Compact image sensor

A compact image sensor is disclosed. This sensor comprises an insulating, imperviously sealed package, provided with a window which is transparent to light rays and is located on a front face. The inside of the package contains a stack arrangement which comprises, between the back of the package and the window, an insulating support, a set of photosensitive elements with a semiconducting substrate, surrounded by contacts. Connection terminals go through the back of the package and the insulating support. Connection means are interposed between these contacts and the ends of the terminals. The light rays are received by the photosensitive elements after they have crossed the semiconducting substrate which is thinned down beforehand. This arrangement of the photosensitive elements, the substrate and the connection means enables an appreciable increase in the area of the photosensitive surface and in the number of connection terminals, without an increase in the area of the front surface of the sensor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns a compact image sensor. This sensor can be 
applied notably in endoscopy. In this application, it is clearly necessary 
for the sensor to occupy as little space as possible while, at the same 
time, preserving acceptable electro-optical performance characteristics 
with regard to photosensitive surface, resolution, sensitivity, etc. 
2. Description of the Prior Art 
FIGS. 1 and 2 give a schematic view of a compact image sensor of the prior 
art. FIG. 1 shows a schematic cross-section of this sensor and FIG. 2 
shows a schematic section along AA of the sensor of FIG. 1. The same 
references are repeated for the same elements in FIGS. 1 and 2. 
This sensor has an imperviously sealed package 1, made of an electrically 
insulating material, for example a molded plastic material. This package 
has a front face 2, with a window 3, made of an electrically insulating 
material, which is transparent to light rays R coming from a part of an 
organ of the human body or from an object for which it is sought to obtain 
an image. The material forming the window 3 is made, for example, of glass 
or of a transparent plastic material. It is clear that when this sensor is 
used to obtain images of the inside of a human organ, the inside of this 
organ is illuminated by a light source which is independent of the sensor. 
This sensor also has a rear face 4, forming the back of the package 1. This 
rear face is parallel to the front face. 
Between the back 4 and the window 3, the inside of the package contains a 
stack arrangement comprising an insulating support 5, formed, for example, 
by a ceramic, and a set 6 of photosensitive elements on a semiconducting 
substrate 7, shown in a non-detailed way in the figure. 
This substrate may be, for example, silicon while the photosensitive 
elements form a matrix consisting of photodiodes which may possibly be 
associated with charge-coupled devices or (CCDs). 
As shown more precisely in FIG. 2, the photosensitive elements form a 
surface S bounded by a periphery P having contact elements or contacts 
such as 8. These contacts are connected to the photosensitive elements of 
the set 6, for example by connection tracks such as 9. 
This sensor also has connection means used to connect the contacts 8 with 
connection terminals 10. These connection terminals go through the back 4 
of the package 1. Herein, for each contact 8 and each corresponding 
terminal 10, there s a conducting strap 11 and an intermediate conducting 
part 12. The conducting strap 11 is metallic and flexible: it is made of 
copper for example. This strap is in contact with the contact element 8 
and with the intermediate part 12, which itself is in contact with the 
terminal 10. This intermediate part too may be made of copper. Electrical 
conduction is thus set up between the terminal 10 and the contact 8. The 
various terminals of the sensor provide for the electrical supply to the 
photosensitive elements corresponding to the different contacts. The 
terminals 10 also enable the collection of the electrical signals given by 
the photosensitive elements when the photosensitive surface S is 
illuminated by light rays R. The electrical contacts between each contact 
element 8, strap 11, intermediate part 12 and terminal 10 are provided 
through the mechanical pressure that the package exerts, as it is being 
molded, on the window 3. 
