Method of fabrication of a light image detector and a two-dimensional matrix detector obtained by means of said method

In a light image detector, a substrate is covered with a first layer of conductive material on which is formed a two-dimensional matrix array of photodiodes in the form of pads arranged in rows and columns and each comprising a layer of amorphous semiconductor material doped with a predetermined type (n-type or p-type), a layer of undoped amorphous semiconductor material, a layer of amorphous semiconductor material doped with another predetermined type (n-type or p-type), a second layer of conductive material, each photodiode being insulated from adjacent photodiodes by means of insulating material. On the insulating material, columns of material are disposed along the columns of photodiodes and are each formed by a layer of metallic material and a layer of doped amorphous semiconductor material. Connection elements are each connected to a photodiode through the layer of conductive material of the photodiode, are located in proximity to a column and each formed by a layer of metallic material and a layer of doped amorphous semiconductor material. Rows of material are disposed along the lines of photodiodes and overlap the columns as well as at least one connection element at each point of intersection of a row and a column. Each row is formed by a layer of undoped amorphous semiconductor material, an insulating layer and a layer of metallic material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of fabrication of a light image detector 
and to a two-dimensional matrix detector which is obtained by means of 
said method in the general field of fabrication of electronic circuits in 
thin films on large surfaces. 
The application contemplated in the present invention is the integrated 
control of each elemental point of a two-dimensional matrix image 
detector. 
2. Description of the Prior Art 
At the present time, the principal applications in which the high 
photoconductivity of amorphous silicon is associated with the possibility 
of fabricating electronic control circuitry from the same material and on 
the same substrate are concerned with linear image detectors of large size 
(up to 20 cm) which are necessary for reading documents corresponding to 
the international page format A4 by lateral displacement of the document 
with respect to the sensor. 
Three different structures have been contemplated for the construction of 
linear detectors of this type: 
the association of a photoconductor, of a storage capacitor, and of a 
thin-film transistor (TFT) of amorphous silicon (a-Si) as described in the 
article by M. Matsumura in the review entitled "IEEE Electron Device 
Letters", vol. EDL - 1, No. 9, September 1980, pages 182 to 184; 
the association of a photodiode of amorphous silicon and of a TFT of 
amorphous or polycrystalline silicon as described in "Extended Abstracts 
of the 15th Conference on Solid State Devices and Materials", Tokyo 
(1983), pages 201 to 204, by F. Okumura; 
the association of a photodiode and of a blocking diode of a-Si as 
described by Y. Yamamoto in "Extended Abstracts of the 15th Conference on 
Solid State Devices and Materials", Tokyo (1983), pages 205 to 208. 
A certain number of these structures make it possible to obtain devices 
having the following characteristics: 
Length: 50 mm 
Resolution: 8 to 10 bits/mm 
Size of pixel: 100 ?m.times.70 ?m 
Reading time: 2 ?s/bit without multiplexing. 
On the basis of these results, it would appear reasonable to expect the 
forthcoming construction of linear sensors which permit reproduction of 
documents under good conditions. By way of example, it will be feasible to 
reproduce documents having a width of 216 mm at a rate of 8 bits per 
millimeter and at a paper transfer rate of 5 ms per line. 
A two-dimensional image detector having a matrix addressing and reading 
system has also been developed such as the detector described in the 
article by S. Uya in Extended Abstracts of the 16th Conference on Solid 
State Devices and Materials, Kobe, (1984), pages 325 to 328. The image 
detector described in this article is formed of a matrix of photodiodes of 
amorphous silicon superposed on a two-dimensional array of charge-coupled 
devices (CCDs) for reading the charges created by light. Since the CCD is 
formed of monocrystalline silicon, the size of the sensor is limited by 
requirements which are specific to the monocrystalline silicon technology, 
namely a prototype of less than 1 cm on each side. 
In contrast, the present invention relates to a large-sized two-dimensional 
image detector with matrix addressing and reading. 
