Mosaic of radiation detectors read by a semiconductor device and a picture pickup system comprising a mosaic of this type

Electrical signals delivered by detectors at the surface of a first high-capacitance insulating layer which provides a separation between a first array of metallic electrodes and the detectors are collected by the electrodes by capacitive coupling. The first array of electrodes is electrically connected to a second array of electrodes having smaller dimensions. The two arrays are separated by a second insulating layer and the second array is located at the interface between the second insulating layer and the oxide layer which covers the semiconductor readout substrate.

This invention relates to a mosaic of radiation detectors read by a 
semiconductor device. The invention is also concerned with a picture 
pickup system equipped with a mosaic of this type. 
Mosaics of electromagnetic radiation detectors are already known in the 
prior art. As a general rule, a mosaic of this type consists of an array 
of detectors formed of pyroelectric material or of junctions formed on a 
semiconductor substrate. Another type of mosaic which is also known is 
formed solely of a wafer of pyroelectric material. 
The array of detectors is exposed to electromagnetic radiation such as 
infrared radiation, for example, and an electrical readout or sense signal 
is collected from each detector. Said readout signal can be collected by 
means of an electron beam or by means of a semiconductor device, thus 
providing the advantageous possibility of reducing the bulk of the picture 
pickup system which is equipped with the mosaic. 
The invention relates to a mosaic of detectors which are read by means of a 
semiconductor device. 
The problem presented by this application lies in the electrical coupling 
and mechanical coupling between the detection and readout portions, that 
is, between the mosaic of detectors and the semiconductor readout device. 
It is necessary on the one hand to collect from the readout portion an 
electrical signal corresponding to the signal generated in the detection 
portion by the electromagnetic radiation with a signal-to-noise ratio 
which is as high as possible: this is the problem of electrical coupling. 
It is necessary on the other hand to join the detection and readout 
portions together while reducing thermal leakages between the two portions 
and at the same time permitting thermal expansion of the detection and 
readout substrates which are often different: this is the problem of 
mechanical coupling. 
The present invention provides a simple and effective solution to the 
problems of electrical and mechanical couplings of the detection and 
readout portions. 
In accordance with the invention, a first insulating layer having a high 
capacitance per unit area provides a separation between the mosaic surface 
on which the detectors are formed and the oxide layer which covers the 
semiconductor substrate of the readout device. Finally, a first array of 
metallic electrodes collects by capacitive coupling the electrical signals 
delivered by the detectors at the surface of the first insulating layer on 
the side corresponding to the readout device.

In the different figures, the same references designate the same elements 
but it will be apparent that the dimensions and proportions of the 
different elements have not been observed for reasons of enhanced clarity. 
In the embodiment of a mosaic which is illustrated by way of example in 
FIG. 1, the detectors d are mesa-structure pn junctions formed on a 
semiconductor substrate 3. The detectors can also be constituted by 
Schottky diodes and can have either a mesa or planar structure. The mosaic 
can also be constituted in known manner by a continuous layer of 
semiconductor material in which the detectors are induced in the form of 
potential wells. Said layer is provided with an insulating layer and with 
signal-collecting electrodes which have a special geometry and 
composition. 
The electromagnetic radiation applied to the mosaic of detectors usually 
comes from the side of the substrate opposite to the side which carries 
the detectors, as indicated by the wavy arrow 7. In order to ensure 
practically complete absorption of the electromagnetic radiation in the 
vicinity of the detectors, the substrate of the mosaic is either given a 
small thickness or else chosen so as to be lightly doped and to have a 
well-determined width of forbidden band. 
