CN102829884B - High-speed SNSPD with strong absorption structure and its preparation method - Google Patents

Abstract

The invention discloses a high-speed superconducting nanowire single-photon detector (SNSPD) with a strong absorption structure and a preparation method of the high-speed SNSPD with the strong absorption structure. The SNSPD is capable of further improving the photon absorptivity of superconducting nanowires based on an incident medium with a high refractive index and an air cavity structure. Compared with the prior art and according to the high-speed SNSPD, under the condition that the nanowires are made of superconducting ultrathin membranes with the same material and the same thickness, nearly 100% of absorptivity can be realized through a lower duty ratio, and the difficulty of electron beams in the exposure steps is reduced greatly, thereby particularly being more beneficial to the preparation of the ultrathin nanowires; and meanwhile, by adopting a silicon on insulator (SOI) substrate, the high-quality growth of the superconducting ultrathin membranes can be ensured simultaneously without affecting the intrinsic quantum efficiency of the detector. In addition, under the condition that the large effective detection area is ensured equally, as the total length of the required nanowires is reduced obviously, the maximum counting rate of the detector can be improved, and the probability of occurring defects during preparation process is decreased notably.

Description

High-speed SNSPD with strong absorption structure and its preparation method

technical field

The invention belongs to the field of single-photon detection, is suitable for realizing ultra-fast and high-efficiency single-photon detection in the near-infrared band, and relates to a high-speed SNSPD with a strong absorption structure and a preparation method thereof.

Background technique

In recent years, G.N.Gol'tsman et al., "Picosecond superconducting single-photon optical detector," Applied Physics Letter, vol.79, pp.705–707, 2001. The superconducting nanowire single-photon detector (SNSPD), Due to its excellent single-photon detection ability in the visible and infrared bands, ultra-high count rate, low dark count, and small time jitter, it has attracted more and more attention, especially its quantum efficiency in the near-infrared band. Both the count rate and the highest count rate have exceeded the existing avalanche photodiodes based on compound semiconductor materials, making them the most powerful candidate detectors in the fields of quantum communication and long-distance optical communication. At present, the intrinsic quantum efficiency of SNSPD made of the most commonly used niobium nitride (NbN) superconducting material can reach more than 90%, but its limited light absorption rate has become a bottleneck limiting the total system quantum efficiency of SNSPD. Since the core photosensitive area of SNSPD is composed of ultra-thin nanowires, its absorption rate for incident photons is very limited, and photons will pass through the gaps between nanowires with a considerable probability, or directly pass through the film, Or reflected back from the superconducting thin film. K.M.Rosfjord et al., "Nanowire single-photon detector with an integrated optical cavity and anti-reflection coating," Optics Express, vol.14, pp.527–534, 2006. It is recorded that adding an optical cavity structure to SNSPD can significantly improve Its photon absorptivity method. However, for a typical NbN nanowire with a thickness of 4nm and a duty ratio of 50%, this method can only obtain an absorption rate of about 70%. If the absorption rate is to be further improved, the duty cycle or thickness of the nanowire needs to be increased, but the former imposes more stringent requirements on sample preparation, while the latter will lead to a decrease in the intrinsic quantum efficiency of the detector. US 2012/0077680A1 "Nanowire-based detector" K.K.Berggren, X.Hu, D.Masciarelli et al. The method of increasing the absorption rate based on nano-antennas can be used under the conditions of 4nm thick and 50% duty cycle NbN nanowires. It achieves an absorption rate close to 100%, but this solution also puts forward relatively high requirements on sample preparation, and the final experimental results show that its yield is not high.

Contents of the invention

In order to overcome the deficiencies of the above-mentioned prior art, the object of the present invention is to provide a high-speed SNSPD with a strong absorption structure and its preparation method, which can achieve high absorption rate under the condition of low duty cycle, and has the advantages of simple structure and controllable process. specialty.

