The present invention concerns the field of radiation imagers, for example for ionising radiation.
Ionising radiation imagers are intended to detect ionising radiation, such as for example X or gamma rays. One type of ionising radiation imager uses a scintillator, also referred to as a “detector”, which converts the ionising radiation into visible radiation. It is this visible radiation that is then detected by photodetectors disposed downstream of the scintillator in the direction of propagation of the radiation. Photodetectors are generally divided into matrices.
Photodetectors may be of the CMOS (“Complementary Metal Oxide Semiconductor”) type. Each photodetector comprises an active part, which serves to detect the light radiation forming the signal, and electronic means. The whole forms the reading block. Electronic means are assembled in the immediate vicinity of the photodetectors and are attached to the sides.
The scintillator is disposed on a transparent substrate that forms a mechanical support for it; this substrate is chosen so as to be transparent to visible radiation. This assembly, referred as the detector block, is situated above the photodetectors.
The reading block and the detection block are separated by a layer of air. However, the effect of this layer of air is that a measured part of the visible radiation is trapped in the detector block. The detection efficiency is therefore very low.
For example, in the case where the scintillator has an optical index equal to 1.82, 92% of the visible radiation is trapped in the detector block.
The document WO 2009/024895 describes a radiation detector comprising light concentrators between a scintillator and a light-sensitive area.
Consequently one aim of the present invention is to offer a method for producing a radiation imager with improved detection efficiency and a radiation imager with improved detection efficiency.
The aim stated above is achieved by a radiation imager and a method for producing said imager, the imager comprising a substrate and a reading block formed by several photodetectors, the photodetectors being disposed at a distance from the substrate, and light guides disposed between the substrate and one or more photodetectors in order to capture the visible photons of the radiation and to bring them to the photodetectors, the waveguides being formed by portions made from a first material transparent to visible radiation having a first optical index connecting the substrate to N photodetectors, and a second material having a second optical index lower than the first optical index or being a reflective material, said second material at least partly surrounding one of the portions made from the first material. The waveguides are produced directly on a substrate by photolithography or by imprinting. Prior to the assembly of the substrate and the reading block, a layer made from a third material is deposited on the reading block so that said layer of third material wets the free ends of the waveguides made from a first material during assembly, thus forming rectifiers.
By means of the invention, the guiding structures collect more photons by virtue of the beam rectifiers added at the foot of the waveguides.
The invention therefore increases the detection efficiency. It may also increase the spatial resolution by guiding the visible photons to the photodetector or photodetectors closest to the generation area thereof in the detector block. The spatial precision of the image thus obtained is therefore improved.
Advantageously, the first material of the waveguides is formed by an adhesive, for example a glue, also serving to fix together the detector block and the reading block. The second material is advantageously air.
Highly advantageously, the first material is structured so that the transverse section thereof reduces from the detection block towards the photodetector or photodetectors.
The subject matter of the present invention is then a method for producing a radiation imager comprising a reading block intended to convert the radiation into an electrical signal, comprising a plurality of photodetectors, said method comprising the steps of:
a) forming a plurality of portions of a first material, with a first index, on a first substrate, the portions comprising, at the periphery thereof, a second material, said second material having a second optical index lower than the first optical index or being a reflective material,
b) forming a flat layer made from a third material on said reading block,
c) aligning the first substrate with respect to the reading block, so that said portions formed on the detector block are disposed opposite the photodetectors of the reading block,
d) assembling said substrate and said reading block by means of the portions made from a first material, so that the third material is wetted on said portions of the first substrate,
e) hardening the third material.
In one embodiment, step a) comprises:
In another embodiment, step a) comprises:
Preferably, during step b), the thickness of the layer made from a third material is between h/10 and 3/h/4, h being the height of the portions made from the first material. For example, the thickness of the layer from the third material is between 100 nm and 3 μm.
Preferably, the optical index of the third material is greater than or equal to that of the second material.
