DESC:
FORM 2
The Patents Act 1970 (39 of 1970)
&
The Patent Rules 2003
NON-PROVISIONAL SPECIFICATION
(See Section 10 and rule 13)

TITLE
A METHOD FOR FABRICATING SUPERCONDUCTING NANOWIRE SINGLE-PHOTON DETECTORS USING SILICON ISOTROPIC ETCHING PROCESS

APPLICANT:
SUPERQ TECHNOLOGIES INDIA PVT LTD
No 18, Gayatri Krupa, Somanna Gardens, Vidyaranyapura Bangalore Karnataka India 560097

PREAMBLE:
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION

A) CROSS- REFERENCE TO RELATED APPLICATION

This application claims the priority of the provisional application with serial number 202341023152 filed on 29th March 2023 with the title, “A METHOD FOR FABRICATING SUPERCONDUCTING NANOWIRE SINGLE-PHOTON DETECTORS USING SILICON ISOTROPIC ETCHING PROCESS” and the contents of which is incorporated in entirety.

B) TECHNICAL FIELD
[0001] The present invention is generally related to the field of superconductors. The present invention is particularly related to single-photon detectors using High-Temperature Superconductors (HTS) having critical temperature Tc > 77K. The present invention is more particularly related to the fabrication of the superconducting nanowire on a patterned substrate using a silicon isotropic etching process in an efficient manner to yield high-quality nanowires suitable for detecting single photons with improved noise performance.

C) BACKGROUND OF THE INVENTION
[0002] The energy of a single photon in the visible or near-infrared range is around 10-19 J. A Single-photon detector is a device sensitive enough for registering these quantum objects. The Single-photon detector is optical detector that provides an electrical signal (voltage pulse) as an output upon detection of a single photon of incident light. Superconducting Nanowire Single Photon Detectors (SNSPDs), also known just as Superconducting Single Photon Detectors (SSPDs), have developed into some of the most advanced superconducting photodetectors, and are key enabling technologies in the
development of quantum communication protocols. SNSPDs detect single photons and particularly stand out in terms of broadband operation with high

detection efficiency (>90%), exceptional signal-to-noise ratio, ultra-high temporal resolution, and fast recovery time.
[0003] SNSPD operates at a current close to the critical current and a temperature below critical temperature. In the region of single photon absorption, local suppression of superconductivity occurs, and a “hotspot” is formed. If at this time a current close to the critical current is passed through the strip, then it redistributes over the part of the film remaining in the superconducting state. Consequently, an increase in current density above the critical current around the hotspot leads to the spreading of the effect. As a result, the entire cross-section of the strip goes into a normal, that is a non- superconducting state, the current is displaced into the high-frequency line and is detected by the pulsed output voltage signal through electronic hardware. To observe the single-photon detection effect in the visible and infrared wavelength ranges, the active element of the detector is a nanowire made of a thin
superconducting film. The nanowire thickness should be of the order of the coherence length of the superconducting material, and the strip width should be less than the penetration depth of the magnetic field.
[0004] Currently, SNSPDs based on Low-Temperature Superconductors (LTS) have several drawbacks, mainly associated with the type.
of superconducting material used for forming the superconducting nanowire or nano-strip which makes them operable only under ultra-low operation temperatures. Presently, the demonstrated SNSPDs perform the function at a very low temperature, nearly less than the liquid helium temperature (< 4.2 K). This summonses the setting up of bulky and costly cooling systems, which results in not only high initial investment but also high maintenance costs. This makes the packaging of SNSPDs extremely impractical and expensive, and their application is limited to academic research.
[0005] Evidently, the choice of an HTS is a practical and economical choice for large-scale deployment of SNSPDs. The operation temperature of the
SNSPDs are mainly decided by two factors i.e., the selection of the functional materials having high transition temperature and the way of fabricating the array of nanowires as this ultimately affects the superconducting properties of the material. The steps involved in the fabrication of nanowire/nanostrips of the
existing SNSPDs are the deposition of the superconducting thin films on the substrate followed by etching to obtain the required nano-patterns. However, in HTS films, degradation of the superconducting properties occurs when the thickness approaches the nanometer scale.
[0006] The fabrication of such thin and narrow high critical temperature
nanowires with homogeneous superconducting properties are challenging, both given the material’s growth mode and the limitations of current thin-film patterning technology. The etching techniques which are employed to fabricate the nanowires/nanostrip deteriorate the properties of the superconducting material. Moreover, the use of a protective layer usually degrades the
superconducting properties due to the chemical and surface chemistry of the compound. Atomic diffusion with another material can degrade the transition temperature drastically. Hence, the post-processing of the deposited superconducting thin films deteriorates the indispensable properties and causes not only a decline in the system efficiency but also lowers the temperature at
which the detection of photons is to be performed.
[0007] Furthermore, an approach to depositing high-temperature-based superconducting materials on the sapphire substrate could further increase the cost of fabrication. In addition, patterning of the superconducting layer was always a challenging task, keeping the device performance into account.
Selective Yttrium Barium Copper Oxide (YBCO) growth was one of the aspects which were tried in forming microwires on Strontium titanate and Magnesium oxide substrates. However, these efforts would further increase the cost and were not quite scalable.
[0008] Hence, in view of this, there is a need for an improved superconducting nanowire single photon detector device system made up of an HTS material having a superconducting critical temperature above 77 K. Moreover, there is a need for the fabrication of an SNSPD, that preserves the
quality of the superconducting material thereby solving the problem relating to the fabrication process and allowing for high-quality films to be deposited and used for superconducting applications.
[0009] The above-mentioned shortcomings, disadvantages, and problems are addressed herein, and which will be understood by reading and
studying the following specification.

