Charge transfer imaging device with blooming overflow drain beneath transfer channel

A charge trasfer imaging device is disclosed, which comprises photosensitive sites formed in and continuous to the surface of a semiconductor substrate for generating and storing signal charge in response to an incident light signal, a shift electrode for controlling the transfer of the signal charge through the semiconductor substrate, and a charge transfer shift register for reading out the signal charge by transferring the same from the shift electrode through transfer channels formed in and continuous to the semiconductor substrate surface. Semiconductor region of the opposite conductivity type to the semiconductor substrate is formed in the substrate other than a portion of the substrate below the photosensitive sites and at least under the transfer channels of the shift register, and a reverse bias voltage is applied between these semiconductor regions and the semiconductor substrate.

BACKGROUND OF THE INVENTION 
This invention relates to a charge transfer imaging device. 
Solid-state imaging devices using charge transfer devices (CTD) such as 
charge coupled devices (CCD) are recently extensively used because they 
have many advantages such as ease of size reduction. 
The CTD has photosensitive sites such as PN diodes formed in superficial 
portions on a p-type semiconductor substrate for generating and storing 
signal charge according to incident light. The signal charge stored in the 
photosensitive sites is transferred to a charge transfer shift register by 
opening a shift electrode or transfer gate after a predetermined period of 
time. Then, after closing the shift electrode, the transferred signal 
charge can be read out as an electric signal from an output terminal by 
impressing a clock pulse on transfer electrodes. 
This CTD, however, has a drawback that where high intensity light is 
irradiated and the period of reading from the shift register is long or 
where the incident light to be photoelectrically converted is of a long 
wavelength range near infrared, electrons generated in the semiconductor 
substrate other than the photosensitive sites are liable to flow into the 
shift register to deteriorate the image signal. 
To overcome this drawback, there has been proposed an overflow drain, i.e., 
a reverse biased n-region buried under photosites arranged in p-layer, as 
disclosed in H. Goto et al, "CCD Linear Image Sensor with Buried Over-flow 
Drain Structure," Electronics Letter, 26th November 1981, vol. 17, No. 24, 
pp 904-905. The overflow drain here serves to discharge electrons 
generated outside the photosites. Thus, undesired carriers can be 
prevented from entering the shift register, and thus it is possible to 
alleviate the deterioration of image. Further, it is possible to prevent a 
blooming effect that may otherwise result when the photosites are 
saturated. 
In this structure of CTD, however, it is liable that the carriers which are 
to be stored in the photosites partly flow into the overflow drain. 
Particularly, where the incident light to be detected is of a long 
wavelength range near infrared, the charge tends to be discharged to the 
overflow drain, thus lowering the sensitivity. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a charge transfer imaging device, 
which is highly sensitive to incident light of even a long wavelength 
range and which can reduce deterioration of the image signal due to 
instruction of carriers generated in the semiconductor substrate into the 
charge transfer shift register. 
To attain the above object, according to the invention a semiconductor 
region of the opposite conductivity type to the semiconductor substrate is 
formed in a portion of the substrate other than the portion of said 
substrate below the photosensitive sites and at least beneath the transfer 
channel of charge transfer shift register. A reverse bias voltage is 
applied between this semiconductor region and the substrate. 
With the above construction, it is possible to prevent unnecessary carriers 
generated in the semiconductor from reaching the shift register and also 
to prevent effective carriers generated by the incident light of long 
wavelength range from being drained away. 
Particularly, with an auto focusing camera a light source which emits light 
near infrared is often used to obtain data of a foreground subject when 
the background is dark while using a light source emitting visible light 
when the background is bright. 
With the charge transfer imaging device according to the invention, when 
used for the auto focusing camera as noted above, it is possible to 
prevent unnecessary carriers generated in the semiconductor substrate from 
reaching the shift register and also to prevent the blooming effect due to 
saturation of photosensitive sites when intense visible light or light 
near infrared is used. 
