 times in a certain electrode place, to accurately coincide with that which is obtained by repeating electron multiplication 100 times in another electrode place. Namely, an electron multiplication factor dispersion that is inherent in a solid-state image pickup device is produced. There arises a problem in that this dispersion appears as fixed pattern noises and the quality of a taken image is impaired.
It is a first object of the invention to provide a method of driving a CCD solid-state image pickup device, and image pickup apparatus in which the production cost can be decreased, and the reliability of a solid-state image pickup device can be improved.
It is a second object of the invention to provide a method of driving a CCD solid-state image pickup device, and image pickup apparatus in which electron multiplication driving can be performed for a short time, generation of electrons that are not electron-multiplied is suppressed to enable accurate electron multiplication to be realized, and an electron multiplication factor dispersion that is inherent in an image pickup device can be suppressed.
According to the present invention, a stress which is due to application of a multiplying pulse of high voltage, and which is applied to a semiconductor or an electrode is relieved. Therefore, the resistance of a CCD solid-state image pickup device to a multiplying pulse can be enhanced, and the reliability of the CCD solid-state image pickup device can be improved.
According to the present invention, electron multiplication driving can be performed for a short time, generation of electrons that are not electron-multiplied is suppressed to enable accurate electron multiplication to be realized, and an electron multiplication factor dispersion that is inherent in an image pickup device can be suppressed.
The invention disclosed herein will be understood better with reference to the following drawings of which:
Hereinafter, embodiments of the invention will be described with reference to the accompanying drawings.
Among the illustrated transfer electrodes adjacent to the pixels PD, electrodes V2, V6 are transfer electrodes which function also as a readout electrode. When a high voltage is applied to the readout electrodes, accumulated charges in pixels PD are read out to the vertical charge transfer path. As shown in
When a high voltage is applied to the electrodes V4, V8, accumulated charges in the pixels are read out to the vertical charge transfer path. In the image pickup device, even when a high voltage is applied to electrodes V1, V3, V5, V7 other than the readout electrodes, therefore, signal charges are not read out to the vertical charge transfer path.
In
Referring to
In next state T2, an intermediate-depth potential well 32 is formed under the electrodes V6, V7, and the potential well 31 under the electrodes V1, V2, V3, V4, V5 is eliminated in the sequence of the electrodes, so that the signal charges in the potential well 31 are moved and accumulated in the potential well 32.
In next state T3, a potential well 33 is formed under the electrodes V2, V3, V4. At this time, a high voltage (multiplying pulse) is applied to the middle electrode V3 to deepen a potential well 34 under the electrode V3.
In next state T4, when the voltage VM in applied to the electrode V5, a barrier 35 between the potential wells 32 and 33 disappears, and the signal charges in the potential well 32 are moved toward the deep potential well 34 to fall into the potential well 34.
The potential difference between the intermediate-depth potential well 32 and the deep potential well 34 is large. When signal charges (electrons) fall from the potential well 32 to the potential well 34, therefore, electron multiplication is caused by avalanche breakdown, and the number of electrons is increased.
In next state T5, an intermediate-depth potential well 36 is formed under the electrodes V5, V6, and the potential wells under the other electrodes V1, V2, V3, V4, V7, V8 are eliminated. Then, the signal charges which have been electron-multiplied are collected and accumulated in the potential well 36.
In next state T6, a potential well 37 is formed under the electrodes V8, V1, V2. At this time, a high voltage (multiplying pulse) is applied to the middle electrode V1 to deepen a potential well 38 under the electrode V1.
In next state T7, when the voltage VM is applied to the electrode V3, a barrier 39 between the potential wells 36 and 37 disappears, and the signal charges in the potential well 36 are moved toward the deep potential well 38 to fall into the potential well 38. As a result, electron multiplication is again caused, and the number of electrons is increased.
In next state T8, in the same manner as state T2, the signal charges are moved and accumulated in the potential well 32 formed under the electrodes V6, V7, and then the state is returned to state T3. Namely, states T3 to T8 which have been described above are repeated in a predetermined number of times to obtain a desired electron multiplication factor.
