Patent Description:
It also relates to methods for manufacturing a sensor according to the invention.

The invention primarily aims at improving the Quality Assurance (QA) and radiobiological optimization of hadron therapy treatment plans. Preferred applications include determining essential input parameters such as the dose-mean lineal energy for the estimation of the relative biological effectiveness of an ion beam in the framework of hadron therapy and assessing the impact of cosmic radiation on a living organism.

Currently, most external radiotherapy treatments use photons to irradiate tumors. However, hadron therapy (using protons or carbon ions for example) damages tumor cells more effectively than photon therapy.

Charged particles have an inverse depth-dose distribution in comparison to photons, delivering a dose increasing with penetration depth up to a maximum at the end of the particles path (Bragg peak). This maximizes the dose transmitted to the cancerous area and reduces the dose transmitted to the healthy tissue.

Compared to protons, heavier ions such as carbon ions additionally show an increased Relative Biological Effectiveness (RBE). The RBE is defined as the ratio of the irradiated dose of ions to that of a reference photon energy that would be required to result in the same biological effect for both.

To ensure a safe dose delivery and to fully take advantage of ion therapy, a reliable estimation of the RBE of the ion beams is required. A microdosimetric kinetic model (MKM) may be used to predict the RBE of ion beams, based on microdosimetry, which rely on measurements of stochastic energy-deposition distributions in micro-sensitive volumes comparable in size to a human cell.

In routine clinical proton therapy, a constant RBE value of <NUM> is usually assumed. However, the RBE has been shown to vary with different Linear Energy Transfer (LET) values.

The LET is used to describe the average energy deposited per unit length (in keV/µm) in an absorber material. Limitations of the LET for precise assessment of the RBE have resulted in the development of a set of measurable stochastic quantities, such as the lineal energy which provides the fundamental basis for microdosimetry.

The lineal energy is used to describe the energy deposited by a single event, namely ε, in a micron-size sensitive volume, or microvolume, along the mean particle track length, l, which is given by: <MAT>.

Several devices such as the tissue equivalent proportional counter (TEPC) or silicon-based semiconductor microdosimeter may be used for the measurement of microdosimetric spectra.

Known TEPC consist of a tissue equivalent plastic sphere in a grounded aluminum shell. The sphere is filled with a tissue equivalent gas to simulate interactions with human tissue. This kind of detectors usually use centimeter-sized sensors, which is much larger than the size of a typical human cell.

Mini-TEPCs have been developed with cylindrical sensitive volumes having sizes of the order of a few tens of millimeters. Recently, different miniaturized tissue-equivalent gas detectors corresponding to operation at a nanometric level down to <NUM> have been developed for microdosimetry applications in radiotherapy.

TEPC detectors have had some success while tested in different therapeutic heavy ion beams but are still being further developed. Currently, to the knowledge of the inventors there is no commercial product and the construction of a mini-TEPC is a demanding operation requiring special knowledge and maintenance during use, e.g. for the supply of gases.

Limitations of TEPCs compared to microdosimetry include the need for high-voltage and for a gas supply system. Further limitations include so-called pulse pile-up problems, as well as a low spatial resolution due to a relatively large size of the sensitive volumes compared to other solid-state microdosimeters (which are in the µm size range).

Silicon-based microdosimeters, better known as silicon-on-insulator (SOI) microdosimeters, have also been developed over the last <NUM> years.

The use of SOI microdosimeters in lineal energy measurements offers high spatial resolution and counting rates at low costs. Using the latest generation of silicon-based microdosimeters, a good agreement with the results from the well-established TEPCs was achieved. However, a frequently raised issue is the question of tissue equivalence in the case of silicon (with Z = <NUM>), leading to complex correction factors when compared to other materials such as diamond (Z = <NUM>). Silicon-based sensors are therefore not optimal in this respect. Diamond, with its unique physical properties (e.g. a large-band gap, temperature stability, fast drift velocity and low capacitance), has been identified as a promising candidate material to produce microdosimetry devices.

Recently, significant developments in diamond-based microdosimetry have been made with various fabrication and operational approaches.

In <NPL>) a diamond-based microdosimeter is disclosed, which includes a multi-layered structure obtained by a two-step diamond growing technique. The fabrication process essentially consists in growing a highly boron-doped diamond layer on a thick High Pressure High Temperature (HPHT) diamond substrate, followed by a thin intrinsic diamond layer and a metal electrode on the top. A Schottky contact is formed between the metal and the intrinsic layer allowing zero bias voltage operation of the device. The metal electrode defines the dimensions of the sensitive volumes.

