JP2780294B2 - Solid-state imaging device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state imaging device, and more particularly, to a structure of a signal charge readout unit thereof.

[Conventional technology]

In a solid-state imaging device including a photoelectric conversion element and a charge transfer element, signal charges are usually read out by applying a pulse voltage to a readout electrode provided between the photoelectric conversion element and the charge transfer element. It is. That is, a reading means is employed in which a channel is formed below the reading electrode by applying a pulse voltage, and the signal charge from the photoelectric conversion element is transferred to the charge transfer element.

Is a partial cross-sectional view of FIG. 1 is a conventional solid-state imaging device, the photoelectric conversion element and the charge transfer device N ^- P junction photodiode and a buried channel type CCD shift register using respectively, The light-receiving portion is vertical over- 3 shows a structure of a reading section of a solid-state imaging device having a flow / drain structure. This solid-state imaging device is formed on an N-type silicon substrate 1, an N ^- P junction of a photodiode is formed by providing an N ^- type region 3 in a P-type well 2, and a CCD shift register is similarly formed in a P-type well 2. It is formed with an N-type region 4 provided therein. Here, the read electrode 5 has a MOS structure over the two regions through a silicon oxide film 7 having a uniform thickness so that a read channel region 6 can be formed between the N ⁻ type region 3 and the N type region 4. Formed. Generally, this read electrode 5
Is often formed integrally with the transfer electrode 5 'of the CCD shift register using polycrystalline silicon. In consideration of misalignment of alignment in the photoresist process and processing variation of polycrystalline silicon, these are somewhat Even if it occurs, it is usual to provide the end so as to hang over the N ^- type region 3 of the photodiode so that the read channel region 6 can be reliably covered. Here, 8, 9, and 10 indicate an interlayer insulating film, a light-shielding aluminum film, and a P ⁺ channel stopper, respectively.

[Problems to be solved by the invention]

As described above, in the conventional solid-state imaging device, the readout pulse is applied since the readout electrode 5 is formed so as to extend over the N ⁻ type region 3 of the photodiode via the silicon oxide film 7 having a uniform thickness. In this case, the channel potential formed under the read electrode is higher than the potential of the read channel 6 in the P-type well 2 in the N ⁻ type region 3 of the photodiode region.
As shown in FIG. 1 (b), the potential is always lower than the potential of the partial region on which the electric potential falls due to the negative charge. Therefore, the conventional solid-state imaging device has a disadvantage that a part of the signal charge e is left behind at the overlapping portion between the readout electrode 5 and the N ⁻ type region 3 of the photodiode region, resulting in a large residual image.

In view of the above circumstances, an object of the present invention is to provide a solid-state imaging device which has solved the drawback that a conventional readout electrode forms a valley of a channel potential at a portion overlapping with an N-type region of a photoelectric conversion device.

[Means for Solving the Problems] A solid-state imaging device according to the present invention includes a photoelectric conversion element and a charge transfer element, and an MO between the photoelectric conversion element and the charge transfer element.
A read electrode that intervenes to form an S structure, and that transfers a signal charge by photoelectric conversion from the photoelectric conversion element to the charge transfer element in a pulsed manner, and an insulating film in an overlapping portion between the read electrode and the photoelectric conversion element region. The thickness of the insulating film on the read channel formed between the two element regions is larger than that of the insulating film.

[Operation] According to the present invention, the thick insulating film formed at the overlapping portion between the photoelectric conversion element region and the read electrode reduces the MOS effect of the pulse voltage applied to the read electrode at this portion. In the case of an insulating film having a uniform thickness, the valley of the channel potential with respect to the signal charge generated in the overlapping portion is eliminated, so that the transfer of the signal charge from the photoelectric conversion element to the charge transfer element is facilitated.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

FIGS. 4 (a) and 4 (b) are a partial sectional view of a solid-state imaging device and a channel potential diagram immediately below a read electrode at the time of reading out signal charges, according to an embodiment of the present invention. Solid-state image sensor of the present embodiment, as in the conventional example, the N-type silicon substrate 1, the photodiode formed respectively on the P-type well 2 on the substrate 1 (photoelectric conversion element) N ^-
The entire surface of the mold region 3 and the N-type region 4 of the CCD shift register (charge transfer element) and the P ⁺ channel stopper 10 are covered with a silicon oxide film 7, and the end portion of the silicon oxide film 7 is an N ⁻ -type region. A read electrode 5 hanging on the peripheral edge of the upper electrode 3 and a transfer electrode 5 'integrated with the read electrode 5 are formed of polycrystalline silicon. However, unlike the conventional example, as shown, the silicon oxide film 7 on the N ⁻ type region 3 is much thicker than other regions. Here, 9 and 10 are an interlayer insulating film and a light-shielding aluminum film, respectively.

When the thickness of the silicon oxide film 7 is set as described above,
When a read pulse voltage is applied to the read electrode 5, the effect of the voltage on the channel potential decreases as the thickness of the oxide film increases. Therefore, the channel potential on the N ⁻ type region 3 of the photodiode can be made lower than the read channel potential 6. That is, as shown in FIG. 4 (b), the valley of the potential existing in the related art is eliminated, so that the signal charge e is transferred to the N-type region 4 of the CCD shift register and read out with almost no signal charge left. In this way, the after-image defect of the solid-state imaging device, which has been conventionally regarded as a problem, is greatly improved. At this time, since the silicon oxide film is originally a light-transmitting material, even if the N ⁻ type region 3 is covered with the thick silicon oxide film, the light conversion efficiency of the photodiode hardly decreases. There is no danger that the solid-state imaging device will impair the signal conversion characteristics that are vital.

