JP2552923B2 - Imaging device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image pickup apparatus using a charge-coupled solid-state image pickup device (FIT-CCD) of a frame interline transfer system, and particularly to an electronic shutter function. The present invention relates to an imaging apparatus that performs imaging by using

[Conventional technology]

A charge-coupled solid-state image pickup device used in such an image pickup apparatus has a structure shown in FIG.

That is, a drain unit 1 for discharging unnecessary charges, a light receiving unit 2 for receiving an optical image of an object, a storage unit 3 for temporarily holding signal charges generated in the light receiving unit 2 for each pixel, and a storage unit 3 The horizontal charge transfer path 4 and the like for reading out the signal charges of the above is formed by a semiconductor manufacturing technique.

To further describe the structure of each part, first, the light receiving section 2
A plurality of photodiodes are arranged in a matrix along the vertical scanning direction V and the horizontal scanning direction H. For example, in the case of a primary color stripe filter, red (R) is formed on the surface of the photodiode group arranged in the first column in the figure. A color filter, a green (G) color filter is stacked on the surface of the photodiode group arranged in the second row, and a blue (B) color filter is stacked on the surface of the photodiode group arranged in the third row. The photodiodes are repeatedly arranged as a set in the horizontal scanning direction H, and each photodiode corresponds to a pixel. Assuming that these photodiodes are arranged in M rows (M is an even number) in the vertical scanning direction V, a photodiode group located in an odd row when viewed from the drain section 1 side is located in an even row in the first field. The photodiode group is treated as a second field.

Further, vertical charge transfer portions l _{1 to} l _N are formed adjacent to the photodiode groups in each column, and a four-phase drive signal is applied to transfer electrode groups (not shown) stacked on the upper surfaces of the vertical charge transfer paths. φ
_I1, φ _I2, φ _I3, by applying a phi _I4, to generate a transfer element for transferring the signal charges to the vertical scanning direction V. In addition, a light-shielding layer for blocking the incidence of light is formed on the upper surfaces of all the vertical charge transfer paths l _{1 to} l _N.

Further, transfer gates (for example, represented by TG in the figure) are provided between the respective photodiodes and the transfer transfer elements of the vertical charge transfer paths adjacent to the photodiodes. The signal charges generated in the respective photodiodes by being conducted are transferred to the charge transfer elements of the vertical charge transfer paths.
The gate electrode for driving the transfer gate is formed integrally with the transfer electrode of the vertical charge transfer path, and the transfer gate is set to the conductive state by setting the drive signal to a high voltage at a predetermined timing. It is supposed to do.

The drain portion 1 is formed of a predetermined impurity layer formed to be connected to one end of all the vertical charge transfer paths l _{1 to} l _N.
Unnecessary charges transferred through the vertical charge transfer path are discharged to the semiconductor substrate.

The storage unit 3 is composed of a group of charge transfer paths extending from the other ends of all the vertical charge transfer paths l _{1 to} l _N , and a group of transfer electrodes (shown in the figure) stacked on the upper surfaces of the charge transfer paths. ), Four-phase drive signals φ _S1 , φ _S2 , φ _S3 , φ _S4 are applied to transfer the signal charges transferred from the light receiving unit 2 in the vertical scanning direction V and temporarily drive the signals. Stopping has a function of holding signal charges in a predetermined charge transfer element group. Therefore, for M / 2 lines (in other words,
It is possible to generate a transfer element group capable of temporarily holding the signal charge generated in the photodiode for one field). Further, a light shielding layer for blocking the incidence of light is formed on the upper surfaces of all the charge transfer paths.

The horizontal charge transfer path 4 is connected to all the ends of the charge transfer path group of the storage unit 3 and receives two-phase drive signals φ _H1A and φ _H2 applied to a transfer electrode group (not shown) formed on the upper surface. The signal charges are transferred in the horizontal scanning direction H in synchronization. The signal charges transferred in synchronization with the two-phase drive signals φ _H1A and φ _H2 are impedance-converted by the floating diffusion amplifier 5 in synchronization with the gate signals φ _RS and φ _H1B and output as color signals for each pixel. Output to the terminal 6.

