JP2723854B2 - Solid-state imaging device and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a solid-state imaging device and a method for manufacturing the same.

[0002]

2. Description of the Related Art A solid-state imaging device accumulates information input such as incident light in the form of electric charges, sequentially transfers the electric charges by a large number of transfer electrodes, and can take out the electric signals.
Widely used for video cameras, faxes, etc.

FIG. 5 is a plan view of a line sensor as an example of a conventional solid-state imaging device, and FIG. 6 is a sectional view taken along line XX of FIG.

In this conventional example, the P on the surface of a silicon substrate is
A photodiode including a first N-type region 205 selectively formed on the surface of the P-type region (P-well 202); and a P-well 202 at a predetermined distance from the first N-type region 205.
Buried channel 203 formed of a second N-type region selectively formed on the surface of the silicon substrate and the first channel crossing the buried channel 203 via the gate oxide film 206 provided on the surface of the silicon substrate. And a charge coupled to the first charge transfer electrode 208-1 and selectively covering the gate oxide film 206 from the above-mentioned predetermined interval to the first N-type region 205. Readout electrode 210
And an opening formed on the first N-type region 205 via the interlayer insulating film 211 and the first N-type region 205
A light shielding film 212 covering a part of the upper part of the mold region 205;

Next, a description will be given of a manufacturing method of this conventional example.

First, as shown in FIG. 8A, a P-well 202 is formed on the surface of an N-type silicon substrate 201, and then an N-type region (buried channel 203) of a charge transfer section (hereinafter referred to as a CCD section). , A P ⁺ type channel stopper 204, a gate oxide film, and a first charge transfer electrode 208-1 made of a first polysilicon film.
-1 is implanted into the buried channel 203 not covered with -1 to form a barrier layer, and the second charge transfer electrode 208-2 and the charge readout electrode 21 made of a second polysilicon film are used.
0 is formed.

Next, as shown in FIG. 8B, as a measure against dark current, an accelerating voltage of 50 is applied to the surface of the N-type region 205.
P-type impurities are ion-implanted at a low energy of about kev using the resist film 215 as a mask, and an ion implantation region
After forming a, the resist film 215 is removed,
The annealing process is performed to a degree to form a P-type region 207 shown in FIG. Here, the P-type region 207 is also formed below the charge transfer electrode 208-1 by thermal diffusion during the annealing process. Finally, as shown in FIG. 8D, an interlayer insulating film 211, a light-shielding film 212 made of an aluminum film, and the like 210 are formed.

[0008]

In the above-described conventional solid-state imaging device, the period during which charge is accumulated in the photodiode (0 volt is applied to the charge readout electrode 210 and φ1 is 5)
A transfer region (a period in which volts φ2 is 0 volt) and a charge readout period (a period in which 5 volts is applied to the charge readout electrode 210 and φ1 is 5 volts and φ2 is 0 volt) (shown schematically by a two-dot chain line in FIG. 6) 7 (a) and (b), the electric charge Q accumulated in the photodiode (205) during the electric charge accumulation time
Is transferred to the buried channel 203 during the charge readout period.

In the conventional solid-state imaging device, since the impurity in the N-type region 205 of the photodiode immediately below the charge readout electrode 210 is dense, a potential well W is formed immediately below the charge readout electrode 210, and only a part of the charge Q2 = Q-Q1 is formed. Is read. Due to this, the conventional solid-state imaging device has a drawback that the readout efficiency and the afterimage characteristics are deteriorated.

Further, since the side surface of the edge of the charge readout electrode has a vertical shape with respect to the semiconductor substrate, the step is large and the step coverage of the aluminum film of the light shielding film 212 is deteriorated. As shown in the figure, there is a disadvantage that the film thickness is locally reduced, and when the film thickness is severe, a step occurs, and the light shielding property is deteriorated.

A first object of the present invention is to provide a solid-state imaging device with improved readout efficiency and a method of manufacturing the same. Further, a second object of the present invention is to provide a solid-state imaging device with improved light-shielding properties and a method of manufacturing the same.