In order to achieve the above object, the technical solutions adopted in the present invention are respectively:

A high-speed SNSPD with a strong absorption structure, comprising a bottom Si substrate-1, a Bragg reflector 2 formed by a periodic arrangement of multilayer Si/SiO2 is deposited on the bottom Si substrate- ₁ , and the top of the Bragg reflector 2 is provided with The bottom resonator 3 formed by epitaxial single crystal Si has a superconducting nanowire 4 above the bottom resonator 3, an upper air resonator 5 on the superconducting nanowire 4, and a Si sheet above the upper air resonator 5 6. An anti-reflection film-7 is provided on the Si sheet 6.

The Bragg reflector 2 is formed of multilayer Si/SiO ₂ periodically arranged at intervals, the number of periods is more than 3, the thickness of each layer is equal to a quarter of the equivalent wavelength of the incident light in the medium, and the bottom one Layer SiO ₂ on Si substrate one 1 .

The thickness of the superconducting nanowire is generally between 4-6nm, and the width is generally between 20-200nm, and the superconducting material used is NbN, NbTiN, TaN, NbSi, Nb or W _x Si _1-x .

The bottom resonant cavity-3 is made of the epitaxial single crystal Si layer of the SOI substrate, and the thickness needs to be optimized through simulation in advance. The optimal value is about half of the equivalent wavelength of the incident light in the medium, but it will There are slight differences in the material, thickness and duty cycle of the conductive nanowires.

The upper air resonant cavity 5 is completed by Au-Au bonding process, and the thickness needs to be optimized through simulation in advance, and the optimal value is about a quarter of the wavelength of the incident light, but it will be determined according to the material, thickness and The duty cycle is slightly different.

The anti-reflection film-7 has a refractive index between 1.7-2.0 and a thickness equal to a quarter of the equivalent wavelength of the incident light in the medium, and can be made of materials such as Al ₂ O ₃ .

The present invention also provides a method for preparing a high-speed SNSPD with the above-mentioned structure, comprising the following steps:

(a) Prepare the SOI substrate, obtain the precise thickness of the required epitaxial single crystal Si layer through simulation in advance, and mechanically thin the Si layer on the back;

(b) Oxidation of the SOI substrate, controlling the thickness of _SiO2 during the process;

(c) grow a polycrystalline Si layer by CVD, and partially oxidize the Si layer to obtain a SiO ₂ layer, and repeat this n times to obtain a Si/SiO ₂ Bragg reflector with n+1 periods;

(d) Using the method of Si-Si bonding, bond the above substrate and another Si sheet together as a new substrate;

(e) The SiO ₂ and Si layers on the back of the SOI substrate were sequentially etched with hydrofluoric acid buffered etching solution and KOH etching solution, and then the SiO ₂ buried layer at the bottom of the single crystal Si layer was removed with hydrofluoric acid buffered etching solution to expose the single crystal Si layer. Crystal Si layer;

(f) Superconducting thin films were grown on single crystal Si layer, and superconducting nanowires were formed by electron beam exposure and reactive ion etching;

(g) Fabricate Au/Ti patterns on top of the superconducting nanowires as a coplanar waveguide readout circuit for the detector and prepare for the subsequent Au-Au bonding;

(h) Prepare a double-sided polished Si wafer, first prepare an Al ₂ O ₃ film on one side by ALD or sputtering, and make an Au/Ti pattern on the other side;

(i) Through the Au-Au bonding method, the upper air resonant cavity is finally formed, and the thickness of the upper air resonant cavity is determined by controlling the thickness of the Au/Ti layers on both sides.

The second structure of a high-speed SNSPD with a strong absorption structure in the present invention includes a metal thin film reflector 8, an upper resonant cavity 9 made of a transparent medium material is arranged below the metal thin film reflector 8, and a superconducting nanometer is placed below the upper resonant cavity 9. Line 2 10, under the superconducting nanowire 10 is an epitaxial single crystal Si layer 11, below the epitaxial single crystal Si layer 11 is a Si substrate 2 12, and the Si substrate 2 12 faces the epitaxial single crystal Si layer 11 with a bottom resonant cavity 13, an anti-reflection film 14 is placed under the Si substrate 12.