FIG. 9 shows the image pickup timing in an electronic still camera having an electronic shutter function, and FIG.
~ (H) shows the operation of each time point t ₁ ~t ₈ in FIG. 9. For simplification of description, FIG. 10 shows three columns of red (R), blue (B), and green (G) which are adjacent to each other and two columns.
A read scan for a pixel group in a row is shown as a representative.

The operation will be described with reference to FIGS. 9 and 10. First, VD in FIG. 9 becomes "H" every 1/60 second.

When the shutter release button of the electronic still camera at time t ₁ is to be pressed, it is the charge transfer mode in the first field, it four-phase drive signals synchronously phi _I1,
By keeping only the drive signal φ _I2 at the high voltage “HH” while keeping φ _I3 and φ _I4 at “M” level (voltage for generating transfer elements in the vertical charge transfer path), As shown in a), the transfer gate corresponding to the first field is made conductive (that is, the potential level becomes deeper than the potential level of the photodiode), and unnecessary charges in all the photodiodes of the first field are adjacent to each other. Transfer to the transfer element of the vertical charge transfer path. At the same time, the charge transfer operation of the horizontal charge transfer path 4 starts, and unnecessary charges in the horizontal charge transfer path 4 are discharged to the outside via the floating diffusion amplifier 5 within a predetermined period.

Next, by switching to the charge transfer mode of the second field and setting only the drive signal φ _I4 to the high voltage “HH” at time t ₂ , the second field is supported as shown in FIG. 10 (b). The transfer gates are turned on, and the unnecessary charges in all the photodiodes of the second field are transferred to the transfer element of the adjacent vertical charge transfer path.

The transfer operation of these time points t ₁ and t _2, unnecessary charges remaining in all the photodiodes moves to the vertical charge transfer paths.

Next, within a predetermined period of the vertical retrace period, the light receiving unit 2
The vertical signal transfer path and the charge transfer path of the storage unit 3 discharge unnecessary signal charges by transferring them to the drain unit 1 in synchronization with the drive signals φ _{I1 to} φ _I4 and φ _{S1 to} φ _S4 . FIG. 10 (c) shows a time point t ₃ during the transfer operation.

Next, at the time point t ₄ when the discharging operation of all unnecessary charges is completed, the mode is switched to the charge transfer mode of the first field again, and the driving signal φ _{I 2 is set} to the high voltage “HH” as at the time point t ₁ . As shown in FIG. 10 (d), the transfer gates corresponding to the first field are made conductive, and the signal charges in all the photodiodes in the first field are transferred to the transfer element of the adjacent vertical charge transfer path. That is, a period tau ₁ from the time of non-conductive the transfer gate after time t ₁ until the transfer gate to a conducting at time t ₄ again, the exposure time of the photo diode group corresponding to the first field.

Next, the signal charges in the vertical charge transfer path are transferred to the charge transfer path of the storage unit 3 by a high-speed charge transfer operation during a predetermined period. FIG. 10 (e) shows the operation at a certain time point t ₅ during the transfer.
When this transfer operation is completed, all signal charges of the photodiode group corresponding to the first field are held in the storage unit 3.

Next, the mode is switched to the charge transfer mode of the second field again, and the driving signal φ _{I4 is set} to the high voltage “HH” at the time t ₆ when the period τ ₁ has passed from the time t ₂ so that FIG. As shown in, the transfer gate corresponding to the second field is made conductive, and the signal charges in all the photodiodes in the second field are transferred to the transfer element of the adjacent vertical charge transfer path. That is, the period τ ₁ from when the transfer gate is turned off after time t ₂ until when the transfer gate is turned on again at time t ₆ is the exposure time of the photodiode group corresponding to the second field.

