A method for optimizing photoresist uniformity using ultrasonic waves

Abstract

The application discloses a photoresist coating method for optimizing photoresist uniformity by using ultrasonic waves. In the photoresist spin coating process, ultrasonic waves are added for assistance. The ultrasonic waves can generate high-frequency vibration to provide external energy and transmit the energy to liquid solution, thereby locally decomposing large droplets of coating material into small droplets and helping the small droplets to be injected into small pores existing on a substrate. In addition, the ultrasonic waves can effectively eliminate bubbles, flow layers and agglomerates in the coating liquid by micro-flow in the liquid, so that the coating liquid is uniformly distributed. In this way, a more uniform coating can be obtained, the variability of the coating thickness is reduced, and the coating quality is improved. The energy effect of the ultrasonic waves can locally decompose large droplets of photoresist material into small droplets, so that the coating liquid can more easily penetrate into the surface and small gaps of the coated object in the spin coating process, the coating coverage is improved, the penetration and diffusion process of the coating liquid is accelerated, the coating speed is increased, and therefore the coating efficiency is improved.

Description

Technical Field

[0001] This invention belongs to the field of photolithography technology and relates to a coating method that utilizes ultrasound to optimize the uniformity of photoresist. Background Technology

[0002] In semiconductor manufacturing, photolithography is the core of planar integrated circuit manufacturing. As a sub-process of photolithography, photoresist homogenization affects the uniformity of critical linewidths, linewidth roughness, and overlay accuracy, thus further impacting chip yield. The uniformity of photoresist is crucial for pattern clarity and accuracy, as uneven photoresist coatings can lead to pattern distortion or errors. For example, uneven homogenization will affect exposure alignment, resulting in defects and color differences.

[0003] Photoresist coating is a deposition process in which a photoresist layer is applied to the surface of a wafer. Spin coating is a widely used method for preparing photoresist thin films. Spin coating utilizes the centrifugal force generated when the wafer rotates to distribute the photoresist across the entire wafer surface. When a small amount of liquid is applied to the rotating disk, most of the liquid is swept off the disk by centrifugal force. Some liquid remains on the disk, and due to the interaction of viscous forces and centrifugal force, its thickness decreases. Ultimately, a very thin, uniform film is produced. This process is called spin coating and is widely used in the manufacture of microelectronic devices.

[0004] This method is quite effective for solutions with low surface tension, but spin coating can cause problems when the coating material has high surface tension. For example, solutions with high surface tension often form large droplets during coating, leading to uneven coating on the substrate surface, such as ring deposits or central bulges. Liquids with high surface tension are more likely to form dewdrops and flow back during spin coating, resulting in uneven coating speed and coating thickness fluctuations. High surface tension also causes significant tension differences at the substrate edges, where the liquid tends to shrink and form higher coating layers. This can lead to unevenness at the coating edges.

[0005] To address the aforementioned issues, current methods for adjusting the surface tension of photoresist mainly include controlling solvent selection and ratio, and adding surfactants. However, these methods may alter the chemical properties, viscosity, or curing characteristics of the photoresist, thereby affecting its performance during the photolithography process. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a coating method that utilizes ultrasound to optimize the uniformity of photoresist.

[0007] This invention is implemented by providing a method for optimizing photoresist uniformity using ultrasound, the method comprising the following steps:

[0008] Step S1: Wafer preprocessing;

[0009] Step S2: Add photoresist;

[0010] Step S3: Ultrasonic-assisted rotational homogenization

[0011] S3-1: Place the ultrasonic vibrator at a distance of λ / 2 from the center of the upper surface of the wafer, where λ is the ultrasonic wavelength;

[0012] S3-2: Turn on the ultrasonic vibrator to generate ultrasonic waves to irradiate the photoresist. At the same time, increase the rotation speed of the wafer in step S2 where the photoresist has been applied and keep it rotating at a constant speed. Use ultrasonic vibration to locally decompose the large droplets of photoresist material into tiny droplets, and let them penetrate into the narrow area on the substrate under the action of centrifugal force. Meanwhile, the rotation of the wafer expands it into a uniform thin film.

[0013] S3-3: Adjust the orientation of the ultrasonic vibrator so that the ultrasonic waves are parallel to the photoresist film after initial diffusion. At this time, the direction of the traveling wave generated by the ultrasonic vibrator is perpendicular to the direction of movement of the photoresist. Continue to increase the wafer rotation speed in step 3-2 and maintain uniform rotation. Use the radiation force of the ultrasonic waves to push the photoresist surface and help the photoresist form a smoother film.