D) OBJECTS OF THE INVENTION:
[0010] The primary object of the present invention is to provide a superconducting nanowire single photon detector fabricated using a silicon.
isotropic etch method.
[0011] Another object of the present invention is to provide a superconducting nanowire single photon detector (SNSPD) device system made up of an HTS material having a superconducting critical temperature at or above 77K.
[0012] Yet another object of the present invention is to provide a method for the fabrication of SNSPD that preserves the quality of the superconducting material thereby solving the problem relating to the fabrication process and allowing for high-quality films to be deposited and used for superconducting applications.
[0013] Yet another object of the present invention is to provide an SNSPD made up of HTS material such as Rare-earth Barium Copper Oxide (ReBCO) having a critical temperature above 77K.
[0014] Yet another object of the present invention is to provide an SNSPD fabricated on a silicon substrate.
[0015] Yet another object of the present invention is to provide an SNSPD with a suitable buffer layer, such as Strontium Titanate (STO), Cerium oxide (CeO2), and Magnesium oxide (MgO).
[0016] Yet another object of the present invention is to provide a system
supporting the SNSPD comprising a light source, cryo-packaging, amplifier, and a data acquisition system.
[0017] Yet another object of the present invention is to provide a method for the fabrication of an SNSPD, by patterning of silicon substrate followed by isotropic etch profile modification and further, deposition of a
suitable buffer layer for the growth of the superconducting material.
[0018] Another object of the present invention is to provide a method for fabricating an SNSPD, that eliminates any further post-processing which adversely affects the quality of the superconducting material, specifically, a surface quality that degrades the critical temperature considerably and affects the
efficiency of the SNSPD.
[0019] These and other objects and advantages of the embodiments herein will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