The first material is an SU8 resin or a resin of the Epotek353ND, Epotek360ND or polycarbonate type.
The first material advantageously has an index close to that of the material of the detector, preferably between 1.4 and 3.
For example, the cavities of the mould have a shape of revolution or polygonal. The cavities of the mould have a variable cross-section reducing as from the face wherein they emerge.
Preferably, the mould and the substrate comprising the layer of resin are heated before the pressing step.
The first substrate is advantageously a transparent material, for example glass.
In an example embodiment, the method may comprise the additional step of producing the detector block on said substrate, after assembly of the substrate and reading block.
In another example embodiment, the first substrate is a detector block having a detector block, comprising at least one detector able to emit an optical signal from an incident radiation to be imaged.
The method may comprise a step of surface treatment of said portion so as to modify the surface energy thereof.
The layer of the first material can be deposited by centrifugal coating.
The manufacturing method comprises for example a step of producing a via by means of metal balls.
The present invention will be better understood by means of the following description and the accompanying drawings, on which:
In
The imager comprises a detector block 1 that is formed in the example shown by a scintillator 2 and a substrate 4 transparent to visible radiation, for example made from glass on which the detector is deposited, and a reading block 6, disposed at a distance from the substrate 4 opposite to the detector 2. The detector converts the ionising photons into visible photons.
The substrate 4 provides the rigidity of the detector, in particular when the latter is thin. The substrate may however be omitted in the case where the thickness of the detector 2 is sufficient to ensure its own rigidity.
The reading block 6 comprises a plurality of photodetectors 8; in the example shown, these are advantageously distributed in one plane. The photodetectors are for example avalanche photodiodes, for example SPADs (Single Photon Avalanche Diodes), or simple photodiodes.
The photodetectors 8 are, in our example, SPAD photodetectors disposed at a distance from one another and separated by a guard ring 9. The photodetectors are grouped together in pixels.
Each pixel 10 has electronics. The pixels 10 are, themselves, disposed in a matrix. In
The imager also comprises portions made from a first material 12 disposed between the detection block and the reading block, each portion made from a first material 12 optically connecting the substrate 4 and one or more photodetectors.
The portions of first material 12 are separated from one another by a second material 11, the optical index of which is lower than that of the first material. In the example shown in
The portions of material covering several photodetectors also have the advantage of improving the mechanical strength of the structure.
The first material has an optical index close to that of the material of the substrate 4 and of the detector. Preferably the optical index of the first material is between 1.4 and 3.
The end of each portion 12 in contact with the reading block is surrounded by an area 14 made from a third material forming a rectifier. This third material is advantageously a glue or a resin. The area 14 is also referred to as the “foot”. The third material has an optical index preferably greater than or equal to that of the second material, further amplifying the effect of rectifying the radiation.
This area 14 around each pad 12 forms an area for rectifying beams towards the photodetection area. The active detection area is in general buried several micrometres under the surface with several levels of metal of electrical connections on the sides of the detection area, shown schematically by broken lines and
The presence of these rectifying areas at the contact between the waveguide pads and the photodetectors is all the more advantageous since the silicon has high reflection at high angles. By virtue of the rectification of the incident beams in the waveguides, the entry thereof is promoted towards the detection areas.
Advantageously, the first material is an adhesive material, for example a resin used in microelectronic processes. As will be seen hereinafter, the use of resin is particularly advantageous for producing these waveguides since it is commonly used in microelectronic processes, but for other purposes.
In
In
The surface area of the cross-section of each column is substantially equal to the surface area of a photodetector.
Preferably, the bottom surface of the pad (in the representation in
The smaller the number of photodetectors per portion made from a first material, until a single photodetector per portion made from a first material as shown in
In this example, a pixel comprises 64 photodetectors, in the embodiment in
An imager may comprise certain photodetectors not covered by a portion made from a first material does not depart from the scope of the present invention.
