JPS62145865A - Solid-stage image pickup device - Google Patents

Abstract

PURPOSE:To eliminate an after-image of a signal charge when transferring an image from a photosensor to a transfer unit by forming the depths of junctions of a first conductivity type well region at the lower center and the lower end of the photosensor substantially the same and forming the depth of the junction of the photosensor deeper than that of the junction of the transfer unit. CONSTITUTION:The depth of the junction of a p-type well region 2 is, for example, 4mum, the depths of a shallow well region (the depth of the junction is, for example, 3mum) and a deep well region (the depth of the junction is, for example, 5mum) are provided in a conventional example. In comparison, uniform depths of junctions are provided. An n<+> type impurity region 3 is formed as a photosensor for storing a signal charge generated by an incident light on the surface of the region 3. The depth of the junction of the region 3 is, for example, 3mum which is to be deeper than the conventional example in which the depth of the junction has been, for example, 2mum.

Description

[Detailed description of the invention] [Technical field of invention] The present invention relates to a solid-state imaging device.

[Technical background of the invention and its problems]

A conventional solid-state imaging device in which a photosensitive section and a transfer section are separate has a photosensitive layer on the surface of a semiconductor substrate 1, as shown in FIG.
A type well region 22 is formed, and the p-type well region 22 has a shallow junction depth of, for example, 3μ, and has a low integrated impurity concentration.
ll) An n+ type impurity region 23 is provided on the surface of the region as a photosensitive region, and a transfer region is provided on the surface of a deep well region where the junction depth of the p-type well region 12 is as deep as, for example, 5 μm, and the integrated impurity concentration is high. n+ as
Type impurity regions 25 and 26 are provided.
The mold well region 22 is grounded and the semiconductor substrate 1 has a voltage of 15V.
A substrate voltage of is applied.

These are vertical overflow drains (Vertica
l Overflow Drain ) structure is formed.

That is, if the junction depth of the n+ type impurity region 23 as a photosensitive part is 2μ, the depth of the junction below the n+ type impurity region 23 is 2 μm.
Since the p-type well region 22 is as thin as 1 μm in thickness (and the integrated impurity concentration is low, the depletion layers at the upper and lower junctions of the p-type well region 22 are coupled to each other regardless of the amount of signal charge, and the The potential of the p-type well region 22 becomes higher than oV, and the excess charge generated in the n+ type impurity region 23 is discharged to the semiconductor substrate 1.
- Since the p-type well region 22 below the type impurity region 25 is thick and has a high integrated impurity concentration, the signal charge in the n+ type impurity region 25 may be lost or the n+
Charges are prevented from being injected into the type impurity region 25.

Further, as shown in FIG. 4, a p+ type impurity region 24 is provided on the surface of an n+ type impurity region 23 serving as a photosensitive portion.
B P D (Burried Photo Diod)
e buried photodiode) structure is formed.

This is to increase the storage capacity of the buried n+ type impurity region 23 and to enable complete depletion. Furthermore, this is to prevent signal charges accumulated in the p+ type impurity region 24 from remaining without being completely transferred during reading. This residual signal charge flows out during the next readout due to the energy of thermal motion, thereby causing an afterimage.

Therefore, the signal charge accumulated in the n+ type impurity region 23 as a photosensitive portion is read out as completely as possible during readout, thereby preventing an afterimage from occurring.

FIG. 5 shows the potential distribution in the depth direction of a cross section taken along the line A-A' in the central part of the photosensitive area at the time of complete depletion. Since the maximum value of the potential of the n-type impurity region 23 as a photosensitive region when fully depleted is designed to be approximately 7.5V, for example, the potential of the n+-type impurity region 25 as a transfer region is set to be 7.5V or more. When a read voltage of, for example, about 8 V to 10 V is applied to the transfer electrode 9, the signal charge accumulated in the n+ type impurity region 23 as a photosensitive part is completely read out to the n+ type impurity region 25 as a transfer part at the time of reading. It is thought that

However, in the above-mentioned conventional solid-state imaging device, as shown in FIG. It's getting deeper. Furthermore, the p-type impurity region at the same depth also has a similar tendency to become high. These extend from the deep well region of the p-type well region 22 adjacent to the outer periphery of the n-type impurity region 23 toward the shallow well region of the p-type well region 22 below the photosensitive area.
This occurs because p-type impurities are laterally diffused by a thermal process.

