CCD apparatus for preventing a smear phenomenon

A CCD for detecting images includes a substrate, a well region formed on the semiconductor substrate, a horizontal CCD (HCCD) formed in the well region, a photodiode region formed in the well region at a prescribed spacing from the HCCD, a channel stop layer, an impurity diffusion layer which serves as a potential barrier region around the side and lower portions of the photodiode region so as to completely separate the photodiode region from the well region, a gate insulating layer formed on the substrate, a polygate formed on the gate insulating layer above the HCCD, an insulating layer formed on portions of the gate insulating layer, and a metal shielding layer formed on the insulating layer, whereby a smear phenomenum is prevented.

BACKGROUND OF THE INVENTION 
The present invention relates to a CCD (charge coupled device) for 
detecting images, and more particularly to a CCD device which is 
appropriate for reducing smear phenomena by forming a potential barrier 
around a photoelectric conversion region. 
Generally, a CCD for detecting images is a two-dimensional array of 
charge-coupled photoelectric conversion devices on a substrate of a 
semiconductor such as silicone, and includes a horizontal CCD decoder 
(hereinafter "HCCD") for transferring image signal charges formed by the 
photoelectric conversion devices arranged in the horizontal direction, a 
vertical CCD decoder (hereinafter "VCCD") for sequentially transferring 
different rows of image signal charges transferred by the HCCD decoder, 
and a sensing amplifier for sensing the transferred image signal charges. 
A method for manufacturing a conventional CCD for detecting images will be 
described in detail below with reference to the attached drawing. 
In FIG. 1, reference numerals 1-10 are summarized as follows: 
1: N-type semiconductor substrate 
2: low concentration P-type well 
3: HCCD region 
4: photo diode region 
5: gate insulating layer 
6: polygate 
7: insulating layer 
8: metal shielding layer 
9: PDP region 
10: signal charge 
In addition, the circled minus signs adjacent reference numeral 10 indicate 
signal charges incident to the photo diode region. 
As shown in FIG. 1, which is a sectional view of a conventional CCD for 
detecting images, a P.sup.- -type well region 2 of low impurity 
concentration is formed on an N-type semiconductor substrate 1, and an 
N.sup.+ -type photodiode region 4 of high impurity concentration and an 
N.sup.+ -type HCCD 3 of high impurity concentration are formed at a 
prescribed spacing from each other in the P-type well region 2 of low 
impurity concentration. 
The N.sup.+ -type photodiode region 4 of high impurity concentration is 
formed by ion implantation of a high concentration of an N-type impurity 
into a P.sup.- -type well region 2 of low impurity concentration. On the 
surface of the photodiode region 4, a P.sup.+ -type layer 9 of high 
impurity concentration for potential barrier formation is manufactured so 
as to form a double P-N junction consisting of PNPN. 
In FIG. 1, only a single photodiode region 4 and a single HCCD 3 are 
illustrated and labelled in the P-type well region 2 of low impurity 
concentration. However, multiple photodiode regions 4 and multiple HCCD's 
3 are actually formed in the P.sup.- -type well region 2 of low impurity 
concentration. 
Subsequently, a gate insulating layer 5 is formed on the entire substrate 
and a polygate 6 is formed on the gate insulating layer 5 above each HCCD 
3. An insulating layer 7 is formed on the gate insulating layer 5 and 
polygate 6 but not over the photodiode regions 4. A metal shielding layer 
8 is formed on the insulating layer 7 but not over the photodiode regions 
4 so as to prevent irradiation of light to regions other than the 
photodiode regions 4. 
The operation of a conventional CCD formed through the above-mentioned 
process is as follows. 
When light irradiates the photodiode regions 4, image-wise charges are 
produced at the photodiode regions 4 of the P-N junction structure and are 
accumulated there. 
The accumulated image-wise charges at the photodiode regions 4 are 
transferred to the respective HCCD 3 by transfer signals applied to the 
polygate 6. 
However, in the above-mentioned conventional CCD, when light irradiates the 
photodiode regions 4, the produced charges are not limited to those caused 
by the direct irradiation of the photodiode regions 4. Charges are also 
produced in the P-type well region 2 of law impurity concentration by 
irregular reflection of the light. These charges produced by the irregular 
reflection of light become noise signals when the image-wise charges at 
the photodiode regions 4 are detected. 
That is, since the potential barrier at the P.sup.- -type well region 2 of 
low impurity concentration is low (as shown in FIG. 2), the signal charges 
produced at each P.sup.- -type well region 2 of low impurity concentration 
are input to the respective photodiode regions 4. These noise signals are 
transferred to the HCCD 3 (FIG. 1) with the image signal charges produced 
at the respective photodiode regions 4, and are passed to the VCCD (not 
shown in FIG. 1) and the sensing amplifier (not shown in FIG. 1) 50 to 
produce an image. Accordingly, a problem of smear phenomenon (i.e. image 
spreading) occurs. 
SUMMARY OF THE INVENTION 
To solve the above-mentioned problem, an object of the present invention is 
to provide a CCD for detecting images in which such a smear phenomena is 
reduced by preventing input of signal charges due to the irregular 
reflection of light into the photodiode regions. 