[0012]

According to the present invention, there is provided a solid-state imaging device comprising: a photodiode including a first N-type region selectively formed on a surface of a P-type region on a surface of a semiconductor substrate;
A buried channel for charge transfer comprising a second n-type region selectively formed on the surface of the p-type region at a predetermined distance from the first n-type region; A register including a charge transfer electrode that intersects via a gate insulating film provided in the semiconductor device, and selectively covers the gate insulating film from the predetermined interval to the first N-type region by coupling with the charge transfer electrode. An aperture on the first n-type region via a charge readout electrode and an interlayer insulating film, and a light shield that covers an upper part of the first n-type region from an edge of the charge readout electrode In the solid-state imaging device having the first and second films, the first N
An N-type impurity concentration on a potential minimum surface at which the potential for electrons in the depth direction is minimized from an arbitrary point on the surface of the mold region has a gradient that decreases toward the second N-type region below the charge readout electrode. That is.

By doing so, it becomes possible to make the potential well that reduces the readout efficiency shallow.

Here, the thickness of the conductive film constituting the charge readout electrode may be increased as the distance from the end on the first N-type region increases.

Then, the step in the region where the light shielding film is formed is reduced.

[0016]

According to the present invention, there is provided a solid-state imaging device comprising: a photodiode including a first N-type region selectively formed on a surface of a P-type region on a surface of a semiconductor substrate;
A buried channel for charge transfer comprising a second n-type region selectively formed on the surface of the p-type region at a predetermined distance from the first n-type region; A register including a charge transfer electrode that intersects via a gate insulating film provided in the semiconductor device, and selectively covers the gate insulating film from the predetermined interval to the first N-type region by coupling with the charge transfer electrode. An aperture on the first n-type region via a charge readout electrode and an interlayer insulating film, and a light shield that covers an upper part of the first n-type region from an edge of the charge readout electrode In the solid-state imaging device having the first and second films, the first N
Has a lowest potential side potential for electrons in the depth direction viewed from any point corresponding to the surface of the mold region the surface is a set of points which becomes local minimum, said potentiometer
The N-type impurity concentration in the first N-type region has a gradient that decreases toward the second N-type region below the charge readout electrode within the minimum surface of the potential. .

Here, a step of depositing an interlayer insulating film after forming the N-type region of the photodiode and forming a light-shielding film covering a part of the N-type region from the extreme end of the charge readout electrode is added. can do.

[0018]

FIG. 1 is a plan view showing an embodiment of a solid-state imaging device according to the present invention, and FIG. 2 is a sectional view taken along line XX of FIG.

In this embodiment, a photodiode including a first N-type region 105 selectively formed on the surface of a P-type region (P-well 102) on the surface of a silicon substrate;
At a predetermined interval from the first N-type region 105, the P-type region (1
02), a buried channel 103 for charge transfer, which is a second N-type region selectively formed on the surface portion, and a first channel crossing the buried channel 103 via a gate oxide film 106 provided on the surface of the silicon substrate. Including the first charge transfer electrode 108-1 and the first charge transfer electrode 108-1 and the first N-type region 105 from the predetermined interval (readout region 102a).
A charge readout electrode 110 for selectively covering the gate oxide film 106 and an opening on the first N-type region 105 via the interlayer insulating film 111 to extend from the edge of the charge readout electrode 110 to the first. In the solid-state imaging device having the light-shielding film 112 that covers a part of the N-type region 105, the first N-type region 105
Potential for electrons in the depth direction viewed from any point of the point to be the minimum of the corresponding to the surface of said surface
A potential minimum surface which is a set,
The n-type impurity concentration of the first n-type region has a gradient that decreases toward the second n-type region (103) below the charge readout electrode within the minimum plane . This potential minimum surface is equal to the impurity concentration maximum surface at which the N-type impurity concentration in the depth direction from any point on the surface of the first N-type region becomes maximum.

Next, an embodiment of a method for manufacturing a solid-state imaging device according to the present invention will be described.

First, as shown in FIG. 4A, a P well 102 is formed on the surface of an N-type silicon
A buried channel 103 (second N
Mold region), a P ⁺ -type channel stopper 104 is formed, and a gate oxide film 106 having a thickness of 80 nm is formed.
Next, a first charge transfer electrode 108-1 intersecting the buried channel 103 is formed by depositing and patterning a first-layer polysilicon film having a thickness of about 500 nm. Next, boron ions for forming a buried channel and a barrier layer which are not covered with the first charge transfer electrode 108-1 are implanted, and the gate oxide film 106 is formed using the first charge transfer electrode 108-1 as a mask. Is removed and thermal oxidation is performed to form a gate oxide film 109. Next, a second polysilicon film having a thickness of about 500 nm is deposited and patterned using the resist film 113 as a mask, so that the second charge readout electrode (108-2 in FIG. 1) and the charge readout electrode 110 are formed. Form. This patterning, by isotropic plasma etching using a gas such as SF _6. The edges of these electrodes are tapered at about 45 °.

