Sensor matrix

A matrix of light-sensitive or x-ray sensitive sensors (S.sub.1,1, . . . S.sub.2000,200) are arranged in rows and columns and generate charges in dependence on the amount of incident radiation. The sensors comprise a respective electric switch (3) and are constructed, like the electric switches (3), of thin-film technology. Each sensor row has a switching line (5, 6, . . . , 7) via which the switches can be activated so that the charges of the relevant activated sensor row are simultaneously output via read lines (8, 9, . . . , 10, . . . ). Transfer means convert the signals read in parallel into a serial output signal; in order to achieve an as favorable as possible noise behavior. An amplifier (11, 12, . . . , 13) is in each read line and is constructed as a silicon crystal and precedes the transfer means. During the read operations, the amplifiers simplify the signals read from the sensors (S.sub.1.1, . . . S.sub.2000,2000) connected to the relevant read line (8, 9, . . . , 10, . . . ).

FIELD OF THE INVENTION 
The invention relates to a device comprising light-sensitive or X-ray 
sensitive sensors which are arranged in rows and columns in a matrix and 
which generate charges in dependence on the incident amount of radiation, 
each of said sensors comprising an electric switch and being constructed, 
like the electric switches, using a thin-film technique, for each sensor 
row there being provided a switching line via which the switches can be 
activated so that the charges of the relevant activated sensor row are 
simultaneously output via read lines, and also comprising transfer means 
for converting the signals read in parallel into a serial signal. 
BACKGROUND OF THE INVENTION 
Of interest is commonly owned copending application Ser. No. 644,712 
entitled "Sensor Matrix" by the present inventor and filed concurrently 
herewith. 
A device of this kind is known from European Patent Specification 0 028 960 
which corresponds to U.S. Pat. No. 4,382,187. In the known device the 
charges of the sensors of a row are simultaneously read. To this end, 
there is provided a circuit for the sensors which activates the electric 
switches of the sensors so that their charges can be output via a read 
line provided for each column. The charges of a row are thus 
simultaneously read, i.e. in parallel. The charges thus read in parallel 
are applied to transfer means which convert the parallel signals into a 
serial signal. To this end, according to the cited European Patent 
Specification a common multiplexer or a common shift register is therefore 
provided for all read lines. 
Notably for X-ray applications only very small X-ray doses are incident on 
the sensors. Consequently, the electric charge generated in the sensor 
elements in dependence on the amount of incident of radiation, is also 
very small. Due to these very small charges to be read, problems are often 
encountered, i.e. a comparatively strong noise is superposed on the signal 
read. In order to counteract this problem, the cited European Patent 
Specification proposes a respective amplifier for each sensor. This 
amplifier amplifies the charges generated in the sensor, the charges 
subsequently being readable in amplified form, via an electric switch and 
the associated read line. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved device of the kind 
set forth. 
This object is achieved in accordance with the invention in that in each 
read line there is provided an amplifier which is composed of crystalline 
semiconductors and which precedes the transfer means, which amplifier 
amplifies, during the read operations, the signals read from the sensors 
connected to the relevant read line. 
In sensor matrices of the kind set forth several problems are encountered 
which should be mitigated as concurrently as possible. 
Notably for X-ray applications, where radiation doses are to be minimized, 
problems are encountered as regards sensor sensitivity. These problems 
arise because the individual sensor must have an as large as possible 
sensitive surface area for an as large as possible radiation sensitivity. 
Moreover, for each sensor there should be provided not only a photosensor 
element, but also a capacitance which stores the charge as well as an 
electric switch which is activated when the charge is read. When an 
individual amplifier is provided for each sensor as in the state of the 
art, the individual sensor element becomes complex and the 
radiation-sensitive surface area of the sensor is further reduced. The 
other possibility, i.e. arranging the amplifier and the actual sensor one 
over the other in a thin-film substrate, is also problematic because in 
that case, like in the case of juxtaposed arrangement on the substrate, a 
higher rejection percentage is to be expected rejects of the substrates 
during manufacture, because the complexity of the substrate is seriously 
increased by the presence of the amplifiers. For example, when a sensor 
matrix comprising 2000 .times.2000 elements is to be formed, an acceptable 
rejection percentage during such manufacture would be impossible to 
achieve by means of state of the art manufacturing techniques if an 
additional amplifier were provided for each sensor element. 
