Image sensor

An image sensor is disclosed which comprises a plurality of image sensor elements arranged in rows and columns. Each of the image sesnor elements includes a a CCD. In order to provide an image sensor which can be used to image in different image formats, the image sensor includes imaging planes on edge surfaces as well as on a top surface. The top and bottom layers of the sensor are of an increased doping level, and these layers serve to guide charge carriers into CCD's located adjacent the edges of the sensor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image sensor, and more particularly, to 
an image sensor which includes multiple imaging planes. 
2. Description of the Prior Art 
Typical charge-coupled device (CCD) image sensors are formed on flat, 
circular semiconductor substrates known as wafers. Image sensor elements 
are fabricated on either a top or bottom surface of the wafer by means of 
various doping layers of several microns in depth. Upon completion of the 
image sensor elements, the die is cut from the wafer and mounted in a 
package where either the top or bottom surface is exposed for 
illumination. 
In U.S. Pat. No. 4,031,315, there is shown a CCD image sensor which 
comprises image sensor elements arranged in matrix form on one surface of 
the image sensor. In one embodiment, the image sensor can be irradiated on 
a top surface, and in a second embodiment, the substrate body is made 
sufficiently thin that the sensor can be irradiated on a bottom surface. 
There is a problem in using this image sensor in certain applications; for 
example, the sensor cannot be used in apparatus where it is desired to 
simultaneously image two opposing surfaces. In such an application, two 
separate image sensors must be used, and this adds to the expense and 
complexity of the apparatus. 
U.S. Pat. No. 4,665,420, discloses a CCD image sensor which is adapted to 
receive illumination on a top surface. In order to prevent the injection 
of undesirable charge carriers into the CCD registers, the image sensor 
includes a means of passivating the edges of the sensor. The image sensor 
includes a plurality of space detectors arranged in columns extending 
along one of the major surfaces of the sensor substrate. Between the edge 
of the substrate and the adjacent column of detectors, an edge drain is 
provided for receiving any charge carriers generated at the edge in order 
to prevent the charge carriers from being injected into the adjacent 
detectors. Although charge carriers are being collected in this image 
sensor from an edge of the sensor, there is no provision for using the 
charge carriers to record information. Thus, the sensor can only be used 
to image in a single plane. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to overcome the problems in the 
prior art discussed above by providing an image sensor in which 
information can be recorded through an edge surface of the sensor. 
In accordance with one aspect of the present invention, there is provided 
an image sensor comprising: a semiconductor substrate of a first 
conductivity type, the substrate having opposed major surfaces and opposed 
edges between the surfaces, one of the edges having a surface which is 
receptive to light; an image sensor element formed in the substrate and 
located adjacent the one edge, the image sensor element including a charge 
collection and transfer means and an image sensing region in which charge 
carriers are generated by light impinging on the surface of the one edge; 
and means in the substrate for guiding charge carriers in the image 
sensing region toward the charge collection and transfer means. 
In one embodiment of the present invention, an image sensor includes an 
elongated substrate of a P-type material. The image sensor includes an 
imaging plane on the top surface and imaging planes on two edge surfaces 
bordering the top surface. The substrate comprises a bottom layer which is 
highly doped with a P-type material and an upper layer which is more 
lightly doped with a P-type material. Three columns of image sensor 
elements are formed in the substrate. One column is adapted to function 
with the top imaging plane, and the other two columns are adapted to 
function with the two imaging planes on the edge surfaces. Each of the 
image sensor elements includes an image sensing region and a charge 
collection and transfer means which is a buried-channel CCD. The image 
sensor element which functions with the top imaging plane includes a 
photodiode in the image sensing region. 
A P.sup.+ layer is formed in the top surface of the image sensor between 
each edge surface and the column of image sensor elements adjacent to the 
edge surface. The increased doping level in the P.sup.+ layers on the top 
and bottom surfaces gives rise to a barrier to electron movement in the 
direction of the P.sup.+ layers. Thus, charge carriers which are created 
as a result of light energy absorbed through the side edges of the sensor 
are effectively guided back toward the middle of the substrate where they 
will diffuse laterally toward the CCD's located along the two edges. 
A principal advantage of the present invention is that the image sensor can 
be used to image in a plurality of planes. This permits the sensor to be 
used in many image formats such as formats involving the simultaneous 
imaging of two opposing surfaces or the surface imaging of passageways. 
Detection of images through the edges is made possible by means of doped 
layers which create a waveguide-like structure adjacent the edges of the 
sensor. The present invention can be formed using processing techniques 
which are compatible with techniques used in forming other types of image 
sensors, and thus, the invention can be easily incorporated in various 
types of known image sensors. 
Other features and advantages will become apparent upon reference to the 
following description of the preferred embodiment when read in light of 
the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIGS. 1 and 2, there is shown an image sensor 10 
constructed in accordance with the present invention. Image sensor 10 
comprises a P-type substrate 12 having major surfaces 11 and 19 and edge 
surfaces 26 and 28. Substrate 12 includes a highly doped bottom layer 15, 
labeled P.sup.+, and an upper layer 17 which is more lightly doped with a 
P-type material having a high carrier lifetime. Layer 17 can be 
fabricated, for example, by using known epitaxial growth techniques. Image 
sensor elements 14, 16, and 18, are formed in layer 17. Elements 14, 16, 
and 18 are arranged in rows, as shown in FIG. 2, and in columns along the 
length of image sensor 10. A P.sup.+ layer 25 is formed between the image 
sensors 14 and 18 and substrate edges 28 and 26 respectively. 
