Optical characterization of highly inhomogeneous thin films D.M. Rosu, A. Hertwig, P. Petrik, U. Beck Department 6.7 - Surface Modification and Measurement Technique BAM - Federal Institute for Materials Research and Testing 8th Workshop Ellipsometry March 11-13, 2014
Outline Motivation Experimental set-up Patterned samples Spectroscopic ellipsometry large scale inhomogeneity small scale inhomogeneity Conclusions
Motivation quality and consistency control: defects, thickness and structural variation, optical constants thin film materials: ideal (database optical constants) real (inhomogeneity, roughness, non-stoichiometric, depolarization, anisotropy, multilayers)
Experimental set-ups EP 3 -SE (Accurion GmbH) Imaging- Null-Ellipsometer + Microscope Spectral range: 400 nm- 1000 nm Objectives: 10x, 20x, 50x Field of view: 1 mm 2 Region of interest 5µm 2 M2000DI (J. A. Woollam Co.) Rotating compensator (RC) ellipsometer Spectral range: 192 nm- 1697 nm Variable angle spectroscopic ellipsometry Spot size ~5 mm mapping ellipsometry
Patterned samples Fraunhofer IISB: Si(100) substrate Ø 150 mm lithographically patterned SiO 2 and photoresist(az 5214E) overview of the sample and a fraction of the patterned, obtained using a Polyvar MET metal microscope
SiO 2 : spectroscopic ellipsometry- visible range 300 1.56 1.54 refractive index extinction coefficient 0.2 200 1.52 n 1.50 0.1 k / 100 1.48 0 1.46 1 2 3 4 5 6 0.0-100 75 Experiment 65 Experiment 70 Experiment 75 Experiment 60 Simulations Energy/eV 50 Ψ/ 25 0 1 2 3 4 5 6 Energy/eV thickness of the SiO 2 layer: optical constants of SiO 2 from C.M. Herzinger, B. Johs, W. A. McGahan and J. A. Woollam, J. of App. Phys. 83 (1998) 3323
SiO 2 : spectroscopic ellipsometry- visible range Spot ID Thickness/nm x y 1 298.9(298.3) 300.7(300.9) 2 299 300.4 3 299.2 300 4 299.2 299.7 5 299.3 (299.3) 6 299.3 298.8 7 299.2 298.4 8 299.2 297.8 9 299(299.1) 297.2(297.3) Woollam M2000DI ellipsometer; AOI: 60-75 thickness inhomogeneity: ~1.3% across the sample total uncertainty for the thickness determination: ± 2 nm
SiO 2 : imaging ellipsometry Image to be mapped, grabbed at λ=533 nm map, measured at λ=533 nm Ψ map, measured at λ=533 nm Imaging ellipsometry in the visible spectral range Map of a 374 µm x 289 µm area of 5 and 10µm native SiO 2 stripes 7 wavelength in the visible range were used
SiO 2 : imaging ellipsometry 300 200 100 Pixel = 1 50 100 150 200 250 300 350 298 299 300 301 50 100 148 150 200 250 Pixel = 1 Image to be mapped, grabbed at λ=533 nm 187 Calculated thickness map Imaging ellipsometry in the visible spectral range Map of a 374 µm x 289 µm area of 5 and 10µm native SiO 2 stripes 7 wavelength in the visible range were used Thickness determined using both Ψ and maps
Photoresist : spectroscopic ellipsometry- visible range Woollam M2000DI ellipsometer; AOI: 60-75 thickness of the resist: Cauchy dispersion model up to 3eV multi-sample analysis (optical constants do not vary) thickness inhomogeneity: 8% across the sample thickness inhomogeneity: 1.28% (~20 nm variation inside the measured spot) Spot Thickness/nm ID x y 1 1447 1457 2-1480 3 1461 1506 4 1488 1528 5 1510 1557 6 1534 1574 7 1559 1560 8-1534 9 1563 1506 10 1546 1490 11 1519 1466 12 1503 1458 13 1474 14 1456
Photoresist : spectroscopic ellipsometry- visible range Middle spot on the sample total uncertainty for thickness calculation: 6 nm (middle spot: 1574 ±6 nm) e.g. experimental and simulation for the spot in the center of the sample: single spot and multispot analysis optical constants of the layers: General oscillator model with 1 Gauss oscillator up to 6 ev
Photoresist : spectroscopic ellipsometry- visible range Depolarization% 100 80 60 40 20 measured 55 measured 65 measured 70 Simulation 0 1 2 3 4 5 6 Energy/eV thickness inhomogeneity: 1.28% (~20 nm variation inside the measured spot)
Photoresist: local inhomogeneity 1515 Pixel = 1 1510 8,176 x / pixel 50 100 150 1500 1510 1520 1474 1481 1487 50 Imaging ellipsometry in the visible spectral 1494 1500 1507 1513 100 y /pixel range Map of a 187 µm x 184 µm area of a 1520 1526 150 photoresist covered area 1532 93,52 Pixel = 1 15 wavelength used Thickness determined using a Cauchy dispersion model for the calculated map
Photoresist: local inhomogeneity Stripe width 150 100 50 25 Thickness of SiO 2 layer/nm 1.5 1.4 1.8 1.9 Measurement of the native oxide covered areas in the patterns using nulling ellipsometry with a 20X objective Structures smaller than 25 µm are difficult to measure/ analyze; any small defect strongly influences the measurement and the calculations
Conclusions Local and global inhomogeneity were studied using spectroscopic ellipsometry The homogeneity of the SiO 2 film qualifies it as a patterned large-area reference sample for film thicknesses Patterns with dimensions in the range of 10 µm were well resolved Imaging ellipsometry in visible spectral range detected the smallest deviations in the photoresist layer 8% inhomogeneity was calculated across the photoresist layer
Acknowledgments BAM Berlin Fraunhofer IISB/MTA Hungary Andreas Hertwig Matthias Weise Uwe Beck Gudrun Rattmann Schellenberger Martin Peter Petrik This work was funded through the European Metrology Research Programme (EMRP) Project IND07 Thin Films.
Calculation of thickness uncertainty relative error 2.0 xc = 1574.34906 1.8 relmse 1.6 1.4 1.2 1.0 1540 1550 1560 1570 1580 1590 1600 1610 thickness/nm
Variation of the optical constants due to thickness uncertainty single spot analysis vs. multiple spot anylysis (Gauss oscillator model) 2.0 1.9 n, Gauss single spot n, Gauss - multispot k, Gauss single spot k, Gauss - multispot n 1.8 1.7 1.6 1 2 3 4 5 6 Energy/eV