Next, while the signal charges corresponding to the second field are stopped in the vertical charge transfer path, the signal charges corresponding to the first field in the storage unit 3 are vertically transferred to the horizontal charge transfer path 4 and one horizontal The horizontal charge transfer path 4 transfers the horizontal charges each time the vertical charges for the lines are transferred, so that color signals corresponding to the pixels corresponding to the first field are output (period Δτ _{1 in the} figure).

When the reading of the signal charges from the storage section 3 is completed,
The signal charge in the vertical charge transfer path of the light receiving section 2 is transferred to the storage section 3. This state is shown in FIG. 10 (g) corresponding to the time point t ₇ .

Next, when all the signal charges corresponding to the second field are transferred to the storage unit 3, the signal charges are vertically transferred to the horizontal charge transfer path 4 and at the same time, the vertical charges for one horizontal line are transferred. By diverting the horizontal charge transfer path 4, the color signal corresponding to each pixel corresponding to the second field is output (period Δτ _{2 in the} figure). This is shown in FIG. 10 (h).

As described above, the reset operation is performed in synchronization with the shutter release button, and the signal charge accumulated in the photodiode is transferred to the vertical charge transfer path according to the shutter speed, thereby obtaining a pseudo frame electronic shutter function. I can do it.

[Problems to be Solved by the Invention] However, in such a conventional imaging device,
There is a problem that the field flicker becomes noticeable as the shutter speed is increased.

That is, as shown in the timing chart of FIG. 9, after the signal charge corresponding to the first field is read out, the second charge is read out.
In order to read out the signal charge corresponding to the field, the signal charge corresponding to the second field is temporarily stopped in the vertical charge transfer path in the light receiving portion. Therefore, the smear generated during this stop period corresponds to the second field. Mix into the signal charge to
This smear component was the cause of field flicker. In FIG. 9, the smear component is mixed in the signal charge corresponding to the second field in the period Δτ ₁ ,
Compared with the influence of smear on the signal charge corresponding to the first field, the influence of smear on the signal charge corresponding to the second field is larger.

This smear component is generated when light incident from the opening of the photodiode reaches the vertical charge transfer path while reflecting in the silicon oxide film below the charge transfer electrode, or light of high light intensity is generated in the semiconductor substrate. It may be caused by generation of unnecessary charges or by light incident through the light shielding film formed on the upper surface of the vertical charge transfer path.

The present invention has been made in view of the above problems, and an object thereof is to provide an imaging device in which the occurrence of field flicker is reduced.

[Means for solving the problem]

In order to achieve such an object, the present invention sets the potential level when a transfer gate formed between a photoelectric conversion element corresponding to a pixel and a vertical charge transfer path adjacent to the pixel is conductive according to the shutter speed. Decided to control.

Further, the principle of the present invention will be described with reference to FIGS. First, FIG. 1 shows a signal charge of a certain pixel corresponding to the first field with respect to the shutter speed (horizontal axis) and a certain field corresponding to the second field when the image is captured by the conventional pseudo frame electronic shutter function. The level difference from the signal charge of the pixel is an actually measured value shown in percentage, and is shown for each of red (R), blue (B) and green (G).

As is clear from the figure, when the shutter speed becomes higher, the incident light intensity becomes higher accordingly, so that the smear component in the pixel signal corresponding to the second field rises with respect to that of the pixel corresponding to the first field. However, when the difference reaches about 1% or more, the flicker can be easily confirmed by human eyes. Since the three primary colors show different characteristics with respect to the shutter speed, not only the field flicker due to the brightness but also the
Color flicker due to color also occurs.

Therefore, the inventor of the present application has found that the conditions under which smear increases are as follows:
As the shutter speed becomes faster, the potential level of the transfer gate is made shallower to limit the amount of signal charge transferred from the photoelectric conversion element to the vertical charge transfer path, so that the color signal corresponding to the first field to be finally read out can be obtained. The control is performed so that the smear mixing ratios of the color signals corresponding to the second field become equal.

