Optimization Scheme of SU-8 Photoresist Micro-Nano Processing

The Shortcomings of Traditional SU-8 Photoresist Processing and the Advantages of Ultrasonic Spray Coating Technology

In high-end manufacturing fields such as micro/nano manufacturing, MEMS devices, microfluidic chips, and precision sensing elements, SU-8 negative photoresist, with its high resolution, high aspect ratio, excellent mechanical properties, and chemical stability, has become a core material for thick-film microstructure processing. Currently, traditional SU-8 photoresist processing mainly uses spin coating as the mainstream coating method. This process is mature, easy to operate, and suitable for processing conventional planar substrates. With its standardized workflow, it is widely used in the mass production of simple two-dimensional microstructures and is currently the most widely adopted photoresist coating technology in the industry. However, as micro/nano devices iterate towards ultra-high precision, complex 3D irregular structures, deep trenches, and micro-hole arrays, the inherent limitations of traditional spin coating processes have become fully apparent, exposing many unavoidable process shortcomings that can no longer meet the processing requirements of high-end precision devices.

The core principle of spin coating is to utilize the centrifugal force generated by the high-speed rotation of the substrate to spread and level the liquid SU-8 photoresist dripped onto the substrate surface, ultimately forming a thin adhesive layer. This method, relying on mechanical centrifugal force, inherently suffers from physical defects: during spin coating, the difference in centrifugal force gradient between the center and edges of the substrate is significant. The photoresist tends to accumulate and pool at the edges, resulting in a thickness deviation where the adhesive layer is too thick at the edges and too thin at the center, drastically reducing the uniformity of the adhesive layer across the entire substrate. While slight thickness unevenness can be compensated for in conventional planar 2D structure processing through subsequent fine-tuning, the defects of spin coating are amplified in scenarios involving deep trenches, high aspect ratio micropores, curved surfaces, and irregularly shaped 3D microstructures. The photoresist cannot penetrate deep into the structure or into corners using centrifugal force, easily leading to coating gaps, air bubbles, missed areas, and adhesive layer breakage—fatal defects that directly cause incomplete microstructure formation, dimensional inaccuracies, and significantly reduce product yield.

Besides the inherent problems of the coating process, traditional contact-mask lithography also has significant drawbacks. This process requires a tight bond between the mask and the photoresist layer on the substrate surface to ensure accurate pattern transfer. However, during long-term, continuous batch processing, SU-8 photoresist has a certain degree of adhesion, making it prone to sticking to the mask. On the one hand, the compression deformation and adhesion caused by the adhesion between the photoresist layer and the mask directly blur the edge contours of the lithographic pattern, reducing lithographic resolution and pattern replication accuracy, making it impossible to process micron- and sub-micron-level precision patterns. On the other hand, repeated adhesion and peeling of the photoresist to the mask surface causes irreversible physical damage such as wear, peeling, and scratches to the chromium light-shielding layer, leading to a higher mask scrap rate. Furthermore, the high cost of masks, high replacement costs, and long replacement cycles not only significantly increase the production material and maintenance costs for enterprises but also increase the production defect rate and rework rate, severely restricting the efficient and stable mass production of precision micro- and nano-devices.

To completely overcome the limitations of traditional spin coating + contact lithography processes and solve industry pain points such as uneven coating, missed coating, low lithography accuracy, and high material consumption in complex 3D microstructures, ultrasonic spraying technology, with its unique non-contact precision coating advantages, has been gradually applied to the high-precision coating process of SU-8 photoresist and has now become a core process solution for complex microstructures and ultra-high precision micro-nano fabrication scenarios. This technology abandons the traditional centrifugal spreading and bonding process, relying on the high-energy vibration of a high-frequency ultrasonic vibration module to precisely atomize liquid SU-8 photoresist, breaking it down into uniform, highly controllable micron-sized droplets, thus fundamentally solving the problems of poor fluidity and uneven spreading of the photoresist.

During the coating process, the system uses precise and controllable low-pressure airflow to guide and uniformly deliver atomized droplets, allowing the fine droplets to settle and adhere evenly to the entire surface of the substrate in a gentle, uniform, and impact-free manner. This includes areas that are difficult to cover with traditional processes, such as deep trenches, the inner walls of micropores, irregular curved surfaces, and tiny corners. The entire process achieves contactless, extrusion-free, and dead-angle-free precision coating. This technology not only completely solves the defects of traditional spin coating processes, such as uneven adhesive layer thickness, edge accumulation, structural gaps, and missed coating in dead corners, but also avoids the problems of adhesive layer adhesion, mask damage, and accuracy reduction caused by contact photolithography. It can stably adapt to the SU-8 photoresist coating requirements of various complex 3D microstructures, significantly improving the processing accuracy and yield of micro-nano devices, and providing reliable process support for high-end precision micro-nano manufacturing.

About Cheersonic

Cheersonic is the leading developer and manufacturer of ultrasonic coating systems for applying precise, thin film coatings to protect, strengthen or smooth surfaces on parts and components for the microelectronics/electronics, alternative energy, medical and industrial markets, including specialized glass applications in construction and automotive.

Our coating solutions are environmentally-friendly, efficient and highly reliable, and enable dramatic reductions in overspray, savings in raw material, water and energy usage and provide improved process repeatability, transfer efficiency, high uniformity and reduced emissions.

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