Superconducting thermal detector (bolometer) of terahertz (sub-millimeter wave) radiation

Abstract

A superconducting thermal detector (bolometer) of THz (sub-millimeter) wave radiation based on sensing the change in the amplitude or phase of a resonator circuit, consisting of a capacitor (Csh) and a superconducting temperature dependent inductor where the said inductor is thermally isolated from the heat bath (chip substrate) by micro-suspensions. The bolometer design includes a thin film inductor located on the membrane, a single or/and multi-layered thin film capacitor, and a thin film absorber of incoming radiation. The bolometer design can also include a lithographic antenna with antenna termination and/or a back reflector beneath the membrane for optimal wavelength detection by the resonance circuit. The superconducting thermal detector (bolometer) and arrays of these detectors operate in a temperature range from 1 Kelvin to 10 Kelvin.

Claims

1. A superconducting thermal detector comprising: a. an absorbing element for absorbing electromagnetic radiation, b. a membrane on a silicon substrate, c. a superconducting inductive element thermally isolated from the substrate and in thermal contact with the absorbing element, and d. a read-out circuit for indicating the absorbed electromagnetic radiation, e. wherein the superconducting inductive element is thermally isolated from the substrate by insulator layer micro-suspensions, f. wherein the superconducting inductive element is electrically connected to a capacitor in order to form a resonator circuit, and g. wherein the read-out circuit is implemented by sensing the change in amplitude and/or phase of the resonator circuit.

2. The superconducting thermal detector in accordance with claim 1, wherein a superconducting inductor phonon system is located on the membrane, whereas the said membrane is thermally isolated from the phonon system of the substrate by micro-suspensions.

3. The superconducting thermal detector in accordance with claim 1, wherein the said superconducting inductive element comprises a superconducting material having a high normal state resistivity.

4. The superconducting thermal detector in accordance with claim 1, wherein the said superconducting inductor comprises a superconducting material such as Aluminium (Al), Niobium (Nb), Vanadium (V), Tungsten silicide (WSi), Magnesium diboride (MgB.sub.2) or superconducting material having superconducting transition temperatures in temperature range 1 Kelvin-20 Kelvin.

5. The superconducting thermal detector in accordance with claim 1, wherein the said superconducting inductor comprises a superconducting material comprising nitrogen (N) and a metal selected form the group consisting of Niobium (Nb), Titanium (Ti) and Vanadium (V).

6. The superconducting thermal detector in accordance with claim 1, further comprising utilizing kinetic inductance thermometry which is read out by a scattering parameter measurement which can be used to determine the amplitude or phase change in the resonator induced by impinging optical power.

7. The superconducting thermal detector in accordance with claim 1, wherein it utilizes kinetic inductance thermometry and incorporates an impedance matching surface for efficient absorption of incident optical power.

8. The superconducting thermal detector in accordance with claim 1, wherein it utilizes kinetic inductance thermometry and incorporates an antenna and an antenna termination which dissipates the incident optical power and translates it to heat to be sensed by the kinetic inductance thermometer.

9. A bolometer array comprising linear or 2-dimensional matrix of superconducting thermal detectors having individual bolometer resonant circuits with different resonant frequencies coupled to a superconducting transmission line by either via a capacitance or via an inductance or via a circuit containing both inductive and capacitive elements, wherein each superconducting thermal detector has; an absorbing element for absorbing electromagnetic radiation, superconducting inductive element which is located on a membrane and thermally isolated from a substrate and in thermal contact with the absorbing element, and a read-out circuit for indicating the absorbed electromagnetic radiation, wherein the superconducting inductive element is thermally isolated from the substrate by insulator layer micro-suspensions, wherein the superconducting inductive element is electrically connected to a capacitor in order to form a resonator circuit, and wherein the read-out circuit is implemented by sensing the change in amplitude and/or phase of the resonator circuit.

