Solid-state imaging device capable of removing influence by false signals

A solid-state imaging device capable of removing undesired influences, includes a semiconductor substrate having one of conductive types, a well layer arranged on the substrate and having the other conductive type opposite to the substrate, photo-sensitive pixels recessed in a matrix having a predetermined number and having the conductive type opposite to the well layer to generate signal charges corresponding to an incident light amount, a transfer channel formed along one direction of the photosensitive pixels arranged by the conductive type as the same as that of the substrate to transfer the signal charges generated by the photosensitive pixels, an electrode provided to the transfer channel on a side opposite to the substrate to supply an electric field to the transfer channel, and a barrier well formed of the impurity semiconductor material of the conductive type opposite to the conductive type of the semiconductor substrate in the manner that an impurity density of the well layer becomes longer along the transfer channel and for covering both ends in a width direction of the transfer channel at a plane opposite to the electrode, and for preventing an invasion of signal charges occurring in the well layer into the transfer channel. In such a construction, there is partially formed higher portion of the fringe electric field to the channel in a bulk along the transfer channel, and the partially higher portion of the field is kept as a path in which the signal charges are moving in a high-speed. Therefore, it is possible suppress the decrease of the transfer efficiency of the signal charges in the high-speed operation and to prevent the invasion of false signals such as smear charges by the barrier well provided around.

BACKGROUND OF THE INVENTION 
The present invention relates to solid-state imaging devices capable of 
removing undesired influences by false signals resulting in smear and 
blooming phenomena, and more particularly, relates to solid-state imaging 
device such as a charge coupled device (CCD) capable of excluding 
undesired influences caused by false signals even when signal charges 
having a low level are transferred. Here, a smear phenomenon means that 
white points are extended in vertical stripes in a reproduced picture 
plane, which are caused by light impinging onto signal lines and the CCD, 
or signal charges occurring inside a semiconductor substrate, which extend 
by diffusion and affect adjacent pixels and a transfer portion. Blooming 
is a phenomenon which resembles smear phenomenon and often accompanies a 
smear. Blooming is a phenomenon in which a white portion is extended 
around a light in the manner that a white flower is blooming on the 
picture plane, because an incident strong light makes the pixels saturated 
and signal charges overflow to affect the adjacent pixels, signal lines 
and vertical transfer CCD. 
There is described an example of a conventional solid-state imaging device 
with reference to attached drawing. FIG. l is a sectional view showing a 
sensitive portion of an interline transfer (IT) type solid-state imaging 
device, in which a p-type impurity well 2 is formed on a surface of an 
n-type semiconductor substrate 1. The p-well 2 has a recessed photodiode 3 
for generating signal charges corresponding to an incident light amount, 
and a vertical transfer channel 4 for transferring the signal charges in 
the vertical direction on the paper. A metal electrode 8 is formed on the 
vertical transfer channel 4 through an insulating layer such as a silicon 
oxide layer (not shown). The metal electrode 8 functions in part as a gate 
for transferring the signal charges accumulated in the photodiode 3 to the 
vertical transfer channel 4 by adding a high electric field, and in part 
for delivering the signal charges in the vertical transfer channel 4 in 
one direction by adding a moving electric field of a plurality of 
electrodes. The photodiode 3 has a construction in which a p.sup.+ -type 
impurity layer is provided on an upper surface of a semiconductor device 
in order to reduce a dark current occurring on the upper surface of the 
semiconductor device. A p-type barrier well 5 is formed around the 
transfer channel 4 for preventing the transfer channel 4 from an invasion 
of smear charges. The p-type barrier well 5 has an impurity density of one 
digit higher than the p-well 2 in order to suppress an extension of a 
depletion layer in the transfer channel 4. Furthermore, an element 
separation layer 6 is formed by a p-type impurity and adjacent to the 
transfer channel 4. A description of an insulation layer and a protective 
layer is omitted. 
There are two types of solid-state imaging devices, namely, an IT type and 
a frame interline transfer (FIT) type. The IT type has a sensitive portion 
in which a photodiode and transfer portion are horizontally arranged to 
one another, while the FIT type has accumulating portion formed from the 
transfer portion in addition to the sensitive portion. In the IT type 
solid-state imaging device, the signal charges are transferred from the 
photodiode to the adjacent transfer portion during a vertical retrace line 
interval of a television signal. On the contrary, in the FIT type 
solid-state the signal charges are moved from the photodiode to the 
adjacent transfer portion, and at the same time, the signal charges are 
transferred to the accumulating portion. Therefore, since the moving 
distance of the signal charges in FIT type devices is longer than in the 
IT type device, it is necessary to move the charges very quickly in the 
transfer channel. 
