PHEMT grid electrode manufacturing method based on composite photoresist and improved lift-off process

Abstract

The invention relates to a pHEMT grid electrode manufacturing method based on a composite photoresist and an improved lift-off process. The pHEMT grid electrode manufacturing method comprises the steps that a wafer is rotated at a low speed, E-beam photoresist liquid is evenly spread on the whole wafer, the rotating speed is increased to 3000-4000 rpm for spin coating, and a hot plate is used for conducting soft baking on the E-beam photoresist; developing through electron beam writing to form a Y gate bottom; continuously rotating the wafer at a low speed, uniformly spreading the i-line photoresist liquid on the whole wafer, increasing the rotating speed to 1500-2500rpm, and spinning glue; carrying out soft baking on the i-line photoresist by using a hot plate; continuously rotating the wafer at a low speed, uniformly spreading the LOR photoresist liquid on the i-line photoresist, increasing the rotating speed to 2000-3000 rpm for spin coating, and performing soft baking on the LOR photoresist by using a hot plate; exposing and developing the grid drawing area, and exposing and developing the I-line photoresist of the non-E-beam drawing area at the same time; performing metal evaporation on the operation grid bars; the wafer is soaked in a developing solution, and gate metal on the surface layer is removed; and removing the light resistance by adopting an efficient composite solvent.

Description

Technical Field

[0001] This invention relates to the field of semiconductor technology, and in particular to a method for manufacturing a pHEMT gate based on composite photoresist and an improved lift-off process. Background Technology

[0002] In the fabrication process of gallium arsenide (GaAs)-based pseudomorphic high electron mobility transistors (pHEMTs), gate formation is a critical step that determines device performance. In particular, the fabrication process faces significant challenges for submicron gates with feature sizes (CD) less than 0.2 micrometers.

[0003] Currently, a commonly used process in the industry is the composite photoresist and metal lift-off technique. This process typically involves: first, coating an electron beam photoresist onto a GaAs substrate, and then precisely defining submicron-level (e.g., 0.15 μm) gate lines through electron beam exposure and development. Subsequently, a thicker (e.g., 1.0 μm) i-line negative photoresist is coated onto this structure, and a window with a positive trapezoidal morphology is formed through exposure and development. The purpose of this positive trapezoidal structure and the use of negative photoresist is to create natural metal fracture gaps after the subsequent evaporation of gate metal (e.g., Ti / Pt / Au, with a total thickness of approximately 5500 Å), thereby effectively avoiding metal bridging.

[0004] After metal evaporation, the photoresist and its superimposed redundant metal need to be removed. Traditional processes employ a two-step method combining adhesive film-assisted mechanical lifting and organic solvent immersion. Specifically, an adhesive film is first applied, and pressure is applied manually using rollers to ensure it adheres tightly to the wafer surface. The film is then peeled off, using its adhesive force to remove most of the redundant metal from the photoresist layer. This step (adhesive film lifting) is crucial. Its primary purpose is to create a contact channel between the photoresist and the subsequent organic solvent (such as N-methylpyrrolidone, NMP), significantly increasing the contact area between the photoresist and the solvent. Secondly, due to the small gate window area, without this step, NMP would need to penetrate and dissolve the entire large-area, thick photoresist layer, which is extremely inefficient and almost impossible to complete.

[0005] However, due to the extremely small gate diameter (CD) (0.15μm), it is difficult to ensure uniform and constant pressure by manually applying pressure to the rollers during the film lifting process. Excessive or uneven pressure can be transmitted through the film to the fragile gate, causing the gate metal strip to be squeezed, pushed over, or even broken, resulting in fatal defects such as gate "leaning" or "collapsing." This physical damage directly leads to abnormal electrical characteristics of the device (such as saturation current, transconductance, and threshold voltage), severely reducing the product yield. Summary of the Invention

