Thin Mirrors and Silicon Wafer Plates

Low-Cost, High-Precision Fabrication Method for Thin Mirrors and Silicon Wafers Technologies based on lightweight, high-precision optical systems such as space telescopes, X-ray mirrors and display panels have advanced significantly over the past several decades, but further progress has been limited by seemingly simple challenges. For example, the surfaces of micro-structured mirrors and wafers required in these optical systems can be damaged by tensioned surface coating materials, compromising optical quality. This is particularly true for ultra-lightweight optical systems such as space optics, where traditional optical manufacturing methods struggle to meet demanding shape requirements. Now, researchers from MIT's Space Nanotechnology Laboratory (SNL) within the Kavli Institute for Astrophysics and Space Research—MIT researchers Youwei Yao, Ralf Heilmann and Mark Schattenburg, and Brandon Chalifoux, who graduated from the doctoral program in 2019—have developed new methods to overcome this barrier. Yao, a research scientist and lead author of the paper, describes in an article published in the 20 April issue of Optica the team's new approaches to reshaping thin-plate materials in a way that eliminates distortion and enables researchers to bend surfaces into precise and complex shapes as needed more arbitrarily. Thin-plate shaping is typically used for high-level, complex systems such as mirrors that can deform during semiconductor manufacturing or wafer-flattening processes, but this innovation means that future manufacturing will be more precise, scalable and inexpensive. Yao and the rest of the team envision that these thinner and more easily deformable surfaces could be useful in broader applications such as augmented reality headsets and larger telescopes that can be sent to space at lower cost. "Using stress to deform optical or semiconductor surfaces is not new, but by applying modern lithographic technology, we can overcome many of the difficulties of existing methods," said Yao. The team's work builds on research by Brandon Chalifoux, now an assistant professor at the University of Arizona. As part of his doctoral research in mechanical engineering, Chalifoux worked with the team in earlier papers to develop a mathematical formalism for relating surface tension conditions to the deformations of thin plates. In this new approach, Yao developed a new stress pattern arrangement to fully control overall stress. For optical surfaces, substrate layers are first coated with thin high-stress film layers made of materials such as silicon dioxide on the back side. New stress patterns were lithographically printed onto the film to allow researchers to vary the material properties in specific areas. Selective processing of the film coating in different areas controls where stress and strain are applied across the surface. Since the optical surface and coating are bonded together, manipulation of the coating material accordingly reshapes the optical surface. Schattenburg, senior research scientist and director of the Space Nanotechnology Laboratory, said, "You are not adding stress to create a shape; rather, you are selectively removing stress in specific directions using carefully designed geometric structures such as dots or lines." "This is a specific way to reduce the target strain at a single location on the mirror and then bend the material accordingly." An Idea from Correcting Space Mirrors Since 2017, the SNL team has worked with NASA's Goddard Space Flight Center (GSFC) to develop a process to correct shape distortion of X-ray telescope mirrors caused by coating stress. The research stemmed from the NASA Lynx next-generation X-ray telescope mission concept project, which requires tens of thousands of high-precision mirrors. Due to the task of focusing X-rays, mirrors must be very thin to efficiently collect X-rays. However, as mirrors become thinner, they rapidly lose stiffness and are easily distorted by stress from reflective coatings—a nanometer-thick iridium layer coated on the front surface to reflect X-rays. "My team at GSFC has been making and coating thin X-ray mirrors since 2001," said William Zhang, head of the X-ray optics group at GSFC. "With technological advances and the continuous improvement in the quality of X-ray mirrors over the past several decades, distortion caused by coatings has become an increasingly serious problem." Yao and the team developed a lithographic stress patterning method that successfully combined several different techniques to ensure that distortion was perfectly corrected when applied to X-ray mirrors made by the group. Following this initial success, the team decided to expand the process to more general applications such as free-form shaping of mirrors and thin substrates, but they encountered a major obstacle. "Unfortunately, the process developed for GSFC can fully control only a single type of surface stress, called 'co-axial' or rotationally uniform stress," said Chalifoux. "Equibiaxial stress conditions cannot correct potato-chip or saddle-shaped distortions; they can only produce local bowl-like bending of the surface. Achieving arbitrary control of surface bending requires control of all three terms called the 'surface stress tensor.' Yao and the team further developed the technology to achieve full control of the stress tensor and ultimately invented what they call stress tensor mesostructures (STMs). These consist of quasi-periodic cells arranged on the back surface of thin substrates, composed of grids superimposed on stressed coatings. "By rotating the direction of the grid in each unit cell and varying the area fraction of selected regions, all three components of the stress tensor field can be controlled simultaneously through a simple modeling process," Yao explained. The team spent more than two years developing this concept. "We encountered a series of challenges in the process," said Schattenburg. "Free-form shaping of silicon wafers to nanometer precision requires a synergy of metrology, mechanics and manufacturing. By combining the laboratory's decades of experience in surface metrology and microfabrication with thin-plate modeling and optimization tools developed by graduate students, we were able to demonstrate a general substrate shape control method that is not limited to bowl-like surface bending alone." A Promising Technique for Many Applications This approach has enabled the team to envision new applications beyond the initial task of correcting distorted coated X-ray mirrors. Jian Cao, a mechanical engineering professor at Northwestern University who was involved in the work, said, "When creating thin plates using traditional methods, most of the methods produce parasitic or residual stresses that lead to secondary distortion and post-process springback, making it difficult to be precise. "However, the STM stress bending method is quite stable, which is particularly useful for optics-related applications." Yao and colleagues also hope to dynamically control stress tensors in the future. "Piezoelectric actuation of thin mirrors used in adaptive optics technology has been in development for many years, but most methods can control only one component of stress," Yao explained. "If we can model STMs on thin, piezo-actuated plates, we could extend these techniques beyond optics to interesting applications such as work on microelectronics and soft robotics." This work was funded by NASA. Source: https://news.mit.edu/2022/mit-scientists-develop-low-costhigh-precision-fabrication-method-thin-mirrors-and-silicon-0420