Photoetching mask method of air bridge microstructure chip

Abstract

The invention provides a photoetching mask method of an air bridge microstructure chip, which comprises the following steps of: controlling spin coating and soft baking conditions of first photoresist, and realizing a trapezoidal section after photoetching without a gray exposure technology; due to the fact that the first photoresist and the second photoresist are compatible in technology, accurate transfer of patterns and formation of a supporting layer or a stripping layer can be achieved at the same time through one-time exposure and development, technological operation is simple, and technological conditions are loose; when the mask provided by the invention is used for metal deposition, the adhesion of the air bridge pier and the substrate can be ensured, and meanwhile, the situation that the arched supporting layer is covered by a metal film in all directions and cannot be peeled off is avoided.

Description

Technical Field

[0001] The present invention relates to the fields of integrated circuit manufacturing technology and quantum chip, and particularly relates to a photolithography mask method for an air-bridge microstructure chip. Background Art

[0002] The air-bridge micro-nano structure is a key structure for high-frequency integrated circuit chips and quantum resonance readout circuit chips. It is a three-dimensional bridge-shaped structure formed by photolithography, and its manufacturing process is much more complex than that of planar structures. In high-frequency and ultra-high-frequency devices, the air-bridge is used to realize the bridging of planar circuits. Its core feature is that the medium between the bridge and the circuit is air or vacuum, thus effectively reducing parasitic effects. In superconducting quantum chips, the application of air-bridges is particularly important. For example, the readout cavity and control signal lines in superconducting quantum bit circuits usually adopt coplanar waveguide transmission lines. However, due to the splitting of the transmission line to the ground layer, a potential difference will be generated between the ground layers on both sides of the transmission line when a signal is input, exciting the parasitic slot-line mode, which in turn affects the coherence of the quantum bits. The air-bridge connects the split ground layers through three-dimensional metal wires, which can eliminate the potential difference, shield signal interference, weaken the crosstalk between different coplanar waveguides, and thus improve the decoherence time of the quantum bits.

[0003] In addition, the air-bridge is a three-dimensional bridge-shaped structure. Compared with planar structures, the bridge-shaped structure can effectively increase the area of micro-nano scale devices, and the heat capacity of air is small, which is convenient for realizing more effective thermal management, higher sensitivity and better performance of micro-nano devices.

[0004] However, there are still many challenges in realizing a stable and large-scale air-bridge preparation process. First, the process complexity is relatively high, usually requiring multiple photolithography, etching and release steps, which not only increases the manufacturing cost but also reduces the production efficiency. Second, during the manufacturing process, it is usually necessary to etch the excess coating on the substrate, and this step is prone to introducing contaminants, which has a negative impact on the chip performance.

[0005] Therefore, developing a stable air-bridge micro-nano structure manufacturing process that can be prepared on a large scale is of great significance for promoting the progress of high-frequency integrated circuits and quantum chips. By adopting new process manufacturing methods and photolithography mask technologies, the preparation accuracy, structural stability and yield of air-bridges can be improved, while reducing the pollution risk and manufacturing cost. Summary of the Invention

[0006] The purpose of the present invention is to provide a photolithography mask method for an air-bridge microstructure chip to solve the problems of complex existing air-bridge preparation processes, poor reliability and inability to be manufactured on a large scale.

[0007] The present invention provides a photolithography mask method for an air-bridge microstructure chip, including: Providing a substrate; Coat a first photoresist on the substrate; Coat a second photoresist on the first photoresist; Use a first mask for lithography, development, and thermal reflow to form a support layer pattern; Re - coat the first photoresist and the second photoresist on the support layer pattern; Use a second mask for overlay lithography and development to form a metal film growth window; Grow the metal film as the main body and piers of the bridge; Strip the support layer and the metal film of non - bridge main body and piers to form a three - dimensional air - bridge structure.

[0008] Optionally, both the first photoresist and the second photoresist are positive photoresists.

[0009] Among them, the first photoresist is a stripping resist, supporting thick - resist process.

