JPH0738083A - Charge transfer imaging device and manufacture thereof - Google Patents

Abstract

PURPOSE:To provide a charge transfer imaging device and a method for the manufacture thereof wherein a wide dynamic range is maintained and yet a high charge transfer efficiency is achieved. CONSTITUTION:The impurity concentration of a well layer 3b for vertical register is increased to a value higher than that of a well layer 3a for horizontal register. The buried layer 2a formed in the well layer 3b for vertical register is composed of a first and a second buried layer 2a and 2b; the first buried layer 2a is identical to the buried layer for horizontal register, and the second buried layer 2b is formed on the first buried layer 2a and has an impurity concentration higher than that thereof. This prevents degradation in signal charge transfer efficiency, and enables the simplification of the manufacturing process.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charge transfer image pickup device and a method of manufacturing the same, and more particularly to a vertical channel and horizontal register embedded channel CCD (Charge Coupled Derices).
The present invention relates to this type of image pickup device and a manufacturing method thereof.

[0002]

2. Description of the Related Art 196
There were various matrix-type proposals in the first half of the 0's, but none of them came into practical use. In the 1970s, MOS
The development of LSI technology and the announcement of charge transfer devices and CCDs have revolutionized the research of solid-state imaging devices. As a result, the photoelectric conversion element, the storage element, and the charge reading element corresponding to a large number of pixels required for the image pickup device are L
It was made into an SI chip.

[0003] In the 1980s, various characteristics were remarkably improved, the situation became sufficiently practical, and the application field utilizing the advantages peculiar to the solid-state image pickup device was opened up, and the research and development was rapidly activated. Above all, CC adopting charge transfer method
The D type is the most advanced, and is currently HDTV (High Defi
Its use extends to those compatible with nition TV).

The CCD type solid-state image pickup device is, for example, E
XTENDED ABSTRACTS OFTHE 1
991 International Conference
nce on Solid State Device
s and Materials, Yokohama,
1991, P.I. 666-P. 668 FIG. 1 and Fi
g. As shown in FIG. 3, basically, a plurality of columns of vertical registers composed of CCDs, photoelectric conversion units arranged adjacent to each vertical register, and signal charges from the photoelectric conversion units to the corresponding vertical registers are stored. The transfer gate includes a transfer gate for controlling transfer, a horizontal register electrically coupled to one end of each vertical register, and a charge detection unit provided at one end of the horizontal register.

In such a structure, conventionally, a well layer having a conductivity type opposite to that of the substrate is formed on a semiconductor substrate, and a buried layer having a conductivity type opposite to that of the well layer is further formed in the well layer. This is realized by forming transfer electrodes of vertical registers and transfer electrodes of horizontal registers on the main surface of this semiconductor layer via a gate insulating film.

By the way, generally a buried channel type CC
In D, the maximum transfer charge amount tends to increase when the well layer has a high impurity concentration and the buried layer is shallowly formed. On the other hand, the lower the impurity concentration of the well layer, the stronger the fringe electric field in the transfer direction, and the more the transfer efficiency tends to improve.

[0007]

However, in the above-mentioned conventional CCD type solid-state image pickup device, since the buried layer and the well layer of the vertical register and the horizontal register have a common structure, the maximum transfer charge amount of the vertical register is ensured. However, it was not possible to select an impurity distribution that simultaneously satisfies the requirement of improving the transfer efficiency of the horizontal register. Therefore, there is a problem that the maximum signal charge amount cannot be obtained sufficiently for the vertical register, and the transfer efficiency of the horizontal register is deteriorated, resulting in deterioration of image quality. Therefore, a first object of the present invention is to provide a charge transfer image pickup device having a high charge transfer efficiency while maintaining a wide dynamic range. A second object of the present invention is to provide a method for manufacturing the above-mentioned charge transfer imaging device, which can prevent deterioration of a reproduced image due to charge transfer failure.

