Detector of electromagnetic radiation

Abstract

A detector of electromagnetic radiation (RL) is described. The detector comprises: an oriented polycrystalline layer (2) of thermoelectric material, a substrate (1) superimposed on the top surface of the oriented polycrystalline layer so that the back surface (10) is in contact with the oriented polycrystalline layer, first and second electrodes spaced the one from the other and in electrical contact with the oriented polycrystalline layer. The substrate comprises at least one ceramic layer and the oriented polycrystalline layer has a crystal orientation at an angle comprised between 30 degrees and 55 degrees relative to a normal to the top surface of the substrate.

Claims

1. A detector of electromagnetic radiation, comprising: an oriented polycrystalline layer of thermoelectric material, which is an active layer of the detector, a substrate superimposed on a top surface of the oriented polycrystalline layer so that a back surface of the substrate is in contact with the oriented polycrystalline layer, the substrate is a target of incident electromagnetic radiation on a front side, first and second electrodes spaced from one another and in electrical contact with the oriented polycrystalline layer, wherein the substrate comprises at least one ceramic layer and the oriented polycrystalline layer has a crystal orientation at an angle of between 30 degrees and 55 degrees relative to a normal to a top surface of the substrate, which is non-oriented.

2. The detector according to claim 1, wherein the substrate comprises a metallic layer which has been electrically passivated by a ceramic layer.

3. The detector according to claim 1, comprising at least one strip formed by the oriented polycrystalline layer which extends between the first and the second electrodes.

4. The detector according to claim 1, comprising a plurality of strips wherein each strip is formed by the oriented polycrystalline layer, the strips of the plurality of strips are spaced apart and are parallel to each other, the strips of the plurality of strips being arranged in succession from a first strip to a n-th strip, each strip of the plurality of strips having a first extremity and a second extremity, the first electrode being connected to the first extremity of a first strip of the plurality of strips and the second electrode being connected to the second extremity of the n-th strip of the plurality of strips, the second extremity of each strip of the plurality of strips being in electrical contact with the first extremity of a successive strip of the plurality of the strips.

5. The detector according to claim 1, comprising a plurality of strips wherein each strip is formed by the oriented polycrystalline layer but wherein the oriented polycrystalline layers of adjacent strips have opposite grain orientations, the strips of the plurality of strips are spaced apart and are parallel to each other, the strips of the plurality of strips being arranged in succession from a first strip to a n-th strip and wherein, each strip of the plurality of strips having a first extremity and a second extremity, the first electrode being connected to the first extremity of the first strip of the plurality of strips and the second electrode being connected to the second extremity of the n-th strip of the plurality of strips, the second extremity of each strip of the plurality of strips being in electrical contact with the first extremity of a successive strip of the plurality of the strips.

6. The detector according to claim 1, wherein a top surface of the substrate present a roughness lower than 2 μm.

7. The detector according to claim 1, comprising a passivation layer under and in contact with the oriented polycrystalline layer.

8. The detector according to claim 7, wherein an adhesion layer is placed between the oriented polycrystalline layer and the passivation layer.

9. The detector according to claim 7, wherein there is an absorber layer superimposed on a top surface of the substrate.

10. The detector according to claim 1, wherein a front surface of the substrate is texturized.

11. The detector according to claim 1, wherein the at least one ceramic layer of the substrate is formed of a material selected from a group of materials which comprises sintered aluminum nitride (AlN), silicon nitride (Si.sub.3N.sub.4), silicon carbide (SiC) boron nitride (BN), boron carbide (B.sub.4C), or alumina (Al.sub.2O.sub.3).

12. The detector according to claim 1, wherein the oriented polycrystalline layer is formed of a material selected from a group of materials which comprises bismuth (Bi), bismuth telluride (Bi.sub.2Te.sub.3), aluminum doped zinc oxide (Al:ZnO) or antimony (Sb).

13. The detector according to claim 1, wherein the substrate is made of a material that is opaque to the electromagnetic radiation.

