Preparation method of flux quantum bit

Abstract

The invention belongs to the technical field of crossing of nano electronics, superconducting technology and quantum information science, and relates to a preparation method of flux quantum bits, which comprises the following steps: 1, processing a substrate; 2, depositing and oxidizing an aluminum film; 3, photoetching and etching to form a large structure; 4, forming a double-layer electron beam adhesive and a suspension bridge structure; 5, carrying out first dip angle evaporation and oxidation; 6, second-time dip angle evaporation; 7, removing photoresist; parameter consistency, yield and microwave performance of the prepared device all reach the industry leading level, the packaging mode is compatible with an existing system, and a reliable solution is provided for large-scale integration of a quantum computing system.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of nanoelectronics, superconducting technology and quantum information science, and particularly relates to a method for preparing a magnetic flux qubit. BACKGROUND

[0002] A superconducting magnetic flux qubit is a basic unit of quantum computation based on superconducting physics, which uses the magnetic flux in a superconducting loop to encode and store quantum information. A superconducting magnetic flux qubit is usually composed of a superconducting loop and one to three Josephson junctions connected in series.

[0003] The existing superconducting magnetic flux qubit generally adopts a planar process of an aluminum thin film on an insulating substrate. The substrate is usually sapphire or high-resistance silicon. A uniform aluminum film is first deposited by evaporation or sputtering, and then large-scale structures such as transmission lines, readout resonant cavities, ground planes and alignment marks are formed by photolithography and etching processes. In this type of process, the Josephson junction usually adopts a suspension bridge structure with double-layer electron beam glue, and the aluminum / aluminum oxide / aluminum multilayer structure is realized by double-angle evaporation. First, a bridge-shaped window with an undercut structure is formed in the electron beam glue, then a first layer of aluminum film is evaporated at a certain inclination angle to generate a thin aluminum oxide barrier through static oxygen oxidation, and then a second layer of aluminum film is evaporated at another inclination angle, and finally the junction area pattern is defined by removing the glue.

[0004] For Transmon and other superconducting bits, the existing technology has formed a relatively mature process flow. For example, after high-temperature degassing on a sapphire substrate, about 100 nm of aluminum film is evaporated and oxidized, large structures such as transmission lines, resonant cavities and markers are defined by photolithography and wet etching, and Josephson junctions are prepared by electron beam exposure and double-angle evaporation process. If necessary, an air suspension bridge can also be prepared to cross the coplanar waveguide gap to reduce parasitic modes and losses. For the magnetic flux qubit structure, three to four Josephson junctions are usually connected in series in a superconducting loop, one to two of which are composed of SQUID structures to adjust the equivalent junction parameters by applying an external magnetic flux, thereby changing the dimensionless parameter a in the loop to achieve adjustable energy level structure and potential well shape. The overall design and finished scanning electron microscope photo are generally similar to a ring-shaped or approximately ring-shaped superconducting loop opened on a section of coplanar waveguide, and two to three junction islands are arranged on the loop.

[0005] However, the existing superconducting magnetic flux qubit preparation has the following defects: First, for different types of devices such as magnetic flux qubits and Transmon bits, the existing process often designs independent layout structures and process parameters for each type of device. The substrate processing, aluminum film deposition, etching and packaging steps lack unified planning, resulting in poor consistency of different devices on the same wafer and difficulty in high-yield mixed preparation of different types of quantum bits on the same process line.

[0006] Second, in the preparation of Josephson junction, the selection of evaporation angle, aluminum film thickness and oxidation process parameters in the traditional double-angle evaporation is usually optimized for a single device, lacking of systematic design for accurate adjustment of parameter a of magnetic flux quantum bit. As a result, the matching of critical current of two large junctions and a set of SQUID junctions in the loop is limited, the adjustable range and resolution of a are limited, and the repeatability between different chips and different batches is poor.

[0007] Third, in the structural design, although the existing magnetic flux quantum bit also adopts the form of two large junctions plus one adjustable SQUID, the relative layout and connection mode of the on-chip readout resonant cavity, transmission line and marker lack unified process constraints, which is easy to cause problems such as long wiring, uncontrollable parasitic capacitance and parasitic inductance, thereby adversely affecting the energy level structure and decoherence time of the quantum bit.

