This document describes a system for polarization-resolved grazing angle scatterometry to monitor roughness for diffusers used in light-emitting device manufacturing. The system offers several advantages including immunity to vibration and stray light, eye-safe operation, integration with OEM solutions, and vacuum compatibility. Simulation results show polarization effects are important for characterizing very rough samples in backscatter geometry. Experimental data demonstrates excellent agreement between measurements of GaN diffusers and atomic force microscopy. The system provides rapid acquisition times of 0.1-0.7 seconds with high accuracy, linearity, and repeatability for roughness characterization from 200nm to 5um.

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Polarization resolved grazing angle scatterometry for
in-situ monitoring of roughness for diffusers for light-
emitting device manufacturing
W. J. Walecki, Peter S. Walecki1,
Eve S. Walecki2, Abigail S. Walecki3
Frontier Semiconductor
1 now at Brown University
2 now at University of Florida
3 now at Florida Atlantic University
Frontier Semiconductor

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Main Advantages
● Immunity to vibration
● Immunity to stray light
● Eye safe measurements
● OEM specific solutions
● Non-contact
● Robust
● Simple
● Vacuum compatible

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Stray Light Rejection (same as EPD platform)
No Lamp Fluorescent 15 W
LWD = 20''
Fluorescent 15 W
40 = 40'' '' LWD
Incandescent 40W
LWD = 20''
Incandescent 40W
LWD = 40''
0
1
2
3
4
Experimental Conditions
AverageSignalatN=100,t=0.1sec(0.1xVolt)
Dependence of signal on
stray light illumination for
various sourced and
various lamp-wafer
distances (LWD)
One can place a typical
desk lamp 50 cm from
probe without affecting
signal
Stray Light Rejection – Signal is not affected by the presence of stray
light

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Stray Light Rejection (same as EPD platform)
Stray Light Rejection –
we observed only very small increase of noise by presence of the very
strong stray light sources
No Lamp Fluorescent 15 W
WD = 20''
Fluorescent 15 W
40 = 40'' '' WD
Incandescent
40W WD = 20''
Incandescent
40W WD = 40''
0
0.01
0.02
0.03
0.04
0.05
Experimental Conditions
RelativeNoiseofsingleshotN=100,t=0.1sec
Dependence of noise on
stray light illumination for
various sourced various
lamp-wafer distances
(LWD)
One can place a typical desk
lamp 50 cm – 100 cm
from probe without affecting
noise too much

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Principle of Operation
• Low angle of incidence: GaN appears matte
– DIFFUSE Reflection
GaN Diffuser
Diffuse Reflection
Matte, non-mirror-like
appearance
• Oblique angle: GaN appears mirror-like
– SPECULAR reflection
Same GaN Diffuser
But now we observe
SPECULAR reflection

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Same Principle works for Metals too!
• Low angle of incidence: Metal appears diffusing (make reference
to other paper too)
– DIFFUSE Reflection
Rough metal
Diffuse Reflection
• Oblique angle: Metal appears mirror-like
– SPECULAR reflection
Same surface
But now we observe
SPECULAR reflection

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Calibration
• System was calibrated used a nickel
roughness standard meeting
ANSI/ASME B46.1
• Data on GaN was compared with data
from measured atomic force
microscopy (AFM) scans
• Excellent metrology for measurement
of very rough samples

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Calibration of system on GaN diffusers vs AFM
Excellent agreement between AFM and optical data

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Simulation results: Grazing Scattering
1.00E+00
1.00E+01
1.00E+02
0.0001 0.001 0.01 0.1
PeakIntensity(arb.units)
sigma(mm)
75º
No significant difference for S and P polarization

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Results of simulations dependence on polarization
75º
No significant difference for S and P polarization.
No significant depolarization in specular regime.
Speckle in S polarization Speckle in P polarization
σ = 1.0 µm λ = 0.50 µm

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Results of simulations: Beam spreading as function of
roughness
75º
No significant difference for S and P polarization.
No significant depolarization in specular regime.
σ = 1.0 µm
λ = 0.50 µm
σ = 4.0 µm σ = 10 µm

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Results of simulations: Backscatter geometry
45º
Significant difference for S and P polarization.
Significant depolarization for rough samples.
Sample tilted
along X axis
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
0.01 0.1 1
Signal(arb.units)
Sigma (mm)
Incident polarization X Detector
polarization Y
Incident polarization X Detector
polarization X
Depolarization caused
by multiple reflections

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Hardware constraints

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Rapid improvement in the century old interest in rough surface
scatterometry measurements
ℎ <
𝜆
8 cos 𝜃
𝜃 𝜃𝜃 𝜃
𝜃 𝜃
Condition for specular reflection
For grazing angle h can be much larger than wavelength

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Large roughness metrology
RMS roughness range: 200 nm - 5000 nm (equivalent 0.2 um - 5
um)
wavelength selected for application in range 650 nm - 820 nm
Total System Accuracy: 5%
Total System Linearity: 2%
Repeatability: 2%
Data acquisition time 0.1 s - 0.7 s
For much larger roughness up to 50 um we may use special mid infrared
beam solution
Please note: all specifications subject to change

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Summary
Main conclusions: We have developed a system with the following advantages:
– The polarization effects are important for very rough samples in backscattering geometry, can be
used for characterization of very rough samples
– Forward scattered light at grazing angle is not significantly depolarized – polarization can be
used to rejects some stray light
In addition: We have developed a system featuring:
– Rapid Data Acquisition (<1s/point)
– Vibration Immunity
– Stray light rejection
– Eye safe operation
– Ease of integration (Commercial Solution Avail.)
– Compatible with vacuum
– Can work through windows
– Immunity to stray light through modulation
– Small footprint (~4 sq. ft.)
– Large working distance

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References
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[3] Ngan, Addy et al. "Experimental Analysis of BRDF Models." Rendering
Techniques 2005.16th (2005): 2.
[4] Speicher, Andy. Identification of geostationary satellites using polarization data
from unresolved images. Diss. UNIVERSITY OF DENVER, 2015.
[5] Belcour, Laurent, et al. "Bidirectional reflectance distribution function
measurements and analysis of retroreflective materials." JOSA A 31.12 (2014): 2561-
2572.
[6] Elfouhaily, Tanos Mikhael, et al. "A critical survey of approximate scattering wave
theories from random rough surfaces." Waves in Random Media 14.4 (2004): R1-R40.
[7] Walecki, Wojciech, et al. "Robust diffuser and roughness metrology tool for LED
manufacturing." SPIE OPTO. International Society for Optics and Photonics, 2015.