The material of the transparent medium is SiO ₂ , and the thickness of the upper resonant cavity 9 needs to be optimized through simulation in advance, and the optimized value is about a quarter of the equivalent wavelength of the incident light in the medium, but it will be determined according to the material of the superconducting nanowire, There are slight differences in thickness and duty cycle.

The metal thin film reflector 8 is made of an Au film with a thickness of more than 60 nm, and there is a 1-2 nm thick Ti as an adhesion layer between it and the dielectric material constituting the upper resonant cavity 9 .

The bottom resonant cavity 2 13 is composed of an epitaxial single crystal Si layer 11 and an air layer. The thickness of the air layer is 1/4 of the wavelength of the incident light. The thickness of the epitaxial single crystal Si layer 11 needs to be optimized by simulation in advance, and the optimized value is It is about one-half of the wavelength of the incident light, but it will vary slightly depending on the material, thickness and duty cycle of the superconducting nanowire.

The method for preparing the above-mentioned second structure high-speed SNSPD comprises the following steps:

(a) Prepare a double-sided polished Si wafer with grooves carved on one side;

(b) Prepare the SOI substrate, obtain the precise thickness of the required epitaxial single crystal Si layer through simulation in advance, mechanically thin the Si layer on the back, and use the Si-Si bonding method to connect the SOI substrate and the above-mentioned concave The Si sheets of the groove are bonded together;

(c) Etch the back Si layer of the SOI substrate with KOH etching solution, and then remove the _SiO2 buried layer with hydrofluoric acid buffer etching solution to expose the single crystal Si layer;

(d) Sputtering and growing a superconducting thin film on a single crystal Si layer, and forming superconducting nanowires by electron beam exposure and reactive ion etching; the other side of the substrate is prepared with an Al ₂ O ₃ thin film as an anti-reflection film;

(e) Fabricate Au/Ti pattern on the superconducting nanowire as the coplanar waveguide readout circuit of the detector; finally fabricate the upper resonant cavity and mirror.

Compared with prior art, the beneficial effect of the present invention is:

Based on the high refractive index incident medium and air cavity structure, the photon absorption rate of superconducting nanowires is further improved. Similarly, under the condition of 4nm thick NbN nanowires, the simulation results show that with these two schemes, only about 25% of the nanometer The absorptivity close to 100% can be achieved when the line duty cycle is small, which greatly reduces the difficulty of the electron beam exposure step, which is especially beneficial for the preparation of ultra-fine nanowires (with a width below 50nm). The use of SOI substrates can simultaneously ensure the high-quality growth of superconducting thin films without affecting the intrinsic quantum efficiency of the detector. In addition, under the condition of ensuring the same large effective detection area, since the total length of the nanowires we need is significantly reduced, the maximum count rate of the detector can be increased, and the probability of defects during the preparation process is significantly reduced.

Description of drawings

Fig. 1 is a schematic diagram of the structure of the first SNSPD with a strong absorption structure.

Fig. 2 is a schematic diagram of the structure of the second SNSPD with a strong absorption structure.

Fig. 3 is a flow chart of the preparation of the first SNSPD with a strong absorption structure.

Fig. 4 is a flow chart for the preparation of the second SNSPD with a strong absorption structure.

Fig. 5 is the simulation result of the variation of the photon absorption rate of the first SNSPD with a strong absorption structure with the duty cycle of the nanowire.

Fig. 6 is the simulation result of the photon reflectance and transmittance of the first SNSPD with a strong absorption structure varying with the duty cycle of the nanowire.

Fig. 7 is a comparison between the photon absorption rate of the two SNSPDs with strong absorption structures and other existing technologies.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings and embodiments.

