JP2002057325A - Solid state imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve charge transfer efficiency by increasing the handling charge amount. SOLUTION: A solid state imaging device comprises a charge transfer unit V-CCD for transferring a charge stored in a pixel S, a first layer electrode 1 poly, a second layer electrode 2 Poly and a third layer electrode 3 Poly of a plurality of transfer electrodes provided corresponding to the transfer unit V-CCD. Impurity injection concentrations of parts corresponding to the respective transfer electrodes of the unit V-CCD are respectively set in response to potentials formed at the unit V-CCD by voltages applied to the first layer electrode 1 poly, the second layer electrode 2 Poly and the third layer electrode 3 Poly.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a solid-state imaging device in which electric charges received and accumulated in a pixel portion are sequentially transferred by electric charge transfer means by a voltage applied from a transfer electrode.

[0002]

2. Description of the Related Art A charge transfer section in a CCD (solid-state image pickup device) forms, for example, a p-type well in an n-type silicon substrate and injects n-type impurities into the well to form a charge transfer channel region. are doing.

A plurality of transfer electrodes are sequentially formed above the charge transfer section. For example, in the case of three transfer electrodes, a first layer electrode, a second layer electrode,
The layer electrodes are sequentially formed in a state of partially overlapping above the charge transfer section, and by applying a drive voltage to these at a predetermined timing, the charges accumulated in the pixel section can be sequentially transferred by the charge transfer section. Become.

[0004]

However, in such a solid-state image pickup device, the channel region forming the charge transfer section is formed by implanting impurities of a uniform concentration, so that the electrodes are formed under actual driving conditions. A phenomenon occurs in which the potentials corresponding to the respective electrodes are different due to the propagation delay due to the material of the above or the effect of coupling from other clocks. This difference in potential causes a limitation in the amount of charge handled in charge transfer and causes a decrease in transfer efficiency.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve such problems. That is, the present invention is a solid-state imaging device including a charge transfer unit that transfers charges accumulated in a pixel unit, and a plurality of transfer electrodes provided corresponding to the charge transfer unit. Accordingly, the impurity implantation concentration of the portion corresponding to each transfer electrode of the charge transfer means is set in accordance with the potential formed in the charge transfer means.

According to the present invention, since the impurity implantation concentration of each portion corresponding to each transfer electrode of the charge transfer means is set, each potential of the charge transfer means formed by the voltage applied to each transfer electrode is reduced. Be able to align.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. 1A and 1B are diagrams illustrating a solid-state imaging device according to the present embodiment, in which FIG. 1A is a schematic plan view, FIG. 1B is a schematic cross-sectional view taken along line AA ′ of FIG. 1A, and FIG. B-
FIG. 3 is a schematic sectional view taken along line B ′.

In this embodiment, a solid-state imaging device of an all-pixel readout system driven mainly by three-layer electrodes and three phases will be described as an example.

This solid-state imaging device has a plurality of pixels S arranged in a matrix, a charge transfer section V-CCD formed between the pixels S along a vertical direction, and a charge transfer section VC.
A first layer electrode 1poly, a second layer electrode 2Poly, and a third layer electrode 3Poly are sequentially formed on the CD.

Here, the first layer electrode 1Poly, the second layer electrode 2Poly, and the third layer electrode 3Poly are formed of, for example, polycrystalline silicon, and are stacked between the respective electrodes via an interlayer insulating film.

The pixel S of the solid-state image pickup device accumulates charges corresponding to the amount of incident light and sends out the charges to the charge transfer section V-CCD at a predetermined timing. The charge transfer unit V-CCD
The charge read out from the pixel S is sequentially transferred by the driving voltage Vφ1 applied to the first layer electrode 1poly, the driving voltage Vφ3 applied to the second layer electrode 2Poly, and the driving voltage Vφ2 applied to the third layer electrode 3Poly. Go.

A drive voltage Vφ generated at a predetermined timing
1, to realize charge transfer at Vφ2 and Vφ3,
The layer electrode 1poly is wired between the pixels S along the horizontal direction, the second layer electrode 2Poly is superimposed on the first layer electrode 1poly, and projects half a pixel downward in the figure of the charge transfer unit V-CCD. Wired, the third layer electrode 3Poly
Is a second layer electrode 2Po protruding above the charge transfer section V-CCD.
ly is also overlapped with the first layer electrode and the second layer electrode 2Poly.

