MIT scientists develop an affordable, high-precision fabrication approach for thin mirrors and silicon wafers.
A novel photolithography method could be a production game-changer for optical applications.
Technologies that depend on lightweight, high-precision optical systems, like space telescopes, X-ray mirrors, and show panels, have established substantially over the previous numerous decades, however more innovative progress has been restricted by seemingly simple obstacles. For instance, the surfaces of plates and mirrors with microstructures that are needed in these optical systems can be misshaped by stressed out surface coating products, degrading optics quality. This is specifically real for ultra-lightweight optical systems like area optics, where normal optical production techniques struggle to meet exacting shape requirements.
Silicon mirrors with stress correction patterns engraved into a thermal oxide layer. Credit: Youwei Yao
Now, MIT researchers Youwei Yao, Ralf Heilmann, and Mark Schattenburg of the Space Nanotechnology Laboratory (SNL) within MITs Kavli Institute for Astrophysics and Space Research, as well as recent graduate Brandon Chalifoux PhD 19, have actually developed brand-new approaches to work past this barrier.
In a paper released in the April 20, 2022, concern of the journal Optica, Yao, a research researcher and the papers lead author, explains their new approach to reshaping thin plate materials in such a way that eliminates distortion and enables scientists to flex surface areas more arbitrarily into the exact and complex shapes they might require. Thin plate shaping is typically used for high-level, complicated systems, like deformable mirrors or wafer-flattening processes during semiconductor production, however this innovation means future production will be more exact, scalable, and cheap. Yao and the rest of the group picture that these thinner and more quickly deformable surfaces can be useful in more comprehensive applications, like enhanced truth headsets and larger telescopes that can be sent into space at lower expense. “Using stress to deform optical or semiconductor surface areas is not brand-new, however by using contemporary lithographic technology, we can overcome a number of the obstacles of existing methods,” says Yao
The groups work constructs on the research of Brandon Chalifoux, who is now an assistant professor at the University of Arizona. Chalifoux dealt with the team on earlier papers to develop a mathematical formalism to link surface stress states with deformations of thin plates, as part of his doctorate in mechanical engineering.
Determined topography of a silicon wafer, showing surface area distortion prior to and after 2D stress correction. Wafer flatness was enhanced by over an aspect of 20. Wafer distortion can be issue in sophisticated semiconductor manufacturing, triggering pattern overlay errors and decreasing yields. Credit: Youwei Yao.
In this brand-new method, Yao has established an unique arrangement of tension patterns for precisely managing basic tension. Selectively treating the movie coating in different areas controls where tension and tension is used across the surface.
” Youre not adding stress to make a shape, youre selectively removing tension in specific instructions with thoroughly created geometric structures, like lines or dots,” says Schattenburg, senior research scientist and director of the Space Nanotechnology Laboratory. “Thats simply a certain way to give a target tension relief at a single location in the mirror which can then flex the product.”
An idea from correcting area mirrors
Because 2017, the SNL team has actually dealt with NASA Goddard Space Flight Center (GSFC) to develop a process to fix the shape distortion of X-ray telescope mirrors brought on by covering tension. The research stemmed from a job of structure X-ray mirrors for NASAs Lynx next-generation X-ray telescope objective concept, which requires tens of thousands of high-precision mirrors. Due to the task of focusing X-rays, the mirrors need to be extremely thin to gather X-rays efficiently. Mirrors lose tightness rapidly as they are thinned, becoming easily distorted by the stress from their reflective coatings– a nanometers-thick iridium layer covered on the front side for the purpose of reflecting X-rays.
Optical micrographs of a variety of surface tensor mesostructure cells, each 0.5 x 0.5 mm in size, producing a wide range of surface area tension states. Credit: Youwei Yao
” My group at GSFC has been making and coating thin X-ray mirrors because 2001,” states William Zhang, X-ray optics group leader at GSFC. “As the quality of X-ray mirrors has improved continuously in the last numerous years following technological advancements, distortion brought on by coverings has ended up being an increasingly major issue.” Yao and his team developed a lithographic tension patterning technique, effectively integrating several different methods, to attain exceptional removal of distortion when applied to X-ray mirrors made by the group.
“Unfortunately, the procedure established for GSFC can just precisely control a single type of surface area tension, the so-called equibiaxial, or rotationally uniform, stress,” says Chalifoux. To achieve approximate control of surface flexing requires control of all 3 terms in the so-called surface area tension tensor.”.
To accomplish full control of the stress tensor, Yao and his team further established the technology, eventually developing what they call stress tensor mesostructures (STMs), which are quasi-periodic cells arrayed on the back surface area of thin substrates, made up of gratings superimposed on stressed coverings. “By turning the gratings orientation in each system cell and altering the area portion of chosen locations, all three parts of the stress tensor field can be controlled simultaneously with a basic pattern procedure,” explains Yao.
The group invested more than two years developing this concept. By combining the laboratorys years of experience in surface area metrology and microfabrication with graduate-student-developed thin plate modeling and optimization tools, we were able to show a general substrate shape control method that is not limited to only bowl-like surface bending.”.
An appealing technique for many applications.
This technique allowed the team to imagine brand-new applications beyond the initial job of correcting coating-distorted X-ray mirrors. “When forming thin plates utilizing standard approaches, it is hard to be accurate because most of the methods create recurring or parasitic 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 involved with the work. “But the STM stress-bending approach is rather steady, which is particularly beneficial for optics-related applications.”.
Yao and his associates are also anticipating to manage stress tensors dynamically in the future. “Piezoelectric actuation of thin mirrors, which is utilized in adaptive optics technology, has been under development for several years, however many techniques can just control one element of the stress,” Yao explains. “If we can pattern STMs on thin, piezo-actuated plates, we would be able to extend these strategies 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 K. Heilmann and Mark L. Schattenburg, 14 April 2022, Optica.DOI: 10.1364/ OPTICA.445379.
This work was moneyed by NASA.
In this new technique, Yao has actually established a novel arrangement of stress patterns for precisely managing basic stress. Mirrors lose stiffness rapidly as they are thinned, ending up being quickly distorted by the stress from their reflective finishes– a nanometers-thick iridium layer coated on the front side for the function of reflecting X-rays.
“Unfortunately, the procedure developed for GSFC can just specifically control a single type of surface tension, the so-called equibiaxial, or rotationally consistent, tension,” says Chalifoux. To attain approximate control of surface area bending requires control of all 3 terms in the so-called surface area tension tensor.”.
“Piezoelectric actuation of thin mirrors, which is utilized in adaptive optics technology, has actually been under development for lots of years, but a lot of approaches can only manage one component of the stress,” Yao describes.