Thus in the case of detection of infrared radiation, for example, the 
substrate of the mosaic of detectors can be constituted by a semiconductor 
having a narrow forbidden-band width such as lead sulphide PbS, lead 
selenide PbSe or lead telluride PbTe. It is also possible to employ 
ternary compounds such as lead and tin telluride PbSnTe or mercury and 
cadmium telluride HgCdTe. In order to reduce absorption of radiation by 
the substrate of the mosaic, it is possible to employ a substrate 
consisting of two portions, namely a first portion of semiconductor 
material having a narrow forbidden-band width on which the junctions are 
formed, and a second portion covering the first, also of semiconductor 
material and having a higher forbidden-band width. In this case, the 
photons collected are limited to those having an energy range between the 
two forbidden-band widths. Thus, again in the case of infrared radiation 
detection, the detectors can be fabricated from lead and tin telluride 
PbSnTe covered with a layer of lead telluride PbTe. 
It has been noted earlier that the detectors could consist of pyroelectric 
material and that the mosaic of detectors could consist solely of a plate 
of pyroelectric material. In accordance with customary practice, the 
pyroelectric material consists of lithium tantalate LiTaO.sub.3 or 
triglycine sulphate (TGS) and its derivatives. 
The semiconductor readout device is formed on a semiconductor substrate 4 
which is separate and distinct from the substrate provided for the mosaic 
of detectors. Said semiconductor substrate 4 is covered with an oxide 
layer 5. Silicon covered with a layer of silica is usually employed for 
the readout operation. 
The mosaic in accordance with the invention is also capable of operating 
when the electromagnetic radiation arrives at the detectors via the 
semiconductor readout device. 
In accordance with the invention, a first insulating layer 1 having a high 
capacitance per unit area provides a separation between that surface of 
the mosaic on which the detectors d are formed and the oxide layer of the 
semiconductor substrate. 
It is also advantageous to ensure that said first insulating layer ensures 
very low thermal conductance between the detection and readout portions 
and permits matching of the coefficients of expansion of the detection and 
readout substrates which are often different. This is achieved when said 
first insulating layer is a vacuum layer having a thickness of 1 micron, 
for example. In order to ensure matching of the coefficients of expansion 
of the substrates, it is also possible to employ a plastic dielectric such 
as a polymer film. 
A first array of metallic electrodes E collects by capacitive coupling the 
electrical signals delivered by the detectors d at the surface of the 
first insulating layer 1 on the same side as the readout device. 
In the embodiment of the invention which is illustrated in FIG. 1, there is 
shown a second array of metallic electrodes e which is separated from the 
first by a second insulating layer 2 and which is located at the interface 
between the second insulating layer and the oxide layer 5. Each electrode 
of the second array is connected electrically to one electrode of the 
array which is located opposite and has larger dimensions. These two 
arrays of electrodes which are connected to each other serve to reduce the 
surface area of the electrodes e in the readout device which is intended 
to consist of a large number of elements while ensuring that the 
electrodes employed for collecting the signal E have a large surface area. 
The second insulating layer 2 can be formed of the same material as the 
first insulating layer 1. 
In the embodiment shown in FIG. 1, a metallic screen 6 which is brought to 
a fixed potential V.sub.f is inserted in the second insulating layer 2. 
This screen serves to provide electrical insulation of the detection 
portion from the parasitic signals emitted by the readout portion. 
Leads 8 and 9 are usually connected to the detection substrate 3 and to the 
readout substrate 4. As a general rule, these leads serve to apply bias 
voltages V.sub.D and V.sub.L to said substrates. 
FIG. 2 is a top view which illustrates a detail of FIG. 1. There are shown 
in FIG. 2 the position and respective dimensions of the first array of 
electrodes E and of the array of detectors d. The metallic electrodes of 
the first array E can be deposited by evaporation directly on either the 
first or the second insulating layer. The elementary pitch b of said 
electrodes corresponding to the spatial resolution in the picture pickup 
system equipped with the mosaic is greater than the elementary pitch a of 
the detectors d. This configuration does not entail the need for any 
registering of the detectors d with respect to the first array of 
electrodes E which serves to collect the signal from the detectors. The 
operation of the mosaic in accordance with the invention will therefore 
not be disturbed by any displacement of the array of detectors or of the 
first array of electrodes under the action of differential expansion, for 
example. 
FIG. 3 is an electric circuit diagram equivalent to each elementary point 
of the mosaic shown in FIG. 1, namely to the surface of an electrode E of 
the first array. 