[0008]

In the charge transfer image pickup device of the present invention, the vertical register and the horizontal register are constituted by the embedded channel CCD. Each well layer of the vertical register and the horizontal register is formed separately, and the former has a shallower impurity distribution and a higher concentration than the latter. Further, the buried layer of the vertical register is formed by stacking the second buried layer on the first buried layer common to the buried layer of the horizontal register. The impurity concentration of this second buried layer is also made higher than that of the first buried layer. According to a method of manufacturing a charge transfer imaging device according to the present invention, a well layer having a conductivity type opposite to that of a semiconductor substrate of one conductivity type is first formed on a semiconductor substrate of one conductivity type so as to form a vertical register and a horizontal register. A first buried layer and a second buried layer having a conductivity type opposite to that of the well layer are formed in the layer. The final transfer electrode of the vertical register is formed of the electrode material of the semiconductor substrate, the transfer electrodes of the horizontal register and the vertical register are formed of the electrode material of the well layer and the buried layer, and the second buried layer forming the vertical register is the vertical register. It is characterized in that it is formed in a self-aligned manner with respect to the final transfer electrode of.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 5, which shows a general interline charge transfer image pickup device, the present device is provided with a plurality of columns of vertical registers 10 and adjacent vertical registers 10 arranged in an array. A plurality of photoelectric conversion units 12, a plurality of transfer gates 13 arranged in an array so as to control the transfer of signal charges from each of the photoelectric conversion units 12 to the corresponding stage of the vertical register 10,
The vertical register 10 includes a horizontal register 11 electrically connected to one end of each of the vertical registers 10, and a charge detection unit 14 provided at one end of the horizontal register 11.

The signal charges accumulated in the photoelectric conversion section 12 in accordance with the amount of incident light incident during a predetermined period are read out to the corresponding vertical register 10 by turning on the transfer gate 13 during the vertical blanking period. By applying a drive pulse to the transfer electrode (not shown) of the vertical register 10 during the horizontal blanking period, the signal charges are transferred in parallel in the vertical registers 10 of a plurality of columns one by one.
Further, it is transferred from the final transfer electrode of the vertical register 10 to the horizontal register 11. The signal charges transferred in the horizontal register 11 in the horizontal direction during the effective video period are converted into a voltage in the charge detection unit 14 and output as a video signal.

By the way, HD currently under development
The charge transfer imaging device compatible with the TV system has 1.3 to 2 million pixels and requires 5 to 8 times as many pixels as the NTSC system. Since the unit pixel area is reduced due to the increase in the number of pixels, the maximum signal charge amount of the vertical register is reduced and the dynamic range of the image pickup device is limited. Also, HD
The horizontal transfer frequency of the TV type solid-state imaging device is about 25-
Since it requires 50 to 2 times as high speed operation as the NTSC system, the deterioration of transfer efficiency becomes a problem.

The present invention has been made to solve such a technical problem, and its contents are as specifically described below.

Referring to FIG. 1 showing a cross-sectional view of a horizontal register and a vertical register in a charge transfer image pickup device according to a first embodiment of the present invention, the present embodiment shows that a p-type well layer and an n-type silicon substrate are provided in an n-type silicon substrate. A horizontal register (FIG. 1A) and a vertical register (FIG. 1B) are constituted by an N-channel buried channel CCD in which a mold buried layer is formed.

In the horizontal register, the first well layer 3a having a relatively deep impurity distribution and a low concentration is formed, and then the first well layer 3a is formed.
To form the buried layer 2a. By forming the well layer having a low concentration in this way, the fringe electric field in the transfer direction becomes strong, and high transfer efficiency without transfer residue can be obtained even at high-speed charge transfer.

On the other hand, the vertical register has a second well layer 3 having a shallower impurity distribution and a higher concentration than the horizontal register.
b is formed. The buried layer of the vertical register is composed of a first buried layer 2a common to the horizontal register and a second buried layer 2b formed on the buried layer 2a and having a high impurity concentration specific to the vertical register. In the vertical register, since the channel layer is shallow as described above, the maximum transfer charge amount per unit area is large, and the dynamic range can be sufficiently large.