D) SUMMARY OF THE INVENTION
[0020] The following details present a simplified summary of the embodiments herein to provide a basic understanding of the several aspects of the embodiments herein. This summary is not an extensive overview of the embodiments herein. It is not intended to identify key/critical elements of the
embodiments herein or to delineate the scope of the embodiments herein. Its sole purpose is to present the concepts of the embodiments herein in a simplified form as a prelude to the more detailed description that is presented later.
[0021] The other objects and advantages of the embodiments herein will become readily apparent from the following description taken in conjunction
with the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the
embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.
[0022] The various embodiments of the present invention provides a superconducting nanowire single photon detector device, system, and method for fabrication thereof. A High-Temperature Superconductivity (HTS) based superconducting nanowire single photon detector device and the system is described. The superconducting nanowire single photon detector (SNSPD) device comprises a silicon substrate, that functions as a base. A lithography patterning technique such as electron beam lithography is carried out on the silicon substrate to define nano-patterns. Further, the device is subjected to an
isotropic modification of the side wall profile of defined lithography patterns using a plasma etching process. There are two types of isotropic sidewall profile modification possible using the plasma etching process, such as dumbbell- shaped, or steep bottom narrow profile. The device further comprises a suitable buffer layer such as Strontium Titanate (STO), Cerium Oxide (CeO2), or
Magnesium Oxide (MgO), which is deposited on top of the patterned silicon substrate using a suitable deposition method such as Radio Frequency (RF) Magnetron Sputtering. The thickness of the buffer layer deposited on the patterned substrate is more than 80 nm. The device further comprises a superconducting thin film such as Rare-earth Barium Copper Oxide (ReBCO) for
example: Yttrium Barium Copper Oxide (YBCO) or Bismuth Strontium Calcium Copper Oxide (BSCCO). Further, the superconducting thin film or the HTS material is deposited on the buffer layer using deposition techniques such as sputtering or pulsed laser deposition. The HTS material utilized is made up of a cuprate-based superconductor material having a critical temperature (Tc) greater than 77 K, which is above the temperature corresponding to the boiling point of liquid nitrogen. Finally, the device comprises a gold (Au) contact pad layer, which is fabricated ex-situ using a silicon shadow mask fabricated using Deep- Reactive Ion etching (DRIE) to protect the quality of the superconducting thin film. The gold (AU) layer also helps to avoid another lithography patterning step, that further degrades the quality of the superconducting thin film.
[0023] According to one embodiment of the present invention, the superconducting nanowire single photon detector (SNSPD) device is further supported by a system. The system comprises a light source consisting of suitable optics such as an attenuated laser source or a single photon source coupled with an optic fiber. The light source is placed optionally as a part of the system supporting the SNSPD device. Further, the ReBCO chip, or the HTS material consisting of a meandered nanowire, is attached to a fiber-optic ferrule followed by an appropriate cryo-packaging such as liquid nitrogen cryostat, in a modular
form. The function of the SNSPD device supported by the described system is to generate an electrical pulse signal upon absorption of an incident photon, which is then amplified by a suitable amplifier such as a low-noise amplifier (LNA), or with broadband. Further, each output signal is recorded and counted by a data acquisition system (DAQ) comprising a high-bandwidth oscilloscope or time correlation counter.
[0024] According to one embodiment of the present invention, the method for the fabrication of SNSPD is provided. The method comprises procuring a suitable substrate such as silicon, which acts as a base. The method further involves cleaning the silicon substrate using piranha solution to remove organic residues from the substrate. The piranha solution, also known as piranha etch, is a mixture of sulfuric acid and hydrogen peroxide, used to clean organic residues off substrates. Post-cleaning the substrate, the method involves defining patterns on the substrate using lithographic techniques like electron beam lithography. Further, the method involves etching the lithographic patterns into a silicon isotropic ally, to modify its side-wall profile. The method further involves depositing a suitable buffer layer such as strontium titanate (STO), Cerium oxide (CeO2), or Magnesium oxide (MgO) on top of the patterned substrate using suitable deposition techniques such as Radio Frequency (RF) Magnetron Sputtering. The thickness of the buffer layer deposited on the patterned substrate is more than 80 nm. Furthermore, the method involves annealing the buffer layer at a high temperature to form a crystalline buffer layer thin film suitable for the growth of the superconducting thin film. The method further involves depositing a suitable HTS material or a superconducting thin film such as Rare-earth Barium Copper Oxide (ReBCO), Yttrium Barium Copper Oxide (YBCO), or Bismuth Strontium Calcium Copper Oxide (BSCCO) on the crystalline buffer layer thin film using deposition techniques such as sputtering or pulsed laser deposition. The method further involves annealing of the HTS material such as ReBCO film to obtain a crystalline ReBCO thin film at high temperature. Finally, the method involves fabricating gold (Au) contact pads ex-situ using a silicon shadow mask fabricated by Deep Reactive-Ion Etching (DRIE) technique.
[0025] According to an embodiment of the present invention, a method for integrating the SNSPD obtained into a system for detecting photons is provided. The method involves providing a light source consisting of suitable optics such as an attenuated laser source or a single photon source coupled with an optical fiber. The light source is placed optionally as a part of the system supporting a superconducting nanowire single photon detector device. Further, the method involves, coupling a ReBCO chip or the HTS material consisting of
a meandered nanowire, to a fiber-optic ferrule, then followed by cryo-packaging such as cryostat, in a modular form. The function of the SNSPD device supported by the described system includes generating an electrical pulse signal upon absorption of an incident photon, which is then amplified by a suitable amplifier
such as a low-noise amplifier (LNA) or broadband. The method further involves amplifying the electric pulse signal generated by the SNSPD. Further, the method involves counting and recording the output signals by a data acquisition system comprising a high-bandwidth oscilloscope or time correlation counter.
[0026] From the foregoing discussion, it is apparent that the present invention makes use of an SNSPD fabricated using the silicon isotropic etch method, which includes patterning on silicon followed by isotropic etch profile modification and further deposition of the suitable buffer layer for the growth of the superconducting material. The superconducting material used in the present invention has a critical temperature >77K. Furthermore, the method of the present invention excludes any further post-processing steps that may degrade the quality of the superconducting material, specifically, a surface quality that deteriorates the critical temperature significantly and affects the efficiency of the device. Thus, the method provides high-quality superconducting devices suitable for single-photon level detection. Overall, the adoption of a high-temperature- based superconductor provides lower power consumption and the cost of the device reduces drastically as the operating temperature is higher (>10K) compared to the existing superconducting single-photon detectors in the market.
[0027] It is to be understood that the aspects and embodiments of the
disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined to form a further embodiment of the disclosure.
[0028] The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments,
and features described above, further aspects, embodiments, and features will become apparent in reference to the drawings and the following detailed description.

E) BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings in which:
[0030] FIG. 1 illustrates the fabrication process flow of an HTS (High-
Temperature Superconductivity) based superconducting single photon detector, according to an embodiment of the present invention.
[0031] FIG. 2 is a flowchart illustrating a method of fabrication of SNSPD using the silicon isotropic etch method, according to an embodiment of the present invention.
[0032] FIG. 3 is a flowchart illustrating a method of integrating the silicon isotropic etch fabricated SNSPD, into a system for detecting a single photon, according to an embodiment of the present invention.
[0033] Although the specific features of the present invention are shown in some drawings and not in others. This is done for convenience only as
each feature may be combined with any or all of the other features in accordance with the present invention.

F) DETAILED DESCRIPTION OF THE DRAWINGS:
[0034] In the following detailed description, a reference is made to the
accompanying drawings that form a part hereof, and in which the specific embodiments that may be practised are shown by way of illustration. These embodiments are described in sufficient detail to enable those skilled in the art to practise the embodiments and it is to be understood that other changes may be made without departing from the scope of the embodiments. The following
detailed description is therefore not to be taken in a limiting sense.
[0035] In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be apparent, however, to one skilled in the art that the present disclosure can be practiced without these specific details.
[0036] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings.
It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the
appended claims.
[0037] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific
examples, are intended to encompass equivalents thereof.
[0038] While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however, that it is not intended to limit the disclosure to the forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure.
[0039] The various embodiments of the present invention provides a superconducting nanowire single photon detector device, system, and method for fabrication thereof. A High-Temperature Superconductivity (HTS) based
superconducting nanowire single photon detector device and the system is described. The superconducting nanowire single photon detector (SNSPD) device comprises a silicon substrate, that functions as a base. A lithography patterning technique such as electron beam lithography is carried out on the silicon substrate to define nano-patterns. Further, the device is subjected to an isotropic modification of the side wall profile of defined lithography patterns using a plasma etching process. There are two types of isotropic sidewall profile modification possible using the plasma etching process, such as dumbbell- shaped, or steep bottom narrow profile. The device further comprises a suitable
buffer layer such as Strontium Titanate (STO), Cerium Oxide (CeO2), or Magnesium Oxide (MgO), which is deposited on top of the patterned silicon substrate using a suitable deposition method such as Radio Frequency (RF) Magnetron Sputtering. The thickness of the buffer layer deposited on the patterned substrate is more than 80 nm. The device further comprises a
superconducting thin film such as Rare-earth Barium Copper Oxide (ReBCO) for example; Yttrium Barium Copper Oxide (YBCO) or Bismuth Strontium Calcium Copper Oxide (BSCCO). Further, the superconducting thin film of the HTS material is deposited on the buffer layer using deposition techniques such as sputtering or pulsed laser deposition. The HTS material utilized is made up of a
cuprate-based superconductor material having a critical temperature (Tc) greater than 77 K, which is above the temperature corresponding to the boiling point of liquid nitrogen. Finally, the device comprises a gold (Au) contact pad layer, which is fabricated ex-situ using a silicon shadow mask fabricated using Deep- Reactive Ion Etching (DRIE) to protect the quality of the superconducting thin film. The gold (AU) layer also helps to avoid another lithography patterning step, that further degrades the quality of the superconducting thin film.
[0040] According to one embodiment of the present invention, the superconducting nanowire single photon detector (SNSPD) device is further supported by a system. The system comprises a light source consisting of suitable optics such as an attenuated laser source or a single photon source coupled with an optic fiber. The light source is placed optionally as a part of the system supporting the SNSPD device. Further, the ReBCO chip, or the HTS material consisting of a meandered nanowire, is attached to a fiber-optic ferrule followed by an appropriate cryo-packaging such as liquid nitrogen cryostat, in a modular
form. The function of the SNSPD device supported by the described system is to generate an electrical pulse signal upon absorption of an incident photon, which is then amplified by a suitable amplifier such as a low-noise amplifier (LNA), or with broadband. Further, each output signal is recorded and counted by a data acquisition system (DAQ) comprising a high-bandwidth oscilloscope or time correlation counter.
[0041] According to one embodiment of the present invention, the method for the fabrication of SNSPD is provided. The method comprises procuring a suitable substrate such as silicon, which acts as a base. The method
further involves cleaning the silicon substrate using piranha solution to remove organic residues from the substrate. The piranha solution, also known as piranha etch, is a mixture of sulfuric acid and hydrogen peroxide, used to clean organic residues off substrates. Post-cleaning the substrate, the method involves defining patterns on the substrate using lithographic techniques like electron beam lithography. Further, the method involves etching the lithographic patterns into a silicon isotropic ally, to modify its side-wall profile. The method further involves depositing a suitable buffer layer such as strontium titanate (STO), Cerium oxide (CeO2), or Magnesium oxide (MgO) on top of the patterned substrate using suitable deposition techniques such as Radio Frequency (RF) Magnetron
Sputtering. The thickness of the buffer layer deposited on the patterned substrate is less than 100 nm. Furthermore, the method involves annealing the buffer layer at a high temperature to form a crystalline buffer layer thin film suitable for the growth of the superconducting thin film. The method further involves depositing a suitable HTS material or a superconducting thin film such as Rare-earth Barium
Copper Oxide (ReBCO), Yttrium Barium Copper Oxide (YBCO), or Bismuth Strontium Calcium Copper Oxide (BSCCO) on the crystalline buffer layer thin film using deposition techniques such as sputtering or pulsed laser deposition. The method further involves annealing of the HTS material such as ReBCO film to obtain a crystalline ReBCO thin film at high temperature. Finally, the method involves fabricating gold (Au) contact pads ex-situ using a silicon shadow mask fabricated using Deep Reactive-Ion Etching (DRIE) technique.
[0042] According to an embodiment of the present invention, a method
for integrating the SNSPD obtained into a system for detecting photons is provided. The method involves providing a light source consisting of suitable optics such as an attenuated laser source or a single photon source coupled with an optical fiber. The light source is placed optionally as a part of the system supporting a superconducting nanowire single photon detector device. Further, the method involves, coupling a ReBCO chip or the HTS material consisting of a meandered nanowire, to a fiber-optic ferrule, then followed by cryo-packaging such as liquid nitrogen cryostat, in a modular form. The function of the SNSPD device supported by the described system includes generating an electrical pulse signal upon absorption of an incident photon, which is then amplified by a