This equivalent circuit diagram comprises four capacitances in series 
between a point A and a point B. There are shown in succession: 
a capacitance C.sub.D representing the capacitance of the detectors 
relative to an elementary point of resolution; 
a capacitance C.sub.O representing the capacitance relative to the same 
surface of the first insulating layer 1 which has a high value of 
capacitance; 
the capacitance C.sub.OX representing the capacitance of the oxide layer 5 
which covers the semiconductor substrate 4 of the readout device and 
relative to the surface of one electrode e of the second array; 
the capacitance C.sub.SC representing the depletion capacitance within the 
semiconductor 4 of the readout device and again relative to the surface of 
one electrode e of the second array. 
A direct-current voltage source which delivers the voltage V.sub.D is 
usually connected between the point A and ground. A direct-current voltage 
source which delivers the voltage V.sub.L is usually connected between the 
point B and ground. 
Between the point M which is common to the capacitances C.sub.O and 
C.sub.OX, and ground, there is shown the stray capacitance C.sub.P which 
is caused by the fixed screen and the direct-current voltage source 
V.sub.f which biases said screen. 
Consideration will now be given to the operation of the mosaic according to 
the invention in the case in which the detectors are constituted by 
junctions and then in the case in which the detectors are of pyroelectric 
material (or when the mosaic is constituted by a strip of pyroelectric 
material). 
In the case in which the detectors are constituted by junctions, the 
semiconductor readout device performs the following successive operations 
in the case of each junction: 
during a first step, reverse-biasing of the junction during which 
integration of the radiation received by this latter takes place during 
the integration time interval t.sub.i ; 
during a second step, forward-biasing of the junction during which the 
charges generated optically and stored during the integration time 
interval t.sub.i within the junction are reinjected into the detection 
substrate 3. 
In the case of detection by pyroelectric material, the semiconductor 
readout device carries out the following successive operations: 
during a first step, integration of the radiation received by the 
pyroelectric material whose potential is allowed to remain floating during 
the integration time interval t.sub.i ; 
during a second step, the potential which has previously been allowed to 
remain floating is stabilized at a reference potential. 
If the reference Q.sub.D designates the quantity of charges generated 
optically by the detectors corresponding to an elementary point during the 
integration time interval t.sub.i, the variation in potential induced on 
each electrode E of the first array and therefore at the point M of FIG. 3 
is written: 
##EQU1## 
with C=C.sub.p +1/(1/C.sub.OX +1/C.sub.SC). 
In practice, C.sub.D varies from 30 to 1000 nF/cm.sup.2 in the case of 
junction detectors and from 0.1 to 10 nF/cm.sup.2 in the case of 
pyroelectric detectors and C.sub.O is typically 1 nF/cm.sup.2, which 
corresponds to approximately 1 micron of vacuum, whilst C.sub.OX is of the 
order of 30 nF/cm.sup.2, which corresponds to 1200 A of silica and 
C.sub.SC is of the order of 5 nF/cm.sup.2. 
In the case of values of C.sub.D which are higher than 30 nF/cm.sup.2 we 
have approximately: 
##EQU2## 
When carrying out a voltage readout by measuring .DELTA.V, it is therefore 
found advantageous to ensure that the capacitance C.sub.O is as high as 
possible and consequently to ensure that the first insulating layer 1 has 
a very high capacitance per unit area. 
During the second step, the quantity of charges Q.sub.D is reinjected into 
the detection substrate 3. The quantity of charges which then circulates 
within the external circuit or in other words between points A and B and 
ground is written: 
##EQU3## 
The quantity of charges Q.sub.ext is therefore much smaller than the 
photo-charge Q.sub.D. Hence it is also advantageous to have a very high 
capacitance C.sub.O when carrying out a current readout by measuring 
Q.sub.ext. 
It was noted earlier that the operation of mosaics in accordance with the 
invention takes place in two steps. If consideration is given to the 
example of junction detectors, the junctions are reverse-biased during the 
first step and are forward-biased during the second step. 