FIG. 2 is a plan view showing the structure of the well layer in the connecting portion between the vertical register and the horizontal register according to the second embodiment of the present invention. Reference numeral 9a is a vertical register channel, 1a is a vertical register transfer electrode, 9b is a horizontal register channel, and 1b is a horizontal register transfer electrode.
The feature of this embodiment is that the well layer 3a of the horizontal register has a shape in which the width gradually narrows from the horizontal register side in the vertical register.

Referring to FIGS. 3A and 3B showing the cross-sectional view taken along the line AA 'of FIG. 2 and the impurity concentration distribution of the well layer, respectively, the wells of the horizontal register and the vertical register are shown. When the impurity concentration distributions are different from each other, the region where the first well layer 3a of the horizontal register and the second well layer 3b of the vertical register overlap (indicated by L in the drawing) is the second well layer 3b. Since the potential becomes shallower than the region where only the charge is formed, the potential becomes lower in the former than in the latter, and a potential gradient that prevents transfer is generated along the charge transfer direction (right to left in the figure), resulting in charge transfer efficiency. Deteriorates.

To prevent this, in this embodiment, the first well layer 3a is formed in the channel 9 of the vertical register as shown in FIG.
The width is gradually reduced from the horizontal register side over the length L in a so as to relax the above-mentioned potential gradient. It is desirable that the length L be as long as possible in the vertical register corresponding to the ineffective light receiving region so as not to affect the characteristics of the photoelectric conversion unit. For example, in the interline transfer type image pickup device, in the vertical register between the effective light receiving region and the horizontal register, and in the frame interline transfer type image pickup device having the image pickup region and the storage region, in the vertical register of the storage region. If so, L can be arbitrarily set.

As an example, the impurity concentration of the first well layer 3a is 1 × 10 ¹⁵ to 2 × 10 ¹⁵ cm ^-3 and the junction depth is 4 to 10.
5 μm, the impurity concentration of the second well layer 3b is 5 × 10 ^15.
Assuming that the first well layer 3a and the second well layer 3b are overlapped with each other, only the second well layer 3b has a potential of 1 × 10 ¹⁶ cm ⁻³ and a junction depth of 2 to 3 μm. It is about 0.5 V lower than the formed region. When the electrode length of the transfer electrode is 4 μm and the first well layer 3a is linearly formed at the connection portion, a potential barrier of 0.5 V is formed under one electrode. Since this value is larger than the thermoelectromotive force (26 mV) of electrons at room temperature, the electrons cannot pass through this barrier and a transfer failure occurs.

On the other hand, the first well layer 3a has a length of 100 μm.
When gradually thinning over m (L in FIGS. 3 and 4), the potential difference under one electrode is 20 mV,
Since it is smaller than the thermoelectromotive force of electrons, no transfer failure occurs. In the 1-inch format HDTV compatible interline transfer type solid-state imaging device, 100 μm is 12
It is a value that corresponds to ~ 13 pixels and can be sufficiently realized. Further, in the case of the frame interline transfer type imaging device, the value of L can be 100 μm or more, and the potential difference under one electrode is further reduced.

With the above structure, it is possible to avoid deterioration of the charge transfer efficiency at the junction between the vertical register and the horizontal register having different well configurations.

Referring to FIG. 4, which shows a sectional view of the second buried layer in the vertical register according to the third embodiment of the present invention in the order of manufacturing steps, a first register forming a horizontal register on the n-type semiconductor substrate 4 will be described. The p-type well layer 3a and the second well layer 3b forming the vertical register are formed, and the first n-type buried layer 2a common to the horizontal register is formed thereon.
Further, after forming a gate insulating film on the surface, the final transfer electrode 1c of the vertical register is formed by the first layer of polycrystalline silicon (FIG. 4A).

Next, a portion other than the vertical register region is covered with a photoresist film 7, and the second buried layer 2b is formed on the electrode 1 in the vertical register region using this photoresist film as a mask.
It is formed by ion implantation in self-alignment with c (FIG. 4).
(B)). As a result, the boundary of the second buried layer 2b coincides with the edge of the final electrode 1c.

Finally, the transfer electrodes of the horizontal register and the vertical register are formed by the second and third layers of polycrystalline silicon (FIG. 4C).

