FEATURE OPTICAL FABRICATION
“As more devices come to market, there should be more feedback on what features are of real interest for customers”
when developing a novel enhanced AR near- eye system, which pioneers the application of what he describes as ‘the first volume holographic optical element (VHOE) to guide the incoming image through the glass, coupled with a second VHOE to extract the image for the user’s eyes.’
Susceptible to drawbacks “Our approach adopts a diffractive method. An alternative technique involves the use of a thin holographic element known as a surface relief grating (SRG), which shares similarities with VHOE but lacks angular and wavelength selectivity. While SRG, due to its absence of angular and wavelength selectivity, effectively directs a colour image to a user’s eyes, it is susceptible to a significant drawback – leakage mode,” he told Electro Optics. “This means that individuals in front
of the user can inadvertently observe the image being viewed by the user, posing a notable limitation. In contrast, VHOE
Morphotonics is developing processes for the large area imprinting of optical waveguides. Here, 480 waveguides can be seen on 16 separate substrates imprinted with a single area mould
eliminates the leakage mode, but it introduces a challenge in the form of a colour breakup problem, which our team is actively addressing in our current research efforts,” he adds. For Sun and others following this line
of work, one of the key extant challenges facing researchers in the continued development and fabrication of enhanced AR near-eye systems like this is the fact
Imprint and etch-based glass patterning
Another interesting area in the ongoing development of waveguides and optical combiners for AR devices is the evolution of technologies and processes that enable the intricate patterning of glass wafers. One company involved in this area is the Belgian research & development organisation Imec, which has recently developed a new process for the imprint-and-etch-based patterning of 300mm high-refractive index glass wafers. As Matt Traub, Research and Development Project Lead at IMEC, explains, the goal of the new process is to ‘bring the precision and
repeatability of 300mm CMOS processes to new optical applications, including optical combiners for augmented reality.’ By taking advantage of existing advanced 300mm tooling capabilities, he says the approach also ensures compliance with wafer contamination and handling rules and avoids impacting other processes. Even so, he notes that the introduction of high-refractive index glasses to these tools can pose several challenges. In particular, the metal oxides introduced to the glass to raise the refractive index ‘also increase the weight and brittleness of the wafers and can introduce contamination issues if the glass is etched directly.’ “To enable processing of these materials in our 300mm line, the high-index glass is encapsulated in an optically transparent layer, followed by deposition and patterning of the optically functional surface layer,” he says. “By careful selection of the thickness and refractive index of the encapsulation layer, the etched surface features can interact with light in the
substrate without actually exposing the high-index material to sensitive tools,” he adds. In Traub’s view, this approach brings the advantages of semiconductor scaling to AR optics, in the process ‘taking advantage of CMOS capabilities in advanced materials, small feature patterning, and repeatability over large areas.’ He also notes that inorganic dielectrics deposited in CMOS tools can achieve higher refractive indices and lower optical losses than the nanoimprint lithography (NIL) resins typically used in optical combiner fabrication. “Fabrication of NIL masters with immersion DUV tools allows consistent critical dimension (CD) control below 100nm across many copies of the same master, while using NIL to perform the patterning on device substrates keeps the number of production process steps limited,” he says. “Highly uniform etching across a 300mm substrate with sub-100 nm is already standard in the semiconductor industry, and applying this control to imprint patterned wafers provides a cost-effective approach to mass-manufacturing of high-performance designs,” he adds. In terms of challenges moving forward, Traub observes that ‘optical specifications are ultimately driven by overall device requirements, and
there is still tremendous uncertainty on what the requirements will be for a mass adoption.’ However, regardless of the details of the final form factor, he says “it is clear that high brightness will need to be combined with lower power consumption in order to manage weight, heat, and battery life, and that it all must be achieved at reasonable cost.” “For optics fabrication, this means continuing to push the performance of not just materials and dimensional control, but of scalability and
yield. Ultimately, scaling is going to be part of the solution to hit a consumer price point, and it is much easier to target a manufacturable process from the beginning than to try to reinvent existing processes while in high-volume production,” he says. In the coming years, Traub also predicts that advancing capabilities in metasurfaces for visible wavelengths should offer a wide variety of
opportunities for innovation, not only in waveguides and optical combiners, but also for ‘the miniaturisation of optics for light engines and for world cameras and sensors’ - i.e. those optical sensors or cameras that ‘make the device interact with the outside world so that it is more than just a display.’ “As more devices come to market over the next few years, there should be more feedback on what features are of real interest for customers, allowing development to be more targeted at the most critical problems,” he adds.
32 Electro Optics February 2024
www.electrooptics.com
that ‘the achievement of the ideal image comes at the expense of energy loss’. “As a consequence, the primary
challenge lies in conserving energy or enhancing system efficiency,” he says. “[In the future], it is imperative to generate a high-quality image with an ample field of view. Secondly, there is a need to conserve energy and minimise the electrical power load on the battery,” he adds. EO
Morphotonics
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