Etchant and method for selectively etching titanium dioxide

Abstract

One method involves selectively etching titanium dioxide using an etchant in the presence of alumina. The etchant is an aqueous solution of an alkali and a hydroperoxide. Surprisingly, this etchant has been found to exhibit better selectivity for titanium dioxide than for alumina, and to effectively remove titanium dioxide without damaging the alumina layer. An etchant for etching titanium dioxide is also provided, which is an aqueous solution of ammonia and hydrogen peroxide, wherein the molar ratio of the ammonia to the hydrogen peroxide is in the range of 1:34 to 1:38.

Description

Technical Field

[0001] This disclosure relates to etchants and methods for selectively etching titanium dioxide. Prior Technology

[0002] If conditions are right, the topological phase of matter carried by semiconductor nanowires approaching superconductors is expected. This makes them promising candidates as building blocks for fault-tolerant quantum computers.

[0003] The topological phase manifests itself as a pair of Majorana zero modes (MZMs) at the ends of the nanowires. Along the body of the line, away from the ends, there are gaps in the single-electron spectrum. Experiments typically use the tunneling spectrum at the nanowire ends to detect the zero-bias peak (ZBP) in the tunneling conductance.

[0004] By forming a network of such nanowires and sensing topology in different parts of the network, qubits can be created that can be manipulated for quantum computing purposes. A qubit is an element to which a measurement can be performed with two possible outcomes, but at any given time (when not measured), it can actually be in a quantum superposition of two states corresponding to different results.

[0005] Nanowires can take the form of elongated portions of semiconductor materials, with their length being many times their width and thickness. Nanowires are quasi-one-dimensional systems. Conventional superconductor layers are arranged on at least a portion of the nanowire.

[0006] To induce the topological phase, nanowires are cooled to the temperature at which superconductors (such as aluminum) exhibit superconducting behavior. The superconductor induces a proximity effect in adjacent semiconductors, causing the semiconductors to also exhibit superconductivity in the region near the interface with the superconductor, i.e., inducing a superconducting pairing gap in adjacent semiconductors. When a magnetic field is applied, MZMs form at both ends of the semiconductor-superconductor hybrid.

[0007] The role of a magnetic field is to enhance spin degeneracy in semiconductors. In the context of quantum systems, degeneracy refers to the situation where different quantum states have the same energy level. Enhancing degeneracy means making such states adopt different energy levels. Spin degeneracy refers to the situation where different spin states have the same energy level. Spin degeneracy can be enhanced by a magnetic field, resulting in energy level segmentation between electrons with different spin polarizations. This is known as the Zeeman effect. The Zeeman energy, i.e., the magnitude of the energy level segmentation, should be at least as large as the superconducting gap in order to close the negligible superconducting gap and reopen the topological gap in the system.

[0008] Inductively coupled micro-ZM (MZM) can also involve adjusting the electrostatic potential of charge carriers in nanowires by using electrostatic potential gates. An electrostatic potential is applied using gate electrodes. Applying an electrostatic potential manipulates the number of charge carriers in the conduction or valence band of a semiconductor device.

[0009] The electronic properties of hybrid systems are highly dependent on the fabrication method of the components. In particular, the interface quality between semiconductor and superconductor components greatly affects the behavior of the finished components. Fabrication of nanowire heterostructures containing InAs and Al layers has been reported (Krogstrup et al., Nat. Mater. 14, 400 (2015)). Summary of the Invention

[0010] In one embodiment, this disclosure provides a method comprising selectively etching titanium dioxide in the presence of alumina using an etchant. The etchant is an aqueous solution of an alkali and hydroperoxide. Surprisingly, this etchant has been found to exhibit better selectivity for titanium dioxide than for alumina, and to effectively remove titanium dioxide without damaging the alumina layer.

[0011] In another embodiment, an etching solution for etching titanium dioxide is provided, which is an aqueous solution of ammonia and hydrogen peroxide, wherein the molar ratio of the ammonia to the hydrogen peroxide is in the range of 1:34 to 1:38.

