Presentation #301.06 in the session Addressing Manufacturing Challenges for Future X-ray Missions.
Due to the preferable grazing-incidence mirror geometry of focusing optics for x-ray telescopes, a large telescope aperture must be filled with many (hundreds to tens of thousands) similar optical elements. The total surface area of these optics is roughly 1/sin(graze angle) ≈ 25-250 times larger than the telescope aperture, with typical graze angles in the range of 0.2 – 2.0 degrees. The same situation applies to x-ray reflection gratings. In contrast, x-ray transmission gratings such as critical-angle transmission (CAT) gratings, operate at near-normal incidence; therefore, the area that needs to be fabricated is the same as the desired coverage of the telescope aperture. However, even with relatively large CAT gratings (≈ tens of cm2) it can take on the order of 1,000 gratings to fill a square-meter aperture.
The Chandra High Energy Transmission Grating Spectrometer has 336 normal-incidence gold transmission gratings of 2.5 cm2 each. Each grating was made individually from start to finish, involving countless manual steps. In today’s much more cost- and schedule-constrained environment a new approach is required to manufacture the many hundreds of identical CAT gratings that would be needed for powerful instruments on future x-ray missions on scales ranging from Explorers (Arcus) to Probes and (Lynx-like) Great Observatories.
To this end, we have teamed with the MIT Lincoln Laboratory foundry to perform the crucial grating line and support structure patterning (4X optical projection lithography) and production of the high aspect-ratio deep-etch oxide mask continuously over the surface of whole 200-mm silicon-on-insulator (SOI) wafers in batch mode. This leads to considerable savings in labor and time and provides consistent results. Depending on size, up to two dozen gratings could be made from a single wafer. Alternatively, the size of individual gratings could be increased tenfold. Tooling for 200-mm wafers is increasingly available on campus at MIT.nano for downstream processing steps.
Unlike the “beaker and tweezer” approach used for the Chandra gratings, higher automation with industrial 200-mm wafer tools lends itself better to modeling of the process flow based on queuing theory. Such modeling could be used for key decisions in advance of production, and produce meaningful probability distributions for cost, schedule, and risk.