[0006] The purpose of this invention is to provide a pHEMT gate manufacturing method based on composite photoresist and improved lift-off process, which can completely eliminate the mechanical stress caused by human factors in the encapsulant lift-off process and completely avoid the risk of gate squeezing, tilting or collapse.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for manufacturing a pHEMT gate based on composite photoresist and an improved lift-off process, the method comprising the following steps: Step S1: Rotate the wafer at a low speed of 500 rpm for 5 to 10 seconds to spread the E-beam photoresist evenly over the entire wafer. Increase the speed to 3000 to 4000 rpm and maintain this speed for 30 to 60 seconds to spin-coat the E-beam photoresist. Use a hot plate to soft bake the E-beam photoresist. Step S2: After electron beam writing, development is performed to form the bottom of the Y-gate; Step S3: Continue rotating the wafer at a low speed of 500 rpm for 5-10 seconds to evenly spread the i-line photoresist across the entire wafer. Increase the speed to 1500-2500 rpm and maintain this speed for 30-45 seconds to spin-coat the i-line photoresist. Use a hot plate to soft-bake the i-line photoresist. Step S4: Continue to rotate the wafer at a low speed of 500 rpm to evenly spread the LOR photoresist onto the i-line photoresist. Increase the rotation speed to 2000-3000 rpm and maintain this speed for 30-45 seconds to spin the photoresist. Use a hot plate to soft bake the LOR photoresist. Step S5: Expose and develop the grid pattern area, and at the same time expose and develop the I-line photoresist in the area without E-beam patterning; Step S6: Metal vapor deposition of the grid strips; Step S7: Immerse the wafer in the developer to remove the gate metal on the surface; Step S8: Remove photoresist using a high-efficiency composite solvent.

[0008] Furthermore, the thickness of the E-beam photoresist in step S1 is 2100Å-2500Å; the E-beam photoresist is baked at a temperature of 170℃~180℃ for 60~90 seconds.

[0009] Furthermore, in step S3, the thickness of the i-line photoresist is 9000Å-13000Å; the temperature for soft baking the i-line photoresist is 90℃~110℃, and the time is 60℃~90 seconds.

[0010] Furthermore, in step S4, the thickness of the LOR photoresist is 3000Å-6000Å, and the LOR photoresist is baked at a temperature of 150℃-180℃ for 60-120 seconds.

[0011] Furthermore, in step S5, the developing solution used is 2.38% TMAH, the developing temperature is 23°C, and the developing time is 80 seconds.

[0012] Furthermore, the efficient composite solvent in step S8 is a mixed solution of NMP and DMSO, with a volume ratio of NMP:DMSO = 7:3.

[0013] Furthermore, step S8 further comprises: heating the high-efficiency composite solvent to 80°C, immersing the wafer in the heated high-efficiency composite solvent and supplementing it with gentle megasonic cleaning for 30 to 60 minutes.

[0014] Furthermore, in step S6, the metal has a three-layer structure, consisting of a Ti layer, a Pt layer, and an Au layer.

[0015] The beneficial effects of this invention are: It completely eliminates the mechanical stress caused by human factors in the film lifting process, completely avoids the risk of gate extrusion, skewness or collapse, and revolutionizes the yield for gate manufacturing of CDs of 0.15μm and below.

[0016] Compared with traditional NMP immersion, the photoresist removal time of the composite hot solvent system can be shortened by more than 50%, which significantly improves production cycle and capacity.

[0017] Excellent process consistency and stability: By utilizing LOR (Light Loss Removal) to dissolve in photoresist and increasing the contact area between the photoresist and NMP solution, the metal and photoresist are stripped away, eliminating the uncertainties of manual operation and batch-to-batch variations, resulting in excellent process repeatability. Gentle megasonic wave assistance ensures the cleanliness of the wafer surface after resist removal, reducing residues. Attached Figure Description

[0018] Figure 1 This is a schematic diagram showing the coating of E-beam photoresist. Figure 2 This is a schematic diagram showing the coating of i-line photoresist and LOR photoresist; Figure 3 A schematic diagram showing the developed area after exposure of the grid pattern area; Figure 4 This is a schematic diagram of metal vapor deposition; Figure 5 This is a schematic diagram before the removal of redundant metals; Figure 6 This is a schematic diagram of the redundant metal removal section; Figure 7 This is a schematic diagram showing the complete removal of E-beam photoresist.

[0019] The components are: 1. wafer, 2. E-beam photoresist, 3. i-line photoresist, 4. LOR photoresist, and 5. metal. Detailed Implementation

[0020] The invention will now be further described with reference to the accompanying drawings.

