A laser micromachining technique for removing x-ray mirror stress

The advanced space telescope currently envisioned by NASA and its collaborators requires advanced mirrors to operate. However, manufacturing these high-resolution thin shell coated mirrors remains challenging. Thin mirrors are more difficult to produce accurate shapes than thick mirrors. In order to achieve the required reflectivity, the thin film coating on the mirror surface often has an internal stress related to it, which increases the deformation.

The research team combined femtosecond laser micromachining with a previously developed digital correction technology called stress based. Source: Heng Zuo/Kavli Institute of Astrophysics and Space Studies, MIT.

A project of the Massachusetts Institute of Technology's Kavli Institute of Astrophysics and Space Research has now developed a laser micromachining technology that can help solve this problem by selectively removing the area of the pressed film and modifying the stress of the entire mirror. As the article published in Optica said, this technology should be able to provide high-throughput thin shell mirror calibration for space-based telescopes, especially for X-ray missions such as the Lynx X-ray measuring instrument currently planned to be launched by NASA in 2036.

Heng Zuo of the Massachusetts Institute of Technology said: "It is difficult to make ultra-thin mirrors with precise shapes because the manufacturing process often causes severe bending of thin materials. In addition, the mirrors of telescopes usually have coatings to increase reflectivity, and these coatings usually further deform the mirrors. Our current technology can solve these two challenges."

The project builds on previous research at MIT. In April 2022, MIT described a mathematical method by which standard semiconductor manufacturing methods can be used to create stress tensor intermediate structures on coating structures, affecting the stresses experienced on these surfaces after coating application.

"Femtosecond lasers can create extremely accurate holes, channels and marks with very little collateral damage. Compared with traditional methods, the high repetition rate of these lasers allows faster processing speed and output. This can help speed up the manufacturing of a large number of ultra-thin mirrors needed for the next generation of X-ray telescopes."

In the experiment of patterning the thermal oxide layer used for planar silicon mirrors, the femtosecond processing using a 515 nm source was used for two different structural changes: patterning removal of uniformly distributed holes and fine spacing removal of periodic wave troughs.

Zuo said: "The results show that the patterned removal of periodic holes leads to an equiaxed bowl like stress state, while the directional removal of the fine spacing of periodic wave troughs leads to a non equiaxed potato like stress component. Combining these two characteristics with the direction of the appropriate rotating groove, we can create various stress states, which can be used to correct any type of errors in the mirror in principle."

Source: OFweek