A novel photolithography approach may very well be a producing game-changer for optical purposes.
Technologies that rely upon light-weight, high-precision optical programs, like house telescopes, X-ray mirrors, and show panels, have developed considerably over the previous a number of a long time, however extra superior progress has been restricted by seemingly easy obstacles. For instance, the surfaces of mirrors and plates with microstructures which can be mandatory in these optical programs could be distorted by harassed floor coating supplies, degrading optics high quality. This is particularly true for ultra-lightweight optical programs like house optics, the place typical optical manufacturing strategies battle to satisfy exacting form necessities.
Now, MIT researchers Youwei Yao, Ralf Heilmann, and Mark Schattenburg of the Space Nanotechnology Laboratory (SNL) inside MIT’s Kavli Institute for Astrophysics and Space Research, in addition to latest graduate Brandon Chalifoux PhD ’19, have devised new strategies to work previous this barrier.
In a paper revealed within the April 20, 2022, problem of the journal Optica, Yao, a analysis scientist and the paper’s lead writer, explains their new strategy to reshaping skinny plate supplies in a method that eliminates distortion and allows researchers to bend surfaces extra arbitrarily into the exact and complicated shapes they could want. Thin plate shaping is often used for high-level, complicated programs, like deformable mirrors or wafer-flattening processes throughout semiconductor manufacturing, however this innovation means future manufacturing will probably be extra exact, scalable, and low-cost. Yao and the remainder of the workforce think about that these thinner and extra simply deformable surfaces could be helpful in broader purposes, like augmented actuality headsets and bigger telescopes that may be despatched into house at decrease price. “Using stress to deform optical or semiconductor surfaces is not new, but by applying modern lithographic technology, we can overcome many of the challenges of existing methods,” says Yao.
The workforce’s work builds on the analysis of Brandon Chalifoux, who’s now an assistant professor on the University of Arizona. Chalifoux labored with the workforce on earlier papers to develop a mathematical formalism to attach floor stress states with deformations of skinny plates, as a part of his doctorate in mechanical engineering.
In this new strategy, Yao has developed a novel association of stress patterns for exactly controlling common stress. Substrates for optical surfaces are first coated on the bottom with skinny layers of high-stress movie, manufactured from supplies like silicon dioxide. Novel stress patterns are lithographically printed into the movie in order that researchers can change the properties of the fabric in particular areas. Selectively treating the movie coating in several areas controls the place stress and stress is utilized throughout the floor. And as a result of the optical floor and the coating are adhered collectively, manipulating the coating materials additionally reshapes the optical floor accordingly.
“You’re not adding stress to make a shape, you’re selectively removing stress in specific directions with carefully designed geometric structures, like dots or lines,” says Schattenburg, senior analysis scientist and director of the Space Nanotechnology Laboratory. “That’s just a certain way to give a target stress relief at a single place in the mirror which can then bend the material.”
An concept from correcting house mirrors
Since 2017, the SNL workforce has labored with NASA Goddard Space Flight Center (GSFC) to develop a course of to right the form distortion of X-ray telescope mirrors attributable to coating stress. The analysis originated from a mission of constructing X-ray mirrors for NASA’s Lynx next-generation X-ray telescope mission idea, which requires tens of hundreds of high-precision mirrors. Due to the duty of focusing X-rays, the mirrors have to be very skinny to assemble X-rays effectively. However, mirrors lose stiffness quickly as they’re thinned, turning into simply distorted by the stress from their reflective coatings — a nanometers-thick iridium layer coated on the entrance aspect for the aim of reflecting X-rays.
“My team at GSFC has been making and coating thin X-ray mirrors since 2001,” says William Zhang, X-ray optics group chief at GSFC. “As the quality of X-ray mirrors has improved continually in the last several decades following technological advancements, distortion caused by coatings has become an increasingly serious problem.” Yao and his workforce developed a lithographic stress patterning technique, efficiently combining a number of totally different methods, to attain wonderful removing of distortion when utilized to X-ray mirrors made by the group.
After this preliminary success, the workforce determined to increase the method to extra common purposes, similar to free-form shaping of mirrors and skinny substrates, however they met a serious impediment. “Unfortunately, the process developed for GSFC can only precisely control a single type of surface stress, the so-called ‘equibiaxial,’ or rotationally uniform, stress,” says Chalifoux. “Equibiaxial stress states can only achieve bowl-like local bending of the surface, which cannot correct potato-chip or saddle shape distortions. To achieve arbitrary control of surface bending requires control of all three terms in the so-called ‘surface stress tensor.’”
To obtain full management of the stress tensor, Yao and his workforce additional developed the expertise, finally inventing what they name stress tensor mesostructures (STMs), that are quasi-periodic cells arrayed on the again floor of skinny substrates, composed of gratings superimposed on harassed coatings. “By rotating the grating’s orientation in each unit cell and changing the area fraction of selected areas, all three components of the stress tensor field can be controlled concurrently with a simple patterning process,” explains Yao.
The workforce spent greater than two years growing this idea. “We encountered a series of difficulties in the process,” Schattenburg says. “Free-form shaping of silicon wafers with nanometer precision requires a synergy of metrology, mechanics, and fabrication. By combining the lab’s decades of experience in surface metrology and microfabrication with graduate-student-developed thin plate modeling and optimization tools, we were able to demonstrate a general substrate shape control method that is not limited to only bowl-like surface bending.”
A promising approach for a lot of purposes
This strategy enabled the workforce to think about new purposes past the preliminary job of correcting coating-distorted X-ray mirrors. “When forming thin plates using traditional methods, it is difficult to be precise because most of the methods generate parasitic or residual stresses which lead to secondary distortion and spring-back after processing,” says Jian Cao, a professor of mechanical engineering at Northwestern University, who was not concerned with the work. “But the STM stress-bending method is quite stable, which is especially useful for optics-related applications.”
Yao and his colleagues are additionally anticipating to regulate stress tensors dynamically sooner or later. “Piezoelectric actuation of thin mirrors, which is used in adaptive optics technology, has been under development for many years, but most methods can only control one component of the stress,” Yao explains. “If we can pattern STMs on thin, piezo-actuated plates, we would be able to extend these techniques beyond optics to interesting applications such as actuation on microelectronics and soft robotics.”
Reference: “Stress tensor mesostructures for deterministic figuring of thin substrates” by Youwei Yao, Brandon Chalifoux, Ralf Ok. Heilmann and Mark L. Schattenburg, 14 April 2022, Optica.
This work was funded by NASA.