[0021] Please see Figures 1 to 7 The present invention provides an embodiment: a method for manufacturing a pHEMT gate based on composite photoresist and an improved lift-off process, the method comprising the following steps: Step S1: Rotate wafer 1 at a low speed of 500 rpm for 5 to 10 seconds to spread the E-beam photoresist 2 evenly over the entire wafer 1. Increase the speed to 3000 to 4000 rpm and maintain this speed for 30 to 60 seconds to spin the coating. Use a hot plate to soft bake the E-beam photoresist 2. Step S2: Electron beam writing followed by development to form the bottom of the Y-gate; the developing solution used in this step is MIBK:IPA=1:3, the development time is 90s, and the temperature is 23°C.

[0022] Step S3: Continue rotating wafer 1 at a low speed of 500 rpm for 5-10 seconds to evenly spread the i-line photoresist 3 liquid across the entire wafer 1. Increase the speed to 1500-2500 rpm and maintain this speed for 30-45 seconds to spin-coat the i-line photoresist 3. Use a hot plate to soft bake the i-line photoresist 3. Step S4: Continue to rotate wafer 1 at a low speed of 500 rpm to evenly spread the LOR photoresist 4 liquid onto the i-line photoresist 3. Increase the rotation speed to 2000-3000 rpm and maintain this speed for 30-45 seconds to spin the adhesive. Use a hot plate to soft bake the LOR photoresist 4. Step S5: Expose and develop the grid patterned area, and simultaneously expose and develop the i-line photoresist 3 in the area without E-beam patterning. Specifically, this patent opens the i-line photoresist 3 in the area without E-beam patterning. The purpose of this is to greatly increase the contact area between the NMP and the photoresist for subsequent photoresist removal, thereby improving the photoresist removal efficiency. LOR stands for Lift-Off Resist. Unlike standard photoresist, the exposed area of ​​LOR is dissolved in the developer, and the unexposed area also has a relatively stable dissolution rate in the developer.

[0023] Step S6: Vapor deposition of the grid metal 5; Step S7: Immerse wafer 1 in developer to remove the gate metal 5 on the surface; Immerse wafer 1 in developer and use the characteristic of LOR dissolving in developer to remove the gate metal 5 on the surface.

[0024] Step S8: Remove photoresist using a high-efficiency composite solvent. NMP: As the main solvent, it has good solubility for photoresist. DMSO (dimethyl sulfoxide): As a strongly polar aprotic solvent, it has stronger penetration and swelling capabilities for highly cross-linked photoresist (such as E-beam photoresist 2). Working synergistically with NMP, it significantly accelerates the stripping and dissolution process of the photoresist. Heating and megasonic waves: Thermal energy accelerates molecular motion and chemical reactions, while gentle megasonic waves generate microfluidics, helping the stripped photoresist fragments leave the surface of wafer 1 and preventing redeposition. The post-removal view of the photoresist is shown below. Figure 6 .

[0025] Please continue reading. Figure 1 As shown, in one embodiment of the present invention, the thickness of the E-beam photoresist 2 in step S1 is 2100 Å-2500 Å; the baking temperature of the E-beam photoresist 2 is 170℃~180℃, and the time is 60~90 seconds. The thickness of the E-beam photoresist 2 is preferably 2300 Å or 2500 Å, and the target thickness of 2300 Å usually falls near the range of 3500 rpm / 45s.

[0026] Please continue reading. Figure 2 As shown, in one embodiment of the present invention, the thickness of the i-line photoresist 3 in step S3 is 9000Å-13000Å; the temperature for soft baking of the i-line photoresist 3 is 90℃~110℃, and the time is 60℃~90 seconds. Preferably, the thickness of the i-line photoresist 3 is 1µm.

[0027] Please continue reading. Figure 2 As shown, in one embodiment of the present invention, the thickness of the LOR photoresist 4 in step S4 is 3000 Å-6000 Å, and the LOR photoresist 4 is subjected to a soft-baking temperature of 150℃-180℃ for 60-120 seconds. The thickness of the LOR photoresist 4 is preferably 4500 Å. Please continue reading. Figure 2 As shown in Figure 3, in one embodiment of the present invention, the developing solution used in step S5 is 2.38% TMAH, the developing temperature is 23°C, and the developing time is 80 seconds.

[0028] Please continue reading. Figure 5 , Figure 6As shown, in one embodiment of the present invention, the high-efficiency composite solvent in step S8 is a mixed solution of NMP and DMSO, with a volume ratio of NMP : DMSO = 7 : 3.