[0014] S3-4: Turn off the ultrasonic vibrator and continue to rotate the wafer at a constant speed. The constant speed rotation will continue to evaporate the remaining solvent in the photoresist, making the photoresist thinner and more durable.

[0015] Step S4: Wafer edge and backside flushing;

[0016] Step S6: Post-processing.

[0017] The beneficial effects of this invention are:

[0018] This invention incorporates ultrasonic assistance during the photoresist spin coating process. Ultrasonic waves generate high-frequency vibrations, providing external energy that is transferred to the liquid solution. This locally breaks down large droplets of the coating material into tiny droplets, helping them penetrate into the small pores on the substrate. Furthermore, the micro-flow within the liquid effectively eliminates bubbles, flow layers, and agglomerates, resulting in a more uniform distribution of the coating liquid. This leads to a more uniform coating, reduces coating thickness variability, and improves coating quality. The energy of the ultrasonic waves locally breaks down large droplets of photoresist material into tiny droplets, making it easier for the coating liquid to penetrate the surface and small crevices of the substrate during spin coating, increasing coverage. It also accelerates the penetration and diffusion process of the coating liquid, increasing coating speed and thus improving coating efficiency. Attached Figure Description

[0019] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the photoresist droplet application process;

[0021] Figure 2 This is a schematic diagram of the photoresist spin coating process;

[0022] Figure 3 This is a comparison between the traditional photoresist spin coating method and the photoresist spin coating method with ultrasonic assistance, where (a) is the traditional photoresist spin coating method and (b) is the photoresist spin coating method with ultrasonic assistance.

[0023] Figure 4 This is a schematic diagram of photoresist coating on a patterned wafer, where (a) is a traditional photoresist spin coating method and (b) is a photoresist spin coating method with ultrasonic assistance.

[0024] Figure 5 A schematic diagram illustrating the process of using the radiative force of ultrasound to drive the photoresist surface;

[0025] Figure 6 This is a schematic diagram illustrating the principle of how ultrasound affects the flatness of photoresist.

[0026] Figure 7 This is a comparison of the results of two adhesive application methods. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0028] This embodiment provides a photoresist coating method that utilizes ultrasound to optimize photoresist uniformity. This method is only applicable to 12-inch wafer fabrication processes. It optimizes issues such as incomplete substrate coating, uneven photoresist thickness, and abnormal photoresist edge thickness that occur during spin coating, resulting in a more uniform photoresist thickness covering the wafer surface. For critical layers in advanced dimensions or when the photoresist surface tension is too high, this coating method can be used to improve the reliability of subsequent photolithography and etching. The method includes the following steps:

[0029] Step S1: Wafer Preprocessing

[0030] First, a 12-inch wafer pre-baking process is performed. The wafer is placed in a pre-treatment reaction chamber, and the hot plate temperature is set to 150°C for at least 60 seconds to remove moisture adsorbed on the wafer surface. During pre-baking, HMDS will evaporate into the pre-treatment reaction chamber and deposit onto the wafer surface. The HMDS vapor deposition time is 30 seconds. After the pre-treatment process is complete, the wafer must be transferred to a cooling plate to cool down. The cooling time to room temperature must be at least 45 seconds.

[0031] Step S2: Add photoresist

[0032] Photoresist is dropped onto the center of the upper surface of the wafer after pretreatment in step S1, as shown below. Figure 1 As shown, during the addition process, maintain a rotation speed of 2000rpm-3000rpm and control the dripping time within 7s-8s; then reduce the rotation speed to below 100rpm and maintain the rotation duration for less than or equal to 1s;

[0033] Figure 1 In the process of adding photoresist, the wafer is held in place by a fixture and rotated at a uniform and slow speed. The photoresist is then quantitatively and rapidly added to the wafer through a nozzle.

[0034] Step S3: Ultrasonic-assisted rotational homogenization

[0035] 3-1: Remove the photoresist nozzle from above the wafer and move it to the center of the ultrasonic vibrator above the wafer. Position the ultrasonic vibrator horn parallel to the wafer and align the horn with the photoresist... Figure 2 As shown, the ultrasonic vibrator is then slowly brought close to the wafer surface until the distance to the wafer surface is λ / 2, which helps to ensure that the ultrasonic vibration can be effectively transmitted to the photoresist.

[0036] The ultrasonic vibrator is securely fixed inside the spin coater cavity to ensure stability during the spin coating process. Furthermore, the ultrasonic vibrator is not connected to the rotator and will not rotate during the coating process.

[0037] Figure 2In the photoresist spin coating process, the wafer is clamped by a fixture and rotated at a constant speed.