As described above, by forming the second buried layer 2b in a self-aligned manner with respect to the vertical final transfer electrode, it is possible to avoid generation of a potential barrier in the transfer direction below the transfer electrode and prevent charge transfer failure. it can. It is also possible to use a material other than polycrystalline silicon as the electrode material.

[0026]

As described above, in the image pickup device according to the present invention, it is possible to form the buried layer of the vertical register and the horizontal register and the well layer in the optimum impurity distribution for each register. Therefore, in the vertical register, it is possible to secure a sufficient transfer charge amount and
In the horizontal register, high transfer efficiency can be obtained even when the transfer frequency is high. As a result, it is possible to realize an imaging device having a wide dynamic range and high resolution.

Further, when the well layers of both the horizontal and vertical registers are formed with different impurity distributions, the first well layer forming the horizontal register is gradually narrowed in width in the vertical register corresponding to the ineffective light receiving region. By taking the above, it is possible to reduce the potential gradient in the well connection portion without affecting the characteristics of the photoelectric conversion portion and prevent the deterioration of the signal charge transfer efficiency.

Further, by forming the second buried layer constituting the vertical register in a self-aligned manner with the final electrode of the vertical register, formation of a potential barrier under the transfer electrode is prevented, and reproduction due to defective transfer of signal charge is prevented. Image deterioration can be prevented.

[Brief description of drawings]

FIG. 1 is a sectional view of a horizontal register and a vertical register according to a first embodiment of the present invention.

FIG. 2 is a plan view of a connecting portion between a vertical register and a horizontal register according to a second embodiment of the present invention.

FIG. 3 is a schematic view showing a cross-sectional view taken along line AA ′ in FIG. 2 and an impurity distribution in a well layer.

FIG. 4 is a cross-sectional view showing a manufacturing process of a buried layer in the charge transfer imaging device of the present invention.

FIG. 5 is a schematic plan view of a general interline charge transfer imaging device.

[Explanation of symbols]

 1a Vertical transfer electrode 1b Horizontal transfer electrode 1c Final transfer electrode 2a First burying layer 2b Second burying layer 3a First well layer 3b Second well layer 4 Semiconductor substrate 5 Gate insulating film 6 Interlayer insulating film 7 Photoresist Film 8 Barrier layer 9a Vertical register channel 9b Horizontal register channel 10 Vertical register 11 Horizontal register 12 Photoelectric conversion unit 13 Transfer gate 14 Charge detection unit

Claims (3)

[Claims] 1. A vertical register formed by a well layer of opposite conductivity type formed on a semiconductor substrate of one conductivity type and a buried layer of opposite conductivity type to the well layer formed in the well layer. In a charge transfer imaging device including a horizontal register, the second well layer has a higher impurity concentration than the first well layer formed for the horizontal register and the first well layer formed for the vertical register. Of the well layer, and the buried layer of the vertical register has a high impurity concentration formed on the first buried layer common to the horizontal register and on the first buried layer of the vertical register. A charge transfer image pickup device, comprising: 2. In the connecting portion between the vertical register and the horizontal register, the first well layer forming the horizontal register has a shape having a width that gradually narrows from the horizontal register side in the ineffective light receiving region in the vertical register. The charge transfer imaging device according to claim 1, wherein the charge transfer imaging device is formed. 3. A well layer of a conductivity type opposite to the substrate is formed on a semiconductor substrate of one conductivity type for the vertical register and the horizontal register, and the well layer is formed inside the well layer and is opposite to the well layer formed inside the well layer. In a method of manufacturing a charge transfer image pickup device in which a conductive type buried layer is formed, and a vertical register and a horizontal register are formed by the well layer and the buried layer, a final transfer electrode of the vertical register is formed of an electrode material of a first layer. , The transfer electrodes of the horizontal register and the vertical register are formed of the electrode materials of the second and third layers, and the second buried layer forming the vertical register is formed in self-alignment with the final transfer electrode of the vertical register. A method of manufacturing a charge transfer imaging device, comprising: