Air bridge preparation method

Abstract

The application discloses an air bridge preparation method and belongs to the technical field of chip preparation. The air bridge preparation method comprises the following steps: obtaining a substrate with a support layer; forming a first metal layer covering the support layer; forming a second metal layer covering the first metal layer, the oxidation resistance of the second metal layer is higher than that of the first metal layer, and the critical current of the first metal layer is higher than that of the second metal layer; and removing the support layer to obtain a target air bridge. In the method, the first metal layer is used to improve the critical current of the air bridge, the second metal layer is used to improve the oxidation resistance of the air bridge, and the air bridge has both high critical current and high oxidation resistance.

Description

Technical Field

[0001] This invention belongs to the field of chip fabrication technology, specifically a method for fabricating an air bridge. Background Technology

[0002] An air bridge is a suspended structure used in quantum chips to connect two sides of a potential imbalance point. By suspending itself above the chip surface, it avoids direct contact with the circuitry or resonant cavity below, thereby reducing electromagnetic losses.

[0003] Currently, air bridges are commonly fabricated using a photoresist sacrificial process, which involves: forming an arched support structure on a substrate using photoresist reflow; depositing metal on the entire substrate surface; then using photolithography to retain the bridge surface metal and etch away excess metal; finally, stripping away the support structure. However, existing methods for fabricating air bridges often face the problem of easy oxidation when achieving high performance with critical current.

[0004] Therefore, how to prepare an air bridge that has both high critical current and high oxidation resistance is an urgent problem to be solved. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing an air bridge to overcome the shortcomings of the prior art. This method can effectively prepare an air bridge that has both high critical current and high oxidation resistance.

[0006] The solution presented in this application is implemented through the following steps.

[0007] In a first aspect, the present application provides an example of a method for preparing an air bridge, the method comprising:

[0008] Obtain a substrate with a support layer;

[0009] A first metal layer is formed to cover the support layer;

[0010] A second metal layer is formed covering the first metal layer, the second metal layer has higher oxidation resistance than the first metal layer, and the critical current of the first metal layer is higher than the critical current of the second metal layer;

[0011] Remove the support layer to obtain the target air bridge.

[0012] According to some examples of this application, obtaining a substrate with a support layer includes:

[0013] Obtain the initial substrate;

[0014] A first photoresist layer is formed by coating a photoresist layer onto the upper surface of the initial substrate.

[0015] The first photoresist layer is sequentially exposed, developed, and reflowed to obtain the support layer.

[0016] According to some examples of this application, prior to forming the first metal layer covering the support layer, the air bridge fabrication method further includes:

[0017] On the upper surface of the substrate where the support layer is located, a first photoresist mask with a target via is prepared. The distance from each wide side of the target via to the wide side of the adjacent support layer is equal. Along the direction perpendicular to the upper surface of the substrate, the projected length of the target via is greater than the length of the support layer, and the projected width of the target via is less than the width of the support layer.

[0018] According to some examples of this application, forming a first metal layer covering the support layer includes:

[0019] A first metal is sputtered into the target through-hole until a first metal layer covering the support layer is formed.

[0020] According to some examples of this application, the formation of the second metal layer covering the first metal layer includes:

[0021] A second metal is sputtered into the target via until a second metal layer is formed that covers the first metal layer.

[0022] According to some examples of this application, the air bridge fabrication method further includes, prior to removing the support layer:

[0023] The first photoresist mask is removed using an organic solvent.

[0024] According to some examples of this application, forming a first metal layer covering the support layer includes:

[0025] A first metal is sputtered onto the upper surface of the substrate where the support layer is located until a first metal layer covering the support layer and the upper surface of the substrate is formed.

[0026] According to some examples of this application, the formation of the second metal layer covering the first metal layer includes:

[0027] Sputter a second metal until a second metal layer is formed that covers the first metal layer.

[0028] According to some examples of this application, the air bridge fabrication method further includes, prior to removing the support layer:

[0029] A second photoresist layer is formed by coating a photoresist layer onto the surface of the second metal layer.

[0030] Based on the position, length, and width of the support layer, the second photoresist layer is exposed and developed to obtain a second photoresist mask; the distance from each wide side of the second photoresist mask to the adjacent wide side of the support layer is equal, and along the direction perpendicular to the surface of the second metal layer, the projected length of the second photoresist mask is greater than the length of the support layer, and the projected width of the second photoresist mask is less than the width of the support layer.

[0031] According to some examples of this application, the air bridge fabrication method further includes, prior to removing the support layer:

[0032] Based on the second photoresist mask, wet etching is used to remove the first metal layer and the second metal layer that are not covered by the second photoresist mask.

