JP2666684B2 - Charge transfer imaging device and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charge transfer imaging device and a method of manufacturing the same, and more particularly, to a channel CCD (Charge Coupled Derices) in which a vertical register and a horizontal register are embedded.
And a method of manufacturing the same.

[0002]

2. Description of the Related Art A solid-state imaging device is described in 196.
In the early 0's, there were various proposals for a matrix type, but none of them came into practical use. In the 1970s, MOS
The development of LSI technology and the introduction of charge transfer devices and CCDs have revolutionized the research on solid-state imaging devices. As a result, the number of photoelectric conversion elements, storage elements, and charge read elements corresponding to a large number of pixels necessary for the imaging device are L
SI chip.

[0003] In the 1980's, various characteristics were remarkably improved, the situation became sufficiently practical, and application fields utilizing the advantages of the solid-state imaging device were pioneered, and research and development were rapidly activated. Above all, CC adopting charge transfer method
The D-type is the most advanced, and currently HDTV (High Defi
Its use extends to those that support nition TV).

A CCD type solid-state imaging device is, for example, an E-type.
xtended Abstracts ofthe 1
991 International Confere
sense on Solid State Device
s and Materials, Yokohama,
1991, p. 666-P. 668, see 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 the respective vertical registers, and signal charges from the photoelectric conversion units to the corresponding vertical registers. It comprises a transfer gate for controlling the transfer, a horizontal register electrically connected to one end of each vertical register, and a charge detector provided at one end of the horizontal register.

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

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

[0007]

However, in the above-described conventional CCD type solid-state imaging 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. In addition, it is impossible to select an impurity distribution that simultaneously satisfies the requirement of improving the transfer efficiency of the horizontal register. Therefore, there is a problem in that the maximum signal charge amount cannot be sufficiently obtained for the vertical register, and the transfer efficiency of the horizontal register deteriorates, resulting in deterioration of image quality. Accordingly, a first object of the present invention is to provide a charge transfer imaging device having high charge transfer efficiency while maintaining a wide dynamic range. It is a second object of the present invention to provide a method of manufacturing the above-described charge transfer imaging device, which can prevent a reproduced image from deteriorating due to charge transfer failure.

[0008]

In the charge transfer imaging apparatus according to the present invention, the vertical register and the horizontal register are constituted by embedded channel CCDs. Each well layer of the vertical register and the horizontal register is individually formed, and the former has a shallower impurity concentration distribution and a higher concentration than the latter. The buried layer of the vertical register overlaps the second buried layer on the first buried layer common to the buried layer of the horizontal register. Thus, the impurity concentration of the buried layer of the vertical register is made higher than the impurity concentration of the first buried layer. The method of manufacturing a charge transfer imaging device according to the present invention comprises first forming a well layer of a conductivity type opposite to the semiconductor substrate 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 electrode of the horizontal register and the vertical register is formed of the electrode material of the well layer and the buried layer, and the second buried layer constituting the vertical register is formed of the vertical register. Is formed in a self-aligned manner with respect to the final transfer electrode.

[0009]

Referring to FIG. 5, which shows a general interline type charge transfer image pickup apparatus, the present apparatus has a plurality of columns of vertical registers 10 composed of CCDs and is arranged adjacent to the vertical registers 10 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 horizontal register 11 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 stored in the photoelectric conversion unit 12 in accordance with the amount of incident light 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 one by one in the vertical registers 10 in a plurality of columns in parallel.
Further, the data 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 voltages by the charge detection unit 14 and output as video signals.

By the way, the HD which is currently being developed is
A charge transfer imaging device compatible with the TV system has 1.3 to 2 million pixels, and requires five to eight times as many pixels as the NTSC system. The reduction in the unit pixel area due to the increase in the number of pixels reduces the maximum signal charge amount of the vertical register, and limits the dynamic range of the imaging device. Also, HD
The horizontal transfer frequency of the solid-state imaging device of the TV system is about 25 to
Since it is 50 MHz and requires a high-speed operation twice to four times as high as that of the NTSC system, deterioration of transfer efficiency becomes a problem.

The present invention has been made to solve such a technical problem, and the contents thereof will be specifically described below.

Referring to FIG. 1 which shows a sectional view of a horizontal register and a vertical register in a charge transfer imaging device according to a first embodiment of the present invention, a p-type well layer and an n-type well are formed 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 type CCD in which a mold buried layer is formed.

The horizontal resistor is formed by forming a first well layer 3a having a relatively deep impurity distribution and a low concentration.
Embedded layer 2a is formed. By forming a well layer having such a low concentration, the fringe electric field in the transfer direction is increased, and high transfer efficiency with no transfer residue even at high-speed charge transfer can be obtained.

On the other hand, in the vertical register, the second well layer 3 having a shallower impurity distribution and a higher concentration than the horizontal register is provided.
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 having a high impurity concentration unique to the vertical register formed on the buried layer 2a. Since the vertical register has a shallow channel layer as described above, the maximum transfer charge per unit area increases, and the dynamic range can be sufficiently increased.

FIG. 2 is a plan view showing a configuration of a well layer at a connection portion between a vertical register and a horizontal register according to a second embodiment of the present invention. 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 decreases from the horizontal register side in the vertical register.

