JP2532619B2 - Solid-state imaging device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a solid-state imaging device.

2. Description of the Related Art In recent years, particularly in the field of commercial use, an increasing number of image pickup devices use a FIT (frame interline transfer) CCD that is effective in reducing smear, which is one drawback of solid-state image pickup devices.
Figure 4 shows the block diagram of the FITCCD.

In FIG. 4, 1 is a photodiode, 2 is a vertical CCD, 3
Is a memory section having the same number of storage sections as the vertical CCD 2, 4 is a drain for discharging unnecessary charges, 5 is a horizontal CCD, and 6 is an FDA (floating diffusion amplifier) for detecting signal charges.

The operation will be briefly described below with reference to FIGS. 5 to 9.

FIG. 5 shows drive pulses given to the transfer electrodes .PHI.A1 to .PHI.A4 of the vertical CC2 of FIG. 4 by a drive circuit (not shown). Signal charges are accumulated in each photodiode 1 in each odd field and even field, but at the same time, since the vertical CCD of the light receiving part is mostly stopped for one field period, it is caused by the charge due to the dark current of the vertical CCD. FPN (fi
xed pattern noise, hereinafter referred to as surface roughness) occurs. This surface is especially noticeable when the screen is dark. Therefore, during the sweeping out period during the vertical blanking period, the unnecessary charges causing the surface roughness for reading out the signal charges of the photodiode 1 to the vertical CCD 2 are first swept out. Next, the signal charge of each photodiode 1 is read out to the vertical CCD 2 by a read-out pulse and transferred to the memory section 3 in the high-speed transfer period. As a result, the time for charge transfer in the vertical CCD 2 is significantly reduced, so the amount of unnecessary charges that leak into the vertical CCD 2 during transfer is reduced, and smear is significantly reduced. At the time of high-speed transfer, the same transfer pulse is applied to the transfer electrodes ΦB1 to ΦB4 of the memory section 3.

FIG. 6 is an enlarged view of the vertical blanking period of FIG. When a transfer pulse is applied as shown in Fig. 6, potential wells are formed in the order of ΦA4, ΦA3, ΦA2, ΦA1 electrodes during sweeping, and in the order of ΦA1, ΦA2, ΦA3, ΦA4 electrodes during high-speed transfer. ) Occurs and the charge flows as shown by the arrow. As a result, the unnecessary charges are swept out in the direction of the drain 4 and the signal charges are transferred in the direction of the memory section 3. Of course, unnecessary charges may be swept out to the horizontal CCD 5 via the memory unit 3.

The signal charges transferred to the memory section 3 by the high speed transfer are transferred in the horizontal CCD5 direction pixel by pixel in the horizontal blanking period by the transfer pulse shown in FIG. In the drive pulse shown in FIG. 6, the signal charge is the lowermost electrode ΦB4 of the memory section 3.
Is transferred from under the electrodes of ΦH1 and ΦH2 forming the horizontal CCD5. The signal charges transferred to the horizontal CCD 5 are sequentially transferred in the same manner as vertical transfer by transfer pulses given to the electrodes of ΦH1 to ΦH4 during the horizontal scanning period, and are detected as voltage signals by the FDA6.

The FITC CCD having the above-described configuration has excellent anti-texture and smear measures, and is very effective for an image pickup device for business use.

There is a photodiode mix (PD mix) method as a signal reading method from the solid-state image sensor. This PD
The mix is a drive pulse shown in FIG. 5, and all pixel signals are read out for each field by adding a read pulse of a dotted line in addition to a read pulse of a solid line at the time of reading. In odd field, photodiode A and photodiode B, photodiode C and photodiode D
The signals are mixed by the combination of the above, the positions are shifted in the even field, and the signals are mixed by the combination of the photodiode B and the photodiode C, and the interlace scanning is performed. This PD mix signal readout method can prevent frame afterimages (system afterimages) due to normal interlaced scanning.

It has been conventionally said that there is no afterimage in the solid-state imaging device, but in reality, there is an afterimage in the light receiving section. This is because in the method of reading a signal through the channel under the transfer electrode like the imaging device of the interline CCD or FITCCD, after the majority of the signal charge is transferred to the vertical CCD, the residual charge in the photodiode decreases below a certain value. The channel is weakly inverted and the signal charge is transmitted by diffusion. This is a so-called incomplete transfer operation, and it takes a long time to read out the signal charges. This afterimage can be prevented by reducing the diffusion impurity concentration of the photodiode as shown in FIG. 9 and depleting the entire diffusion layer after all the accumulated signal charges have been transferred.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention With the above configuration and driving method, smear and vertical CCD
FPN (surface roughness) due to dark current accumulated inside can be prevented, but afterimage can be prevented, but the storage capacity of the photodiode becomes small, and as a result, the saturation signal charge amount is small and the dynamic range cannot be widened. Occurs. This is a big drawback for an image pickup device that requires a high dynamic range.