[0029] Please continue reading. Figure 5 , Figure 6 As shown, in one embodiment of the present invention, step S8 further comprises: heating the high-efficiency composite solvent to 80°C, immersing the wafer 1 in the heated high-efficiency composite solvent with gentle megasonic cleaning, and soaking for 30-60 minutes. After the metal 5 is deposited, the photoresist and the redundant metal 5 covering it need to be removed. The traditional process uses a two-step method of adhesive film-assisted mechanical lifting combined with organic solvent immersion. Specifically, a layer of adhesive film is first used, and pressure is applied by manually operating a roller to make the film tightly cover the surface of the wafer 1. Then the film is peeled off, and most of the redundant metal 5 on the surface of the photoresist layer is peeled off by its adhesion. This step (adhesive film lifting) is crucial. Its primary purpose is to create a contact channel between the subsequent organic solvent (such as N-methylpyrrolidone, NMP) and the bottom photoresist, greatly increasing the contact area between the photoresist and the solvent. Secondly, since the gate window area is small, without this step, NMP would need to penetrate and dissolve the entire large-area thick photoresist layer, which is extremely inefficient and almost impossible to complete. The photoresist removal time using the composite hot solvent system in this application can be shortened by more than 50% compared to traditional NMP immersion, significantly improving production cycle time and capacity.

[0030] Please continue reading. Figure 5 , Figure 6 As shown, in one embodiment of the present invention, the metal 5 in step S6 has a three-layer structure, consisting of a Ti layer, a Pt layer, and an Au layer. The structure of metal 5 is Ti / Pt / Au = 500 / 400 / 4100 Å. Specific Implementation Example 1:

[0031] 1. Coated with E-beam photoresist 2 PMMA, thickness 2500 Å: Step 1 (Spreading): Low speed, typically 500 rpm, for 5-10 seconds. The purpose is to spread the photoresist evenly across the entire wafer 1.

[0032] Step 2 (spinning): Spin at 3000-4000 rpm for 30-60 seconds.

[0033] Step 3 (Soft Baking): The hot plate temperature is 170℃ - 180℃; The duration is 60-90 seconds; After electron beam writing and development (MIBK:IPA=1:3, time 90s, temperature 23°C), the morphology is as follows. Figure 1 As shown; 2. Coating i-line photoresist 3, 1µm thickness: Step 1 (Spreading): 500 rpm, 5-10 seconds.

[0034] Step 2 (spinning): Spin at 1500 - 2500 rpm for 30 - 45 seconds.

[0035] Step 3 (Soft Baking): The hot plate temperature is 90℃ - 110℃; The time is 60-90 seconds.

[0036] 3. Coat with LOR photoresist 4, with a thickness of 4500 Å: Step 1 (Spreading): 500 rpm, 5-10 seconds; Step 2 (spinning): Spin at 2000-3000 rpm for 30-45 seconds; Step 3 (Soft Baking): The hot plate temperature is 150℃ - 180℃; The time is 60-120 seconds, such as Figure 2 As shown; 4. Expose and develop the area where the grid lines are drawn; The developing conditions were as follows: 2.38% TMAH developer, temperature: 23℃, time: 80 seconds. Because LOR dissolves in the developer, the overall morphology after development is as follows: Figure 3 ; 5. The metal 5 of the grid is vapor-deposited, and the structure of metal 5 is Ti / Pt / Au=500 / 400 / 4100 Å, such as... Figure 4 As shown; 6. Redundant Metal 5 Removal Process: Wafer 1 is immersed in developing solution. The surface gate metal 5 is removed by utilizing the characteristic of LOR (Left-of-Role) dissolving in the developing solution. The process is illustrated below: Figures 5 to 6 As shown.

[0037] 7. Use a high-efficiency composite solvent to remove photoresist: Solvent composition: A mixed solution of NMP and dimethyl sulfoxide (DMSO) with a volume ratio of NMP : DMSO = 7 : 3.

[0038] Operating temperature: Heat to 80℃.

[0039] Processing method: Immerse wafer 1 in the above-mentioned hot solvent and supplement it with gentle megasonic cleaning (power density <1 W / cm²). The immersion time can be shortened to 30-60 minutes.

[0040] View after photoresist removal as shown Figure 6 As shown.