The elemental point is constituted by an amorphous silicon photodiode 
connected to the drain of an amorphous silicon control transistor. The 
source of said transistor is formed by a column electrode and the gate is 
controlled by a row electrode. If so required, the second terminal of the 
photodiode which is common to all the diodes can be at a nonzero potential 
in order to reverse-bias the diode. 
The present invention benefits by the progress achieved both in the field 
of photovoltaic diodes and in the recent performances obtained in the 
fabrication of control transistors for addressing liquid-crystal display 
screens of the flat panel type. 
One of the principal advantages of the method in accordance with the 
invention lies in the fact that it requires only three masking levels. 
Furthermore, it permits the construction of large-area detectors of much 
greater size than those in current use. 
By way of example, one advantageous application of said detector could be 
the construction of a two-dimensional x-ray detector on condition that a 
scintillator which absorbs x-rays and re-emits visible rays is interposed 
between the x-ray source and the detector. This would be one way to 
replace the photosensitive plates or the x-ray image intensifiers at 
present employed in radiology. 
SUMMARY OF THE INVENTION 
The present invention therefore relates to a method of fabrication of a 
light image detector and is distinguished by the fact that said method 
comprises the following successive steps: 
(a) a first step of deposition of a first layer of conductive material on a 
substrate; 
(b) a second step of deposition of a layer of amorphous semiconductor 
material doped with a predetermined first type; 
(c) a third step of deposition of a layer of undoped amorphous 
semiconductor material; 
(d) a fourth step of deposition of a layer of amorphous semiconductor 
material doped with a second type opposite to that of a first type of the 
second step; 
(e) a fifth step of deposition of a first layer of metallic material; 
(f) a sixth step of etching through a mask in the different layers 
previously deposited from the second step of deposition of photodiodes; 
(g) a seventh step of deposition of a first layer of insulating material 
having a thickness equal to that of the photodiodes; 
(h) an eighth step of machining of the insulating material located above 
the photodiodes in order to retain the insulating material located between 
said photodiodes; 
(i) a ninth step of deposition of a second layer of metallic material; 
(j) a tenth step of deposition of a layer of n-type doped semiconductor 
material; 
(k) an eleventh step of etching through a mask in the layer of 
semiconductor material deposited during the ninth step and in the second 
layer of metallic material so as to form connection elements each in 
contact with one photodiode and columns located on the insulating medium 
in proximity to said connection elements; 
(l) a twelfth step of deposition of a layer of undoped amorphous 
semiconductor material; 
(m) a thirteenth step of deposition of a second layer of insulating 
material; 
(n) a fourteenth step of deposition of a second layer of metallic material; 
(o) a fifteenth step of etching through a mask in the layers of 
semiconductor material, of insulating material and of metallic material 
deposited during the twelfth, thirteenth and fourteenth steps so as to 
form rows which overlap said connection elements and said columns. 
The invention accordingly relates to a two-dimensional matrix detector 
which essentially comprises: 
on a substrate covered with a first layer of conductive material, a matrix 
of photodiodes in the form of pads arranged in rows and columns and each 
comprising a layer of amorphous semiconductor material doped with a 
predetermined type, a layer of undoped amorphous semiconductor material, a 
layer of doped amorphous semiconductor material doped with another 
predetermined type, a second layer of conductive material, each photodiode 
being insulated from adjacent photodiodes by means of insulating material; 
on the insulating material, columns deposited along the columns of 
photodiodes and each formed by a layer of metallic material and a layer of 
doped amorphous semiconductor material; 
connection elements each connected to a photodiode through the layer of 
conductive material of the photodiode, located in proximity to a column 
and each formed by a layer of metallic material and a layer of doped 
amorphous semiconductor material; 
rows disposed along the rows of photodiodes and overlapping the columns as 
well as at least one connection element at each point of intersection of a 
row and a column, each row being formed by a layer of undoped amorphous 
semiconductor material, an insulating layer and a layer of metallic 
material.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the invention, the method of fabrication of light image 
detectors provides for depositing a layer of conductive material 2 on a 
substrate 1 such as glass during a first step. Depending on the mode of 
utilization considered, said layer of conductive material can be a 
detector of transparent material as will be explained in the following 
description. There is thus deposited a thin film of mixed oxide of tin and 
indium (ITO) or an equivalent material (In.sub.2 O.sub.3, SnO.sub.2). A 
layer of this type will have a thickness of 500 to 1500 Angstroms (1250 
Angstroms, for example) and oxidation of the layer is then performed. 