[0012] This summary is provided to introduce a series of concepts in a simplified form, which will be further described in the detailed embodiments below. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter. The claimed subject matter is also not limited to embodiments that address any or all of the shortcomings mentioned herein. Simple Explanation of the Diagram

[0013] To aid in understanding the embodiments disclosed herein and to show how such embodiments can be implemented, reference is made to the accompanying drawings by way of example only, in which: Figure 1 is a flowchart outlining an exemplary method; Figure 2 is a schematic plan view of an exemplary workpiece. Figure 3 is an annotated scanning electron microscope (SEM) image of the component manufactured according to Example 1; Figure 4 is a schematic plan view of the element used in the transmission spectroscopy study as described in Example 1; Figure 5 is a graph showing the differential conductance of the exemplary element (solid line) and the comparison element (dashed line) as measured by transmission spectroscopy and discussed in Example 1, as a function of the applied source-drain voltage; Figure 6 shows optical microscope images of the two workpieces prior to the etching process described in Example 2; Figure 7 shows optical microscope images of the workpiece after (a) five-minute etching and (b) ten-minute etching as described in Example 2; Figure 8 is a SEM image of the workpiece after 5 minutes of etching as described in Example 2, with annotations to identify the masked (M) and exposed (E) areas of the workpiece; Figure 9 is a SEM image of the workpiece after 10 minutes of etching as described in Example 2, with annotations to highlight the approximate location of the edges of the masked area; Figure 10 is an optical microscope image of the workpiece after 15 minutes of etching, as described in Example 2; Figure 11 is an optical microscope image of the workpiece after 20 minutes of etching as described in Example 2; Figure 12 is an optical microscope image of the workpiece used in the comparative example; and Figure 13 is an optical microscope image of the etched workpiece based on a comparative example.

[0014] All optical microscope images were digitally processed to increase contrast and to convert them from color to grayscale. Implementation

[0015] The verb "include" is used in this text as a shorthand for "includes" or "consisting of". In other words, while the verb "includes" is intended to be an open term, it is explicitly considered that the closed term "consisting of" should be used instead, especially when used in conjunction with chemical compositions.

[0016] The volume was measured at 25°C and atmospheric pressure (1 atm; 101325 Pa).

[0017] The ionization constant (e.g., pKb) can be measured by potentiometric titration at 25°C in a 0.15 M potassium chloride aqueous solution.

[0018] A method for manufacturing a semiconductor-superconductor hybrid device is described in co-pending international patent application PCT / EP2020 / 081723. The method involves forming a mask on an aluminum layer and subsequently performing anodizing to selectively oxidize the portions of the aluminum layer not covered by the mask. An example of a mask material that can be used in this method is titanium, which is converted into titanium dioxide by anodizing.

[0019] Figure 1 illustrates an exemplary component 100 that can be manufactured using this method. The component includes a substrate 110, which may include a semiconductor component; an aluminum component 120; an aluminum oxide layer 130; and a titanium oxide mask 140. The aluminum oxide layer 130 covers the aluminum component 120 and the substrate 110. The aluminum component 120 is disposed beneath the titanium oxide mask 140, wherein the aluminum oxide layer 130 extends between the aluminum component 120 and the titanium oxide mask 140.

[0020] The presence of any residual titanium metal can be problematic during component operation. This can be avoided by selecting the conditions used in anodizing to achieve a complete titanium oxide mask, but this imposes additional constraints on the manufacturing process. It has been found that even the presence of titanium oxide can interfere with or prevent certain types of measurements on semiconductor-superconductor hybrid structures.

[0021] The aluminum oxide layer protects semiconductor and superconductor components. To enhance the performance of hybrid devices, the surface of semiconductor components should be pristine. Therefore, a method is needed to selectively remove the mask 140 while retaining sufficient aluminum oxide layer 130 to protect the aluminum component 120 and the substrate 110.

[0022] Dry plasma etching using fluorine has been proposed as a technique for selectively etching titanium dioxide in the presence of alumina. However, this technique is potentially hazardous and requires specialized equipment.

[0023] The use of wet etching requires selectively removing the titanium dioxide without also removing the aluminum oxide layer.

[0024] The inventors have surprisingly discovered that an etchant in the form of an aqueous solution of alkali and hydroperoxide erodes titanium dioxide at a much higher rate than alumina, and can be used to selectively etch titanium dioxide in the presence of alumina. This etchant can also be used to etch titanium, as titanium readily oxidizes in situ.

[0025] Not wanting to be bound by theory, it is believed that the following reaction may occur when aqueous solutions come into contact with titanium or titanium oxide.

[0026] Any exposed titanium metal is oxidized to titanium dioxide by water, dissolved oxygen, and / or hydroperoxides.

[0027] Titanium dioxide dissolves in the presence of the etchant.

