Low aberration monolithic diffraction gratings for high performance optical spectrometers

Transcription

1 Low aberration monolithic diffraction gratings for high performance optical spectrometers P. Triebel 1, T. Diehl 1, M. Burkhardt 2, L. Erdmann 2, A. Kalies 2,A. Pesch 2, A. Gatto 2 1 Carl Zeiss Spectroscopy GmbH, Carl-Zeiss-Promenade 10, D Jena, Germany 2 Carl Zeiss Jena GmbH, Carl-Zeiss-Promenade 10, D Jena, Germany

2 Agenda Motivation Technological Processes Process Simulation Results Summary and Conclusion

3 Agenda Motivation Technological Processes Process Simulation Results Summary and Conclusion 3

4 Earth observation Sentinel-4 project which is part of the Copernicus program of the ESA Sentinel-4 o Copernicus Geostationary Atmospheric Mission Mission Air Quality measurements Stratospheric Ozone monitoring Climate monitoring Main data products: O3, NO2, SO2, HCHO and aerosol optical depth Source ESA: Spatial resolution: 8 km Satellite: Meteosat Third Generation Sounder 4

5 Earth observation Sentinel-4 project which is part of the Copernicus program of the ESA Sentinel-4 UVN instrument o High resolution spectrometer for ultraviolett ( nm) visible ( nm) Near-infrared ( nm) Spectal resolution in the wavelength bands varies between 0.12 and 0.5 nm Source: S. T. Gulde, et. al, Proc. ICSO 2014, 5

6 Key requirements for diffraction gratings for space applications Major requirements: High efficiency in the defined wavelength range Tailored efficiency characteristics over the entire wavelength range Low polarization sensitivity Low straylight of the microstructure and the substrate Groove alignment and adapted substrate dimension Wavefront Aberration and imaging performance 6

7 Agenda Motivation Technological Processes Process Simulation Results Summary and Conclusion 7

8 The ZEISS competencies for designing and manufacturing diffraction gratings Optical Design Rigorous modeling Wave-optical engineering Customized Optics and Substrates Dicing Grinding & polishing Free Form Surfaces Patterning Holography Mechanical ruling Grey scale lithography Metrology and Testing AFM SEM Confocal microscopy Phase shift interferometer Optical characterization Thermal vacuum testing Certification 500 nm Custom Gratings Coatings Design Metallic coating Reflective coatings Anti-Reflective coatings Dry Etching Processes IBE RIBE Replication Several processes Free choice of substrate material 8

9 Simulation methods for designing diffraction gratings with different structure sizes Rigorous methods incl. effective media approach Rigorous Coupled Wave Analysis (RCWA) Rigorous Rigorous methods methods near near the resonance the resonance domain domain Integral Equation System Method with Parameterization (IESMP) Scalar diffraction theory Iterative Fourier Transform Method (IFTA) Air Fused silica Arbitary discretization Profile Optimization using already manufactured profiles Allow 2-D gratings (cross gratings, etc.) Efficiency calculations from AFM Scans Rigorous methods near the resonance domain [B.Kleemann et al., J. Mod. Opt. (1996)] Fast calculation Efficiency Calculation Echelle Gratings Finite Element Method (FEM) CGH Beam Shaper Micro lenses Deep IR gratings with lower periods 9

10 Optics Component Manufacturing for in-house production of various types of grating substrates Complete process chain from raw glass up to high end optical components The substrates and the exposure optics defining the resulting wavefront aberration of the grating to be produced Lowest wavefront error optics production due to various in-house manufacturing technologies Plane Optics Freeform Optics Aspheres 10

11 Holographic Exposure Setup of Interference Lithography for Symmetric and Blaze profiles comprising lowest wavefront aberrations Process metrology Process metrology Grating 2 punctal sources 2 diverging spherical waves Symmetric profiles into the photoresist 2 punctual sources Converging and diverging spherical waves leading to a blaze profile in the photoresist Blaze profiles into the photoresist 11

12 Diffraction gratings Technological process for holographic blazed type gratings air Holographic Recording Defines: Principal groove shape (blaze or symmetric) resist/substrate Development (Wet-Chemical) Defines: Actual groove shape Profile smoothness Reactive Ion Beam Etching Defines: Groove depth Blaze angle Replica Generation Defines: Groove depth Blaze angle using Mastering technology Master Grating Replica Grating ZEISS Fußzeile Gitterkolloquium

13 Agenda Motivation Technological Processes Process Simulation Results Summary and Conclusion 13

14 Diffraction gratings Technological process for holographic blazed type gratings air Holographic Recording Defines: Principal groove shape (blaze or symmetric) resist/substrate Development (Wet-Chemical) Defines: Actual groove shape Profile smoothness Reactive Ion Beam Etching Defines: Groove depth Blaze angle Replica Generation Defines: Groove depth Blaze angle using Mastering technology Master Grating Replica Grating ZEISS Fußzeile Gitterkolloquium

15 Diffraction gratings Technological process for holographic blazed type gratings air Holographic Recording Defines: Principal groove shape (blaze or symmetric) resist/substrate Development (Wet-Chemical) Defines: Actual groove shape Profile smoothness Reactive Ion Beam Etching Defines: Groove depth Blaze angle Replica Generation Defines: Groove depth Blaze angle using Mastering technology Master Grating Replica Grating ZEISS Fußzeile Gitterkolloquium

16 Etching Simulation for the Holography of a blazed Grating Symmetrical Profile to Blazed Profile Photoresist Ion beam etching Blazed Grating 110min with 500nm Sinus-Resist-Grating AOI Ion beam 42, 220 step simulation 16

17 Detailed Process Overview for Fabrication of blazed gratings using symmetrical profiles Simulation Process Strategy: - Photoresist Grating with adequate layer thickness - Development of a symmetrical grating - Ion etching process defines the final profile in the substrate Adapted Process for Micostructures >2µm depth 17

18 Agenda Motivation Technological Processes Process Simulation Results Summary and Conclusion 18

19 Example Measurement Results lamellar grating High efficiency, low Polarization sensitivity A transmission of around 80% to 90% without Fresnel-reflection on the back surface was measured. The AOI was 20. The polarization sensitivity below 5% for wavelengths smaller than 900nm and 10% for wavelengths between 900nm and 1000nm. 19

20 Example Wavefront Measurements Plane Grating for the used diffraction order Interferometric Wavefront measurement of a plane grating in transmission. An rms value of 3,5nm was measured. PV = 85 nm RMS= 3.5 nm 20

21 Agenda Motivation Technological Processes Process Simulation Results Summary and Conclusion 21

22 Summary and Conclusion The system performance of spectral imaging systems is influenced by the parameters of efficiency/transmission, polarization sensitivity and the wavefront aberrations of its optical components. An advanced manufacturing approach starting with the substrate manufacturing and the holographically recording in combination with ion beam plasma etching was presented in order to minimize the wavefront aberrations. Recent results showing wavefront aberrations 3.5nm rms. Results of an high efficient grating for the NIR range with low polarization sensitivity was shown. Recent results were also used for the manufacturing of Offner gratings, GRISM and imaging gratings. 22

23 Thank you for your attention 23

24