[0033] In the air bridge fabrication method described in the foregoing example of this application, an air bridge is fabricated having a first metal layer and a second metal layer stacked from bottom to top. The second metal layer has higher oxidation resistance than the first metal layer, and the critical current of the first metal layer is higher than that of the second metal layer. In the method provided in this application, the first metal layer is used to increase the critical current of the air bridge, and the second metal layer is used to increase the oxidation resistance of the air bridge, thereby achieving the goal of having both high critical current and high oxidation resistance in the air bridge. Attached Figure Description

[0034] To illustrate this more clearly, the accompanying drawings used in the description will be briefly introduced below.

[0035] Figure 1 Here is a flowchart of an air bridge fabrication method in one example of this application;

[0036] Figure 2 This is a flowchart illustrating the process of obtaining a substrate with a support layer, as shown in one example of this application.

[0037] Figure 3 This is a top-view perspective view of the support layer and the first photoresist mask in one example of this application;

[0038] Figure 4 This is a flowchart of an air bridge fabrication method in another example of this application. Detailed Implementation

[0039] To enable those skilled in the art to better understand the technical solutions in this application, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of this application. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0040] In the description of this invention, it should be understood that the terms "center", "upper", "lower", "left", "right", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0041] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0042] In existing technologies, photoresist sacrificial processes are commonly used to fabricate air bridges during chip manufacturing. The specific steps of air bridge fabrication include: coating a substrate with photoresist to form a support layer structure through photoresist exposure and development; depositing metal onto the entire surface of the substrate with the support layer; and then using photolithography to remove excess metal while retaining only the bridge surface metal, thus obtaining the air bridge. However, existing methods for fabricating air bridges often face the problem of easy oxidation when achieving high performance with critical current.

[0043] Based on this, such as Figure 1 As shown in the illustration, one embodiment of the present invention provides a method for preparing an air bridge, which includes:

[0044] S110, Obtain a substrate with a support layer;

[0045] S120, forming the first metal layer covering the support layer;

[0046] S130, a second metal layer is formed covering the first metal layer. The oxidation resistance of the second metal layer is higher than that of the first metal layer, and the critical current of the first metal layer is higher than that of the second metal layer.

[0047] S140, remove the support layer to obtain the target air bridge.

[0048] In this design, the thickness of the first metal layer is greater than the thickness of the support layer, and the length of the projected area of ​​the first metal layer is greater than the length of the support layer along the direction perpendicular to the upper surface of the substrate. The thickness relationship between the first and second metal layers depends on their oxidation resistance, critical current, and the oxidation resistance and critical current of the target air bridge. It should be noted that the first metal layer partially covers the support layer; for example, along the length direction of the support layer, the length of the first metal layer is greater than that of the support layer, and along the width direction of the support layer, the width of the first metal layer is less than that of the support layer; conversely, along the length direction of the support layer, the length of the first metal layer is less than that of the support layer, and along the width direction of the support layer, the width of the first metal layer is greater than that of the support layer.

[0049] Specifically, before forming the support layer, the thicknesses of the first and second metal layers are determined based on the oxidation resistance and critical current of the target air bridge, the oxidation resistance and critical current of the first metal, and the oxidation resistance and critical current of the second metal. Then, the thickness of the support layer is determined based on the thickness of the first metal layer.

[0050] Furthermore, a support layer of a predetermined thickness is formed at a predetermined location on the substrate surface. Based on the position information of the support layer on the substrate surface and the corresponding thickness of the first metal layer, a first metal is sputtered to form a first metal layer covering the support layer and connected to the substrate surface. Based on the position information of the first metal layer on the substrate surface and the corresponding thickness of the second metal layer, a second metal is sputtered to form a second metal layer covering the first metal layer and connected to the substrate surface. After sequentially preparing the first metal layer with high critical current and the second metal layer with high oxidation resistance, a suitable removal method is selected according to the material type of the support layer to remove the support layer, obtaining the target air bridge of the first metal layer and the second metal layer stacked from bottom to top. The preparation of the first metal layer and the second metal layer must be carried out in situ in a single operation to avoid the formation of an oxide layer on the surface of the first metal layer.

[0051] It should be noted that this application does not impose specific limitations on the types of the first metal layer, the second metal layer, and the support layer; all can be selected according to actual needs, as long as the removal of the support layer does not damage the first metal layer or the second metal layer. For example, the first metal layer can be a niobium layer, the second metal layer can be a tantalum layer, and the support layer can be photoresist.