As shown in Figure 1, it is the first superconducting nanowire single photon detector with a strong absorption structure of the present invention, which includes a bottom Si substrate-1, and is deposited with multilayer Si/SiO ₂ periodic rows on the bottom Si substrate-1. A Bragg reflector 2 composed of cloth, the top of the Bragg reflector 2 is provided with a bottom resonant cavity-3 formed by epitaxial single crystal Si, a superconducting nanowire-4 is arranged above the bottom resonant cavity-3, and a superconducting nanowire-4 has a The upper air resonant cavity 5 has a Si sheet 6 above the upper air resonant cavity 5, and an antireflection film-7 is arranged on the Si sheet 6.

The Bragg reflector 2, which is periodically arranged by multilayer Si/SiO ₂ , has a very high reflectivity in a considerable wavelength range when its period number is large (greater than 6), and the reflectivity is greater than 99%. And because the refractive index difference between air and Si material is large, the interface between the air resonant cavity and the upper Si sheet can also form a good reflective surface. When the thickness of the air resonant cavity on the upper layer and the Si resonant cavity on the bottom layer are just right, the incident light just forms a standing wave between the two reflecting surfaces, and the superconducting nanowire is just at the antinode position of the maximum light intensity, so the structure The photon absorptivity of the nanowires can be significantly increased.

As shown in Figure 3, its preparation process includes the following steps:

(a) Prepare the SOI substrate, obtain the precise thickness of the required epitaxial single crystal Si layer through simulation in advance, and mechanically thin the Si layer on the back.

(b) Oxidation of SOI substrates requires precise control of _SiO2 thickness.

(c) The polycrystalline Si layer is grown by CVD, and the Si layer is partially oxidized to obtain a SiO ₂ layer. Repeat this n (n≥3) times to obtain a Si/SiO ₂ Bragg mirror with n+1 cycles, which requires precise Control the thickness of each layer.

(d) Using Si-Si bonding method, the above substrate and another Si sheet are bonded together as a new substrate.

(e) Use hydrofluoric acid (HF) buffered etching solution and KOH etching solution to etch the SiO ₂ and Si layer on the back of the SOI substrate in sequence, and then use hydrofluoric acid (HF) buffered etching solution to remove the SiO at the bottom of the single crystal Si layer ₂ buried layer, exposing the single crystal Si layer, the single crystal Si layer constitutes the bottom resonant cavity 1, the thickness of the bottom resonant cavity 1 is approximately half of the equivalent wavelength of the incident light in the medium, but will be based on the superconducting nanowire There are slight differences in material, thickness and duty cycle.

(f) High-quality superconducting thin films were grown on the single crystal Si layer by methods such as magnetron sputtering, and superconducting nanowires were formed by electron beam exposure and reactive ion etching (RIE). The thickness of the superconducting nanowire is generally between 4-6nm, and the width is generally between 20-200nm. The superconducting material used is NbN, NbTiN, TaN, NbSi, Nb, W _x Si _1-x or other materials.

(g) Au/Ti patterns are formed by optical exposure, sputtering (or electron beam evaporation), lift-off and other steps, as the coplanar waveguide readout circuit of the detector, and at the same time prepare for the subsequent Au-Au bonding.

(h) Prepare a double-sided polished Si wafer, and first prepare an Al ₂ O ₃ film on one side by ALD or sputtering. The refractive index needs to be between 1.7-2.0, and the thickness is equal to the incident light in the medium, etc. A quarter of the effective wavelength requires precise control. On the other side, Au/Ti patterns are formed by optical exposure, sputtering (or electron beam evaporation), lift-off and other steps.

(i) Through the Au-Au bonding method, the upper air resonant cavity is finally formed. The thickness of the upper air resonant cavity can be determined by controlling the thickness of the Au/Ti layers on both sides, which is approximately a quarter of the wavelength of the incident light. According to the super There are slight differences in the material, thickness and duty cycle of the conductive nanowires. However, the thickness change before and after Au-Au bonding needs to be considered in advance.