That is, according to the present invention, as shown in FIG. 2, a photoelectric conversion element (photodiode) is formed by an impurity layer PD diffused in a well layer in a semiconductor substrate, and a vertical charge transfer path adjacent to the photoelectric conversion element is formed by a transfer gate TG. And the photoelectric conversion element and the vertical charge transfer path are formed such that the potential level of the photoelectric conversion element and the vertical charge transfer path are formed so that the vertical charge transfer path becomes the deepest, as shown in FIG. During exposure, control is performed so that the potential level of the transfer gate becomes the shallowest and becomes a potential barrier.
Then, when the shutter speed is high, as shown in FIG. 4, the potential level under the transfer gate TG is controlled to be shallow so that the signal charge generated in the photoelectric conversion element moves to the vertical charge transfer path. By limiting the amount, the signal charge amount corresponding to the smear component generated in the vertical charge transfer path is left on the photoelectric conversion element side. On the other hand, when the shutter speed is slow, as shown in FIG. 5, the amount of movement of the signal charge generated in the photoelectric conversion element to the vertical charge transfer path is controlled by making the potential level under the transfer gate TG deeper. Relax the restrictions of.

In this way, by controlling the potential level of the transfer gate in the opposite manner to the characteristic curve shown in FIG. 1, the smear components for each field can be made pseudo equal and the occurrence of field flicker is reduced. be able to.

[Action]

In the present invention based on such a principle, when the signal charge corresponding to the smear component mixed with the signal charge corresponding to the second field is transferred from the photoelectric conversion element to the vertical charge transfer path, the shutter speed is changed. Since the potential is not read out by controlling the potential level of the transfer gate in inverse proportion, the apparent smear component of each field becomes equal, and the occurrence of field flicker can be reduced.

〔Example〕

 An embodiment of the present invention will be described below with reference to the drawings.

FIG. 6 shows the structure of an image pickup device in an electronic still camera. Note that the same FIT-CCD as in FIG. 8 is applied to the imaging device.

First, the structure will be described. Reference numeral 7 is a frame interline transfer type charge-coupled solid-state imaging device. Further, as in the case shown in FIG. 8, red (R), blue ( The color filters 8 are formed on the stripes of B) and green (G).

Reference numeral 9 denotes a vertical drive signal driver, which is a four-phase drive system drive signal φ _I1 , φ _I2 , for driving a vertical charge transfer path in the light receiving portion (see FIG. 8) of the charge-coupled solid-state imaging device.
φ _I3 , φ _I4 and four-phase drive driving signals φ _S1 , φ _S2 , for driving the charge transfer paths in the storage section (see FIG. 8).
φ _S3 and φ _S4 are generated in synchronization with the synchronization signal from the timing generation circuit 10.

Reference numeral 11 is a field shift control circuit, which generates a timing of a control signal FS for controlling a transfer gate (see FIG. 8) formed between a photodiode forming a pixel in the light receiving portion and a vertical charge transfer path. It is generated in synchronization with the sync signal from the circuit 10.

Reference numeral 12 denotes a horizontal drive signal driver, which is a two-phase drive type drive signal φ _H1A for driving the horizontal charge transfer path (see FIG. 8) and gate signals φ _H1B , φ for driving the gates.
_RS is generated in synchronization with the synchronization signal from the timing generation circuit 10.

Reference numeral 13 is a sample and hold circuit 14, which samples the color signal output through the horizontal charge transfer path (see FIG. 8) by the correlated double sampling method in synchronization with the timing signal _SSH from the timing generation circuit 10. .

A process circuit 14 performs white balance adjustment and γ correction processing on the color signal output from the sample hold circuit 13, and then forms and outputs a luminance signal and a color difference signal.

