Ultrasonic spray of photoresist for MEMS wafers and other substrates with varying topographies
Direct spray photoresist coating onto MEMS wafers, silicon wafers, and other 3D microstructures has been performed for the past decade, with significant research regarding the advantages of Deep Topography Photoresist Deposition into deep well (high) topographies. Ultrasonic spray photoresist deposition is shown to have advantages over conventional spin coating in producing a more uniform coating, particularly along the top section of sidewalls in high aspect ratio trenches and V-groove structures. In these high aspect ratio features, centrifugal spin makes it difficult to achieve High Aspect Ratio Photoresist coverage along the sidewalls without excessive photoresist pooling at the bottom of cavities.
Ultrasonic spray is a simple, economical, and repeatable spray coating process for photoresist coatings in photolithography wet processes, providing a practical alternative to spin coating for complex wafer topography. Sono-Tek coating systems enable fine control of flow rate, coating speed, solvent delivery, and photoresist deposition amount using an advanced layering technique. Low-velocity spray shaping defines the atomized spray into precise, controllable patterns, avoiding overspray while producing very thin film layers with uniform coating across complex substrate topography. Direct spray coating using ultrasonic technology is proven to be a reliable and effective lithography method for photoresist coating on 3D microstructures, resulting in reduced device failure from overexposure of metal to etchant.
The SPT200 WS coating system incorporates automated handling of two cassettes of 100, 150, or 200 mm wafers (300 mm optional). The WS handler is clean room Class 1 compatible and features pre-alignment sequencing, complete operator-free automated loading of wafers, and comprehensive software tools and utilities. It can be integrated with a single SPT coating system or dual SPT coating systems, as shown.
Read a technical article describing photoresist film coverage improvement on high aspect ratio topography using ultrasonic spray technology here.
The benefits of ultrasonic nozzles for direct spray photoresist coatings in photolithography wet processes include:
- Uniform thin film coverage of various wafer surface profiles and substrate topographies.
- Capable of coating high aspect ratio trenches with excellent uniformity along sidewalls.
- Non-clogging atomized spray for consistent photoresist deposition.
- Ability to deposit thin single-micron layers with high uniformity.
- Repeatable, proven spray coating process.
The heart of all Sono-Tek spray systems is our patented ultrasonic spray nozzle, which uses high-frequency sound vibrations to produce a soft mist of mathematically defined droplets. This eliminates clogging since no pressure is used to produce the spray, and the droplets have a very narrow drop distribution size, further contributing to adhesion and uniformity of the photoresist layer deposited. Read more about how our ultrasonic nozzles work.