Design and Fabrication of a Spin Coater with In-Situ Optical Measurement for Soft Thin Films

Technical Summary: Design and Fabrication of a Spin Coater with In-Situ Optical Measurement for Soft Thin Films

Problem Statement
Spin coating is a standard method for fabricating thin polymeric and elastomer films, particularly for applications like dielectric elastomer actuators (DEAs) where mechanical and functional performance is inversely proportional to film thickness. However, reliable thickness verification for highly compliant materials remains a significant challenge. Conventional contact-based tools (e.g., calipers, micrometers) induce elastic deformation in soft films, leading to measurement errors that often exceed fabrication tolerances. Conversely, high-resolution non-contact optical metrology systems (e.g., interferometry, ellipsometry) are often expensive, require complex calibration, rely on specific optical properties, or necessitate removing samples from the fabrication environment, disrupting the workflow. There is a gap between the need for precise thickness control in soft-film manufacturing and the accessibility of measurement tools that do not deform the sample.

Methodology and System Architecture
The authors present a low-cost, primarily 3D-printed benchtop spin coater integrated with a minimally deforming optical thickness measurement system. The system comprises three subsystems:

1. Mechanical Spin Coater: Constructed largely from PLA, featuring a motor-driven sample plate, a protective housing cup with a material-trapping lip, and a magnetic sample plate retention system. It operates at speeds sufficient for 50–300 µm film fabrication without requiring high-precision industrial motors.
2. Optical Measurement System: This subsystem utilizes a geometric-optical configuration to measure thickness in-situ. A lightweight mirror is placed on the cured sample surface. A laser diode (895 nm) reflects off the mirror onto a quadrant photodetector (Thorlabs PDQ80A). The system tracks the displacement of the reflected beam spot to infer thickness changes. The design ensures the mirror's weight causes negligible deflection (<1.3 µm) compared to the deformation caused by standard micrometers (~19.5 µm).
3. Control and Data Acquisition: An Arduino UNO controls the motor via a SPARK MAX controller using a CAN Bus for closed-loop velocity control. A myRIO-1900 reads the quadrant sensor's analog signals to determine beam displacement, which is converted to thickness via a calibration constant.

Key Contributions
The paper outlines four primary contributions:

1. Integration: The unification of fabrication and minimally deflecting thickness measurement into a single, accessible benchtop system, eliminating the need for sample removal.
2. Measurement Approach: A single-point, material-independent optical method suitable for highly compliant films where contact tools fail.
3. Quantitative Validation: Rigorous testing of measurement accuracy and repeatability using calibrated metal shims and silicone films.
4. Accessibility: Demonstration that thickness-controlled fabrication of compliant films is achievable without specialized industrial metrology equipment, relying instead on 3D printing and standard electronic components.

Experimental Results

• Resolution and Accuracy: Using calibrated stainless steel shims, the system achieved a thickness resolution of 3.6–3.7 µm. The best-case measurement repeatability was 13 µm (95% confidence interval) when calibrated near the target thickness.
• Linearity Constraints: The system operates effectively within the linear response range of the quadrant sensor. Measurements for samples significantly thicker than the calibration reference (e.g., 0.5 mm samples calibrated against 0.2 mm or 0.3 mm references) showed increased error and confidence intervals due to beam displacement exceeding the sensor's linear domain.
• Silicone Fabrication: The system successfully manufactured Ecoflex10T silicone films with a target thickness of 0.3 mm. The produced films were within 9 µm of the target thickness based on comparative micrometer measurements (using a stack-up method to mitigate deformation). The measurement system showed a worst-case repeatability of 58 µm for the silicone, attributed partly to the material's tackiness affecting mirror settling.

Significance and Claims
The authors claim that this work addresses a practical gap in research-scale manufacturing by providing a proof-of-concept platform that integrates fabrication and verification. The system demonstrates that accessible optical metrology can be embedded into low-cost spin coating workflows to achieve thickness control within 10 µm for soft films. While the authors acknowledge limitations regarding sensor linearity for thicker samples and variability in mirror placement on tacky surfaces, they assert that the platform successfully validates the feasibility of in-situ, non-destructive thickness verification for compliant materials without the cost and complexity of industrial instrumentation. The work serves as a foundation for iterative refinement toward more robust fabrication-measurement systems.