Also, when light near infrared is used for imaging, the signal charge 
generated is drained to a lesser extent compared to the prior art charge 
transfer imaging device. Thus, it is possible to improve the sensitivity 
to light near infrared.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, a first embodiment of the charge transfer imaging device according to 
the invention will be described with reference to FIGS. 1 to 4. 
The charge transfer imaging device illustrated comprises a p-type 
semiconductor substrate 10, on which photosensitive sites 12a to 12f for 
generating and storing a signal charge according to the incident light are 
formed. The photosensitive sites 12a to 12f are n-type semiconductor 
regions. A shift electrode (or transfer gate) 14 is formed over the 
semiconductor substrate 10 in the vicinity of the photosensitive sites 12a 
to 12f via an insulating film 30. This shift electrode 14 is common to all 
the photosensitive sites 12a to 12f. A charge transfer shift register 16 
is formed on the side of the shift electrode 14 opposite the 
photosensitive sites 12a to 12f. The charge transfer shift register 16 
includes a plurality of transfer electrodes 18 and transfer channels 20. 
In this embodiment, the charge transfer shift register consists of unit 
cells each containing four electrodes. For example, a unit cell 
corresponding to the photosensitive site 12d includes electrodes 18d1, 
18d2, 18d3 and 18d4. These electrodes are connected to respective power 
supplies P1, P2, P3 and P4. The electrodes 18d1 to 18d4 are formed over 
the substrate 10 via the insulating film 30. Of the electrodes 18, the 
electrodes 18a1, 18b1, . . . , 18f1 for the respective photosensitive 
sites 12a, 12b, . . . , 12f are formed such that they partly overlap the 
shift electrode 14 so that the charge generated in the photosensitive 
sites 12a to 12f may be transferred through the shift electrode 14 to the 
shift register 16. When the shift electrode 14 is activated or opened, a 
charge stored in, for instance, the photosensitive site 12d is transferred 
to a transfer channel 20d1 extending beneath the transfer electrode 18d1. 
The charge that is transferred in the above way can be subsequently read 
out as electric signal by de-energizing the shift electrode 14 and then 
applying a clock pulse to the transfer electrodes 18 in the shift register 
16. 
In the charge transfer imaging device according to the invention, an n-type 
semiconductor region 24 is formed in the semiconductor substrate 10 under 
each transfer channel 20 of the shift register 16. The semiconductor 
region 24 is formed to extend to a depth of several .mu. to 20.mu. from 
the surface of the semiconductor substrate 10. A reverse bias voltage Vb 
is applied between the semiconductor region 24 and substrate 10 through a 
contact hole 26. The gist of the invention resides in that the n-type 
semiconductor regions 24 are formed under the transfer channels 20 of the 
shift register 16 but not under the photosensitive sites 12a to 12f. That 
is, the n-type semiconductor regions 24 may be formed in any other part of 
the semiconductor substrate 10 as well so long as the condition noted 
above is satisfied. 
Further, p.sup.+ -type channel stoppers 28 are formed to extend in the gaps 
between adjacent ones of the photosensitive sites 12a to 12f on the 
surface of the semiconductor substrate 10 and other necessary portions of 
the device. A light shield film 32 is formed on the insulating film 30 
other than the portions thereof on the photosensitive sites 12a to 12f. 
The semiconductor region 24 is formed through a process as shown in FIGS. 
5A to 5C. As shown in FIG. 5A, an n-type semiconductor layer 42 is formed 
in a given superficial portion of a p-type semiconductor substrate 40 by 
ion implanting, phosphorus for instance. Then, a p-type semiconductor 
layer 44, as shown in FIG. 5B, is epitaxially grown on the p-type 
semiconductor substrate including the n-type semiconductor layer 42. Then, 
an n-type layer 46 connected to the n-type semiconductor layer 42, as 
shown in FIG. 5C, is formed by thermally diffusing an n-type impurity into 
a given superficial portion of the p-type semiconductor layer 44. The 
charge transfer imaging device as shown in FIGS. 1 to 4 is formed in the 
surface of the semiconductor substrate 10 thus formed. 