[0010] Among them, the second photoresist is a pattern resist compatible with the process conditions of the first photoresist, and both the soft - bake temperature and the glass transition temperature are lower than those of the first photoresist.

[0011] Among them, the second photoresist being compatible with the process conditions of the first photoresist means that under the same exposure conditions and development conditions, the pattern formed by the second photoresist conforms to the design size, and the pattern formed by the first photoresist also conforms to the design size ignoring the influence of lateral etching.

[0012] Optionally, coat a first photoresist on the substrate and coat a second photoresist on the first photoresist. That is, when coating the photoresist for the first time, due to the requirement of bridge height, the first photoresist needs to be spin - coated multiple times. After each spin - coating, it is soft - baked at the lowest soft - bake temperature of the first photoresist, and the soft - bake time decreases successively from the first time, with the time range being: 100 seconds to 300 seconds, to obtain a larger lateral etching rate; the second photoresist only needs to be spin - coated once, and there are no special requirements for the soft - bake time and temperature.

[0013] Optionally, use a first mask for lithography, development, and thermal reflow to form a support layer pattern, where: Use a first mask for lithography. That is, in the first lithography, first use the first mask to perform hard - contact exposure on the first photoresist and the second photoresist, develop for a time T1 to form a support layer shape; then perform flood exposure on the support layer shape and develop for a time T2 to remove the second photoresist in the support layer shape and retain the first photoresist in the support layer shape.

[0014] Among them, the flood exposure dose is less than the hard contact exposure dose, and the flood exposure dose is only for achieving complete exposure of the second photoresist; after flood exposure, the first photoresist is in an under-exposed state.

[0015] Among them, the development time T1 is greater than the development time T2, to avoid the change of the pattern size of the support layer caused by the continuous reaction of the first photoresist during development.

[0016] Among them, the first mask is a support layer with equally spaced distribution, and the support layer is located on the pattern of the substrate.

[0017] Optionally, after the first lithography, the time range for the thermal reflow process of the first photoresist is 10 minutes to 30 minutes, and the temperature range is 110% to 125% of the glass transition temperature of the first photoresist, so that the fluidity of the first photoresist is enhanced and the adhesion is weakened.

[0018] Optionally, the first photoresist and the second photoresist are coated again on the support layer pattern, that is, when the photoresist is coated for the second time, since the thickness of the first photoresist needs to be greater than the thickness of the support layer, the first photoresist needs to be spin-coated multiple times, and soft baking is performed on the first photoresist after each spin coating, and there are no special requirements for the soft baking time and temperature; the second photoresist only needs to be spin-coated once, and there are no special requirements for the soft baking time and temperature.

[0019] Optionally, a metal film growth window is formed by using a second mask for overlay and development, where: Using a second mask for overlay, that is, in the second overlay, the second mask is used, and the same hard contact exposure as in the first lithography is adopted, and the exposure and development conditions are the same as those in the first lithography.

[0020] Among them, the second mask is a metal film growth window with equally spaced distribution, exposing a part of the support layer and a part of the substrate; there is an overlapping part between the metal film growth window and the support layer in the first direction, and there is a gap in the second direction perpendicular to the first direction.

[0021] Among them, the first direction is the cross-sectional direction of the metal film suspension in the view, and the second direction is the cross-sectional direction of the metal film attached to the substrate in the view.

[0022] Among them, the size of the overlapping part in the first direction depends on the minimum resolution of the adopted exposure method, to avoid the proximity effect during overlay, ensure that there is an overlap in the first direction after exposure and development of the support layer and the metal film growth window, so as to facilitate the peeling of the support layer; the size in the second direction is equal to the size of the support layer in the second direction plus the designed size of the pier in the second direction.

[0023] Among them, the gap size is the designed size of the pier in the first direction.

[0024] Optionally, a second mask is used for overlay lithography and development to form a metal film growth window, where: evaporation is selected for growing the metal film. Since the kinetic energy of the target atoms or molecules deposited on the support layer during magnetron sputtering is greater than that during evaporation, the metal film grown by magnetron sputtering will be in closer contact with the support layer, which is not conducive to peeling; moreover, the step coverage of magnetron sputtering is better than that of the evaporation method, which will cause the support layer to be completely covered by the metal film and unable to be peeled off.