Reference numeral 15 denotes a magnetic recording circuit, which FM-modulates the luminance signal and the color difference signal output from the process circuit 14 and records them on a recording medium such as a magnetic disk.

A central control circuit 16 controls the overall operation timing of each circuit.

Reference numeral 17 denotes a shutter control unit, which outputs a signal indicating that the operator has pressed the shutter release button to the central control circuit.
Supplying to 16 activates a series of imaging operations with an electronic shutter function.

FIG. 7 shows the configuration of the vertical drive signal driver 9 in more detail. That is, the vertical drive signal driver 9 is a circuit (not shown) that generates four-phase drive signals φ _I1 , φ _I2 , φ _I3 , and φ _I4.
In addition, a variable voltage circuit 18 for outputting a voltage designated by the voltage designation signal SV from the central control circuit 16 is provided, and its output voltage V _TG is added via the analog switch 19 to the adder circuits 20 and 2
Made to apply to 1. In addition, the analog switch 19 uses the control signal FS from the field shift control circuit 11.
Accordingly, during the first field read operation, it is switched to the switching contact a side at a predetermined timing, and during the second field read operation, it is switched to the switch contact b side at another predetermined timing. The drive signal φ _I2 is applied to the adder circuit 20, and the drive signal φ _I4 is applied to the adder circuit 21. When the analog switch 19 is switched to the switching contact a side, the adding circuit 20
Outputs a drive signal φ _I2 on which the voltage V _TG is superimposed, and when the analog switch 19 is switched to the switching contact b side, the adder circuit 21 outputs a drive signal φ _I4 on which the voltage V _TG is superimposed.

Incidentally, the voltage V _TG is set so as to correspond to the characteristic curve of FIG. 1 so that the voltage becomes lower as the shutter speed becomes faster, and conversely becomes higher as the shutter speed becomes slower.

Next, the operation of the electronic still camera having such a configuration will be described. The control timing is the same as in FIG. 9, and if the operator presses the shutter release button at time t ₁ after setting the shutter speed f, the shutter control section 17 instructs the central control circuit 16 to start imaging. Is done,
Further, the central control circuit 16 holds shutter speed data.

Then, at time t ₁ , the maximum voltage V _TG that causes the transfer gate to generate a potential level enough to transfer all the signal charges of the photoelectric conversion element corresponding to the pixel to the vertical charge transfer path is output to the variable voltage circuit 18. , Time t ₁
The analog switch 19 is switched to and connected to the contact a for a short time in synchronization with. As a result, the unnecessary charges of the photoelectric conversion element corresponding to the first field are transferred to the vertical charge transfer path.

Next, the output voltage V _TG of the variable voltage circuit 18 is maintained as it is, and the analog switch 19 is switched and connected to the contact b for a short time at time t ₂ . As a result, the unnecessary charges of the photoelectric conversion element corresponding to the second field are transferred to the vertical charge transfer path.

Then, the unnecessary charges are discharged to the drain portion by the charge transfer operation by the vertical charge transfer path and the charge transfer path of the storage section.

Next, while keeping the output voltage V _TG of the variable voltage circuit 18 at the maximum voltage, at time t ₄ (after the exposure time τ ₁ ), the analog switch 19 is connected to the contact a side for a short time. As a result, the voltage V _TG corresponding to the shutter speed is superimposed on the drive signal φ _I2 , the potential under the transfer gate becomes the deepest, and all the signal charges of the photoelectric conversion element corresponding to the first field are transferred to the vertical charge transfer path. Transferred to.

Then, the signal charge corresponding to the first field is transferred to the storage unit and then stopped.

Next, after the central control circuit 16 supplies the variable voltage circuit 18 with a predetermined voltage instruction signal SV corresponding to the shutter speed and outputs the voltage V _TG corresponding to the shutter speed, the time t ₆ At (after exposure time τ ₁ ), the analog switch 19 is connected to the contact b side for a short time. As a result, the drive signal φ _I4 has a voltage V corresponding to the shutter speed.
_TG is superimposed, and the potential under the transfer gate corresponding to the second field has a depth proportional to the voltage V _TG . Therefore, as shown in FIGS. 3 to 5, the signal charges generated in the photoelectric conversion element are made to remain according to the shutter speed, and the remaining signal charges are transferred to the vertical charge transfer path.