In the embodiment, as described above, the electrode to which the electron multiplying pulse that has a high voltage is applied is changed in the sequence of V3→V1→V3→V1→ . . . , or not fixed. Therefore, the resistance to repeated electron multiplication can be enhanced by the improvement in which the timing of driving the vertical charge transfer path is changed as described above, and the reliability of the solid-state image pickup device can be enhanced without increasing the production cost of the solid-state image pickup device.
In the embodiment, the electrode to which the electron multiplying pulse is applied is changed in the sequence of V3→V1→V3→V1→ . . . , or at each time when electron multiplication is performed one time. Alternatively, the interval of the electrode change may be periodically changed, or, for example, at each time when electron multiplication is performed five times, or at each time when electron multiplication is performed 100 times. There is a configuration where electron multiplication is not always performed, such as that where electron multiplication is required only when highly sensitive imaging is conducted in a digital camera or the like. In such a case, preferably, information of the electrode on which previous electron multiplication is performed is stored in a nonvolatile memory or the like, and the electrode on which the electron multiplying pulse in the present electron multiplication is to be applied is determined on the basis of the information.
In the illustrated example, among the transfer electrodes adjacent to the pixels PD, electrodes V1, V3, V5, V7 are transfer electrodes which function also as a readout electrode. When a high voltage is applied to the readout electrodes, accumulated charges in pixels PD are read out to the vertical charge transfer path. In a state where a signal charge is already read out from a corresponding pixel and the pixel is empty, even when a high voltage is applied to a readout electrode, however, no signal charge is read out.
In
Referring to
In next state T2, the intermediate-depth potential well 32 is formed under the electrodes V4, V5, and the potential well 31 under the electrodes V8, V1, V2, V3 is eliminated in the sequence of the electrodes, so that the signal charges in the potential well 31 are moved and accumulated in the potential well 32.
In next state T3, the potential well 33 is formed under the electrodes V8, V1, V2. At this time, a high voltage (multiplying pulse) is applied to the middle electrode VI to deepen the potential well 34 under the electrode V1. The electrode V1 is a readout electrode for the pixel. However, the signal charge is already read out from the pixel. Therefore, it is a matter of course that, even when the high voltage is applied to the electrode V1, no signal charge is read out from the pixel. Consequently, the electrode V1 can be used as an electrode for applying the multiplying pulse.
In next state T4, when the voltage VM is applied to the electrode V3, the barrier 35 between the potential wells 32 and 33 disappears, and the signal charges in the potential well 32 are moved toward the deep potential well 34 to fall into the potential well 34.
The potential difference between the intermediate-depth potential well 32 and the deep potential well 34 is large. When signal charges (electrons) fall from the potential well 32 to the potential well 34, therefore, electron multiplication is caused by avalanche breakdown, and the number of electrons is increased.
In next state T5, the intermediate-depth potential well 36 is formed under the electrodes V5, V6, and the potential wells under the other electrodes V1, V2, V3, V4, V7, V8 are eliminated. Then, the signal charges which have been electron-multiplied are collected and accumulated in the potential well 36.
In next state T6, the potential well 37 is formed under the electrodes V1, V2, V3. At this time, a high voltage (multiplying pulse) is applied to the middle electrode V2 to deepen the potential well 38 under the electrode V2.
In next state T7, when the voltage VM is applied to the electrode V4, the barrier 39 between the potential wells 36 and 37 disappears, and the signal charges in the potential well 36 are moved toward the deep potential well 38 to tall into the potential well 38. As a result, electron multiplication is again caused, and the number of electrons is increased.
In next state T8, in the same manner as state T2, the signal charges are moved and accumulated in the potential well 32 formed under the electrodes V4, V5, V6, and then the state is returned to state T3. States T3 to T8 which have been described above are repeated in a predetermined number of times to obtain a desired electron multiplication factor.
