JPH0795828B2 - Driving method for solid-state imaging device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charge transfer type solid-state imaging device, and more particularly to a driving method of a solid-state imaging device suitable for low smear and high S / N.

[Background of the Invention]

The two-dimensional solid-state imaging device has advantages such as small size, light weight, maintenance-free and low power consumption, and is expected as a next-generation imaging device instead of the electron tube imaging device.
However, when a bright subject is photographed, a vertical smear phenomenon, which is tailed at the top and bottom of the reproduced image, occurs, which causes deterioration of image quality at high illuminance. An excellent method for reducing this vertical smear is the smear differential method described in Ozawa et al., 1984 Television Society National Convention Proceedings 3-15, P67-68.
In this system, the vertical smear charge and the signal charge on which the vertical smear is superimposed are read at the same time, and the difference between the two is output to output the signal charge on which the smear is not superimposed. With this method, vertical smear can be reduced to a level where there is no problem in practical use, but there remains a problem that the random noise increases due to arithmetic processing and the S / N ratio deteriorates.

[Object of the Invention]

It is an object of the present invention to provide a low smear and high-sensitivity solid-state image pickup device, particularly in an interline CCD solid-state image pickup device, by performing the above-mentioned smear differential method without increasing random noise.

[Outline of Invention]

Generally, when the smear differential method is performed, if the smear charge amount read independently is Gq _s and the smear charge amount mixed in the signal is q _s , the smear is removed by the differential operation to obtain the true signal charge. , 1 / G times the smear charge that is read independently,
It should be subtracted from the signal charge on which the smear is superimposed. At this time, if the random noise charge amount mixed in the smear charge amount read independently is n _s and the random noise charge amount mixed in the signal charge on which the smear charge is superimposed is n, random noise mixed in the true signal charge The quantity n _t can be expressed by equation (1).

That is, Only the random noise increases. Therefore, by increasing the smear charge amount read independently from the smear charge amount mixed in the signal (G> 1), an increase in random noise can be prevented. In the present invention, in the interline CCD solid-state imaging device, in order to make the smear charge amount read independently larger than the smear charge amount mixed in the signal, the vertical charge transfer element is smeared within each horizontal scanning period. N + 2 of N electrodes for respectively accumulating N (N: positive integer) optical signal charges mixed with electric charges, 1 electrode for accumulating smear charges and 1 electrode in an empty state
A plurality of sets of electrodes, each of which is configured to transfer the first charge accumulated in the electrode immediately behind the empty electrode to the electrode in the empty state within each horizontal blanking period, as viewed in the charge transfer direction,
Next, the second charge stored in the electrode immediately behind the first charge is transferred to the electrode that has become empty by the transfer of the first charge, and the transfer of the nth charge (n = 1 to N) is generally performed. The operation of transferring the (n + 1) th charge to the electrode in the empty state is repeated, and the smear charge described above is immediately behind the empty electrode in each horizontal scanning period as viewed in the charge transfer direction. An electrode for accumulating is arranged, and during each horizontal retrace period, the smear charge accumulated in the electrode accumulating the smear charge is first transferred to the electrode in the empty state.

Example of Invention

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

FIG. 1 shows an interline type CC showing an embodiment of the present invention.
It is a block diagram of a D solid-state imaging device.

In FIG. 1, 1-1 and 1-2 are photodiodes and 2'is an ion implantation region 8 for vertically performing a triple transfer operation.
CCD, 2'-1, 2'-2, ... Are electrodes forming a vertical CCD 2 ',
3'-1, 3'-2, 3'-3 accepts the three types of signal charges Q _A , Q _B , q _s sent by the vertical CCD 2 ', and outputs each charge.
Horizontal CCDs transferred to 1,4-2,4-3, 5'-1,5'-
2,5'-3,5'-4 are vertical clock pulse generators for driving the vertical CCD 2 '. Here, the horizontal clock pulse for driving the horizontal CCD 3'is omitted in order to simplify the drawing, but it may be two phases, three phases, or four phases. Four electrodes 2'-1, 2'-2, which form a vertical CCD 2 ',
Under 3 electrodes of 2'-3, 2'-4, two series of signal charges Q _A , Q _B and smear q _s are accumulated, and the remaining one electrode is placed in an empty state. Then, the signal charges Q _A , Q _B , and q _s are transferred to the horizontal CCDs 3′-1, 3′-2,
It is sent to 3'-3.

FIG. 2 is an explanatory diagram showing the triple transfer operation of the vertical CCD according to the present invention.

