Scalable self-aligned active matrix IGZO TFT backplane technology and its use in flexible semi-transparent image sensors Albert van Breemen
Image sensors today 1 Dominated by silicon based technology on rigid substrates - High performance - Large area applications difficult and/or expensive - Rigidity hampers integration in products where form factor is important - Mechanical integrity poor CMOS sensor Fingerprint sensor Digital X-ray detectors 1 Market size estimated to reach USD 17.5 Billion by 2020 CAGR of 30% between 2015 and 2020
Flexible image sensors Can offer similar performance as compared to silicon, but: - can be processed over large area and hence have a competitive cost per area ratio - are thin and light weight - are robust - are conformable - can be (semi)-transparent Enable new products and product design
Applications semi-transparent image sensors Stacked image sensors in cameras or lens-less color scanners RGB photodetectors Stacked image sensors Light Reduced photon shot noise Conventional image sensor with color filters Semi-transparent backplane Increased resolution T. Takagiet al. International Symposium on Electronic Imaging 2016, 264.1
Applications semi-transparent image sensors New user interfaces also in combination with displays - Gesture, form & shape recognition - New ways of interaction with smart walls, wearable devices or IoT nodes Security & Authentication 7
Our scalable technology choices O P D Thin-Film Encapsulation Transparent Transparent top electrode top electrode Bottom electrode Oxide semiconductor TFT backplane on plastic - 10x higher mobility than a-si - Ultra-low leakage currents, so higher sensitivity T F T S IGZO G PI foil D Organic photodetector frontplane - Low process temperatures, compatible with plastic substrates - Large area deposition: competitive cost per area ratio Transparent thin-film encapsulation - Developed by Holst Centre, used in products Holst Centre TFT pilot line 8
Organic photodetector (OPD) Interface between the electrodes and the OPD blend layer is of prime importance in order to control the dark current Transparent top anode HTL = MoOx ETL = IGZO Cathode Interface can be modified by: - a Hole Transport Layer (HTL) at the anode Amorphous indium-gallium-zinc-oxide as electron transport layer in organic photodetectors H. Arora et al. Applied Physics Letters, 106, 143301 (2015) - an Electron Transport Layer (ETL) at the cathode Indium Gallium Zinc Oxide (IGZO) proves to be an excellent ETL in OPDs 9
Performance OPD J dark 3x10-8 ma/cm 2 : comparable to a-si EQE 45 % @ 550 nm Transparent top anode HTL = MoOx ETL = IGZO Cathode 10 1 60 10-1 10-4 Bias -2V 50 J [ma/cm 2 ] 10-3 10-5 10-7 J [ma/cm 2 ] 10-5 10-6 10-7 EQE [%] 40 30 20 10 10-9 -2-1 0 1 2 V [V] 10-8 0 1 2 3 4 5 time [min] 0 400 500 600 700 800 900 1000 Wavelength [nm] 10
Semi-transparent backplane (1) Self Aligned (SA) TFT technology 8 mask step process - IGZO - Gate stack - SiN Via - SD - Interlayer - Cathode - ETL - Edge Cover Sequential processing of TFT, interlayer, OPD cathode and ETL Source Edge cover layer Gate Gate dielectric IGZO Drain Barrier Polyimide Glass ITO IGZO Interlayer Inter metal dielectric 11
Semi-transparent backplane (2) Reduction of mask steps by: 1. using IGZO as both the TFT contact, Edge cover layer TFT semiconductor and OPD cathode/etl 2. combining Intermetal Dielectric and Interlayer in one material Source Gate Gate dielectric IGZO Drain IGZO Barrier Polyimide IGZO Interlayer Inter metal dielectric Glass 12
Self aligned semi-transparent backplane 5 mask step process IGZO used as both: - TFT contact - TFT semiconductor - OPD cathode/etl Inter metal dielectric Source Interlayer Gate Gate dielectric IGZO Barrier Polyimide Glass 13
Large area deposition OPD blend S2S based Slot-die Coating process Homogeneous coatings on GEN1 size (320 x 352 mm) Optimization of materials, inks and coating conditions Pinhole free, homogeneous coating 14
Semi-transparent image sensors 15
Transmission measurements 1,0 top encapsulation 0,8 transparent top electrode Transmission 0,6 0,4 0,2 PI + bottom TFE SA P12 PI + bottom TFE + backplane SA P12 PI + bottom TFE + backplane *0.8 0,0 400 500 600 700 800 D IGZO bottom encapsulation OPD TFT Wavelength [nm] PI foil @ 550 nm 86% @ 550 nm 70% TFT backplane 80% glass carrier 16 Albert van Breemen - Scalable self-aligned active matrix IGZO TFT backplane technologyand its use in flexible semi-transparent image sensors
Semi-transparent imager movie Image sensor is turned upside down glass carrier PI foil bottom encapsulation IGZO D transparent top electrode top encapsulation TFT OPD transparency Holst Centre logo illumination 17
Video semi-transparent image sensor 18 Albert van Breemen - Scalable self-aligned active matrix IGZO TFT backplane technologyand its use in flexible semi-transparent image sensors
Near-IR (NIR) photodetectors Applications foreseen in: - pulse oximetry - biometrics - interactive surfaces 19
Near-IR (NIR) photodetectors Applications foreseen in: - pulse oximetry - biometrics Donor 1 Acceptor Donor 2 - interactive surfaces Spectral sensitivity of OPD can be easily tuned by changing donor polymer 60 50 40 Donor 2 + Acceptor EQE [%] 30 20 10 Donor 1 + Acceptor 20 0 400 500 600 700 800 900 1000 λ [nm]
Near-IR (NIR) photodetectors 16 x 16 OPD array 21
Video Near-IR (NIR) photodetector 22
Summary Semi-transparent self-aligned TFT backplanes: - IGZO facilitates the combination of specific TFT and OPD functionalities in a single material - 5 mask step scalable process - Transparency 80% Functional semi-transparent image sensors shown Near-IR photodetectors will further unleash the applications of semi-transparent image sensors Source Inter metal dielectric Interlayer Gate Gate dielectric IGZO Buffer Barrier Polyimide Glass 23 Albert van Breemen - Scalable self-aligned active matrix IGZO TFT backplane technologyand its use in flexible semi-transparent image sensors
Acknowledgements Holst Centre TFT pilot line team Santhosh Shanmugam Holst Centre thin film encapsulation team Hylke Akkerman Bart Peeters, Ilias Katsouras Gerwin Gelinck and many others! Bart Peeters 24
albert.vanbreemen@tno.nl