Infrared wire grid polarizers: metrology, modeling, and laser damage threshold Matthew George, Bin Wang, Jonathon Bergquist, Rumyana Petrova, Eric Gardner Moxtek Inc. Calcon 2013
Wire Grid Polarizer (WGP) Wire Grid Polarizer (WGP) Common optical component with applications in: o Optical isolation, beamsplitting, spectroscopy, imaging, and display array of metallic lines with sub-wavelength period transparent substrate form birefringence & diattenuation (dichroisim) anisotropic reflectivity Advantages of wire grid polarizers: broadband performance angle insensitive (+/- 20 ) o large angular aperture compact size (thin substrate) resilient to high flux, environmentally stable WGP schematic Existing WGP products for MWIR & LWIR are inadequate: Moxtek WGP addresses needs low contrast between passing & blocking transmittance (large wire grid period) Expensive, small clear apertures (exotic substrates) 200 mm diameter wafers 144 nm pitch
Aluminum nanowire polarizer technology High aspect ratio nanowires 200 mm wafer scale processing Standard Moxtek 144 nm pitch Silicon IR (SIR) WGP AR Coating Silicon wafer AR Coating Al nanowires on AR-coated Silicon 1 mm Highly regular Al nanowire grid on Silicon 300 nm
Silicon IR (SIR) WGP transmittance Instruments/Equipment: Cary 670 FTIR (Moxtek) Nexus 870 FTIR ESP (Ball Aerospace) Doubled SIR WGP as source pre-analyzer Rotation Stage Methods: Ratio measurements: empty compartment N 2 -purged reference baseline (100% T ) Add part, rotate to minimize signal (white light interferogram) at detector Measure blocking state transmittance (T s ) Measure passing state transmittance (T p ) Remove part, check for baseline drift wire grid polarizer pre-analyzer rotation mount Transmission Measurement Setup
T p [%] Moxtek Silicon IR (SIR) WGP performance 100 90 Transmittance comparison - Moxtek SIR product family SIR3-5 SIR7.5 SIR7-15, 45 AOI SIR7-15 SIR10.6 80 70 60 50 40 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Wavelength [mm] Product Wavelength (µm) Minimum T p Minimum Contrast Ratio SIR3-5 3.0-3.7 85% 5,000 (37 db) 3.7-5.0 94.5% 7,000 (38.5 db) SIR7-15 7.0-8.8 78% 7,000 (38.5 db) 8.8-15 68% 10,000 (40 db) 10.6 82% 10,000 (40 db) SIR7.5* 7.5 92% 7,000 (38.5 db) SIR10.6* 10.6 83% 12,600 (41 db) * developmental products
Varying angle transmittance Equipment: Cary 670 FTIR Doubled SIR WGP as pre-analyzer Rotation Stage Variable Angle Transmission Accessory o No compensating substrate to account for beam shift Top-down view T p, 45
T s [%] Contrast Ratio T p [%] Varying angle transmittance 100 Transmittance of SIR3-5 for varying Angle of Incidence 0.08 50,000 Contrast of SIR3-5 for varying Angle of Incidence 90 0.07 80 70 60 50 40 Tp, 0 AOI Tp, 12 AOI Tp, 20 AOI Tp, 30 AOI Tp, 45 AOI Ts, 0 AOI Ts, 12 AOI Ts, 20 AOI Ts, 30 AOI Ts, 45 AOI 0.06 0.05 0.04 0.03 0.02 0.01 10,000 Contrast, 0 AOI (Ts boxcar) Contrast, 12 AOI (Ts boxcar) Contrast, 20 AOI (Ts boxcar) Contrast,30 AOI (Ts boxcar) Contrast, 45 AOI (Ts boxcar) 30 0 2.5 3 3.5 4 4.5 5 5.5 Wavelength [mm] 1,000 2.5 3 3.5 4 4.5 5 5.5 Wavelength [mm] I o q i R 1 R 2 R 3 WGP q o q t 0.62 mm thick Silicon T down ARC layers T 1 T 2 T 3
T s [%] T p [%] Varying angle transmittance Transmittance of SIR7-15 for varying Angle of Incidence 100 0.05 90 0.045 80 0.04 70 0.035 60 Tp, 0 AOI Tp, 12 AOI 0.03 50 Tp, 20 AOI Tp, 30 AOI 0.025 40 Tp, 45 AOI Ts, 0 AOI Ts, 12 AOI Ts, 20 AOI 0.02 30 Ts, 30 AOI Ts, 45 AOI 0.015 20 0.01 10 0.005 0 0 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5 Wavelength [mm] Contrast of SIR7-15 for varying Angle of Incidence 20,000 10,000 Contrast Ratio Contrast, 0 AOI (Ts boxcar) Contrast, 12 AOI (Ts boxcar) Contrast, 20 AOI (Ts boxcar) Contrast,30 AOI (Ts boxcar) Contrast, 45 AOI (Ts boxcar) 1,000 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5 Wavelength [mm] I o q i R 1 R 2 R 3 WGP q o q t 0.62 mm thick Silicon T down ARC layers T 1 T 2 T 3