As described above, also in the CCD solid-state image pickup device in which pixels are arranged in a square lattice pattern, the electrode to which the electron multiplying pulse is applied is periodically changed or not fixed similarly with the first embodiment. Therefore, the resistance of the CCD solid-state image pickup device can be enhanced by the improvement of the method of driving the vertical charge transfer path.
When rising and falling edges of the electron multiplying pulse are steep (approximately vertical), however, the potential of a transfer electrode to which the electron multiplying pulse is applied is steeply changed, and an electric field applied to the transfer electrode and its vicinity is largely changed, thereby producing a possibility that the electrode and a semiconductor portion in the vicinity may be electrostatically broken. Moreover, the electron multiplying pulse is repeatedly applied many times. Even when electrostatic breakdown does not occur, therefore, the physical properties of the portions may he impaired.
In the embodiment, at least one of rising and falling edges of the electron multiplying pulse is inclined as shown in the lower portion of
Preferably, both the pulse rising and falling edges are provided with inclination (time width) of about several nanoseconds to produce an electron multiplying pulse which avoids a steep electric field change. The vertical charge transfer path is driven by the pulse.
When the CCD solid-state image pickup device is driven as described above, even in the case where the transfer electrode V3 is fixed as the electrode to which the electron multiplying pulse is applied as described with reference to
When the high voltage is applied to the transfer electrode for applying the electron multiplying pulse, an electric field applied to the transfer electrode and its vicinity is largely changed, thereby producing a possibility that the electrode and a semiconductor portion in the vicinity may be electrostatically broken. Moreover, the electron multiplying pulse is applied a very large number of times. Even when electrostatic breakdown does not occur, therefore, the physical properties of the portions may be impaired.
In the embodiment, as shown in
A fifth embodiment of the invention is configured by combining the above-described third and fourth embodiments with each other. Namely, the height of the electron multiplying pulse is made lower than that of a read pulse, and at least one, more preferably both, of rising and falling pulse edges is inclined. According to the configuration, the resistance of the CCD solid-state image pickup device can be further enhanced.
Namely, at least one, more preferably both, of rising and falling pulse edges of the multiplying pulse shown in
In the embodiment, the electrode to which the electron multiplying pulse is applied is not fixed, and the edge(s) of the electron multiplying pulse is inclined so that the electric field is gently changed. Therefore, the resistance of the CCD solid-state image pickup device is enhanced, and the reliability is improved.
A seventh embodiment of the invention is configured by combining the above-described first and fourth embodiments with each other. Namely, the electrode to which the electron multiplying pulse is applied is not fixed, and the height of the electron multiplying pulse is made lower than that of a read pulse. Also according to the configuration, the resistance of the CCD solid-state image pickup device is enhanced, and the reliability is improved.
An eighth embodiment of the invention is configured by combining the above-described first and fifth embodiments with each other. Namely, the electrode to which the electron multiplying pulse is applied is not fixed, the height of the electron multiplying pulse is made lower than that of a read pulse, and at least one, more preferably both, of rising and falling edges of the electron multiplying pulse is inclined. Also according to the configuration, the resistance of the CCD solid-state image pickup device is enhanced, and the reliability is improved.
Namely, at least one, more preferably both, of rising and falling edges of the multiplying pulse shown in
According to the embodiment, in the CCD solid-state image pickup device in which pixels are arranged in a square lattice pattern, the electrode to which the electron multiplying pulse is applied is not fixed, and the edge(s) of the electron multiplying pulse is inclined so that the electric field is gently changed. Therefore, the resistance of the CCD solid-state image pickup device is enhanced, and the reliability is improved.
A tenth embodiment of the invention is configured by combining the above-described second and fourth embodiments with each other. In the CCD solid-state image pickup device in which pixels are arranged in a square lattice pattern, the electrode to which the electron multiplying pulse is applied is not fixed, and the height of the electron multiplying pulse is made lower than that of a read pulse. Also according to the configuration, the resistance of the CCD solid-state image pickup device is enhanced, and the reliability is improved.