[0038] 3-2: Set the frequency of the ultrasonic vibrator to 19kHz to 20kHz, turn on the ultrasonic vibrator to generate ultrasonic waves to irradiate the photoresist, and simultaneously increase the rotation speed of the wafer from which the photoresist has been applied in step S2 to 900rpm and maintain a constant rotation speed for 5 seconds. The main purpose of this step is to use ultrasonic vibration to locally break down large droplets of photoresist material into tiny droplets, such as... Figure 3 As shown, it penetrates into a narrow area on the substrate under the action of centrifugal force, such as Figure 4 As shown, the rotation of the wafer causes it to expand into a uniform thin film.

[0039] Figure 3 This compares the traditional photoresist spin coating method with the ultrasonic-assisted photoresist spin coating method. (a) In the traditional method, due to the strong attraction between molecules (high surface tension), the solution tends to form large droplets, which makes it difficult for the photoresist to spread on the substrate, resulting in an uneven photoresist film. In (b), because the ultrasonic vibrator provides external energy to these large droplets through ultrasound, it eventually breaks them down into smaller pieces. Under the action of centrifugation, the photoresist can better cover the substrate, achieving a more uniform coating.

[0040] Figure 4 This diagram illustrates the application of photoresist to a patterned wafer. (a) shows the traditional spin-coating method, where large droplets are unevenly distributed on the substrate, creating gaps with the existing pattern and potentially leading to reduced device performance and yield. (b) utilizes ultrasonic spin-coating, where the ultrasound breaks down the large droplets into smaller ones, allowing for a denser and more uniform photoresist film to form on the patterned substrate.

[0041] 3-3: Adjust the orientation of the ultrasonic vibrator horn. Rotate the ultrasonic vibrator 90° counterclockwise until it is parallel to the initially diffused photoresist film. At this point, the traveling wave generated by the ultrasonic vibrator will propagate in a direction perpendicular to the movement direction of the photoresist. Figure 5 As shown, the wafer rotation speed in step 3-2 is further increased to 1100 rpm and maintained at a constant speed for 5 seconds. The purpose of this step is to use the radiation force of ultrasound to push the photoresist surface, such as... Figure 6 As shown, this helps the photoresist form a smoother film.

[0042] Figure 5 This process utilizes the radiation force of ultrasound to push the surface of the photoresist, helping the photoresist to form a smoother thin film.

[0043] Figure 6 This diagram illustrates the effect of ultrasound on the flatness of photoresist. Ultrasound waves are traveling waves, and their propagation direction is perpendicular to the direction of photoresist movement. An ultrasonic vibrator generates traveling waves, and the acoustic flux is excited between the vibrating surface and the photoresist. This causes the acoustic viscous force to act on the photoresist, effectively applying an outward force along the wafer radius to the photoresist. This thrust allows the photoresist to be coated onto the wafer more quickly and evenly, improving the uniformity of the photoresist film thickness after coating.

[0044] 3-4: Turn off the ultrasonic vibrator, and continue to rotate the wafer at a constant speed of 1100 rpm for 10 seconds, so that the photoresist thickness currently attached to the wafer reaches T+δ; where T represents the target thickness of the photoresist, and δ represents the redundant thickness, preferably... The purpose of this step is to continuously evaporate the remaining solvent in the photoresist using uniform rotation, so that the photoresist is thinner and more robust.

[0045] This invention combines an ultrasonic generator with a traditional photoresist spin coating method, generating ultrasonic waves during the spin coating process. During spin coating, the ultrasonic waves provide external vibrations (periodic perturbations) to the photoresist flowing on the substrate. This vibration generates tiny droplets by overcoming its internal cohesive energy. These droplets can move at very high speeds on the substrate surface and more easily penetrate narrow areas on the substrate. Simultaneously, the rotation of the wafer causes it to expand into a uniform film, resulting in a more uniform coating. At the same time, the acoustic viscous force of the ultrasonic waves acts on the flowing photoresist, and the radiating force pushes the photoresist surface, positively impacting the surface smoothness of the photoresist.

[0046] Step S4: Wafer edge and backside flushing:

[0047] To address the potential photoresist buildup at the wafer edges and bottom surface, step S3, which involves rinsing the wafer edges and bottom surface with ultra-clean water, maintains a rotation speed of 1000 rpm during rinsing, and controls the wafer rotation rinsing time to 5-8 seconds.

[0048] Step S6: Post-processing:

[0049] Increase the wafer rotation speed to 2000 rpm and maintain a constant rotation speed for 5 seconds to remove excess ultrapure water and achieve wafer drying.

