Pushing the Performance of Electro-Mechanical Thin Films Paul - PowerPoint PPT Presentation

Ljubljana, 27 September 2016 STI-PM Pushing the Performance of Electro-Mechanical Thin Films Paul Muralt EPFL Swiss Federal Institute of Technology of Lausanne, Electroceramic Thin Film Laboratory 1015 Lausanne, Switzerland. IEEE UFFC Distinguished Lecturer 1

Downscaling STI-PM Feynman: “There is plenty of room at the bottom” state logic and memory Ultra-high density solid 1dim precision, in pocket and wrist watches 𝜇 * 22 nm IC’s *) by Photolithography World wide web In spite of wavelength limitations 2

There is more than microelectronics! STI-PM More than Moore! MEMS research Example of thin film bulk acoustic wave filters: MEMS products Preceding materials research, process technology, and Smart phones component innovation 1995 Downsizing of RF filters for mobile 2010 2001 phones (courtesy R. Ruby, AVAGO) 3 !

Towards industry 4.0, robot society, etc. STI-PM Myriads of micro sensors, micro actuators, micro transducers are needed …. voltage strain Electro mechanical charge stress transducer + Downscaling = Micro-Electro-Mechanical System (MEMS) 4

Important Role of Piezoelectrics STI-PM Electromechanics of ionic crystals Pb 2+ voltage strain Zr 4+ Ti 4+ Piezo- O 2- electric charge stress material Pb(Zr,Ti)O 3 (PZT) Strongest + piezoelectric ceramics Downscaling = Major competing principle Micro-Electro-Mechanical based on electrostatics forces Systems vacuum/air gap 5

Important Role of Piezoelectrics STI-PM 0.15 AlN G Conductance 30 µm square wurtzite Q=460 B Susceptance 0.1 2 =5.9% k voltage strain Admittance [S] 0.05 2 charge stress 0 Very low loss -0.05 as + 7000 7200 7400 7600 7800 8000 8200 8400 8600 3 compar Frequency [MHz] Downscaling ed to real coil = Δ D(t) E(t) P Micro-Electro-Mechanical Systems Trapped wave within thin film 6

Outline STI-PM • What is piezoelectricity? • How to put materials into microsystems? • Which are the achieved properties? • What are the issues in performance limitation? 1. Nucleation of PZT thin films 2. Piezoelectric cracking 3. Heating by unipolar cycling 4. Imprinting, Aging 7

Basics of piezoelectrics: polar chain example STI-PM +q The effect works in both directions: + 𝐷 = 𝐷/𝑂 '& − 𝑙 %& " 𝑛 𝑂 '& # 𝑙 %& d=charge/force= Direct: 𝑛 k s1 '& − 𝑙 %& " 𝐷 '& # 𝑙 %& d=strain/E= Converse: = 𝑛/𝑊 𝑛 𝐷𝑊 c 𝑛 / 𝑛 -q - k s1 <k s2 k s2 + Brothers Pierre and Jacques Curie - Discovered the direct effect in 1880 Charge separation at Tourmaline known as pyroelectric upon pulling

Consequences and Symmetry requirements STI-PM Piezoelectricity is strongest in ionic crystals (large q), and proportional to the dielectric constant (strain per voltage, charge per stress). The linear relation between second order stress tensor and vector of 3 requires absence of inversion center among the electric field 𝐸 = 𝑒𝑈 symmetry elements, since stress T is invariant, and displacement field D changes sign upon inversion. Absence of inversion center, no The gravitation center of positive polar axis (like in quartz) ions moves differently than the one of negative ions.

Basics of piezoelectrics: polar structures STI-PM Transition to 3d structures: Polar surfaces Example AlN: + + - - N-polar surface Al-polar surface + + - - + + - - - - Coupling with transverse and shear deformations + + - -

Frequently encountered situation with one polar axes (4 and 6 fold rotation axis, random in plane, poled ceramics) STI-PM     E ⊥ P E / / P E Shear deformation in S 3 =d 33 E 3 PP the plane of P P electric field and polarization S 1 =d 31 E 3 S 5 =d 15 E 1 Longitudinal (33) – Transverse (31) Shear (15, 24) Strain, stress in reduced index notation: ii=i, 12=6, 13=5, 23=4

Concept of Coupling Factor K 2 STI-PM Ratio of stored electrical and mechanical energy in piezoelectric material or device structure (MEMS) Created mechanical energy K 2 = ( Converse) Electrical input energy in capacitor Created electrical energy in the capacitor (Direct) K 2 = Mechanical input energy Bulk materials: Champions: PZT ceramics and related single crystals (PMN-PT) > 50 % LiNbO 3 single crystal SAW structures (Surface acoustic wave) 5-7 % Quartz single crystal < 1 % 12

Ferroelectric materials as piezoelectric ceramics STI-PM Most performing ceramics: Pb(Zr,Ti)O 3 52/48: 14 possible domain orientations T > Tc E = 0 Ferroelectric domains T c : critical or Curie temperature E Thin films can be made textured. All grains equivalent. Larger piezoelectric effect?