[0008] Therefore, there is a need for a superconducting magnetic flux quantum bit preparation method that can optimize the design of Josephson junction and magnetic flux loop on the basis of following the mature Transmon process, improve the adjustability, consistency and device performance of parameter a through reasonable evaporation angle, aluminum film thickness and SQUID structure, and solve the above technical problems. SUMMARY

[0009] The present application aims to reuse the mature Transmon process in the preparation of the overall large structure, only use a specific double-angle evaporation process in the preparation of Josephson junction, and design a magnetic flux quantum bit structure containing two large junctions and a SQUID, so as to realize accurate adjustment of parameter a while ensuring process compatibility and yield.

[0010] The present application provides the following technical scheme: a preparation method of a magnetic flux quantum bit, comprising the following steps: Step 1: high-temperature degassing or chemical cleaning of the sapphire substrate or high-resistance silicon substrate to remove surface adsorbed molecules and natural oxide layer; Step 2: depositing an aluminum film on the treated substrate by electron beam evaporation, and then oxidizing to form a uniform surface oxide layer in a static oxygen environment after deposition is completed; Step 3: defining a large structure pattern by laser direct writing or mask exposure, and developing and fixing treatment using a developing solution; wet etching the unprotected area using an aluminum etching solution until the aluminum film is penetrated to form the required pattern, and then removing the photoresist and drying; Step 4, after the large structure is completed on the chip, select the flux quantum bit circuit area, spin-coat a double-layer electron beam glue of lower MMA and upper PMMA, form a suspension bridge window with undercut in the glue layer by high-energy electron beam exposure and development, and provide a three-dimensional structure channel for subsequent double-angle evaporation; Step 5, tilt the sample, keep the fixed inclination angle relative to the evaporation source, evaporate the aluminum film under vacuum conditions, make the aluminum film deposit on one side of the suspension bridge window to form the first layer of junction electrode, and oxidize in a static oxygen environment after evaporation to form an aluminum oxide barrier layer with a target thickness on the surface of the aluminum film to constitute the insulating layer of the tunnel junction; Step 6, tilt the sample again, evaporate the aluminum film at a high speed to make the second layer of aluminum film deposit on the other side of the suspension bridge window and cross the aluminum oxide barrier to form an overlapping area with the first layer of aluminum film, thereby constituting a Josephson junction stack structure; by repeating the double-angle evaporation in the same opening structure, two large junctions and a group of SQUID junctions are continuously formed on the magnetic flux loop, the critical current of the large junction and the SQUID junction is matched by using the different island areas and overlapping lengths in the layout, the parameter α can be pre-set in the process, and fine adjustment is made by applying an external magnetic flux; Step 7, after the two evaporation processes are completed, immerse the sample in an NMP solution and heat to make the electron beam glue fully dissolved, flush away the excess aluminum film on the suspension bridge structure, and only keep the metal pattern on the lower substrate to form the final Josephson junction structure; finally, clean and dry.

[0011] Preferably, in step 1, the sapphire substrate is baked at a temperature of 180-220°C for 5-8 hours to remove gas.

[0012] Preferably, in step 2, the thickness of the deposited aluminum film is 80-120 nm, and the deposition rate is 0.8-1.2 nm / s; the static oxygen environment is 20-80 mTorr.

[0013] Preferably, in step 3, the large structure pattern includes a transmission line, a resonant cavity, a ground plane, and a marker.

[0014] More preferably, based on step 3, an air suspension bridge structure is prepared to cross the aluminum bridge on the coplanar waveguide gap to suppress the slot mode and the parasitic mode and improve the microwave performance of the device.

[0015] Preferably, in step 4, after the double-layer electron beam glue of lower MMA and upper PMMA is spin-coated, a layer of conductive glue is added to improve the charge dissipation effect during exposure.