With the charge transfer imaging device having the construction as 
described above, the photosensitive sites 12a to 12f generate and store 
signal charge according to light incident on them. The signal charge thus 
generated can be read out by applying a clock pulse to the transfer 
electrodes 18 in the shift register 16. According to the invention, 
carriers that may be generated in the semiconductor substrate 10 in the 
vicinity of, for instance, the photosensitive site 12d due to the incident 
of long wavelength light on the site, can be mostly stored in the site due 
to a diffusing effect. This is so because unlike the prior art charge 
transfer imaging device there is no drain region for carriers under the 
photosensitive site 12d. Thus, it is possible to improve the sensitivity 
to long wavelength light. According to the invention, the n-type 
semiconductor region 24, which is reversely biased, is also provided under 
each transfer channel 20 of the shift register 16. When light in the 
ordinary wavelength range or near infrared is incident, some of electrons 
generated in the semiconductor substrate 10 move toward the transfer 
channel 20 due to the diffusion effect. These electrons are captured in 
the semiconductor region 24 and drained. Thus, when detecting highly 
intense light, intrusion of unnecessary electrons into the shift register 
16 can be prevented or greatly reduced to alleviate deterioration of the 
image signal. The charge transfer imaging device according to the 
invention thus is particularly useful in its auto focusing camera 
application where an infrared LED light source used when imaging is done 
in a dark background while a visible light source is used when the 
background is bright. 
Now, a second embodiment of the invention will be described with reference 
to FIG. 6. Each transfer channel 20 of shift register 16 is formed in a 
p.sup.+ -type semiconductor region 50 having a high impurity concentration 
compared to semiconductor substrate 10. For the rest, the embodiment is 
the same as the preceding first embodiment, so like parts are designated 
by like reference numerals and symbols in FIG. 6 and are not described in 
any further detail. Here, the n-type semiconductor region 24 and p.sup.+ 
-type semiconductor region 50 are formed through a process as shown in 
FIGS. 7A and 7B. An n-type semiconductor layer 52 with a thickness of 
several .mu., as shown in FIG. 7A, is formed in a given superficial 
portion of a p-type semiconductor substrate 10 by ion implanting and 
thermally diffusing an n-type impurity. Then, a p-type impurity is ion 
implanted in a superficial portion of the n-type semiconductor layer 52, 
and is then thermally diffused, thereby obtaining the p.sup.+ -type 
semiconductor region 50 and n-type semiconductor region 24, as shown in 
FIG. 7B. The charge transfer imaging device as shown in FIG. 6 is formed 
using the semiconductor substrate 10 which is formed in the above way. 
In the charge transfer imaging device thus formed, each transfer channel 20 
of the shift register 16 is formed in the p.sup.+ -type semiconductor 
region 50 which has a higher impurity concentration than the semiconductor 
substrate 10. Thus, the intrusion of electrons generated in the substrate 
into the shift register 16 can be further reduced by a diffusion potential 
effect that is obtained at the interface between the p-type substrate 10 
and p.sup.+ -type semiconductor region 50. Generally, an effect similar to 
that noted above can be obtained where a p.sup.+ -type semiconductor 
region is present in a route, through which electrons generated in the 
semiconductor substrate 10 are led to the transfer channel 20. 
With the second embodiment of the charge transfer imaging device, it is 
also possible to improve the detection sensitivity when a light source 
emitting long wavelength light is used for imaging and also to prevent 
unnecessary electrons from reaching the transfer channel when a light 
source emitting highly intense light is used. 
While in the previous first and second embodiments the photosensitive sites 
were formed in a one-dimensional arrangement, they may formed in a 
two-dimensional arrangement as well. Further, the transfer channels of the 
shift register 16 may be n-type buried channels. Further, an n-type 
semiconductor may be used as the semiconductor substrate 10. In this case, 
the other semiconductor layers are of correspondingly opposite 
conductivity types.