Charge transfer multi-linear strip

A charge transfer multi-linear strip comprising several lines of photosensitive detectors, each line receiving successively the radiation to be detected and comprising charge transfer shift registers which provide in phase summing of the information collected at the detectors occupying the same position in the different lines, and wherein: PA0 said strip is adapted to radiology, the photosensitive detectors being photodiodes with a scintillator before these photodiodes and a screen being disposed to protect from the x rays the whole strip, except the photodiodes and their connections; PA0 there is a device for charge injection of the information between each connection connected to a detector and an input of a charge transfer shift register, said device comprising especially a charge injection diode connected to one of the connections and a screen grid brought to a constant potential, said grid providing biasing of the detector.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a charge transfer multi-linear strip. 
2. Descrption of the prior art 
Such strips are known in the prior art which will be described with 
reference to FIGS. 1 and 2. 
In these strips, N aligned photosensitive elements are used, called 
D.sub.1, D.sub.2 . . . D.sub.N in FIG. 1. These N photosensitive elements 
receive successively the radiation to be detected. 
In FIG. 1, an arrow going from left to right shows the direction in which 
the strip travels in front of the object or body 1 which is emitting the 
radiation to be detected. The integration time is thus multiplied by a 
factor N provided the information collected by the different detectors is 
summed in synchronism with the travelling movement. For summing in phase 
the information collected, this information must be provided with delays 
of values T, 2T, 3T . . . NT, where T is the time for the passage of the 
object or body which is emitting the radiation in front of each detector, 
it is also the integration time of each detector. In FIG. 1 is shown 
schematically that a delay T is provided for the information coming from 
detector D.sub.1 before being fed to a summator, a delay 2T is provided 
for the information coming from detector D.sub.2 . . . and so on. 
The device shown in FIG. 1--apart from the detectors D.sub.1 to D.sub.N 
--is known under the name of "time delay integration" or TDI and it is by 
these initials that it will be mentioned hereafter. 
Constructing a TDI from a charge transfer shift register with parallel 
inputs and a series output is known. In the example shown in FIG. 1, the 
information coming from detector D.sub.N is fed to the first stage of the 
register, then is transferred to the second stage where it is summed with 
the information coming from detector D.sub.N-1 . . . and so on. 
The U.S. Pat. No. 4,054,797 shows a TDI, for infra-red radiations, 
comprising transfer shift registers with parallel lateral inputs and a 
series output, the stages of these registers having a capacity increasing 
in the direction of the charge transfer. It is explained in this U.S. 
Pat., column 4, lines 6 to 12, that the information is fed to the TDI in 
voltage form, that is to say that the charges stored in the detectors are 
read as voltages and it is these voltages which control the injection of 
the charges into the TDI. 
For forming a charge transfer multi-linear strip of great dimensions, for 
example 30 cm, in length, for radiology, a large number of devices such as 
the one shown in FIG. 1 are used. On a support, M circuits each comprising 
N detector lines and P detector columns are juxtaposed, with the N 
detectors of the same column connected to a TDI as shown in FIG. 1. 
Thus, for example, a strip 50 cm in length is obtained, formed from M=50 
elementary strips each comprising P=40 detector columns, and 1 cm in 
width, with N, the number of lines, equal to 20. 
SUMMARY OF THE INVENTION 
The present invention relates to a charge transfer multi-linear strip for 
radiology in which several problems due to the use of x-rays have been 
solved. 
These problemes are due for instance: 
to the detectors, the characteristics of which have to remain stable when 
they receive x-rays; 
to the polarization of the detectors, which cannot be realized on the 
photosensitive zone which receives the x-rays; 
to the large capacities of the detectors and of their connections. 
The present invention relates to a charge transfer multi-linear strip 
comprising several lines of photosensitive detectors, each line receiving 
successively the radiation to be detected and comprising charge transfer 
shift registers which provide in phase summing of the information 
collected at the detectors occupying the same position in the different 
lines, wherein: 
said strip is adapted to radiology, the photosensitive detectors being 
photodiodes with a scintillator before these photodiodes and a shield 
being disposed to protect from the x-rays the whole strip, except the 
photodiodes and their connections; 
there is a device for charge injection of the information between each 
connection connected to a detector and an input of a charge transfer shift 
register, said device comprising specially a charge injection diode 
connected to one of the connections and a screen grid brought to a 
constant potential, said grid providing biasing of the detector. 