suitable amplifier such as a low-noise amplifier (LNA) or with broadband. The method further involves amplifying the electric pulse signal generated by the SNSPD. Further, the method involves counting and recording the output signals by a data acquisition system comprising a high-bandwidth oscilloscope or time correlation counter.
[0043] FIG. 1 illustrates the fabrication process flow of an HTS (High- Temperature Superconductivity) based superconducting single photon detector, according to an embodiment of the present invention. FIG. 1, 100 illustrates the superconducting nanowire single photon detector device fabricated by the silicon isotropic etch method. The superconducting nanowire single photon detector
(SNSPD) device 100 comprises a silicon substrate 101, that functions as a base. A lithography patterning technique such as electron beam lithography is carried out on the silicon substrate 101 to define nano-patterns. Further, the device is subjected to an isotropic modification of the side wall profile of defined lithography patterns using a plasma etching process 102. There are two types of isotropic sidewall profile modification possible using the plasma etching process, such as dumbbell-shaped, or steep bottom narrow profile. The device 100 further comprises a suitable buffer layer 103 such as Strontium Titanate (STO), Cerium
Oxide (CeO2), or Magnesium Oxide (MgO), which is deposited on top of the patterned silicon substrate 101 using a suitable deposition method such as Radio Frequency (RF) Magnetron Sputtering. The thickness of the buffer layer 103 deposited on the patterned substrate 101 is more than 80 nm. The device, 100 further comprises a superconducting thin film 104 such as Rare-earth Barium
Copper Oxide (ReBCO) for example; Yttrium Barium Copper Oxide (YBCO) or Bismuth Strontium Calcium Copper Oxide (BSCCO). Further, the superconducting thin film 104 or the HTS material is deposited on buffer layer 103 using deposition techniques such as sputtering or pulsed laser deposition. The HTS material 104 utilized is made up of a cuprate-based superconductor material having a critical temperature (Tc) greater than 77 K, which is above the temperature corresponding to the boiling point of liquid nitrogen. Finally, device, 100 comprises a gold (Au) contact pad layer 105, which is fabricated ex-situ using a silicon shadow mask fabricated using the Deep Reactive-Ion Etching (DRIE) technique to protect the quality of the superconducting thin film 104. The
gold (AU) contact pad layer 105 also helps to avoid another lithography patterning step, that further degrades the quality of the superconducting thin film 105.
[0044] FIG. 2 is a flowchart illustrating a method for fabrication of SNSPD using the silicon isotropic etch method, according to an embodiment of
the present invention. The method 200, comprises procuring a suitable substrate such as silicon, which acts as a base, at step 201. The method 200, further involves cleaning the silicon substrate using piranha solution to remove organic residues from the substrate at step 202. The piranha solution, also known as piranha etch, is a mixture of sulfuric acid and hydrogen peroxide, used to clean organic residues off substrates. Post-cleaning the substrate, method 200, involves defining patterns on the substrate using lithographic techniques like electron beam lithography at step 203. Further, method 200 involves etching the lithographic patterns into a silicon isotropic ally, to modify its side-wall profile at step 204. The method 200, further involves depositing a suitable buffer layer such as strontium titanate (STO), Cerium oxide (CeO2), or Magnesium oxide (MgO) on top of the patterned substrate using suitable deposition techniques such as Radio Frequency (RF) Magnetron Sputtering at step 205. The thickness of the buffer layer deposited on the patterned substrate is more than 80 nm. Furthermore, method 200 involves annealing the buffer layer at a high temperature to form a crystalline buffer layer thin film suitable for the growth of the superconducting thin film at step 206. The method 200, further involves depositing a suitable HTS material or a superconducting thin film such as Rare- earth Barium Copper Oxide (ReBCO), Yttrium Barium Copper Oxide (YBCO),
or Bismuth Strontium Calcium Copper Oxide (BSCCO) on the crystalline buffer layer thin film using deposition techniques such as sputtering or pulsed laser deposition at step 207. The method 200, further involves annealing of the HTS material such as ReBCO film to obtain a crystalline ReBCO thin film at a high temperature at step 208. Finally, method 200, involves fabricating gold (Au)
contact pads ex-situ using a silicon shadow mask at step 209 fabricated using Deep Reactive-Ion Etching (DRIE).
[0045] FIG. 3 is a flowchart illustrating a method for integrating the silicon isotropic etch fabricated SNSPD, into a system for detecting photons, according to an embodiment of the present invention. The method 300, involves providing a light source consisting of suitable optics such as an attenuated laser source or a single photon source coupled with an optical fiber at step 301. The light source is placed optionally as a part of the system supporting a superconducting nanowire single photon detector device. Further, method 300 involves, coupling a ReBCO chip or the HTS material consisting of a meandered nanowire, to a fiber-optic ferrule 302, then followed by cryo-packaging such as liquid nitrogen cryostat, in a modular form at step 303. The function of the SNSPD device supported by the described system includes generating an electrical pulse signal upon absorption of an incident photon, which is then amplified by a suitable amplifier such as a low-noise amplifier (LNA) or broadband. The method 300, further involves amplifying the electric pulse signal generated by the SNSPD at step 304. Further, method 300 involves counting and recording each output signal by a data acquisition system comprising a high- bandwidth oscilloscope or time correlation counter at step 305. The system for supporting the SNSPD device in detecting individual low-energy photons with high quantum efficiencies, comprising: the SNSPD chip is mounted in a liquid nitrogen cryostat after fabrication; the SNSPD chip is optically and electrically addressed via a fibre array and an RF probe, respectively; continuous wave or picosecond-pulsed laser sources are used to launch light via calibrated, adjustable optical attenuators into the optical input of the liquid nitrogen cryostat; the optical output is detected with a calibrated photodetector; a current source (battery powered) is used to supply the bias current for the SNSPD; the output pulses are amplified with broadband, low-noise amplifiers (LNA); and the amplified output pulses are registered either with a high-bandwidth oscilloscope or a time correlation counter.
[0046] The foregoing examples and illustrative implementations of various embodiments have been provided merely for explanation and are in no way to be construed as limiting the present invention. While the present invention has been described with reference to various embodiments, illustrative implementations, drawings, and techniques, it is understood that the words, which have been used herein, are words of description and illustration, rather than words of limitation.
[0047] Further, although the present invention has been described herein with reference to particular means, materials, embodiments, techniques, and implementations, the present invention is not intended to be limited to the
particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.
[0048] It will be understood by those skilled in the art, having the benefit of the teachings of this specification, that the present invention is capable of modifications and other embodiments may be affected and changes may be made thereto, without departing from the scope and spirit of the present invention.
G) ADVANTAGES OF THE INVENTION:

