State of the art in silicon immersed gratings for space - Aaldert van Amerongen, Hélène Krol, Catherine Grèzes-Besset, Tonny Coppens, Ianjit Bhatti,

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1 State of the art in silicon immersed gratings for space - Aaldert van Amerongen, Hélène Krol, Catherine Grèzes-Besset, Tonny Coppens, Ianjit Bhatti, Dan Lobb, Bram Hardenbol, Ruud Hoogeveen

2 Climate research by SWIR spectroscopy from space CO 2, CH 4 and H 2 O and HDO observable in m range SWIR 1 at 1.6 micrometer (CO 2, CH 4 ) SWIR 2 at 2.0 micrometer (CO 2, H 2 O) SWIR 3 at 2.3 micrometer (CH 4, H 2 O, HDO, CO) 2

3 Outline Why immersed gratings? How are these gratings made? TROPOMI immersed grating Optical coatings Developments for future missions 3

4 Why immersed gratings? Atmospheric science asks for medium to high resolution spectroscopy in the SWIR wavelength range Classical grating spectrometers become too large Solution: immersed grating

5 Volume reduction

6 Silicon n=3.4, n 3 =40 transmission spectrum high purity mono crystals available

7 V-groove etching <111> <100> potassium hydroxide: etching along <100> 100 times faster than <111> Electron micrograph of etched grating

8 TROPOMI SWIR spectrometer optical layout 8

9 Immersed grating design period = 2500 nm order = -6 blaze = 55 aoi = 60 area = mm 2 > 60% Pol. < 10% 9

10 Production Etch grooves in silicon Industry lithography technology High-end optical manufacturing technology Lean development strategy: concurrent development quick improvement cycles reduce waste

11 Manufacturing flow 11

12 TROPOMI Results realized remark profile perfect Efficiency curve as simulated Efficiency 65 % polarization 8 % ghosts none Stray light 10-5 Beyond PSF WFE 350 nm rms After focus correction 12

13 Efficiency Average = 65 % Polarization = 8 % 13

14 Stray light

15 Optical coating technology Dual Ion Beam Sputtering DIBS deposition technique Ion sputtering of target and substrate yields dense coating insensitive to temperature and atmospheric conditions High level of control by realtime in-situ visible and infrared optical monitoring Paper 100 Hélène Krol at al 15

16 All tests were passed successfully. Spectral measurements and the results of the qualification tests show the reliability of these multi-dielectric and metal-dielectric functions f Optical coating technology design and performance Solution: Multilayer dielectric coating 16

17 All tests were passed successfully. Spectral measurements and the results of the qualification tests show the reliability of these multi-dielectric and metal-dielectric functions f Optical coating technology design and performance 2 On the third facet R<1.5% for 2280nm to 2410nm at 0 to 30 AoI in the silicon prism medium Solution: Metallic-dielectric multilayer coating 17

18 Optical coating technology qualification for space Thermal: 20 cycles -80 C; +50 C at ambient pressure and under N 2 atmosphere Humidity: 48 hours; 40 C and 95% humidity Adhesion: ISO , test 02, level 02 Abrasion: ISO , test 01, level 01 Protons: 40 MeV, and a flux of cm -2 Gamma: 60 krads total radiation dose All tests were passed successfully 18

19 Ongoing developments for future climate missions Needs for the future 1.6 m, 2.0 m and 2.3 m >60%; polarization < 10% Low stray light and low WFE Wafer-to-prism bonding Faster, cheaper, better process Blazed gratings Increased application range High line-density First-order gratings for high efficiency and low stray light Arbitrary blaze angle prototype realized 200 nm lines and spaces prototype realized 19

20 Lean manufacturing flow Create prism Bond grating to prism 20

21 METIS: A Mid-infrared E-ELT Imager and Spectrograph METIS the third planned instrument for the E-ELT contains: Thermal infrared diffractionlimited imager integral field unit spectrograph ( m) with a resolution of SRON leads consortium that manufactures a Demonstrator IG for METIS 140 mm grating Diffraction limited performance

22 Conclusion We are the suppliers for silicon IGs for space We have a reliable and efficient manufacturing flow for IG's TROPOMI gratings are at TRL 8: fully qualified Ongoing grating developments for future missions e.g. Sentinel 5 planned for TRL 5 early

23 Lithography Polish silicon disk to high flatness and smoothness Deposit silicon nitride masking layer Spin on photoresist Pattern photoresist using UV lithography Plasma etch silicon nitride Remove photoresist, anisotropic etch of silicon in KOH Remove silicon nitride mask in HF

24 Mounting 24