Reflectance Reflectance Reflectance measurements Equipment: Source pre-analyzer (Moxtek doubled SIR WGP) Rotation Stage Harrick Variable Angle Reflection Accessory o Harrick 12 Absolute VW Reflectance Sampling Stage rotation mount pre-analyzer (a) 100% Moxtek SIR3-5 WGP 12 Reflectance vs. modeling (b) 100% wire grid polarizer 90% 80% 70% 90% 80% 70% 60% 60% 50% 50% 40% 40% 30% 30% 20% 20% 12 vw stage Reflection Measurement Setup 10% 0% 3 4 5 6 7 8 Wavelength [μm] 10% 0%
Reflectance Reflectance Reflectance measurements Equipment: Source pre-analyzer (Moxtek doubled SIR WGP) Rotation Stage Harrick Variable Angle Reflection Accessory o Harrick 12 Absolute VW Reflectance Sampling Stage (a) rotation Moxtek SIR3-5 WGP 12 pre-analyzer Reflectance vs. modeling 100% mount 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% wire grid polarizer 3 4 5 6 7 8 Reflection Measurement Setup Wavelength [μm] 12 vw stage (b) 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Moxtek SIR7-15 WGP 12 Reflectance vs. modeling modeled Rs, 12 AOI measured Rs, 12 AOI measured Rp, 12 AOI modeled Rp, 12 AOI 3 4 5 6 7 8 9 10 11 12 13 14 15 Wavelength [μm]
Modeling for MWIR & LWIR WGP products Optical modeling parameters G-Solver rigorous coupled wave analysis (RCWA) software o Retain +/-12 orders (evanescent) for calculations Wavelengths varying from 1.5 to 10.0 or 17.5 microns Aluminum wire grid: o 144 nm pitch, 165nm tall, 40 nm wide (rectangular) Normal incidence, 12 and 45 angle of incidence IR constants and layer thickness for ARC materials from coating vendor IR optical constants for aluminum and silicon from JA Woollam: Refractive Index and absorption coefficient vs. wavelength 3.47 0.0014 n 3.46 3.45 3.44 3.43 n_ir_si_avg k_ir_si_avg 0.0012 0.0010 0.0008 0.0006 0.0004 k 3.42 0.0002 3.41 1 3 5 7 9 11 13 15 17 19 Wavelength [mm] 0.0000
WGP models for RCWA simulations Low coherence source
WGP models for RCWA simulations & R total = R down + T down R bottom (R up R bottom ) 0 T up + T down R bottom (R up R bottom ) 1 T up + T down R bottom (R up R bottom ) 2 T up + = R down + T down R bottom T up (1-R up R bottom ) -1 T total = T down (R bottom R up ) 0 T bottom + T down (R bottom R up ) 1 T bottom + T down (R bottom R up ) 2 T bottom + = T down T bottom (1-R up R bottom ) -1
T p [%] Broadband SIR product modeling results 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Transmittance modeling for varying angle of incidence MWIR Tp, 0 AOI MWIR Tp, 12 AOI MWIR Tp, 45 AOI LWIR Tp, 0 AOI LWIR Tp, 12 AOI LWIR Tp, 45 AOI 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Wavelength (μm)
Contrast Ratio Broadband SIR product modeling results 100,000 Contrast modeling for varying angle of incidence 10,000 1,000 MWIR_0 MWIR_12 MWIR_45 LWIR_0 LWIR_12 LWIR_45 100 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Wavelength [μm]
Reflectance Broadband SIR product modeling results 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Reflectance modeling for varying AOI R s R p Product SIR3-5 (MWIR) ------ SIR7-15 (LWIR) Angle of Incidence 0 12 45 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Wavelength [µm]