The charge transfer multi-linear strips used for collecting radiological 
images are especially interesting because they allow the x-ray dose used 
to be reduced for the same exposition time. A strip with N aligned 
photosensitive detectors allows to multiply by N the integration time and 
since the reading noises are added quadratically, a gain on the signal to 
noise ratio is obtained equal to the square root of N. This increasing of 
the signal to noise ratio allows to reduce the x-ray dose used for the 
same exposition time. 
For collecting radiological images, the characteristics of the photodiodes 
used remain stable when said photodiodes receive x-rays. 
It is not the case when photo-MOS are used as detectors. A scintillator is 
placed before the photosensitive zone, with an insulating layer between 
the scintillator and the photosensitive zone. Said scintillator changes 
the x-rays into visible light but the photosensitive zone is all the same 
crossed by x-rays. 
According to the invention, charge injection devices or CIDs inject in the 
form of charges into the TDIs the information collected by the detectors, 
without previous conversion into voltages. 
These CIDs provide impedance matching between on the one hand the large 
capacity of each photodiode and of the connection which connects said 
photodiode to the information processing part of the strip, sheltered from 
the radiation and then far from the photodiodes and on the other hand, the 
low capacity of a charge transfer shift register. An accurate and 
efficient injection is then realized. When the information is fed to the 
TDI in voltage form, only low voltages are available for controlling the 
injection when the capacities of the detectors and of their connections 
are large, as it is the case for radiological type applications and this 
is a disadvantage. 
It is then possible as in the U.S. Pat. No. 4,054,797 to amplify the 
voltage which control the injection but this solution has for disadvantage 
to increase the space necessary to the strip. 
According to the invention, a CID is used which comprises a charge 
injection diode connected to one of the connections and a screen grid 
brought to a constant potential, said grid providing biasing of the 
detector. So the biasing of the photodiodes is realized by CIDs which are 
protected from the x-rays. 
The injection is further improved by different improvements such as the use 
of two driver charges. In a preferred embodiment, a drain is used for 
removing the excess charges placed at the level of the CIDs. The diodes 
used have a high capacity and if there is excessive illumination, it is 
the charge transfer shift registers which are first of all staturated. The 
use of removal drains at the level of the CIDs protects the registers 
without substantially increasing the space required.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 3 is shown the diagram of one embodiment of a TDI. Parallel 
connections, vertical in the Figure, C.sub.1 to C.sub.N are connected to 
detectors D.sub.1 to D.sub.N. It will be recalled that a charge transfer 
multi-linear strip comprises several lines of photosensitive elements. 
Each line receives successively the radiation to be detected. Each TDI is 
connected to N detectors which occupy the same position, i.e. the same 
column, in the different lines. 
Each connection is connected through a charge injection device or CID to 
the input of a stage of an N stages charge transfer shift register. This 
register has parallel and lateral inputs and a series output. 
In the embodiment shown in FIG. 3, a charge transfer shift register is used 
in which each stage is formed by an electrode pair connected to a control 
signal .phi..sub.1 and an electrode pair connected to a control signal 
.phi..sub.2. In each pair, one of the electrodes is a storage electrode 
and the other a transfer electrode. The disymmetry in the surface 
potentials required for making the transfer unilateral is provided for 
example by an extra oxide thickness or by implantation of impurities of 
the same type as the substrate. 
Similarly, a register may be used having any number of control phases. In 
FIG. 3, the injection of the charges takes place on phase .phi..sub.1. 
In the embodiment of FIG. 3, the CIDs are in contact with the right hand 
end of the storage electrodes connected to the clock signal .phi..sub.1. 
The CIDs could also for example, be in contact with the left hand end of 
these electrodes or with their upper part. 
The transfer direction of the charges in the register is vertical, from 
bottom to top, and parallel to the connections. The stages of this 
register have a capacity increasing in the transfer direction. The 
capacity increase is achieved preferably by increasing the area of the 
electrodes. The capacity of the register may be doubled from one stage to 
the next. By means of connection C.sub.N, the charges coming from detector 
D.sub.N are transferred into the register. These charges are transferred 
into the shift register from the CID connected to connection C.sub.N. 
There is transfer of these charges to the following stage, then the 
transfer into this stage of the register through connection C.sub.N-1 of 
the charges coming from detector D.sub.N-1 takes place . . . and so on . . 
. the register assigning decreasing delays of values NT, (N-1). T . . . to 
the charges coming from detectors D.sub.N to D.sub.1 and this register 
also summing the charges. 
The arrangement adopted in FIG. 3 is particularly advantageous from the 
point of view of compactness of the strip. 