[0025] Optionally, the support layer and the metal films of the non-bridge main body and the pier are peeled off to form a three-dimensional air bridge structure, which includes two steps: the first step is to place the side of the substrate without the grown metal film downward in the stripping solution and let it stand until the metal films of the non-bridge main body and the pier are warped and fall off; the second step is to perform oxygen plasma ashing on the side of the substrate with the grown metal film.

[0026] Among them, since the first photoresist will dissolve after standing in the stripping solution, the metal films of the non-bridge main body and the pier will fall off accordingly; when the metal films of the non-bridge main body and the pier fall off, the substrate is placed in an isopropyl alcohol solution until the stripping solution is dissolved, and then the moisture in the substrate is dried with a hot plate, and the temperature of the hot plate is lower than the soft bake temperature of the second photoresist.

[0027] Among them, the oxygen plasma ashing process mainly relies on chemical reactions rather than physical bombardment. The chemical reaction itself is isotropic, so it will perform lateral etching on the support layer under the metal film; in addition, the oxygen plasma ashing process is realized by using a reactive ion etching instrument. By increasing the gas pressure and the oxygen flow rate, the concentration of active oxygen atoms can be increased, thereby enhancing the lateral etching and realizing the peeling of the support layer.

[0028] Compared with the prior art, the significant advantages of the present invention are as follows: by controlling the spin coating and soft bake conditions of the first photoresist, a trapezoidal profile after lithography can be achieved without using a gray-scale exposure technique; since the first photoresist and the second photoresist are process-compatible, precise transfer of patterns, formation of a support layer or a stripping layer can be achieved simultaneously by one exposure and development, the process operation is simple, the process conditions are loose, and large-scale manufacturing can be carried out; by using the mask provided in this application for metal deposition, while ensuring the adhesion between the air bridge pier and the substrate, it is possible to avoid the arch support layer being completely covered by the metal film and unable to be peeled off. Description of the Drawings

[0029] Figure 1 It is a schematic diagram of an air bridge microstructure; Figure 2 Flow chart of a photolithography mask method for an air bridge micro-structure chip; Figure 3 Flow chart of a photolithography mask method for a support layer along the A1A2 section in an embodiment; Figure 4 Flow chart of a photolithography mask method for a metal air bridge along the A1A2 section in an embodiment; Figure 5 Flow chart of a photolithography mask method for a support layer along the B1B2 section in an embodiment; Figure 6 Flow chart of a photolithography mask method for a metal air bridge along the B1B2 section in an embodiment; Figure 7 Schematic diagram of the structures of the first mask and the second mask for fabricating an air bridge; Figure 8 Scanning electron microscope photograph of an air bridge micro-structure processed by the method of the present invention. Detailed implementation manners

[0030] In order to make the objectives, technical solutions and advantages of the present application clearer and more understandable, the present application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0031] Please refer to Figure 1 , which is a schematic diagram of an air bridge micro-structure according to the present invention. Among them, 101 is a substrate with an existing pattern, 102 is a pier, 103 is the main body of the bridge, and 102 and 103 form an air bridge structure suspended in the air.

[0032] Please refer to Figures 2 to 4 , which is a flow chart of a photolithography mask method for an air bridge micro-structure chip according to the present invention, mainly including the following steps: Step S210: Provide a substrate 101 with an existing pattern on it.

[0033] Step S220: Coat a first photoresist 302 on the substrate.

[0034] In an embodiment of the present application, from Figure 1 the perspective of the cross-section of the air bridge micro-structure in the A1A2 direction, the specific process of step S220 is as follows: After spin-coating the first layer of the first photoresist on the substrate 101, bake it at the lowest soft bake temperature for 5 minutes; then spin-coat the second layer of the first photoresist and bake it at the lowest soft bake temperature for 4 minutes; then spin-coat the third layer of the first photoresist and bake it at the lowest soft bake temperature for 3 minutes to obtain the first photoresist 302.