Now, FIG. 6 shows the potential distribution in the depth direction of a cross section taken along the line B-B' at the end of the photosensitive section in the case of complete depletion. P-type well region 2 below n+-type impurity region 23 as a photosensitive region
Since the integrated impurity concentration of 2 is higher than that below the center of the photosensitive area, the maximum value of the potential when the n+ type impurity region 23 shown in FIG. 6 is completely depleted is as shown in FIG. It is lower than that.

Figure 7 also shows the T
1-Ml-M2-T2-T3-T4 line cross section shows a potential distribution at the time of complete depletion when a read voltage of, for example, 9V is applied to the transfer electrode 9. As is clear from FIGS. 5 and 6, the potential at point M2 at the end of n-type impurity region 23 serving as the sensing tip is lower than the potential at point M1 at the center.

Therefore, the signal charge is transferred to the n+ type impurity region 2 as a photosensitive area.
3 to the n+ type impurity region 25 serving as the transfer section, a potential barrier is generated at the end (point M2) of the n+ type impurity region 23, as shown in FIG. A portion of the signal charge remains in the 01 type impurity region 23 due to this potential barrier, and this remaining signal charge gradually flows out due to the energy of thermal motion, which is a major cause of afterimages. However, the above-mentioned conventional solid-state imaging device has the problem of producing an afterimage.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to eliminate afterimages of signal charges when transferred from a photosensitive section to a transfer section, and to provide a solid-state imaging device with less afterimages.

[Summary of the invention]

The present invention includes a semiconductor substrate, a first conductivity type well region formed on the surface of the semiconductor substrate, and a second conductivity type impurity formed by adding a second conductivity type impurity to the surface of the first conductivity type well region. a photosensitive section that accumulates signal charges, a transfer section that is formed by adding a second conductivity type impurity to the surface of the first conductivity type well region and transfers the signal charges, and a transfer section that is provided between the transfer section and the photosensitive section. In a solid-state imaging device having a readout section that reads out signal charges accumulated in the photosensitive section to a transfer section, the depth of the junction of the first conductivity type well region under the center and below the end of the photosensitive section is determined. They are almost the same, and are characterized in that the depth of the junction in the photosensitive part is deeper than the depth of junction in the transfer part.

In addition, a potential barrier is not generated at the edge of the photosensitive area to ensure complete readout of signal charges without residual charges, and the thickness of the first conductivity type well region below the photosensitive area is The structure is made thinner, and the excess charge in the contact area is discharged to the semiconductor substrate.

[Embodiments of the invention]

FIG. 1 shows a cross section of a solid-state imaging device according to an embodiment of the present invention. A p-type well region 2 is formed on the surface of a semiconductor substrate 1 made of, for example, an n-type silicon substrate. The junction depth of this p-type well region 2 is, for example, 4 μm, and the conventional example is a shallow well region (the junction depth is, for example, 3 μm) and the deep φ well region (the junction depth is, for example, 5 μm).
It is characterized in that it has a uniform bonding depth, compared to the conventional method (Ilm). Also, in the impurity degree distribution of the tc p-type well region 2, the impurity concentration is the same at the same depth.

On the surface of the p-type well region 2, an n-type impurity region 3 is formed as a photosensitive portion for accumulating signal charges generated by incident light. The depth of the junction of this n4 type impurity region 3 is, for example, 3 μm, and the depth of the junction of the conventional example is, for example, 2 μm.
The feature is that the depth of the bond is deeper than that of μm.

A p+ type impurity region 4 is provided on the surface of an n+ type impurity region 3 serving as a photosensitive portion, forming a BPD structure. As a result, the buried n+ type impurity region 3 has a large storage capacity and can be completely depleted.