In this type of sensor, the area of the photosensitive surface S amounts to 
less than half of that of the front surface of the package 1, bounded by 
its external rim F. In a package with a front surface area of 16 mm.sup.2 
for example, the photosensitive surface S occupies only 43% of this front 
surface. The remaining part is occupied, to a great extent, by all the 
connection elements (straps 11, intermediate parts 12) connecting the 
contacts 8 to the corresponding terminals 10. In the example considered, 
for a package with a front surface area of 16 mm.sup.2, the number of 
terminals 10 is restricted to six, and it is impossible to increase this 
number without increasing the front surface area of the sensor, precisely 
because of the excessive amount of space that the connection elements take 
up in the package. It is undesirable to increase the front surface area, 
especially for endoscopy. The result of this is that, in the prior art, 
there is no known sensor which, for the type of structure described, and 
for one and the same front surface area, has a photosensitive surface with 
a far greater area and with more connection terminals. This is a major 
drawback, because the performance characteristics of this sensor are 
limited. 
The invention is aimed precisely at overcoming these drawbacks by making a 
compact image sensor wherein, without increasing the area of the front 
surface, it is possible to very substantially increase the area of the 
photosensitive surface as well as the number of connection terminals. 
These aims are achieved, as shall be seen further below in detail, by 
means of a judicious stacking arrangement within the package and through 
the use of far more compact connection elements. 
SUMMARY OF THE INVENTION 
It is an object of the invention provide a compact image sensor, comprising 
an imperviously sealed package made of an electrically insulating 
material, said package having a front face provided with a window made of 
an electrically insulating material which is transparent to light rays, 
and a rear face forming the back of the package, said sensor further 
having, in this package, a stack arrangement comprising, between the back 
of the package and the window, an insulating support, a set of 
photosensitive elements with a semiconducting substrate, said 
photosensitive elements forming a photosensitive surface bounded by a 
periphery, comprising contacts for connection with the photosensitive 
elements and connection means to connect the contacts with connection 
terminals, enabling electrical supply to the photosensitive elements and 
enabling the collection of electrical signals given by these elements when 
they are illuminated, said connection elements going through the back of 
the package, wherein the semiconducting substrate is located so as to face 
the window, the photosensitive elements being located so as to face the 
insulating support and receiving the light rays that have gone through the 
window and the substrate, the connection terminals going through the 
insulating support and coming out, by respective ends, on to a face of 
this support that faces the photosensitive surface and the contacts, the 
connection means being interposed between said ends of the terminals and 
the corresponding contacts. 
According to another characteristic of the invention, the connection means 
comprise spacers made of an electrically conductive material, respectively 
interposed between the ends of the terminals and the corresponding 
contacts, these spacers being in contact with these ends and these 
terminals. 
According to another characteristic, these spacers are indium balls. 
According to another characteristic, said spacers are formed by a 
conductive bonder 
According to another characteristic, the connection means have an 
insulating sheet with zones of conductive material, respectively located 
so as to face said contacts and the corresponding ends of said terminals. 
According to another characteristic, said conducting material is a metal.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The sensor according to the invention, shown schematically in a 
cross-section in FIG. 3, comprises a sealed package 20, made of an 
electrically insulating material, for example a plastic material. This 
package has a front face 21, provided with a window 22, made of an 
electrically insulating and transparent material, such as a plastic 
material or glass. The imperviousness between the package 20 and the 
window 22 is provided by suitable means such as bonding for example. The 
window 22 receives light rays R, coming from a part of the inside of an 
object or an organ of the human body. A rear face 23 of this package, 
parallel to the window 22, forms the back of the package. 
The inside of the package contains, between the back 23 and the window 22, 
a stack arrangement which comprises an insulating support 24, made of 
ceramic for example, and a set of photosensitive elements 25 with a 
semiconducting substrate 26. 
The photosensitive elements form a photosensitive surface S1, which can be 
seen more clearly in the section BB of FIG. 4. This surface is bounded by 
a periphery P1 having contacts such as 27 for connection with the 
photosensitive elements 25. These elements are not shown in detail. They 
may consist of photodiodes, possibly associated with charge-coupled 
devices (CCDs) forming a photosensitive matrix of the surface S1. 
