Frontier Semiconductor develops non-contact stress metrology techniques for flexible electronics using polarized light. Their FSM 128 FPD G6 tool uses photoelastic principles and polarimetry to measure local stress in flexible materials like polyimide films with high sensitivity. Analysis of the polarization state of light passing through a stressed sample allows recovering the three independent components of in-plane stress. This photoelastic metrology approach provides a fast, accurate way to evaluate stress in flexible films and foils to support manufacturing process monitoring.

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Prepared by Wojtek Walecki, Ph.D.
February 2018
Non-Contact Stress Metrology for Flexible
Electronics

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Wojtek Walecki, Ph.D.
CTO
Frontier Semiconductor
waleckiw@frontiersemi.com
Frontier Semiconductor FSM 128 FPD G6 and other
techniques for FPD process monitoring

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FSM Company Profile - Mission
➢ Founded in 1988
➢ Management driven by Customer satisfaction facilitated by
offering the Best-Known-Method in our customized metrology
solutions
➢ Partnership with major Semiconductor makers to enable a
measurement eco-system and to align with customer’s yield
improvement and cost reduction initiatives
➢ World-wide presence: sales & service for semiconductor fabs
➢ Direct onsite support experience in U.S., France, Korea, China,
Philippines, and Taiwan, 24/7

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Philippines
FSMP

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FSM 128 FPD G6 Description: General Tool Layout
FSM 128 FPD G6 Schematic for illustration only.
Support Table
Metrology sensors
above panel
Panel support by
pins
EMO
Sturdy Frame
Room for Computer
Control
Light tower
FEU
(optional)
Glass Entry Slit with
automatic door
User Interface with
Monitor and
Keyboard

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Local Stress: Photo-elastic principle
Polarizer Polarizer
Sample
Light
source
Camera
Use of polarized illumination and
polarization sensitive array camera reveals
Frozen Residual Stresses

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https://thumpertalk.com/forums/topic/1035824-the-power-of-roost/
Local Stress Detection: Photo-elastic principle – stress in googles illustration
Another solution, to cracks in
high stress area
Most abrupt stress variations
are observed close to nose area
Stress in this area often results
in cracks

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Our new G6 tool adopts modified Senarmont method
and meets the requirement of standards
ASTM C 1048 and GB 9656.
LED array
Local Stress: using polarimetry
Angle typically
<21 degree

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Photo-elastic Optical Probes and Software Diagnostic Interface
Simple/robust calibration
polarimeter computer controlled unit
(7 units built and tested)
New fast compact LCD unit
(under development)
Graphical User Interface

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Local Stress: using polarimetryPolyimide is 100x more
sensitive than glass

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Local Stress Metrology: Using polarimetry
Polyimide is 100x more sensitive than glass
System setup. The principal stresses are denoted as σ1 and σ2 . After passing through the sample, which
is birefringence caused by the stress, the circular polarized light becomes elliptical polarized. Each
component of different direction of the elliptical polarized light can be obtained through rotating the
linear polarizer.

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Kapton tape with different stress
0M
Pa
47.9
MPa
71.3
MPa
24.5
MPa
61.7
MPa

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Kapton tape with different polarization angle
-
38
°
2° 52
°
92
°
14
2°

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Light Intensity change with the stress
I : the intensity of light at a selected point on the specimen
a : the amplitude of the source
Θ : the position of the analyzer relative to the fast axes
Δ : represents the relative retardation due to birefringence in the
specimen
Δ
σ1,σ2: principal stresses
C : stress-optic coefficient
λ : wavelength
h : thickness of the Kapton tape 0
0.2
0.4
0.6
0.8
1
1.2
0.00E+00 2.00E+07 4.00E+07 6.00E+07 8.00E+07
Intensity(a.u.)
Stress(Pa)
Stress vs. light Intensity in 532nm
width=2cm
Tapes
break
h : thickness of the Kapton tape=50 µm
C : stress-optic coefficient=1.5E-10 Pa-1

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Light Intensity change with the polarizer angle
I : the intensity of light at a selected point on the specimen
a : the amplitude of the source
Θ : the position of the analyzer relative to the fast axes
Δ : represents the relative retardation due to birefringence in the
specimen
0
50
100
150
200
-60 -30 0 30 60 90 120 150 180
Intensity(a.u)
θ(degree)
Polarizer Angle vs. light Intensity in
532nm

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Local Stress: using polarimetry
Polyimide is 100x more sensitive than glass
Expected magnitude of retardation for the stress
difference of the order of 10 MPa in 5 um film:
3 ∗ 10−10 𝑃𝑎−1 ∗ 107 𝑃𝑎 ∗ 5 ∗ 10−6 𝑚 = 15 ∗ 10−9 𝑚 = 15 𝑛𝑚
This corresponds to rotation of polarization angle by
∆=
15 𝑛𝑚
500 𝑛𝑚
360 𝑑𝑒𝑔 = 11 𝑑𝑒𝑔
In practice we expect two passes through the material,
and some reflection from the top surface but still effect is of
the order of 10 degrees

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Local stress Stress Separation
The photoelastic data can be used to recover stress components.
Historically several mathematical methods have been developed
Including SHEAR DIFFERENCE METHOD:
The glass topography
provides independent on
PI thickness measurement
of stress

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Local Stress: using polarimetry
Polyimide is 100x more sensitive than glass
All three independent
components of
stress recovered
𝜎𝑥𝑥, 𝜎 𝑦𝑦, 𝜎𝑥𝑦,

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Conclusions
1. Photoelastic metrology is available for polyimide
and other similar films and foils
2. Result of measurement of in plane stress tensor
3. Measurement system is fast and accurate
4. We will be happy to demonstrate system and
evaluate on real life samples
Specifications subject to change. Examples are for illustration only – no specification implied.
All acceptance wafers need to be pre-qualified by FSM for suitability.

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THANK YOU
Frontier Semiconductor
2127 Ringwood Ave
San Jose, CA 95131, USA
Ph. +1 408 432 8838
Fax. +1 408 232 1115
Email: fsm100@frontiersemi.com
Web: www.frontiersemi.com