The use of ultrasonic nozzle for spray coating of thin films to various substrates has gained considerable attention in recent years, specifically where a thin, uniform coating and a non-contact application method are required. Examples include, thin, conductive layers on three-dimensional components, thin layers or photoresist on large panels, anti-reflection coating layers on lenses, dopants to silicon wafers, anode and cathode catalyst layers on fuel cell electrolyte substrates, among many others.
However, most spray coating applications require that a uniform wet film to be applied to the substrate; the first step in this process using ultrasonic spray is to develop a uniform and stable spray from the vibrating tip. A major challenge for ultrasonic spray devices is the ability to deliver the liquid to the atomizing surface such that a uniform film forms on the atomizing surface prior to spray formation. If the liquid can be delivered to the atomizing surface in a controlled manner, the resulting spray will have a consistent drop size distribution, spray density and stable shape. This is especially challenging for spray devices, like ultrasonic nozzles, where the liquid must pass through the ultrasonic device through a relatively large tube before it reaches the atomizing surface of the ultrasonic nozzle. Because the diameter of the tube is relative large, it is very difficult to control how the flow path of the liquid onto the atomizing surface. Consequently, the spray tends to move around the atomizing surface to create a pulsating and unstable spray. This is especially true at lower liquid flow rates.
USI’s “nozzle-less” approach avoids these problems by delivering the liquid directly to the atomizing surface of the spray forming tip. This technique, as described in detail below, produces a stable and uniform spray.
Patented ‘Nozzle-less’ Technology
USI’s PRISM series of ultrasonic coating systems utilise a patented ‘nozzle-less’ ultra-Thin Coating Application Technology (tCAT) to deposit thin, uniform, defect-free coatings for lab-scale and industry applications ranging from solar panels, semiconductors, fuel cells, medical devices and electronics assembly markets. The coating system has advanced material handling capabilities for the precise application of a range of expensive coating liquids from suspensions and slurries to flammable solvents. It includes an automated X-Y-Z-θ-Ø motion control platform that reduces operation time and increases precision in repetitions. The ultra-spray head assembly contains the following components.
1. An ultrasonic transducer with a spray forming tip
Unlike an ultrasonic nozzle system, the liquid does not pass through the device. Instead, it is applied externally to the solid surface of the spray-forming tip. The tip is a part of the ultrasonic transducer which vibrates at an ultrasonic frequency to break the liquid into small drops that form the spray. The shape of the spray pattern is rectangular, following the shape of the spraying tip.
An ultrasonic generator drives the transducer which is resonant at ultrasonic frequencies of 35 kHz, 45 kHz, or 60 kHz. The frequency is selected based on the material to be sprayed and the application requirements. For example, 35 kHz is optimal for most suspensions used in the manufacture of fuel cells. It also sets the amplitude of vibration of the spray-forming tip required to atomize the liquid at the operated flow rate.
2. An integrated fluid applicator
The integrated fluid applicator contains separate external passageways for liquid and air. The coating liquid is delivered directly to the vibrating spray-forming tip on the spray head from an external channel. The liquid forms a thin film on the vibrating tip of the spray head and the ultrasonic vibrations form capillary waves in the liquid film and break it into small drops.
In order to produce a thin, uniform coating film on the surface of the substrate, the spray pattern must have a uniform and stable shape. This requires the liquid spray to have a consistent density and drop size distribution, which is determined by the following factors:
- The coating liquid must be delivered to the vibrating tip of the ultrasonic spray head uniformly across the width of the vibrating tip. This forms a “sheet-like” spray with a very tight drop size distribution and a very uniform density.
- The sheet-like spray must be expanded and accelerated into a wider pattern while maintaining the drop size distribution and uniform density. The integrated air director is designed to do this without causing the drops to coalesce into larger drops.
The external application of liquid and air jet ensures that the spray pattern and drop size distribution are very stable.
3. External liquid applicator
The liquid applicator is specially designed to reduce thickness variation of the liquid film applied on the spray forming tip, which minimises the drop size variation.
4. Air director
- The spray produced with ultrasonic energy alone is a very low velocity, narrow “sheet-like” pattern. The width of the spray pattern produced is equal to the width of the spray-forming tip (2 mm to 4 mm).
- The air director impinges a jet of air on the tip of the spray head opposite the liquid feed side.
- The directed air stream is used to entrain, expand and accelerate the ultrasonically produced spray, providing a rectangular, uniform spray pattern.
- The air flow rate can be controlled electronically to adjust the spray velocity.
- The coating pattern width is proportional to the distance between the spray forming tip and the substrate and can be expanded to 5 times the width of the spray forming tip.
5. Precision liquid delivery system
- The coating liquid is fed to the liquid applicator with PRISM’s positive displacement Precision Metering Pump (PMP) which uses a micro-stepping drive to control liquid flow rate and ensure precise, reliable, and repeatable delivery and coating process.
- Additionally available is a dual pump setup in which one pump feeds the spray head while the other pump gets filled from the holding reservoir. This allows continuous coating without any downtime during production.
- The PMP is also available with recirculation and stirring to maintain required properties of the liquid during the coating process.
Advantages of the ‘nozzle-less’ tCAT technology
- The nozzle-less design ensures no clogging such as in traditional methods like Air atomizing spray valves, reducing maintenance and downtime.
- The nozzle-less technology produces a rectangular spray pattern and a rectilinear coating distribution on the wafer surface. This technology overcomes the problems associated with ultrasonic nozzle based spray devices which produce a conical spray pattern (figure). This is why ultrasonic nozzle based devices deliver a non-uniform coating distribution on the substrate surface.
The coating thickness is uniform across about 90% of the width of the sprayed pattern and the edges are “feathered”. This easily produces a uniform coating layer on the substrate surface.- This nozzle-less ultrasonic spray, supplemented with low velocity air streams, provides a transfer efficiency of 95 to 99%, ensuring that very little coating is wasted due to overspray.
The coating thickness is uniform across about 90% of the width of the sprayed pattern and the edges are “feathered”. This easily produces a uniform coating layer on the substrate surface.
Two spray head types are available for different coating applications:
- CAT-ILDS Ultrasonic Head: Produces a spray pattern width in the range of 3 mm to 30 mm and is used for smaller substrates and for substrates with topography.
- Blade Ultrasonic Head: Produces a spray pattern width in the range of 25 mm to over 150 mm and is used to quickly coat larger substrates. An air director in the spray head assembly is used to entrain, expand and accelerate the width of the ultrasonically produced spray up to 5 times the width of the vibrating tip. The pattern width increases as the distance between the spray head tip and the substrate increases.
NanoTechnology Solutions is the exclusive distributor of USI’s PRISM coating systems in Australia and New Zealand, offering installation, service and technical support for instruments used in the whole range of applications. USI’s Ultrasonic Coating systems have been chosen by leading Australian universities and industry.