Transition of the different elementary detection points from the first to 
the second step and conversely can take place either sequentially or in 
parallel, that is, at the same time in the case of all detection points. 
FIGS. 4a and 4b show one embodiment of a mosaic in accordance with the 
invention and a diagram which serves to explain the operation of this 
latter. 
In the case of FIG. 4a, transition from the first to the second step and 
conversely takes place sequentially by displacement of a quantity of 
charges at the interface between the read substrate 4 and the oxide layer 
5 which covers this latter, within a CCD register which is integrated in 
the read substrate and the oxide layer which covers said substrate. 
In FIG. 4a, the addressing of a single line of the mosaic is shown in a 
transverse sectional view. In order to simplify the figures, the detection 
portion is represented schematically by photodiodes d. 
A CCD register is therefore integrated in the readout substrate 4 and in 
the oxide layer 5 which covers said substrate. 
The CCD register can be of the surface transfer type or of the volume 
transfer type and can have two or more phases. Similarly, the creation of 
potential barriers can be effected by ion implantation or by differences 
in oxide level. 
In the example illustrated in FIG. 4a, the CCD register has two phases 
.phi..sub.1 and .phi..sub.2 and the potential barriers are formed by 
differences in oxide level. 
The CCD register shown in FIG. 4a is provided in known manner with two 
types of electrodes which rest either on the oxide layer 5 or on an 
additional oxide layer 10. The electrodes e of the second array constitute 
one electrode out of two of the electrodes which rest on the oxide layer 
5. Among the electrodes which rest on the additional layer 10, one 
electrode out of two is connected to a potential .phi..sub.1 and the other 
electrodes are connected to a potential .phi..sub.2. The electrodes which 
rest on the oxide layer 5 and are not electrodes e of the second array are 
also connected to the potential .phi..sub.2. Depending on the direction of 
charge transfer indicated by an arrow, these electrodes follow the 
electrodes located on the layer 10 and connected to .phi..sub.2. The 
potentials .phi..sub.1 and .phi..sub.2 which are shown in FIGS. 5a and 5b 
are clock signals having the same period T and substantially in phase 
opposition with a slight overlap at the top level as is customary practice 
in the field of CCDs. 
The electrodes e are periodically connected (in a manner not shown in the 
figure) to a constant potential which establishes beneath these electrodes 
a surface potential of intermediate value between those corresponding to 
the bottom level and to the top level of the electrode of the following 
register. 
A quantity of charges Q is introduced at the input of the CCD register with 
a period of the order of the integration time interval t.sub.i and 
considerably longer than T in accordance with one of the well-known 
techniques of charge introduction in a CCD, such as the 
potential-balancing technique, for example. This quantity of charges is 
then transferred within the CCD from one point to the next in known 
manner. The surface potential .phi..sub.S within the substrate 4 is shown 
in FIGS. 4a and 4b when the potential .phi..sub.1 is at the top level and 
then when the potential .phi..sub.1 is at the bottom level. 
The detection and readout substrates being continuously brought to constant 
potentials V.sub.D and V.sub.L, and when the quantity of charges Q arrives 
beneath an electrode of the second array e corresponding to an elementary 
detection point, said quantity causes the charges previously stored at the 
interface between the detectors and the high-capacitance insulating layer 
1 to be reinjected in the detection substrate 3 by capacitive coupling and 
to recombine within said substrate. When it arrives, the charge Q 
therefore produces forward-biasing of the detectors. 
On the contrary, when said charge Q again leaves an electrode e, it causes 
reverse-biasing of the detectors. 
It is worthy of note that this mode of transition from the first to the 
second step and conversely by means of an integrated CCD register can be 
employed only in the case of detectors having pn junctions, Schottky 
junctions or induced junctions. In the case of pyroelectric detectors 
which make use of insulating materials such that only bound charges exist 
in said materials and no free charge is present, this mode of transition 
from one step to the other is not applicable. 
Consideration will now be given by way of example to a number of different 
modes of reading of the mosaic shown in FIG. 4a. 
FIGS. 6a, 6b and 7 show one mode of reading of said mosaic, or so-called 
current readout. 
