Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-07-21
2002-01-01
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370090, C250S367000
Reexamination Certificate
active
06335528
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a process for producing a solid-state radiation detector by assembling unitary elementary slabs in order to obtain a composite slab of large size, and it more particularly relates to a process for adhesively bonding these unitary elementary slabs.
The invention also relates to a detector thus produced.
DISCUSSION OF THE BACKGROUND
The principal application of such detectors is X-ray radiology. To give a concrete idea, in what follows, without limiting its scope in any way by this, the context of the preferred application of the invention will be assumed, unless otherwise indicated.
A scintillator according to the prior art is described by way of nonlimiting example, in French patent application FR-A-2 636 800 (THOMSON-CSF).
The mode of operation and the general structure of a solid-state X-radiation detector will now be sumarized with a reference to the description of
FIGS. 1
a
to
3
appended [lacuna] the present description.
According to the state of the art, radiation detectors are produced on the basis of one or more matrices of solid-state photosensitive elements. Known solid-state photosensitive elements are not directly sensitive to rays with very short wavelengths, as X-rays are. It is necessary to combine them with a scintillator component. The latter is made of a substrate which has the property, when it is excited by X-rays, of emitting light in a range of longer wavelengths: in the visible spectrum (or the near visible spectrum). The particular wavelength depends on the substance used. The scintillator therefore acts as a wavelength converter. The visible light thus generated is transmitted to the photosensitive elements which carries out photoelectric conversion of the received light energy into electric signals which can be processed by suitable electronic circuits.
FIGS. 1
a
and
1
b
represent two mutually orthogonal lateral sections of a matrix of photosensitive elements which is conventionally combined with a sheet of scintillating substance.
Each photosensitive element has a photodiode or a phototransistor which is sensitive to photons in the visible spectrum or the near visible spectrum. By way of example, as illustrated in
FIGS. 1
a
to
1
d
, each photosensitive element consists, for example, of two diodes D
mn1
and D
mn2
which are arranged head-to-tail, and the matrix array RM has column conductors, Cc
1
, to Cc
x
, and row conductors, Cl
1
to Cl
y
. Each of the diodes D
mn1
and D
mn2
constitutes, in a known way, a capacitor when it is reverse-biased. The first diode D
mn1
has a capacitance typically ten times smaller than the capacitance of the second diode D
mn2
. It principally fulfills the function of a switch, whereas the second diode is preferably photodetecting.
At each intersection of a row and a column, for example of row Cl
n
and column Cc
m
(see
FIG. 1
d
), such a set of two diodes head-to-tail, D
mn1
and D
mn2
, is arranged. The diodes can be replaced by transistors produced in “TFT” technology, TFT standing for thin film transistor.
The conductors
12
(
FIGS. 1
a
and
1
b
) consist of a metal deposit on an insulating substrate
10
, preferably glass. The deposition is followed by a photoetching operation in order to obtain parallel conductive tracks of suitable width. The diodes (for example D
mn1
and D
mn2
) are formed by deposition, on the column conductive tracks
12
, then etching, of the layers of amorphous silicon (ASi) which is intrinsic or doped using P or N type semiconductor material. A very thin layer of conducting, preferably transparent material is deposited on the insulating layer
20
in order to form, after etching, the row conductive tracks
22
of the matrix array RM.
The assembly described above forms what is generally referred to as the unitary elementary “amorphous silicon slabs”.
The row conductors Cl
1
-Cl
x
and the column conductors Cc
1
-Cc
y
constitute the electrodes for biasing the diode capacitors. The latter store electric charges when they are subjected to light radiation and deliver an electric signal, proportional to the stored charge, when they are electrically biased. The row conductors Cc
1
-Cc
x
and the column conductors Cc
1
-Cc
y
are addressed in a suitable chronological sequence, so that all the pixels p
mn
are biased sequentially in a predetermined order. The signal delivered by each pixel p
mn
is thus recovered and processed by electronic circuits (not shown) so as to reconstruct (point by point) the image stored in the form of electric charges.
The signals are recovered in respective connection zones
3
and
4
, for the rows Cl
1
-Cl
x
and the columns Cc
1
-Cc
y
. The connections with the external electronic circuits may be made using flexible multiconductor cables,
30
and
40
, respectively.
It is generally necessary to provide sequences referred to as “optical relevelling” of the pixels p
mn
, once the signals have been delivered by them. The chronological sequence of the addressing signals is adapted accordingly. Optical relevelling sequences are inserted between the reading signals. They consist in performing generalized illumination of the pixels p
mn
, after reading. The purpose of these sequences is to re-establish an electrical reference state on the pixels p
mn
which have been perturbed during the phases of storing and reading the charges.
This generalized illumination is performed via the rear face of the glass slab
10
, which must be sufficiently transparent at the wavelengths of the light which is used.
As indicated above, the photosensitive elements need to be illuminated with visible light (or in a range close to visible light). It is necessary to provide a scintillator which converts the X-rays into light energy, in the visible-wavelengths spectrum. To that end, it is sufficient to cover the amorphous silicon slab described above with a layer of scintillating substance
24
. By way of example, for a detector sensitive to X-rays of the order of 60 KeV [sic], the scintillating substance used is cesium iodide (CsI) doped with sodium iodide (NaI) or thallium iodide (TiI), depending on whether the intention is to obtain a light signal of wavelength 390 nm or 550 nm, respectively.
The amorphous silicon slab which has just been described is produced by vacuum evaporation of thin films of material on the glass slab. The dimensions of the glass slab must be compatible with the current dimensional capacities of machines for carrying out the deposition.
However, the need has been felt to provide slabs with large sizes, these sizes being incompatible with the aforementioned deposition machines. It is thus necessary to resort to unitary elementary slabs, of smaller sizes, which are assembled by juxtaposition with one another. By way of nonlimiting example, a chequer-board of four elementary unitary slabs is assembled in order to form a composite slab of large size. Such an assembly process is described, for example, in French patent application FR-A-2 687 494 (THOMSON TUBES ELECTRONIQUES). The unitary slabs preserve their autonomy as regards the addressing of their own pixels p
mn
.
FIGS. 2
a
and
2
b
appended to the present description illustrate such an assembly, respectively in side and plan view. In the example described, the composite slab of large size comprises four unitary elementary slabs
10
a
to
10
d
.
The unitary elementary slabs
10
a
to
10
d
are cut precisely on two sides of their periphery which are free of the connection zone (not shown) so that the active zones of pixels, RM
a
to RM
d
, are flush with the edge of the cut slab. The cut slabs
10
a
to
10
d
are then positioned relative to one another in order to preserve continuity of the active zone of the pixels and their pitch, from one slab to the next.
The assembly is carried out by adhesive bonding (film
6
) of a common support
7
on the cut and positioned slabs
10
a
to
10
d
. This support
7
must also be transparent enough to visible light in order to allow optical relevelling o
Nicollet Andre
Spinnler Vincent
Vieux Gerard
Gabor Otilia
Hannaher Constantine
Thomson Tubes Electroniques
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