A novel photolithography technique could change the way optical applications are manufactured.
Technologies that depend on lightweight, high-precision optical systems, such as space telescopes, X-ray mirrors, and display panels, have advanced significantly over the past few decades, but more advanced advances have been limited by seemingly simple obstacles. For example, the surfaces of mirrors and discs with microstructures necessary in these optical systems can be distorted by stressed surface coating materials and degrade optics quality. This is especially true for ultra-lightweight optical systems such as space optics, where typical optical manufacturing processes struggle to meet demanding shape requirements.
Now, WITH Researchers Youwei Yao, Ralf Heilmann, and Mark Schattenburg of the Space Nanotechnology Laboratory (SNL) at MIT’s Kavli Institute for Astrophysics and Space Research and recent graduate Brandon Chalifoux PhD ’19 have developed new methods to overcome this barrier.
In an article published in the April 20, 2022 issue of the journal Optics, Yao, a research scientist and lead author of the publication, explains her new approach to forming thin sheet materials in a way that eliminates distortion and allows researchers to more arbitrarily bend surfaces into the precise and complex shapes they may need. Thin plate shaping is typically used for sophisticated, complex systems such as deformable mirrors or wafer flattening processes during semiconductor manufacturing, but this innovation means future production will be more precise, scalable and cost-effective. Yao and the rest of the team envision that these thinner and more malleable surfaces could be useful in broader applications, like augmented reality headsets and larger telescopes that can be sent into space at lower cost. “Using stress to deform optical or semiconductor surfaces is not new, but by applying modern lithographic technologies we can overcome many of the challenges of existing methods,” says Yao.
The team’s work builds on the research of Brandon Chalifoux, who is now an assistant professor at the University of Arizona. Chalifoux worked with the team on earlier work to develop a mathematical formalism to relate surface stress states to thin plate deformations as part of his PhD in mechanical engineering.
In this new approach, Yao has developed a novel arrangement of stress patterns to precisely control overall stress. Optical surface substrates are first coated on the back with thin layers of heavy-duty foil made of materials such as silicon dioxide. Novel stress patterns are lithographically printed into the film to allow researchers to alter the material’s properties in specific areas. By selectively treating the film coating in different areas, it controls where stresses and stresses are applied to the surface. And since the optical surface and the coating adhere to each other, manipulating the coating material also reshapes the optical surface accordingly.
“You don’t add stress to create a shape, you selectively remove stress in specific directions with carefully designed geometric structures like points or lines,” says Schattenburg, senior research scientist and director of the Space Nanotechnology Laboratory. “It’s just a specific way to achieve targeted stress relief at a single point in the mirror, which can then bend the material.”
An idea of correcting space mirrors
The SNL team has been involved since 2017 NASA Goddard Space Flight Center (GSFC) to develop a method to correct the shape distortion of X-ray telescope mirrors caused by coating stresses. The research grew out of a project to build X-ray mirrors for the mission concept of NASA’s next-generation Lynx X-ray telescope, which requires tens of thousands of high-precision mirrors. Due to the task of focusing x-rays, the mirrors have to be very thin to collect x-rays efficiently. However, mirrors quickly lose rigidity as they get thinner, and deform easily under the stress of their reflective coatings — a nanometer-thick layer of iridium coated on the front to reflect X-rays.
“My team at GSFC has been manufacturing and coating thin X-ray mirrors since 2001,” says William Zhang, group leader for X-ray optics at GSFC. “As the quality of X-ray mirrors has continuously improved over the past few decades due to technological advances, distortions caused by coatings have become an increasingly serious problem.” Yao and his team developed a lithographic stress patterning process that successfully combined several different techniques to achieve excellent distortion removal when applied to X-ray mirrors manufactured by the group.
After this initial success, the team decided to extend the process to more general applications, such as B. the free-form design of mirrors and thin substrates, but encountered a major obstacle. “Unfortunately, the process developed for GSFC can only accurately control a single type of surface stress, called ‘equibiaxial’ or rotationally uniform stress,” says Chalifoux. “Equal stress states can only achieve shell-like local deflection of the surface, which cannot correct for potato chip or saddle shape distortions. In order to achieve arbitrary control of surface bending, control of all three terms in the so-called ‘surface tension tensor’ is required.”
To achieve complete control over the stress tensor, Yao and his team advanced the technology and eventually invented what are known as stress tensor mesostructures (STMs), which are quasi-periodic cells arranged on the back of thin substrates and made of superimposed lattices consist of hard-wearing coatings. “By rotating the orientation of the lattice in each unit cell and changing the area fraction of selected regions, all three components of the stress tensor field can be controlled simultaneously with a simple patterning process,” explains Yao.
The team spent more than two years developing this concept. “We ran into a number of difficulties,” says Schattenburg. “The free shaping of silicon wafers with nanometer precision requires a synergy of measurement technology, mechanics and production. By combining the lab’s decades of experience in surface metrology and microfabrication with thin plate modeling and optimization tools developed by students, we were able to demonstrate a general method for controlling substrate shape that is not limited to just bowl-like bending of the surface.”
A promising technique for many applications
This approach allowed the team to envision new applications beyond the original task of correcting coating-distorted X-ray mirrors. “When forming thin plate using traditional methods, it is difficult to be precise because most methods generate parasitic or residual stresses that lead to secondary deformation and springback after machining,” says Jian Cao, professor of mechanical engineering at the Northwest University, who was not involved in the work. “But the STM stress bending process is quite stable, which is particularly useful for optical applications.”
Yao and his colleagues also expect to dynamically control stress tensors in the future. “The piezoelectric actuation of thin mirrors used in adaptive optics technology has been under development for many years, but most methods can only control one component of the voltage,” explains Yao. “If we can pattern STMs on thin piezo-actuated plates, we could extend these techniques beyond optics to interesting applications like microelectronic actuation and soft robotics.”
Reference: “Stress tensor mesostructures for deterministic figuring of thinsubstrates” by Youwei Yao, Brandon Chalifoux, Ralf K. Heilmann and Mark L. Schattenburg, April 14, 2022, optics.
This work was funded by NASA.
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