During a second step, a layer 3 of p-doped amorphous silicon is deposited 
by means of a known method of deposition of amorphous silicon such as 
luminescent discharge or reactive sputtering. 
In a third step, a layer 4 of undoped amorphous silicon is deposited by a 
method which is identical to the method of the preceding step. 
In a fourth step, a layer 5 of n-doped amorphous silicon is deposited in 
the same manner as before. 
In a fifth step, a thin metal layer 6 such as aluminum is deposited by 
vacuum evaporation or vacuum sputtering. 
A structure as shown in FIG. 1 is thus obtained. The thickness of the four 
layers (silicon and metal layers) 3, 4, 5 and 6 is approximately 1 micron. 
In a sixth step, photodiodes such as the photodiode E shown in FIG. 2 are 
cut by etching in these four layers. This etch cutting step may be 
performed by photolithography. This process entails the need for masking 
which does not require a high degree of accuracy either in dimensions or 
positioning. The masking operation is followed by dry etching or chemical 
etching of the four layers 3, 4, 5 and 6. 
As shown in FIG. 2, a pin-type photodiode E is thus obtained. However, the 
silicon layer 3 could have been n-type doped and the silicon layer 5 could 
have been p-type doped, in which case a nip-type photodiode would have 
been obtained. 
In the course of a seventh step, there is deposited an insulating layer 7, 
the thickness of which is equal to the thickness of the photodiode E. The 
insulator employed in this step can be silica or a silicon nitride. The 
deposition technique employed can be a luminescent discharge or reactive 
sputtering at a temperature which is compatible with the nature of the 
photodiode E. There is thus obtained a structure of the type shown in FIG. 
3. 
An eighth step then consists in carrying out removal of the insulating 
material 7 located at the surface of the metal layer 6 of the photodiode 
E. To this end, the surface of the insulator 7 is covered with a layer of 
a negative resin. The complete structure is then exposed through the 
substrate 1 to a source of illumination as represented by the arrows in 
FIG. 3. The metal layer 6 of the photodiode E serves as a mask for the 
insulator 7 located above the photodiode E, this insulator being 
subsequently removed after development. The structure thus obtained is 
illustrated in FIG. 4. As shown in the perspective view of FIG. 8, the top 
surface of the electrode E is flush with the top surface of the insulator 
7. 
A ninth step involves deposition of a layer 8 of metal such as chromium, 
palladium or molybdenum in accordance with the process adopted for 
deposition of the metal layer 6, that is to say by vacuum evaporation or 
cathode sputtering. The layer 8 must have a thickness of a few hundred 
Angstroms (600 Angstroms, for example). 
In a tenth step, a heavily doped n-type layer of semiconductor material 9 
such as amorphous silicon is deposited by means of a known method of 
deposition of amorphous silicon such as luminescent discharge or reactive 
sputtering. The thickness of this layer will be approximately 400 
Angstroms (between 300 and 1500 Angstroms). A structure as shown in FIG. 5 
is thus obtained. 
During an eleventh step, etching of the two previously deposited layers 
(metal layer 8 and amorphous silicon layer 9) is carried out in such a 
manner as to form connection elements CX and column elements CL as shown 
in FIG. 6. The connection element CX is constituted by a layer 8X and a 
layer 9X which are etched in the layers 8 and 9 of FIG. 5. Similarly, the 
column CL is constituted by a layer 8L and by a layer 9L which are etched 
in the layers 8 and 9. 
FIG. 9 gives a perspective view of the structure thus obtained. The 
connection element CX has the general shape of an L. One of the arms of 
the L is in contact with the electrode E. The other arm of the L is 
parallel to the column CL. 