As shown in Figure 2, it is the second superconducting nanowire single photon detector with a strong absorption structure of the present invention, including a metal thin film reflector 8, an upper layer resonant cavity 9 made of a transparent dielectric material is arranged below the metal thin film reflector 8, and the upper layer Below the resonant cavity 9 is a superconducting nanowire 210, below the superconducting nanowire 10 is an epitaxial single crystal Si layer 11, below the epitaxial single crystal Si layer 11 is a Si substrate 212, and the Si substrate 212 faces the epitaxial single crystal The Si layer 11 is provided with a bottom resonant cavity 2 13, and an anti-reflection film 2 14 is provided under the Si substrate 2 12.

The principle of the second structure and the first structure to improve the photon absorption rate is exactly the same, except that between the superconducting nanowire and the incident medium of light, the second structure has one more resonant cavity than the first structure, and the mirror consists of Metal films instead of Bragg mirrors, but simulation results show that the absorption rate of the two structures is exactly the same if the loss of incident light in the metal film is not considered.

As shown in Figure 4, the preparation process of the second structure includes the following steps:

(a) Prepare a double-sided polished Si wafer, and use optical exposure, reactive ion etching (RIE) (or traditional bulk silicon etching) to carve grooves on one side, and the thickness of the grooves needs to be accurate control.

(b) Prepare the SOI substrate, obtain the precise thickness of the required epitaxial single crystal Si layer through simulation in advance, mechanically thin the Si layer on the back, and use the Si-Si bonding method to connect the SOI substrate and the above-mentioned concave The Si sheets of the slot are bonded together.

(c) Etch the back Si layer of the SOI substrate with KOH etching solution, and then remove the SiO ₂ buried layer with hydrofluoric acid (HF) buffer etching solution to expose the single crystal Si layer. SiO ₂ constitutes the second upper resonant cavity. The thickness of the upper resonant cavity second needs to be optimized through simulation in advance. The optimized value is approximately a quarter of the equivalent wavelength of the incident light in the medium, but it will vary according to the material of the superconducting nanowire, There are slight differences in thickness and duty cycle.

(d) Grow high-quality superconducting thin films on the single crystal Si layer by magnetron sputtering and other methods, and use electron beam exposure and reactive ion etching (RIE) to form superconducting nanowires; the other side of the substrate is coated with atomic layer Deposition (ALD) or sputtering and other methods to prepare Al ₂ O ₃ film as an anti-reflection film, the refractive index needs to be between 1.7-2.0, the thickness is equal to a quarter of the equivalent wavelength of the incident light in the medium, and it needs to be accurate control.

(e) Form Au/Ti patterns by optical exposure, sputtering (or electron beam evaporation), lift-off and other steps, as the coplanar waveguide readout circuit of the detector; by optical exposure, sequential sputtering (or electron beam evaporation) SiO _2. Steps such as Ti, Au and stripping form the upper resonant cavity and mirror, and the thickness of the SiO ₂ dielectric layer needs to be precisely controlled.

As shown in Fig. 5, as the period number p of the Bragg reflector increases, the photon absorption rate of the first SNSPD with a strong absorption structure is significantly improved. When p is equal to 4, the simulation results show that its absorptivity is very close to the case of using a completely ideal reflective layer (p=∞), so in the actual manufacturing process, it is more appropriate to take the period number of the Bragg reflector as 4 or above . The simulation results of reflectivity and transmittance shown in Figure 6 also show that as the period number p increases, the reflectivity of the Bragg reflector is indeed increased, the closer it is to the ideal reflective surface, thereby reducing the transmittance of the entire structure, and finally The absorptivity of the nanowires is enhanced.