However, the sensitive volumes need to be connected to a readout channel, which requires the formation of interconnecting metal tracks inactive in terms of signal generation but which generate distorting signals. Furthermore, an incomplete signal coming mostly from the edges of the sensitive area can be observed. This incomplete signal has only little influence on the full Charge Collection Efficiency (CCE) signal for a large sensitive volume with an area of 300x300 µm<NUM> as described in the reference cited above. However, this incomplete signal will significantly contribute to the measured signal when the sensitive volumes becomes smaller, e.g. 30x30 µm2, as the area with full CCE is also smaller, thus decreasing the accuracy of the measurement.

<NPL>) discloses a diamond microdosimeter detector featuring a lateral electric field structure, created by using a combination of selective laser ablation and active brazing alloys. The so-called 3D Lateral Electrode Structure is deposited onto an intrinsic single crystal diamond substrate.

However, this design is believed to generate distorted signals also and the detector has not been tested with ion beams to the knowledge of the inventors.

<NPL>) discloses bi/tri-layered diamond structures. A diamond microdosimeter utilizing a combination of laser ablation and active brazing alloys upon a bi-layered structure is proposed. The structure features cylindrical sensitive volumes created by laser ablation and active brazing alloys situated within a thin membrane of electronic grade polycrystal CVD diamond. The microfabrication process is relatively complicated.

<NPL>) discloses a so-called "p+sensor" structure consisting of a patterned heavily boron-doped layer (p+), which is used to create active micro-sensitive volumes within the sensor volume.

Due to the formation of local p+-i-m junctions, this sensor is self-biased with an experimentally measured built-in potential of <NUM> V, corresponding to a built-in electric field of <NUM> V/µm in the <NUM>-thick sensor. This electric field is strong enough to obtain a full Charge Collection Efficiency (CCE) for low LET protons. However, for heavier ions with higher LET (e.g. carbon), incomplete CCE is observed.

<NPL>) discloses a guard ring approach, consisting in depositing a patterned metal-based top electrode onto an intrinsic diamond membrane. The patterned top electrode in combination with a pad electrode on the back of the so-called "GR sensor" is used to create active micro-sensitive volumes within a <NUM>-thick membrane. As a result of the biasing of the sensor, a significant increase in the electric field, <NUM> V/µm and higher is obtained within the micro-sensitive volumes, thereby enabling full CCE to be obtained for various projectiles. Preliminary experiments using clinical beams have demonstrated a very good performance. However, this sensor includes connecting bridges, which are essential for the signal read-out from the micro-sensitive volumes but generate a distorting signal degrading the accuracy of the measurements.

The application <CIT>discloses a diamond-based detector in which micro-sensitive volumes may be formed by degraded patterns in a diamond layer.

The publication <NPL> relates to a diamond microdosimeter prototype comprising sensitive volumes separated through laser milled trenches.

The thesis <NPL> discloses a 3D Lateral Electrode Structure detector in which radiation-damaged patterns may be used to reduce charge sharing between sensitive volumes.

Therefore, there remains a need for improving existing sensors to remedy to all or part of the deficiencies of the prior art mentioned above, and the invention aims to benefit from a reliable and relatively easy to manufacture sensor allowing accurate microdosimetric measurements.

Exemplary embodiments of the invention relate to a sensor according to claim <NUM>, comprising.

Thanks to the absence of charge generation from and to insulation provided by the layer of the non-electrically active material surrounding the micro-sensitive volumes, signal distortion as encountered in the prior art and arising for example due to connecting bridges between micro-sensitive volumes is avoided, thus contributing to improve the accuracy of the measurements.

In particular, as there is substantially no charge diffusion from areas surrounding the micro-sensitive volumes and occupied by the non-electrically active material, the sensor may deliver a high spatial resolution response with no or limited low energy tail in pulse-height spectra arising from charge diffusion directed to micro-sensitive volumes from neighboring material. Therefore, a better microdosimetric spectrum quality can be achieved.

The electrodes may easily be connected to a multichannel readout. In particular, an electrode may be easily connected to a respective isolated single micro-sensitive volume, if desirable. When combined with a multichannel readout, the individual reading of the output of each of the micro-sensitive volumes (or sets of micro-sensitive volumes electrically connected together) allows relatively good mapping capabilities, as well as precise measurements in the case of a high rate of incoming particles.

Due to the physical properties of diamond, the sensor according to the invention is also more tissue-equivalent and radiation hard compared to silicon-based sensors.

In the context of the invention, a "non-electrically active material" refers to a material that creates substantially no directional drift of radiation-induced free charge carriers under an electric field. While incident radiation such as an ion beam may create free charge carriers in the non-electrically active material, the lifetime of the free charge carriers in the non-electrically active material is very short, e.g. on the order of a picosecond.

As a result, under typical conditions, e.g. under electric fields of <NUM>-<NUM> V/µm, no measurable signal is generated in the non-electrically active material.

Examples of suitable non-electrically active materials include radiation hard resists such as epoxy-based resins (e.g. SU-<NUM>), polymers such as PMMA, and some inorganic materials such as glass.

