JP2652225B2 - Imaging device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging device using an electronic flash.

[Conventional technology]

2. Description of the Related Art In recent years, an imaging device that records a still image of a subject has been used as an electronic still video camera or as an industrial camera. In such a camera, a solid-state imaging device is used as an imaging device because of demands for small size, light weight, and high reliability.

A weak point of the solid-state image sensor is the blooming phenomenon, in which strong light is incident on a part of the image area, and the amount of charge generated in that area exceeds the charge storage capacity of the pixel and flows into an adjacent area with a small amount of incident light. There is a phenomenon in which an anti-blooming gate for removing excess charges due to electron-hole recombination is provided in a pixel as a technique for preventing the phenomenon. As such a solid-state imaging device, a frame transfer type virtual phase CCD (Charge
A solid-state imaging device having an anti-blooming gate is known as a coupled device. The mechanism of the anti-blooming gate will be described below.

FIG. 2A is a cross-sectional view in the transfer direction of a virtual phase CCD having an anti-blooming gate. One pixel corresponds to one transfer gate region and one virtual phase region. An anti-blooming gate 18 is provided in the virtual phase domain, which is located between the virtual barrier and the virtual well. Here, 16 is a transfer gate electrode, 17 is an N-type impurity layer, and 19 is a P ⁺ impurity layer.
FIG. 2B is a diagram showing a potential distribution in the charge accumulation mode. At this time, a clock of an arbitrary frequency is applied to the anti-blooming gate.

FIG. 3 shows a potential distribution in the depth direction below the anti-blooming gate 18 when the voltage level applied to the anti-blooming gate 18 is High. In this state, when the signal charge, that is, the charge generated by the photoelectric conversion in the charge accumulation mode is equal to or larger than the capacity determined by the surface potential and the maximum value of the channel potential, the further charge overflows at the interface,
Trapped at surface levels. Here, when the potential level of the anti-blooming gate 18 is set to a low level that causes the surface to be in a pinning state, holes are filled in the surface by using the P ⁺ layer 19 in the channel stop region and the virtual gate region as a supply source, and the surface level The electrons trapped in the holes recombine with holes. By repeating this operation during charge accumulation, it is possible to prevent blooming caused by excessive charge.

FIG. 4 is a diagram showing a configuration of a conventional electronic still video camera. In the figure, 1 is a lens, 2 is a half-open shutter, 3 is a solid-state imaging device having an anti-blooming gate, 6 is a drive circuit for generating a gate pulse of the solid-state imaging device 3, and 4 is a photometry circuit for determining an appropriate exposure. , 5
Is a colorimetric circuit for determining the white balance, 7 is a system control circuit that generates operation timing pulses of the entire apparatus and generates control signals, and 8 is a color difference signal (R− Y and BY)
And a luminance signal + synchronization signal (Y + Sync), an imaging signal processing circuit 9 for FM-modulating a signal from the imaging signal processing circuit 8, a recording amplifier 10 for amplifying a signal from the modulation circuit 9, and a reference numeral 11 A magnetic head for electromagnetically converting a signal from the recording amplifier 10, a magnetic sheet 12 for recording a magnetic signal from the magnetic head 11, a motor 13 for rotating the magnetic sheet 12, and a motor 14 for controlling the rotation of the motor 13. A servo circuit 15 for illuminating a subject is an electronic flash (generally called a strobe, and hence will be referred to as a strobe hereinafter).

FIG. 5 is a time chart showing the operation of the electronic still camera shown in FIG. 4 at the time of flash photography. The operation of a conventional electronic still camera at the time of flash photography will be described below with reference to FIGS. 5 and 4.