[0028] Dissolved titanium reacts with hydroperoxides to form relatively soluble titanium(IV) complexes. Various different Ti(IV) complexes can be formed. Hydroperoxides can provide a source of one or more ligands selected from: oxoligands (O 2-), superoxide ligands (O 2-), peroxide ligands (O 2 2-), hydroperoxide ligands (ROOH), and peroxy ligands (ROO-). Some Ti(IV) complexes may further include one or more aqueous ligands and / or one or more hydroxyl ligands. The properties and relative proportions of the resulting complexes can vary depending on the pH of the solution [Mori et al., Bulletin of the Chemical Society of Japan, Vol. 29, No. 8, pp. 904-907].

[0029] Aluminum oxide can react with alkalis to form aluminum trioxide (Al(OH)₃). Aluminum trioxide is extremely poorly soluble in water. Aluminum trioxide can hydrolyze to form the more soluble aluminum tetraoxide anion (Al(OH)₄⁻). However, the etchant of the present invention has been observed to provide good titanium removal selectivity. It is believed that the reaction of alumina with alkalis is much slower than that involving titanium dioxide, and / or titanium complexes are substantially more soluble than aluminum salts.

[0030] It is believed that alumina does not react with hydrogen peroxide under the conditions used in etching processes. Commercially available concentrated hydrogen peroxide stock solutions are often supplied in aluminum containers.

[0031] In the fabrication of the semiconductor-superconductor hybrid device according to the above scheme, the function of the alumina layer is to protect the surface of the substrate and any semiconductor or superconductor components. If these components remain protected, partial removal of the alumina layer and / or partial conversion of the protective layer from alumina to aluminum hydroxide is well tolerated.

[0032] The thickness of the alumina layer can be selected such that, after etching, the alumina layer is at least 3 nm thick. This corresponds to approximately the depth of the natural alumina layer formed when aluminum is exposed to air. An alumina layer with a thickness of 3 nm or greater provides very good protection for any component underneath. By growing an aluminum layer and then oxidizing it by anodizing, an alumina layer of any thickness can be formed.

[0033] The exemplary method will now be explained with reference to Figure 2. Figure 2 is a flowchart outlining the method.

[0034] At block 201, prepare the etchant. Any suitable technique can be used. For example, a stock aqueous solution of hydroperoxide can be diluted to the desired target concentration and then mixed with a stock solution of alkali.

[0035] At block 202, titanium dioxide is selectively etched using an etchant in the presence of alumina. This operation may include immersing a workpiece comprising both titanium dioxide and alumina in the etchant. Both titanium dioxide and alumina are in contact with the etchant.

[0036] The method of the present invention is particularly useful during the manufacture of semiconductor-superconductor hybrid devices. The workpiece can be, for example, as described above with reference to Figure 1. However, it should be understood that the method of the present invention can also be used in other situations requiring the selective etching of titanium or titanium dioxide in the presence of alumina.

[0037] Etching can be performed at any temperature where the etchant is in a liquid state. For example, etching can be performed at approximately room temperature (20°C to 30°C).

[0038] The etching rate can vary depending on the concentration of the reagent in the etchant and the temperature at which the etching is performed. The duration of etching can be selected based on the etching rate.

[0039] For example, aqueous solutions of ammonia in the concentration range of 60 mM to 70 mM and hydrogen peroxide in the concentration range of 1.8 M to 2.2 M removed a 3 nm titanium dioxide layer in about 15 minutes, with almost no damage to aluminum oxide layers with a thickness greater than 8 nm.

[0040] It is possible to control the thickness of the alumina layer during the manufacturing process of the workpiece. For example, an aluminum layer of the desired thickness can be grown and then oxidized by anodizing to form the alumina layer. If aggressive etching conditions (e.g., high temperature and / or high alkali concentration) are required, starting with a relatively thick alumina layer can mitigate any damaging effects on the alumina layer.

[0041] Etching can be terminated by removing the workpiece from the etchant solution and optionally washing it with a solvent (such as water).

[0042] At block 203, clean the alumina. For example, the workpiece can be immersed in an acid bath. Hydrofluoric acid is an example of a suitable acid.

[0043] The surface of alumina may be slightly damaged by etchants or may be contaminated with pollutants (e.g., precipitated aluminum hydroxide). Therefore, cleaning the surface may be useful. However, cleaning is optional and can be omitted in some embodiments. Example 1

[0044] The workpiece of the type shown in Figure 1 is manufactured according to the method described in PCT / EP2020 / 081723. A semiconductor heterostructure comprising an indium arsenide quantum well layer for accommodating a two-dimensional electron gas is prepared on a substrate. An aluminum epitaxial layer is then grown in situ over the semiconductor component and the substrate. The surface of the aluminum layer is oxidized to form a natural oxide layer. Anodizing is performed to reduce the thickness of the aluminum layer by oxidizing a portion of its thickness.