The structure of the above-described embodiment of the solid-state imaging device of the present invention can be manufactured by the following method. 2 (a) to 2 (i) show a process sequence diagram of one method of the manufacturing method. First, FIG. 2 (a)
As shown in FIG. 1, a P-type well 2 is formed on an N-type silicon substrate 1, and a silicon oxide film 11 and a silicon nitride film 12 are sequentially grown on the entire surface thereof. Are selectively formed by patterning. Next, using the resist film 13 as a mask, a silicon nitride film is used.
12 is selectively etched, and the resist film 14 is re-attached. Then, an N-type impurity 15 is introduced using the two patterns of the resist film 14 and the silicon nitride film 12 as masks (FIG. 2B). Next, the unnecessary resist film 14
Removed, by diffusing N-type impurity 15 is heat-treated, photodiode N ^- after the formation of the mold region 3, the N ^- 2 type region 3 above are covered with the resist film 16, the resist film 16 and a nitride film 12 Using the two patterns as masks, N-type impurities 17 are introduced again (FIG. 2 (c)). Here, if the resist film 16 is removed and heat treatment is performed again, the N-type region 14 of the CCD shift register can be obtained.
Is formed [see FIG. 2 (d)]. Then the pattern of the silicon nitride film 12 as a mask of the photodiode performs thermal oxidation N ^- growing type region 3 and the CCD shift register N-type region 4 on the thick oxide film 18, respectively selective [Figure 2 ( d)]. Thereafter, the silicon nitride film 12 is removed, respectively, then as shown in FIG. 2 (e) N ^- -type region 3
After the resist region 19 is coated on the space between the substrate and the N-type region 4, the resist pattern and the thick silicon oxide film are formed.
P ⁺ impurities are introduced using the mask 18 as a mask to form P ⁺ channel stoppers 10 respectively. Next, the resist film 19 is removed, and a thick silicon oxide film 18 is formed on the N-type region 4 of the CCD shift register by using the re-attached resist film 20 as a mask.
Are removed by etching [FIG. 2 (f)]. Then, after the unnecessary resist film 20 is removed, the entire surface is subsequently etched so that the thick silicon oxide film 18 has a predetermined thickness [FIG. 2 (g)]. Next,
When the entire surface of the substrate is thermally oxidized, a silicon oxide film 7 having a predetermined thickness is formed in which the thickness on N ⁻ type region 3 is several times the thickness on other regions. Then, a readout electrode 5 and a transfer electrode 5 'made of polycrystalline silicon are formed thereon by patterning (FIG. 2 (h)), and finally, an interlayer insulating film 8 and a light-shielding aluminum film 9 are sequentially formed. 2 is completed [FIG. 2 (i)].

FIGS. 3 (a) and 3 (b) are a partial sectional view of a solid-state imaging device and a channel potential diagram immediately below a read electrode at the time of reading signal charges, respectively, showing another embodiment of the present invention. According to this embodiment, the read electrode 5 and the transfer electrode 5 'of the CCD shift register are formed independently and separately. In this case, the channel potential when the read pulse voltage is applied to the read electrode 5 and the transfer electrode 5 'becomes the same as that of the previous embodiment as shown in FIG. 3B, and the signal charge e remains. Since it does not occur, afterimage defects are improved. Further, since these two electrodes can be independently controlled, there is an advantage that the control voltage range of the transfer voltage of the CCD shift register can be set wide.

〔The invention's effect〕

As described above in detail, according to the present invention, the conventional solid-state imaging device is configured such that the insulating film below the readout electrode is thicker on the photoelectric conversion element than on the readout channel region. Can eliminate channel valleys that have occurred in the overlapping portion with the above, facilitating the transfer of signal charges from the photoelectric conversion element to the charge transfer element. is there.

[Brief description of the drawings]

4 (a) and 4 (b) are a partial cross-sectional view of a solid-state imaging device showing one embodiment of the present invention, a channel potential diagram immediately below a read electrode when reading out signal charges, and FIG.
FIGS. 3A to 3I are process sequence diagrams showing one method of manufacturing the solid-state imaging device of the present invention, and FIGS. 3A and 3B are each a solid-state imaging device showing another embodiment of the present invention. 1 and a channel potential diagram immediately below a read electrode when reading out signal charges. FIGS. 1 (a) and 1 (b) are a partial cross-sectional view of a conventional solid-state imaging device and immediately below a readout electrode when reading out signal charges, respectively. 3 is a channel potential diagram of FIG. 1 ... N-type silicon substrate, 2 ... P-type well, 3 ... N ^- type region of photodiode, 4 ... N-type region of CCD shift register, 5 ... Read electrode, 6 ... Read channel region, 7 ... silicon oxide film, 8 ... interlayer insulating film, 9 ...
Light-shielding aluminum film, 10 ... P ⁺ channel stopper, e ... Signal charge.

Claims (1)

(57) [Claims] An MOS structure is formed between a photoelectric conversion element and a charge transfer element, and a MOS structure is interposed between the photoelectric conversion element and the charge transfer element, and a signal charge by photoelectric conversion is pulsed from the photoelectric conversion element to the charge transfer element. A read electrode to be transferred, wherein the thickness of the insulating film at the overlapping portion between the read electrode and the photoelectric conversion element region is larger than the thickness of the insulating film on the read channel formed between the two element regions. A solid-state imaging device characterized by being set on a film.