Interest in micro-optical components for applications ranging from telecommunications to life sciences has driven
the need for accessible, low-cost fabrication techniques. Most micro-lens fabrication processes are unsuitable for
applications requiring 100% fill factor, diameters around 1 mm, and scalability to large areas with millions of
lenses. We report on a ﬂexible, low-cost mold fabrication technique that utilizes a combination of milling and
microforging. The technique involves first performing a rough cut with a ball-end mill. Final shape and sag
height are then achieved by pressing a sphere of equal diameter into the milled divot. Using this process, we
have fabricated molds for rectangular arrays of 1-10,000 lenses with apertures of 0.25-1.6 mm, sag heights of
3-130 &mgr;m, inter-lens spacings of 0.25-2 mm, and fill factors of 0-100%. Mold profiles have roughness and ﬁgure
error of 68 nm and 354 nm, respectively, for 100% fill factor, 1 mm aperture lenses. The required forging force
was modeled as a modiﬁed open-die forging process and experimentally verified to increase nearly linearly with
surface area. The process is easily adapted to lenticular arrays. Limitations include milling machine range and accuracy.
The Reflection Grating Spectrometer (RGS) on Constellation-X is designed to supply astronomers with high spectral resolution in the soft x-ray band from 0.25 to 2 keV. High resolution, large collecting area and low mass at grazing incidence require very flat and thin grating substrates, or thin-foil optics.
Thin foils typically have a diameter-to-thickness ratio of 200 or higher and as a result very low stiffness. This poses a number of technological challenges in the areas of shaping, handling, positioning, and mounting of such optics. The most minute forces (gravity sag, friction, thermal mismatch with optic mount, etc.) can lead to intolerable deformations and limit figure metrology repeatability. We present results of our efforts in the manipulation and metrology of suitable grating substrates, utilizing a novel low-stress foil holder with friction-reducing flexures.
A large number of reflection gratings is needed to achieve the required collecting area. We have employed nanoimprint lithography (NIL) - which uses imprint films as thin as 100 nm or less - for the high-fidelity and low-stress replication from 100 mm diameter saw-tooth grating masters.
The Reflection Grating Spectrometer (RGS) on Constellation-X will require thousands of large gratings with very exacting tolerances. Two types of grating geometries have been proposed. In-plane gratings have low ruling densities (~500 l/mm) and very tight flatness and assembly tolerances. Off-plane gratings require much higher ruling densities (~5000 l/mm), but have somewhat relaxed flatness and assembly tolerances and offer the potential of higher resolution and efficiency. The trade-offs between these designs are complex and are currently being studied. To help address critical issues of manufacturability we are developing a number of novel technologies for shaping, assembling, and patterning large-area reflection gratings that are amenable to low-cost manufacturing. In particular, we report results of improved methods for patterning the sawtooth grating lines that are required for efficient blazing, including the use of anisotropic etching of specially-cut silicon wafers to pattern atomically smooth grating facets. We also report on the results of using nanoimprint lithography as a potential means for replicating sawtooth grating masters. Our Nanoruler scanning beam interference lithography tool allows us to pattern large area gratings up to 300 mm in diameter. We also report on developments in grating assembly technology utilizing lithographically patterned and micromachined silicon metrology structures ("microcombs") that have achieved submicron assembly repeatability.
We provide an overview of the Constellation-X SXT development program. We describe the performance requirements and goals, and the status of the technology development program. The SXT has a 1.6-meter diameter, a 10-meter focal length, and is to have an angular resolution exceeding 15 arc seconds. It has a modular design, incorporting lightweight, multiply nested, segmented Wolter Type I x-ray mirrors. All aspects of the design lend themselves to mass-production. The reflecting surfaces are produced by epoxy replication off precision mandrels onto glass substrates that have been accurately formed by thermal slumping. Coalignment of groups of relfectors to the required sub-micron accuracy is assisted by precison silicon micorstructures. Optical alignment is performed using the Centroid Detector Assembly originally developed for aligning the Chandra mirror. Recent efforts have concentrated on the producotin of an Engineering Unit, incorporating the components for the first time into a flight-like configuration. We summarize the status of the development of the processes for the key components and the initial metrology results of the Engineering Unit.
The proposed Reflection Grating Spectrometer (RGS) on the Constellation-X mission is designed to provide high-resolution x-ray spectroscopy of astrophysical sources. Two types of reflection grating geometries have been proposed for the RGS. In-plane gratings have relatively low-density rulings (~500 lines/mm) with lines perpendicular to the plane of incidence, thus dispersing x-rays into the plane. This geometry is similar to the reflection grating spectrometer flown on the X-ray Multi-Mirror (XMM) mission. Off-plane, or conical, gratings require much higher density rulings (>5000 lines/mm) with lines parallel to the plane of incidence, thus dispersing x-rays perpendicular to the plane. Both types present unique challenges and advantages and are under intensive development. In both cases, however, grating flatness and assembly tolerances are driven by the mission's high spectral resolution goals and the relatively poor resolution of the Wolter foil optics of the Spectroscopy X-ray Telescope (SXT) that is used in conjunction with the RGS. In general, to achieve high spectral resolution, both geometries require lightweight grating substrates with arcsecond flatness and assembly tolerances. This implies sub-micron accuracy and precision which go well beyond that achieved with previous foil optic systems. Here we present a progress report of technology development for the precision shaping, assembly and metrology of the thin, flat grating substrates.