In view of such a point, the present invention is suitable for an imaging device having a high dynamic range and high performance, such as a conventional FITCCD and a structure that prevents afterimages, while securing a wide dynamic range and removing fixed pattern noise. An object is to provide a solid-state imaging device.

Means for Solving the Problems The invention of claim 1 of the present application Photodiodes (hereinafter referred to as PDs) arranged in a grid pattern, corresponding to each PD row,
A vertical charge coupled device (hereinafter referred to as CCD) that transfers PD signals, a memory that sequentially holds signals obtained from the vertical CCDs in each column, and a FIT that has a horizontal CCD that horizontally transfers signals obtained from the memories. Type solid-state image sensor, a signal level determination circuit for determining the level of the signal level of one screen transferred from the horizontal CCD of the solid-state image sensor, and the signal charge of each PD of the solid-state image sensor at least within one field period. Reads to the vertical CCD twice, and removes unnecessary charges generated in the vertical CCD within the first drive mode and one field period in which the signal charges read to the vertical CCD are transferred to the memory unit at high speed immediately after the final reading. Immediately after the sweep operation, the signal charges of the PDs are read to the vertical CCDs, and immediately thereafter, the signal charges read to the vertical CCDs are transferred to the memory unit at high speed. A driving circuit for driving the solid-state imaging device in any one of the second driving modes, and switching the first and second driving modes in synchronization with vertical scanning in accordance with the magnitude of the signal level determination circuit. And is provided.

In the invention of claim 2 of the present application, the signal level determination circuit of claim 1 is replaced with the first drive mode when the gain of the image pickup level gain adjustment switch is standard gain, and when the gain is increased, the second drive mode is selected. In addition, the first and second drive modes are configured to be switched in conjunction with the gain adjustment switch.

According to the invention of claim 1 of the present application having such a feature, the signal read out from the CCD when the image data is obtained from the solid-state image sensor is given to the signal level discriminating circuit and the magnitude of the signal level of one screen is increased or decreased. The drive circuit is switched between the first drive mode requiring a dynamic range and the second drive mode removing fixed pattern noise depending on the magnitude of the signal.

In the invention of claim 2 of the present application, the standard gain of the gain adjustment switch and the gain driving are switched to the first and second driving modes, respectively.

Embodiment 1 FIG. 1 is a block diagram of a solid-state imaging device according to an embodiment of the present invention.

In FIG. 1, reference numeral 1 denotes photodiodes arranged in a grid pattern in a number corresponding to the number of pixels of the image pickup device, and the number thereof is reduced in the drawing. Reference numeral 2 is a vertical CCD, 3 is a memory section having the same number of storage sections as the vertical CCD 2, 4 is a drain for discharging unnecessary charges, 5 is a horizontal CCD, and the solid-state image sensor is composed of these elements. 6 is an FDA that detects the signal charge read from the solid-state image sensor, 7 is a signal level determination circuit that determines the magnitude of the output signal level of, for example, one screen or a specific part thereof, and 8 is a unit for each part of the solid-state image sensor as described later It is a drive circuit that sends drive pulses in two different modes. 1 to 6 have the same operations and functions as those of the conventional example.

The operation of the solid-state imaging device of the present embodiment configured as described above will be described with reference to FIGS. 2 and 3.

2A and 2B show two drive modes applied from the drive circuit 8 to the transfer electrodes ΦA1 to ΦA2 of the vertical CCD2. 1 in the first drive mode 1 shown in FIG.
The signal charge of the photodiode 1 is read out to the vertical CCD2 under the electrodes of ΦA1 and ΦA3 once almost in the middle of the field period.
Further, in the vertical blanking period, the signal charges are read out again in a form of being added to the read-out signal charges for the first time and transferred to the memory unit 3 by high-speed transfer. By doing so, a signal charge amount that is almost twice as large as that in the case of the normal single read can be obtained. This is because it is possible to increase the impurity concentration in the N region of the vertical CCD shown in FIG. 9 and thus increase the capacitance, and therefore the amount of charge stored in the potential well of the vertical CCD 2 is shown in FIG. It can be larger than twice the saturated charge amount. Since the signal charge is read once in the middle of the field, the sweeping operation is not performed after the signal charge is read once. (It is possible before reading once)
Moreover, reading is PD mixed mode. High speed transfer, PD
The operation of mixing and the like is similar to that of the conventional example, and the description thereof is omitted.

The second drive mode 2 shown in FIG. 2B is exactly the same as the drive mode of the conventional example. Before the signal is read out, the charges due to the dark current accumulated in the vertical CCD are swept out to reduce fixed pattern noise (texture). Reading is PD mixed mode.

In the solid-state imaging device of the present invention, the above-mentioned two driving modes are switched by the signal level judgment circuit 7 according to the signal level. In other words, when the signal level is low and the screen is dark, fixed pattern noise (texture) tends to stand out, so take the countermeasure with the drive mode of mode 2 that has a sweeping operation.
When the signal level is high and the screen is bright, the amount of signal charge that can be handled by the drive mode of mode 1 is increased to expand the dynamic range. Mode 1 and Mode 2
Is switched in synchronism with vertical scanning because the reading of mode 1 is performed in the middle of the field.