This etching step is carried out by photolithography and alone serves to 
define the geometry of the transistor (length of the transistor channel). 
In a twelfth step, a layer 10 of semiconductor material such as undoped 
amorphous silicon is carried out by means of a known method such as 
luminescent discharge. The thickness of the layer thus obtained is 
approximately 3000 Angstroms. 
A thirteenth step provides for deposition of an insulating layer 11 (gate 
insulator) by means of a method which is similar to that of the twelfth 
step. The thickness of the insulating layer 11 is approximately 1500 
Angstroms. By way of example, the insulator employed can be a nitride. 
A fourteenth step consists in depositing a layer 12 of metallic material 
such as chromium, palladium, molybdenum, and so on. The method of 
deposition of this layer will be similar to that of the ninth step of 
deposition of the layer 8. The thickness of the layer 12 will be 
approximately 300 Angstroms. On completion of the twelfth, thirteenth and 
fourteenth steps, a structure as shown in FIG. 7 is obtained. 
In a fifteenth step, it is necessary to etch the metal layer 12, the 
insulating layer 11, the silicon layer 10 in order to obtain rows LG. 
In the perspective view of FIG. 10, there is thus shown a row LG which is 
perpendicular to the column CL as well as a portion of the connection 
element CX. This etching operation is performed by photolithography. 
At this stage, the integrated matrix image detector is formed as 
illustrated in FIG. 11. The substrate 1 carries rows LG, columns CL, 
connection elements CX at each point of intersection of a row and a 
column, as well as a photodiode E connected to each connection element CX. 
Moreover, as shown in FIG. 12, each row LG has a portion of increased width 
at the level of each crossover point (transistor). This makes it possible 
to reduce the parasitic transistor effect which is liable to exist between 
the drain of the principal transistor and the following column. 
It should be added that the depositions of amorphous silicon layers 10 and 
insulating material 11 performed during the twelfth and thirteenth steps 
described in the foregoing are carried out through a mask for protecting 
the periphery of the substrate 1 from these deposits. Thus the peripheral 
contacts of each row LG are formed directly on the substrate by the 
deposited metal layer 12. 
It is also worthy of note that the deposition operation of the twelfth step 
can be an electrically compensated semiconductor or even a lightly p-doped 
semiconductor in order to reduce the photoconductivity and thus to limit 
the influence of light on the characteristics of the transistors. 
The electric connections to the rows and columns are not shown in the 
accompanying figures and are established in accordance with a known 
technique. 
A matrix detector as described in the foregoing permits detection of a 
light image. Since the substrate 1 and the layer 2 are transparent to the 
light beam which is produced by the image to be detected, this light beam 
illuminates a photodiode E through the substrate 1 and the layer 2, thus 
initiating conduction of said photodiode. Scanning of the matrix detector 
in rows and columns makes it possible to detect the luminous flux received 
by each photodiode E and to read the received image which is broken-down 
point by point (or electrode E). 
Under these conditions, the received light image passes through the 
substrate 1. Should it be desired to prevent the light image from passing 
through said substrate in order to prevent any deformation, arrangements 
may accordingly be made to ensure that the metal layer 6 of the photodiode 
4 is a layer of conductive material which is transparent to the luminous 
flux. Deposition of this material in the fifth step mentioned above is 
accordingly carried out by vacuum cathode sputtering and the material 
employed can be ITO or SnO.sub.2. In this type of construction, the layer 
2 may be opaque to the transmitted luminous flux. 
The light image therefore arrives on the top of the formed component 
through the layer 6 of transparent conductive material onto the electrodes 
E. The image is thus formed directly on the photodiodes. Furthermore, the 
metal layer 12 of each row LG serves to screen the transistors at the 
points of intersection of the rows and columns from the incident light. 
The invention will find a particularly advantageous application in 
radiography systems. Thus a received radiographic image may be recorded 
electrically in a memory. The types of material employed for the 
fabrication of layers which may or may not be transparent (substrate 1, 
layer 2, metal layer 6) must therefore be adapted to the nature of the 
radiations emitted by the image to be detected according to the type of 
application.