14. The detector according to claim 1, wherein the substrate is a sintered ceramic layer.

15. The detector according to claim 1, wherein the oriented polycrystalline layer is formed on the substrate by deposition.

16. The detector according to claim 15, wherein the oriented polycrystalline layer is deposited by means of a glancing angle deposition technique.

17. The detector according to claim 1, wherein the first and second electrodes are made of the same material of as the oriented polycrystalline layer of thermoelectric material.

18. A detector of electromagnetic radiation, comprising: an oriented polycrystalline layer of thermoelectric material, which is an active layer of the detector, a substrate superimposed on a top surface of the oriented polycrystalline layer so that a back surface of the substrate is in contact with the oriented polycrystalline layer, the substrate is a target of incident electromagnetic radiation on a front side, first and second electrodes spaced from one another and in electrical contact with the oriented polycrystalline layer, wherein the substrate is a ceramic layer and the oriented polycrystalline layer has a crystal orientation at an angle of between 30 degrees and 55 degrees relative to a normal to a top surface of the substrate, which is non-oriented.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the present invention, some embodiments thereof are now described, purely by way of non-limiting examples and with reference to the annexed drawings, wherein:

(2) FIG. 1 consists of FIGS. 1A, 1B and 1C; FIG. 1A is a cross-section view of a detector for laser radiation in accordance with an embodiment of the present invention in a first step of manufacturing method of the detector;

(3) FIG. 1B is a cross-section view of a detector for laser radiation in accordance with an embodiment of the present invention in a second step of manufacturing method of the detector;

(4) FIG. 1C is a cross-section view of a detector for laser radiation in accordance with an embodiment of the present invention;

(5) FIG. 2 is a bottom view of a serial arrangement of electrodes and patterned sensor layer for the detector of FIG. 1C;

(6) FIG. 3 is a bottom view of another serial arrangement of electrodes and patterned sensor layer for the detector of FIG. 1C.

DETAILED DESCRIPTION OF THE INVENTION

(7) Referring to FIGS. 1-3 a detector of laser radiation according to a preferred embodiment of the present invention is described.

(8) The detector of laser radiation in accordance with the present invention comprises a substrate 1 comprising a ceramic layer, preferably a ceramic layer without the requirement of a specific grain orientation, that is a highly thermally conductive (higher than 100 W/mK) and dielectric material with a high melting temperature (higher than 1800° C.). The substrate 1 may be constituted of the only ceramic layer, preferably a sintered ceramic layer, e.g. sintered aluminum nitride (AlN), sintered silicon nitride (Si.sub.3N.sub.4), sintered silicon carbide (SiC) sintered boron nitride (BN), sintered boron carbide (B.sub.4C) or sintered alumina (Al.sub.2O.sub.3), or the substrate may comprise a metallic substrate (comprising for example Al or Cu) which has been previously electrically passivated by a thin non oriented ceramic layer with a good thermal conduction coefficient (e.g. AlN, Si.sub.3N.sub.4, SiC, BN, B.sub.4C, Al.sub.2O.sub.3). Preferably the thickness of the substrate is comprised in the range between 50-1000 μm; by changing the thickness of the substrate both the response time and the damage threshold of the sensor to laser irradiation can be adapted for a specific application, being the response time inversely proportional to the thickness of the substrate, while the damage threshold of the sensing active layer directly proportional to it.

(9) Preferably, the top surface 10 of the substrate 1 presents a roughness R.sub.a lower than 2 μm.

(10) On the top surface 10 of the substrate 1 in FIG. 1A, a polycrystalline layer 2 is deposited with crystal orientation at an angle α, preferably comprised between 30 and 55 degrees, relative to the normal A to the top surface 10 of the substrate 1. That is, the polycrystalline layer 2 has a columnar grain structure with a crystal axis P tilted at the angle α with respect to the normal axis A. Preferably, the polycrystalline layer 2 is deposited with crystal orientation at 45 degrees for obtaining an optimum deposition.

(11) The polycrystalline layer 2 represents the active layer of the detector of laser radiation and the inclined oriented crystal structure of the polycrystalline layer 2 needs to provide the desired thermoelectric effect. The substrate is not oriented as the thin polycrystalline active layer.