In accordance with European Patent Specification 0 028 960 the amplifiers 
are formed, together with the sensors, on a thin-film substrate. The 
amplifiers are thus also constructed according to the thin-film technique. 
However, because the amplifiers should be as noise-free and as sensitive 
as possible for the reasons described above, these requirements cannot be 
satisfactorily met by the described device, because thin-film amplifiers 
are problematic in this respect. 
The present invention is based on the recognition of the fact that in 
practice the described noise and manufacturing problems cannot be overcome 
by means of a device as disclosed in European Patent Specification 0 028 
960. Therefore, it is proposed to provide only one amplifier in each read 
line of the matrix, which amplifier serves to amplify the signals read 
from all sensors of the relevant column. Thus, instead of providing an 
amplifier for each sensor, a common amplifier is provided for each sensor 
column. This amplifier need no longer be manufactured according to the 
thin-film technique, but is preferably manufactured using the conventinal 
semiconductor crystal technique where, for example silicon can be used as 
the semiconductor material. As regards noise behavior and sensitivity such 
amplifiers exhibit substantially better properties than thin-film 
amplifiers. When such amplifiers in conventional silicon technique are 
used, the device offers a further essential advantage in that the sensor 
matrix or the individual sensors have a simple structure which, from a 
manufacturing-technical point of view, is substantially easier to realize, 
than the structure in accordance with the state of the art. 
These advantages are obtained by combination of the actual sensor matrix, 
manufactured according to the thin-film technique, and the amplifier which 
is manufactured according to the conventional silicon technique and only 
one of which is provided for each read line. 
In one embodiment of the invention, the sensors of each column are 
connected in groups of approximately equal numbers to the various read 
lines of the column, an amplifier being provided in each read line. 
As has already been described, for optimum evaluation, the signal read 
should be situated as well as possible above the noise level of the 
overall device. The noise on the read lines, however, is due notably to 
the lines themselves and to the electric switches of the individual 
sensors connected to the lines, which switches have capacitances, like the 
read lines, which have a negative effect on noise. In order to reduce the 
noise, several read lines may be provided for each column. For example, 
when three read lines are provided, each time one third of the sensor 
elements is connected to an individual read line. Thus, the capacitance 
per read line is also approximately one third of that involved in the 
solution with only one read line per column. However, when several read 
lines are provided for each column, an amplifier must be provided in each 
read line. 
In a very attractive embodiment in accordance with the invention, two read 
lines are provided for each column, half the sensors of each column being 
connected to one read line, the other half of the sensors of the column 
being connected to the other read line. In this configuration, one read 
line for each column can be routed to one side of the substrate, the other 
read line being routed to the other side of the substrate. In this case it 
is not necessary to arrange two read lines one adjacent to the other on 
the substrate, and the length of the read lines is optimized. 
In a further embodiment of the invention, the transfer means are provided 
with several analog multiplexers which are connected to a respective part 
of the read lines and which convert the read signals simultaneously 
occurring on the read lines into serial signals. 
For the conversion of the signals read in parallel into one or more serial 
signals there are provided analog multiplexers which are connected to a 
respective part of the read lines. The read signals occurring on these 
read lines are then converted into a serial signal by the analog 
multiplexer. 
In a further embodiment in accordance with the invention, each analog 
multiplexer is succeeded by an A/D converter, each A/D converter being 
succeeded by a microprocessor or signal processor, in each case two 
microprocessors which process the signals from neighboring sensors are 
connected to a common memory. 
The various analog multiplexers are succeeded by a respective A/D converter 
in which the serial output signal of the analog multiplexer is converted 
into a digital signal. The digital output signals of the A/D converters 
are further processed by microprocessors. In each case, two 
microprocessors, processing signals from neighboring sensors, access a 
common memory. 
