JPS6062154A - Charge transfer device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a charge transfer device, and the present invention relates to a charge transfer device including a photodiode, a charge transfer element, and a MOS gate.
By configuring a region with the same conductivity type as the channel and high impurity concentration, even if the amount of residual signal charge obtained by the photodiode becomes small, the residual signal current flows extremely smoothly, and no unread signal charges in the photodiode are left. Another object of the present invention is to provide a charge transfer device with less afterimage.

In a conventional charge transfer device, the elements for one pixel are shown in Figure 1a.
For example, a P-type substrate 1 has an N-type photodiode 2 and an N-type embedded CCD.
3 are arranged side by side, and both are connected by a transfer gate 4. In the figure, 5 is an oxide film, and 6 is a channel.

In such a charge transfer device, its potential characteristics are shown in FIG. 1, that is, the potential characteristics when the transfer gate 4 is off are shown by a solid line, and the potential characteristics when the transfer gate 4 is on are shown by a dotted line. When the transfer gate 4 is turned on, the potential on the surface of the channel 6 decreases, as shown by the dotted line in FIG. The potential of the photodiode 2 gradually decreases as the charges flow out, and when most of the signal charges flow out, the potential of the photodiode 2 becomes close to the potential of the channel 6.

In this way, when the potentials of photodiode 2 and channel 6 become close to each other, the amount of charge flowing naturally decreases, and it takes a long time for the small amount of charge remaining in photodiode 2 to completely flow out. Put it away.

However, in the actual operation of the charge transfer device, the transfer gate 4 turns from on to off in a limited period of time, so there is an unread signal charge on the photodiode 2, which causes afterimages. It becomes.

As shown in Figure 1, if all the charges in diode 2 flow out, the potential of photodiode 2 will be approximately 0.3 mm lower than the potential of channel 6.
When the transfer gate 4 is turned off in this state, the potential of the photodiode 2 is '8', but when light hits the first diode 2 again, a charge is generated. The potential of photodiode 2 increases.

Therefore, it seems possible to reduce the afterimage by increasing the voltage applied to the transfer gate 4 and deepening the potential of the channel 6. However, if the voltage applied to the transfer gate 4 is increased in this way, the photo Although the potential of the diode 2 is deep, the relative relationship between the two potentials does not change, so the time required to completely transfer the signal charge depends on the magnitude of the voltage applied to the transfer gate 4.

Further, it is possible to reduce the afterimage by making the length of the channel 6 of the transfer gate 4 submicron, for example, but in a charge transfer device with such a submicron channel length, the length of the transfer gate 4 is If the embedded CCD 3 is operated with the photodiode 2 turned off, since the photodiode 2 and the embedded CCD 3 are close to each other, the punch-through will be exceeded and the embedded CCD 3 will not operate properly.

The present invention eliminates the above-mentioned drawbacks, and examples thereof will be described below.

FIG. 2a is an explanatory diagram of one embodiment of the charge transfer device according to the present invention, for example, one pixel of a CCD image pickup plate in which a PN junction is formed in the light receiving part, and FIG. 2a is an explanatory diagram of potential characteristics.

In the figure, 11 is a P- type substrate, and 12 is an N+ type substrate.
・Diode, 13 is an N-type buried COD, 14 is a MOS gate that connects the photodiode 12 and the buried CCD 13) ・Transfer gate, 15 is an oxide film, 16 is a channel length of about 2 to 3 μm, for example.
m channels, and their configuration is almost the same as the conventional one.

Reference numeral 17 denotes a P region having a higher impurity concentration than the channel 16 region and having the same conductivity type as the channel 16, which is a feature of the present invention. Channel length L2 is approximately 1 μm
It is as follows.

When the charge transfer device is configured as described above, the potential characteristics of the charge transfer device are as shown in FIG. As the potential is shown by the dotted line, when the transfer gate 14 is turned on, the potential of the P region 17 becomes the channel 16.
It is in a state higher than the P- region of . Then, the signal charge of the photodiode 12 flows out to the embedded CCD, and the potential of the photodiode 12 gradually decreases, until most of the signal charge of the photodiode 12 flows out, and the potential of the photodiode 12 approaches the potential of the P region 17. Then, the amount of charge flowing out decreases as in the conventional case. However, since the length L2 of the P region is short, for example 0.4 μm,
Even if the potential difference between the two is small, the charge smoothly flows into the P region due to the short channel effect. Since there is a sufficient potential difference between the P region 17 and the channel 16, the charge flowing into the P region 17 immediately flows into the embedded CCD 13, so that the signal charge of the photodiode 12 flows into the embedded CCD 1.
3, it will start flowing without delay.