When such an electrode structure is adopted, the load on the second layer electrode 2Poly sandwiched between the first layer electrode 1Poly and the third layer electrode 3Poly becomes heavy, and is formed by the second layer electrode 2Poly. The amount of charge handled by the charge transfer unit V-CCD is determined by the effective amplitude of the drive voltage Vφ3.

FIG. 2 is a timing chart showing the driving voltages of the three-layer electrodes. As described above, in the portion indicated by a circle in the drawing, there is a portion where the effective amplitude of the drive voltage Vφ3 formed by the second layer electrode 2Poly does not completely reach the High level. This is the second layer electrode 2Pol
This is due to the influence of the propagation delay of y and the coupling from other driving voltages (clocks).

FIG. 3 is a schematic diagram of the potential of the charge transfer section. This is when the channel region forming the charge transfer section is formed with a uniform impurity implantation concentration (conventional example).
Is schematically shown.

As described above, the second layer electrode 2Pol
The effective amplitude of the drive voltage Vφ3 formed by y is completely Hi.
gh level, the drive voltage Vφ of the first layer electrode 1poly and the third layer electrode 3Poly
1. The potential formed by the drive voltage Vφ3 of the second layer electrode 2Poly is lower than the potential formed by Vφ2 (see p in the figure).

As described above, when the potential is only partially reduced, the total amount of charges that can be handled by the charge transfer section is limited by the shallow potential of the portion, resulting in deterioration of transfer efficiency.

Therefore, in the present embodiment, as shown in FIG. 4, the charge transfer portion V- is maintained without changing the electrode structure (arrangement / film thickness).
It is characterized in that the effective amplitude is secured by setting the impurity concentration profile of the channel region in the CCD corresponding to each electrode.

More specifically, the impurity implantation concentration in the channel region corresponding to the electrode whose potential becomes shallower is made higher than that in other portions so that the potential when the same voltage is applied becomes deeper.

In the example shown in FIG. 4, the second layer electrode 2Poly
The impurity implantation concentration of the portion C2 of the charge transfer portion V-CCD corresponding to the above is set to be N + N ', and is higher than the impurity implantation concentration N of the other portions C1 and C3.

As a result, the portion C2 of the charge transfer section V-CCD corresponding to the second layer electrode 2Poly is connected to the second layer electrode 2Poly.
Even if a sufficient amplitude cannot be obtained due to the propagation delay of Poly or the like, the original potential is deep, so that the potential of the other portions C1 and C3 can be made as deep as the potential of the other portions C1 and C3 when the High bias is applied.

On the other hand, since the potential is considered to be almost the same as the other portions C1 and C3 in the low bias because of the pinning, the effective amplitude can be sufficiently secured even under the second layer electrode 2Poly, and the potential of the portion is shallow. , The amount of charge handled by the charge transfer unit V-CCD can be increased.

By setting the impurity implantation concentration corresponding to each electrode, the potentials corresponding to all the electrodes can be made uniform, and the charge transfer efficiency can be increased.

Here, in order to simulate the effect of the present embodiment, FIG. 5 shows measured values of the amount of electric charges handled when only the voltage value formed by the second layer electrode 2Poly is offset. In FIG. 5, the horizontal axis is the second layer electrode 2Pol.
The voltage offset amount of y, and the vertical axis indicates the amount of charge handled.

From this result, it can be seen that when a higher voltage is applied to the second layer electrode 2Poly than the other electrodes (with the same amplitude), the amount of handled charges can be increased. In other words, it can be confirmed that the handling charge amount can be increased by making the potential of the second layer electrode 2Poly at the High level deeper than the other electrode portions.

FIGS. 6A and 6B are views for explaining a solid-state imaging device having a three-layer electrode, four-phase driving structure. FIG. 6A is a schematic plan view, and FIG.
FIG. 3A is a schematic cross-sectional view taken along line CC ′ of FIG.
It is an outline sectional view taken on line -D '.

In the four-phase driving structure, the first layer electrode 1poly
Is wired between the pixels S along the horizontal direction, and the second layer electrode 2
Poly is overlapped on the first layer electrode 1poly in two parts, and each is wired so as to protrude vertically in the drawing of the charge transfer unit V-CCD. In addition, the third layer electrode 3Po
ly is overlapped on the two second layer electrodes 2Poly and also on the first layer electrode 1Poly.