Fig. 7 is a comparison of the photon absorption rate of the above two SNSPDs with strong absorption structures and other existing technologies. Curve "1" in the figure represents the above two structures; curve "2" represents the structure described in E.A.Daul er et al., "Superconducting nanowire single photon detectors," IEEE PhotonicsConference (PHO), 2011. Curve "3" represents B .Baek et al., "Superconducting nanowire single-photon detector in an optical cavity for front-side illumination," Appled Physics Letters, vol.95, p.191110 2009. The structure described; Curve "4" represents K.M.Rosfjord et al ., "Nanowire single-photon detector with an integrated optical cavity and anti-reflection coating," Optics Express, vol.14, pp.527–534, 2006. The structure recorded; Curves "5" and "6" respectively represent without The absorptivity of NbN nanowires with any additional structure under the conditions of backlight and frontlight, the substrate used is the most commonly used sapphire substrate. In the simulation process, the loss of light in the Au metal mirror (less than 4%) is not considered. If this part of the loss is to be eliminated, the metal film can be replaced by a Bragg reflector with a higher period number.

All the simulations and optimizations in Figure 5-7 are only for the most commonly used 1550nm communication wavelength, the superconducting material used is NbN, the incident light is perpendicular to the nanowire, and the polarization direction of the electric field is parallel to the direction of the nanowire. The thickness of the NbN nanowire is 4nm, and the simulation results show that the absorption rate is only related to the duty cycle of the NbN nanowire, but has nothing to do with the width of the nanowire itself. From the comparison of the simulation results, it can be clearly seen that under the same duty cycle conditions, the absorption rate of the two structures proposed by the present invention is significantly higher than that of all existing technologies at present, and can be used with a very low duty cycle of nanowires ( 25%) can achieve a photon absorption rate close to 100%, which greatly reduces the difficulty of the electron beam exposure step in the nanowire preparation process, especially for the preparation of ultra-fine nanowires (with a width below 50nm). for the benefit. In addition, under the condition of ensuring the same large effective detector area, since the total length of the nanowires we need is only half of the 50% duty cycle, the maximum count rate of the detector can be doubled, and the occurrence of The probability of defects is reduced to half.

Claims (1)

1. a method of preparing the high speed SNSPD with strong absorbing structure, described SNSPD comprises bottom Si substrate one (1), at bottom Si substrate one (1), deposits multilayer Si/SiO ₂the Bragg mirror (2) that cycle arranges and forms, Bragg mirror (2) top is provided with the bottom resonator cavity one (3) that epitaxy single-crystal Si forms, in bottom resonator cavity one (3) top, there is superconducting nano-wire one (4), on superconducting nano-wire one (4), there is air resonance chamber, upper strata (5), there is Si sheet (6) top, air resonance chamber, upper strata (5), on Si sheet (6), there is antireflection film one (7)

It is characterized in that, comprise the steps:

(a) prepare SOI substrate, by emulation, obtain in advance the precise thickness of needed epitaxy single-crystal Si layer, the Si layer at the mechanical reduction back side;

(b) oxidation SOI substrate, controls SiO in process ₂thickness;

(c) with CVD method growth polycrystalline Si layer, and partial oxidation Si layer, SiO obtained ₂layer, n time so repeatedly, obtains the Si/SiO in n+1 cycle ₂bragg mirror;

(d) by the method for Si-Si bonding, above-mentioned SOI substrate and another Si sheet are bonded together, as new substrate;

(e) with buffered hydrofluoride acid and KOH corrosive liquid, corrode successively respectively the SiO of SOI substrate back ₂with Si layer, then with buffered hydrofluoride acid, remove the SiO of single crystal Si layer bottom ₂buried regions, exposes single crystal Si layer;

(f) growth of superconductive film in single crystal Si layer, and form superconducting nano-wire by electron beam exposure and reactive ion etching;

(g) above superconducting nano-wire, make Au/Ti figure, as the co-planar waveguide sensing circuit of detector, for follow-up Au-Au bonding, prepare simultaneously;

(h) prepare the Si sheet of a twin polishing, first one side is prepared Al with ALD or sputtering method therein again ₂o ₃film, at another side, makes Au/Ti figure;

(i) by the method for Au-Au bonding, finally form air resonance chamber, upper strata, the thickness in air resonance chamber, upper strata determines by controlling the thickness of both sides Au/Ti layer.