The non-electrically active material may have a planar configuration and/or be of a substantially constant thickness, preferably having a same thickness as the micro-sensitive volumes.

The non-electrically active material preferably extends in contact with and around the entire periphery of each one of the micro-sensitive volumes.

Preferably, the non-electrically active material fills all the space between the micro-sensitive volumes, leaving no voids between them.

The non-electrically active material may be rigid or flexible.

The micro-sensitive volumes may be mechanically supported by the non-electrically active material, and the latter may be supported only at its periphery by a frame, preferably a diamond-based frame made of a same material as the micro-sensitive volumes.

The non-electrically active material may support metal electrodes that covers both the micro-sensitive volumes and the non-electrically active material.

The non-electrically active material may support a doped layer or cover a doped layer, as detailed below.

The non-electrically active material may be superposed entirely to a substrate, for example a HPHT (High Pressure High Temperature) or CVD diamond substrate.

The non-electrically substrate may support conductive tracks extending along its thickness.

According to some embodiments, the sensor comprises a diamond-based frame supporting said non-electrically active layer, preferably an intrinsic diamond layer, more preferably an intrinsic CVD diamond layer.

The diamond-based frame may for instance be U-shaped when seen in a direction orthogonal to the plane of the non-electrically active layer.

Preferably, the sensor comprises electrode bonding pads in contact with said frame.

The bonding pads are advantageously arranged on another part than the resist layer. Due to the possibly flexible nature of the resist layer, bonding pads arranged on the resist may be damaged. By contrast, performing micro-bonding on diamond provides improved reliability. Furthermore, the bonding pads are preferably arranged as far as possible from the back electrode in order to minimize the electric field at the bonding pads' location and to prevent undesired collection of charges.

Preferably, the sensor comprises a common electrode extending over end faces which are on a side of the non-electrically active layer adjacent the diamond-based frame.

This common electrode, arranged on the bottom side of the sensor, may be kept at a ground potential or at some biasing voltage provided by an external source.

According to some embodiments, the sensor comprises a diamond-based substrate over which the plurality of micro-sensitive volumes extends.

Preferably, the sensor comprises a p, p+, n or n+ doped diamond layer. This layer is in contact with end faces of the plurality of micro-sensitive volumes, on one side thereof.

The doped layer may extend over the end faces that are adjacent the diamond-based frame. The doped layer may extend between the plurality of micro-sensitive volumes and the diamond-based substrate and over said substrate on a first area thereof, electrode bonding pads being located respectively on said doped layer over said first area and on the substrate over a second area thereof opposite the first one with respect to the micro-sensitive volumes. Preferably, the p, p+ doped layer is a boron doped layer and the n, n+ doped layer is a phosphorus doped layer, but other dopants may be used.

The doped layer provides a self-biasing of the micro-sensitive volumes, thus creating a built-in potential. The built-in potential of the sensor according to the invention may be strong enough to obtain a full charge collection efficiency (CCE) in the case of a proton beam. Thus, external biasing voltage may not be needed for measurements in proton therapy. However, for heavier ions (e.g. carbon, oxygen, silicon, or neon) an external bias may be applied to create a stronger electric field, thus resulting in a full CCE for a wide range of energies and ions.

Additionally, the doped layer may provide an additional mechanical support for the micro-sensitive volumes.

In a preferred embodiment, the diamond-based micro-sensitive volumes are made of intrinsic diamond, preferably single crystal diamond, more preferably CVD (chemical vapor deposition) diamond.

According to a preferred embodiment, the micro-sensitive volumes have a thickness in a direction perpendicular to a plane defined by the non-electrically active layer comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>, even more preferably between <NUM> and <NUM>.

The micro-sensitive volumes preferably have a shape that has a symmetry of revolution.

The micro-sensitive volumes may have a cylindrical shape with an axis perpendicular to said plane and with a diameter comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>, even more preferably between <NUM> and <NUM>.

In some variant embodiments, the micro-sensitive volumes have a spherical or half-spherical shape with a diameter comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>, even more preferably between <NUM> and <NUM>. A spherical or half-spherical shape may improve accuracy of the measurement when incoming radiation is not oriented in a particular direction. However, in hadron therapy a cylindrical shape is appropriate as the sensor may be oriented so that the axis of the micro-sensitive volume may be substantially aligned with the incoming direction of the ion beam.

The diameter-to-thickness ratio for the micro-sensitive volumes may range from <NUM>:<NUM> to <NUM>:<NUM>, and is preferably of about <NUM>:<NUM>. When the cross-section of the micro-sensitive volume is not circular, the diameter corresponds to the one of the circumscribed circle.

Preferably all the micro-sensitive volumes have a same shape, but in some variants the sensor comprises at least two micro-sensitive volumes with different shapes.