[0035] Step S230: Coat a second photoresist 303 on the first photoresist.

[0036] In one embodiment of the present application, looking at the cross-section of the air bridge microstructure along the A1A2 direction, the specific process of step S230 is as follows: On the first photoresist 302, a layer of second photoresist 303 is spin-coated and soft baked. Figure 1 In one embodiment of the present application, looking at the cross-section of the air bridge microstructure along the A1A2 direction, the specific process of step S240 is as follows: Hard contact exposure in ultraviolet lithography is adopted, and the pattern of the air bridge support layer is transferred onto the photoresist by using the first mask 304; A developer that chemically reacts with both the first photoresist 302 and the second photoresist 303 is used for development for a time T1 to obtain the shape of the support layer; Then, flood exposure in ultraviolet lithography is performed on the support layer shape; A developer that chemically reacts with both the first photoresist and the second photoresist is used for development for a time T2 (T2 < T1) to remove the second photoresist that is completely exposed in the support layer shape and retain the first photoresist in the support layer shape; The trapezoidal support layer 305 is thermally reflowed at 110% of the glass transition temperature of the first photoresist 302 to form an arched support layer 306.

[0037] Step S240, using the first mask for lithography, development, and thermal reflow to form the support layer pattern.

[0038] In one embodiment of the present application, looking at the cross-section of the air bridge microstructure along the A1A2 direction, the specific process of step S250 is as follows: After spin-coating the first layer of the first photoresist on the substrate 101, soft baking is performed, and the spin-coating and soft baking of the first photoresist are repeated four times to form the first photoresist 307. Then, a layer of second photoresist 308 is spin-coated and soft baked. Figure 1 In one embodiment of the present application, looking at the cross-section of the air bridge microstructure along the A1A2 direction, the specific process of step S250 is as follows: After spin-coating the first layer of the first photoresist on the substrate 101, soft baking is performed, and the spin-coating and soft baking of the first photoresist are repeated four times to form the first photoresist 307. Then, a layer of second photoresist 308 is spin-coated and soft baked.

[0039] Step S250, coating the first photoresist and the second photoresist again on the support layer pattern 306.

[0040] In one embodiment of the present application, looking at the cross-section of the air bridge microstructure along the A1A2 direction, the specific process of step S260 is as follows: Hard contact exposure in ultraviolet lithography is adopted, and the pattern of the metal film growth window is transferred onto the photoresist by using the second mask 309; A developer that chemically reacts with both the first photoresist 307 and the second photoresist 308 is used for development for a time T1 to obtain the shape of the metal film growth window. Figure 1 In one embodiment of the present application, looking at the cross-section of the air bridge microstructure along the A1A2 direction, the specific process of step S260 is as follows: Hard contact exposure in ultraviolet lithography is adopted, and the pattern of the metal film growth window is transferred onto the photoresist by using the second mask 309; A developer that chemically reacts with both the first photoresist 307 and the second photoresist 308 is used for development for a time T1 to obtain the shape of the metal film growth window.

[0041] Step S260, using the second mask for overlay lithography and development to form the metal film growth window.

[0042] In one embodiment of the present application, looking at the cross-section of the air bridge microstructure along the A1A2 direction, the specific process of step S270 is as follows: A metal film is grown as the main body of the bridge. Figure 1 In one embodiment of the present application, looking at the cross-section of the air bridge microstructure along the A1A2 direction, the specific process of step S270 is as follows: A metal film is grown as the main body of the bridge.

[0043] Step S270, growing a metal film as the main body of the bridge.

[0044] In one embodiment of the present application, along the cross-section of the air bridge microstructure in the A1A2 direction, the specific process of step S270 is to deposit a 250-nanometer-thick aluminum film by electron beam evaporation. Figure 1

[0045] Step S280: Strip the support layer and the metal films of the non-bridge main body and piers to form a three-dimensional air bridge structure.

