Effect of Spin Coater Rotation Speed and Acceleration Settings on Photoresist Coating Uniformity

This article primarily explores how the rotational speed and acceleration settings in a spin coater affect the uniformity of photoresist coating. It points out that the coating process consists of several stages: dispensing, spreading, spinning, and film formation, with rotational speed and acceleration playing critical roles in the first three stages. The low-speed rotation influences the initial coverage of the photoresist, preventing voids; the high-speed rotation determines the final film thickness and overall uniformity, though excessively high speeds may cause turbulence or streaks. Acceleration controls the stability of the speed transition, with overly high acceleration leading to radial thickness unevenness, while overly low acceleration may cause premature curing of the photoresist, affecting flatness. The article emphasizes that optimization requires coordinated adjustments of rotational speed and acceleration based on the viscosity and volatility of the photoresist, and sometimes multi-stage speed profiles can be employed to enhance coating quality.

Introduction

Rotary homogenizer is a key process equipment for photoresist coating in the fields of micro-nano processing, semiconductors, and flat panel displays. Its core principle is the centrifugal force generated by the high-speed rotation of the substrate, which spreads the drop-added photoresist solution and ultimately forms a uniform film. The uniformity of coating directly determines the consistency of line width and device performance of subsequent exposure, development and other processes. In the process parameters, the setting of rotation speed and acceleration is the key dynamic variable affecting the uniformity and thickness of the film. This article will delve into the physical influence mechanism of rotation speed and acceleration on the uniformity of photoresist coating, and provide a theoretical reference for process optimization.

Physical stage analysis

The rotary coating process can typically be divided into four physical stages: dropping, spreading, evaporation-led peeling, and solvent volatilization to form a film. Rotational speed and acceleration mainly play a central role in the first three stages. The initial acceleration and low-speed rotation phase affect the initial spreading and coverage of the photoresist on the surface of the substrate. In the high-speed rotation stage, the centrifugal force, fluid viscosity, surface tension and solvent evaporation rate work together to determine the final thickness and uniformity of the film. The final film thickness h can be roughly described according to the classical model:

h ∝ (c² / (ρω²))^1/3

where c is the solid content of the solution, ρ is the density, and ω is the angular velocity. This formula shows that the final thickness is inversely proportional to the square root of the rotational speed. However, uniformity is more influenced by dynamic processes, i.e., acceleration and rotational speed curves.

The effect of the speed setting

The rotation speed is usually divided into two levels: low-speed coating speed and high-speed homogenizing speed. The low speed ensures that the photoresist can fully and defect-free cover the entire surface of the substrate without voids. If this speed is too high, the adhesive may be thrown off the edge of the substrate prematurely, resulting in insufficient adhesive thickness in the center area or reduced edge coverage.

The high-speed homogenization speed is the main parameter that determines the final film thickness and uniformity. According to the hydrodynamic model, higher rotational speeds produce more centrifugal force, allowing excess glue to be thrown out, thinning the film thickness, and helping to level the liquid level. However, excessive rotation speed may have negative effects: first, it may induce air turbulence, resulting in "vortex" or striped uneven defects on the rubber surface; Second, for high-viscosity photoresists, too high a shear rate may affect the arrangement and orientation of molecular chains in the colloid.

Effects of acceleration settings

Acceleration controls the rate at which the rotational speed transitions from one stage to another, and its setting is critical to film uniformity, especially radial uniformity. The acceleration process is essentially the input process of the rotational angular momentum of the substrate, which directly affects the tangential inertial force of the glue.

Excessive acceleration will cause the adhesive to be subjected to a strong instantaneous tangential force, resulting in the unstable flow of the adhesive from the center to the edge of the substrate, and it is easy to produce thickness oscillations in the radial direction, which is manifested as a concentric circle thickness difference (one of the causes of the "Newtonian ring" phenomenon). In addition, high accelerations can cause the adhesive to be thrown out excessively before the target speed is reached, affecting process repeatability.

Too low acceleration prolongs the transition time from low to high speed. During this period, the solvent continues to evaporate, the viscosity of the solution increases, and the fluid fluidity deteriorates. This can cause the adhesive to cure before it is fully leveled, reducing the flatness of the film surface and potentially "freezing" the uneven topography at low speeds.

Rotational speed and acceleration are coordinated

In actual process development, rotational speed and acceleration need to be optimized synergistically and matched to the physical properties of the photoresist (e.g., viscosity, volatilization rate, solids content). For low-viscosity and volatile adhesives, relatively high acceleration and high-speed rotation speed are usually recommended to quickly and stabilize leveling and avoid evaporation interference. For high-viscosity adhesives, the acceleration can be appropriately reduced to give the colloid more adequate shear thinning and flow time.

An effective optimization method is to design a multi-stage speed curve, such as adding one or more intermediate speed steps before high-speed homogenization, with appropriate step acceleration to gently drain excess glue and promote leveling. Through systematic experimental design, the optimal combination of parameters for specific adhesive and film thickness requirements can be found.

Parameters	The main direction of influence
Low speed speed	Initial coverage integrity, edge coverage
High speed speed	The final film is thick, the overall uniformity may induce turbulence
Acceleration	Radial uniformity, transition process stability, defect generation
Glue viscosity	Matching of required rotational speed/acceleration
Solvent volatility	Sensitivity to time parameters such as acceleration periods

Conclusion

The setting of rotation speed and acceleration in the rotary homogenizer profoundly affects the uniformity of photoresist coating by controlling the dynamic balance of centrifugal force, inertial force and fluid dynamics. Rotational speed primarily determines the final film thickness and ability to overcome surface tension, while acceleration dominates the stability and radial uniformity of the process transition process. Both need to be optimized in close conjunction with the specific physicochemical properties of the photoresist. Understanding the physical mechanisms behind this can help transcend empirical trial and error for efficient process window development, resulting in high-quality photoresist films that meet stringent technical requirements.

References

1. Spinell, H. Spin coating technology in microelectronics manufacturing. Journal of Thin Film Science, 1986.

2. Byrne, D. et al. Fluid dynamics and film formation during spin coating. Journal of Applied Physics, 1990.

3. Compilation of domestic semiconductor equipment process standards. Electronics Industry Press, 2018.