All micro-sensitive volumes may have a same thickness. However, at least two micro-sensitive volumes may have different thicknesses. The two different thicknesses may differ from each other by at least <NUM>%, or more, for example by at least <NUM> or <NUM>%.

Micro-sensitive volumes of different thicknesses may have a same diameter or a different diameter, preferably a same diameter.

Micro-sensitive volumes having a same thickness may have a same diameter or a different diameter, preferably a same diameter.

Micro-sensitive volumes of different thicknesses allow to vary the sensitivity to energy deposition which may prove useful when the LET of the ion beam varies along the treatment path. At the entrance of the treatment path, far from the Bragg peak, the LET may be very small (e.g. a few keV/µm) whereas around the Bragg peak maximum it may reach up to <NUM> keV/µm. Micro-sensitive volumes of larger thicknesses may thus provide a stronger signal than those of smaller thickness thus improving the signal/noise ratio. Shorter micro-sensitive volumes provide however better spatial accuracy near the Bragg peak, due to their reduced thickness, and their lower sensibility is compensated by the stronger energy deposition near the Bragg peak.

The sensor preferably comprises an array of micro-sensitive volumes.

The volume of one micro-sensitive volume may range from <NUM> to <NUM><NUM>, preferably from <NUM> to <NUM><NUM>, more preferably from <NUM> to <NUM><NUM>.

A top electrode connected to a readout channel may interconnect several micro-sensitive volumes of an array of micro-sensitive volumes of the sensor. The number of interconnected micro-sensitive volumes may range from <NUM> to <NUM>.

The number of micro-sensitive volumes of the sensor may range from <NUM> to <NUM>.

The sensor of the invention can be manufactured according to the various methods defined below, depending its structure as defined above.

One method according to claim <NUM> of manufacturing the sensor of the invention relates to a method for manufacturing a sensor, comprising:.

Another method according to claim <NUM> of manufacturing the sensor of the invention relates to a method for manufacturing a sensor, comprising:.

A further method according to claim <NUM> of manufacturing the sensor of the invention relates to method for manufacturing a sensor, comprising:.

All these microfabrication methods are relatively easy to implement and to manufacture the sensor in a reliable manner at a relatively low cost. They may allow the fabrication of micro-sensitive volumes of different sizes independently of the currently available scCVD diamond plates.

The sensor may be used in hadron therapy, in particular with Pencil Beam Scanning (PBS) ion beams and with passive scattering delivery techniques such as Double Scattering (DS).

The sensor may also be used for monitoring exposition to space radiation, for example during missions to the moon or Mars.

The terms "top" and "back" refer to one orientation of the sensor on the drawings. The sensor may be used in any appropriate orientation with respect to the incident radiation, and the top electrode may or may not be facing the incident radiation.

Referring to <FIG>, there is shown a first embodiment of a micro-dosimeter sensor <NUM> according to the invention.

The sensor <NUM> comprises an intrinsic (i.e. undoped) CVD diamond frame <NUM>. The latter supports on its top side a layer <NUM> of a non-electrically active material. The layer <NUM> is made for example of an epoxy-based resist, for example SU-<NUM>.

Micro-sensitive volumes <NUM> (also referred to as µSVs) are embedded in the non-electrically active layer <NUM>, while having their end faces on the bottom side and on the top side exposed. The µSVs <NUM> are separated from one another by the material of the layer <NUM>.

In the embodiment shown in <FIG>, the µSVs <NUM> are cylindrical in shape, with their axis oriented perpendicularly to the plane along which the layer <NUM> extends. They may have a diameter comprised between <NUM> and <NUM>, but their size and shape may vary.

The layer <NUM> is not electrically active, meaning that the incoming ions do not generate directional drift of free charge carriers therein.

The layer <NUM> contributes to the mechanical stability of the µSVs <NUM>.

In the example shown, all the µSVs <NUM> have a same thickness, but their thickness may vary as will be described in reference to a further embodiment.

The µSVs <NUM> and the layer <NUM> may be <NUM> to <NUM> thick, depending in particular on the dosimetry needs for the sensor.

The µSVs <NUM> are preferably made of single crystal CVD intrinsic diamond.

Top electrodes (only two of them being apparent in the figures) are arranged on the top side of the layer <NUM>, each of the top electrodes being in electrical contact with end faces of a respective subset of µSVs <NUM>.

In the <FIG>, two arrays of µSVs <NUM> are shown to be connected to respective top electrodes <NUM> and <NUM>. For each array, the corresponding µSVs <NUM> are interconnected by the respective top electrode <NUM> or <NUM>.

The top electrodes are made of any appropriate electrically conducive material and may be metal-based, for example aluminum-based, or carbon-based.

Bonding pads are arranged on the frame <NUM> and are electrically connected to the top electrodes. In <FIG>, two bonding pads <NUM>, <NUM> are shown, respectively connected to top electrodes <NUM> and <NUM>. The bonding pads <NUM>, <NUM> are connected to readout electronics by any appropriate wiring (not shown).