[0016] Preferably, in step 5, the sample inclination angle is -15° to -25°; the evaporation rate of the evaporated aluminum film is 0.8 to 1.2 nm / s, and the thickness of the evaporated aluminum film is 35 to 45 nm; in step 6, the sample inclination angle is 15° to 25°; the evaporation rate of the evaporated aluminum film is 0.8 to 1.2 nm / s, and the thickness of the evaporated aluminum film is 55 to 65 nm.

[0017] Preferably, in step 7, the heating time is 5 to 8 hours; the cleaning is performed using acetone and isopropyl alcohol, and the drying is performed using nitrogen.

[0018] Preferably, after step 7 is completed, the chip is cut into a single device or a small array by a cutting machine along a predetermined scribe line; for a sapphire hard substrate, a diamond soft knife is used for resin bonding, and the feed rate and repeated cutting times are controlled to avoid edge collapse; after cutting, the chip is fixed in a metal sample box, a bit grounding area is connected to the box body using a wire bonding instrument, and the driving and reading terminals are connected to the center conductor of the SMA connector, thereby completing packaging.

[0019] Preferably, the magnetic flux quantum bit is arranged on a substrate, and the on-chip includes a readout resonant cavity, a transmission line, a ground plane, an alignment marker, and the magnetic flux quantum bit itself; except for the Josephson junction in the quantum bit loop, the remaining large structure can be designed and processed using the same steps as the Transmon device, and is completed through one-time aluminum film deposition and photolithography etching.

[0020] The present application has the following advantages: 1. The present application realizes process platform unification and reuse; the substrate processing, aluminum film deposition, photolithography, and etching steps of the magnetic flux quantum bit are completely unified with the Transmon device process, and only the specific double-angle evaporation and SQUID structure design are used in the Josephson junction preparation link, so that the magnetic flux quantum bit and various devices such as Transmon can be prepared on the same wafer substrate at the same time, thereby significantly reducing process development costs and improving chip design flexibility.

[0021] 2. The parameter alpha of the present application is highly accurate and adjustable; the present application arranges two large junctions and a SQUID in the loop, cooperates with two layers of 40 nm and 60 nm aluminum films and double evaporation angles of -20 degrees and +20 degrees, combines reasonable island area and overlap length design, makes the critical current ratio of the large junction and the SQUID junction more controllable, and makes the nominal value and adjustable range of alpha more easily and accurately realized. When the external magnetic flux changes, the adjustment curve of alpha is smooth and has good repeatability, which is beneficial to the fine control of the potential well shape and energy level structure of the magnetic flux quantum bit.

[0022] 3. The device of the present application has high consistency and yield; the present application separates the large structure process and the Josephson junction process, on the basis of uniform aluminum film deposition and photolithography etching, a group of junctions with narrow critical current distribution can be obtained on a chip through double-angle evaporation and the same batch of oxidation parameters, thereby improving the consistency of parameters between different magnetic flux qubits.

[0023] 4. The microwave performance and packaging reliability of the present application are improved; the present application reuses the mature transmission line and resonant cavity design in the Transmon process, and can select structures such as air suspension bridge to reduce parasitic modes, the magnetic flux qubit of the present application obtains a better compromise between coupling strength, readout efficiency and sensitivity to environmental noise, and the packaging method is compatible with the existing Transmon packaging, which is conducive to system-level integration. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a general structure diagram of the preparation method of the magnetic flux qubit of the present application; Figure 2 is a schematic diagram of the second inclination evaporation step of the present application; Figure 3 is the effect of the present application Figure 1 ; Figure 4 is the effect of the present application Figure 2 ; Figure 5 is a schematic diagram of the method steps of the present application. DETAILED DESCRIPTION

[0025] The related technologies in the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0026] As shown in Figures 1-5 , the preparation method of the magnetic flux qubit of the present application includes: 1. General structure The magnetic flux qubit of the present application is arranged on a sapphire or high-resistance silicon substrate, and on-chip includes a readout resonant cavity, a transmission line, a ground plane, an alignment marker, and the magnetic flux qubit itself. Except for the Josephson junction part in the quantum bit loop, the rest of the large structure adopts the same design and process steps as the Transmon device, which is completed through one-time aluminum film deposition and photolithography etching, thereby realizing process unification and layout reuse.