[0045] A 3 nm thick titanium mask is formed on the natural oxide layer by evaporation and stripping. The resulting workpiece is then anodized to selectively oxidize the portions of the aluminum layer not covered by the titanium mask. The titanium mask is also oxidized. Oxidation of the titanium mask may occur prior to anodizing by exposing it to air.

[0046] An etchant solution was prepared by mixing 1 volume part of an ammonia stock solution (25 wt% ammonia; 13.4 M), 50 volume parts of a hydrogen peroxide stock solution (30 wt% hydrogen peroxide; 9.8 M), and 200 volume parts of high-purity deionized water. The stock solutions were obtained from Sigma-Aldrich and could be used without further purification.

[0047] Immerse the workpiece in the etchant solution at room temperature for about 15 minutes, then rinse with deionized water to stop the etching.

[0048] The workpiece was then examined using a scanning electron microscope. An image of the workpiece is shown in Figure 3. The edges of the masked areas are annotated with dashed lines. The dark areas in the image correspond to the regions where the titanium mask was removed by the etchant. The bright areas correspond to the regions where the alumina layer was exposed. The color gradient is the effect of the difference in alumina layer thickness between these two regions. It was observed that the titanium oxide mask was completely removed.

[0049] A gate stack is formed on the workpiece to produce a component as shown in the planar view in Figure 4. A gate dielectric is added to cover the surface of the workpiece. Two Ti / Au gate electrodes (430a, 430b) are formed on the gate dielectric. Gate electrodes 430a and 430b are spaced apart from each other to provide quantum dot contacts. Each gate electrode 430a and 430b overlaps with the boundary between the region 410 lacking the superconducting layer and the region 420 having the superconducting layer.

[0050] A comparator element was also fabricated. The comparator element has the layout shown in Figure 4. Its aluminum components were not thinned, but patterned by etching rather than by selective anodizing. No titanium mask was used in the fabrication of the comparator element.

[0051] Transmission spectral measurements were performed on both elements. The results are illustrated in Figure 5. Figure 5 is a graph of differential conductance as a function of voltage across the quantum dot contact. Solid lines illustrate the results for the exemplary element. Dashed lines illustrate the results for the comparative element.

[0052] Zero differential conductance is a sign of the absence of fermion states, in which case it is due to induced superconducting pairing. The spacing Δ between the peaks in the differential conductance plot provides a measure of the size of the induced superconducting gap in the semiconductor. It is desirable for the induced gap to be as large as possible, as this allows for greater stability of states of interest (e.g., MZM).

[0053] Superconducting gaps were induced in both components. Removing the titanium shield did not damage the underlying superconducting component, further confirming that the protective alumina layer remained above the superconducting component.

[0054] In fact, devices fabricated by removing the Ti mask after anodizing have a larger inductive superconducting gap than comparative devices. This is believed to be due to the reduction in the thickness of the aluminum layer, which can be achieved using anodizing. Example 2

[0055] Four workpieces were fabricated, each workpiece comprising a substrate, an alumina layer on the substrate, and a titanium dioxide mask on the alumina layer. The titanium dioxide mask was formed by depositing a 3 nm thick titanium layer and patterning the mask by peeling. The titanium layer was oxidized to titanium dioxide by exposure to air.

[0056] Optical micrographs of two parts of the workpiece were captured. A contrast adjustment filter was applied, and the images were converted to black and white. The images are illustrated in Figure 6. The mask is visible as a dark gray area on the lighter alumina.

[0057] An etchant solution was prepared by mixing 1 volume part of ammonia water stock solution (25% ammonia water; 13.4 M), 50 volume parts of hydrogen peroxide stock solution (30% hydrogen peroxide; 9.8 M), and 200 volume parts of high-purity deionized water.

[0058] The workpieces were immersed in the etchant at room temperature. Individual workpieces were removed after 5, 10, 15, and 20 minutes and rinsed to terminate the etching. Optical micrographs and SEM images of each workpiece were captured. The images were processed by applying a contrast adjustment filter and then converted to black and white.

[0059] Figure 7 shows optical micrographs of the workpieces removed after (a) 5 minutes and (b) 10 minutes.

[0060] Five minutes later (workpiece (a) in Figure 7), the titanium mask was still visible. The mask was also clearly visible in the SEM image (Figure 8).

[0061] After 10 minutes, (workpiece (b) in Figure 7) the etchant had eroded the titanium mask. Some remnants of the titanium mask were still visible. This was confirmed by SEM. The SEM image is illustrated in Figure 9, where the approximate location of the mask is highlighted with a white dashed line. The Ti mask appears incomplete in some places, but is still almost visible.