T p [%] T p [%] Transmittance modeling vs. measurement (a) MWIR Transmittance modeling and measurement for varying AOI 100% 90% 80% 70% 60% modeled Tp, 0 AOI 50% modeled Tp, 12 AOI 40% modeled Tp, 45 AOI 30% measured Tp, 0 AOI measured Tp, 12 AOI 20% measured Tp, 45 AOI 10% 0% 2 3 4 5 6 7 8 Wavelength [μm] (b) 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% LWIR Transmittance modeling and measurement for varying AOI modeled Tp_0 AOI modeled Tp_12 AOI modeled Tp_45 AOI measured Tp_0 AOI measured Tp_12 AOI measured Tp_45 AOI 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Wavelength [μm]
Contrast modeling vs. measurement
Contrast modeling vs. measurement
T p [%] Comparison of FTIR and dispersive spectrometer results for SIR3-5 Contrast 100 Transmittance vs. Wavelength, FTIR vs. CARY5000 0.25 25,000 Contrast vs. Wavelength, FTIR vs. CARY5000 90 Tp, FTIR 0.2 10,000 80 Tp_CARY5000 0.15 Ts, FTIR 5,000 70 Ts_CARY5000 0.1 T s [%] 60 50 2.5 3 3.5 4 Wavelength [mm] 0.05 0 Contrast, FTIR (boxcar) Contrast, CARY5000 (boxcar) 1,000 2.5 3 3.5 4 Wavelength [mm] Nexus 870 FT-IR ESP from Thermo Nicolet DTGS TEC detector, KBr beam-splitter. 256 scan avg., 6 cm -1 Resolution, ~7 mm beam, ~ 5 min. N 2 purge. CARY5000 from Agilent 10 s int. time (300 chopper revolutions) per measurement, 20 nm SBW, 2700-3300nm scan. Zero / baseline correction. ~5 min N 2 purge.
IR Wafer Production Metrology Tool lock-in amp. Customized dispersive spectrometer with chopper, liquid N 2 cooled detector, and lock-in amplifier.
Laser Damage Threshold (LDT) testing Product / Description LDT Results [kw/cm 2 ] Pulsed LDT Results [J/cm 2 ] blocking passing blocking passing λ [μm] test type LDT Test Parameters Pulse Rep Rate Duration [khz] 1/e 2 Beam Exposure Diameter [μm] duration SIR_7-15μm WGP 100 10 -- -- 10.6 cw -- -- 360 30 sec. 7-15μm AR-coated substrate -- 397 -- -- 10.6 cw -- -- 124 60 sec SIR_3-5μm WGP 0.646 >14 0.026 > 0.56 3.3 pulsed 7 ns 25 150 20 min. 3-5μm AR-coated substrate -- >14 -- > 0.56 3.3 pulsed 7 ns 25 100 12 msec. For LWIR AR-coating, damage initiated at defects 20 kw/cm 2, passing 60kw/cm 2, passing (a) (b) (c) Improved LWIR bare AR-coating before after (d) (e) (f) 110 kw/cm 2, blocking 130 kw/cm 2, blocking ~400 kw/cm 2 LDT @ 10.6 µm
SEM analysis of defective LWIR LDT sample Pre-existing AR-coating defects (a) (b) (c) 2 µm 2 µm 2 µm Passing state laser damage initiated at AR-coating defects (d) (e) (f) 100 µm 20 µm 2 µm
SEM analysis of defective LWIR LDT sample blocking state damage spot, quasi-pulsed exposure (a) (b) (c) 5 µm 2 µm (e) (d) 50 µm (~7 s, 67 kw/cm2) 8 µm 2 µm
Cross section of blocking state LDT spot blocking state damage spot, quasi-pulsed exposure Second Pt infill (a) (b) (c) 1 µm (~7 s, 67 kw/cm2) First Pt infill
SEM analysis of improved LWIR AR-coated sample (no wire grid) >440 kw/cm 2 LDT (a) (b) (c) (d) 40 µm 5 µm 500 nm 2 µm Improved LWIR AR-coating shows different damage mechanism 397 kw/cm 2 LDT
T p [%] 100 90 80 70 Transmittance comparison - Moxtek SIR product family SIR3-5 SIR7.5 SIR7-15, 45 AOI SIR7-15 SIR10.6 Conclusions Normal incidence Transmittance Varying angle Transmittance Reflectance at 12 AOI 60 50 40 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Wavelength [mm] 60% RCWA modeling 50% 40% FTIR vs. dispersive spectrometer 30% 20% Laser Damage Threshold 10% Laser damage characterization 0% (a) Second Pt infill First Pt infill Reflectance 100% 90% 80% 70% Reflectance modeling for varying AOI R s R p Product SIR3-5 (MWIR) ------ SIR7-15 (LWIR) Angle of Incidence 0 12 45 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Wavelength [µm]
Acknowledgements