However, when the solid-state imaging device has the construction including 
the barrier well 5 effective to prevent the smear phenomenon, it is 
possible to prevent the extension of the depletion layer in the vertical 
transfer channel 4 into the semiconductor device, thereby decreasing the 
transfer efficiency by reducing a fringe electric field added to the 
transfer channel in a bulk. In the FIT type device for moving the signal 
charges quickly, when the barrier well is provided, there occur problems 
that the vertical resolution is reduced at transferring low level signal 
charges. Accordingly, the barrier well is not provided for the FIT type 
device to keep the high-speed operation of the vertical transfer of the 
signal charges to increase the fringe electric field in the bulk. 
Alternatively, the FIT type device is used in a range for obtaining the 
necessary transfer efficiency by decreasing the transfer frequency at the 
sacrifice of the high-speed operation. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a solid-state 
imaging device capable of reducing undesired influences such as smear and 
blooming phenomena caused by false signals without decreasing the vertical 
resolution even when the low level signal charges are transferred. 
In order to achieve the above object, a solid-state imaging device 
according to the present invention comprises a semiconductor substrate 
formed on the semiconductor substrate in the manner of having one of 
conductive types, a sensitive pixel having another conductive type 
opposite to one conductive type of the substrate for generating signal 
charges corresponding to an incident light amount, a transfer channel 
having another conductive type opposite to the substrate for transferring 
the signal charges, an electrode for adding to the transfer channel with 
an electric field, and a barrier well formed in the manner of covering the 
transfer channel and having another conductive type opposite to the 
substrate for preventing the transfer channel from an invasion of the 
charges occurring in the semiconductor substrate, wherein the barrier well 
has a gap region or a low impurity density region which is formed along 
and under the transfer channel. 
The present invention has the construction of providing the low impurity 
density region or the gap region in the barrier well under the transfer 
channel which causes the signal charges of the solid-state imaging device 
to be moved. Therefore, it is possible to partially increasing the fringe 
electric field of the channel in the bulk. The partially high fringe 
portion is formed along the transfer channel to keep the high moving path 
of the signal charges in the channel by the high fringe electric field, 
thereby it is possible to suppress the reduction of the transfer 
efficiency of the signal charges in high-speed operation, and to prevent 
the invasion of the smear charges by means of the barrier well provided 
around the transfer channel. 
As described above, the present invention can partially form the high 
fringe electric field region in the bulk even though the barrier well is 
provided under the vertical transfer channel. Accordingly, even in the FIT 
type solid-state imaging device, it is possible to provide the barrier 
well layer around the vertical transfer channel, thereby extremely 
reducing undesired influences such as a smear and blooming caused by false 
signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
There will now be described preferred embodiments of a solid-state imaging 
device according to the present invention with reference to the attached 
drawings. 
FIG. 2 is a sectional view showing a sensitive portion of the solid-state 
imaging device according to a first embodiment. In the figure, the same or 
corresponding portions as or to those shown in FIG. 1 are labeled by the 
same numerals in FIG. 1 and the description thereof is omitted. 
In FIG. 2, a p-type impurity well 2 is formed in an n-type semiconductor 
substrate 1. The p-well 2 comprises a recessed photodiode 3, and a 
vertical transfer channel 4 for transferring signal charges in a vertical 
direction on the paper. The vertical transfer channel 4 has an impurity 
density such as "1.times.10.sup.17 " cm.sup.-3, and an electrode 8 is 
provided through an insulating layer on the vertical transfer channel 4. 
The photodiode 3 has the construction having p.sup.+ -type impurity layer 
on the upper surface of the semiconductor substrate in order to reduce a 
dark current occurring on the upper surface of the semiconductor 
substrate. 
Furthermore, an element separation layer 6 is formed by a p-type impurity 
adjacent to the transfer channel 4. There is provided a p-type barrier 
well layer 51 as a barrier well having a conductive type opposite to that 
of the semiconductor substrate around the transfer channel 4 in order to 
prevent the invasion of the smear charges. The p-type barrier well 51 has 
an impurity density such as "1.times.10.sup.16 " cm.sup.-3 which is one 
digit higher than that of the p-well 2 for suppressing the extension of 
the depletion layer of the transfer channel 4. The barrier well layer 51 
has a construction of including a gap provided under the vertical transfer 
channel, and the gap thereof is provided in parallel with the transfer 
direction of the signal charges by the transfer channel 4 (namely, in the 
vertical direction against the paper surface). The gap may provided 
anywhere under the transfer channel. 