[0052] In the air bridge preparation method provided in this application, a first metal with a high critical current is used as the lower metal and a second metal with high oxidation resistance is used as the upper metal. The second metal can effectively protect the first metal from oxidation. While improving the oxidation resistance of the air bridge, it also increases the critical current of the air bridge, so that the air bridge has both high critical current and high oxidation resistance.

[0053] like Figure 2 As shown, in one embodiment of this application, step S110, obtaining a substrate with a support layer, includes:

[0054] Step S210: Obtain the initial substrate;

[0055] Step S220: Photoresist is applied to the upper surface of the initial substrate to form a first photoresist layer;

[0056] In step S230, the first photoresist layer is exposed, developed, and reflowed sequentially to obtain the support layer.

[0057] The initial substrate can be a silicon wafer. Alternatively, the initial substrate can be a silicon wafer with coplanar waveguide signal lines, which include a center band and ground bands located on both sides of the center band.

[0058] Specifically, before applying the photoresist, the initial substrate is cleaned to remove surface contaminants and impurities, ensuring that the photoresist adheres evenly and firmly to the initial substrate. A suitable photoresist is selected based on process requirements and the type of light source used (e.g., ultraviolet, deep ultraviolet, extreme ultraviolet, etc.). The photoresist is evenly applied to the substrate surface using methods such as spin coating, spraying, or blade coating. The thickness of the photoresist layer is controlled by adjusting the rotation speed and the viscosity of the photoresist. After coating, the photoresist layer is dried to obtain the first photoresist layer. A lithography machine is used to project the pattern on a preset mask onto the first photoresist layer for exposure. During exposure, the light reacts chemically with the photoresist, changing its solubility. After exposure, a developer is used to remove the photoresist from the exposed (or unexposed, depending on the type of photoresist) areas, leaving the desired pattern structure. The developed pattern structure is reflowed to improve the edge shape of the pattern, resulting in a support layer.

[0059] When the initial substrate is a silicon wafer with coplanar waveguide signal lines, photoresist is applied to the upper surface of the initial substrate to form a first photoresist layer; based on the position information of the coplanar waveguide signal lines on the silicon wafer, the first photoresist layer is exposed and developed to form a support layer connecting the grounding bands on both sides; the support layer obtained after development is reflowed to form a support layer with an arched surface.

[0060] When the support layer is made of photoresist, step S140 above, removing the support layer, includes: immersing the silicon wafer with the support layer in a selected organic solvent, the immersion time depending on the type and thickness of the photoresist and the type and temperature of the solvent; after removing the photoresist support layer, the silicon wafer surface needs to be thoroughly rinsed with deionized water to remove residual solvent and photoresist fragments; after rinsing, the silicon wafer surface can be dried with nitrogen or other drying methods to obtain the target air bridge of the stacked first metal layer and second metal layer.

[0061] In one embodiment of this application, before forming the first metal layer covering the support layer in step S120 above, the air bridge fabrication method further includes:

[0062] On the upper surface of the substrate where the support layer is located, a first photoresist mask with a target via is prepared. The distance from each wide side of the target via to the wide side of the adjacent support layer is equal. Along the direction perpendicular to the upper surface of the substrate, the projected length of the target via is greater than the length of the support layer, and the projected width of the target via is less than the width of the support layer.

[0063] The first photoresist mask has only one target via, used to expose part of the substrate and part of the support layer. For example... Figure 3 As shown, along the direction perpendicular to the upper surface of the substrate, the long side of the target via projection is parallel to the long side of the support layer, the wide side of the target via projection is parallel to the wide side of the support layer, the length of the target via projection is greater than the length of the support layer, the width of the target via projection is less than the width of the support layer, and the axis of symmetry of the target via coincides with the axis of symmetry of the support layer.

[0064] Specifically, based on the size and position information of the support layer on the substrate, the size and position information of the target via is determined; based on the size and position information of the target via, the corresponding photomask is determined; a uniform layer of photoresist is coated on the upper surface of the substrate where the support layer is located; using a microscope and alignment device, the designed photomask is aligned onto the photoresist-coated substrate to ensure correct pattern alignment; the photolithography machine is started for exposure, so that the target via pattern is projected onto the photoresist through the photomask. During the exposure process, the photoresist in the exposed area undergoes a chemical reaction, its properties change, and it is easily dissolved and removed; the photoresist in the exposed area is removed using a developer, leaving the target via pattern, forming the first photoresist mask with the target via.