(12) The polycrystalline layer 2 is deposited on the top surface 10 of the substrate 1 and is not grown by means of an epitaxial growth. For this reason, the material of the substrate 1 does not require a specific grain orientation, as in the prior art, and the substrate 1 may be constituted of the only ceramic layer, preferably a sintered ceramic layer.

(13) The polycrystalline layer 2 can be deposited adopting either physical vapor deposition technique (PVD) or electron beam physical vapor deposition (EBPVD) and by placing the substrate 1 at the slant angle α with respect to the evaporation source, by means of the well-known glancing angle deposition technique (GLAD). The latter forms a tilted highly multi-crystalline grain structure of the deposited active layer. The GLAD technique is well described in Chapter 13—Glancing Angle Deposition, In Handbook of Deposition Technologies for Films and Coatings (Third Edition), edited by Peter M. Martin, William Andrew Publishing, Boston, 2010, Pages 621-678, ISBN 9780815520313, https://doi.org/10.1016/B978-0-8155-2031-3.00013-2. In order to obtain the polycrystalline layer 2 with crystal orientation at the angle α, the partial pressure inside the deposition chamber during evaporation must be in the range of 10.sup.−3-10.sup.−7 mbar, the deposition rate in the range of 0.1-1000 nm/s and the substrate temperature between 293-500 K.

(14) Preferably, the polycrystalline layer 2 is deposited with crystal orientation at 45 degrees for obtaining an optimum deposition. The polycrystalline layer 2 is a thin layer; preferably the thickness of the polycrystalline layer 2 is comprised between 0.3 μm and 10 μm, being this a suitable range to obtain a high enough sensitivity combined with a quick restoring time of thermal gradient across the polycrystalline layer 2.

(15) The material of the polycrystalline layer 2 is a material selected from a group of thermoelectric material comprising bismuth (Bi), bismuth telluride (Bi.sub.2Te.sub.3), aluminum doped zinc oxide (Al:ZnO) and antimony (Sb). Preferably the material containing bismuth is deposited over the top surface 10 of the substrate 1 by a thermal evaporation technique.

(16) The roughness R.sub.a of the top surface of the substrate 1 helps the adhesion of the active material layer 2 to the substrate 1 preventing delamination and increases the diffused reflection of the detector.

(17) Even a relatively high roughness (>0.6 μm) can be adopted without affecting the transverse thermoelectric properties of films with thickness lower than 500 nm. In fact, as long as the average orientation of the tilted grains remains unaltered, the response of the sensor remains globally stable, using a spot with a diameter >>R.sub.a.

(18) An adhesion layer 3 is formed on the top surface of the polycrystalline layer 2; the adhesion layer 3 has preferably a thickness lower than 20 nm and preferably consists of titanium.

(19) A passivation layer 4, preferably a highly non-reactive and high melting temperature layer, is deposited on the top surface of the adhesion layer 3; the passivation layer 4 has preferably a thickness ranging from 80 nm to 100 nm and preferably consists of platinum or palladium.

(20) The adhesion layer 3 is configured to act as adhesion layer between the active layer 2 and the passivation layer 4.

(21) A metallic conductive contact layer 5 is deposited on the layer 2 or on the layer 4 in the case wherein the layers 3 and 4 are conductive. The metallic conductive layer 5 is in electrical contact with the polycrystalline layer 2.

(22) The metallic conductive layer 5 is deposited so as to form two contacts spaced the one from the other; the contacts of a single strip represent the electrodes 6, 7. The transverse electric field between the electrodes results in a voltage V(t).

(23) The material of the metallic conductive layer 5 may be gold (Au), copper (Cu), silver (Ag) or molybdenum (Mo). Alternately, the metallic conductive layer 5 is made of the same material of the active layer, in order to avoid thermocouple effects at the junctions between the active and the conductive layer 5, which sum up in the case of the serial interconnection of the strips, moving the baseline of the output voltage signal of the sensor. That is, the metallic conductive contact layer 5 may be bismuth (Bi), bismuth telluride (Bi.sub.2Te.sub.3), aluminum doped zinc oxide (Al:ZnO) or antimony (Sb).