Description

DESCRIPTION OF FIGURES

(1) The following reference numbers will be used in connection with the following terms:

(2) 1 absorbing element

(3) 2 superconducting inductor

(4) 3 substrate

(5) 4 read-out circuit

(6) 5 membrane layer

(7) 6 superconducting inductor layer (1.sup.st superconducting electrode)

(8) 7 insulator layer

(9) 8 2.sup.nd superconducting layer (2.sup.nd superconducting electrode)

(10) 9 optical cavity

(11) 10 back reflector

(12) 11 micro-suspensions

(13) 21 membrane perforations

(14) Csh shunting thin film capacitor

(15) The features of the invention can be understood with reference to the drawings and the graphs described below, and the claims. The drawings are not necessarily to scale, emphasizing instead the principles and key features of the invention. In the drawings, numerals are used to indicate throughout the views.

(16) In the following, the invention is examined with the aid of examples and with reference to the accompanying drawings.

(17) FIG. 1 shows a general principle scheme of THz radiation detection with a superconducting thermal detector using the inductance in accordance with the invention.

(18) FIGS. 2A-2B show a schematic cross-section of thin film multi-layered structure of a superconducting thermal detector (bolometer) in accordance with the invention.

(19) FIG. 3 shows an equivalent electrical circuit of an array of N superconducting thermal detectors (bolometers) capacitively coupled to a superconducting transmission line in accordance with the invention.

(20) FIG. 4 shows a graph of calculated noise-equivalent power (NEP) contributions with optical efficiency of unity for a superconducting thermal detector (bolometer) in accordance with the invention at reduced temperatures T/T.sub.C=0.3 . . . 1.

(21) FIG. 5 shows a graph of measured transmission S21 parameter in decibels for 8-pixel array of superconducting thermal detectors (bolometers) in accordance with the invention at a reduced temperature T/T.sub.C=0.9.

(22) FIG. 6 shows a graph of measured temperature dependence of the lowest frequency resonance in the 8-pixel array of superconducting thermal detectors (bolometers) in accordance with the invention.

(23) FIG. 7 shows a graph of deduced from data in FIG. 6 temperature dependence of magnetic penetration length of the superconducting inductor of the superconducting thermal detector (bolometer) in accordance with the invention.

DETAILED DESCRIPTION OF FIGURES

(24) FIG. 1 shows general principle scheme of THz radiation detection with a superconducting thermal detector using the superconducting inductance. The incoming radiation is absorbed by the absorbing element 1 which is in a tight thermal contact with a superconducting inductor 2. The superconducting inductor is thermally isolated from the thermal bath (substrate) by micro-suspension legs, whose total thermal conductance is denoted by G. The signal is detected by the read-out circuit 4.

(25) FIG. 2A shows a schematic cross-section of multi-layer structure of a superconducting thermal detector (bolometer). The detector is micro-machined using standard thin film deposition and micro-lithography methods. The substrate 3 represents a silicon wafer. Deposited thin films on top of the silicon substrate are: 1.sup.st layer is a silicon etch-stop layer (optional in the process), 2.sup.nd layer is a membrane layer 5, 3.sup.rd layer is a superconducting inductor layer 2 and 1.sup.st, 4.sup.th layer is an insulating layer (dielectric) 7, 5.sup.th layer is a superconducting layer 8 (optional in the process). The perforations 21 (optional in the process) in the membrane layer 5 are applied to form narrow micro-suspension legs for thermal insulation. The deep silicon etch through the substrate 3 is applied to form resonant optical /4 cavity 9.

(26) FIG. 2B shows the superconducting thermal detector (bolometer) as in FIG. 1 with an etched resonant optical /4 cavity 9 and a back reflector 10 to enhance optical absorptivity.

(27) FIG. 3 shows an equivalent electrical circuit of an array of N superconducting thermal detectors (bolometers) capacitively coupled to a superconducting transmission line. Each pixel represents an inductance Li(T) embedded into a resonant circuit with the shunting capacitor Csh_i, where i denotes the number of the i.sup.th bolometer pixel. The microwave losses of each inductor are depicted with resistor R.