[0049] The present invention provides a superconducting nanowire single photon detector (SNSPD) device system and a silicon isotropic etch fabrication method thereof. An SNSPD is an optoelectronic device that converts incident light or other electromagnetic radiation in the ultra-violet, visible, and
infrared spectral regions into electrical signals thereon. A single-photon detector is a quantum detector that can detect one incident photon at a time. A single- photon detector comprises a Superconductor strip biased near its critical current. The Superconductor strip provides a detectable output signal upon absorption of a single incident photon. The Superconductor strip meanders to increase its probability of receiving a photon from a light source. Hence, the SNSPD of the present invention is suitable for many applications including free-space and satellite communications, quantum communications, quantum cryptography, weak luminescence, and semiconductor device testing.
[0050] Low dark count rate, high detection efficiency, and accurate
timing resolution are the three most desired features of a single-photon detector. These characteristics can be equivalently described using figure-of-merit, for example, the Noise Equivalent Power (NEP) or Signal to Noise Ratio (SNR). Detectors with low noise performances are increasingly sought after, for applications in both quantum and classical technologies. In particular, linear
optics quantum information processing crucially relies on the availability of low dark count rate single photon detectors. Most prominently, quantum key distribution implementations are currently limited in rate and range by imperfect detector characteristics. Other applications which would greatly benefit from improved single-photon detection systems include but not limited to the
characterization of quantum emitters, optical time domain reflectometry as well as picosecond imaging circuit analysis.
[0051] Therefore, the present invention provides an SNSPD fabricated using the silicon isotropic etch method, which includes patterning on silicon followed by isotropic etch profile modification and further deposition of the suitable buffer layer for the growth of the superconducting material. The superconducting material used in the present invention has a critical temperature >77K. Furthermore, the method of the present invention excludes any further post-processing steps that may degrade the quality of the superconducting material, specifically, a surface quality that deteriorates the critical temperature significantly and affects the efficiency of the device. Thus, the method provides high-quality superconducting devices suitable for single-photon level detection. Overall, the adoption of a high-temperature-based superconductor provides lower power consumption, and the cost of the device reduces drastically as the operating temperature is higher (>10K) compared to the existing superconducting single-photon detectors in the market.
[0052] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments.
[0053] It is to be understood that the phraseology or terminology
employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.
[0054] Although the embodiments herein are described with various specific embodiments, it will be obvious for a person skilled in the art to practice the invention with modifications. However, all such modifications are deemed to be within the scope of the claims.
[0055] It is also to be understood that the following claims are intended to cover all of the generic and specific features of the embodiments described herein and all the statements of the scope of the embodiments which as a matter of language might be said to fall there between.
[0056] The scope of the embodiments of the present invention is ascertained by the claims to be submitted at the time of filing the complete specification.