[0046] In one embodiment of the present application, along the cross-section of the air bridge microstructure in the A1A2 direction, the specific process of step S280 is as follows: Place the side of the substrate 101 without the grown metal film 310 facing down in the N-methyl solution and let it stand for 3 hours until the metal films of the non-bridge main body and piers warp and fall off; then place the side of the substrate 101 with the grown metal film 310 facing up in a reactive ion etching instrument for oxygen plasma ashing to obtain a suspended air bridge structure 311.

[0046] Figure 1

[0047] Please refer to Figure 5 , which is Figure 1 the flowchart of the photolithography mask method for the support layer along the cross-section of the air bridge microstructure in the B1 and B2 directions. 101 is the substrate with the existing pattern provided in step S210, 302 is the first photoresist coated in step S220, 303 is the second photoresist coated in step S230; 304 is the first mask used in step S240; 305 is the trapezoidal support layer; 306 is the arched support layer formed after S240. The specific processes of steps S210 to S240 are all the same as those in the Figure 3 illustrated embodiment.

[0048] Please refer to Figure 6 , which is Figure 1 the flowchart of the photolithography mask method for the metal air bridge along the cross-section of the air bridge structure in the B1 and B2 directions. 101 is the substrate with the existing pattern provided in step S210, 306 is the arched support layer formed after step S240, 307 is the first photoresist coated in step S250, 308 is the second photoresist coated in step S250; 309 is the second mask used in step S260; 311 is the air bridge structure formed after step S280, that is, Figure 1 the air bridge structure suspended in the air composed of 102 and 103 shown in Figure 4 the illustrated embodiment. The specific processes of steps S250 to S280 are all the same as those in the

[0049] Optionally, the first photoresist 302 can be a LOR series lift-off photoresist; the second photoresist can be an AZ series pattern photoresist.

[0050] Please refer to Figure 7, (a) is a schematic structural diagram of the first mask for fabricating the air bridge microstructure according to the present invention, (b) is a schematic structural diagram of the second mask, and (c) is an enlarged schematic diagram of the pattern in (b). The first mask (a) includes support layers 305 evenly distributed on the patterned substrate 101. The second mask (b) includes metal film growth windows 702 evenly distributed, exposing a part of the support layer 305 and a part of the substrate 101; the first direction is the Figure 1 B1 and B2 directions in Figure 1 , and the second direction is the A1 and A2 directions in Figure 1 ; in (c), there is an overlapping part 703 between the metal film growth window 702 and the support layer 305 in the B1 and B2 directions, and a gap 704 in the A1 and A2 directions; the gap 704 is the pier 102 part.

[0051] Please refer to Figure 8 , which is a scanning electron microscope photo of the air bridge microstructure processed by the lithography mask method of the air bridge microstructure chip according to the present invention.

[0052] The embodiments of the present invention are not limited by the described examples. Any changes, simplifications, substitutions, and combinations made without departing from the essence and principle of the present invention should be included in the protection scope of the present invention.

[0053] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0054] The above-described embodiments only represent several implementation manners of the present application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A photolithography mask method for an air bridge microstructure chip, characterized in that: The following steps are involved: providing a substrate; coating a first photoresist on the substrate; coating a second photoresist on the first photoresist; Using a first mask for photolithography, development, and thermal reflow to form a support layer pattern; coating the first photoresist and the second photoresist on the support layer pattern again; Using a second mask to overlay and develop to form a metal film growth window; growing the metal film as the main body and piers of the bridge; The supporting layer and the metal films of the non-bridge main body and the bridge piers are peeled off to form a three-dimensional air bridge structure.

2. The photolithography mask method of an air bridge microstructure chip according to claim 1, characterized in that: The first photoresist and the second photoresist are both positive photoresists; Wherein, the first photoresist is a stripping glue, which supports thick glue process; The second photoresist is a pattern glue compatible with the process conditions of the first photoresist, and its soft baking temperature and glass transition temperature are lower than those of the first photoresist.