[0062] After 15 minutes, the titanium mask appeared to have been completely removed, as can be seen in Figure 10. SEM images confirmed this observation.

[0063] After 20 minutes, it was observed that the etchant had begun to corrode the exposed aluminum in areas not covered by titanium. The areas where aluminum had been etched are visible as dark gray areas in Figure 11. SEM images confirm this observation.

[0064] The black spots visible in optical micrographs are dirt fragments introduced when handling the workpiece with tweezers. Comparison Examples

[0065] Prepare a workpiece of the type described in Example 2. Capture an optical micrograph of the workpiece and process it as previously described. The micrograph is illustrated in Figure 12.

[0066] The workpiece was immersed in concentrated sulfuric acid at approximately room temperature. The workpiece was examined using an optical microscope after 10, 100, and 700 seconds.

[0067] No visible changes in the Ti mask were observed after 10 or 100 seconds.

[0068] Figure 13 illustrates a processed optical micrograph of the workpiece after 700 seconds of etching. Some titanium dioxide has been removed from the area indicated by the arrow, where the mask has become at least partially transparent. However, most of the mask remains intact. The etching using sulfuric acid is considered unsuccessful due to etching inhomogeneity.

[0069] Three pits can be seen in Figures 12 and 13. These pits are the result of using wire bonding to provide electrical connection to allow anodizing to be performed, and correspond to the location of the wire connectors.

[0070] It should be understood that the above embodiments are described by way of example only.

[0071] More generally, according to one example disclosed herein, a method is provided that involves selectively etching titanium dioxide in the presence of alumina using an etchant. The etchant is an aqueous solution of an alkali and hydroperoxide. It has been surprisingly found that wet etching using a combination of alkali and hydroperoxide can remove titanium dioxide without excessively damaging the alumina.

[0072] Etching can be performed by bringing titanium dioxide and aluminum oxide into contact with the etchant.

[0073] Without being bound by theory, it is believed that the etchant transforms titanium dioxide into one or more relatively soluble titanium (IV) complexes. These complexes are believed to contain at least one ligand derived from hydroperoxide. Some complexes may further contain one or more hydroxyl (OH-) ligands and / or one or more water (H₂O) ligands.

[0074] Hydroperoxides can be any hydroperoxide that provides a source of one or more ligands capable of coordinating with titanium, the source being selected from the following: oxoligands (O 2-), superoxide ligands (O 2-), peroxide ligands (O 2 2-), hydroperoxide ligands (ROOH), and peroxy ligands (ROO-).

[0075] The hydroperoxide may be selected from hydrogen peroxide and C1 to C4 alkyl hydroperoxides. Particularly preferred is hydrogen peroxide. An example of a C1 to C4 alkyl hydroperoxide is tertiary butyl hydroperoxide.

[0076] In embodiments where titanium dioxide is formed in situ, the peroxide may additionally act as an oxidizing agent.

[0077] Alkali adjusts the pH of the etchant. Alumina is easily corroded by acidic solutions, so avoiding such conditions may help improve the selectivity of the etchant. Alkali can also help dissolve titanium dioxide.

[0078] Reliably measuring the pH of solutions containing hydroperoxides can be difficult. For example, hydrogen peroxide can interfere with the operation of a pH meter (Kolczynski et al., J. Am. Chem. Soc 1957, 79, 3, 531-533). The apparent pH may differ from the true pH of the solution by two log units or even more: misleading pH readings can be obtained unless the presence of hydroperoxides is calibrated very carefully.

[0079] The inventors believe that optimal selectivity is achieved when the etchant is moderately alkaline. Increasing the pH may favor the formation of more soluble titanium (IV) complexes. However, it is predicted that excessively high pH levels may make controlling the etching process more difficult, as the erosion rate of alumina may increase.

[0080] The properties of alkalis are not particularly limited, provided that a suitable pH is obtained. The concentration of alkalis can be optimized through routine testing.

[0081] The base can be unary. The pKb of the base can be in the range of 4.5 to 5.5. Particularly preferred is ammonia.

[0082] Alkali and hydroperoxide can be present in the etchant in a molar ratio ranging from 1:25 to 1:45. For example, alkali and hydroperoxide can be present in the etchant in a molar ratio ranging from 1:30 to 1:40, or from 1:34 to 1:38. In particular, alkali and hydroperoxide can be present in the etchant in a molar ratio of 1:36 to 1:37, or about 1:36.5.

