JPH0728032B2 - Charge coupled device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a charge-coupled device, and more particularly to an improved charge transfer efficiency of signal charges in a signal output circuit section.

[Prior Art] As is well known, a charge coupled device (CCD), which is a typical charge transfer device (CTD), signals to a depletion region (depletion layer) formed on the surface side of a semiconductor substrate of a MOS structure. To periodically change the voltage of the clock, which is injected with electric charges and is applied to the electrode formed on the surface of the semiconductor substrate through the thin insulating film, to periodically change the potential of the depletion region. Is configured to transfer the injected signal charge.

The transfer channel formed in the CCD regulates the width of the transfer channel by forming a channel stop region (inactive region) into which an impurity having a sign opposite to that of the signal charge is generally injected at a high concentration. The channel stop region is configured to be conductive with the semiconductor substrate,
This channel stop region is normally set to the reference potential, usually the ground potential (GND).

Now, in the CCD having such a structure, the transfer channel in the vicinity of the signal output portion is often configured as shown in FIG. 8A.

In the figure, 1 is a transfer channel (active region), and the outside of the transfer channel 1 is a channel stop region (one-dot chain line region) 2. 3 is a lower electrode, and 4 is an upper electrode. Reference numeral 5 denotes a drain region, which functions as a signal charge detection unit, and the signal charge is converted into a voltage and taken out. 6
Is a buffer amplifier.

The area of the transfer channel 1 is made narrower in the transfer direction, in order to make the capacity of the detecting section 5 as small as possible to improve the charge / voltage conversion efficiency. That is, when the signal charge amount in the detection unit 5 is ΔQ, the detection capacitance is C, and the output signal voltage is ΔV, the signal voltage ΔV is represented by ΔV = ΔQ / C, and therefore the detection capacitance C is made as small as possible. By doing so, the conversion efficiency can be improved. Therefore, the transfer channel width in the detection unit 5 is narrowed as shown in the figure.

[Problems to be Solved by the Invention] As described above, since the channel stop region 2 is in conduction with the semiconductor substrate and is fixed at the reference potential, the potential distribution in the width direction of the transfer channel is the eighth.
As shown in FIG. B, both ends of the well type are fixed to the ground potential.

This potential is for a signal charge, and when electrons are used as a signal, the potential direction is a negative voltage direction.

FIG. 3B shows the potential distribution state of the cross section on the line II-III-III of FIG.
The voltages applied to the same are assumed to be the same.

As the width of transfer channel 1 decreases from W1 to W3,
The potential well near the center of the channel gradually becomes Δφ1,
It becomes shallow with Δφ2.

The reason why the potential well becomes gradually shallower in this way is that both ends in the width direction of the transfer channel 1 are fixed to the ground potential, so that the potential well near the center is pulled in the width direction by the narrow channel effect. The extent of pulling in the width direction increases as the channel width of the transfer channel 1 decreases.

When the depth of the potential well changes depending on the channel width of the transfer channel 1, a potential barrier is formed in the transfer direction, which results in poor transfer of signal charges, the frequency response of the output signal and S / N of the signal. Cause deterioration.

Then, this invention solves such a problem,
We propose a transfer channel structure that improves the transfer efficiency by eliminating the potential barrier.

[Means for Solving the Problems] In order to solve the problems described above, in the present invention, the transfer channel width in the vicinity of the signal output portion is gradually narrowed in the transfer direction of the signal charges. In the coupling element, the potential of at least one potential barrier region that defines the transfer channel width gradually becomes deeper in the transfer direction.

As means for increasing the potential of the potential barrier region, in addition to controlling the width of the potential barrier region itself, the width of the potential barrier region itself is formed so narrow that a narrow channel effect appears, and the potential barrier region is sandwiched. A pseudo transfer channel region may be provided on the opposite side of the transfer channel and the channel width of the pseudo transfer channel region itself may be gradually expanded in the transfer direction.

[Operation] When the width of the potential barrier region itself is gradually widened in the transfer direction of the signal charges, the potential of the potential barrier region becomes gradually deeper in the transfer direction.

This deepening of the potential cancels the narrow channel effect of the transfer channel, so that a potential barrier in the transfer direction does not occur.