[0050] Furthermore, this invention also employs ultrasound to adjust the flatness of the photoresist. While the photoresist is still in a viscous fluid state during spin coating, ultrasound waves are irradiated onto the photoresist surface. The radiation force of the ultrasound waves pushes the photoresist surface, causing changes in its contours. This ultrasonic effect has a positive impact on the surface flatness of the photoresist. Figure 4As shown, acoustic flow is excited between the vibrating surface and the substrate. This causes acoustic viscous forces to act on the flowing photoresist, positively affecting the flatness of the photoresist film thickness.

[0051] In this embodiment, UV1610 photoresist was selected for homogenization, and the target film thickness was [missing information]. The same wafer was used for the experiment. First, photoresist was spin-coated using the conventional method. After measuring the film thickness, the wafer was reworked, and photoresist was spin-coated again using the method provided in this patent. The corresponding film thickness data was then measured. A total of 49 points were used for film thickness measurement. The results of the two experiments are as follows: Figure 7 As shown, curve 2 represents the film thickness data measured by ordinary spin coating, and curve 1 represents the film thickness data optimized by ultrasonication. It can be seen that the uniformity of the photoresist has been significantly improved.

[0052] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for optimizing photoresist uniformity using ultrasound, characterized in that... The method includes the following steps: Step S1: Wafer preprocessing; Step S2: Add photoresist; Step S3: Ultrasonic-assisted rotational homogenization S3-1: Place the ultrasonic vibrator at a distance of λ / 2 from the center of the upper surface of the wafer, where λ is the ultrasonic wavelength; S3-2: Turn on the ultrasonic vibrator to generate ultrasonic waves to irradiate the photoresist. At the same time, increase the rotation speed of the wafer in step S2 where the photoresist has been applied and keep it rotating at a constant speed. Use ultrasonic vibration to locally decompose the large droplets of photoresist material into tiny droplets, and let them penetrate into the narrow area on the substrate under the action of centrifugal force. Meanwhile, the rotation of the wafer expands it into a uniform thin film. S3-3: Adjust the orientation of the ultrasonic vibrator so that the ultrasonic waves are parallel to the photoresist film after initial diffusion. At this time, the direction of the traveling wave generated by the ultrasonic vibrator is perpendicular to the direction of movement of the photoresist. Continue to increase the wafer rotation speed in step S3-2 and maintain uniform rotation. Use the radiation force of the ultrasonic waves to push the photoresist surface and help the photoresist form a smoother film. S3-4: Turn off the ultrasonic vibrator and continue to rotate the wafer at a constant speed. The constant speed rotation will continue to evaporate the remaining solvent in the photoresist, making the photoresist thinner and more durable. Step S4: Wafer edge and backside flushing; Step S6: Post-processing.

2. The method according to claim 1, characterized in that... Step S1 involves pre-baking the wafer, during which hexamethyldisilazane (HMDS) vapor is passed through it; after pre-baking, the wafer temperature is lowered to room temperature.

3. The method according to claim 2, characterized in that... The deposition time for hexamethyldisilazane (HMDS) vapor in step S1 is 30 seconds.

4. The method according to claim 2, characterized in that... The pre-baking temperature for step S1 is 150°C, and the baking time is at least 60 seconds.

5. The method according to claim 2, characterized in that... After the pre-baking in step S1 is completed, the wafer temperature is lowered to room temperature and the cooling time is greater than or equal to 45 seconds.

6. The method according to claim 1, characterized in that... Step S2 specifically involves dropping photoresist onto the center of the wafer surface after the pretreatment in step S1, maintaining a rotation speed of 2000rpm-3000rpm during the dropping process, and controlling the dropping time to 7s-8s; then reducing the rotation speed to below 100rpm and maintaining the rotation for a duration controlled to 0.01s-1s.

7. The method according to claim 1, characterized in that... In step S3-2, the rotation speed of the wafer for which photoresist has been applied in step S2 is increased to 900 rpm, and this process lasts for 5 seconds.

8. The method according to claim 1, characterized in that... In step S3-3, the wafer rotation speed in step 3-2 is further increased to 1100 rpm and maintained at a constant speed for 5 seconds.

9. The method according to claim 1, characterized in that... In steps S3-4, the wafer rotation speed is maintained at 1100 rpm for 10 seconds.

10. The method according to claim 1 or 9, characterized in that... After step S3-4, the thickness of the photoresist attached to the wafer reaches T+δ. Where T represents the target thickness of the photoresist, and δ represents the redundant thickness.

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