φSS1 is a pulse of an opening command for the half-open shutter 2. With this command, the half-open type shutter 2 starts operating, and after a delay time with respect to the electric signal due to the mechanical structure, the shutter 2 starts to open to an arbitrary aperture diameter. The anti-blooming gate 18 of the solid-state imaging device 3 has f ₀ that prevents blooming from occurring in the subject image when the strobe light is emitted, particularly, reflected light of the strobe light itself.
Clock φAB frequency H _Z is applied after having φSS1 pulse the occurrence of, or delay in anticipation of the delay of the shutter 2. φSS2 is a pulse that is generated at the same time that the shutter 2 has a predetermined aperture diameter, and the pulse stops the opening operation so that the shutter 2 is maintained at the aperture diameter. At the same time as the stop of the shutter 2 stops at the specified diameter, a flash command is issued to the strobe 15 to emit light with a flash time of 1/1000 sec at the latest. After the strobe light is emitted, the shutter 2 has a closing command φ
SS3 is issued and starts closing. After the shutter is closed, the charge stored in the solid-state imaging device 3 is read out, and signal processing, modulation,
It is amplified and recorded on the magnetic sheet 12.

[Problems to be solved by the invention]

However, in the above conventional example, if a sufficient anti-blooming ability is to be obtained, the dark current becomes extremely high,
There is a problem that image quality deteriorates.

 The details will be described below.

The operation of the anti-blooming gate for the removal of excess charge has already been described, but in the course of its operation, after surface pinning, holes return to the channel stop region and the virtual gate region of the P ⁺ layer from which they are the source. There is a process. Immediately after the bias of the anti-blooming gate changes from a level at which the surface potential under the gate is pinned to a positive bias, most of the holes remain under the gate. Since the gate bias is high thereafter, a large fringing field is formed between the edge of the anti-blooming gate region and the edge of the virtual gate region or the edge of the channel stop region. The holes remaining in the anti-blooming gate region return to the channel stop region (P ⁺⁾ and the virtual gate region along this field, but the holes become hot holes due to the large fringing field, resulting in impact ionization. Cause The resulting electrons are collected as a false signal. The dark current generated by such a mechanism is hereinafter referred to as a hot-hole dark current.

In a solid-state imaging device having such an anti-blooming gate, the amount of removal of excess charges and the amount of generation of a false signal can be defined by one clock cycle of the anti-blooming gate. Therefore, if the accumulation time is constant, it can be said that the anti-blooming ability and the dark current are proportional to the clock frequency.

In flash photography, the flash time of a discharge tube (using a xenon tube) that generates flash light is very short, and therefore, the amount of light in a very short time is extremely high.
As described above, in a solid-state image sensor having an anti-blooming gate, the anti-blooming capability is determined by the amount of excess charge removed in a very short time, so a very high frequency clock must be added to the anti-blooming gate during flash photography. As a result, the dark current becomes extremely large, and the image quality deteriorates.

The present invention has been made under such circumstances,
It is an object of the present invention to provide an imaging device in which image quality is less likely to be degraded due to activation of an anti-blooming gate when a strobe is used.

[Means for solving the problem]

In the present invention, in order to achieve the above object, the imaging apparatus is configured as in the following (1) and (2).

(1) a solid-state imaging device having an anti-blooming means for removing excess charge and an electronic flash, and a clock of a predetermined frequency is applied to the anti-blooming means during the emission period of the electronic flash; Clock generation means for applying a clock having a frequency lower than the predetermined frequency is provided just before the flash of the electronic flash and after the flash is completed.

(2) In the above (1), the clock generation means can externally switch the frequency of the clock applied to the anti-blooming means until immediately before the emission of the electronic flash and after the completion of the emission.

[Action]

According to the above configurations (1) and (2), the dark current is reduced except when the strobe light is emitted. According to the configuration (2),
When the strobe is not emitting light, an appropriate clock frequency can be selected according to the usage of the strobe.

〔Example〕

 Hereinafter, the present invention will be described with reference to examples.

FIG. 1 is a timing chart showing the operation of the electronic still video camera according to the first embodiment of the present invention. The configuration of this embodiment is almost the same as that shown in FIG.
The difference is that the system control 7 is provided with a clock generating means for generating a frequency-variable clock as described later.

Therefore, this embodiment will be described with reference to FIG. 4 and FIG.