Further, on the surface of the p-type well region 2, n+-type impurity regions 5 and 6 are formed as transfer portions for transferring signal charges. A readout section is provided between the n4 type impurity region 3 as a photosensitive section and the n+ type impurity region 5 as a transfer section, for reading out the signal charges accumulated in the n1 type impurity region 3 to the n+ type impurity region 5. ing. Roughly speaking, there is a di-tunnel layer doped with p+ type impurities between the n+ type impurity region 3 and the n+ type impurity region 6 serving as a transfer portion of an adjacent pixel.
A stopper 7 is formed.

Further, a transfer electrode 9 is formed above the n+ type impurity regions 5 and 6 serving as the transfer section and the readout section with an insulating film 8 interposed therebetween. A light shielding film (not shown) made of Al-8i covers the entire surface except for the photosensitive area. Furthermore, it is covered with a surface protective film (not shown). Furthermore, the p-type well region 2 is grounded, and a substrate voltage of 15V is applied to the semiconductor substrate 1.

Next, the manufacturing process of the solid-state imaging device according to this embodiment will be explained. First, an insulating film 8 is formed by oxidizing the surface of a semiconductor substrate 1 made of an n-type silicon substrate. Then, boron B is ion-implanted over the entire surface, and annealing is performed to form a p-type trench region 2. Phosphorus P is ion-implanted into the surface of this p-type well region 2, and annealing is performed to form an n''-type impurity region 3 as a photosensitive portion.

Furthermore, arsenic A3 is ion-implanted to form an n+-type impurity region 5.6 as a transfer region on the surface of the p-type well region 2, and then boron B is ion-implanted to form a di-V channel stopper 7. Further, a transfer electrode 9 made of polysilicon doped with vAP is formed above the transfer section and the readout section via an insulating film 8, and a p+ type impurity region is further formed on the surface of the n+ type impurity region 3 serving as a photosensitive section. After ion implantation of boron B to form 4, an annealing process is performed.

Furthermore, after forming a light shielding film made of Al-8i on the entire surface, the portion corresponding to the photosensitive area is opened and sintered (5i
terino). Finally, a surface protective film is formed on the entire surface.

Next, T1-Ml-M2-T-T-
FIG. 3 shows the potential distribution at the time of complete depletion in the T4 line cross section. Point M1 and point M2 are maximum potential points at the center and end portions of n+ type impurity region 3 as a photosensitive portion, respectively, and are located in the middle of the thickness of n+ type impurity region 3. Point T1 is located at the boundary between n-type impurity region 3 and channel stopper 7, and at the same depth as point M1. Point T2 is located at the boundary between n+ type impurity region 3 and the readout section and at the same depth as point M2. The point T4 is the maximum potential point applied to the n+ type impurity region 5 as a transfer portion, and is located in the middle of the thickness of the n+ type impurity region 5. The point T3 is located at the boundary between the n+ type consumable area 5 and the reading section, and is located at the same depth as the point T4.

The maximum value of the potential when fully depleted of the n+ type impurity region 3 as a photosensitive area is designed to be, for example, 8■, so the transfer is performed so that the potential of the n+ type impurity region 5 as a transfer area is 8■ or more. When a read voltage of, for example, about 10 V is applied to the electrode 9, the signal charges accumulated in the n+ type impurity region 3 as a photosensitive part are transferred to the n+ type impurity region 3 as a transfer part during reading.
The signal is read out to the + type impurity region 5. At this time, the p-type well region 2 is formed under the center and under the end of the n4-type impurity region 3 as a photosensitive region, both in the junction depth and in the impurity concentration distribution in the depth direction. Since there is no difference, as shown in Fig. 3, the maximum value of the potential at the time of complete depletion is different between the center (point M) and the end point (-2) of the n+ pear-shaped -11- impurity region 3. However, unlike the conventional example, no potential barrier is generated at the end (point M2) of the n+ type impurity region 3 that hinders the movement of signal charges. 'Therefore, the signal charge does not remain in the n+ type impurity region 3 and is completely read out. In this way, according to this embodiment, it is possible to prevent the occurrence of afterimages without generating residual charges in the photosensitive portion.