(24) The detector in FIG. 1A is then flipped so that the back surface 11 of the substrate 1 becomes the top surface of the substrate 1, as shown in FIG. 1B.

(25) The sensor chip is then attached on its back side, that is the surface 10 of the substrate 1, to a suitable heatsink 60 using a thermally conductive and dielectric adhesive layer 40, as shown in FIG. 1C.

(26) The substrate 1 acts as absorber of incident laser radiation on the front side. When required, an additional absorption layer 9 of different materials (e.g. SiC, Si.sub.3N.sub.4, carbon nanotubes or black textured metallic layers) and thickness (0.1-10 μm) may be deposited on the surface 11, when the chosen substrate results optically transparent to laser radiation (e.g. AlN, Al.sub.2O.sub.3). For further reducing the reflectivity of the substrate to the incident electromagnetic radiation, laser texturing to form surface morphologies able to enhance the optical light trapping on the surface 11 of the substrate. Therefore, the substrate 1 is made of a material that is opaque to the electromagnetic radiation or is covered with the additional absorption layer 9 placed on the surface 11.

(27) Thin films of suitable materials (e.g. Bi, Bi.sub.2Te.sub.3, Al:ZnO, Sb), deposited with a slant angle between the evaporation source and the substrate show a transverse thermoelectric response to laser-irradiation. That is, if a thermal gradient is present along the normal direction to the film surface, a thermoelectric response longitudinally to the plane of the film surface is generated.

(28) The intensity of the generated electric signal, extractable along the longitudinal plane, has a well-defined direction and it is parallel to the projection of the angled grains of the film of active material grown onto the substrate, showing a minimum in the coplanar perpendicular direction.

(29) The sensor acts as an electromotive force (e.m.f.) generator when irradiated by a radiation laser RL. Its signal is proportional to the temperature gradient formed between the external surface of the active film and its interface with the substrate.

(30) The response of the sensor can be expressed by the formula:
V(t)∝ sin(2α).Math.(S.sub.∥−S.sub.⊥).Math.ΔT(t),

(31) where α, S.sub.∥, S.sub.⊥ and ΔT are, respectively, the tilt-angle of the grains of the active layer 2 with respect to the normal direction to the substrate plane, the Seebeck coefficient in the direction parallel to the projection of the angled grains of the active layer 2 onto the substrate surface 10 in FIG. 1A, the Seebeck coefficient perpendicular to the substrate surface 10 and the temperature gradient across the film.

(32) Preferably the detector according to the invention must be patterned into a single strip 100 or a plurality of strips 100, preferably in the form of adjacent closely packed and electrically isolated strips as illustrated in FIGS. 2-3.

(33) Each strip 100 has preferably a width W1 of about 0.5 to 30 mm and a length L1 in the range of 5 to 30 mm. Each strip is parallel to the direction of the projection of the tilted grains onto the substrate 1. The space W2 between two adjacent strips of a plurality of strips must be as small as possible, preferably comprised between 10 μm and 100 μm, in order to achieve the maximum relative coverage of the substrate by the sensing material. The metal layer 5 provides to form the contacts at the extremities S1 and S2 of each strip; the contacts are spaced the one from the other. The isolation of the strips can be performed either by laser ablation of the detector in FIGS. 1A, 1B, 1C or by a suitable masking of the substrate 1 prior to the deposition of the layers 2-5. The formation of strips 100 serves for two different reasons. First, for better collecting the electric signal at the extremities of the strip. Second, for matching the impedance of the fast sensor according to the invention with the electronics which will digitalize the analogic signal originated from the sensor. Given a fixed rectangular area of the active material and a fixed thickness of the deposited films, the lateral density of the strips is dictated by the electric impedance of the fully interconnected device.

(34) The design of the conductive circuit is chosen in order to optimize the impedance matching with the electronics used for reading the output signals, extracted from the electrodes 6 and 7.

