Making gratings more compact
GoalTo develop an immersed grating in silicon for the Mid-infrared E-ELT Imager and Spectrograph, METIS.
The technology is based on the combination of advanced lithography and etching to create the grating grooves and high-end optical manufacturing techniques for polishing, bonding and fusing.The grating is manufactured via etching into a Si-wafer. The grating is merged with molecular bonding wih and Si-prism. Bonding is followed by a thermal fuse step that strengthens the bond. In the end the performance of the wafer is tested.
We develop an immersed grating in silicon for the Mid-infrared E-ELT Imager and Spectrograph, METIS. Our grating meets the required diffraction-limited performance at a resolution of 100000 for the L and M spectral bands. Compared to a conventional grating, the immersed grating drastically reduces the beam diameter and thereby the size of the spectrometer optics. As diffraction takes place inside the high-index medium, the optical path difference and angular dispersion are boosted proportionally to the index of refraction, thereby allowing a smaller grating area and a smaller spectrometer size. The METIS immersed grating is lithographically produced on a 150 mm industry standard for wafers and replaces a classical 400 mm echelle. Our approach provides both a feasible path for the production of a grating with high efficiency and low stray light and improves the feasibility of the surrounding spectrometer optics.
METIS contains a diffraction-limited imager for the thermal-infrared wavelengths and a high-resolution spectrograph for the band 2.9 mm - 5.3 mm. A spectacular example of science with METIS is the spectroscopy of exo-planet atmospheres, expanding the applicability of recently developed methods [Snellen et al. Nature 465, 2010]. The spectrograph resolution of 100000, combined with the diffraction-limited performance results in optical components with demanding specifications. The baseline design contains a 400 mm wide classical echelle grating and a three-mirror anastigmat collimator/camera assembly with similar dimensions. Grating and mirrors shall have wave-front errors below 100 nm and 15 nm rms respectively. We have made an alternative spectrometer design based on an immersed grating (IG) of only 140 mm width. Our technology is based on the combination of advanced lithography and etching to create the grating grooves and high-end optical manufacturing techniques for polishing, bonding and fusing. We build on IG technology that was pioneered by the group of Dan Jaffe in the USA and that we have further developed in the Netherlands for the TROPOMI space spectrometer. Furthermore we make use of knowledge of lithography and process technology as well as optical bonding and fusing techniques that are available at Philips Innovation Services. This was possible since Philips has turned their research facilities into an open campus in line with the "open innovation" paradigm. Our IG-based spectrometer design has a four times smaller volume as compared to the classical solution with equal optical performance. Our grating is manufactured via etching into the Si with the surface cut along the <100> crystal plane. The grooves thus will have a natural blaze angle close to 55 degrees. The angle of incidence on the grating will be close to this blaze angle. The grating is integrated with a Si prism. The prism is ordered from an optics supplier, while the grating is produced on an industry-standard Silicon wafer. Wafer and prism are then merged with molecular bonding. Bonding is followed by a thermal fuse step that strengthens the bond. The bond and fuse process is specially developed for METIS. The IG principle is depicted in Figure 1; the light enters (black arrows) and exits (red and blue arrows) the prism through the same facet.
Figure 1 Sketch of immersed grating principle: light enters the prism through a polished entrance surface (black arrows); after diffraction at the grating surface the dispersed light leaves the prism through the same surface (red and blue arrows).
Figure 2 SEM image of a cross-section of a sample immersed grating after completion of the anisotropic etching and nitride removal. The black shows the silicon cross section, while the gray shows the grooves outer surfaces. The high level of control over the groove shape, the perfect blaze and the smooth groove surfaces are striking.
Figure 2 shows an electron micrograph of a cross-section of a sample immersed grating after completion of the etching. The high level of control over the groove shape, the perfect blaze and the smooth groove surfaces are striking. We have simulated the grating performance and find peak efficiency above 80% for all orders in the operational waveband. Some parameters of the METIS immersed grating are listed in Table 1.
Table 1 grating parameters
|Operational wavelength band||2.9 µm - 5.3 µm|
|Diffraction orders||20 - 42|
|Efficiency in diffraction peak||>80%|
|Resolution||> 100000 @ 4.65 µm|
The assembly of prism and grating wafer is illustrated in Figure 3. The groove profile is illustrated with a zoom in. The grating period is 20 micrometer.
Figure 3 Prism and wafer prior to bonding. The groove profile is illustrated with a zoom in. The grating period is 20 micrometer.
Figure 4 shows the optical layouts of the classical and IG-based spectrometer designs on the same scale. The pre-disperser configuration (not shown on the images) is identical for both designs. The optical system of the two designs is based on the same double pass three-mirror astigmatic (TMA) configuration, but the optical characteristics of the relay mirror, the TMA components and separations between the mirrors were re-optimized in order to minimize WFE on the detector. In the classical design the first TMA mirror (MM1) and the last mirror before the detector (MM4) are separated, in the case of the IG design they are merged (as in a traditional TMA). It is clearly visible that the IG based solution requires a significantly smaller space envelope.
Figure 4 Optical layout of the main dispersers of the CL and IG solutions on the same scale. It is clearly visible that the space envelope needed for the latter is significantly less.
After completion of a first preliminary design phase and a second phase where we developed critical technology and made a detailed design we are now in the third and last project phase where we build and test a real-size demonstrator model immersed grating designed to meet all performance requirements.
The technology of etching silicon gratings of the current size and of bonding a prism to a wafer followed by thermal fusion were developed in this project. Immersed gratings based on these technological steps have been proposed for various planned space missions for climate and air-quality research.
SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, the Netherlands; NOVA Optical Infrared Instrumentation Group at ASTRON, P.O. Box 2, 7990 AA Dwingeloo, the Netherlands;
TNO - Technical Sciences, Stieltjesweg 1, 2628 CK, Delft, the Netherlands;
Philips Innovation Services, High Tech Campus 4, 5656 AE Eindhoven, the Netherlands;
Piece of material with a regular line structure, by which the incoming light will be refracted.