In FIG. 1, a command pulse for opening the φSS1 half-open shutter 2 is shown. With this command pulse, the half-open type shutter 2 starts operating, and after a delay time with respect to the electric signal due to the mechanical structure, the shutter 2 starts opening so as to have an arbitrary aperture diameter. The anti-blooming gate 18 of the solid-state imaging device 3 is supplied with a pulse of φSS1 through the drive circuit 6 by the clock generation means of the system control 7 or after a delay time in consideration of the delay of the shutter. an image, a very low frequency f ₁ clock φAB than the clock frequency f ₀ as blooming does not occur is applied in particular by the reflected light of the strobe light itself. The φSS2 pulse is generated at the same time when the shutter 2 reaches the predetermined aperture diameter. The opening operation of the shutter is stopped by this pulse so that the shutter is maintained at the aperture diameter. The clock generating means obtains the pulse to the object image at the time of emission of the strobe light is switched to the clock generation of the frequency f ₀ H _Z which do not occur blooming particular by light reflected strobe light itself. After the strobe light emission, a φSS3 pulse, which is a command to close the shutter 2, is issued. With this pulse, the shutter 2 starts the closing operation, and eventually closes. Clock φAB to the anti-blooming gate is switched on again the frequency f ₁ H _Z in pulse FaiSS3.

As described above, by increasing the clock frequency of the anti-blooming gate only during strobe light emission with a high instantaneous incident light amount, and lowering the clock frequency at other times, it is possible to maintain a high anti-blooming ability as in the past, and In addition, the hot-hole dark current can be reduced, and the deterioration of image quality can be reduced.

 Next, a second embodiment of the present invention will be described.

In the present embodiment, the clock frequency to the gate in the first embodiment can be externally switched by using a strobe light.

In other words, strobes can be used in the following ways: a. Illuminate the subject in a dark place, b. Correct the exposure of an image that has become dark due to backlight, etc. The frequency of the clock φAB of the blooming gate is different, and the latter requires a higher frequency.

Therefore, a switch for designating the usage of the strobe is provided outside, or when the amount of peripheral light is reduced, the above a is automatically set.
The condition b is to allow the photographer to select by an external switch, and to switch the clock frequency of the anti-blooming gate before and after the strobe light emission depending on the purpose of the strobe light.

〔The invention's effect〕

As described above, according to the present invention, the clock frequency to the anti-blooming means for removing excess charge is increased only during strobe light emission, and the clock frequency to the anti-blooming means is reduced except during strobe light emission. While maintaining high anti-blooming ability,
A high-quality image can be obtained with a low dark current.

Further, since the clock frequency to the anti-blooming means can be switched and selected from outside according to the use of the strobe, the deterioration of the image quality can be further reduced.

[Brief description of the drawings]

FIG. 1 is a timing chart showing the operation of the first embodiment,
FIG. 2 shows a virtual phase CC having an anti-blooming gate
FIG. 3 is a diagram showing a potential distribution in a depth direction below an anti-blooming gate, FIG. 4 is a block diagram showing a configuration of a conventional electronic still camera, and FIG. 5 is a timing chart showing an operation of a conventional example. It is. 3. Solid-state image sensor having anti-blooming gate 7. System control 15 Strobe 18 Anti-blooming gate

Continued on the front page (72) Inventor Tsutomu Takayama 770 Shimonoge, Takatsu-ku, Kawasaki-shi, Kanagawa Prefecture Inside the Tamagawa Works of Canon Inc. (56) References JP-A-61-293078 (JP, A) JP-A-59-153384 (JP) , A)

Claims (2)

(57) [Claims] 1. A solid-state imaging device having an anti-blooming means for removing excess charge and an electronic flash, wherein the anti-blooming means comprises:
A clock generating means for applying a clock of a predetermined frequency during the emission period of the electronic flash, and applying a clock of a frequency lower than the predetermined frequency until immediately before emission of the electronic flash and after completion of emission. Characteristic imaging device. 2. The clock generating means according to claim 1, wherein
2. The image pickup apparatus according to claim 1, wherein the frequency of a clock applied to the anti-blooming means can be switched from the outside immediately before the emission of the flash and after the completion of the emission.