That is, the dog portion of the signal charge of the photodiode 12 is read out, and even if the remaining signal charge is small, the charge flows smoothly to the transfer gate 14, so that there is little unread charge and there is almost no afterimage.

Furthermore, even if the P region 17 is submicron, the P-channel 16 is long and connected to the P region 17, and there is sufficient distance between the photodiode 12 and the buried (,'CI) 13. Photodiode and embedded COD
Punch-through does not occur as would occur if the distance between the

FIG. 3 is an explanatory diagram of main parts of another embodiment of the charge transfer device according to the present invention.

In the figure, 21 is a P-substrate, 22 is an N+-type first diode, 23 is an N-type embedded CCD, 24 is a transfer gate, 25 is an oxide film, 26 is a channel, and 27 has higher impurity than the channel 26. channel 2 in concentration
The P region has the same conductivity type as No. 6, and most of the configurations thereof are almost the same as those of the previous embodiment.

However, in the above embodiment, the P region is formed at the entire boundary between the N++ photodiode and the P- type substrate, whereas in this embodiment, the P region is formed at the entire boundary between the N++ photodiode and the P- type substrate.
It is only partially constructed in the vicinity.

However, even if the P area 27 is only partially configured in this way, the same effects as in the embodiment described above can be achieved.

Next, a method for manufacturing the charge transfer device of the present invention will be briefly described.

First, as shown in Fig. 4a-c, N-type embedded C
Using the polysilicon and oxide film 15 as a mask,
For example, after boron ions are implanted and heat treated to form the P region 17, arsenic ions are implanted from the same opening and heat treated.

Then, the P region 17 will be formed by the difference in the lateral diffusion distances of boron ions and arsenic ions. In other words, the length L2 can be adjusted to about 1μ by controlling the amount of boron ions and arsenic ions implanted and the temperature and time during heat treatment.
A P region of less than m can be created under the transfer gate with good reproducibility.

However, when configuring the P area partially as shown in Figure 3,
The structure can be similarly easily constructed by providing a special photoresist mask.

Further, the potential difference between the P region and the P- channel can be controlled with good reproducibility by controlling the impurity concentrations of both.

The same is true even if all P and N in the above explanation are replaced, and the same is true even if the buried channel C'CD is replaced with the surface channel COD or BBD.Further, the l transfer gate and COD The electrodes may be separated and removed.

As described above, in the charge transfer device according to the present invention, which includes a photodiode, a charge transfer element, and a MOS gate, an impurity concentration of the same conductivity type as that of the channel is provided in the MOS gate channel on the photodiode side. Since it consists of a high area, even if the amount of residual signal charge obtained by the photodetector diode becomes small, the residual signal current flows out extremely smoothly, and there is less unread signal charge in the photodetector diode, resulting in an excellent feature of less afterimage. have

[Brief explanation of the drawing]

Figure 1a is a cross-sectional view showing the element configuration for one pixel of a conventional charge transfer device.
Figure a is a cross-sectional view showing the element configuration for one pixel of the charge transfer device according to the present invention, the same figure shows the potential difference of the same element, and Figure 3 shows another example of the charge transfer device according to the present invention. FIGS. 4a, 4b, and 4c are cross-sectional views showing the element structure of each pixel, and are process explanatory views showing a method of manufacturing a charge transfer device according to the present invention. 11.21...P-type substrate, 12.22...N-type photodiode (light receiving diode), 13,23...
N-type buried COD (charge transfer device), 14.24
...Transfer gate (MOS gate), 15.2
5... Oxide film, 16.26... Channel, 17
．． 27...P area. Patent applicant: Katsumi Utaka, agent of Victor Japan Co., Ltd.

Claims (1)

[Claims] A charge transfer device comprising a light receiving diode, a charge transfer element, and a MOS gate, characterized in that a region having the same conductivity type as the channel and having a high impurity concentration is formed in the MOS gate channel on the side of the light receiving diode. .