In
For the case of the detection of photons at a small angle, it was found that the pads where the cross-section decreases, between the detector block and the matrix of photodetectors, made it possible to increase the collection efficiency. Thus, in this case, the pads formed so that their base, that is to say the surface in contact with the detector block, is wider than their end in contact with the photodetector, are preferred. Straight pads, i.e. with a constant transverse section, can be used for detection at higher angles. The foot of glue 14 in the case of photons with a small angle and with a greater angle rectify the beams towards the detection area.
The height of the pads, that is to say the distance separating the photodetectors from the detector block, may vary, for example, between 1 μm and 100 μm, preferably between 5 μm and 30 μm. The height of the pads, that is to say the distance separating the photodetectors from the detector block or the substrate (or the base of the pads from their end) may vary, for example between 1 μm and 100 μm, preferably between 5 μm and 30 μm.
The surface area of the small base (or their end) is preferably substantially equal to the surface area of the active part of a photodetector.
In
The example embodiments in
Preferably, according to this embodiment, the pads are delimited by a second reflective material, for example a metal, or one with an index lower than that of the material of the pads, so that some of the photons emerging towards the outside of a pad are re-admitted in this pad. “Reflective” means a material for which the majority of the incident light is reflected rather than being absorbed or transmitted.
If a structure is considered wherein the first material of the pads is SiO2 and the second material is copper or another metal; the photons are reflected: the guidance of the light is effected by reflection, because of the presence of metal, and therefore of the reflected material, at the interface between the first material and the second material. When the radiation is in the visible range, metals, and for example copper, are good reflectors.
In
A Lambertian emitter in an infinite medium of index 1.51 is considered, which is formed by the detector 2 and the substrate 4. The first material is a glue of index 1.51. The detector block and the photodetection block are separated by a distance of 10 μm. As a reminder, the detector block comprises the scintillator material, the latter being able to be mechanically supported by a layer of transparent material, for example glass.
The curve I represents the case where the layer of air separates the substrate 4 from the photodetectors.
The curve II represents the fraction of light collected by one pixel in the case where the entire pixel is covered with glue, which corresponds to the imager in
The curve III represents the fraction of light collected by the device in
The curve IV represents the fraction of light collected by the device in
The curve V represents the fraction of light collected by the device in
In all cases, the fill factor of the matrix of the photodetectors is 50%. Thus it is found with curve I that the fraction of light collected in normal incidence is equal to the fill factor of the sensor. This falls for an angle greater than 33°, this angle corresponding to the total internal reflection angle.
The curve II shows the fraction of light collected in the case where several photodetectors are covered by the same first material.
It is found that, because of the variation in the total internal reflection angle at the interface between detector block and air, there is more light collected by the matrix of photodetectors. For example, a beam emerging from the detector block at an angle greater than the total reflection angle, with respect to the vertical, is not re-emitted to the detector when the first material is air; the total reflection angle is around 33.3° considering that the index of the scintillator is 1.82. On the other hand, when the air is replaced with adhesive resin, the index of which is higher than the index of air, the total reflection angle increases. Thus the quantity of light collected by the elementary photodetectors making up the matrix is increased. The sensitivity of the device is then increased.
The curve III shows that the fraction of the light collected increases substantially for angles around 20° passing from 50% to 70%, which is obtained by means of the guidance of the light by the columns.
It is also found that the fraction of light collected by the devices in
Thus not only does the structure substantially increase the resolution in terms of energy, by increasing the quantity of light collected, but also substantially improves the spatial resolution of the conversion point of the gamma or X-ray photon into a visible photon, the light being collected at small angles. The addition of rectifying areas 14 at the end of the pads further increases the quantity of light collected.
The portions made from a first material may be deposited either only on the photodetectors, for example patterns from a few microns to a few hundreds of microns depending on the size of the photodetector, or on a set of photodetectors in order to mask an electronic part situated alongside these photodiodes, routing, etc., and the patterns may then be from a few hundreds of μm to a few mm.