(28) FIG. 4 shows a calculated noise-equivalent power (NEP) contributions for a superconducting thermal detector (bolometer) at reduced temperatures T/T.sub.C=0.3 . . . 1 assuming unity for optical efficiency: NEP of phonon noise (solid line), thermal noise (circles), generation-recombination noise (triangles), and the background limited noise level of a blackbody with temperature 300 K in an optical bandwidth 100 GHz (dash-dotted line). The thermal conductance of micro-suspensions is assumed to be temperature dependent as G(T)=50 nW/K*(T/T.sub.C).sup.3 assuming temperature dependent lattice thermal isolation T.sup.3 of an insulating material of microsuspensions [Ref. 12].

(29) FIG. 5 shows measured transmission S21 parameter in decibels for 8-pixel array of superconducting thermal detectors (bolometers) at a reduced temperature T/T.sub.C=0.9. The quality factor of the resonators is about 300.

(30) FIG. 6 shows measured temperature dependence of the lowest frequency resonance in the 8-pixel array of superconducting thermal detectors (bolometers).

(31) FIG. 7 shows deduced from data in FIG. 5 temperature dependence of magnetic penetration length of the superconducting inductor of the superconducting thermal detector (bolometer).

(32) Some preferred embodiments of the invention are listed in the following paragraphs:

(33) Manufacturing Method of Superconducting Thermal Detector (Bolometer) of Terahertz (Sub-millimeter) Wave Radiation in Accordance with the Invention

(34) In the following, manufacturing method of a superconducting thermal detector (bolometer) of the present invention is explained with reference to FIG. 2A-2B with the same reference numerals as earlier.

(35) The typical manufacturing process of a superconducting thermal detector (bolometer) includes the following micro-fabrication steps of thin film depositions and microlithography patterning methods: 1. Deposition of etch-stop layer such as silicon oxide or onto a silicon substrate 3 (optional step). 2. Deposition of the membrane layer 5 onto the silicon substrate 3. The membrane layer comprises a 100 nm to 1 m thick film of a material such as a silicon nitride (SiN, Si.sub.3N.sub.4), a silicon oxide (SiO, SiO.sub.2), or other-like materials used to form membranes. 3. Deposition of superconducting inductor layer 6 is done by sputtering method in argon atmosphere. The superconducting inductor comprises a 3 nm to 500 nm thick film of a superconducting material such as niobium (Nb), niobium nitride (NbN) or niobium titanium nitride (NbTiN). The material of superconducting inductor layer 6 is not limited to Nb, NbN, or NbTiN, and other superconducting materials such as aluminium (Al), vanadium (V), vanadium nitride (VN), tungsten silicide (WSi), magnesium diboride (MgB.sub.2) and other-like can be used. The superconducting inductor layer 6 is patterned by micro-lithography and by wet etching or by plasma etching. 4. Deposition of insulator layer 7. The insulator layer 7 material comprises a 10 nm to 1 m thick film of silicon nitride (SiN, Si.sub.3N.sub.4), a silicon oxide (SiO, SiO.sub.2), aluminium oxide (AlO, Al.sub.2O.sub.3), or other-like insulator materials. The insulator layer 7 is patterned by micro-lithography and by wet etching or by plasma etching. 5. (Optional step) Deposition of 2.sup.nd superconducting layer 8 comprising a 3 nm to 1 m thick film comprising any of superconducting materials listed in the step 3 of deposition of superconducting inductor layer 6. The 2.sup.nd superconducting layer 8 is patterned by micro-lithography and by wet etching or by plasma etching. 6. Deposition of absorber layer followed by wet etching or by dry etching to form absorbing element 1. The material of an absorbing element 1 comprises a normal metal 100 nm thick film of titanium tungsten (TiW). The material of absorbing element 1 is not limited to TiW, and other metallic materials such as Mo, Ti and the like can be used to form the absorbing element 1. 7. (Optional step). Forming membrane perforations 21 and micro-suspension legs 11 to enhance thermal isolation of a superconducting inductor 2 or/and absorbing element 1. The perforations 21 in the membrane layer 5 are patterned by micro-lithography and by wet etching or by plasma etching. 8. Deep etching of silicon to release the membrane layer 5 by anisotropic silicon ICP etching or by wet etching. To enhance absorption of THz radiation, an optical cavity 9 of a gap of /4 or odd multiples of /4: (2n+1)/4, (n=1,2, . . . ) can be formed between an absorbing element 1 and the attached back reflector 10. To form /4214 m or 3/4643 m or optical cavity (the wavelength 857 m for incoming 350 GHz radiation) the silicon wafer substrates 3 with corresponding approximate thicknesses of /4 and 3/4 are used in the microfabrication process of a superconducting thermal detector (bolometer). A back reflector 10 can comprise a reflective surface or a reflective film deposited onto another substrate, which are attached to the substrate 3. 9. (Optional step). Removing etch-stop silicon oxide layer in a buffered HF solution or in a dry HF vapour etcher.