Dated this 26th day of March 2024

For, SUPERQ TECHNOLOGIES PRIVATE LIMITED

BY THEIR AGENT

(DR. BABITHA THARAPPAN)
IN/PA-1614
ATV-LEGAL

,CLAIMS:WE CLAIM:

1. A superconducting nanowire single photon detector (SNSPD) device, comprising:

a. selecting a substrate typically a silicon, functioning as a base;

b. nano lithographic patterns defined on the substrate using lithography techniques, said techniques including photolithography, electron beam lithography, X-ray lithography, extreme UV lithography, focused ion beam lithography, neutral atomic beam lithography, soft lithography, colloidal lithography, nano-imprint lithography, scanning probe lithography, and atomic force microscope nano-lithography;

c. isotropic etching of the nano lithographic patterns into silicon to modify its side-wall profile;

d. a suitable buffer layer selected from a group consisting of Strontium Titanate (STO), Cerium Oxide (CeO2), or Magnesium Oxide (MgO), deposited on the patterned substrate by a Radio Frequency (RF) Magnetron Sputtering, wherein the thickness of the buffer layer deposited on the patterned substrate is more than 80 nm;

e. a high temperature superconductivity (HTS) material selected from the group consisting of Rare-earth Barium Copper Oxide (ReBCO) materials, deposited on the buffer layer; and

f. a gold (Au) contact pad deposited using a silicon shadow mask fabricated ex-situ using a Deep-Reactive Ion Etching (DRIE).