[0027] The flux qubit body is composed of a superconducting loop and three Josephson structures on the loop, two of which are large-area Josephson junctions on both sides, and the middle is a SQUID structure composed of two small structures, which is used to adjust the equivalent alpha value. The whole loop is coupled with a co-planar waveguide or a readout resonant cavity to realize frequency readout and control operation.

[0028] 2. Substrate and large structure preparation Step 1: Substrate treatment Sapphire or high-resistance silicon substrate is selected, and the substrate is subjected to high-temperature degassing or chemical cleaning to remove surface adsorbed molecules and natural oxide layer, ensuring the adhesion of the subsequent aluminum film and the interface quality. For sapphire substrates, baking at about 200 degrees Celsius for several hours can remove gas.

[0029] Step 2: Aluminum film deposition and oxidation On the treated substrate, an aluminum film about one hundred nanometers thick is deposited by electron beam evaporation at a deposition rate of about one nanometer per second. After deposition, oxidize in a static oxygen environment of tens of millitorr for several minutes to form a uniform surface oxide layer to improve surface stability and superconducting contact performance.

[0030] Step 3: Photolithography and etching to form large structures Spin-coat positive photoresist and bake, define the large structure patterns such as transmission lines, resonant cavities, ground planes, markers, etc. by laser direct writing or mask exposure, and develop and fix the patterns using the corresponding developer. Then use aluminum etching solution to wet etch the unprotected areas, until the aluminum film is penetrated, forming the required pattern, and finally remove the photoresist and clean and dry.

[0031] If necessary, an air-bridge structure can be further prepared to cross the aluminum bridge on the co-planar waveguide gap to suppress the slot mode and parasitic mode, and improve the microwave performance of the device. The specific process can reuse the mature air-bridge process.

[0032] 3. Flux qubit Josephson junction preparation Step 4: Double-layer electron beam glue and suspension bridge structure formation On the chip with completed large structures, select the flux qubit loop area, spin-coat double-layer electron beam glue of MMA and PMMA, and further add a layer of conductive glue to improve the charge dissipation effect during exposure. Through high-energy electron beam exposure and development, a suspension bridge window with undercut is formed in the glue layer, providing a three-dimensional structure channel for subsequent double-angle evaporation.

[0033] Step 5: First angle evaporation and oxidation The sample is tilted to about -20 degrees, and a 40 nm thick aluminum film is evaporated at a rate of about 1 nm / s under high vacuum conditions, with the relative evaporation source fixed at a certain angle. The aluminum film is deposited on one side of the suspended bridge window to form the first layer of the junction electrode.

[0034] After the evaporation is completed, oxidation is performed in a static oxygen environment. The oxygen pressure and time are selected to form an aluminum oxide barrier layer of a suitable thickness on the surface of the aluminum film, which constitutes the insulating layer of the tunnel junction. The oxidation step can be adjusted in terms of time and oxygen pressure according to the target critical current density.

[0035] Step 6: Second angle evaporation The sample is tilted to about +20 degrees, and a 60 nm thick aluminum film is evaporated at a rate of about 1 nm / s. The second layer of aluminum film is deposited on the other side of the suspended bridge window and across the aluminum oxide barrier, forming an overlapping region with the first layer of aluminum film to form a Josephson junction stack structure.

[0036] By repeating the above double-angle evaporation in the same opening structure, two large junctions and a set of SQUID junctions can be continuously formed on the magnetic flux loop. By adjusting the island area and overlap length in the layout, the critical current of the large junction and the SQUID junction can be matched, allowing the parameter α to be pre-set in the process and fine-tuned by an external magnetic flux.

[0037] Step 7: Lift-off After the two evaporation steps are completed, the sample is immersed in an NMP solution and heated for several hours to fully dissolve the electron beam glue. The excess aluminum film on the suspended bridge structure is washed away by a syringe or soft brush, leaving only the metal pattern on the underlying substrate to form the final Josephson junction structure. Subsequently, acetone and isopropyl alcohol are used for cleaning, and nitrogen is used for drying.