Such a construction can be obtained through the steps of forming the 
barrier well 50 by ion injection near both ends of the vertical transfer 
channel 4 by a high-acceleration ion injection apparatus, and connecting 
the barrier well to the element separation layer 6, is provided at the 
side wall of the vertical transfer channel 4. In this case, since the 
thermal diffusion of the barrier well can be suppressed, it is 
advantageous that the n-type impurity profile does not substantially 
change in the vertical transfer channel 4. Furthermore, the barrier well 
layer 51 may be formed by thermal diffusion at both ends of the vertical 
transfer channel 4. In this case, since the n-type impurity of the 
vertical transfer channel 4 is cancelled by the p-type impurity layer of 
the barrier well 51, it is necessary to compensate a density of the 
transfer channel. 
When such the construction is provided, a potential distribution from the 
semiconductor substrate surface toward inside is shown in FIG. 3. This 
figure is shown such that an under side of the paper is deeper from the 
surface into the inside, and a right side of the paper has a higher 
potential. In this figure, a solid line shows the potential distribution 
in the A--A direction which does not include the barrier well 51 in FIG. 
2, and a dotted line shows the potential distribution in the B--B 
direction including the barrier well 51. 
Therefore, since the depletion position to the well side of the transfer 
channel reaches to the semiconductor substrate surface at both sides of 
the transfer channel where the barrier well 51 is formed that is, the 
density of the barrier well is such that it does not enter a depletion 
condition, and since the depletion position from the n-type substrate to 
the well side reaches to the semiconductor substrate, the barrier well 51 
functions to prevent the invasion of smear charges into transfer channel 
4. In the portion under the transfer channel 4 where the barrier well 51 
is not formed, since the barrier position of the lowest well potential 
resides inside the semiconductor substrate, the fringe electric field is 
higher, thereby promoting the signal charges to move in the charge 
transferring direction. In the above embodiment, there is shown an example 
where a well potential is depleted in the potential distribution of the 
A--A direction. The present invention is not limited to this example and a 
depleted position from the transfer channel to the well side may be deeper 
from a B--B line into the inside of the semiconductor substrate. 
In the construction that the barrier well is separately provided at both 
sides of the transfer channel, it is possible to respectively set the 
particular impurity density of the respective barrier well by using 
different masks for forming the barrier well, for example. By using this 
technology, it is possible to increase the impurity density of the barrier 
well opposite to the read-out gate and the like. Therefore, it is possible 
to increasingly reduce the smear because the depleted position from the 
n-type transfer channel to the well side becomes shallower (nearby the 
upper surface of the semiconductor substrate). 
FIG. 4 shows a solid-state imaging device according to a second embodiment. 
In the figure, a description of duplicated portions corresponding to FIG. 
2 will be omitted. In the construction of the second embodiment shown in 
FIG. 4, a barrier well layer 52 is constituted in a double-well 
constitution comprising a high density region and a low density region of 
impurities. A part for contacting to the vertical transfer channel 4 
having the double-well constitution, is partially constructed from only 
the low density region of the impurity. Therefore, since there is a high 
potential portion of the well barrier potential in the semiconductor 
substrate which deepens a depleted position of the substrate at the well 
side of the transfer channel and the fringe electric field activates the 
signal charges, it is possible to expect the same effect as that by the 
construction of the first embodiment shown in FIG. 2. 
FIG. 5 shows a solid-state imaging device according to a third embodiment 
of the present invention. In FIG. 5, a description of duplicated portions 
corresponding to FIG. 2 is omitted. In a construction shown in FIG. 5, 
there is shown an example in which an n-type impurity region 55 is 
partially formed in a uniform p-type barrier well layer 53 to provide a 
barrier well 50 having an impurity density partially reduced in the p-type 
barrier well layer 53. In this case, since the depleted position of the 
transfer channel at the well side is deepened by an occurrence of the high 
well barrier potential in the low impurity density portion, the fringe 
electric field also has an effect on the signal charges. 
In such a manner, since the several embodiments of present invention have 
the construction in that the gap or the low-impurity density region is 
formed in parallel with the transfer channel in the barrier well layer 
under the vertical transfer channel, it is possible to prevent the 
invasion of the noise changes from the adjacent region to the transfer 
channel, and at the same time, to increase the moving efficiency by acting 
on the fringe electric field to the moving direction of the signal 
charges. 
The present invention can be applied to transfer path of signal charges in 
various kinds of semiconductor devices, and it is possible to plan a 
high-speed operation with respect to the operational frequency and prevent 
smear by applying the present invention not only with the above-mentioned 
FIT solid-state imaging devices but also with a linear image sensor and 
the like.