Referring to FIGS. 3 (a) and 3 (b) showing a cross-sectional view taken along line AA 'of FIG. 2 and an 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 is lower than that of the region where only the charge is formed, the potential is lower in the former than in the latter, and a potential gradient is generated that can prevent the transfer in the charge transfer direction (from right to left in the figure), resulting in charge transfer efficiency. Deteriorates.

In order to prevent this, in this embodiment, as shown in FIG. 3, the first well layer 3a is connected to the channel 9 of the vertical register.
The width is gradually reduced from the horizontal register side over the length L in FIG. It is desirable that the length L be as long as possible in the vertical register corresponding to the ineffective light receiving area so as not to affect the characteristics of the photoelectric conversion unit. For example, in an interline transfer type imaging apparatus, a vertical register between an effective light receiving area and a horizontal register, and in a frame interline transfer type imaging apparatus having an imaging area and a storage area, Then, L can be arbitrarily set.

As an example, the first well layer 3a has an impurity concentration of 1 × 10 ¹⁵ to 2 × 10 ¹⁵ cm ⁻³ and a junction depth of 4 to 10 × 10 ¹⁵ cm ⁻³ .
5 μm, the impurity concentration of the second well layer 3 b is 5 × 10 ¹⁵
〜1 × 10 ¹⁶ cm ⁻³ and a junction depth of 2 to 3 μm, the potential of the region where the first well layer 3a and the second well layer 3b overlap is only the second well layer 3b. It becomes lower by about 0.5 V than the formed region. If the first well layer 3a is formed linearly at the connection portion with the electrode length of the transfer electrode being 4 μm, a potential barrier of 0.5 V is formed under one electrode. Since this value is larger than the thermal electromotive force (26 mV) of electrons at normal temperature, electrons cannot cross this barrier and transfer failure occurs.

On the other hand, the first well layer 3a has a length of 100 μm.
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, transfer failure does not occur. In a 1-inch format HDTV-compatible interline transfer solid-state imaging device, 100 μm is 12
This is a value that can be sufficiently realized, corresponding to 13 pixels. In the case of a frame interline transfer type imaging device, the value of L can be set to 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 a second buried layer in a vertical register according to a third embodiment of the present invention in the order of manufacturing steps, a first register forming a horizontal register on an n-type semiconductor substrate 4 is shown. Is formed, and a second well layer 3b constituting a vertical register is formed, and a first n-type buried layer 2a common to a horizontal register is formed thereon.
Further, after a gate insulating film is formed on the surface, the final transfer electrode 1c of the vertical register is formed by the first layer of polycrystalline silicon (FIG. 4A).

Next, the area other than the vertical register area is covered with a photoresist film 7, and using this photoresist film as a mask, a second buried layer 2b is
4c by ion implantation in a self-aligned manner with FIG.
(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 and vertical registers are formed of 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, the generation of a potential barrier in the transfer direction under the transfer electrode can be avoided, and the charge transfer failure is prevented. it can. It is also possible to use a material other than polycrystalline silicon as the electrode material.

[0026]

As described above, in the imaging device according to the present invention, the buried layer and the well layer of the vertical register and the horizontal register can be formed with the optimum impurity distribution for each register. Therefore, in the vertical register, a sufficient transfer charge amount can be secured, and
In the horizontal register, high transfer efficiency can be obtained even when the transfer frequency is high. Thereby, a wide dynamic range and high resolution imaging device can be realized.

When the well layers of both the horizontal and vertical registers are formed with different impurity distributions from each other, the first well layer constituting the horizontal register is gradually narrowed in the vertical register corresponding to the ineffective light receiving area. Thus, the potential gradient at the well connection part can be reduced without affecting the characteristics of the photoelectric conversion unit, and deterioration of the signal charge transfer efficiency can be prevented.

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 poor transfer of signal charges is performed. Image degradation can be prevented.

[Brief description of the 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 connection portion between a vertical register and a horizontal register according to a second embodiment of the present invention.

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

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

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

[Explanation of symbols]

 1a Vertical transfer electrode 1b Horizontal transfer electrode 1c Final transfer electrode 2a First buried layer 2b Second buried 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)

(57) [Claims] A vertical register formed of a well layer of an opposite conductivity type formed on a semiconductor substrate of one conductivity type and a buried layer of an opposite conductivity type to the well layer formed in the well layer; in charge transfer imaging device having a horizontal register, the said well layer has the impurity concentration higher than that of the first well layer and the first well layer formed for the vertical registers are formed for the horizontal register A second buried layer of the vertical register and a second buried layer of the vertical register formed on the first buried layer common to the horizontal register and the first buried layer of the vertical register . A charge transfer imaging device comprising a buried layer . 2. In a connection portion between the vertical register and the horizontal register, a first well layer constituting the horizontal register has a shape having a width gradually narrowing from a horizontal register side in an ineffective light receiving area 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 a vertical register and a horizontal register, and a well layer formed inside the well layer and opposite to the well layer is formed. In a method for manufacturing a charge transfer imaging 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 a first layer electrode material. The transfer electrodes of the horizontal register and the vertical register are formed of the second and third electrode materials, and the second buried layer forming the vertical register is formed in a self-aligned manner with the final transfer electrode of the vertical register. A method for manufacturing a charge transfer imaging device, comprising: