Micro-optics and nanostructures are key technologies for the latest optoelectronic components in smartphones, smart glasses and vehicles. Some examples used in consumer electronics include microlenses in time-of-flight or ambient light sensors, diffractive optical elements (DOE) for structured light generation, as well as surface relief gratings with nanometer precision in diffractive waveguides that enable new applications like 3D sensing and augmented reality glasses.
This technology is also used in vehicles in microlens-based ultra-compact projection systems for headlamps or decorative and functional lighting applications that can be integrated anywhere in a car. With that said, there is a need for robust and economic manufacturing solutions in consumer electronics and automotive applications where several hundred thousand to a million pieces are produced each year.
Injection molding is an established and efficient manufacturing process that could theoretically be used as a solution. However, it is not able to fulfill the precise alignment needed for such miniaturized optoelectronics. It also requires thermoplastic materials, like PC or PMMA. These materials are not able to meet the optical and mechanical requirements needed, especially at elevated temperatures. Glass seems like a good choice because it meets all the requirements; however, it is difficult to handle, leading to more complex and expensive manufacturing and assembly processes.
That explains why, for miniaturized optical applications, fast UV-curing materials are used almost exclusively. These materials, with their defined transmission properties, are essentially optical-grade adhesives. Since they are able to take on many other tasks besides bonding, with the most prominent being optical functions, these unique adhesives are often called UV-curable polymers.
With these materials, lenses and nanostructures are produced using nano-imprint lithography (NIL). NIL is an assembly process that has become indispensable within the semiconductor industry. It is defined as a high throughput, high-resolution parallel patterning method where a surface pattern of a stamp is replicated into a material by mechanical contact and 3D material displacement. The achievable structural resolution can be as low as 10 nm. Lateral alignment accuracies below 5 μm can be achieved with standard industrial equipment.
In a typical UV-NIL process, a liquid UV-curing (adhesive) material is deposited onto a substrate. The stamp is brought into contact and the liquid material flows into the stamp's cavities. Next, UV exposure begins, curing the liquid material which becomes solid. After this solidification has finished, the stamp can be removed, resulting in an imprinted structure on the substrate. While a lot of variation can be found in the details, generally every UV imprint process follows this scheme.
UV-Curable Polymer Specifications
Imprint materials for optical applications can have a multitude of different properties, dictated by the function they must fulfill. The refractive index is one of the key properties when selecting a UV-curable optical imprint material. While some applications might work with a broad range of refractive indices, optical designers often prefer one refractive index over another due to design or manufacturing constraints, and often applications require a specific refractive index.
For example, grating couplers for augmented reality or core materials for light guide applications need a high refractive index (>1.7); whereas cladding materials for optical waveguides demand a low refractive index (<1.45). These requirements are met by chemically different formulations, based on different polymer families with individual overall strengths. The refractive index of epoxy-based materials typically ranges from 1.45 to 1.6. For acrylic materials, the range can be from 1.35 to 1.9 and above.
Other optical properties are also important. A diffusor material, for example, needs to contain a precise amount of shaped particles to realize the desired scattering profile. A lens material with low haze and high transmission ratio requires a high level of purity. Completely light blocking materials have to combine high optical densities with small layer thicknesses. For other applications, these materials need to act as optical filters in a specific wavelength range. The combination of such optically tailored properties offers almost complete design freedom as well as enabling complex device architectures that are easy to fabricate.
Another fundamental aspect, which is frequently disregarded, is the thermo-mechanical behavior of an imprint material. Take, for example, a monolithic lens design. It requires a very hard material with a high glass transition temperature because the hardness directly translates to good mechanical stability and scratch resistance. In contrast, a material for large area imprint of thick optical structures on a glass carrier needs to be softer so that it can compensate for thermal stress created by different coefficients of thermal expansion of glass and the imprint material.
For these different scenarios, users can choose from a wide range of soft and hard materials. Generally speaking, their Young's modulus can be designed to be as low as 400 MPa (soft) and as high as 6,500 MPa (hard). Regarding temperature stability, UV-curable epoxies can be equipped with a glass transition temperature that can reach almost 200°C if necessary.
Both acrylic and epoxy materials pass industry-standard temperature and humidity tests (THT test, typically at 85°C and 85% r.h. with storage times varying from 72h to 1000h or more), as well as temperature cycling and temperature shock tests (-40°C to +120°C or higher) for consumer electronics and other applications that are used at ambient temperatures. For demanding high-temperature applications, like those found in automotive scenarios with their 150°C requirements, UV-curable epoxies are normally the material of choice. These polymers can withstand reflow soldering, as they prove in MSL (moisture sensitivity level) testing (THT followed by 3x reflow process with 260°C peak temperature), making them suitable for optoelectronic components that face a reflow process in their downstream final assembly.
Given these options, and many more that have not been mentioned, including non-yellowing, low shrinkage and low outgassing, it is obvious how important it is to balance all these properties to achieve the best performance in different application and production scenarios, based on simulations and experimental tests. Both acrylic and epoxy-based materials have their individual strengths. As a rule of thumb, acrylic materials tend to be stronger in covering a wide refractive index range, whereas epoxies are beneficial with higher temperature and low shrinkage requirements. In the end, the final application will dictate which to use to achieve the best performance.
True Power of Wafer Level Optics
UV imprint of single optical elements is just a small aspect of wafer-level optics. Its true power comes into play when full optical systems are manufactured in parallel on a wafer-level. Besides the optical elements, such as a lens or a DOE, an optical system needs additional components, like spacers, apertures and a housing. Wafer-level optics deal not only with fabricating each single element, but also how to stack these elements to form the full system.
UV-Curable Polymers Help To Reach Optoelectronic Innovations
UV-curable polymers, which are essentially adhesives, are increasingly used for mass production of micro- and nanoscale optical elements via nanoimprint lithography. A matching refractive index, high transmission, and balanced thermo-mechanical behavior, paired with an overall excellent reliability, are their most important features.
Given the large number of optical functions which can be implemented in the design of a new device, these, and many other properties need to be considered for, and tailored to, a specific application. When this is done properly, the optical-grade materials offer an almost complete freedom of design, enable complex device architectures and are easy to fabricate at the same time. With their help, consumer electronics and automotive OEMs are able to successfully reach optoelectronic innovations with regard to 3D sensing, automotive headlamps and projectors as well as AR.
This article was written by Karl Bitzer, PhD, Head of Product Management, DELO (Windach, Germany). For more information, visit here .