At the level of the analog multiplexers the signals read in parallel are 
thus converted into several serial signals. These serial signals are 
further processed in parallel again. When a sensor matrix comprises, for 
example 2,000.times.2,000 separate sensors, an analog multiplexer is 
capable of processing 256 columns of the matrix. Each analog multiplexer 
then converts 256 parallel signals into one serial signal. In that case 8 
analog multiplexers are required, so that at the level of the A/D 
converters and the microprocessors 8 serial signals are processed in 
parallel. Such an approach where the parallel signals are converted into 
serial signals quasi within individual signal groups and the serial 
signals thus produced are further processed in parallel again, is 
particularly attractive because the large data volume involved can only be 
handled in this manner. In the case of a signal read rate of, for example 
25 Hz for a complete image, i.e. for all sensors of the matrix, only an 
extremely short period of time is available for the processing of the 
signals during which serial processing of the signals by a single 
microprocessor is not possible. Therefore, the signals are combined in 
sub-groups in accordance with the invention, after which they are further 
processed in parallel. 
Subsequent to this parallel processing, in a final stage conversion into a 
common overall video signal containing the signals from all sensors of the 
matrix takes place. To this end, in a further embodiment of the invention 
the sensor signals processed by the microprocessors are combined so as to 
form an overall video signal in a digital multiplexer succeeding the 
microprocessors. 
In a further embodiment of the invention, correction of the signals which 
are supplied by the individual sensors and which are subsequently 
amplified is performed through the microprocessors and their accessing of 
memories which are common to neighboring sensors, so that compensation is 
made for differences in sensitivity of the sensors, failure of individual 
sensors, or differences between gain factors of the amplifiers. 
Because two microprocessors which process signals from neighboring sensors 
in the matrix can access a common memory, it is possible for each 
microprocessor to compare the signals supplied by its associated sensors 
with those of neighboring sensors. A deviating sensitivity of an 
individual sensor as well as that of a complete sensor group, being due to 
a different gain factor of the associated amplifier, can thus be 
determined and corrected by means of the microprocessors. In the case of a 
breakdown of a sensor or a failure of a sensor to supply a useful signal, 
the microprocessor can replace its signal by a calculated signal which may 
result from, for example the mean value of the signals of the neighboring 
sensors. 
In a further embodiment of the invention, in response to a corresponding 
control signal the microprocessors combine each time the signals of 
several sensors which adjoin one another in the column direction and/or 
the row direction in order to form one signal. 
Should the radiation incident on the sensors have a particularly low 
intensity, noise problems could still occur, despite the improved noise 
behavior of the device in accordance with the invention, because the 
signals of the individual sensor elements are too close to the noise 
level. In that case the signals of neighboring sensors can be combined, 
i.e. added by the microprocessors. A stronger signal having an improved 
signal-to-noise ratio is then obtained. For given applications such an 
improved noise behavior may be more important than the reduction of the 
matrix resolution due to the combination of signals of neighboring 
sensors. 
Notably in an X-ray examination apparatus the device in accordance with the 
invention can be advantageously used because a particularly low X-ray dose 
is desirable for X-ray examinations, optimum benefit then being derived 
from the favorable noise behavior of the device in accordance with the 
invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a part of a device 54 which is used in connection with x-ray 
apparatus 50 which generates x-ray radiation 52, the radiation passing 
through an object 53 being examined. The device 54 comprises a matrix 
which is only partly shown. The matrix consists of sensors which are 
arranged in rows and columns in the matrix. Such a matrix may comprise, 
for example 2,000.times.2,000 sensors, only a few of which are indicated 
in the Figure. 
In the first row of the matrix shown in the Figure there are indicated 
sensors S.sub.1,1 and S.sub.1,2. In the same row, the sensor S.sub.1,2 is 
succeeded by sensors which are not shown. However, the Figure shows the 
sensor S.sub.1,128 which is the 128th sensor of the first row. In this row 
there are provided further sensor elements, up to approximately 2,000, 
which are not shown in the Figure. 
The same is applicable to the second row of the matrix; the Figure shows 
the sensors S.sub.2,1, S.sub.2,2 and S.sub.2,128 of this second row. 
However, this second row again comprises a total of approximately 2,000 
sensor elements. 
These first two rows of sensors are followed by further rows which are not 
shown in the Figure. The Figure merely shows the last row, i.e. row 2,000. 
The first sensor in this row is referred to as S.sub.2000,1, the second 
sensor bearing the reference S.sub.2000,2. Like in the rows 1 and 2, the 
Figure does not show further sensors, except for the sensor 
S.sub.2000,128. The first sensors of each row together constitute the 
first column, the second sensors of each row constituting the second 
column etc. 