This sensor also has connection means to connect the contacts 27 with 
connection terminals 28, enabling electrical supply to the photosensitive 
elements and enabling the collection of the electrical signals given by 
these elements when they are illuminated by the light rays R, coming from 
inside the object or organ for which it is sought to obtain an image. Of 
course, this picture is obtained by processing the signals collected at 
the terminals 28. The connections 28 go through the back 23 of the package 
20 and, according to the invention, also go through the insulating support 
24. Each end 30 of each terminal emerges from the insulating support and 
reaches that face of this support which faces the photosensitive elements 
25 and the contacts 27. 
According to the invention, the semiconducting substrate 26 (made of 
silicon for example) is placed facing the window 22 while the 
photosensitive elements are placed facing the insulating support 24. In 
the above-described, prior art type of sensor, a reverse arrangement is 
chosen as can be seen in FIG. 1. The semiconducting substrate 7 is facing 
the insulating support 5 while the photosensitive elements 6 are facing 
the window 3. The result thereof is that, in the prior art sensor, the 
light rays R are directly received by the photosensitive elements 6 after 
they have gone through the window 3. By contrast, in the sensor of the 
invention (FIGS. 3 and 4), the light rays R are received by the 
photosensitive elements 25, after they have crossed the window 22 and the 
semiconducting substrate 26, obtained by thinning down the standard 
semiconducting substrate which, herein, has a thickness which is far 
smaller than that of the known sensor (10 to 15 microns instead of several 
tens of microns). It is precisely this reverse arrangement that makes it 
possible to very greatly increase the area of the photosensitive surface 
within the package as can be seen in FIG. 4. For, the contacts 27 are 
located so that they face the back of the package and the corresponding 
connection terminals 28. This makes it possible, as shall be seen further 
below in detail, to reduce the space occupied by the connection means used 
to connect these contacts and these terminals respectively. 
The connection means are, herein, respectively interposed between these 
contacts 27 and the terminals 28 which correspond to them. 
In a first embodiment of the sensor of the invention, as shown in FIGS. 3 
and 4, these connection means comprise spacers 29 made of a conductive 
material These spacers are respectively interposed between the contacts 27 
and the ends 30 of the corresponding terminals 28. Each spacer is in 
contact with the end 30 of the corresponding terminal 28, owing to the 
mechanical pressure exerted by the package on the window 22 which itself 
rests on the substrate 26. These spacers may be formed, in this 
embodiment, by indium balls obtained by growth under vacuum on the 
contacts 27. When the package is molded around the stack arrangement, 
these balls are slightly crushed owing to the mechanical pressure exerted 
by the package on the window, and good electrical contact is thus provided 
between these balls and the ends 30 of the terminals 28. 
These spacers may also be formed by a conductive bonder, such as an epoxy 
resin containing silver for example. 
FIG. 5 gives a schematic view, in perspective, of another embodiment of the 
connection means interposed between the contacts 27 and the ends 30 of the 
terminals 28. In this embodiment, these connection means are formed by an 
insulating sheet 31 which has zones 32 of a conducting material. These 
zones are respectively located so that they face contacts 27 and the ends 
30 of the corresponding terminals 28. The conducting zones are metallic 
(made of copper for example). The insulating sheet 31 may be made of an 
elastomer material. During the assembly of the sensor, this insulating 
sheet 31 is interposed between the set of photosensitive elements 25 and 
the ends 30 of the terminals 28. The pressure exerted by the package on 
the window 22 provides for good contact among the conducting zones 32, the 
ends 30 of the terminals 28 and the contacts 27. By means of the structure 
which has just been described, the area of the surface S1 of the set of 
photosensitive elements can be at least two times smaller than the area of 
the surface S of these elements in the prior art sensor. The same is true 
for the number of connection terminals which is far greater than in the 
prior art sensor. These goals are achieved without modifying the frontal 
surface of the sensor.