As shown in FIG. 2 (a), the clock pulse generator 5 '
Each line of -1 to 5'-4 has electrodes 2'-1, 2'-
2, 2'-3, 2'-4, 2'-1 ... Are sequentially connected. The potential under the electrodes 2'-1, 2'-2, 2'-3, 2'-4, ... At the times t ₁ to t _n of the pulse timing shown in FIG. The positions of the electrodes shown in FIG. 2B correspond to those in FIG. 2A.

The triple transfer operation will be described with reference to FIG.

(I) t = t ₁ : 1 Due to the light incident during the field,
Signal charges Q _A , Q accumulated in the photodiodes 1-1, 1-2
_B is sent to the vertical CCD 2'side through the transfer gates 7-1 and 7-2 which are conductive (lower in potential) by the high voltage clock pulse, and the electrodes 2'-1 and 2'-.
Accumulated under 3.

(Ii) t = t ₂ : The signal charge Q _A moves one electrode in the vertical CCD 2 ′ from below the electrode 2′-1 to below the electrode 2′-2 due to the low voltage of the electrode 2′-1. To do.

(Iii) t = t ₃ : The clock pulse applied to the electrode 2′-1 has a high voltage and the potential under each electrode is low, but the ion-implanted layer 8 formed partly under each electrode is formed. The electric charges Q _A and Q _B are accumulated under the same electrode as at the time t = t ₂ due to the barrier V _B.

(Iv) t = t ₄ : Since the vertical scanning period is entered, the charges Q _A and Q _B are transferred toward the horizontal CCD. Note that t = t ₁ and t ₂ are times within the vertical blanking period.

First, the low voltage is applied to the electrode 2'-4, is transferred under the smear charges accumulated in the electrode 2'-4 q _s or electrodes 2'-1, the electrode 2'-1 under smear q _'s Is added. Here, the smear charge is generated by the charge generated by the incident light leaking to the vertical CCD side, and leaks during the subsequent transfer period (t _{4 to} t _n ). The amount of smear charge increases with the passage of time. On the other hand, since this leaked electric charge enters any of the electrodes, smear electric charge is mixed into the signals Q _A and Q _B, and the smear component in the signal also increases with the passage of time.

(V) t = t ₅ : The charge Q _B accumulated so far on the electrode 2′-3 becomes lower than the voltage on the electrode 2′-3, so that it is below the electrode 2′-4 to which high voltage is applied. Transferred.

(Vi) t = t ₆ : The electrode 2′-3 has a high voltage and the electrode 2′-2 has a low voltage, and the charge Q _A stored in the electrode 2′-2 is transferred to the lower side of the electrode 2′-3. To be done.

Due to the above operation from time t _{4 to} t ₆ , the charges Q _B , Q _A , and q _s are applied to the electrode 1
Only this number will be transferred to the horizontal CCD 3'side.

Similarly, the three signals Q _B , Q _A , and q _s are transferred to the horizontal CCD 3 ′ side, and the Q _B , Q _A , and q _s that reach the final electrode of the vertical CCD 2 ′ have their respective horizontal CCD 3′−1. , 3'-2,3'-3. The charges are sent to the horizontal CCD 3'in the horizontal blanking period, and when the horizontal scanning period is started, the signals Q _B , Q _A , and q _s are simultaneously output in the horizontal CCD 4-1, 4-2, 4 -3. Since the horizontal CCDs 3'-1, 3'-2, 3'-3 are given to each signal one by one, the charge of the horizontal CCD is the same single transfer operation (3 The transfer may be performed by expressing the normal operation for the double transfer operation).

Now, in the above operation, the vertical CCD2 ′ within the vertical scanning period
The charge transfer within is not continuous. That is, in the horizontal scanning period, the charges in the vertical CCD are stationary in the state of t = t ₃ in FIG. By repeating the operation of t = t _{4 to} t ₆ four times within the horizontal blanking period, the three signals Q _B , Q _A , and q _s are transferred to the horizontal CCD 3 ′ side, and at the same time, a set of Q _{B is set.} (N), Q _A (n), and q _s (n) are sent to the horizontal CCD. This cycle is repeated to read out all pixels. In the current NTSC system, the horizontal scanning period is about 63.5 μsec, which is longer than the horizontal retrace period of about 10 μsec, and smear charge is generally vertical.
During the period of t = t ₃ where no charge transfer occurs in the CCD 2 ′, it is mixed in the vertical CCD. As a result, smear charges of almost equal amount are mixed under the four electrodes within one scanning period. Therefore, as described in the present embodiment, during the horizontal retrace period, smear charges are first transferred to collect smear charges for two electrodes, and the smear charges that are independently read are mixed into the signal. Can be almost doubled, and S / N deterioration due to smear differential can be prevented. Assuming that the random noise n _s mixed in the signal charge and the random noise amount n mixed in the smear charge are equal, the S / N deterioration is 1 dB according to the equation (1) according to the driving method of this embodiment. On the other hand, when the driving method of the present embodiment is not adopted, conversely, one signal charge in the two sets of signal charges is mixed with twice as much smear as the smear charges that are individually read. As a result, S / N degradation reaches 7 dB. Therefore, when the smear differential method is performed, the driving method of the present embodiment can improve the sensitivity by about 6 dB.