Next, using the resist film 114 as a mask,
Phosphorus ions are implanted at an acceleration voltage of 200 to 500 keV. This is for forming the first N-type region 105 of the photodiode. The resist film 114 serves as the charge readout electrode 11
Of the 0 tapered edges, the upper part of the P-type well 102 surrounded by the P ⁺ -type channel stopper is exposed. Then, P at the edge of the charge readout electrode 110
The first N-type region 105 having a shallow N-junction position can be formed. The tilted structure of the first N-type region 105 adjusts the thickness of the second polysilicon film and the accelerating voltage of phosphorus ions to form the charge readout electrode 110.
And can be formed in a self-aligned manner. Next, the resist film 114 is removed. Next, as shown in FIG.
A resist film 115 is formed so as not to expose the tapered edge of the charge readout electrode 110, boron ions are implanted at an acceleration voltage of about 50 keV, the resist film 115 is removed, and an annealing process at about 900 ° C. is performed. FIG. 4 (d)
Is formed on the surface of the first N-type region 105. This P-type region 107 is for suppressing dark current, but is also formed below the charge readout electrode 110 by thermal diffusion during annealing. Next, FIG.
(D), as shown in FIGS. 1 and 2, a light-shielding film 2 is formed by forming an interlayer insulating film 111 of a BPSG film having a thickness of about 1.2 μm, depositing and patterning an aluminum film.
12 and an electrode wiring for supplying clock pulses φ1 and φ2 to the transfer electrodes. Next, a cover film (not shown) is formed, and a second light-shielding film covering the CCD register portion is formed.

Since the edge of the charge readout electrode 110 is tapered, the step ledge of the aluminum film is improved, the uniformity of the light shielding film 112 is good, and no step break occurs.

FIG. 3 is a potential diagram of a transfer region (a potential minimum surface schematically shown by a two-dot chain line) in one embodiment of the solid-state imaging device of the present invention.

A period during which charges are accumulated in the photodiode (0 volts is applied to the charge readout electrode 110 and φ1 is 5
The potential に お け る in the volts and φ2 is 0 volts) is as shown in FIG. The potential minimum surface (maximum N-type impurity concentration surface) in the first N-type region 105 becomes shallow because the PN junction surface of the first N-type region 105 and the P-well 102 is inclined toward the charge readout region 102a. Accordingly, the depth becomes shallower toward the charge readout region 102a. Since the PN junction surface of the first N-type region 105 is inclined below the charge readout electrode, the N-type impurity concentration in the vicinity of the surface immediately below the charge readout electrode is lower than in the conventional example. The potential well becomes shallower.

Next, when a voltage of 5 volts is applied to the charge readout electrode 110, the charge Q stored in the first N-type region 105 is partially removed except for a part Q1a.
3 is read. Since the potential well is shallow, Q1a is smaller than Q1 of the conventional example. The read charge amount Q2a = Q-Q1a is larger than Q2 (improvement in readout efficiency). As a result, the afterimage, which was conventionally about 80 mV in output voltage, could be reduced to at least half.

As described above, the edge of the charge readout electrode is tapered by performing isotropic dry etching, and the ion implantation using the tapered edge is used to improve the readout efficiency and the readout efficiency. The light-shielding property could be improved.

In the above, the example in which the charge readout electrode is separated from the first charge readout electrode has been described. However, the present invention can be applied to those having the same charge readout / transfer electrode. In that case, the second charge readout electrode is formed of the first-layer polysilicon film, and then an N-type impurity such as phosphorus is selectively implanted into the buried channel to form a storage region. It is sufficient to deposit a silicon film and perform isotropic etching to form a first charge readout electrode which also serves as a charge readout electrode with a tapered edge.

Although a line sensor has been described as an example, it is apparent that the present invention can be applied to a two-dimensional sensor without further detailed description.