[0038] 4. Slicing and packaging Multiple magnetic flux quantum bit devices can be arranged on a whole sample. After micro-nano processing is completed, the chip is cut into individual devices or small arrays along the predetermined scribe line by a cutting machine. For hard substrates such as sapphire, a diamond soft knife bonded with resin can be used to control the feed rate and repeated cutting times to avoid edge collapse. After cutting, the chip is fixed in a metal sample box, and a wire bonder is used to connect the grounding area of the bit to the box body, and the driving and reading terminals are connected to the center conductor of the SMA connector, completing the packaging.

[0039] Further, the substrate and large structure material of the present application can be replaced by other materials. The substrate can be replaced by high-resistance silicon or other low-loss insulating materials instead of sapphire, as long as it has good interface quality with the aluminum film and can maintain small dielectric loss at low temperature. The large structure metal material can also be replaced by niobium or aluminum-niobium laminates instead of aluminum, and the deposition and etching processes need to be adjusted accordingly.

[0040] Further, the alternative scheme of the evaporation angle and the aluminum film thickness of the present application includes: the specific angle of the double-angle evaporation can be selected as other symmetric or asymmetric angle combinations, such as negative thirty degrees and positive thirty degrees, and the aluminum film thickness can also be designed as other ratios according to the target critical current density, as long as a stable overlapping area and a tunneling potential barrier can be formed in the suspension bridge structure, which constitutes an equivalent alternative scheme of the present application.

[0041] Further, the alternative of the SQUID structure form of the present application includes: the SQUID structure for adjusting alpha can be arranged on the other side of the loop instead of the one side, or an incomplete symmetric double-junction structure can be used, and even extended to a multi-junction SQUID, as long as the adjustable equivalent critical current can be realized by the applied magnetic flux, and the technical effect is equivalent to the present application scheme.

[0042] Further, the alternative scheme of the suspension bridge and the etching process of the present application includes: the double-layer electron beam glue for forming the suspension bridge structure can be replaced by other multi-layer glue systems that can form undercut. The substrate under the Josephson junction can be first shallow etched or a thin dielectric layer is added to optimize the electric field distribution and leakage path. These changes belong to the process detail alternatives, and do not change the core technical idea of the present application.

[0043] In summary, the present application realizes the co-fabrication of the flux quantum bit and the Transmon device through a unified process platform, which significantly reduces the process development cost. The core innovation is to use the double-angle evaporation technology, to accurately control the evaporation angle, the aluminum film thickness and the oxidation parameters while keeping the consistency of the large structure process, to realize the high-precision matching of the critical current of the Josephson junction. This design not only makes the adjustment curve of the parameter alpha have excellent smoothness and repeatability, but also through the combination of the SQUID structure and the two large junctions, the best balance between the coupling strength, the readout efficiency and the environmental noise suppression is achieved. Experiments show that the devices prepared by this scheme have reached the industry leading level in terms of parameter consistency, yield and microwave performance, and the packaging method is compatible with the existing system, which provides a reliable solution for the large-scale integration of quantum computing systems.

[0044] It should be emphasized that: the above is only the preferred embodiment of the present application, and does not limit the present application in any form, and any simple modification, equivalent change and modification of the above embodiment according to the technical essence of the present application still belongs to the scope of the technical scheme of the present application.