In this example, since the voltage of the electrode for accumulating charges is increased to perform the transfer, the potential of the semiconductor surface under the empty electrode having a low electrode voltage can be made lower than the substrate potential. . As a result, the dark current can be reduced.

Moreover, it goes without saying that the present invention can be applied regardless of the configuration and driving method of the horizontal CCD. Further, although FIG. 1 shows the case where the charges read in one horizontal scanning period are arranged in the order of (q _s , Q _B , Q _A ) from the side closer to the horizontal CCD in the vertical CCD. (Q _B , q
_s , Q _A ), (Q _B , Q _A , q _s ) may be arranged in this order.

FIG. 3 shows another embodiment of the present invention. In FIG.
The same parts as those in FIG. 1 are designated by the same reference numerals. Vertical CCD
It differs from FIG. 1 in that 2 ′ is composed of three electrodes. In this embodiment, in the horizontal blanking period, the smear charge is first moved to the empty electrode, then the smear charge is moved, and the signal charge is transferred to the empty electrode, whereby the smear charge amount read independently. Is set to twice the amount of smear charge mixed in the signal, and the S / N deterioration due to the differential can be limited to 1 dB.

It should be noted that the present invention is generally applicable to the case of carrying N signal charges and M smear charges.

〔The invention's effect〕

According to the present invention, an increase in random noise associated with smear differential can be reduced to 1 dB, and thus a solid-state imaging device with low smear and high S / N can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing a basic structure of a charge transfer type solid-state image pickup device showing an embodiment of the present invention, and FIG. 2 is a vertical CCD 3 of the present invention.
FIG. 3 is a diagram showing a double transfer operation, and FIG. 3 is a diagram showing a basic configuration of a charge transfer type solid-state imaging device showing another embodiment of the present invention. 1-1, 1-2 ... Photo diode, 2 '... Vertical CCD, 2'
-1,2'-2,2'-3,2'-4 ... electrode, 3'-1,3'-2,
3'-3,3 "-2,3" ... Horizontal CCD, 4,4-1,4-2,4-3 ...
... Output terminals, 7-1, 7-2 ... Transfer gates, 5-1 and 5-2,
6-1, 6-2 ... Clock pulse generator.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Norio Koike 1-280 Higashi Koigakubo, Kokubunji City, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd. (72) Toshiyuki Akiyama 1-280 Higashi Koigakubo, Kokubunji, Tokyo Hitachi Ltd. Central Research Laboratory (56) References JP-A-60-150384 (JP, A) JP-A-61-117980 (JP, A)

Claims (2)

[Claims] 1. A photoelectric conversion element group arranged two-dimensionally on the same semiconductor substrate, and a plurality of vertical charge transfer elements for vertically transferring the optical signal charges accumulated in the photoelectric conversion element group, The optical signal charge transferred from the vertical charge transfer element is integrated with a horizontal charge transfer element that horizontally transfers and outputs it once in each horizontal scanning period, and is transferred to and after the output end of the horizontal charge transfer element. In the charge transfer type solid-state imaging device in which a subtracter for subtracting the smear charge amount mixed during the charge transfer in the vertical charge transfer element from the optical signal charge is arranged, each vertical charge transfer element in each horizontal scanning period N electrodes (N: positive integer) with smear charges mixed therein, N electrodes for accumulating optical signal charges, M electrodes for accumulating smear charges, and 1 electrode in an empty state N + M +
A plurality of pairs of one electrode is formed, and the first charge accumulated in the electrode immediately behind in the charge transfer direction is transferred to the electrode in the empty state within each horizontal retrace line period, Then, the second electrode accumulated in the electrode which is emptied by the movement of the first electric charge and further in the electrode immediately behind the second electrode.
The transfer of the electric charges of (1) to (n = 1 to N) is repeated, and the operation of transferring the (n + 1) th charges to the electrode that is in the empty state by the movement of the nth (n = 1 to N) charges is repeated, and One of the M electrodes for accumulating the smear charges is arranged immediately behind the electrodes in the state in which the smear charges are accumulated, and the smear charges are first accumulated in each horizontal blanking period. A method of driving a solid-state image pickup device, wherein the smear charge accumulated in one electrode is transferred to the electrode in the empty state. 2. The number M of electrodes for accumulating the smear charge.
The method for driving a solid-state imaging device according to claim 1, wherein is M = 1.