Furthermore, in the case where each portion made from a first material covers only one photodetector, it may have a form other than a column with a circular cross-section and may be a column with a square cross-section, for example with sides of 12 μm.
By way of example, the first material may be a SU8 resin or a resin of the EPO-TEK®353ND, EPO-TEK®360ND, polycarbonate, SiO2, etc. type.
In the case where the second material is a reflective material, a metal may be chosen, for example copper with an index N=0.95, or aluminium.
By way of example, a detector according to this second embodiment can be implemented as follows:
According to a variant, the deposition of a metal layer can also be effected before carrying out lithography, the spaces left free by the lithography being filled in by means of a first material.
The guides thus described can be applied to types of images other than to ionising radiation images, such as for example infrared or UV imagers or wavelength shifters.
We shall now describe various embodiments of a method for producing an imager according to the present invention in the case where the portions made from a first material have a frustoconical shape, the steps of which are shown schematically in
Firstly the reading block is produced, which is formed from a substrate comprising matrices of photodetector pixels. The reading block without its electrical connections, which will be produced subsequently by vias, is shown in
Moreover, a layer of thermoplastic or thermosetting or UV-setting polymer is formed on a glass substrate 4. For example, it may be a thermoplastic such as PMMA or PS and/or a UV-setting polymer such as the SU8 resin manufactured by the company MicroChem®, for example by spin coating.
Alignment crosses were produced in advance on the glass substrate 4 for aligning the substrate with a mould 16. The mould 16 comprises a plurality of frustoconical recesses corresponding to the shape of the portions made from a first material 12. The truncated cone is in contact with the glass substrate 4.
The element thus obtained is shown in
During a following step, preferably the mould and the substrate comprising the layer of resin are heated before the pressing step, at a temperature higher than the glass transition temperature of the polymer, typically 20° to 50° above the glass transition temperature of the polymer. The mould 16 is aligned with the substrate 2 (
For example, if the pads are produced in SU-8, which is a UV-setting resin, after imprinting, the SU-8 pads are exposed to UV radiation and annealed in order to finalise the hardening of the resin.
During a following step, a layer of a third material is deposited on the reading block, for example by spin coating. Let h be the height of the pads, the thickness of the layer of glue is then advantageously between h/10 and 3h/4.
For example, h is equal to 4 μm and the thickness of the glue is between 100 nm and 3 μm.
During the following step, the element obtained after imprinting is turned over and is aligned with the reading block provided with the layer of third material; more particularly each portion made from a first material is aligned with the active part of a photodetector so that each pad 12 is centred on a photodetector. During this application, each pad 12 penetrates the layer of third material 13, the thickness of which is such that the third material 13, which is adhesive, wets the walls of each pad 12 and forms an area 14 surrounding each end of a pad 13.
The gluing is then carried out. The portions of resin are then in contact with the glass substrate 2 and the photodetectors 8. In
The element obtained is shown in
Next a step of thinning the substrate of the reading block takes place, for example by polishing, this is for example made from silicon. The mechanical rigidity of the assembly is provided mainly by the glass substrate 4.
The element obtained is shown in
The electrical connections of the reading block to the vertical connection means or via (or TSV “through-silicon via”) are then made through the substrate and connection balls. The element obtained is shown in
Next the detector 2 is connected to the element shown in
According to this embodiment, the second material 11 may be air.
In
The use of a mould makes it possible to produce portions of resin with a free shape, for example pads not having a constant cross-section, for example in the form of truncated lenses, truncated cones (
We shall now describe another embodiment of the production method according to the invention.
This method differs from the one described with reference to
This method is not a contact lithography, unlike the imprinting method. The forms of the pads that can be produced are forms with a constant cross-section, such as those in
The production of the areas 14 also makes it possible to produce a stronger assembly of the pads on the substrate carrying the photodetectors because of the presence of a relatively great thickness of the glue 13 and therefore to obtain a more robust device. This production method is therefore all the more advantageous when the first material constituting the pads is not sufficiently adhesive, and then a third material is used, the refractive index of which is close to that of the first material. This third material is adhesive, so that it affords good adhesion between the pads and the matrix of photodetectors.
