MIT researchers establish a low-priced, high-precision fabrication approach for thin mirrors and silicon wafers.
An unique photolithography method might be a production game-changer for optical applications.
Technologies that depend upon light-weight, high-precision optical systems, like area telescopes, X-ray mirrors, and show panels, have actually established substantially over the previous numerous years, however advanced development has actually been restricted by relatively easy challenges. For example, the surface areas of mirrors and plates with microstructures that are needed in these optical systems can be misshaped by stressed out surface area covering products, degrading optics quality. This is particularly real for ultra-lightweight optical systems like area optics, where common optical production approaches have a hard time to fulfill exacting shape requirements.
Silicon mirrors with tension correction patterns engraved into a thermal oxide layer. Credit: Youwei Yao
Now, < period class ="glossaryLink" aria-describedby ="tt" data-cmtooltip ="<div class=glossaryItemTitle>MIT</div><div class=glossaryItemBody>MIT is an acronym for the Massachusetts Institute of Technology. It is a prestigious private research university in Cambridge, Massachusetts that was founded in 1861. It is organized into five Schools: architecture and planning; engineering; humanities, arts, and social sciences; management; and science. MIT's impact includes many scientific breakthroughs and technological advances.</div>" data-gt-translate-attributes= "[{"attribute":"data-cmtooltip", "format":"html"}]" > MIT scientistsYouweiYao,RalfHeilmann, andMarkSchattenburg of theSpaceNanotechnologyLaboratory( SNL) within MIT’sKavliInstitute forAstrophysics andSpaceResearch, along with current graduateBrandonChalifoux PhD’19, have actually created brand-new approaches to work past this barrier.
In a paper released in theApril20,2022, concern of the journalOptica, Yao, a research study researcher and the paper’s lead author, discusses their brand-new technique to improving thin plate products in such a way that removes distortion and allows scientists to flex surface areas more arbitrarily into the accurate and intricate shapes they may require.Thin plate shaping is normally utilized for top-level, intricate systems, like deformable mirrors or wafer-flattening procedures throughout semiconductor production, however this development indicates future production will be more accurate, scalable, and low-cost.Yao and the rest of the group envision that these thinner and more quickly deformable surface areas can be beneficial in more comprehensive applications, like increased truth headsets and bigger telescopes that can be sent out into area at lower expense. “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,” states Yao.
The group’s work constructs on the research study of Brandon Chalifoux, who is now an assistant teacher at the University ofArizona Chalifoux dealt with the group on earlier documents to establish a mathematical formalism to link surface area tension states with contortions of thin plates, as part of his doctorate in mechanical engineering.
Measured topography of a silicon wafer, revealing surface area distortion prior to and after 2D tension correction. Wafer flatness was enhanced by over an aspect of20 Wafer distortion can be issue in innovative semiconductor production, triggering pattern overlay mistakes and reducing yields. Credit: Youwei Yao
In this brand-new technique, Yao has actually established an unique plan of tension patterns for specifically managing basic tension. Substrates for optical surface areas are very first covered on the behind with thin layers of high-stress movie, made from products like silicon dioxide. Novel tension patterns are lithographically printed into the movie so that scientists can alter the homes of the product in particular locations. Selectively dealing with the movie covering in various locations manages where tension and stress is used throughout the surface area. And due to the fact that the optical surface area and the covering are adhered together, controling the covering product likewise improves the optical surface area appropriately.
“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,” states Schattenburg, senior research study researcher and director of the Space NanotechnologyLaboratory “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 fixing area mirrors
Since 2017, the SNL group has actually dealt with < period class ="glossaryLink" aria-describedby ="tt" data-cmtooltip ="<div class=glossaryItemTitle>NASA</div><div class=glossaryItemBody>Established in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States Federal Government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program, as well as aeronautics and aerospace research. It's vision is "To discover and expand knowledge for the benefit of humanity."</div>" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" > NASAGoddardSpace(**************************************************************************************************************************************************************************************************************** )(********************************************************************************************************************************************************************************************************************************* )( GSFC )to establish a procedure to fix the shape distortion of X-ray telescope mirrors triggered by covering tension.(*********************************************************************************************************************************************** )research study stemmed from a job of structure X-ray mirrors for NASA’sLynx next-generation X-ray telescope objective idea, which needs 10s of countless high-precision mirrors.Due to the job of focusing X-rays, the mirrors should be extremely thin to collect X-rays effectively.However, mirrors lose tightness quickly as they are thinned, ending up being quickly misshaped by the tension from their reflective coverings– a nanometers-thick iridium layer covered on the front side for the function of showing X-rays.
Optical micrographs of a range of surface area tensor mesostructure cells, each 0.5 x 0.5 mm in size, producing a wide variety of surface area tension states. Credit: Youwei Yao
“My team at GSFC has been making and coating thin X-ray mirrors since 2001,” states William Zhang, X-ray optics group leader 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 group established a lithographic tension pattern approach, effectively integrating numerous various strategies, to accomplish outstanding elimination of distortion when used to X-ray mirrors made by the group.
After this preliminary success, the group chose to extend the procedure to more basic applications, such as free-form shaping of mirrors and thin substrates, however they satisfied a significant barrier. “Unfortunately, the process developed for GSFC can only precisely control a single type of surface stress, the so-called ‘equibiaxial,’ or rotationally uniform, stress,” statesChalifoux “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 accomplish complete control of the tension tensor, Yao and his group even more established the innovation, ultimately creating what they call tension tensor mesostructures (STMs), which are quasi-periodic cells arrayed on the back surface area of thin substrates, made up of gratings superimposed on stressed out coverings. “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,” discusses Yao.
The group invested more than 2 years establishing this idea. “We encountered a series of difficulties in the process,” Schattenburg states. “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.”
An appealing method for numerous applications
This technique allowed the group to envision brand-new applications beyond the preliminary job of fixing 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,” states Jian Cao, a teacher of mechanical engineering at < period class ="glossaryLink" aria-describedby ="tt" data-cmtooltip ="<div class=glossaryItemTitle>Northwestern University</div><div class=glossaryItemBody>Established in 1851, Northwestern University (NU) is a private research university based in Evanston, Illinois, United States. Northwestern is known for its McCormick School of Engineering and Applied Science, Kellogg School of Management, Feinberg School of Medicine, Pritzker School of Law, Bienen School of Music, and Medill School of Journalism. </div>" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" >NorthwesternUniversity , who was not included with the work.“But the STM stress-bending method is quite stable, which is especially useful for optics-related applications.”
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Yao and his coworkers are likewise anticipating to manage tension tensors dynamically in the future.“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 discusses.“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” byYouweiYao,BrandonChalifoux, Ralf K.Heilmann andMark L. Schattenburg,14April2022,Optica
DOI:101364/ OPTICA.445379
This work was moneyed by NASA.
