2014 J.A. Woollam Co., Inc. www.jawoollam.com 1 3B: Review Nina Hong U Penn, February 2014
2014 J.A. Woollam Co., Inc. www.jawoollam.com 2 Review 1A: Introduction to WVASE: Fun Quiz! 1B: Cauchy 2A: Pt-by-Pt and GenOsc 2B: Advanced GenOsc 3A: Non-Idealities
2014 J.A. Woollam Co., Inc. www.jawoollam.com 3 Q1 If the left column shows the Psi and Delta for an ideal glass substrate, can you identify the complexity of the right column data? Ideal Surface Roughness
2014 J.A. Woollam Co., Inc. www.jawoollam.com 4 Q2 If the left column shows the Psi and Delta for an ideal glass substrate, can you identify the complexity of the right column data? Ideal Backside Reflection
2014 J.A. Woollam Co., Inc. www.jawoollam.com 5 Q3 If the left column shows the Psi and Delta for a Si substrate, can you identify the complexity of the right column data? Ideal Backside Reflection
2014 J.A. Woollam Co., Inc. www.jawoollam.com 6 Q4 What is the substrate type of the following ellipsometric data? 1Transparent substrate (dielectric) 2 Semi-absorbing substrate (semiconductor) 3 Absorbing substrate (Metal)
2014 J.A. Woollam Co., Inc. www.jawoollam.com 7 Q5 What does ellipsometric measurement probe? 1 Size of electric field 2 Shape of electric field Less Intense Y E Different Size (Intensity). More Intense Y E X Same Shape! (Polarization) X
2014 J.A. Woollam Co., Inc. www.jawoollam.com 8 Q6 Pseudo Optical Constants <n>, <k> Which one is corresponding to a single reflection on surface? 1 2
2014 J.A. Woollam Co., Inc. www.jawoollam.com 9 Q7 Unknown Film on Glass Which spectral range does the film look transparent?
2014 J.A. Woollam Co., Inc. www.jawoollam.com 10 Data Analysis Strategies Substrates S-1: Opaque Substrates S-2: Semiconductor substrates S-3: Transparent substrates Films F-1: Transparent films F-2: Semi-Absorbing films F-3: Absorbing films
Psi in degrees Delta in degrees 2014 J.A. Woollam Co., Inc. www.jawoollam.com 11 S-1 Opaque Substrates Psi stays near 45 Delta away from 0 or 180 46 180 44 150 42 40 38 65 75 85 120 90 60 30 65 75 85 36 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200
2014 J.A. Woollam Co., Inc. www.jawoollam.com 12 S-1 Opaque Substrates Two Categories Bulk Samples No overlayers. Polished metal. Optically thick films. Fit Strategy Invert psi and delta for n and k. Normal fit from reference values 2 Known: Ψ, Δ 2 Unknowns: n, k Example 1 Optically Thick Cr Film
2014 J.A. Woollam Co., Inc. www.jawoollam.com 13 S-2 Semiconductor Substrates Psi follows shape of absorption Delta away from 0 or 180 when absorbing 50 200 Y in degrees 40 30 20 10 55 65 75 D in degrees 150 100 50 55 65 75 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200
2014 J.A. Woollam Co., Inc. www.jawoollam.com 14 S-2 Semiconductor Substrates Bulk semiconductor n&k are well known Crystalline structure insures repeatable optical/electrical properties. Doping NOT important at VIS wavelengths Doping Is important in the MID-IR, however (5 micron or longer). Fit Strategy Use published index values. Fit for oxide thickness. Example 2 Bare Si Wafer
2014 J.A. Woollam Co., Inc. www.jawoollam.com 15 S-3 Transparent Substrates Psi flat and smooth- follows shape of index Delta = 0,180 - except for surface films Y in degrees 30 25 20 15 10 5 0 0 300 600 900 1200 1500 1800 45 55 65 75 D in degrees 180 150 120 90 60 30 0.5nm roughness 0 0 300 600 900 1200 1500 1800 45 55 65 75
2014 J.A. Woollam Co., Inc. www.jawoollam.com 16 S-3 Transparent Substrates Fit Strategy Cauchy Complexities Surface roughness (Add srough) Backside reflection (Suppress or Model correction) How to obtain small k-values Add Transmission data and fit k only. 25 Example 3 10mm bk7 (Two dat files: SE and T) Experimental Data 200 Experimental Data 1.0 Experimental Data Y in degrees 20 15 10 Exp E 55 Exp E 57.5 Exp E 60 Exp E 62.5 Exp E 65 Exp E 67.5 Exp E 70 D in degrees 150 100 50 Exp E 55 Exp E 57.5 Exp E 60 Exp E 62.5 Exp E 65 Exp E 67.5 Exp E 70 Transmission 0.8 0.6 0.4 Exp pt 0 5 0 0.2 0 0 300 600 900 1200 1500 1800-50 0 300 600 900 1200 1500 1800 0.0 200 400 600 800 1000
2014 J.A. Woollam Co., Inc. www.jawoollam.com 17 F-1 Transparent Films Each reflected wave will have a different phase and amplitude. ~n 0 ~n 1 ~n 2 Delay caused by both index and thickness