A doped diamond layer <NUM> is arranged on the bottom side of the non-electrically active layer <NUM> and is in electrical contact with the bottom end faces of the µSVs <NUM>.

The doped layer <NUM> may be a boron-doped p+ diamond layer. It may have a thickness comprised between <NUM> and <NUM>.

The doped layer <NUM> provides additional mechanical support to the µSVs <NUM> in addition to self-biasing the µSVs <NUM> as illustrated in <FIG>.

With a boron p+ doped layer <NUM>, intrinsic scCVD diamond µSVs <NUM>, and aluminum top electrodes <NUM>, <NUM>, the built-in potential may be above 1V, for example of about <NUM> V.

A bottom electrode <NUM> is arranged on the bottom side of the doped layer <NUM>. This bottom electrode <NUM> may be metal-based, for instance aluminum-based, and may be electrically connected to the electrical ground.

When the sensor <NUM> is irradiated by incident radiation, ionizing particles may interact with the µSVs <NUM> as illustrated in <FIG>. When an ionization event inside one of the µSVs <NUM> occurs, an electrical signal is generated, which may be processed by the readout electronics to provide micro-dosimetric or dose rate measurement.

In the embodiment of <FIG>, the built-in potential is strong enough to obtain a full charge collection efficiency (CCE) in the case of a proton beam. Thus, no external biasing is needed for measurements in proton therapy in this configuration.

However, for heavier ions (e.g. carbon, oxygen or neon) a stronger electric field is needed. An external bias from a voltage source may then be applied to the electrodes, thus resulting in a full CCE for a wide range of energies and ions.

Despite the external bias applied to the µSVs <NUM>, no charge transport takes place in the layer <NUM>. Thus, no signal distortion is induced by charge diffusion from the layer <NUM> into the µSVs <NUM>.

<FIG> illustrate successive microfabrication steps for manufacturing the sensor of <FIG>.

Firstly, a diamond plate <NUM> is provided (<FIG>). The diamond plate is preferably between <NUM> and <NUM> thick and may be made of single crystal chemical vapor deposition (scCVD) raw synthetic diamond.

As shown in <FIG>, the diamond plate <NUM> is then sliced in thin plates <NUM> with a preferred thickness of <NUM> to <NUM>, and polished. The surface area of the thin plates may be comprised between 3x3 mm<NUM> and 1x1 cm<NUM>.

Next, the thin plate <NUM> is etched to create an ultra-thin membrane <NUM> with a thickness smaller than or equal to <NUM>, suspended over a bulky frame <NUM> (<FIG>). The ultra-thin membrane <NUM> may have the same surface area as the original thin plate. The bulky frame <NUM> may be U-shaped when viewed in a direction orthogonal to the plane of the membrane.

In order to obtain the ultra-thin membrane <NUM>, a deep Ar/O<NUM> plasma etching is used, following the method described e.g. in <NPL>).

The ultra-thin diamond membrane <NUM> is advantageously cleaned with hydrofluoric acid and a hot acid treatment in order to eliminate possible surface contamination.

As shown in <FIG>, a thin and heavily doped diamond layer <NUM> is grown on a bottom side of the diamond membrane <NUM> and of the frame <NUM>. The diamond layer <NUM> is advantageously p+ boron-doped and may be <NUM> to <NUM> thick.

The doped layer <NUM> is produced via chemical vapor deposition (CVD).

Preferably, prior to further processing, a hot acid cleaning treatment is applied on the sample to remove possible surface contamination.

Optionally, the sample is fixed onto a silicon wafer in order to achieve a better structural stability for the following processing steps. To do so, a thin layer of photoresist is spin coated on the silicon wafer and the sample placed on it with its top side facing up, i.e. with the doped layer <NUM> facing the silicon wafer. The photoresist is then hardened by soft baking for a few minutes, for instance at a temperature of <NUM>.

In the next step, as shown in <FIG>, a positive epoxy-based resist <NUM> is deposited on the diamond membrane. Photolithographic patterning is then performed using UV light. In other words, only specific areas of the resist layer <NUM> are exposed to the UV light, such that the resist from these specific areas may be removed. This creates empty volumes <NUM> that allow a direct access to the diamond membrane <NUM> through the resist layer <NUM>.

Next, a layer of chromium (Cr) which may be <NUM> to <NUM> thick is deposited onto the top side of the diamond membrane <NUM>. The sample is then gently rinsed with acetone in order to detach the residual photoresist layer (lift-off technique). A metallic Cr mask <NUM> on the diamond membrane <NUM> is thus obtained (<FIG>).

A wet etch approach may also be used to obtain the Cr mask <NUM>.

Acetone rinsing is used to remove the sample from the silicon wafer.

The sample is then rinsed with isopropanol and blow-dried.