To achieve the above object of the invention, there is provided a CCD for 
detecting an image, comprising a first-conductivity type semiconductor 
substrate, a second-conductivity type well region of low impurity 
concentration formed on the semiconductor substrate, a second-conductivity 
type HCCD of high impurity concentration formed in the second-conductivity 
type well region of low impurity concentration, a photodiode region formed 
of high impurity concentration of the second-conductivity type and formed 
in the well region at a prescribed distance from the HCCD, a channel stop 
layer of the second-conductivity type with high impurity concentration, 
and an impurity diffusion layer of the second-conductivity type with high 
impurity concentration which serves as a potential barrier region and is 
formed on the upper portion and side portions, respectively, of the 
photodiode regions. Also, an impurity diffusion layer of the 
second-conductivity type is formed in the well region of low impurity 
concentration, which wraps each photodiode region, the associated impurity 
diffusion layer of the second-conductivity type of high impurity 
concentration, and the channel stop region. Further, a gate insulating 
layer is formed on the entire surface of the substrate, and a polygate is 
formed on the gate insulating layer above each HCCD region. Finally, an 
insulating layer is formed on the gate insulating layer so as to wrap each 
polygate but not cover the respective associated photodiode region, and a 
metal shielding layer is formed so as to wrap the gate insulating layer 
but not cover the respective associated photodiode region. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 is a structural sectional view of a CCD for detecting images 
according to an embodiment of the invention. 
The CCD for detecting images according to the present invention includes an 
N-type semiconductor substrate 21, a P.sup.- -type well region 22 of low 
impurity concentration formed on the N-type semiconductor substrate 21, an 
N.sup.+ -type photodiode region 24 of high impurity concentration formed 
in the P.sup.- -type well region 22 of low impurity concentration, a HCCD 
region 26 formed in the P.sup.- -type well region 22 of low impurity 
concentration so as to maintain a prescribed spacing with the photodiode 
region 24, a P.sup.+ -type layer 25 of high impurity concentration for 
potential barrier formation formed on the surface of the photodiode region 
24, a P-type impurity diffusion layer 23 for preventing smear, formed so 
as to wrap the N.sup.+ -type photodiode region 24 of high impurity 
concentration and P.sup.+ -type layer 25 of high impurity concentration in 
the P.sup.- -type well region 22 of low impurity concentration, a gate 
insulating layer 27 formed on the entire substrate, a polygate 28 for 
transferring signals formed on the gate insulating layer 27 opposite to 
the HCCD region 26, an insulating layer 29 formed on the gate insulating 
layer 27 so as to wrap the polygate 28 but leave uncovered a region above 
the photodiode region 24, and a metal shielding layer 30 formed on the 
surface of the insulating layer 29 but not over the photodiode region 24, 
as shown in FIG. 3. 
The CCD of the present invention further includes a P.sup.+ -type channel 
stop region 31 formed on the side of the photodiode region 24 in the 
P-type impurity diffusion layer 23. 
A method for manufacturing a CCD for detecting images according to the 
present invention having the above-mentioned structure will be described 
below. 
First, a P.sup.- -type well region 22 of low impurity concentration is 
formed in the N-type semiconductor substrate 21 by ion implantation of a 
low concentration of P-type impurity into the N-type semiconductor 
substrate 21, and the HCCD 26 is formed in the P.sup.- -type well region 
22 of low impurity concentration. 
Then, a P-type impurity diffusion layer 23 is formed by implanting P-type 
impurities, using a high energy ion implantation equipment, into the 
P.sup.- -type well region 22 of low impurity concentration between the 
HCCDs 26 so as to have a wider width than the subsequently-formed 
photodiode region 24. 
Then, the photodiode region 24 is formed by ion implantation of N.sup.+ 
-type impurity of high concentration into the P-type impurity diffusion 
layer 23. 
Then, a channel stop layer 31 is formed on the side of the photodiode 
region 24 by ion implantation of a P.sup.+ -type impurity of high 
concentration into the photodiode region 24, and a P.sup.+ -type layer 25 
of high impurity concentration is formed on the surface of the photodiode 
region 24. Therefore, the structure (from the layer 25 to the region 22) 
is made to have a PNPP structure on the N-substrate 21. 
The gate insulating layer 27, including of ONO (oxide-nitride-oxide), is 
formed by successively depositing an oxide layer, a nitride layer and an 
oxide layer on the substrate, and the polygate 28 is then formed on each 
portion of the gate insulating layer 27 which is respective HCCD 26. 
The insulating layer 29 is formed on the gate insulating layer 27 so as to 
wrap the polygate 28, but is not formed above the photodiode region 24. 
The metal shielding layer 30 is formed on the surfaces of insulating layer 
29 to prevent irradiation of light onto the HCCD 26, but is not formed 
above the photodiode region 24 thereto. 
FIGS. 4A and 4B show examples of the electric potential distribution and 
dose profile of each region cut along the line B-B' in FIG. 3. 
As illustrated in FIGS. 4A and 4B, since a the P-type impurity diffusion 
layer 23 is formed around the photodiode region 24 (i.e., a photoelectric 
conversion region plays the role of a potential barrier layer,), a 
potential "a" than which is higher than that formed in the conventional 
CCD for is formed. Therefore, although noise charges are produced in the 
P.sup.- -type well region 22 by the irregular reflection of light, etc., 
the noise charges do not transfer to the photodiode regions 24 due to the 
potential barrier formed by the P-type impurity diffusion layer 23. 
Instead, the noise charges are passed to the semiconductor substrate 21. 
Accordingly, smear phenomenon is prevented by restraining the transfer of 
charges which are not due to the image signals to the photodiode regions 
24. 
According to the present invention as described above, inflow of noise 
charges into the photodiode regions can be prevented by forming a 
potential barrier region around the photodiode regions. Thus, even if 
noise charges are produced in the P.sup.- -well regions by the irregular 
reflection of light, a smear phenomenon is prevented from occurring, and 
this results in an improvement of the resolution of the CCD device.