An eleventh embodiment of the invention is configured by combining the above-described second and fifth embodiments with each other. In the CCD solid-state image pickup device in which pixels are arranged in a square lattice pattern, the electrode to which the electron multiplying pulse is applied is not fixed, the height of the electron multiplying pulse is made lower than that of a read pulse, and at least one, more preferably both, of rising and falling edges of the electron multiplying pulse is inclined. Also according to the configuration, the resistance of the CCD solid-state image pickup device is enhanced, and the reliability is improved.
The embodiment relates to a driving method in the motion-picture imaging mode (including the preliminary operation). The illustrated pixel arrangement of the solid-state image pickup device is a square lattice arrangement, and color filters (R=red, G=green, B=blue) are arranged in a Bayer pattern.
The readout electrodes are the electrodes V1, V3, V5, V7. In the still-picture imaging mode, signal charges of all pixels are read out by the multi-field reading. In the motion-picture imaging mode, usually, signal charges are read out while performing pixel decimation. In the illustrated example, the reading out of signal charges to the vertical charge transfer path is performed only on every third pixel line.
In the motion-picture imaging mode, in each frame, the reading out of signal charges to the vertical charge transfer path is repeatedly performed on the readout pixel row. Namely, a read pulse of high voltage is repeatedly applied to the readout electrode of the pixel row.
In the embodiment, in the motion-picture imaging mode including the preliminary operation, therefore, an electrode adjacent to a pixel other than a readout row is used as the transfer electrode to which the electron multiplying pulse is to be applied. In the example shown in
Alternatively, electron multiplication may be performed while the embodiment is combined with the other embodiment(s).
According to the configuration, the resistance of the CCD solid-state image pickup device is enhanced, and the reliability of the device is improved The embodiment can be applied not only to a CCD solid-state image pickup device of the square lattice arrangement, but also to a CCD solid-state image pickup device of the honeycomb pixel arrangement.
The above-described method of driving a CCD solid-state image pickup device is performed in the following manner. In an image pickup apparatus on which a CCD solid-state image pickup device is mounted, such as a digital camera, an image pickup device driving portion which drives and controls the CCD solid-state image pickup device, such as a timing generator produces a driving pulse such as shown in
The imaging portion 51 comprises: an optical lens system 51a which collects light incident from an object field; an aperture or a mechanical shutter 51b which converges the light that has passed through the optical lens system 51a; and a CCD solid-state image pickup device 100 which receives the light that has been collected by the optical lens system 51a and converged by the aperture, and which outputs taken-image data (analog image data).
The digital camera of the embodiment further comprises: a digital signal processing portion 56 which receives the digital image data output from the A/D 53, and which performs the interpolating process, the white balance correction, the RGB/YC converting process, and the like; a compression/expansion processing portion 57 which compresses image data to image data of the JPEG format or conversely expands image data; a displaying portion 58 which displays a menu, a through image, and a taken image; the system controlling portion (CPU) 59 which controls the whole of the digital camera; an internal memory 60 such as a frame memory; a media interface (I/F) portion 61 which performs an interface process with respect to a recording medium 62 that sores JPEG image data or the like; and a bus 70 which interconnects these portions. An operating portion 63 through which the user inputs instructions is connected to the system controlling portion 59.
In the thus configured digital camera, when the user input “highly sensitive imaging instruction” through the operating portions 63, for example, the CPU 59 instructs the driving portion 54 to perform electron multiplication. Then, the driving portion 54 produces the multiplying pulse which has been described in the embodiments, drives the CCD solid-state image pickup device 100 so that a very small amount of signal charges obtained in an imaging process is multiplied on the vertical charge transfer path, and outputs the multiplied signal charges.
As described above, even in the case where a solid-state image pickup device is not produced by a material having a high physical resistance to a high voltage, when an electrode to which a high-voltage pulse is to be applied, and the high-voltage pulse are improved as described above, places to which a high voltage (a read pulse and a multiplying pulse) is applied is not unevenly distributed, and hence stress concentration due to application of a high voltage can be avoided. Even in an existing CCD solid-state, image pickup device, therefore, electron multiplication can be easily performed, and highly sensitive imaging or the like is enabled.