In a further processing step, depicted in <FIG>, single-standing micro-sensitive volumes (µSVs) <NUM> are created within the diamond membrane <NUM>.

To that end, Ar/O<NUM> reactive ion etching (RIE) is used. Due to the higher etching selectivity for diamond compared with the Cr mask <NUM>, micro-sized volumes of diamond are etched beneath the metal mask, down to the doped layer <NUM>, thus forming isolated µSVs <NUM> in the diamond membrane <NUM>.

The diamond membrane <NUM> in between the µSVs <NUM> is completely etched.

Before further processing, a chromium etchant followed by a hot acid cleaning treatment is applied to the etched membrane in order to remove the residual Cr mask and possible surface contamination. The etched membrane is also rinsed with isopropanol and blow-dried.

Preferably, the sample is then again fixed on a silicon wafer as previously described.

Ar plasma may be used to ensure that the surface of the membrane is clean from any organic contamination.

As shown in <FIG>, a layer <NUM> of non-electrically active material is then spin coated between the µSVs <NUM>. The layer <NUM> may be made of SU-<NUM> photoresist.

To limit the layer <NUM> to regions closely surrounding the µSVs <NUM>, photolithography may be used in the same way as described with reference to <FIG>.

Should the sample be attached to a silicon wafer with photoresist, the photoresist is then dissolved with acetone.

Next, the layer <NUM> is hard baked, for instance at a temperature of <NUM>, to cure the resist <NUM>. The layer <NUM> subsequently becomes solid and impossible to dissolve in acetone.

Once the layer <NUM> is cured, photolithography cannot be used anymore to modify its shape or to remove it. Therefore, shallow Ar/O2 plasma etching is used to remove the resist layer <NUM> which may lie on top of the end faces of the µSVs <NUM>.

As shown in <FIG>, the next step consists in depositing a strip of Cr layer <NUM> onto the back side of the sample. The Cr layer <NUM> acts as a mask to protect the underlying doped layer during the next etching step, where the portion of the doped layer <NUM> outside the masked area is etched from the frame <NUM> to obtain the structure of <FIG>.

Etching the doped layer prevents the formation of local p+-intrinsic diamond junction and thus the creation and collection of charge from the bonding pads and the connection tracks.

The Cr layer <NUM> additionally acts as a strip pad back electrode for the diamond sensor after plasma etching.

Next, one or several electrical contacts <NUM>, <NUM> are deposited on the top side of the sample (<FIG>). The contacts <NUM>, <NUM> may be metal- or carbon-based as mentioned above.

To this end, the sample may be fixed on a silicon wafer following the procedure described previously. To create multiple patterned electrodes <NUM>, <NUM>, covering only the area closest to the µSVs <NUM>, a standard resist is spin coated onto the sample. Using photolithography, resist patterns can be created which will define the top electrical contacts <NUM>, <NUM> and the bonding pads <NUM>, <NUM>.

In the case of metal-based electrodes, a wet etch technique may be applied: using a specific metal etchant, all metallization except the regions covered by the resist is removed. In the case of carbon-based electrodes, a lift-off technique may be used instead.

The sample is then gently rinsed with acetone to remove the resist patterns from the newly created electrodes and to remove the sample from the silicon wafer.

The processed sample is then rinsed with isopropanol and blow-dried.

The resulting sensor <NUM> is then ready for mounting onto an appropriate sensor carrier and for wire bonding (<FIG>).

Turning to <FIG>, a second embodiment of a sensor <NUM> according to the invention is described.

The sensor <NUM> of <FIG> differs from that of <FIG> by the absence of the doped layer <NUM>, the bottom electrode <NUM> being directly in contact with the bottom end face of the µSVs <NUM> and the bottom face of the layer <NUM>.

When the micro-dosimeter sensor <NUM> of <FIG> is irradiated with an ion beam, ionizing particles may interact with a µSV <NUM> as illustrated in <FIG> and excess charge carriers' electrons and holes <NUM>, <NUM> are generated and start to drift in the presence of an externally generated electric field as illustrated in <FIG>.

<FIG> illustrate successive steps of a manufacturing method to obtain the sensor <NUM>.

Firstly, the preliminary steps previously described with reference to <FIG> may be carried out in order to obtain a thin intrinsic diamond membrane <NUM> suspended over an intrinsic diamond frame <NUM>, as shown in <FIG>.

<FIG> illustrate processing steps identical to the processing steps already described with reference to <FIG>.

Next, as shown in <FIG>, Ar/O<NUM> reactive ion etching is carried out to create the single-standing micro-sensitive volumes <NUM> within the diamond membrane <NUM>, similarly to the processing step described with reference to <FIG>. However, in the present case, the diamond membrane <NUM> is not fully etched through: a residual layer <NUM> of intrinsic diamond, which may be approximately <NUM> thick, is left as a layer supporting the µSVs <NUM>.