Each of the sensors, a few of which are shown in FIG. 1, comprises a 
photosensor element. When suitable semiconductors are used, this 
photosensor element itself may already be sensitive to X-rays. However, it 
may also be a light-sensitive photodiode which receives light when X-rays 
are incident on a phosphor layer provided on the photodiode. In the Figure 
the photosensor elements are indicated as photodiodes I. Furthermore, each 
sensor has a storage capacitance 2. The anode of the photodiode 1 and an 
electrode of the storage capacitance 2 are both connected to a direct 
voltage source 4 providing negative direct bias. The cathode of the 
photodiode I and the other electrode of the storage capacitance 2 are both 
connected to a source terminal of a switching field effect transistor 3. 
All sensors of the matrix comprise a respective photodiode 1, a storage 
capacitance 2 and a field effect transistor 3 and are manufactured 
entirely by means of the thin-film technique. 
When radiation is incident on the photodiode 1, the photodiode becomes 
conductive and, due to the bias introduced by the direct voltage source 4, 
charge is transferred to the storage capacitance 2, the amount of charge 
being dependent on the intensity of the radiation incident on the 
photodiode 1. The charge stored in the capacitance 2 after a given period 
of time, therefore, is a measure of the radiation intensity. This charge 
can be individually read for each sensor element via the switching 
transistors 3. 
To this end, for each row of the sensor matrix there is provided a 
switching line. In the representation of FIG. 1, a switching line 5 is 
provided for the first line, a switching line 6 being provided for the 
second row and a switching line 7 for the row 2000. These switching lines 
are connected to the gate terminals of the field effect transistors 3 in 
the sensors. A switching line thus activates the transistors 3 of the 
associated row. For example, the switching line 5 activates all 
transistors 3 of the first row of the matrix. 
The switching lines 5, 6, 7 and the further switching lines which are not 
shown in the Figure can be controlled by means of a digital decoder 30. 
The digital decoder 30 serves to activate the rows of the sensor matrix 
successively during a read operation for the charges stored in the 
sensors. This is realized so that, for example first the switching line 5 
for the first row is activated so that the transistors thereof become 
conductive, subsequently the switching line 6 being activated in order to 
activate the transistors 3 of the second row, and so on until the row 2000 
is reached. The digital decoder 30 itself is controlled via a control line 
31. This can be realized, for example by means of a microprocessor which 
is not shown in the Figure and which provides overall control for the read 
operation. 
For each column of the matrix which is partly shown in FIG. 1 there is 
provided a respective read line. For example, the first column, of which 
only the sensors S.sub.1,1, S.sub.2,1, and S.sub.2000,1, are indicated in 
the Figure, comprises a read line 8. Similarly, the second column 
comprises a read line 9, and the column 128 indicated in the Figure 
comprises a read line 10. The columns which are not shown in the Figure 
also comprise a respective read line. The read lines are connected 
together to the drain terminals of the field effect transistors 3 of the 
associated column. For example, the read line 8 of the first column is 
connected to the drain terminals of the field effect transistors 3 of all 
sensors arranged in this column. 
In each read line, only the read lines 8, 9 and 10 being indicated in the 
Figure, there is provided an amplifier. In FIG. 1, an amplifier 11 is 
provided in the read line 8, an amplifier 12 being provided in the read 
line 9 while a amplifier 13 is provided in the read line 10. The 
amplifiers are each arranged in the read line so that they amplify the 
charges emanating from the individual sensors. The amplifiers precede an 
analog multiplexer 14 whose inputs are connected to the outputs of the 
amplifiers. The amplifiers are connected as current integrators and, with 
their associated analog multiplexer, are manufactured as an integrated 
circuit using the conventional silicon crystal technique. 
For example, when the first row is read, the field effect transistors 3 of 
the sensors present in this row are activated via the switching line 5. 
The charges stored in the capacitances 2 of the sensors present in this 
row are then output via the field effect transistor 3 of the relevant 
sensor and via the read line. Thus, in this case all sensors of this row 
are simultaneously activated and the charges stored in the sensors are 
output simultaneously via the read lines. For the sensors indicated in the 
Figure this means that charges reach the analog multiplexer 14 via the 
read lines 8, 9 and 10 and the subsequent amplifiers 11, 12 and 13. In the 
analog multiplexer 14 the charges, arriving simultaneously and in 
parallel, are converted into a serial signal which is available on a 
serial output 15 of the multilexer. The multiplexer 14 is controllable via 
a control line 16 which can be controlled, for example like the digital 
decoder 30, by means of an external microprocessor which is not shown in 
the Figure. 