Hereinafter, the color signal corresponding to all the pixels is output by reading out the signal charges at the timing shown in FIG.

According to this embodiment, the signal charge corresponding to smear mixed while the second field read signal charge is temporarily stopped in the vertical charge transfer path in the light receiving portion is previously photoelectrically converted. The transfer is limited from the conversion element to the vertical charge transfer path, and the restriction is performed according to the shutter speed. Therefore, the smear of the color signal corresponding to the first field and the color signal corresponding to the second field to be finally read is performed. The mixing ratio can be made equal to the level that human eyes cannot perceive flicker.

The gate voltage generating means for making the transfer gate conductive is not limited to the configuration shown in FIG. 7, and other circuits having the same function may be used.

In addition, the transfer electrode of the vertical charge transfer path and the gate electrode for the transfer gate are separated, and the gate voltage V applied to the transfer gate for each pixel for red (R), blue (B), and green (G) Color flicker may be prevented by controlling _TG .

〔The invention's effect〕

As described above, according to the present invention, the potential level when the transfer gate formed between the photoelectric conversion element corresponding to the pixel corresponding to the second field and the vertical charge transfer path adjacent to the photoelectric conversion element is conductive is set. By controlling according to the shutter speed, the signal charge of the second field is prevented from being read out in advance by the signal charge corresponding to the smear component mixed in the period in which the signal charge is stopped in the vertical charge transfer path. The smear mixing rates of the signal charges corresponding to the field and the second field can be made substantially equal, and the occurrence of flicker can be reduced.
It is possible to provide a clear reproduced image.

[Brief description of drawings]

1 to 5 are explanatory views for explaining the principle of the present invention; FIG. 6 is an embodiment configuration diagram showing a configuration of an embodiment of the present invention; FIG. 7 is a main part of FIG. FIG. 8 is a structural diagram of a charge-coupled solid-state imaging device of frame interline transfer system; FIG. 9 is a timing chart for explaining the operation of the charge-coupled solid-state imaging device of FIG. 8; FIG. 10 is an operation explanatory view for explaining the operation at the appropriate time point in FIG. Reference numerals in the figure: 1; drain section 2; light receiving section 3; storage section 4; horizontal charge transfer path 7; charge-coupled solid-state imaging device 8; color filter 9; vertical drive signal driver 10; timing generation circuit 11; field Shift control circuit 12; horizontal drive signal driver 13; sample hold circuit 14; process circuit 15; magnetic recording circuit 16; central control circuit 17; shutter control unit 18; variable voltage circuit 19; analog switch 20, 21; adder circuit l _{1 to} l _N ; Vertical charge transfer path

Claims (1)

(57) [Claims] 1. A signal corresponding to the first field after the unnecessary charge generated in each pixel of a frame interline transfer type charge coupled solid-state imaging device is once discharged and after the discharging time corresponding to the shutter speed. While the charge is transferred to the storage section and the signal charge corresponding to the second field is transferred to the vertical charge transfer path of the light receiving section, respectively, and then the signal charge corresponding to the second field is stopped to the vertical charge transfer path of the light receiving section. In an imaging device that performs pseudo frame electronic shutter imaging by reading the signal charge corresponding to the first field and then reading the signal charge corresponding to the second field, the signal charge corresponding to the second field is converted into the vertical charge. When transferring to the transfer path, a transistor formed between the photoelectric conversion element corresponding to the pixel and the vertical charge transfer path adjacent to the photoelectric conversion element is formed. An imaging device, wherein a potential level when the transfer gate is turned on is controlled according to a shutter speed.