[Embodiment] An example of the charge coupled device according to the present invention will be described in detail below with reference to FIG.

The present invention is directed to a CCD having a structure in which the channel width of a transfer channel is narrower than the channel width of a previous transfer channel in the transfer direction.

Then, as shown in FIG. 3, the potential of the potential barrier region on both sides or one side of the transfer channel where the channel width of the transfer channel starts to narrow becomes
As it becomes narrower from W1 to W3, it gradually becomes deeper in the transfer direction as ΔV1 and ΔV2. Then, even if the narrow channel effect acts on the transfer channel 1 itself, a potential sufficient to cancel this effect acts from both sides or one side of the transfer channel. Therefore, the depth of the potential well near the center in the transfer direction is Is almost constant regardless of the change in the channel width.

Even if only the potential of the potential barrier region on one side of the transfer channel is gradually increased toward ΔV1 and ΔV2 in the transfer direction as the channel width of the transfer channel 1 becomes narrower from W1 to W3, the action is almost the same. is there.

FIG. 1 is one of the specific examples for realizing such a potential relationship.

In the figure, the transfer channel 1 is gradually narrowed and connected to a signal charge detection portion (cross-hatched area) 5. 3, 4 are transfer electrodes.

In this example, the channel stop regions (one-dot chain line regions) 2 are formed on both sides of the original channel width T of the transfer channel 1, and are the same as the channel width T on both sides of the transfer channel 1 formed narrowly. Thus, a barrier region (illustrated as a hatched region) 20 is formed. Therefore, the barrier region 20 is gradually formed wider in the transfer direction.

The barrier region 20 regulates the substantial channel width of the transfer channel 1. The barrier region 20 is provided with a constant potential difference with respect to the transfer channel 1. The potential difference is obtained from the potential structure in the semiconductor substrate as described later.

FIG. 2 is a sectional view taken along the line II-II in FIG. 1A. In addition,
In the following description including this figure, the case where the signal charge is an electron and the case where the signal charge has a buried channel structure will be described.

In that case, as shown in the figure, the transfer channel 1 is formed on the surface side of the P-type semiconductor substrate 11 by the N layer 12 doped with a predetermined concentration over the region of the channel width T. P ⁺ layers 13 are formed on both sides of the N layer 12 and function as channel stop regions 2.

The surface of the N layer 12 is doped with a predetermined amount of P-type impurities 16 in a predetermined region, and this is used as a barrier region 20.
By doping with P-type impurities 16, this region 20 acts as a barrier to signal charges.

The transfer electrode 4 is formed on the surface of the semiconductor substrate 11 via an insulating layer 14 such as SiO2.

When the barrier region 20 is formed in this way, the potential of the potential barrier portion formed by the barrier region 20 gradually becomes deeper toward the detection unit 5. In contrast to the transfer channel 1 formed so that the channel width becomes narrower in the transfer direction, the barrier region 20 is formed wider in the transfer direction, so that the narrow channel effect is generated in the barrier region 20. Works in the opposite direction to that in the area of the transfer channel 1.

Therefore, the potential distribution in the transfer channel 1 with the narrowed channel is as shown in FIG. 1B. Curve I in FIG. 9B is the potential distribution of the cross section on line II in FIG.
Curve II is the potential distribution of the cross section on the line II-II.

As is clear from FIG. 9B, the potential distribution near the center becomes uniform, and the potential barrier generated toward the detection unit 5 disappears.

FIG. 4 and subsequent figures show another embodiment of the present invention.
2 shows the case where the barrier region is also a buried channel type CCD, but FIG. 4 shows an example in which the barrier region is a surface channel type CCD.

In that case, the sectional view taken along the line II-II in FIG. 1 is as shown in FIG. A transfer channel 1 is formed by the N layer 17, and a barrier region 20 is formed between the N layer 17 and the P ⁺ layer 13. N layer 17
The surface side of the semiconductor substrate 11 between the P ⁺ layer 13 and the P ⁺ layer 13 has a surface channel structure which is not doped.

Also in such a CCD, if the region of the N layer 17 is narrowed in the transfer direction, the same operation as described above is achieved.

