Silicon Semiconductor Wafer

MIT scientists effect a low-attach, excessive-precision fabrication system for skinny mirrors and silicon wafers.

A peculiar photolithography system in total is a producing sport-changer for optical capabilities.

Technologies that depend on lightweight, excessive-precision optical systems, relish space telescopes, X-ray mirrors, and say panels, beget developed considerably all the diagram in the course of the last plenty of a long time, but extra stepped forward development has been restricted by seemingly straightforward obstacles. As an example, the surfaces of mirrors and plates with microstructures which may well well maybe perhaps be a truly principal in these optical systems may well well maybe also be distorted by pressured ground coating materials, degrading optics quality. Right here is in particular appropriate for extremely-lightweight optical systems relish space optics, the put same old optical production solutions fight to meet exacting form necessities.

Silicon Mirrors With Stress Correction Patterns

Silicon mirrors with stress correction patterns etched right into a thermal oxide layer. Credit: Youwei Yao

Now, MIT researchers Youwei Yao, Ralf Heilmann, and Designate Schattenburg of the Spot Nanotechnology Laboratory (SNL) inner MIT’s Kavli Institute for Astrophysics and Spot Be taught, to boot to most up-to-date graduate Brandon Chalifoux PhD ’19, beget devised contemporary how one can work previous this barrier.

In a paper printed in the April 20, 2022, challenge of the journal Optica, Yao, a learn scientist and the paper’s lead creator, explains their contemporary diagram to reshaping skinny plate materials in one diagram that eliminates distortion and enables researchers to bend surfaces extra arbitrarily into the suppose and intricate shapes they may well well maybe need. Thin plate shaping is on the entire used for excessive-stage, advanced systems, relish deformable mirrors or wafer-knocking down processes at some stage in semiconductor manufacturing, but this innovation system future production shall be extra right, scalable, and low-attach. Yao and the comfort of the crew agree with that these thinner and extra without speak deformable surfaces may well well maybe also be priceless in broader capabilities, relish augmented actuality headsets and better telescopes that may well well maybe also be despatched into space at decrease attach. “Using stress to deform optical or semiconductor surfaces is no longer contemporary, but by applying new lithographic abilities, we can overcome most of the challenges of existing solutions,” says Yao.

The crew’s work builds on the learn of Brandon Chalifoux, who’s now an assistant professor at the University of Arizona. Chalifoux labored with the crew on earlier papers to effect a mathematical formalism to glue ground stress states with deformations of thin plates, as share of his doctorate in mechanical engineering.

Silicon Wafer Measured Topography

Measured topography of a silicon wafer, exhibiting ground distortion earlier than and after 2D stress correction. Wafer flatness used to be improved by over a a part of 20. Wafer distortion may well well maybe also be arena in stepped forward semiconductor manufacturing, causing pattern overlay errors and decreasing yields. Credit: Youwei Yao

On this contemporary system, Yao has developed a unusual affiliation of stress patterns for precisely controlling frequent stress. Substrates for optical surfaces are first coated on the backside with skinny layers of excessive-stress movie, product of materials relish silicon dioxide. Sleek stress patterns are lithographically printed into the movie in instruct that researchers can substitute the properties of the subject materials in particular areas. Selectively treating the movie coating in varied areas controls the put stress and tension is applied across the bottom. And since the optical ground and the coating are adhered together, manipulating the coating arena materials also reshapes the optical ground accordingly.

“You’re no longer in conjunction with stress to manufacture a form, you’re selectively doing away with stress in particular directions with fastidiously designed geometric constructions, relish dots or lines,” says Schattenburg, senior learn scientist and director of the Spot Nanotechnology Laboratory. “That’s honest a undeniable diagram to present a target stress relief at a single put in the have which would perhaps then bend the subject materials.”