In
In
The pads in a pyramidal, truncated pyramid and truncated paraboloid form are produced by imprinting. The pads of cylindrical form can be produced by imprinting or by UV lithography as described previously.
In general, according to this embodiment, the pads extend between a top base and a bottom base, the bottom base being up against the photodetector; the transfer section of said pads increases over the bottom part of the pads, that is to say on the part adjacent to the bottom base.
The imprinting technique is particularly suited for structuring non-conventional substrates in microelectronics, such as for example substrates of the scintillator type.
A method for producing a mould for imprinting will now be described briefly.
A hard mask is deposited on a substrate, for example made from silicon, provided with alignment marks. This is then structured by the deposition of a resin forming a pattern on the mask and by mask etching.
The silicon is then etched through the mask and the mask is removed. This is wet or dry etching or a combination of the two.
Preferably the mould thus formed is covered with a layer having non-adhesive properties, for example a single layer of molecules containing fluorinated atoms. This type of treatment is well known to persons skilled in the art and will not be described in detail. Such a layer facilitates the separation from the mould and from the substrate after imprinting.
In the case where the pads are produced on the detector block, they are assembled with the photodetectors, preferably with a machine of the flip-chip type, which enables the waveguides to be aligned with the photodetectors. An alignment of less than 1 μm can be achieved for aligning the waveguides with a photodetector substrate.
The form of the rectification areas 14 can advantageously be controlled during the gluing. For example, control of the time, the gluing temperature, the surface energies of the waveguides and the thickness of the glue makes it possible to control the form of the rectification areas 313.
The choice of the temperature of the third material, during pressurisation, has an effect on its viscosity. The higher the temperature, the more viscous the third material, and in addition this has a tendency to rise along the pads and therefore to wet them further. Controlling the temperature of the third material controls the wettability of the third material. The wetter the pads, the more pronounced is the beam rectification effect.
For example, in the case where the layer 313 is made from SU8, by pressing pyramidal waveguides with a slope of 80° at a temperature 50° higher than the glass transition temperature of the SU8 resin and at a temperature 10° higher than the glass transition temperature of the SU8 resin, rectification areas with very different forms are obtained. The areas obtained at a temperature 50° higher than the glass transition temperature of the resin has a greater height along the pads than that obtained with a temperature 10° higher than the glass transition temperature.
It is also possible to greatly accentuate the forms of the rectification area 14 by controlling the surface energy of the waveguides and controlling the height at which the third material wets the pads by modifying the wetting angle of the third material on the pads. For this purpose, chemical treatments are applied to the surface of the pads. These treatments are aimed at modifying the hydrophilic or hydrophobic character of the pads, by hydrophobic treatments, for example of the OpTool® type, or hydrophilic treatments, for example of the plasma argon or plasma argon and acetic acid vapour type.
Like the control of the temperature of the third material by promoting the wetting of the pads by the third material by virtue of suitable surface treatment, it is possible to amplify the effect of rectification of the beams.
As described above, the cylindrical waveguides can be produced by photolithography. It is also possible to produce waveguides in the form of cones with a slight slope. For this purpose, a substrate 4 is for example coated with a photosensitive resin, for example JR 335 resin. Next, by controlling the doses and the focusing distances, it is possible to obtain various types of structure having a slight slope, as presented in
After development of the non-exposed areas, the scintillator block is assembled on the waveguides.
In
By way of example, the transparent or silicon substrate is coated with a 3 μm layer of JR 355. The latter undergoes UV lithography. The resin not exposed by the UV radiation is then developed.
Number | Date | Country | Kind |
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11 61645 | Dec 2011 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP12/75662 | 12/14/2012 | WO | 00 | 6/13/2014 |