As has already been noted in the description relating to FIG. 3, this mode 
of reading consists in integrating the current which flows within the 
detection substrate or readout substrate supply circuit, that is to say 
for example between point A and ground, between the end of the first step 
and the beginning of the following first step, and therefore in the case 
of FIG. 4a between the instant preceding the arrival of the addressing 
charge beneath an electrode e and the instant followings its departure. 
As shown in FIGS. 6a and 6b, there is employed for this purpose an 
operational amplifier A.sub.p mounted as an integrator with a capacitor 
C.sub.R. The positive input of said amplifier is connected to the voltage 
of the detection substrate V.sub.D. Its negative input is connected to the 
detection substrate 3. 
In FIG. 6a, the capacitor C.sub.R is short-circuited after each current 
readout by an MOS transistor T.sub.1 which is mounted in parallel with 
said capacitor. A periodic signal V.sub.RAZ which is shown in FIG. 5c is 
applied to the gate of said transistor T.sub.1. The signal V.sub.RAZ has a 
period equal to T and is present at the top level whereas .phi..sub.1 is 
at the bottom level. The disadvantage of the embodiment shown in FIG. 6a 
lies in the fact that the transistor T.sub.1 constitutes a source of noise 
which may interfere with current readout. 
In the embodiment shown in FIG. 6b, the source of noise mentioned above is 
suppressed. In this embodiment, a transistor T.sub.2 controlled by the 
signal V.sub.RAZ is mounted between a point D and a constant voltage 
V.sub.C, this point D being separated from the output of the operational 
amplifier by a coupling capacitor C.sub.L. Thus, after each current 
readout, the output of the readout device is brought to a continuous 
reference level. 
In the case of FIGS. 6a and 6b, the output of the readout device can be 
followed by a conventional sampling and holding circuit which makes it 
possible to obtain a continuous video signal at the output. 
FIG. 7 illustrates an improvement which can be made in the mode of current 
readout illustrated in FIGS. 6a and 6b. Each electrode of the second array 
e can be brought periodically to a reference potential V.sub.R by MOS 
transistors T.sub.31, T.sub.32, T.sub.33 . . . which are connected on the 
one hand to an electrode e and on the other hand to the voltage V.sub.R 
and to which is applied a periodic signal having a period T. This periodic 
signal triggers the transistors T.sub.31, T.sub.32, T.sub.33 into 
conduction when the phase .phi..sub.2 is at the top level or in other 
words during the first step in which integration of the photo-charges 
takes place in the detectors. The signal V.sub.RAZ can be employed for 
this purpose. Since the potential of the electrodes e is substantially 
constant during the integration time interval t.sub.i, it is thus possible 
to store a greater quantity of charges in respect of the same initial bias 
voltage applied to the detectors while preventing saturation; the 
integration time interval t.sub.i can thus be increased. 
The MOS transistors employed in the readout device can be integrated in the 
semiconductor readout substrate 4 which is already employed for the CCD 
register. 
FIG. 8 illustrates a mode of reading known as voltage readout of the mosaic 
which is shown in FIG. 4a. 
Each output of an address register having multiple outputs R is connected 
to the gate of an MOS transistor T.sub.41, T.sub.42, T.sub.43 . . . . Each 
MOS transistor is connected between one electrode of the second array e 
and an output preamplifier 9. The address register R and the transistors 
T.sub.41, T.sub.42, T.sub.43 . . . can be integrated in the readout 
substrate 4. When the address register addresses one of its outputs, there 
is obtained at the output of the preamplifier 9 the voltage on the 
electrode e to which said input is connected via an MOS transistor. 
It is possible to measure the difference in voltages on each electrode e 
between the beginning of the first step and the end of the first step or 
in other words during the integration time interval t.sub.i. In this case, 
the address register R having multiple outputs initiates conduction of the 
MOS transistor which is connected to the electrode e to be read between 
the instant following departure of the quantity of addressing charges and 
the instant preceding transfer of the following quantity of addressing 
charges. A sampling and holding circuit is usually connected to the 
preamplifier 9. The integration time interval t.sub.i can be adjusted by 
varying the instant of addressing by the register R of the electrode e 
which is read. 