2. The said SNSPD device of claim 1, wherein the HTS material comprises Yttrium Barium Copper Oxide (YBCO) or Bismuth Strontium Calcium Copper Oxide (BSCCO).

3. The said SNSPD device of claim 1, wherein the isotropic modification of the side wall profile of the defined nano-lithography patterns are done by a plasma etching process, and wherein the isotropic sidewall profile modification is achieved with a dumbbell shaped profile or a steep bottom narrow profile.

4. The said SNSPD device of claim 1, wherein the HTS material is deposited on the buffer layer by sputtering or pulsed laser deposition.

5. The said SNSPD device of claim 1, wherein the HTS material is made up of a cuprate-based superconductor material having a critical temperature (Tc) greater than 77 K, which is above the temperature corresponding to the boiling point of liquid nitrogen.

6. The said SNSPD device of claim 1, further comprising a system for supporting the SNSPD device in detecting individual low-energy photons with high quantum efficiencies, comprising:

a. the SNSPD chip is mounted in a liquid nitrogen cryostat after fabrication;

b. the SNSPD chip is optically and electrically addressed via a fibre array and an RF probe, respectively;

c. continuous wave or picosecond-pulsed laser sources are used to launch light via calibrated, adjustable optical attenuators into the optical input of the liquid nitrogen cryostat;

d. the optical output is detected with a calibrated photodetector;

e. a current source (battery powered) is used to supply the bias current for the SNSPD;

f. the output pulses are amplified with broadband, low-noise amplifiers (LNA); and

g. the amplified output pulses are registered either with a high-bandwidth oscilloscope or a time correlation counter.

7. A method for the fabrication of a superconducting nanowire single photon detector (SNSPD), comprising:

a. procuring a substrate typically a silicon, wherein the substrate acts as a base;

b. cleaning the substrate using a piranha solution to remove organic residues from the substrate;

c. defining patterns on the substrate using lithographic techniques;

d. performing the isotropic modification of the sidewall profile of silicon by plasma etching;

e. depositing a buffer layer such as strontium titanate (STO), Cerium oxide (CeO2), or Magnesium oxide (MgO) on the patterned substrate using Radio Frequency (RF) Magnetron Sputtering, and wherein the thickness of the buffer layer is more than 80 nm;

f. annealing the buffer layer at a high temperature to form a crystalline buffer layer thin film suitable for the growth of the superconducting thin film;

g. depositing a high-temperature superconducting (HTS) material selected from the group consisting of Rare-earth Barium Copper Oxide (ReBCO), Yttrium Barium Copper Oxide (YBCO), and Bismuth Strontium Calcium Copper Oxide (BSCCO) on the crystalline buffer layer thin film using deposition techniques such as sputtering or pulsed laser deposition;

h. annealing the HTS material such as ReBCO film to obtain a crystalline ReBCO thin film at high temperature; and

i. fabricating gold (Au) contact pads using a silicon shadow mask fabricated by the Deep Reactive-ion etching (DRIE) technique.

8. The method of claim 7, wherein the said piranha solution, also known as a piranha etch comprising sulfuric acid and hydrogen peroxide.

9. The method of claim 7, wherein the lithographic techniques include photolithography, electron beam lithography, X-ray lithography, extreme UV lithography, focused ion beam lithography, neutral atomic beam lithography, soft lithography, colloidal lithography, nano-imprint lithography, scanning probe lithography, or atomic force microscope nanolithography.

10. The method of claim 7, wherein the plasma etching techniques are employed for isotropically etching the sidewall of the nano-patterned silicon substrate.

Dated this 26th day of March 2024

For, SUPERQ TECHNOLOGIES PRIVATE LIMITED

BY THEIR AGENT

(DR. BABITHA THARAPPAN)
IN/PA-1614
ATV-LEGAL