2014 J.A. Woollam Co., Inc. www.jawoollam.com 18 Interference shifts toward red as film grows. More Interference oscillations as film grows. 100 F-1 Thickness Effects 0nm 25nm 50nm 75nm 100nm 200nm 300nm 400nm 500nm Y in degrees 80 60 40 20 0 0 300 600 900 1200 1500 1800
2014 J.A. Woollam Co., Inc. www.jawoollam.com 19 F-1 Effect of Film Index Index difference affects Interference oscillations (Mostly Psi amplitude). n=1.5 n=1.75 n=2.0 n=2.25 n=2.5 n=2.75 n=3.0 n=3.25 n=3.5 n=3.75 n=4.0 Y in degrees 100 80 60 40 20 0 0 300 600 900 1200 1500 1800
2014 J.A. Woollam Co., Inc. www.jawoollam.com 20 F-1 Using This Information Adjust Index to match Psi peak height. Adjust Thickness to match oscillation period. 90 Experimental Data Y in degrees 75 60 45 30 15 n=1.5 n=2 n=2.5 n=3 Exp E 75 0 600 700 800 900 1000 1100 Y in degrees 30 25 20 15 10 5 Generated and Experimental Model Fit Exp E 75 0 600 700 800 900 1000 1100
F-1 Cauchy Procedure 1. Adjust An parameter to approximate index. Psi amplitude can help estimate. 2. Adjust thickness to match # of oscillations. 3. Fit Thickness, An and Bn. 4. Add Cn. Does it improve the MSE? 5. Normal dispersion??? Normal Fit IF Model is Close to Answer! RESET if fit fails!!! Example 4 Al2O3 on Si 2014 J.A. Woollam Co., Inc. www.jawoollam.com 21
2014 J.A. Woollam Co., Inc. www.jawoollam.com 22 F-2 Semi-Absorbing Films If light is absorbed before returning to surface, only top reflection is seen (Film appears as Substrate) 300 250 D in degrees 200 150 100 50 0-50 120 150 180 210 240 270 300
2014 J.A. Woollam Co., Inc. www.jawoollam.com 23 Fit Cauchy to Transparent region determine thickness. F-2 Choosing a Model Fix thickness and fit n,k at all wavelengths using point-by-point fit. When in doubt, convert to Genosc to insure Kramers-Kronig consistency. Surface roughness, grading, and anisotropy can be added if they improve MSE. Example 5 Semi Absorbing Film on Si
2014 J.A. Woollam Co., Inc. www.jawoollam.com 24 F-2 Surface Roughness / Index Grading Roughness sensitivity when material is absorbing or has high index. Grading and roughness often correlated when both lower index toward surface. When grading increases index toward surface, may also be sensitive to roughness.
F-3 Absorbing Films Most metal films are opaque above 50-100nm thickness. Absorption at all wavelengths prevents periodic oscillations from thickness. Data will appear similar to absorbing substrates: Psi large and never close to zero. Delta between 0 and 180. Pseudos from different angles the same. 2014 J.A. Woollam Co., Inc. www.jawoollam.com 25
2014 J.A. Woollam Co., Inc. www.jawoollam.com 26 F-3 Multiple angles Y in degrees D in degrees 40 35 30 25 20 Exp E 45 Exp E 55 Exp E 65 Exp E 75 15 200 400 600 800 1000 1200 180 160 140 120 100 80 60 Exp E 45 Exp E 55 Exp E 65 Exp E 75 40 200 400 600 800 1000 1200 < 1 > No new information! 5.0 2.5 0.0-2.5-5.0 0 200 400 600 800 1000 1200 Exp < 1 >-E 65 Exp < 1 >-E 75 Exp < 2 >-E 65 Exp < 2 >-E 75 30 25 20 15 10 5 < 2 >
2014 J.A. Woollam Co., Inc. www.jawoollam.com 27 F-3 Modeling? Challenge: More unknown sample properties than measured values. Solution: Measure additional information or reduce number of unknown properties. n(l), k(l) Known Substrate Y(l), D(l) d
2014 J.A. Woollam Co., Inc. www.jawoollam.com 28 Opaque Layer Transparent Region Optical Constant Parameterization Multiple Angles Interference Enhancement SE + Intensity Multiple-Sample In-Situ F-3 Methods for Absorbing Films Multiple-Ambient J. Hilfiker, Survey of methods to characterize thin absorbing films with Spectroscopic Ellipsometry Thin Solid Films 516 (2008) 7979-7989.
Preferred Method #1 Multi-Layer Strategies Measure n,k from single-layers: use dispersion models. Fit thickness only. If poor fit, add dispersion parms. for least stable film. Film A Film B Film C Substrate Substrate Film B Film C Film A Substrate Substrate 2014 J.A. Woollam Co., Inc. www.jawoollam.com 29
Preferred Method #2 Multi-Layer Strategies Measure n,k for each new layer with previous layers fixed: use dispersion models. If poor fit, add thickness and then dispersion parameters for previous layers. Film C Film B Film B Film A Film A Film A Substrate Substrate Substrate 2014 J.A. Woollam Co., Inc. www.jawoollam.com 30
6 Example 6-poly Si on 100nm TOx on Si Two Film layers Unknown thickness poly Si ~100nm Thermal Oxide Build your own method to fit the data. 2014 J.A. Woollam Co., Inc. www.jawoollam.com 31
2014 J.A. Woollam Co., Inc. www.jawoollam.com 32 Thank You & Good Luck!