3. The photolithography mask method of an air bridge microstructure chip according to claim 1, characterized in that: A first photoresist is coated on the substrate, and a second photoresist is coated on the first photoresist, that is, when the photoresist is coated for the first time, the first photoresist needs to be repeatedly spin-coated to a preset height, and after each spin coating, the first photoresist is soft-baked at the lowest soft-baking temperature of the first photoresist, and the soft-baking time decreases from the first time onwards, and the time range is: 100 seconds to 300 seconds; The second photoresist only needs to be spin-coated once.

4. The photolithography mask method for an air bridge microstructure chip according to claim 1, characterized in that: The support layer pattern is formed by photolithography, development and thermal reflow using a first mask, wherein: Using a first mask for photolithography, that is, in the first photolithography, firstly use a first mask to perform hard contact exposure on the first photoresist and the second photoresist, and develop for a time T1 to form a support layer shape; then perform flood exposure on the support layer shape, and develop for a time T2 to remove the second photoresist in the support layer shape and retain the first photoresist in the support layer shape; wherein the flood exposure dose is less than the hard contact exposure dose, and the flood exposure dose is only for achieving full exposure of the second photoresist; after the flood exposure, the first photoresist is in an under-exposed state; wherein the developing time T1 is greater than the developing time T2; Wherein, the first mask is a support layer distributed at equal intervals, and the support layer is located on the pattern of the substrate.

5. The photolithography mask method for an air bridge microstructure chip according to claim 4, characterized in that: After the first photolithography, the first photoresist is subjected to a thermal reflow process for a time range of 10 minutes to 30 minutes at a temperature range of 110% to 125% of the glass transition temperature of the first photoresist.

6. The photolithography mask method for an air bridge microstructure chip according to claim 1, characterized in that: The first photoresist and the second photoresist are coated again on the supporting layer pattern, that is, when the photoresist is coated for the second time, the first photoresist is repeatedly spin-coated for multiple times until the thickness of the first photoresist is greater than the thickness of the supporting layer, and the first photoresist is soft-baked after each spin coating; The second photoresist only needs to be spin-coated once.

7. The photolithography mask method for an air bridge microstructure chip according to claim 4, characterized in that: A metal film growth window is formed by overlaying and developing using a second mask, wherein: Overlaying with a second mask, that is, during the second overlaying, the second mask is used, and the same hard contact exposure as that during the first photolithography is adopted, and the exposure and development conditions are the same as those during the first photolithography; Wherein, the second mask is a metal film growth window distributed at equal intervals, exposing a portion of the support layer and a portion of the substrate; there is an overlap between the metal film growth window and the support layer in a first direction, and there is a gap in a second direction perpendicular to the first direction; Wherein, the first direction is the cross-sectional direction of the metal film being suspended in the view, and the second direction is the cross-sectional direction of the metal film being attached to the substrate in the view; The size of the overlapping portion in the first direction depends on the minimum resolution of the adopted exposure method; the size in the second direction is equal to the size of the supporting layer in the second direction plus the design size of the bridge pier in the second direction; The gap size is the design size of the pier in the first direction.

8. The photolithography mask method for an air bridge microstructure chip according to claim 1, characterized in that: A metal film growth window is formed by overlaying and developing using a second mask, wherein: Evaporation method is selected for growing metal films.

9. The photolithography mask method for an air bridge microstructure chip according to claim 1, characterized in that: The support layer and the metal film of the non-bridge main body and the bridge pier are peeled off to form a three-dimensional air bridge structure, including two steps: The first step is to place the substrate with the side on which the metal film is not grown downward in a stripping solution and leave it to stand until the metal films of the non-bridge body and the bridge pier are lifted up and fall off; The second step is to perform oxygen plasma stripping on the side of the substrate where the metal film is grown; After the metal films of the non-bridge body and the bridge piers fall off, the substrate is placed in an isopropanol solution until the stripping solution is dissolved, and then the moisture in the substrate is dried using a hot plate, the temperature of the hot plate being lower than the soft baking temperature of the second photoresist.

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