In the embodiment shown in FIG. 5, the potential in the potential barrier region is controlled by the barrier region 20 and the pseudo transfer channel 21.

As shown in FIG. 1A, a barrier region 20 is formed adjacent to and parallel to the transfer channel 1 with a predetermined width, and the transfer channel 1 is formed outside the barrier region 20 so as not to exceed the channel width T. A region of the pseudo transfer channel 21 is formed on the side opposite to.

Since the pseudo transfer channel 21 forms a potential equal to or close to that of the transfer channel 1, the drain end 22 formed as a drain region is provided on the output end side of the pseudo transfer channel 21. Each drain 22 has a terminal 23
A predetermined DC voltage VD is applied, and the pseudo transfer channel 21
The electric charge inside is discharged.

FIG. 6 is a cross-sectional view taken along the line II-II of FIG. 5A, and in this example, the barrier region is also a buried channel type CCD. The barrier region 20 is doped with P-type impurities 16.

In the illustrated example, the pseudo transfer channel 21 and the transfer channel 1 have the same potential.

As shown in FIG. 5, in this example, the channel width of the transfer channel 1 sandwiched between the barrier regions 20 is gradually narrowed, and the width of the barrier region 20 itself remains constant, while the channel width of the pseudo transfer channel 21 is changed in the transfer direction. On the other hand, it is structured so that it can be gradually expanded.

According to this structure, the potential of the potential barrier region is controlled by the cooperation of the barrier region 20 and the pseudo transfer channel 21. That is, in the pseudo transfer channel 21, the narrow channel effect acts in reverse to the transfer channel 1 formed inside, and the potential of the narrow barrier region is gradually lowered in the transfer direction. Therefore, since the potential well near the center is deepened by the amount of the pulling down, the potential distribution in the transfer direction near the center becomes uniform as shown in FIG. 5B.

FIG. 7A shows a case where only the barrier region 20 is narrowed in the transfer direction in the configuration of FIG. 5A.

By doing so, the height of the barrier of the barrier region 20 itself is gradually narrowed in the transfer direction. As a result, the potential distribution is strongly uniformed or rather deepened in the transfer direction, especially near the center of the transfer channel 1, as shown in FIG. 7B.

FIG. 7B illustrates the case where the detection unit 5 side is rather deepened.

Also in FIGS. 6 and 7, it can be applied when the barrier region is used as a surface channel.

In the above description, the case where the signal charge is an electron and the case where the buried channel structure is described, but the present invention is not limited thereto, and when the signal charge is a hole,
Alternatively, the same discussion can be made in the case of the surface channel structure.

[Effects of the Invention] As described above, according to the present invention, the potential of at least one of the potential barrier regions that define the transfer channel width is made gradually deeper in the transfer direction. It is a thing.

According to this, since the potential barrier near the center of the transfer channel is completely eliminated, most of the transferred signal charges can be converted into a voltage by the signal detection unit and taken out. Therefore, the charge transfer becomes smooth, and the frequency response and S / N of the output signal can be greatly improved as compared with the conventional case.

[Brief description of drawings]

FIG. 1A is a configuration diagram showing an example of a charge coupled device transfer channel according to the present invention, and FIG. 1B is a potential distribution diagram thereof.
2 is a sectional view taken along the line II-II in FIG. 1A, FIG. 3 is a diagram showing a potential control state of the potential barrier region, and FIG.
5 is a sectional view similar to that of FIG. 1 and a potential distribution diagram, FIG. 6 is a sectional view of FIG. 5A, and FIG. 8 is a conventional example. FIG. 2 is a sectional view and a potential distribution diagram similar to FIG. 1 …… Transfer channel 3,4 …… Transfer electrode 5 …… Signal detector 11 …… Semiconductor substrate 12,17 …… N layer 13 …… P ⁺ layer 16 …… P-type impurity 20 …… Barrier region 21 …… Pseudo transfer channel 22 ... Emission drain

Claims (1)

[Claims] 1. A charge coupled device in which a transfer channel width in the vicinity of a signal output portion is gradually narrowed in a signal charge transfer direction, and at least one of potential barrier regions defining the transfer channel width. A charge-coupled device, characterized in that the potential of one potential barrier region gradually becomes deeper in the transfer direction.