Although the embodiments have been described by exemplifying a CCD solid-state image pickup device serving as an area sensor, the invention may be applied also to a line sensor.
The digital camera of this embodiment is configured in a similar manner as the digital camera of the thirteenth embodiment. Moreover, the CCD solid-state image pickup device 40 shown in
Although not illustrated, in order to perform horizontal pixel addition, a buffer which is called a line memory (LM), and which temporarily stores signal charges received from the vertical charge transfer paths 11 and transfers the signal charges to the horizontal charge transfer path 13 may be sometimes disposed between end portions of the vertical charge transfer paths 11 and the horizontal charge transfer path 13, as described in, for example, JP-A-2002-112119.
Although, in the following, the CCD solid-state image pickup device having the configuration shown in
In the figure, a hollow octagon indicates a photodiode from which a signal charge has been read out to the vertical charge transfer path 11, and a solid octagon indicates a photodiode from which a signal charge has not been read out.
In the figure, a hollow circle in a potential well indicates a signal charge (in this example, electrons), and a hatched area indicates an area where a signal charge (electrons) exists.
In the figure, V1 to V8 denote transfer electrodes to which a transfer pulse φVi (i=1 to 8) is applied Each of pairs V1A and V1B, V2A and V2B, V3A and V3B, and V4A and V4B indicates that the same pulse is sometimes applied to corresponding electrodes, and different pulses are sometimes applied to the electrodes. In the electron multiplication driving, different pulses are applied to the electrodes.
Signal charges are read out from the photodiodes 12 respectively flanked on the transfer electrodes V1A, V2A, V3A, V4A, to the adjacent vertical charge transfer path 11. As shown in state T0, the signal charges are confined in a potential well 41 which is formed under the electrodes V6, V7, and a high voltage (for example, +15 V) is applied to the electrode V2B to form a deep potential well 43 under the electrode V2B.
Then, a barrier 42 (a barrier which is formed by applying a voltage of, for example, −8 V to the electrode V8) that divides between the potential well 41 which stores the signal charges, and which is formed under the electrodes V6, V7, and the deep potential well 43 under the electrode V2B is eliminated For example, the barrier is eliminated by setting the voltage applied to the electrode V8 to 0 V).
As shown in state T1, as a result, the signal charges in the potential well 41 flow toward the deep potential well 43, and fall into the potential well 43.
In states T0 and T1, in order to form the potential well 41, −8 V is applied to the electrodes V5, V4A, and the harrier 44 is formed. In next state T2, −8 V is applied also to the electrode V6 which is adjacent to the electrode V5 on the side of the potential well 43, thereby increasing the height of the barrier under the electrode V6. This causes the signal charges which flow toward the potential well 43, to be further pushed toward the potential well 43 by the barrier 44.
In next state T3, −8 V is applied also to the electrode V7 which is adjacent to the electrode V6 on the side of the potential well 43, thereby increasing the height of the barrier under the electrode V7. This causes the signal charges which flow toward the potential well 43, to be further pushed toward the potential well 43 by the barrier 44 to rapidly fall into the potential well 43. In this way, in electron multiplication, the driving is performed so that signal charges are pushed toward a deep potential well, and hence the time of multiplication driving can be shortened.
In the illustrated example, the pushing by the barrier 44 toward the potential well 43 is stopped with leaving a length corresponding to two electrodes or the electrodes V1B and V8. When the pushing by the barrier 44 is not stopped with leaving a length corresponding to at least one electrode, the potential difference between the electrode V2B for forming the potential well 43 and the electrode V1B adjacent to the electrode is excessively large (+15 V−(−8 V)=+23 V), thereby causing electrostatic breakdown. Therefore, the pushing by the barrier 44 to the vicinity of the potential well 43 is not performed.
In next state T4, this multiplication driving is ended, and hence the deep potential well is eliminated (the voltage applied to the electrode V2B is returned from +15 V to 0 V). In next state T5, the position of the barrier 44 is returned to under the electrodes V5, V4A. Then the state is returned to state T0, and the above-described operations are repeated.
