No technology is totally new. Most amazing consumer devices are actually from some “grandpa” technology invented several decades ago, but just gets re-invented in miniaturization with modern fabrication process. From personal interest, I share technical slides like this, to introduce the context of the technology of sensing system that have been adopted in consumer product. Hope it helps and inspires those who are doing R&D in related fields. Note that all market/technical inference here are based on non-confidential information easily searchable on websites.

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(USFP) Ultrasonic
Fingerprint Scanner
jAMES Wang
Dec 2019

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Preface
• No technology is totally new. Most amazing consumer
devices are actually from some “grandpa” technology
invented several decades ago, but just gets re-invented in
miniaturization with modern fabrication process.
• From personal interest, I share technical slides like this, to
introduce the context of the technology of sensing system
that have been adopted in consumer product. Hope it helps
and inspires those who are doing R&D in related fields.
• Note that all market/technical inference here are based on
non-confidential information easily searchable on websites.

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Content
• Market Demand
• Technology Overview
• Modeling Is Believing
• Hacker Summary

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MARKET DEMAND

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Full Screen
• Trend of smartphone full-screen display

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Full Screen
• Miscellaneous technology to satisfy full-screen

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Full Screen
• Miscellaneous technology to satisfies full screen
– biometrics module is the most challenging one

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Biometrics Solution
• Apple:
– IR Face ID (iPhone-X/XI)
– Under-display fingerprint (rumors say 2020 or 2021? …
could be optical or ultrasonic?)
• Android Alliance:
– Under-display optical fingerprint (Oppo, VIVO, Huawei)
– Under-display USFP (Samsung S10)
– Note that capacitive & thermal type fingerprint scanner
cannot detect through typical thickness (1~1.5 mm)
formed by display module and cover glass

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Biometrics Solution
• Under-display fingerprint: optical vs. ultrasonic
Cover
Glass
Fingerprint
OLED

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Biometrics Solution
• USFP has advantages to penetrate grease & water
Optical Ultrasonic
Dirty Finger
Dry Finger
Wet Finger

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Market Summary
• USFP perfectly fits two demands:
• Full-screen smartphone
– FPS needs to penetrate deeper
– Optical/ultrasonic appears since 2016
• Wet/Dirty finger user scenario
– Automotive/house door lock
– Ultrasonic performs best here

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TECHNOLOGY

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Finger Physiology
• Cross-Section • Surface Profile
• USFP distinguishes finger ridge
and valley by their acoustic
impedance difference
(MRayl)
Ridge Valley
(Air)
Valley
(liquid)
Acoustic
Impedance
1.99 4.2E-4 1.48 (water)
1.38 (grease)

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Seismology vs. USFP
• The key principle of USFP is the elastic wave generation, propagation
and reflection in solid platen (OLED + Cover Glass)
• Most required knowledge can be found in seismology of "earth science"
where they care about finding natural gas layer or source of earthquake
• In USFP, we care about finding the locations of finger ridges & valleys

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SAM vs. USFP
• SAM (scanning acoustic
microscope) is often used as a
inspection tool for non-destructive
testing of IC
• Basic principle: discontinuity
imaging of acoustic impedance
• SAM shares common principles
with USFP

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Physical Limit
• Image Contrast Ratio: R_valley / R_ridge
• Finger valley could be filled with air, water or grease
• Depending on the material of platen, the image contrast ratio of USFP is
calculated as below, which is the physical limit of performance

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USFP History
• First prototype existed almost 30 years ago (Dr. J.K. Scheneider)
• Consumer development started since 2013, and only Qualcomm
succeeds in mass production (for Samsung Galaxy S10 in 2019)

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Tech. Comparison
• In principle, 4 kinds of transducer can be used for
USFP
Tx/Rx Transducer Company
Piezo Pillar/Post Sonavation
Piezo Thin Film Qualcomm
PMUT
(piezoelectric-driven membrane)
TDK-Invensense
CMUT
(electrostatics-driven membrane)
N/A