A chromium etchant followed by a gentle hot acid cleaning treatment may then be applied to the etched membrane, in order to remove the residual Cr-mask and possible surface contamination of the sample. Isopropanol rinsing and blow-drying may be carried out. Additionally, Ar plasma may be used to ensure that the surface of the sample is clean from any organic contamination before further processing.

Next, a layer <NUM> of non-electrically active material is spin coated onto the supporting layer <NUM> as illustrated in <FIG>. This step is carried out in the same way as detailed above with reference to <FIG>.

The following step, depicted in <FIG>, consists in removing the residual diamond layer <NUM> from the bottom side of the non-electrically active layer <NUM>. To do so, Ar/O<NUM> reactive ion etching may be used.

Then, as shown in <FIG>, the bottom electrode <NUM> which may be metal-based, preferably aluminum-based, is deposited on the bottom side of the layer <NUM> and of the µSVs <NUM>. The electrode <NUM> acts as a full pad back electrode for the sensor and provides improved stability. Finally, as shown in <FIG>, top electrodes on the top side of the layer <NUM> and of the µSVs <NUM> are deposited, following the method described with reference to <FIG>.

The sensor <NUM> is then ready for mounting onto an appropriate sensor carrier and for wire-bonding, as illustrated in <FIG>.

Turning to <FIG>, a third embodiment of a sensor <NUM> according to the invention is described.

Referring to <FIG>, the sensor <NUM> comprises an intrinsic (i.e. undoped) thick diamond bulk substrate <NUM>. The substrate <NUM> may be <NUM> to <NUM> thick and supports on its top side a doped diamond layer <NUM> and a non-electrically active layer <NUM>.

The main purpose of the diamond bulk substrate <NUM> is the mechanical stabilization of the sensor. The substrate <NUM> has no impact on the measured microdosimetric spectra.

Micro-sensitive volumes <NUM> are embedded in the layer <NUM>, while leaving their end faces on the bottom side and on the top side exposed. As in the previous embodiments, the µSVs <NUM> are separated from one another by the material of the layer <NUM>.

In the embodiment shown in <FIG>, the µSVs <NUM> are cylindrical in shape. As for the sensors <NUM> and <NUM>, the µSVs <NUM> are preferably made of single crystal CVD intrinsic diamond. The doped diamond layer <NUM> may be a p+ boron-doped layer, with a thickness comprised between <NUM> and <NUM>.

The bulk substrate <NUM> may be a HPHT (High Pressure High Temperature) diamond due to its low cost. It may also be CVD grown diamond, for example heteroepitaxial diamond grown by CVD on iridium.

On the bottom side, the end faces of the µSVs <NUM> are in electrical contact with the doped layer <NUM>. Thus, the latter may serve as a bottom electrode for the µSVs <NUM>. The doped layer <NUM> also allows self-biasing of the µSVs <NUM>.

A common contact pad <NUM> which may be carbon- or metal-based lies on top the doped layer <NUM> aside the µSVs <NUM> and may be kept at a given potential, for example ground potential. On the top side, the end faces of the µSVs <NUM> are in electrical contact with top electrodes, only two of them, referred to as <NUM> and <NUM> being shown. These top electrodes are in electrical contact with end faces of a respective subsets of µSVs <NUM>.

As in previous embodiments, the µSVs <NUM> of each array are interconnected by a respective top electrode.

Bonding pads <NUM>, <NUM> are electrically connected to the top electrodes <NUM>, <NUM>. The bonding pads <NUM>, <NUM> are connected to readout electronics by any appropriate wiring (not shown).

The bonding pads are deposited directly on the diamond substrate <NUM> rather than on the non-electrically active layer <NUM>.

In the example shown, the bonding pads lie opposite to the contact pad <NUM> with respect to the µSVs <NUM>.

When the micro-dosimeter sensor <NUM> of <FIG> is irradiated with an ion beam, ionizing particles may interact with a µSV <NUM> as illustrated in <FIG> and excess charge carriers' electrons and holes <NUM>, <NUM> are generated and start to drift in the presence of a built-in electric field as illustrated in <FIG>.

As described with reference to the embodiment of <FIG>, the built-in potential may be strong enough to obtain a full charge collection efficiency (CCE) in the case of a proton beam. An external bias may also be applied to obtain a stronger electric field.

Turning to Figures 12A to <NUM>, a method for manufacturing the sensor <NUM> is described.

Firstly, a diamond bulk substrate <NUM> is provided (<FIG>). The substrate <NUM> may be a CVD grown diamond plate or HPHT diamond plates. The substrate <NUM> may be <NUM> to <NUM> thick.

A thin and heavily doped diamond layer <NUM> is then grown on the top side of substrate <NUM> as shown in <FIG> via CVD. The layer <NUM> may be <NUM> to <NUM> thick and may be p+ boron-doped.