[0065] In one embodiment of this application, step S120, forming a first metal layer covering the support layer, includes:

[0066] Within the target via, a first metal is sputtered until a first metal layer covering the support layer is formed.

[0067] Specifically, based on the location information of the target via on the substrate, the sputtering direction is controlled, and a predetermined number of first metals are sputtered to form a first metal layer covering the support layer inside the target via and connecting to the substrate surface. The sputtering angle is different for each sputtering, and all sputtering directions are located in the same plane perpendicular to the upper surface of the substrate.

[0068] In one embodiment of this application, step S130, forming a second metal layer covering the first metal layer, includes:

[0069] Inside the target via, a second metal is sputtered until a second metal layer is formed that covers the first metal layer.

[0070] Specifically, based on the location information of the target via on the substrate, the sputtering direction is controlled, and a predetermined number of second metals are sputtered to form a second metal layer covering the first metal layer within the target via. The sputtering angle is different for each sputtering operation, and all sputtering directions are located in the same plane perpendicular to the upper surface of the substrate.

[0071] In one embodiment of this application, before removing the support layer in step S140, the air bridge fabrication method further includes:

[0072] The first photoresist mask is removed using an organic solvent.

[0073] When the support layer is also made of photoresist, the first photoresist mask can be removed at the same time as the support layer.

[0074] Specifically, the silicon wafer with the first photoresist mask is immersed in a selected organic solvent. The immersion time depends on the type and thickness of the photoresist, as well as the type and temperature of the solvent. After removing the photoresist support layer, the silicon wafer surface needs to be thoroughly rinsed with deionized water to remove residual solvent and photoresist fragments. After rinsing, the silicon wafer surface can be dried with nitrogen or other drying methods.

[0075] In one embodiment of this application, step S120, forming a first metal layer covering the support layer, includes:

[0076] On the upper surface of the substrate where the support layer is located, a first metal is sputtered until a first metal layer covering the support layer and the upper surface of the substrate is formed.

[0077] Specifically, a support layer is located on the upper surface of the substrate. A first metal is sputtered multiple times on the upper surface of the substrate to form a first metal layer with a thickness greater than that of the support layer. The first metal layer covers the upper surface of the support layer and the upper surface of the substrate excluding the support layer. The first metal layer can be formed by multiple sputtering operations, with each sputtering operation having a different angle and direction. All sputtering directions are located in the same plane perpendicular to the upper surface of the substrate.

[0078] In one embodiment of this application, step S130, forming a second metal layer covering the first metal layer, includes:

[0079] Sputter a second metal until a second metal layer is formed that covers the first metal layer.

[0080] Specifically, a second metal is sputtered multiple times onto the upper surface of the first metal layer to form a second metal layer that covers the upper surface of the first metal layer. The sputtering angle is different for each sputtering operation, and all sputtering directions are located in the same plane perpendicular to the upper surface of the substrate.

[0081] like Figure 4As shown, in one embodiment of this application, before removing the support layer in step S140, the air bridge fabrication method further includes:

[0082] S410, Photoresist is applied to the surface of the second metal layer to form a second photoresist layer;

[0083] S420, based on the position, length and width of the support layer, the second photoresist layer is exposed and developed to obtain a second photoresist mask; the distance from each wide edge of the second photoresist mask to the wide edge of the adjacent support layer is equal, and along the direction perpendicular to the surface of the second metal layer, the projected length of the second photoresist mask is greater than the length of the support layer, and the projected width of the second photoresist mask is less than the width of the support layer.

[0084] The positional relationship between the second photoresist mask and the first photoresist mask includes: the size and position information of the target via on the first photoresist mask, i.e., the size and position information of the second photoresist mask. For details, please refer to [link to relevant documentation]. Figure 3 .

[0085] Specifically, before applying the second photoresist layer, the surface of the second metal layer needs to be cleaned to remove oxides, contaminants, and impurities, ensuring that the photoresist adheres uniformly and firmly to the metal layer. A uniform layer of photoresist is applied to the surface of the second metal layer using spin coating or spraying. Based on the position, length, and width of the support layer, as well as the required shape and size of the second photoresist mask, a corresponding photomask is designed and prepared. The photomask is precisely aligned onto the photoresist-coated second metal layer using a microscope and alignment tool. The lithography machine is started for exposure. During exposure, light passes through the transparent area of ​​the photomask and illuminates the photoresist, causing a chemical reaction in the exposed area. After exposure, a suitable developer is used to remove the photoresist from the exposed (or unexposed, depending on the type of photoresist) areas, leaving the required second photoresist mask.

