At its core, nanocoining is a new process that covers a metal surface with 3D features that are smaller than the wavelength of visible light. This metal surface can form a seamless drum mold that can replicate the features onto a glass or plastic panel via roll-to-roll manufacturing. The features create a nano-structured surface that is invisible to the eye but can manipulate light or repel water, giving the surface desirable properties including:
- Anti-reflective and anti-glare
- Increased optical transmission
- Structured (pigment-free) color
Roll-to-roll (R2R) processes are attractive not only because they are fast, but also because they can create continuous patterns into a large-format film or panel. In the industry today, creating the drum mold remains a significant challenge because the mold must be wear-resistant and have a uniform pattern over areas exceeding 1 square meter. Micro-structured molds are made on a precision drum lathe, but traditional cutting tools are unable to create sub-wavelength features because any tool sharp enough to create them would be extremely fragile and susceptible to rapid wear.
Nanocoining includes a custom tool that solves this problem by indenting the features rather than scraping the metal away, enabling features significantly smaller than the wavelength of light to be created at a rate of over 300 million features or 30 square millimeters per second. The custom tool is mounted to commercially available drum lathe with precision axes that move the indenting tool along the drum as it spins on an air-bearing spindle.
The nanocoining tool consists of a diamond die mounted to an ultrasonic actuator. The diamond die is first patterned with a few thousand features (1) using a focused ion beam. The die is attached to the ultrasonic resonant beam actuator (2) that is used to generate about 50,000 indents per second, with each indent replicating the thousands of features on the die. Indents can be made into either a flat or cylindrical metal mold (3) that becomes a robust stamp to replicate the structure into polymer films. In the cylindrical form it can be used as a drum mold in a roll-to-roll process (4) that can roll film at more than 20 square meters per minute, ten million times faster than the ion beam that began the process.
Nanocoining has a distinct competitive advantage for making robust metal molds with sub-wavelength features, particularly in its speed and scale. The advantage of nanocoining is enhanced by its ability to pattern cylinders. Laser-interference lithography can pattern flat shims that can be wrapped around a base cylinder, but that method creates a seam. Further, even slight deviation in the flat will disrupt the entire interference pattern. E-beam lithography can produce high-resolution nano-features but is extremely limited by speed and scale.
SMS recently completed a successful STTR Phase I project with the National Science Foundation and NC State University. Diamond dies were created with 160 nm tall structures on a 320 nm pitch over a 10x10 µm area. Also, the nanocoining process was used to create 38 mm diameter metal cylinders uniformly coated with 320 nm features at a rate of 50 million features per second, and those cylinders have been used to replicate a plastic film. The structure on the film gives a 2-2.5% reduction of reflection and, more importantly, a 2-2.5% transmission increase across the entire visible band, indicating that the indented structure is small and uniform enough to not cause scattering. A wide array of custom patterns can be created into the diamond.
Applications in toDay's world
There are countless applications for structured structures. Some, such as the "moth-eye," are multifunctional - simultaneously giving a surface anti-reflective and non-wetting properties.
Non/Enhanced Wetting Applications:
Self-cleaning windows or solar cells
Anti-fouling and oleophobic surfaces
Brighter OLED displays and lighting
More efficient solar panels
Wire grid polarizers