In addition, the junction depth is 4 μmg) 3 μm on the surface of the p-type well region 2.
Since the n+ type impurity region 3 is formed as a photosensitive region with a deep junction depth of μm, the thickness of the p type well region 2 below the n+ type impurity region 3 is smaller than that of the conventional example having a vertical overflow drain structure. It is also thinner at 1 μm. Therefore, regardless of the amount of signal charge, the depletion layers at the junction between the n+ type impurity region 3 and the p-type well region 2 and the junction between the p-type well region 2 and the semiconductor substrate 1 are combined with each other, The potential of p-type well region 2 becomes higher than Ov, and excess charges generated in n+-type impurity region 3 are discharged to semiconductor substrate 1. In this way, this embodiment has the function of discharging excess charge generated in the photosensitive area to the semiconductor substrate 1, as in the case of having a vertical overflow drain structure, and can prevent the smear blooming phenomenon. can.

Furthermore, since the n+ type impurity region 5 serving as a transfer portion has a relatively shallow junction depth compared to the n+ type impurity region 3 serving as a photosensitive portion, the p type well region 2 below the n+ type impurity region 5 is thicker and has a higher integrated impurity concentration. Therefore, loss of signal charges in n+ type impurity region 5 and injection of charges from semiconductor substrate 1 into n+ type impurity region 5 can be prevented.

Note that p+ forming the BPD structure in the above example
The type impurity region 4 may be smaller than the nF type impurity region 3 serving as a photosensitive portion on the reading section side.

A cross section of the solid-state imaging device in this case is shown in FIG. Furthermore, due to manufacturing errors, the p-type well region 4 may be larger than the n+-type consumable region.

Further, in the above embodiment, the transfer portion is the n+ type impurity region 5.
It has an embedded transfer channel structure consisting of
An m structure without the n+ type impurity region 5 may be used. The depth of the junction of the channel stopper 7 made of the p+ type impurity region is
There is no need to be limited to the depth shown in FIGS. 1-2, or alternatively there may be no channel stopper 7 itself. Further, the substrate voltage applied to the semiconductor substrate 1 does not need to be limited to 15V as in the above embodiment.

Further, in the above embodiment, the transfer electrode and the light shielding film are made of polysilicon and Al-8i, respectively, but they may be made of different materials.

In addition, although the embodiment described above is a solid-state imaging device that uses electrons as signal charges, the n-type conductivity type and the n-type conductivity type of all the regions constituting this device are switched, and holes are used as signal charges. It may also be a solid-state image carrier. In this case, the polarity t of the voltage applied to the semiconductor substrate 8[1, the transfer electrode 9, etc.] is reversed.

〔Effect of the invention〕

As described above, according to the present invention, no signal charges remain in the transfer of signal charges from the photosensitive section to the transfer section, and it is possible to prevent afterimages from occurring.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a solid-state imaging device according to an embodiment of the present invention, FIG. 2 is a sectional view showing a solid-state imaging device according to another embodiment of the invention, and FIG. 3 is a sectional view showing a solid-state imaging device according to another embodiment of the invention. 4 is a sectional view showing a conventional solid-state imaging device, and FIGS. 5 to 7 are potential distribution diagrams showing characteristics of a conventional solid-state imaging device. 1... Semiconductor substrate, 2, 22... P-type well region,
3.5.6.23.25.26...n+ type impurity region, 4.14.24...p+ type impurity region, 7...channel stopper, 8...insulating film, 9...・Transfer electrode. Fig. 3 P1 shout QIOd 〉 [F] 9! ! 2 PotentwL

Claims (1)

[Claims] A semiconductor substrate, a first conductivity type well region formed on the surface of the semiconductor substrate, and a second conductivity type well region formed by adding a second conductivity type impurity to the surface of the first conductivity type well region, a photosensitive section that accumulates generated signal charges; a transfer section that is formed by adding a second conductivity type impurity to the surface of the first conductivity type well region and transfers the signal charges; and the transfer section and the photosensitive section. In the solid-state imaging device, the solid-state imaging device includes a readout section that is provided between the photosensitive section and reads out the signal charges accumulated in the photosensitive section to the transfer section, wherein the first conductive conductor is located below the center and below the end of the photosensitive section. A solid-state imaging device characterized in that the junction depths of the mold well regions are substantially the same, and the junction depth of the photosensitive section is deeper than the junction depth of the transfer section.