(35) As shown in FIG. 2, the conductive metallic layer 50 is arranged for interconnecting in series the extremities of the strips of the active layer 2. Considering FIG. 2, the first extremity S1 of each strip of the plurality of strips 101 . . . 10n—arranged in succession from the first strip 101 to the last strip 10n—is the bottom extremity. Analogously, the second extremity S2 of each strip of the plurality of strips is the top extremity in FIG. 2. The electrode 6 is placed in contact with the polycrystalline layer 2 of the first extremity S1 of the first strip 101 of the plurality of strips. Electrode 7 is placed in contact with the polycrystalline layer 2 of the second extremity S2 of the last strip 10n of the plurality of strips. The second extremity S2 of the first strip 101 is placed in electrical contact, by means the metallic conductive layer 50, with the first extremity S1 of the second strip 102 of the plurality of strips, and so on.

(36) As shown in FIG. 3, the conductive metallic layer 50 is arranged for interconnecting in series the extremities of the strips of the active layer 2. However, in this case, differently from the detector in FIG. 2, a plurality of alternated strips 201, 301 . . . 20n, 30n with opposite grain orientations are present. That is, the oriented polycrystalline layers of the adjacent strips (201, 301; 202, 302 . . . 20n, 30n) have opposite grain orientations. The dual orientation of the grains is obtained by performing two subsequent depositions of the active material. With the first deposition step, all the alternated strips with one same grain orientation are made, by masking properly the substrate 1 and successively by adopting either physical vapor deposition technique (PVD) or electron beam physical vapor deposition (EBPVD) and placing the substrate 1 at the slant angle α with respect to the evaporation source, by means of the well-known glancing angle deposition technique (GLAD). The second deposition is performed after rotating the substrate 1 by 180 degrees around the axis, which passes from the center of the substrate and is perpendicular to the surface plane. Masking the substrate 1 properly and successively by adopting either physical vapor deposition technique (PVD) or electron beam physical vapor deposition (EBPVD) and placing the substrate 1 at the slant angle α with respect to the evaporation source, by means of the well-known glancing angle deposition technique (GLAD, all the strips with opposite grain orientation, with respect to those obtained during the first deposition, are made.

(37) Said plurality of alternated strips 201, 301 . . . 20n, 30n form a succession of alternated strips 201, 301, 202, 302 . . . 20n, 30n. In this way, the electrode 6 is placed in contact with the polycrystalline layer 4 of the first extremity S1 of the first strip 201 of the plurality of strips 201, 301 . . . 20n, 30n. Electrode 7 is placed in contact with the polycrystalline layer 2 of the second extremity S2 of the last strip 30n of the plurality of strips 201, 301 . . . 20n, 30n. The second extremity S2 of the first strip 201 is placed in electrical contact, by means the metallic conductive layer 50, with the adjacent first extremity S1 of the first strip 301 of the plurality of strips 201, 301 . . . 20n, 30n in the bottom of FIG. 3. The second extremity S2 of the strip 301 is placed in electrical contact, by means the metallic conductive layer 50, with the adjacent first extremity of the strip 202 of the plurality of strips 201, 301 . . . 20n, 30n in the top of FIG. 3, and so on. The arrangement of the detector in FIG. 3 allows a more compact detector with respect to the detector in FIG. 2.

(38) According to a variant of the embodiment of the present invention, the surface 10 of the substrate 1 is texturized (i.e. by laser scribing). In this way, it is obtained a reduction of the reflectivity and a consequent enhancing of the optical light trapping.

(39) According to a further variant of the fast detector of the invention, an additional absorber layer 9 is deposited onto the top surface 11 of the substrate 1. Suitable materials for this layer are SiC, Si.sub.3N.sub.4, Al.sub.2O.sub.3, TiO, carbon nanotubes or black textured metallic layers. Preferably, the range of thickness of this layer ranges from 0.1 to 20 depending on the refractive index of the material and the spectral working region of the sensor. The increase of thickness increases the response time of the sensor, while increasing its damage threshold to laser radiation.