The last stage of the register comprises a device for reading the charges. 
This device may be formed for example by an output grid G.sub.S, followed 
by a reading diode D.sub.L to which are connected a follower stage S.sub.i 
which supplies the output signal S and by a reset transistor T.sub.RAZ 
whose grid is controlled by the signal .phi..sub.2 and which receives a DC 
voltage V.sub.R. 
As it is shown in FIG. 3, for the use in radiology, a shield opaque to the 
x-rays is disposed on the TDI. 
In FIGS. 4, 5 and 6, is shown a preferred embodiment of the CIDs of FIG. 3. 
In FIG. 4, which is a top view, it can be seen that each CID comprises: 
a charge injection device d.sub.i connected to a connection C.sub.I ; 
a screen grid P brought to a constant potential; 
a storage grid C.sub.1 ; 
an injection grid C which controls the passage of the charges to the input 
of a charge transfer shift register; 
a diode D.sub.B for removing the charges connected laterally to the storage 
grid C.sub.1. 
FIG. 5a is a sectional view along BB' of the device of FIG. 4. 
It will be noted that the CID is formed on a N/P diffusion. A P/N diffusion 
could also be used. The transfers therefore take place in volume, which 
reduces the noise. Similarly, the registers preferably comprise diffusions 
so that the transfers take place in volume. Under the grip P there exists 
a compensation 1 which partially raises the threshold and the purpose of 
which will explained further on. Similarly, there exists a compensation 2 
under grid C. 
In FIG. 5b, the profiles of potentials at two periods a and b have been 
shown. Increasing positive potential is in the downward direction. 
During the period a, the transfer of charges takes place towards the stage 
of the register which is shown in FIGS. 5a and b. 
Grids C.sub.1 and C are at a low level. 
At the injection diode d.sub.i and under the screen grid P is to be found 
the signal charge Q.sub.S coming from detector D.sub.i. 
During period b, grids C.sub.1 and C are brought to high levels. There is a 
transfer towards these grids of a charge amount equal to Q.sub.S +Q.sub.O, 
where Q.sub.O forms a first driver charge for improving the transfer of 
the charges at the level of grid P. The driver charge Q.sub.O is retained 
under grid C.sub.1 whereas the signal charge Q.sub.S is transferred into 
the register where it is added to the signal charges coming from other 
detectors. The high levels applied to grid C.sub.1 and C are chosen so 
that the driver charge Q.sub.O is retained under grid C.sub.1. 
The screen grid P serves essentially for biasing the detectors. If defines 
the reference potential of the detectors at the beginning of their 
integration time. That occurs at each read out when the charges in excess 
with respect to the reference potential are transferred under grid 
C.sub.1. 
The compensation 1 straddled under grip P and which partially raises its 
threshold improves this decoupling between the reference potentiel and the 
potential under grid C.sub.1. 
As biasing of the detectors is realized by TDIs, a photosensitive zone is 
obtained with no MOS transistor and no charge device. 
Under grid C, there is also a compensation 2 which raises its threshold. 
Thus, when grids C and C.sub.1 go to the low level, the driver charge 
Q.sub.O is restored under grid P and diode d.sub.i and not in the 
register. 
To improve the transfer of signal charges Q.sub.S at the level of grid C, a 
second driver charge Q.sub.1 is used. 
The driver charge Q.sub.1 is preferably transferred into the CID at the 
same time as the signal charge Q.sub.S. It may be generated by additional 
illumination of the detectors or, for example, by means of a charge 
transfer shift register or MOS transistors providing injection of an 
additional charge amount at the level of connections C.sub.1 . . . C.sub.N 
for example. 
The second driver charge Q.sub.1 is of the order of 1/10 or 1/20 of the 
average signal charge, whereas the first driver charge Q.sub.O is of the 
order of 3 to 5 times the maximum signal charge. It is transferred into 
the register at the same time as the signal charges Q.sub.S. 
The CID of FIGS. 4 and 5 forms a preferred and very much improved 
embodiment. 
The basic embodiment of a CID comprises the diode D.sub.i, the screen grid 
P, the storage grid C.sub.1 and the injection grid C. In more improved 
embodiments, the compensations 1 and 2 are added under grids P and C, the 
system for generating the second driver charge . . . From the embodiment 
shown in FIGS. 4 and 5, other CID embodiments may be derived which perform 
less well. Similarly, the CIDs and the registers may operate with surface 
transfers. 