Strong piezoelectric effects are found in inorganic crystals close to morphotropic phase transitions STI-PM Perovskites: usually cubic phase at high temperatures. Phase Pb 2+ transition to lower symmetry required. In PZT=Pb(Zr,Ti)O 3 , transition at 250 to 490 ° C, depending on Zr/Ti ratio. At 52/48 = MPB transition, T c at around 360 ° C. Zr 4+ Ti 4+ a/c domain O 2- 90 ° domains in {100} film pattern P // [111] P // [001] Sol-gel thin film, piezo AFM 14

+ Ferroelectricity, adding highly polarizable ions (e.g., Pb) and polarization switching on the nano scale STI-PM Ferroelectric domains provide amplification on the nanoscale: Charge and strain amplification by domain effects Sol-Gel CV-curves N. Chidambaram 40 PZT 1st cycle 0.14 1600 2nd cycle 20 3rd cycle 2 ) Polarization ( µ C/cm thin film 1400 0.12 ε r 0 1200 P 0.10 P -20 1000 0.08 tan d -40 800 e r PZT 0.06 600 -600 -400 -200 0 200 400 600 Electric field to top electrode (kV/cm) 0.04 tan( δ ) 400 Transverse 500 MPa ! 200 0.02 stress loops @ 300 kV/cm 0 -200 -150 -100 -50 0 50 100 150 200 1 µm Bias field to top electrode (kV/cm) However: • Non linearity 1 = − e 31, f ⋅ E 3 T • Increased Losses Recently: • Back switching – Poling issues PZT e 31,f > 20 C/m 2 • Fatigue (bipolar operation) • Irreversibility – history dependence 15 A.Mazzalai

Thin film situation on elastic layer/substrate STI-PM T 2 T 3 S 2 T 3 T 1 A t p S 1 Q 3 T 1 h n z S 1 S 1 ( z ) = z R T 2 T 3 R S 2 x T 3 x Piezoelectric Bending Piezoelectric bulk sample controlled by e 31,f thin film on substrate Mixed boundary conditions: d 33 à d 33,f : only thickness changes e 31 à e 31,f : T 3 does not change (=0). P. Muralt, Integrated Ferroelectrics 1997

Piezoelectric MEMS structures and devices STI-PM Piezoelectric micromachined ultrasonic transducers (pMUT) in motion, 2-dim array, and as ultrasound probe in emission-receive experiment 17

More motion STI-PM 1995 Towards ink-jet 2006 18

Immense Frequency range of piezoelectric thin films STI-PM Auto-focus lens Loudspeaker and for mob.phone Microphone for mob.phones Inkjet printing head a Flexural devices Energy (a) b harves Contour ting modes (b) Lamb wave S A c c modes (c) 0 0 Thickn modes d (d) Limited by Qf 0 1 2 3 4 5 6 7 8 9 10 = 5’000 GHz Log(frequency(Hz)) Bio-MEMS: particle filters / transport RF filters for mob.phone

Materials “Road Map” STI-PM Category BULK Thin Film Category Bulk Thin film Time and High Q, Low K 2 Quartz single AlN/SiO 2 multilayer frequency control High stability crystal High Q, medium K 2 , LiNbO 3 single RF filters medium stability (in crystal AlN, AlScN T) Energy harvesting AlScN, high Sc content Acoustics Lower to low Q, Large K 2 PZT ceramics PZT thin film Low frequency Less stable (in T, AlScN thin film, high Sc devices and properties) ….........ev. lead free substitute 20

Progress in PZT and related films STI-PM sputter 3 Baek et al, Science 2011 1 Pb(Mg,Nb)O 3 -PTO Lugienbuhl et al 1996 d 31 e 31, f = Muralt et al, 1996 E + s 12 E s 11 Kanno et al 1997 Shepard et al, 1998 σ 1 = − e 31, f E 3 Dubois et al. 1999 Yoshimura et al, 2001 D 3 = e 31, f ( x 1 + x 2 ) Seifert et al. 2001 Zhang et al. 2002 Ledermann et. al. 2003 Zhou et al, 2003 Kanno et al. 2004 Nino et al. 2004 Sol-gel Maria et al. 2005 Bassiri et al. 2005, Yokohama et al. 2006 Tyholdt et al. 2007 Krume et al. 2007 Calame et al. 2007 ( till 2008: see Muralt, J.Am.Cer.Soc. May 2008) 21

AlScN: Comparison of e 31,f values STI-PM Industrial project with EVATEC About d 33

How to put materials into microsystems STI-PM 23

Chemical solution deposition (sol-gel): Automatic CSD tool STI-PM Rotating covered chuck and media arm s o l - g e l s o l u t i o n wafer Hot plate (350oC) Spin coat Wafer on chuck with media arm Piezo-MEMS workshop, Lausanne, 2011 High volume piezoelectric thin film production process for microsystems (2010-2013)

STI-PM 600 to 700 ° C Piezo-MEMS workshop, Lausanne, 2011

In-situ Sputter deposition of PZT thin films STI-PM PZT Module at EPFL cluster

Sputter deposition of AlN STI-PM