In the above-described embodiment, when multiplication driving is performed by causing signal charges to fall into a deep potential well, the driving in which signal charges are pushed toward the potential well for multiplication is performed. Therefore, the multiplication driving can be performed for a short time, and the time of outputting a taken-image signal from the CCD solid-state image pickup device can be quickened.
In the modification, when, in states T2 and T3, signal charges are push-driven toward the potential well 43, a barrier portion 44a of the harrier 44 used in the pushing, on the side of the potential well 43 is made slightly higher than the barrier 44. For example, a voltage of −9 V is applied to the electrode(s) forming the barrier portion 44a.
When this driving method is employed, the signal charges in the potential well 41 can be surely pushed toward the potential well 43.
In the modification, the process which is performed until, in state T3, the barrier 44 is push-driven toward the potential well 43 and signal charges fall into the potential well 43 is identical with that of the fourteenth embodiment. As described also in state T3 of
In the modification, therefore, the potentials of the electrodes V1B, V8 are set to the level at which electrostatic breakdown does not occur, in state T4 subsequent to state T3. For example, the height of the barrier 44 is formed by applying −8 V to the corresponding electrodes. At this time, the voltage applied to the electrodes V1B, V8 is limited to, for example, −6 V or −4 V, thereby accelerating the rapid fall of signal charges into the potential well 43.
In the conventional driving method, electron multiplication is performed in the following manner. Accumulated electrons of photodiodes are read out to a vertical charge transfer path by the interlace method, signal charges are stored in the potential well 41 formed under the electrodes V6, V7 on the vertical charge transfer path, and the deep potential well 43 is formed under the electrodes V2A, V2B.
In this case, when the barrier between the potential wells 41, 43 is eliminated and electrons in the potential well 41 fall into the potential well 43, however, there is a possibility that some of the electrons jump into electrons in the potential well 41 in the precedent stage as shown by the arrow a in
In the embodiment, therefore, a countermeasure for preventing mixture of electrons into the precedent stage is taken in the following manners The reading of signal charges from the photodiodes 12 is performed by 4-field reading. An empty packet 46 is previously disposed in a place where the mixture into the previous stage may possibly occur in electron multiplication. Electrons which jump over the deep potential well 43 toward the precedent stage are captured by the empty packet 46, and mixture of signal electrons into the stage two stages before is avoided.
The empty packet is illustrated also in
It is not preferable that electrons which are once captured into the empty packet 46 are moved to another empty potential well. Therefore, preferably, a high voltage is applied to the electrodes V6, V7 which form the empty packet 46 (the electrodes function also as electrodes for forming the potential well 41) so that a strong electric field is not produced above the empty packet.
After the multiplication driving, the electrons which are captured in the empty packet 46 are combined with the original electrons which have been multiplied, whereby the accuracy of a taken-image signal can be enhanced.
In the illustrated example, when, in state T0, the barrier 42 is eliminated and electrons in the potential well 41 fall into the deep potential well 43, electrons 47 jump into the empty packet 46 with a certain probability. When electrons are pushed toward the potential well 43 in states T2 and T3 which are not shown in
In the embodiment, after state T4 transfers to state T5 and electron multiplication is ended, a barrier 48 (a barrier under the electrodes V5, V4B) which separates the potential well 41 storing the multiplied electrons from the empty packet 46 is eliminated, and a barrier 49 (a harrier under the electrode V8) which is on the opposite side of the empty packet 46 is moved toward the potential well 41 (state T6), whereby, as shown in state T7, the multiplied electrons and the electrons 47 in the empty packet 46 can be combined with each other in the potential well 41.
Then, the state is again returned to state T0, and the next electron multiplying operation is performed. According to the configuration, electron multiplication can be performed also on electrons which have jumped into an empty packet, and the accuracy of a taken-image signal can be further enhanced.