An belief from correcting space mirrors

Since 2017, the SNL crew has labored with ” knowledge-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>NASA Goddard Spot Flight Center (GSFC) to effect a activity to honest the form distortion of X-ray telescope mirrors precipitated by coating stress. The learn originated from a challenge of building X-ray mirrors for NASA’s Lynx subsequent-generation X-ray telescope mission belief, which requires tens of hundreds of excessive-precision mirrors. Because of the duty of focusing X-rays, the mirrors must be very skinny to bag X-rays efficiently. On the opposite hand, mirrors lose stiffness all without prolong as they are thinned, changing into without speak distorted by the stress from their reflective coatings — a nanometers-thick iridium layer coated on the front side for the map of reflecting X-rays.

Optical Micrographs Surface Tensor Mesostructure Cells

Optical micrographs of a diffusion of ground tensor mesostructure cells, every 0.5 x 0.5 mm in dimension, producing a huge differ of ground stress states. Credit: Youwei Yao

“My crew at GSFC has been making and coating skinny X-ray mirrors since 2001,” says William Zhang, X-ray optics neighborhood leader at GSFC. “As the usual of X-ray mirrors has improved persistently in the final plenty of a long time following technological trends, distortion precipitated by coatings has turn into a further and extra serious arena.” Yao and his crew developed a lithographic stress patterning system, efficiently combining plenty of various solutions, to carry out extra special elimination of distortion when applied to X-ray mirrors made by the neighborhood.

After this preliminary success, the crew decided to elongate the diagram in which to extra frequent capabilities, equivalent to free-make shaping of mirrors and skinny substrates, but they met a prime obstacle. “Sadly, the system developed for GSFC can most effective precisely relieve an eye on a single vogue of ground stress, the so-known as ‘equibiaxial,’ or rotationally uniform, stress,” says Chalifoux. “Equibiaxial stress states can most effective accomplish bowl-relish native bending of the bottom, which would perhaps no longer honest potato-chip or saddle form distortions. To accomplish arbitrary relieve an eye on of ground bending requires relieve an eye on of all three terms in the so-known as ‘ground stress tensor.’”

To accomplish fleshy relieve an eye on of the stress tensor, Yao and his crew extra developed the abilities, at final inventing what they call stress tensor mesostructures (STMs), which may well well maybe perhaps be quasi-periodic cells arrayed on the encourage ground of thin substrates, quiet of gratings superimposed on pressured coatings. “By rotating the grating’s orientation in every unit cell and altering the impart a part of selected areas, all three substances of the stress tensor self-discipline may well well maybe also be controlled at the same time as with a straightforward patterning activity,” explains Yao.

The crew spent bigger than two years setting up this belief. “We encountered a sequence of difficulties in the system,” Schattenburg says. “Free-make shaping of silicon wafers with nanometer precision requires a synergy of metrology, mechanics, and fabrication. By combining the lab’s a long time of abilities in ground metrology and microfabrication with graduate-student-developed skinny plate modeling and optimization tools, we were in a put to expose a frequent substrate form relieve an eye on system that’s no longer restricted to most effective bowl-relish ground bending.”

A promising system for many capabilities

This system enabled the crew to mediate contemporary capabilities previous the preliminary job of correcting coating-distorted X-ray mirrors. “When forming skinny plates the utilization of primitive solutions, it’s though-provoking to be right because most of the solutions generate parasitic or residual stresses which lead to secondary distortion and spring-encourage after processing,” says Jian Cao, a professor of mechanical engineering at Northwestern University, who used to be no longer enthusiastic with the work. “However the STM stress-bending system is rather stable, which is in particular priceless for optics-connected capabilities.”

Yao and his colleagues are also expecting to relieve an eye on stress tensors dynamically in the long term. “Piezoelectric actuation of thin mirrors, which is used in adaptive optics abilities, has been under development for decades, but most solutions can most effective relieve an eye on one ingredient of the stress,” Yao explains. “If we can pattern STMs on skinny, piezo-actuated plates, we would be in a put to elongate these solutions previous optics to inviting capabilities equivalent to actuation on microelectronics and gentle robotics.”

Reference: “Stress tensor mesostructures for deterministic figuring of thin substrates” by Youwei Yao, Brandon Chalifoux, Ralf K. Heilmann and Designate L. Schattenburg, 14 April 2022, Optica.
DOI: 10.1364/OPTICA.445379

This work used to be funded by NASA.

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