As will be apparent from the discussion that follows, the thickness of 
layer 17 is important since it determines the effective vertical aperture 
dimension in edges 28 and 26 of the sensor 10. Common thicknesses for 
epitaxial layers range from 1-20 microns, and this range covers the range 
of dimensions needed for most CCD detectors. The carrier (electron) 
lifetime in the P.sup.+ layers is low due to the doping level, and hence, 
these layers will not contribute significantly to the aperture. One 
suitable thickness for layer 17 is about 10 microns. 
Each of the image sensors includes an image sensing region and a charge 
collecting and transfer means. Image sensor element 16 includes photodiode 
20 which functions as the image sensing region and is formed in layer 17 
by an N-type region 21. The charge collecting and transfer means in sensor 
16 is a buried-channel CCD 23 which is also formed by an N-type region 27. 
Image sensor element 16 further includes a storage gate 36, a transfer 
gate 38, and a clock phase terminal 40. 
Image sensors 14 and 18 are generally similar to each other, and their 
charge collecting and transfer means are spaced several microns from the 
edges 26 and 28 of sensor 10. Image sensor element 14 comprises an image 
sensing region 27, a buried-channel CCD 31 formed by an N-type region 33, 
a storage gate 30, a transfer gate 32, and a clock phase terminal 34. 
Image sensor element 18 comprises an image sensing region 29, a 
buried-channel CCD 35 formed by an N-type region 37, a storage gate 42, a 
transfer gate 44, and a clock phase terminal 46. 
As shown in FIG. 1, image sensor 10 has a first imaging plane 50 on edge 
28, a second imaging plane 52 on a top surface of the sensor 10, and a 
third imaging plane 54 on edge 26 of the sensor. Image sensor element 16 
functions in a conventional manner in response to illumination on plane 
52. That is, photons .lambda. impinging on imaging plane 52 will result in 
charge carriers "e" being collected in photodiode 20. When a voltage is 
supplied to transfer gate 38, the charge carriers are transferred to CCD 
23 (see FIG. 3), and the charge carriers are shifted from the CCD 23 in a 
direction perpendicular to the surface to the drawing in FIG. 3 by means 
of clock terminal 40. 
Photons .lambda. impinging on imaging planes 50 and 54 generate charge 
carriers in regions 27 and 29, respectively. The charge carriers are 
guided toward the CCD's 31 and 35 by the P.sup.+ layers 15 and 25 which 
combine to form a type of electron waveguide in substrate 12. There is no 
local depletion region adjacent edges 26 and 28 so the carriers are not 
readily collected. Instead, the carriers are free to diffuse laterally. 
The effect of the P.sup.+ layers in guiding the charge carriers is 
illustrated by the energy band diagram in FIG. 4 in which E.sub.c 
represents the conduction band energy level, E.sub.v is the valence band 
energy level, E.sub.i is the intrinsic energy level, and E.sub.f is the 
Fermi level. As demonstrated in FIG. 4, the increased doping level in 
layers 15 and 25 gives rise to a barrier to electron movement in the 
direction of these layers. Consequently, the carriers are effectively 
guided back, as indicated by arrow 39, toward the middle portion of 
substrate 12 where they will continue to diffuse laterally. Carriers which 
diffuse toward the edges 26 and 28 will be lost to recombination, but 
those which diffuse inward toward the CCD's 31 and 35 will eventually 
encounter the drift field from the storage gate depletion and be collected 
as signal charge. Since there is no patterning on the edges 26 and 28, the 
effective horizontal aperture on the edge surfaces is continuous. Pixel 
separation does not occur until the charge is collected within CCD storage 
regions 61. The probability of collection is highest in the storage region 
nearest the point of photon absorption. Charge carriers which are 
collected adjacent to edges 26 and 28 are transferred to CCD's 31 and 35 
by means of voltages supplied to transfer gates 32 and 44, respectively. 
The carriers are shifted out of the CCD's 31 and 35 in a well-known manner 
by means of voltages supplied to clock phase terminals 34 and 46. 
A second embodiment of the present invention is shown in FIG. 5. Shown 
therein is an image sensor 10' in which elements similar to elements in 
image sensor 10 are identified with the same reference numeral with a 
prime added. Image sensor 10' is generally similar to sensor 10, except 
for CCD's 14' and 18' in which the lightly-doped N-type regions 37' and 
33' have been extended to the edges 26' and 28', respectively. This 
structure effectively increases the lateral extent of the depletion region 
since the N-type region is coupled to the higher potential from the 
storage region. As a result, charge collection is improved and crosstalk 
is reduced. 
With reference to FIG. 6, there is shown a suitable mounting arrangement 
for image sensor 10. In order to avoid the interference of external 
connections on an imaging surface, bond pads 60 are shifted to ends 62 and 
64 of the sensor 10. An opaque coating 66 can be applied on die edges 26 
and 28 to prevent stray light from being absorbed on the die ends. Image 
sensor 10 is mounted on an insulated support block 70, and interconnects 
71 on block 70 are used to make the connections between bond wires 72 and 
leads 74. An imaging lens 80 is indicated schematically for each of the 
imaging planes 50-54. 
It will be apparent that the image sensors 10 and 10' can be used in 
various applications. For example, the sensors could be used to 
simultaneously image a plurality of planes. Further, the concept of 
forming an imaging plane on an edge of a sensor can be used in an image 
sensor having any number of columns of image sensor elements on a top 
surface thereof. The image sensors of the present invention can also be 
used for color applications in which a red filter is placed over the image 
sensor elements in one imaging plane, a green filter covers the elements 
in a second imaging plane, and a blue filter covers the elements in a 
third imaging plane. In the use of a sensor with the color filters and 
suitable optics, a color document could be scanned in a single pass of the 
sensor. 
The invention has been described in detail with particular reference to the 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.