FIG. 3 shows the relationship between the amount of incident light and the amount of output signal in Mode 1 and Mode 2. For example, if the standard state is set to lens aperture F5.6 subject illuminance 20001x, then mode 2 (1 If the saturation amount in (reading twice) is 12,0001x, which is 6 times the standard, in the case of mode 1, 12 times 24,0
The dynamic range can be expanded up to 001x. If the maximum charge amount that can be handled by the vertical CCD is at least twice the saturation charge amount of the two photodiodes, it is possible to further increase the number of times of reading in mode 1 and further expand the dynamic range.

In addition, when the gain is increased by 9 dB, the standard state is 7101x, so even in mode 2, the dynamic range is 17 times larger, and conspicuous fixed pattern noise (texture) can be removed when the gain is increased.

By switching the drive mode according to the signal level as described above, it is possible to easily take measures against surface roughness and expand the dynamic range.

Needless to say, the same effect can be obtained even if the signal level is judged not by directly judging the signal level, for example, by judging the aperture value of the lens. Further, instead of the actual signal level, the noise reduction or the high dynamic range may be selected by interlocking with the normally used gain up switch such as 9 dB up or 18 dB up to select the drive mode. Alternatively, a dedicated switch for mode switching may be provided without interlocking with the gain up switch.

As described above, according to the present invention, the dynamic range is expanded in the case of a bright screen where the signal level is large by switching the drive mode in conjunction with the signal level or the gain up switch, and the signal level is increased. When a small and dark screen is used, noise countermeasures such as surface roughness are taken, so that it is possible to provide a solid-state imaging device having a high dynamic range and high performance, which is suitable for business use, and its practical effect is great.

[Brief description of drawings]

FIG. 1 is a block diagram of a solid-state imaging device according to an embodiment of the present invention, and FIGS. 2 (a) and 2 (b) are two circuits applied to the transfer electrodes ΦA1 to ΦA4 of the vertical CCD 2 of FIG. FIG. 3 is a diagram showing the drive pulse of each mode, FIG. 3 is a diagram showing the relationship of the output signal amount with respect to the incident light amount in the two drive modes, and FIG.
Structure diagram of TCCD, Fig. 5 is transfer electrode ΦA of vertical CCD of Fig. 4.
FIG. 6 is a diagram showing drive pulses applied to 1 to ΦA4, FIG. 6 is an enlarged view of the vertical blanking period of FIG. 5, and FIG. 7 is a diagram of transfer electrodes ΦB1 to ΦB4 and horizontal CCD5 of the memory section 3 of FIG. FIG. 8 is a diagram showing drive pulses applied to transfer electrodes ΦH1 to ΦH4.
The figure shows the outline of the photodiode 1, the vertical CCD2, and the transfer electrodes ΦA1 to ΦA4 in FIG. 4, and FIG. 9 shows the afterimage prevention structure.
It is a schematic diagram of CCD. 1 ... Photodiode, 2 ... Vertical CCD, 3 ... Memory section, 4 ... Drain, 5 ... Horizontal CCD, 6
...... FDA, 7 …… Signal level judgment circuit, 8 …… Drive circuit.

Claims (2)

(57) [Claims] Claims: 1. Photodiodes (hereinafter referred to as PDs) arranged in a grid pattern, vertical charge coupled devices (hereinafter referred to as CCDs) that transfer the signals of the PDs of the PDs corresponding to the columns of the PDs, and A FIT type solid-state image sensor having a memory that sequentially holds signals obtained from a vertical CCD, and a horizontal CCD that horizontally transfers signals obtained from the memory, and one screen transferred from the horizontal CCD of the solid-state image sensor. A signal level determination circuit for determining the magnitude of the signal level, and a signal read out to the vertical CCD immediately after the final readout by reading out the signal charge of each PD of the solid-state image sensor at least twice within one field period. Immediately after the first drive mode for transferring charges to the memory section at high speed and the sweeping operation for removing unnecessary charges generated in the vertical CCD within one field period, the signal charges of each PD are immediately Each vertical CCD
Immediately after the reading, the solid-state imaging device is driven in any one of the second drive modes in which the signal charge read out to the vertical CCD is transferred to the memory unit at high speed,
A solid-state imaging device, comprising: a drive circuit that switches the second drive mode in synchronization with vertical scanning in accordance with the size of the signal level determination circuit. 2. A gain adjusting switch for an image pickup level is provided instead of the signal level judging circuit, and the drive circuit is set to the first drive mode when the gain of the gain adjusting switch is a standard gain, and the second drive is set when the gain is increased. 2. The solid-state imaging device according to claim 1, wherein the first and second drive modes are switched so as to be in a mode in cooperation with a gain adjustment switch.