In the case of use in radiology, the detectors have a high capacity. In the 
case of over illumination, the registers are saturated first, they may for 
example have a storage capacity ten times less than that of the detectors. 
For space saving reasons, it is not very desirable to use removal drains at 
the level of the registers. It is preferred to dispose a removal drain at 
the level of the CIDs. In the embodiment of FIG. 4, this drain is formed 
by a diode D.sub.B connected to the grid C.sub.1. The diode D.sub.B is 
placed laterally with respect to grid C.sub.1. FIG. 6a is a sectional view 
in the direction DD', D'B' shown in FIG. 4 and FIG. 6b shows the potential 
profile in the case of overillumination for the device of FIG. 6a. When 
there is a charge excess under grid C.sub.1, there is transfer towards 
diode D.sub.B. 
To obtain a good low noise transfer, the assembly of grids P, C.sub.1, C as 
well as the charge transfer shift register are formed on a diffusion, for 
example N/P in FIG. 6a, so as to effect all the transfers in volume, but 
the diffused zone N is interrupted in zone E--see FIG. 6a--in the vicinity 
of diode D.sub.B. The potential in this zone is such that the transfer 
takes place on the surface. A potential barrier is obtained which defines 
the maximum charge amount storable under grid C.sub.1. In a variant, the 
diffused zone N is not interrupted in zone E, in the vicinity of diode 
D.sub.B. In this case, there is formed under grid C.sub.1, in the vicinity 
of diode D.sub.B, a compensation which is higher than the compensation 2 
existing under grid C, which allows a higher threshold in the vicinity of 
diode D.sub.B for modulating the charge amounts stored under grid C.sub.1. 
It can be seen from FIG. 3 that the diodes for injecting charges D.sub.1 to 
D.sub.N-1 all begin at the same level on the connections. It is possible 
to limit the height of the injection diodes to the size of the grids P, 
C.sub.1, C . . . of the CIDs. The increase in area of these diodes allow 
the passage of connections such as those which convey the clock signals 
.phi..sub.1 and .phi..sub.2 to the adjacent TDIs. 
In FIG. 7, there is shown in greater detail one of the elementary strips 
shown in FIG. 2 and M copies of which much be joined together so as to 
obtain a charge transfer multi-linear strip. The whole of the strip is 
formed preferably on the same semiconductor substrate. 
This strip comprises a photosensitive zone formed by N lines and P columns 
of detectors. 
It may be considered that each column forms a resolution point. In the 
embodiment of FIG. 7, the detectors are photodiodes having a rectangular 
shape about 250 microns long and 500 microns wide. The gap between the 
photodiodes is about 10 microns so as to increase the sensitivity to the 
maximum. On each detector column there are N vertical connections C.sub.1 
to C.sub.N which connect one of the detectors to an input of a TDI. 
These connections are made from a semi transparent conductor material, 
polycrystalline silicon for example. They are as fine as possible so as 
not to reduce the sensitivity too much, for example their length is 5 
microns and they are separated by a gap of 5 microns. These connections 
extend over the whole width of the photosensitive zone so as to balance 
the sensitivities of all the photodiodes. 
The circuit of FIG. 7 thus comprises, following the photosensitive zone, P 
CID members and P TDI members, i.e. a CID and a TDI per photodiode column. 
A shift register addresses each of these TDIs one after the other. 
Addressing takes place through the grid of MOS switching transistors 
T.sub.1 to T.sub.N connected between a common point K and the output of 
the follower stage S.sub.1 to S.sub.N connected to the output of a TDI. 
These follower stages deliver a voltage across an average impedance. The 
point K receives successively the information from each TDI and delivers 
it to the output S through a low impedance output amplifier A. 
The same shift register may be used for successively addressing all the 
TDIs of the strip comprising several elementary strips such as the one 
shown in FIG. 7. 
The CIDs, the TDIs, and the multiplexing and output portions of the strip 
are protected by a screen. A dead zone is provided between the 
photosensitive zone and the TDIs for positioning the edge of the screen 
without difficulty. One advantage of the device of the invention is that 
biasing of the detectors is provided by CIDs which are protected from the 
radiation. 
The photosensitive zone is preferentially formed on an epitaxied substrate, 
a zone P on a substrate P.sup.+ for example, as can be seen in FIG. 8 
which is a sectional view of a detector along the direction AA' shown in 
FIG. 7. The effective thickness of the epitaxied layer is small, about 15 
microns for example. The x-rays have only a small chance of being absorbed 
in this zone which would generate a flow of parasite charges.