Claims

1. A method for preparing magnetic flux qubits, characterized in that, Includes the following steps: Step 1: Perform high-temperature degassing or chemical cleaning on the sapphire substrate or high-resistivity silicon substrate to remove surface adsorbed molecules and the natural oxide layer. Step 2: An aluminum film is deposited by evaporation on the prepared substrate using an electron beam. After deposition, the film is oxidized in a static oxygen environment to form a uniform surface oxide layer. Step 3: Define the large structure pattern by laser direct writing or mask exposure, and perform development and fixing processes using a developer; Wet etching is performed on the areas not protected by photoresist using aluminum etching solution until the aluminum film is penetrated to form the desired pattern. The photoresist is then removed and the area is cleaned and dried. Step 4: On the wafer with the large structure already completed, select the magnetic flux quantum bit circuit region and spin-coat a double layer of electron beam resist consisting of a lower MMA layer and an upper PMMA layer; through high-energy electron beam exposure and development, form a cantilever window with a downward cut in the resist layer; Step 5: Tilt the sample and keep it at a fixed angle relative to the evaporation source. Evaporate the aluminum film under vacuum conditions so that the aluminum film is deposited on one side of the suspension window to form the first junction electrode. After evaporation, oxidize the aluminum film in a static oxygen environment to form an aluminum oxide barrier layer of the target thickness on the surface of the aluminum film, which constitutes the insulating layer of the tunnel junction. Step 6: Tilt the sample again and evaporate the aluminum film at a higher rate, so that the second aluminum film is deposited on the other side of the cantilever window and crosses the alumina barrier, forming an overlapping area with the first aluminum film, thus forming a Josephson junction stacked structure; by repeating double-angle evaporation in the same open structure, two large junctions and a set of SQUID junctions are continuously formed on the magnetic flux loop. By utilizing the different island areas and overlap lengths in the layout, the ratio design of the critical current of the large junctions and SQUID junctions is realized. Step 7: After completing the two evaporations, immerse the sample in NMP solution and heat it to fully dissolve the electron beam gel. Rinse off the excess aluminum film on the suspension structure, leaving only the metal pattern on the substrate below, to form the final Josephson junction structure. Finally, clean and dry the sample.

2. The method for preparing a magnetic flux quantum bit according to claim 1, characterized in that, In step 1, the sapphire substrate is baked at 180℃~220℃ for 5~8 hours to remove gas.

3. The method for preparing a magnetic flux quantum bit according to claim 1, characterized in that, In step 2, the thickness of the deposited aluminum film is 80–120 nm, and the deposition rate is 0.8–1.2 nm / s; the static oxygen environment is 20–80 mTorr.

4. The method for preparing a magnetic flux quantum bit according to claim 1, characterized in that, In step 3, the large structural pattern includes: transmission line, resonant cavity, ground plane, and marker.

5. The method for preparing a magnetic flux quantum bit according to claim 4, characterized in that, Based on step 3, an air suspension bridge structure is prepared, and an aluminum bridge is connected across the coplanar waveguide gap.

6. The method for preparing a magnetic flux quantum bit according to claim 1, characterized in that, In step 4, after spin-coating the double-layer electron beam adhesive of the lower MMA and upper PMMA, a layer of conductive adhesive is added.

7. The method for preparing a magnetic flux quantum bit according to claim 1, characterized in that, In step 5, the sample tilt angle is -15° to -25°; the evaporation rate of the aluminum film is 0.8 to 1.2 nm / s; and the thickness of the aluminum film is 35 to 45 nm. In step 6, the sample tilt angle is 15° to 25°; the evaporation rate of the aluminum film is 0.8 to 1.2 nm / s; and the thickness of the aluminum film is 55 to 65 nm.

8. The method for preparing a magnetic flux quantum bit according to claim 1, characterized in that, In step 7, the heating time is 5 to 8 hours; the cleaning is carried out using acetone and isopropanol, and the drying is carried out using nitrogen.

9. The method for preparing a magnetic flux quantum bit according to claim 1, characterized in that, When slicing and packaging are performed after step 7, the chip is cut into individual devices or small arrays along the predetermined scribing lines using a cutting machine; for sapphire hard substrates, a soft diamond blade bonded with resin is used. After dicing, the chip is fixed in a metal sample box. A wire bonding device is used to connect the grounding area of ​​the bits to the ground of the box. The drive and read terminals are connected to the center conductor of the SMA connector to complete the packaging.

10. The method for preparing a magnetic flux quantum bit according to claim 1, characterized in that, The magnetic flux qubits are arranged on a substrate, and the on-chip includes: a readout resonant cavity, a transmission line, a ground plane, an alignment marker, and the magnetic flux qubits themselves; except for the Josephson junction in the qubit loop, the other large structures can adopt the same design and process steps as the Transmon device, and are completed by one aluminum film deposition and photolithography etching.

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