With such an electrode structure, the load on the second layer electrode 2Poly sandwiched between the first layer electrode 1Poly and the third layer electrode 3Poly becomes heavy, and the second layer electrode 2Poly is formed. This means that the effective amplitudes of the drive voltages Vφ1 and Vφ3 cannot be sufficiently secured.

Therefore, as shown in FIG. 6B, the impurity implantation concentration of the portion C2 corresponding to the two second layer electrodes 2Poly of the channel region constituting the charge transfer portion V-CCD is set to N + N ', and It is set higher than the impurity implantation concentration N of the portions C1 and C3.

As a result, the portion C2 of the charge transfer section V-CCD corresponding to the second layer electrode 2Poly is connected to the second layer electrode 2Poly.
Even if a sufficient amplitude cannot be obtained due to the propagation delay of Poly or the like, the original potential is deep, so that the potential of the other portions C1 and C3 can be made as deep as the potential of the other portions C1 and C3 when the High bias is applied.

In the above embodiment, the propagation delay of the second layer electrode 2Poly is mainly described. However, the propagation delay of the third layer electrode 3Poly may be large depending on the electrode structure. In this case, the third
What is necessary is just to increase the impurity implantation concentration of the portion C3 of the channel region corresponding to the layer electrode 3Poly.

That is, if the impurity implantation concentration is set so as to equalize the potential of the channel region formed corresponding to each drive electrode, efficient charge transfer can be performed without being restricted by any one potential. become able to.

In this embodiment, the solid-state imaging device of the all-pixels reading system has been mainly described. However, the same applies to solid-state imaging devices of other reading systems.

[0034]

As described above, the present invention has the following effects. That is, the potentials of the charge transfer means formed corresponding to the transfer electrodes can be made uniform, and the amount of the saturation signal can be increased and the charge transfer efficiency can be improved. As a result, it is possible to reduce the unit pixel size, improve the sensitivity, reduce the read voltage, improve the margin for blooming, and reduce the shutter voltage.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a solid-state imaging device according to an embodiment.

FIG. 2 is a timing chart showing respective drive voltages of a three-layer electrode.

FIG. 3 is a schematic diagram of a potential of a charge transfer unit.

FIG. 4 is a schematic diagram illustrating an impurity implantation concentration distribution in the solid-state imaging device according to the embodiment.

FIG. 5 is a diagram showing measured values of the amount of handled charges when only a voltage value formed by 2Poly is offset.

FIG. 6 is a diagram illustrating a solid-state imaging device having a three-layer electrode, four-phase driving structure.

[Explanation of symbols]

1poly ... first layer electrode, 2Poly ... second layer electrode, 3
Poly: third layer electrode, S: pixel

Claims (4)

[Claims] 1. A solid-state imaging device comprising: a charge transfer unit configured to transfer charges accumulated in a pixel unit; and a plurality of transfer electrodes provided corresponding to the charge transfer unit. Solid-state imaging device, wherein the impurity implantation concentration of a portion corresponding to each transfer electrode of the charge transfer means is set according to the potential formed in the charge transfer means. 2. When the potential formed by the applied voltage of one transfer electrode is smaller than the potential formed by the applied voltage of another transfer electrode,
2. The solid according to claim 1, wherein an impurity implantation concentration of a portion corresponding to said one transfer electrode of said charge transfer means is set higher than an impurity implantation concentration of a portion corresponding to said another transfer electrode. Imaging device. 3. The method according to claim 1, further comprising: setting an impurity implantation concentration of a portion corresponding to each transfer electrode of said charge transfer means so that potentials formed in said charge transfer means are equalized by respective applied voltages of said plurality of transfer electrodes. The solid-state imaging device according to claim 1, wherein: 4. When the plurality of transfer electrodes are formed in the order of a first-layer electrode, a second-layer electrode, and a third-layer electrode, an impurity is implanted into a portion of the charge transfer unit corresponding to the second-layer electrode. 2. The solid-state imaging device according to claim 1, wherein the concentration is set higher than an impurity implantation concentration of a portion corresponding to another electrode.