Because the Figure shows only a comparatively small part of the matrix 
comprising a total of 2000.times.2000 sensors, the further circuitry of 
the analog multiplexer 14 is not completely shown either. A total of 128 
read lines are connected to the analog multiplexer 14, the signals of the 
read lines being converted by the multiplexer so as to form one serial 
output signal which is present on the output 15. Because the matrix 
comprises a total of 2000 columns, 16 of such analog multiplexers are 
required, but the Figure shows only one multiplexer. 
During a read operation for the sensor matrix, the rows are successively 
activated, upon activation of a row the charges of all sensors present in 
the relevant row being output. Subsequently, the next row is activated, so 
that again the charges stored in the sensors of this row are output. This 
operation is repeated until row 2000 is reached. During each read 
operation the sensor signals arriving in parallel are converted into a 
serial signal by the analog multiplexer. Because of the multitude of 
sensors, this is done in groups of 128 sensors. Because 16 analog 
multiplexers are provided in total, 16 serial signals are thus obtained, 
each of the signals representing the signals of 128 sensors of a row. 
FIG. 2 shows further transfer means associated with the device shown in 
FIG. 1. FIG. 2 shows the amplifiers 11, 12 and 13 and the analog 
multiplexer 14 according to FIG. 1. However, because 2000 sensor elements 
are provided per row as stated above, a total of 16 analog multiplexers 
are included in the transfer means. Of these multiplexers FIG. 2 shows 
only the multiplexer 14, a further multiplexer 17 and final a multiplexer 
18. Each of these analog multiplexers 17 and 18 as well as the further 
analog multiplexers which are not shown in the Figure comprises, like the 
multiplexer 14, 128 inputs which are preceded by amplifiers whose inputs 
are connected, via the read lines, to the relevant column. 
Each of the analog multiplexers is succeeded by a respective 
analog-to-digital converter. FIG. 2 shows an analog-to-digital converter 
19 which succeeds the multiplexer 14, as well as an analog-to-digital 
converter 20 which succeeds the analog multiplexer 17. The same structure 
is provided the further analog multiplexers which are not shown in the 
Figure. The Figure shows the multiplexer 18 as the last multiplexer which 
is succeeded by an A/D converter 21. 
The A/D converter 19 is succeeded by a microprocessor 22, the A/D converter 
20 by a microprocessor 23, and the A/D converter 21 by a microprocessor 
24. The other A/D converters which are not shown in the Figure are also 
succeeded by a respective microprocessor. The microprocessors 22 and 23 
access a common memory 25. The microprocessor 23 also accesses a memory 26 
which is also accessed, in a manner not shown in the Figure, by a further 
microprocessor. The microprocessor 24 also accesses two memories, of which 
only the memory 27 is shown in the Figure. 
The microprocessors serve to process the digital signals which are supplied 
by the A/D converters and which originate from the signals read in the 
sensors. For example, different sensitivities of the sensors, different 
sensitivities of the amplifiers or other errors can thus be compensated 
for. This is possible because each of the microprocessors can access a 
memory in which not only the signals of its associated sensors are stored, 
but also the signals of the neighboring sensors which are processed per se 
by another microprocessor. For example, the microprocessor 22 can access 
the memory 25 in which, however, also the signals are stored which are 
processed by the microprocessor 23 and which stem from sensors which 
neighbor the sensors in the matrix whose signals are processed by the 
microprocessor 22. Differences in sensitivity of the sensors or the 
amplifiers can be detected by comparison of the signals of neighboring 
sensors. Furthermore, the microprocessors are capable of detecting the 
breakdown of individual sensor elements. The signals thereof can then be 
replaced, for example by averaged signals from the neighboring sensors. 
Subsequent to this operation, each of the microprocessors supplies an 
output signal of the sensors of the associated columns. These digital 
output signals are applied to a digital multiplexer 28 in which the 16 
signals from the 16 microprocessors are combined so as to form an overall 
signal having a serial character. This signal becomes available on an 
output 29 of the digital multiplexer 28 and represents the overall signal 
of the device which contains the signals of all sensors of the matrix.