[0083] The alkali can be present in the etchant at a concentration ranging from 25 mM to 100 mM. Alternatively, the alkali can be present in the etchant at a concentration ranging from 30 mM to 70 mM, and more preferably from 50 mM to 60 mM.

[0084] Hydroperoxides can be present in the etchant at concentrations ranging from 1 M to 3 M. Optionally, hydroperoxides can be present in the etchant at concentrations ranging from 1.5 M to 2.5 M, and more preferably from 1.8 M to 2.2 M.

[0085] The etching temperature can be selected as needed. For example, etching can be performed at a temperature of 30°C or less. Some semiconductor materials used in semiconductor-superconductor hybrid devices can degrade under high temperatures. Performing etching at or below room temperature may help avoid this degradation. Furthermore, good selectivity has been observed in etching performed at room temperature.

[0086] The etching duration can be appropriately selected depending on the concentration of the reagent in the etchant, the temperature, and the thickness of the titanium or titanium dioxide layer to be removed.

[0087] The method may further include oxidizing titanium to form titanium dioxide.

[0088] Titanium can be pre-oxidized. A titanium film with a thickness of up to about 7 nm may be oxidized by exposure to air. Alternatively or additionally, titanium can be oxidized by anodizing.

[0089] Titanium can be oxidized by exposure to water, oxygen, and / or hydroperoxides. Titanium can also be oxidized by contacting it with an etchant. Because titanium readily transforms in situ into titanium dioxide, the methods presented herein can be used to etch titanium metal.

[0090] Some of the titanium can be pre-oxidized, and some can be oxidized in situ by exposure to an etchant. In embodiments where titanium and / or titanium dioxide are used as a mask for controlling the anodizing of aluminum, this allows for a wider range of anodizing conditions because anodizing does not require converting all the titanium into titanium dioxide.

[0091] Etching can be performed in the presence of semiconductor components. In such embodiments, aluminum oxide can be in the form of a layer covering the surface of the semiconductor components. The aluminum oxide layer can protect the semiconductor components from the etchant.

[0092] Semiconductor components may contain III-V group semiconductor materials, such as the material of Formula 1: InAs xSb 1-x (Equation 1) Where x is in the range of 0 to 1. In other words, the semiconductor component may contain indium antimonide (x=0), indium arsenide (x=1), or a ternary mixture containing 50% indium and a variable proportion of arsenic and antimony (x=1) in moles. <x<1)。

[0093] Another class of materials that can be used as semiconductor components are group II-VI semiconductor materials. Examples of group II-VI semiconductor materials include cadmium telluride, mercury telluride, lead telluride, and tin telluride.

[0094] Semiconductor components can be in the form of nanowires. Semiconductor components can be in the form of nanowire networks.

[0095] Semiconductor components can take the form of semiconductor heterostructures. Semiconductor heterostructures include quantum wells disposed between a lower potential barrier and an upper potential barrier. This structure is called a heterostructure because the quantum well contains a different material than the lower and upper potential barriers. The materials of the lower and upper potential barriers can be chosen independently.

[0096] There are no particular restrictions on the construction of the lower and upper potential barriers, provided that these layers allow electrons to be trapped in the quantum well. The lower barrier may comprise one or more layers of one or more different materials. The upper barrier may comprise one or more layers of one or more different materials. Constructing a barrier from multiple layers can provide defect filtering, that is, it can reduce the influence of dislocations in the crystal structure of the material used.

[0097] Quantum wells may include a layer of semiconductor material having a relatively small band gap compared to the materials of the lower and upper barriers. Illustrative materials that can be used to form quantum wells are described, for example, in Odoh and Njapba, ⸢A Review of Semiconductor Quantum Well Devices⸥, Advances in Physics Theories and Applications, Vol. 46, 2015, pp. 26–32; and S. Kasap, P. Capper (eds.), ⸢Springer Handbook of Electronic and Photonic Materials⸥, DOI 10.1007 / 978-3-319-48933-9_40.

[0098] Etching can be performed in the presence of the superconducting component. Alumina can cover the superconducting component and protect it from the etchant. There are no particular limitations on the properties of the superconductor, and it can be selected appropriately. The superconductor is typically an S-wave superconductor. Any S-wave superconductor known in the art can be used. Examples include aluminum, indium, tin, and lead. In particular, the superconducting component may contain aluminum.

[0099] Titanium dioxide can be arranged on aluminum oxide. The aluminum component can be located below the titanium dioxide, with the aluminum oxide situated between the aluminum component and the titanium dioxide.