[0086] In one embodiment of this application, before removing the support layer in step S140, the air bridge fabrication method further includes:

[0087] Based on the second photoresist mask, wet etching is used to remove the first and second metal layers that are not covered by the second photoresist mask.

[0088] Specifically, a suitable etching solution is selected based on the materials of the first and second metal layers; the silicon wafer coated with the second photoresist mask is placed in an etching tank containing the etching solution; the etching solution reacts chemically with the first and second metal layers not covered by the second photoresist mask, gradually dissolving and removing the metal layers; the thickness of the metal layer to be removed is determined based on the thickness of the first and second metal layers, and the thickness of the metal layer to be removed is equal to the sum of the thicknesses of the first and second metal layers; based on the determined removal thickness, the etching depth is precisely controlled by controlling the etching time and etching conditions.

[0089] Furthermore, after wet etching is completed, the second photoresist mask is removed using a selected organic solvent, followed by the removal of the support layer. The silicon wafer with the support layer removed is then thoroughly cleaned and dried to remove residual etching solution and other contaminants, thereby obtaining the target air bridge of the first metal layer and the second metal layer stack.

[0090] It should be understood that although the steps in the flowcharts of the above embodiments are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowcharts of the above embodiments may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the steps or stages of other steps.

[0091] In the description of this specification, references to terms such as "some embodiments" or "example" indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0092] The above are merely preferred embodiments of the present invention and do not constitute any limitation on the present invention. Any equivalent substitutions or modifications made by those skilled in the art to the technical solutions and content disclosed in the present invention without departing from the scope of the present invention shall be deemed to have remained within the protection scope of the present invention.

Claims

1. A method for preparing an air bridge, characterized in that, The method includes: Obtain a substrate with a support layer; A first metal layer is formed to cover the support layer; A second metal layer is formed covering the first metal layer, the second metal layer has higher oxidation resistance than the first metal layer, and the critical current of the first metal layer is higher than the critical current of the second metal layer; Remove the support layer to obtain the target air bridge.

2. The method for preparing an air bridge according to claim 1, characterized in that, The process of obtaining a substrate with a support layer includes: Obtain the initial substrate; A first photoresist layer is formed by coating a photoresist layer onto the upper surface of the initial substrate. The first photoresist layer is sequentially exposed, developed, and reflowed to obtain the support layer.

3. The method for preparing an air bridge according to claim 1, characterized in that, Prior to forming the first metal layer covering the support layer, the air bridge fabrication method further includes: On the upper surface of the substrate where the support layer is located, a first photoresist mask with a target via is prepared. The distance from each wide side of the target via to the wide side of the adjacent support layer is equal. Along the direction perpendicular to the upper surface of the substrate, the projected length of the target via is greater than the length of the support layer, and the projected width of the target via is less than the width of the support layer.

4. The method for preparing an air bridge according to claim 3, characterized in that, The first metal layer forming the support layer includes: A first metal is sputtered into the target through-hole until a first metal layer covering the support layer is formed.

5. The method for preparing an air bridge according to claim 4, characterized in that, The second metal layer forming the first metal layer includes: A second metal is sputtered into the target via until a second metal layer is formed that covers the first metal layer.

6. The method for preparing an air bridge according to claim 5, characterized in that, Before removing the support layer, the air bridge fabrication method further includes: The first photoresist mask is removed using an organic solvent.

7. The method for preparing an air bridge according to claim 1, characterized in that, The first metal layer forming the support layer includes: A first metal is sputtered onto the upper surface of the substrate where the support layer is located until a first metal layer covering the support layer and the upper surface of the substrate is formed.

8. The method for preparing an air bridge according to claim 7, characterized in that, The second metal layer forming the first metal layer includes: Sputter a second metal until a second metal layer is formed that covers the first metal layer.

9. The method for preparing an air bridge according to claim 8, characterized in that, Before removing the support layer, the air bridge fabrication method further includes: A second photoresist layer is formed by coating a photoresist layer onto the surface of the second metal layer. Based on the position, length, and width of the support layer, the second photoresist layer is exposed and developed to obtain a second photoresist mask; the distance from each wide side of the second photoresist mask to the adjacent wide side of the support layer is equal, and along the direction perpendicular to the surface of the second metal layer, the projected length of the second photoresist mask is greater than the length of the support layer, and the projected width of the second photoresist mask is less than the width of the support layer.

10. The method for preparing an air bridge according to claim 9, characterized in that, Before removing the support layer, the air bridge fabrication method further includes: Based on the second photoresist mask, wet etching is used to remove the first metal layer and the second metal layer that are not covered by the second photoresist mask.

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