As described in the embodiments above, the operation is repeated in which, after electron multiplication driving is performed by using the deep potential well 43 formed in a certain place A in the vertical charge transfer path, electros increased by the multiplication driving are shifted (transferred) to a place B of the next line (the transfer electrodes of the vertical charge transfer path consist of repetition of the electrodes V1 to V8, and the next line is a line configured by the subsequent electrodes V1 to V8), and electron multiplication driving is performed by using the deep potential well 43 formed in the place B.
The line shift may be conducted each time electron multiplication is performed one time. Alternatively, the operation may be repeated in which electron multiplication driving is performed a predetermined number of times in a certain place, and electron multiplication driving is again performed the predetermined number of times in the next line-shifted place.
As described above, electron multiplication driving is repeated while line-shifting the place where the driving. In the embodiment, therefore, a dispersion of the multiplication factor depending on a place can be suppressed, and fixed pattern noises can be reduced.
In the embodiment, by contrast, an operation is repeated in which, as shown in
In the case where electron multiplication driving is repeated 100 times, the transfer electrode to which a high voltage is applied in order to form a deep potential well may be sequentially switched over on the same line. In place that the position where the deep potential well is formed is switched over each time the driving is performed, the position may be switched over each time the driving is performed a predetermined plural number of times. Alternatively, the embodiment may be combined with the above-described sixteenth embodiment so that electrodes for forming a deep potential well are switched over while performing the line-shift.
According to the configuration, fixed pattern noises can be reduced to (1/√N) as compared with the case where electron multiplication driving is repeated in the same place.
The route of charge movement in multiplication driving, particularly, the potential difference in the interface through which electrons fall a deep potential well largely affects the electron multiplication factor. This is caused by the phenomenon that, even when the same voltages are applied to electrodes, the potential difference between electrodes across a boundary or the electric field strength is different depending on whether the boundary is formed vertically or obliquely.
When, as in the seventeenth embodiment, the place (electrodes) where multiplication driving is performed is moved each time electron multiplication is performed or each time electron multiplication is performed a predetermined number of times, the electrode dependency of the electron multiplication factor can be eliminated, and a uniform electron multiplication factor can be obtained.
The above description is directed to the example shown in FIG. 21A. Also in the CCD solid-state image pickup device shown in
An electrode which functions also as a readout electrode is formed so as to be suitable for application of the readout voltage which is a high voltage. Therefore, a high voltage can be readily applied to such an electrode, and a deep potential well can be easily formed. By contrast, an electrode dedicated to transfer is formed on the assumption that a high voltage is not applied. Even in the case where the same high voltage is applied, therefore, the depth of a formed potential well is different from that formed by an electrode which functions also as a readout electrode.
Also in the CCD solid-state image pickup device of
Although the first to seventeenth embodiments have been individually described, it is a matter of course that, when plural or all of the embodiments are applied to driving of a CCD solid-state image pickup device, the stability, reliability, accuracy, noise reduction, and the like of electron multiplication driving can be further enhanced.
According to the driving method of the present invention, the resistance to electron multiplication can be enhanced, and, even when electron multiplication is performed, the reliability of the device can be improved. Therefore, the method is useful in application to a CCD solid-state image pickup device which is mounted on a digital camera or the like.
Moreover, according to the driving method of the present invention, the electron multiplication can be performed for a short time and accurately, the location dependency of the electron multiplication factor can be reduced, and fixed pattern noises due to the location dependency of the electron multiplication factor can be reduced. Therefore, the method is useful as a method of driving a CCD solid-state image pickup device which is mounted on a digital camera that performs highly sensitive imaging.
This application is based on and claims priority under 35 U.S.C. 119 from Japanese Patent Application No. 2007-160199 filed Jun. 19, 2007, Japanese Patent Application No. 2007-256224 filed Sep. 28, 2007, Japanese Patent Application No. 2007-256226 filed Sep. 28, 2007 and Japanese Patent Application No. 2007-256227 filed Sep. 28, 2007.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2007-160199 | Jun 2007 | JP | national |
| P2007-256224 | Sep 2007 | JP | national |
| P2007-256226 | Sep 2007 | JP | national |
| P2007-256227 | Sep 2007 | JP | national |