[0100] The method may further include, prior to etching: A titanium mask is formed on an aluminum layer, wherein the titanium mask covers a first portion of the aluminum layer and exposes a second portion of the aluminum layer; and Perform anodizing, The titanium shield protects the first portion of the aluminum layer from complete anodizing; and The second part of the aluminum layer is completely oxidized by anodizing to form aluminum oxide.

[0101] An aluminum layer may extend over a semiconductor component. The semiconductor component may be a semiconductor heterostructure configured to carry a two-dimensional electron gas or a two-dimensional hole gas.

[0102] The titanium mask can be formed by peeling. In such embodiments, the method further includes: A sacrificial mask is formed over the aluminum layer, the sacrificial mask having at least one opening; Deposit a titanium layer on the sacrificial mask and in at least one opening; Remove the sacrificial mask to selectively remove titanium from the sacrificial mask, thereby forming a titanium mask; and Perform anodizing.

[0103] The mask can be formed by electron beam lithography. The titanium layer can be grown by evaporation.

[0104] The method may further include a portion of the aluminum oxide layer thickness before forming the titanium mask. This partial oxide thickness may include exposing the aluminum layer to oxygen and allowing the formation of a native oxide layer. The partial oxide thickness may further include performing anodizing. For example, growing a relatively thick aluminum layer is useful in fabricating semiconductor-superconductor hybrid devices because growing high-quality thin films is very challenging. Subsequently reducing the film thickness through partial oxidation can improve the superconducting properties of the aluminum layer. For example, a thinner layer has a higher critical magnetic field than a thicker layer.

[0105] Titanium dioxide can be in the form of a layer with a thickness of less than or equal to 5 nm.

[0106] Alumina can be in the form of a layer. Alumina can cover the surface of a substrate (e.g., a wafer of semiconductor material). One or more components can be present between the alumina and the substrate surface. For example, one or more components can include a semiconductor-superconductor hybrid structure. The thickness of the alumina layer can be selected such that, after etching, the thickness of the alumina layer is at least 3 nm. This can provide good protection for the components beneath the aluminum layer. For example, alumina can be in the form of a layer having a thickness of at least 8 nm.

[0107] The method may further include, after etching, cleaning the alumina with an acid solution, optionally a hydrogen fluoride solution. Contact between the alumina and the etchant can lead to surface contamination. Cleaning the surface of the alumina layer with an acid solution can remove such contaminants.

[0108] The method may further include preparing the etchant by mixing an aqueous solution of an alkali, an aqueous solution of hydroperoxide, and water. In such embodiments, the aqueous solution of alkali may have a pH in the range of 10 to 13 and is included in the etchant at a volume ratio in the range of 1:125 to 1:500, optionally 1:225 to 1:275, and more optionally 1:245 to 1:255, based on the total volume of the etchant. The aqueous solution of hydroperoxide may have a concentration in the range of 8 M to 12 M and may be included in the etchant at a volume ratio in the range of 1:3 to 1:6, based on the total volume of the etchant.

[0109] The etchant can be obtained by mixing an aqueous ammonia solution, a hydrogen peroxide solution, and water. The aqueous ammonia solution can be an aqueous solution containing 25% by weight of ammonia based on the aqueous ammonia solution. The hydrogen peroxide solution can be an aqueous solution containing 30% by weight of hydrogen peroxide based on the hydrogen peroxide solution. The aqueous ammonia solution, hydrogen peroxide solution, and water can be mixed in a volume ratio in the range of (0.75 to 1.25):(37.5 to 62.5):200, optionally 1:50:200.

[0110] In another embodiment, an etching solution for etching titanium dioxide is provided, which is an aqueous solution of ammonia and hydrogen peroxide, wherein the molar ratio of the ammonia to the hydrogen peroxide is in the range of 1:34 to 1:38.

[0111] Ammonia can be present at concentrations ranging from 50 mM to 60 mM, and hydrogen peroxide concentrations can range from 1.8 M to 2.2 M.

[0112] The etchant may consist essentially of an aqueous solution of ammonia and hydrogen peroxide, or in other words, essentially a mixture of ammonia, hydrogen peroxide, and water. The etchant may be free of solutes other than ammonia, hydrogen peroxide, dissolved carbonates, and trace impurities. The etchant may contain no more than 50 ppm, optionally 25 ppm, of solutes other than ammonia and hydrogen peroxide.

[0113] Once the disclosure herein is given, other variations or use cases of the disclosed technology may become apparent to those skilled in the art. The scope of this disclosure is not limited to the described embodiments, but only to the claims appended.