(36) Some preferred embodiments of the invention are listed in the following paragraphs: 1) A superconducting thermal detector (bolometer) where the thermometry is carried out by sensing the change in the amplitude or phase of a resonator circuit, consisting of a capacitor and a superconducting inductor where the said superconducting inductor is thermally isolated from the heat bath of the system. 2) A superconducting thermal detector (bolometer) described in paragraph 1) where the inductor phonon system is located on the membrane thermally isolated from the phonon system of the lattice by micro-suspensions. 3) A superconducting thermal detector (bolometer) of paragraph 1), wherein the said inductor comprises a superconducting material having a high normal state resistivity. 4) A superconducting thermal detector (bolometer) of paragraph 1), wherein the said superconducting inductor comprises a superconducting material such as aluminium (Al), Niobium (Nb), Vanadium (V), Tungsten silicide (WSi), Magnesium diboride (MgB.sub.2), and other-like superconducting materials. A superconducting thermal detector (bolometer) of paragraph 1), wherein the said superconducting inductor comprises a superconducting material comprising nitrogen (N) and a metal selected form the group consisting of Niobium (Nb), Titanium (Ti) and Vanadium (V). 5) A superconducting thermal detector (bolometer) utilizing kinetic inductance thermometry which is read out by a scattering parameter measurement which can be used to determine the amplitude or phase change in the resonator induced by impinging optical power. 6) A superconducting thermal detector (bolometer) of paragraph 1) which utilizes kinetic inductance thermometry and incorporates an impedance matching surface for efficient absorption of incident optical power. 7) A superconducting thermal detector (bolometer) of paragraph 1) which utilizes kinetic inductance thermometry and incorporates an antenna and an antenna termination which dissipates the incident radiation power and translates it to heat to be sensed by the kinetic inductance thermometer. 8) The read-out circuit of incident THz radiation signal that is implemented by sensing microwave transmission/reflection parameters via a superconducting transmission line to which the said superconducting thermal detector (bolometer) of any of paragraphs 1) to 7) is coupled either via a capacitance or via an inductance or via a circuit containing both an inductance and a capacitance. The bolometer array circuit comprising linear or 2-dimensional matrix of superconducting thermal detectors (bolometers) consists of individual bolometer resonant circuits with different resonant frequencies coupled to a superconducting transmission line via a capacitance or via an inductance or via a circuit containing both inductive and a capacitive elements.
Disclaimer

(37) Although the theoretical description described herein is thought to be correct, the operation of the devices/detectors described here does not depend upon the accuracy or validity of the theoretical description. That is, latter theoretical developments that may explain the observed results on a basis different from the theory presented herein will not detract from the inventions described herein.

(38) Any patent, patent application, or publication identified in the specification is hereby incorporated by reference herein in its entirety. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material explicitly set forth herein is only incorporated to the extent that no conflict arises between the incorporated material and the present disclosure material. In the event of a conflict, the conflict has to be resolved in favour of present disclosure as the preferred disclosure.

(39) While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be affected therein without departing from the spirit and scope of the invention as defined by the claims.

(40) Cited Patent Document

(41) D. G. McDonald, U.S. Pat. No. 4,869,598, issued Sep. 26, 1989.

REFERENCES AND RELATED LITERATURE

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