[0041] This invention operates on the following principles: The process of removing the LOR photoresist 4 completely eliminates the mechanical stress caused by human factors in the resist film removal process, completely avoiding the risks of gate compression, skewness, or collapse. For gate manufacturing of CDs with a thickness of 0.15μm and below, the yield is revolutionaryly improved. The photoresist removal time using the composite hot solvent system can be shortened by more than 50% compared to traditional NMP immersion, significantly improving production cycle time and capacity. By utilizing the dissolution of LOR in the photoresist and increasing the contact area between the photoresist and the NMP solution to peel off the metal 5 and the photoresist, the uncertainty of manual operation and batch-to-batch differences are eliminated, resulting in excellent process repeatability. Gentle megasonic wave assistance ensures the cleanliness of the wafer 1 surface after resist removal, reducing residues.

[0042] The above description is only a preferred embodiment of the present invention and should not be construed as a limitation of this application. All equivalent changes and modifications made in accordance with the scope of the patent application of the present invention should be covered by the present invention.

Claims

1. A method for manufacturing a pHEMT gate based on composite photoresist and an improved lift-off process, characterized in that: The method includes the following steps: Step S1: Rotate the wafer at a low speed of 500 rpm for 5 to 10 seconds to spread the E-beam photoresist evenly over the entire wafer. Increase the speed to 3000 to 4000 rpm and maintain this speed for 30 to 60 seconds to spin-coat the E-beam photoresist. Use a hot plate to soft bake the E-beam photoresist. Step S2: After electron beam writing, development is performed to form the bottom of the Y-gate; Step S3: Continue rotating the wafer at a low speed of 500 rpm for 5-10 seconds to evenly spread the i-line photoresist across the entire wafer. Increase the speed to 1500-2500 rpm and maintain this speed for 30-45 seconds to spin-coat the i-line photoresist. Use a hot plate to soft-bake the i-line photoresist. Step S4: Continue to rotate the wafer at a low speed of 500 rpm to evenly spread the LOR photoresist onto the i-line photoresist. Increase the rotation speed to 2000-3000 rpm and maintain this speed for 30-45 seconds to spin the photoresist. Use a hot plate to soft bake the LOR photoresist. Step S5: Expose and develop the grid pattern area, and at the same time expose and develop the I-line photoresist in the area without E-beam patterning; Step S6: Metal vapor deposition of the grid strips; Step S7: Immerse the wafer in the developer to remove the gate metal on the surface; Step S8: Remove photoresist using a high-efficiency composite solvent.

2. The method for manufacturing a pHEMT gate based on composite photoresist and improved lift-off process according to claim 1, characterized in that: The thickness of the E-beam photoresist in step S1 is 2100Å-2500Å; the E-beam photoresist is baked at a temperature of 170℃~180℃ for 60~90 seconds.

3. The method for manufacturing a pHEMT gate based on composite photoresist and improved lift-off process according to claim 1, characterized in that: The thickness of the i-line photoresist in step S3 is 9000Å-13000Å; the temperature for soft baking the i-line photoresist is 90℃~110℃, and the time is 60℃~90 seconds.

4. The method for manufacturing a pHEMT gate based on composite photoresist and improved lift-off process according to claim 1, characterized in that: In step S4, the thickness of the LOR photoresist is 3000Å-6000Å, and the LOR photoresist is baked at a temperature of 150℃-180℃ for 60-120 seconds.

5. The method for manufacturing a pHEMT gate based on composite photoresist and improved lift-off process according to claim 1, characterized in that: In step S5, the developing solution used is 2.38% TMAH, the developing temperature is 23°C, and the developing time is 80 seconds.

6. The method for manufacturing a pHEMT gate based on composite photoresist and improved lift-off process according to claim 1, characterized in that: The efficient composite solvent in step S8 is a mixed solution of NMP and DMSO with a volume ratio of NMP : DMSO = 7 :

3.

7. The method for manufacturing a pHEMT gate based on composite photoresist and improved lift-off process according to claim 1, characterized in that: Step S8 further comprises: heating the high-efficiency composite solvent to 80°C, immersing the wafer in the heated high-efficiency composite solvent and supplementing it with gentle megasonic cleaning for 30 to 60 minutes.

8. The method for manufacturing a pHEMT gate based on composite photoresist and improved lift-off process according to claim 1, characterized in that: In step S6, the metal has a three-layer structure, consisting of a Ti layer, a Pt layer, and an Au layer.

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