It is also possible to measure the difference in voltages on each electrode 
e between the end of the first step and the beginning of the following 
first step. In this case also, the address register having multiple 
outputs R must initiate conduction of the MOS transistor which is 
connected to the electrode e to be read between the instant preceding 
transfer of the quantity of addressing charges and the instant which 
follows departure of this quantity of charges. In this case, the voltage 
readout time is distinctly shorter than the integration time interval 
t.sub.i, thus permitting the advantageous possibility of reducing the 
influence of noise, low-frequency variations or drift. 
An MOS transistor T.sub.S connected between the input of the preamplifier 9 
and the reference voltage V.sub.R makes it possible to bring each 
electrode e to the reference voltage V.sub.R immediately after reading of 
said electrode. 
FIG. 9 shows another embodiment of a mosaic in accordance with the 
invention. 
The transition from the first to the second step and conversely takes place 
sequentially and no longer through a CCD register but simply through an 
address register having multiple outputs R. Each output is connected to 
the gate of an MOS transistor T.sub.41, T.sub.42, T.sub.43 . . . which is 
connected between one of the electrodes e of the second array and a common 
point R.sub.o to which a pulse i is applied periodically via a capacitor 
C.sub.i, thus initiating changeover from one step to the next of the 
detectors which receive said pulse. 
The detection substrate 3 and the readout substrate 4 are brought 
continuously to constant potentials V.sub.D and V.sub.L. 
Current readout of the electrodes e can be performed by employing an 
operational amplifier A.sub.p mounted as an integrator with a capacitor 
C.sub.R, as in the case of FIGS. 6a and 6b. 
Current readout may be carried out as in the case of FIGS. 6a and 6b by 
integrating the current which flows within the bias circuit of the 
detection substrate between the end of the first step and the beginning of 
the following first step. It is also possible to integrate the development 
of current flow within the bias circuit which is connected to the 
electrodes, that is, to the output of the amplifier A.sub.p. 
Voltage readout can also be performed in the manner explained in the 
description relating to FIG. 8. The readout device thus provided is more 
simple and the device obtained in the case in which the transition from 
one state to another takes place by means of a CCD register but does not 
make it possible to obtain an adjustable integration time interval t.sub.i 
as was the case with the CCD register. 
The transition from the first to the second step and conversely can also 
take place in parallel or in other words at the same time in the case of 
all the detectors. This result can be obtained by modifying the bias of 
the detection and readout substrates or by modifying the bias of the 
electrodes e of the second array which are connected in parallel. In the 
case of pyroelectric detectors, it is possible to employ only variable 
biasing of the electrodes e of the second array. FIG. 10 shows another 
embodiment of a mosaic in accordance with the invention in which the 
transition from one step to the other takes place in parallel. There is 
employed in this case a CCD register R' which carries out a lateral 
transfer in the direction indicated by an arrow in the figure, of the 
quantities of charges corresponding to the readout performed at the same 
instant on the electrodes e. This CCD register therefore carries out a 
multiplexed readout of the detectors of one line of the mosaic. A 
preamplifier 10 is connected to this register. The register R' and the 
preamplifier 10 can be integrated in the readout substrate. Between each 
electrode e of the second array and one input of the CCD register is 
interposed a device f which carries out conversion of the voltage on each 
electrode e to charges which can be introduced into the CCD. In fact, 
during the integration time interval t.sub.i, the electrodes e are 
floating or in other words are not connected to any bias voltage. Their 
potential therefore undergoes a progressive variation and it is the 
measurement of this progressive variation in the voltage on the electrodes 
e during the integration time interval which constitutes the readout 
signal of the mosaic. Conversion of the voltage on the electrodes e to 
charges within the CCD register can be carried out in different known ways 
and especially by the method of balancing of potentials which is described 
in the IEEE Journal of Solid State Circuits, volume SC 10, No 2, April 
1975.