[0030]

As described above, in the solid-state imaging device according to the present invention, a charge readout electrode having a tapered edge is formed using isotropic etching, and the charge readout electrode corresponding to the tapered edge is formed. By forming the N-type region having the impurity concentration distribution as the photodiode portion by using ion implantation, the N-type impurity concentration at the surface of the N-type region of the photodiode below the charge readout electrode can be reduced. As a result, the N-type impurity concentration on the potential minimum surface where the potential for electrons in the depth direction is minimized from an arbitrary point on the surface of the N-type region of the photodiode during the period in which charge is accumulated in the photodiode is lower than the charge readout electrode. Therefore, the potential well in the N-type region of the photodiode can be made shallow, the charge readout efficiency can be improved, and the afterimage can be reduced. Further, since the edge of the charge readout electrode is tapered, a light-shielding film covering this edge can be formed with good step coverage via an interlayer insulating film, so that light-shielding properties can be improved and disconnection can be prevented.

[Brief description of the drawings]

FIG. 1 is a plan view illustrating an embodiment of a solid-state imaging device according to the present invention.

FIG. 2 is a sectional view taken along line XX of FIG.

FIG. 3 is a potential diagram (FIG. 3A) during a charge accumulation period and a potential diagram at the time of charge reading (FIG. 3B) for describing a charge reading operation in one embodiment of the solid-state imaging device of the present invention. )).

FIGS. 4A to 4D are cross-sectional views illustrating a method of manufacturing a solid-state imaging device according to an embodiment of the present invention, which are illustrated separately in (a) to (d).

FIG. 5 is a plan view illustrating an example of a conventional solid-state imaging device.

FIG. 6 is a sectional view taken along line XX of FIG. 5;

7A and 7B are a potential diagram during a charge accumulation period (FIG. 7A) and a potential diagram at the time of charge reading (FIG. 7B) for explaining a charge reading operation in an example of a conventional solid-state imaging device.

FIGS. 8A to 8D are cross-sectional views in the order of steps, illustrating an example of a conventional method for manufacturing a solid-state imaging device.

[Explanation of symbols]

101, 201 N-type silicon substrate 102a, 202a Charge readout area 102, 202 P well 103, 203 Buried channel (second N-type area) 104, 204 P ⁺ type channel stopper 105, 205 First N-type area 106 , 206 Gate oxide film 107, 207 P-type region 108-1, 208-1 First charge transfer electrode 108-2, 208-2 Second charge transfer electrode 109, 209 Gate oxide film 110, 210 Charge readout electrode 111 , 211 Interlayer insulating film 112, 212 Light shielding film 113 Resist film 114 Resist film 115, 215 Resist film

Claims (4)

(57) [Claims] A photodiode including a first N-type region selectively formed on a surface of a P-type region on a surface of a semiconductor substrate; and a photodiode provided at a predetermined distance from the first N-type region. A buried channel for charge transfer comprising a second N-type region selectively formed on the surface of the P-type region, and a charge transfer crossing the buried channel via a gate insulating film provided on the surface of the semiconductor substrate A register including an electrode; a charge readout electrode coupled to the charge transfer electrode and selectively covering the gate insulating film from the predetermined interval to the first N-type region; A light-shielding film that has an opening on the N-type region and covers a part of the first N-type region from the edge of the charge readout electrode. During the period in which the first N-type region is accumulated. Corresponding to the surface have a potential minimum surface potential for electrons is a set of points which becomes local minimum in its depth direction as seen from any point of the surface
The N-type impurity concentration of the first N-type region within the potential minimal plane is lower than the second N-type region below the charge readout electrode.
A solid-state imaging device having a gradient that decreases toward a mold region. 2. The solid-state imaging device according to claim 1, wherein the thickness of the conductive film forming the charge readout electrode increases as the distance from the end on the first N-type region increases. 3. A buried channel is formed by selectively introducing an N-type impurity into a surface portion of a semiconductor substrate having a P-type region on a surface portion, a gate insulating film is formed on the surface, and a conductive film is deposited. Performing anisotropic etching to form a charge readout electrode having a tapered peripheral region whose thickness decreases toward an end; and adjoining the buried channel at a predetermined interval by using ion implantation. When the N-type region of the photodiode is selectively formed on the surface of the semiconductor substrate, the PN with the P-type region can be improved by utilizing the tapered peripheral region.
The bonding surface where the bonding surface is inclined corresponding to the tapered portion.
Forming a mold region. 4. A step of depositing an interlayer insulating film after forming the N-type region of the photodiode and forming a light-shielding film covering a part of the N-type region from the edge of the charge readout electrode. Item 4. A method for manufacturing a solid-state imaging device according to Item 3.