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Transducer Foundry Options
Ref: Ultrasound sensor market set for take off – 2018/08/08 (Link)
Foundry Piezoelectric CMUT Piezo Film
TSMC AlN, PZT Medical Imaging (for
Butterfly)
---
G.F. AlN (for Invensense) --- ---
GIS --- PVDF adhesion for Qualcomm
X-Fab AlN, PZT --- ---
SilTerra AlN --- ---
Hitachi --- Medical imaging ---
Philips --- Medical imaging ---
FujiFilm Thin-film PZT (Inkjet head) --- ---
STM Thin-film PZT (with USound) --- ---
Silex Thin-film PZT, AlN --- ---
Siemens --- Medical imaging ---
Dimatix Thin-film PZT (Inkjet head) --- ---

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Sonavation
• Representative Patent: US20140219521A1 Biometric
sensing device for three dimensional imaging of
subcutaneous structures embedded within finger tissue

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Qualcomm
• Representative Patent: US9323393B2 Display with
peripherally configured ultrasonic biometric sensor (Link)
• describes the material stacking and Tx/Rx operation

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TDK-Invensense
• Representative Patent: US20180107854A1 INTEGRATED
PIEZOELECTRIC MICROELECTROMECHANICAL ULTRASOUND
TRANSDUCER (PMUT) ON INTEGRATED CIRCUIT (IC) FOR
FINGERPRINT SENSING

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S10 Hacked
• Samsung S10+ Assembly Reference Images (from
iFixit)

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S10 Hacked
• According to the public information from Qualcomm
and GIS, it’s believed that they use PVDF film (4x9
mm^2, DPI = 508) as ultrasonic transceiver (pixel
size 50 um)

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Design Challenges
• Transducer directivity improvement
• Multiple-layer reflection & transmission optimization
• Pixel interference
• Cost-effective transducer/ASIC process integration
• Power consumption and CKT protection
• Thickness limitation for ultrasound penetration
• Screen protector artifact (S10)

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MODELING IS BELIEVING

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Motivation
• To understand how MHz-level elastic wave is
generated (by piezoelectricity), propagated
(transient mechanics) and reflected (impedance
difference) among the stacking layers of material
• To estimate how much current and voltage will be
generated by stimulation of echo waves
• You can use any multiphysics simulation tools like
COMSOL, ANSYS or ABACUS etc.

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3D Model
• Based on the collected information, we build up a
3D model which mimics S10’s USFP under display

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Driving Voltage
• A short pulse of driving voltage is described by
• 𝑉𝑑 𝑡 = 𝑉𝑎𝑐 𝑒
−(
𝑡−𝑇0
0.2∗𝑇0
)2
sin(2𝜋𝑓𝑑 𝑡), 𝑓𝑑 =15 (MHz)

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Elastic Wave
• Based on the collected information, we may build
up a 3D model which mimics USFP under display
Plane wave rising
from PVDF film
Plane wave
reflected from two
interfaces
Echoes from
OLED/Cover
arrives PVDF
OLED
Glass
OLED
Glass
OLED
Glass

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Sensing Voltage
• A typical 20~30 (mV) output is expected, which
makes the front-end circuit design much easier

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Analysis
• The 3D Multiphysics modeling shows that the
PVDF transceiver generates clear elastic-wave
echoes from glass/finger interface due to the
incident plane wave
• Echo amplitude signal will be used to correlate the
intensity of impedance mismatch, which will be
extracted by envelope detection with either analog
or digital approach

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SUMMARY

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Now
• USFP appeared around 1990 and started its consumer
miniaturization since 2013 by Qualcomm and Invensense
due to the market demand for full screen of smartphone.
• The first and the only successful mass production is
QTI’s SensorID which uses piezo film as elastic wave
transceiver to generate plane wave.
• 3D Multiphysics modeling shows that its echo signals
can be easily separated from background reverberations,
which makes the sensing circuit design much easier.

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Future
• To achieve full screen, under-display optical
fingerprint sensor is the major competitor of
USFP, no matter in cost and area
scalability.
• Except for smart phone market, outdoor
locks (door or car) could be another
scenario which strongly suitable for USFP,
to tolerate dirty/wet fingers.

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dibao.wang@gmail.com
www.linkedin.com/in/jdbwang/