Prior to further processing, a hot acid cleaning treatment may be used to remove possible surface contamination.

As shown in <FIG>, a layer <NUM> of intrinsic diamond is then grown via CVD on the top side of the doped layer <NUM>.

The obtained multi-layer sample may then be cleaned using a hot acid treatment to remove possible surface contamination. In order to improve its stability, it may also be fixed on a silicon wafer following the method described previously.

As shown in <FIG>, a resist layer <NUM> is subsequently deposited on the diamond layer <NUM> and photolithographic patterning is performed as described with reference to <FIG> and <FIG>.

As described with reference to <FIG> and <FIG>, a <NUM> to <NUM> thick layer of Cr is then deposited and the layer <NUM> is removed to create a metallic hard mask on the layer <NUM> as shown in <FIG>.

Next, as described with reference to <FIG>, Ar/O<NUM> reactive ion etching is used to form the µSVs <NUM> in the diamond layer <NUM> as shown in <FIG>. The diamond layer <NUM> is etched over the entirety of its thickness, i.e. down to the doped layer <NUM>.

Before further processing, a chromium etchant followed by a hot acid cleaning treatment may be applied to the etched sample in order to remove the residual Cr mask and possible surface contamination. Isopropanol rinsing and blow-drying may follow.

Next, to remove the doped layer <NUM> from a region of the diamond substrate <NUM> and thus define the shape of the doped electrode, a polycrystal CVD diamond layer is used as a shadow mask <NUM> as shown in <FIG>. To this end, the shadow mask <NUM> is arranged on the sample without attaching it to the sample.

Ar/O<NUM> reactive ion etching is then used to remove the area of the doped layer <NUM> uncovered by the mask <NUM> (<FIG>).

Alternatively, the shape of the doped electrode may be defined by growing the doped layer <NUM> only on selected areas of the diamond substrate <NUM>.

Before further processing, a hot acid cleaning treatment may be applied to remove any possible surface contamination during the etching process. The sample may also be rinsed with isopropanol and blow-dried, and once more fixed on a silicon wafer.

Next, as depicted in <FIG>, a layer <NUM> of non-electrically active material is spin coated onto the sample. Photolithography may be used as described with reference to <FIG> to limit the layer <NUM> to regions closely surrounding the µSVs <NUM>.

Should the diamond substrate <NUM> be fixed to a silicon wafer, the photoresist used to this end is removed from the edges of the diamond substrate <NUM> with acetone to detach the sample. The sample is then hard baked in order to cure the layer <NUM>.

Once the resist layer <NUM> is cured, shallow Ar/O2 plasma etching is used to remove the layer <NUM> which may lie on top of the end faces of the µSVs <NUM>.

In the next step illustrated in <FIG>, electrical contacts comprising electrodes <NUM>, <NUM> and contact pads <NUM>, <NUM>, <NUM> are patterned on the top side of the of the layer <NUM> and of the µSVs <NUM>, following the method described with reference to <FIG>.

Finally, the obtained multi-layer diamond sensor <NUM> should be rinsed with isopropanol and blow-dried. The sensor <NUM> is then ready for mounting onto an appropriate sensor carrier and for wire bonding as shown in <FIG>.

<FIG> illustrates a further embodiment of a sensor <NUM> according to the invention. As depicted, the micro-sensitive volumes <NUM> may form several subsets of respective different thicknesses. Each subset may have µSVs <NUM> all having a same thickness and interconnected by a same top electrode or by a plurality of top electrodes. One subset may have µSVs <NUM> having a thickness that differs by at least <NUM>% from the thickness of another subset.

The number of µSVs <NUM> may be the same for each subset or may be different.

<FIG> illustrates the response of a sensor according to the invention to a raster scan with a +<NUM> V bias voltage and a <NUM> MeV alpha particle beam. As shown by the image, the non-electrically active material <NUM> does not produce any measurable signal, contrary to the µSVs <NUM>.

The invention is not limited to the examples described above; notably, features of the illustrated examples may be combined with one another in variants which have not been illustrated.

The doped layer may be a n type, instead of p type, and be a p, p+, n or n+ doped layer.

The non-electrically active material may be a material other than an epoxy-based resist, for example any other appropriate any hard-baked stable resist or polymer, such as PMMA, or an inorganic material, such as glass.

Claim 1:
A sensor (<NUM>; <NUM>; <NUM>; <NUM>) comprising:
- a plurality of isolated, single-standing diamond-based micro-sensitive volumes (<NUM>) having opposite end faces (12a, 12b),
- a layer (<NUM>) of a non-electrically active material extending around each micro-sensitive volume (<NUM>) of said plurality,
- electrodes (<NUM>, <NUM>; <NUM>; <NUM>) electrically connected to said end faces of said plurality for signal readout.