[0114] 100: Components 110:Substrate 120: Aluminum components 130: Alumina layer 140: Titanium oxide mask 201: Square 202: Square 203: Square 410: Region lacking a superconducting layer 420: Region with superconducting layer 430a: Gate electrode 430b: Gate electrode (a): Workpiece (b): Workpiece E: Exposed area M: Masking area

[0115] Domestic storage information (please note in order of storage institution, date, and number) none Overseas storage information (please note in the order of storage country, institution, date, and number) none

Claims

1. A method for selectively etching titanium dioxide, comprising the steps of: oxidizing a portion of the thickness of an aluminum layer; forming a titanium mask over the aluminum layer, wherein the titanium mask covers a first portion of the aluminum layer and exposes a second portion of the aluminum layer; performing anodizing to convert the titanium mask into titanium dioxide, wherein the titanium mask protects the first portion of the aluminum layer from complete oxidation by the anodizing, and wherein the second portion of the aluminum layer is completely oxidized by the anodizing to form aluminum oxide; and selectively etching the titanium dioxide in the presence of the aluminum oxide using an etchant, wherein the etchant is an aqueous solution of an alkali and hydroperoxide.

2. The method as claimed in claim 1, wherein the hydroperoxide is selected from hydrogen peroxide and a C1 to C4 alkyl hydroperoxide.

3. The method as described in claim 1, wherein the hydroperoxide is hydrogen peroxide.

4. The method as described in claim 1, wherein the base is monobasic and has a pKb in the range of 4.5 to 5.

5.

5. The method as described in claim 1, wherein the alkali is ammonia.

6. The method as described in claim 1, wherein the base and the hydroperoxide are present in the etchant in a molar ratio in the range of 1:25 to 1:

45.

7. The method as described in claim 1, wherein the base and the hydroperoxide are present in the etchant in a molar ratio in the range of 1:30 to 1:

40.

8. The method as described in claim 1, wherein the base and the hydroperoxide are present in the etchant in a molar ratio in the range of 1:34 to 1:

38.

9. The method as described in claim 1, wherein the alkali is present in the etchant at a concentration in the range of 25 mM to 100 mM.

10. The method as described in claim 1, wherein the alkali is present in the etchant at a concentration in the range of 30 mM to 70 mM.

11. The method as described in claim 1, wherein the alkali is present in the etchant at a concentration in the range of 50 mM to 60 mM.

12. The method as described in claim 1, wherein the hydroperoxide is present in the etchant at a concentration in the range of 1 M to 3 M.

13. The method as claimed in claim 1, wherein the hydroperoxide is present in the etchant at a concentration in the range of 1.5 M to 2.5 M.

14. The method as claimed in claim 1, wherein the hydroperoxide is present in the etchant at a concentration in the range of 1.8 M to 2.2 M.

15. The method as described in claim 1, wherein the etching is performed at a temperature less than or equal to 30°C.

16. The method as described in claim 1, wherein the etching is performed at a temperature of 20°C to 30°C.

17. The method as described in claim 1, wherein the etching is performed at a temperature of 23°C to 27°C.

18. The method as claimed in claim 1, wherein the etching is performed in the presence of a semiconductor component and / or a superconducting component, and the alumina protects the semiconductor component and / or the superconducting component from the etchant.

19. The method as described in claim 1, wherein: i) The thickness of the titanium dioxide is less than or equal to 5 nm; and / or ii) The thickness of the aluminum oxide is at least 8 nm.

20. The method as described in claim 1 further includes the step of: cleaning the alumina with an acid solution after the etching.

21. The method as described in claim 1, wherein the acid solution is a hydrogen fluoride solution.

22. The method as described in claim 1 further includes the step of preparing the etchant by mixing an aqueous solution of the base, an aqueous solution of the hydroperoxide, and water.

23. The method as claimed in claim 22, wherein the aqueous solution of the alkali has a pH in the range of 10 to 13 and is contained in the etchant at a volume ratio in the range of 1:125 to 1:500 based on the total volume of the etchant.

24. The method as claimed in claim 22, wherein the aqueous solution of the alkali has a pH in the range of 10 to 13 and is contained in the etchant at a volume ratio in the range of 1:225 to 1:275 based on the total volume of the etchant.

25. The method as claimed in claim 22, wherein the aqueous solution of the alkali has a pH in the range of 10 to 13 and is contained in the etchant at a volume ratio in the range of 1:245 to 1:255 based on the total volume of the etchant.

26. The method as described in any one of claims 22 to 25, wherein the aqueous solution of the hydroperoxide has a concentration in the range of 8 M to 12 M and is contained in the etchant at a volume ratio in the range of 1:3 to 1:6 based on the total volume of the etchant.

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