Measurement of Gas Sensor Performance Gas sensing materials: 1. Sputtered ZnO film (150 nm (Massachusetts Institute of Technology) 2. Sputtered SnO 2 film (60 nm) (Fraunhofer Institute of Physical Measurement Techniques) Target gases: H 2, CO, NH 3, NO 2, CH 4 Electrical Measurement Operating temperature: 320-460 ºC Pt electrode SiO 2 layer H 2 H 2 H 2 H 2 ZnO film Si wafer 1 Mechanisms in Semiconducting Gas Sensor Bulk: Change in stoichiometry O O = 2e + V O.. + 1/2 O2 (g) Induce shallow donors: density related to PO 2 n 2 [V O.. ] PO2 1/2 = K R (T) n = (2 K R (T)) 1/3 PO 2-1/6 modulate Bulk electronic conduction 2 1
Resistive Oxygen Sensors Based on SrTiO 3 semiconducting oxide E kt σ e A po 1 ± m 2 p Exhaust O 2 U I Electrode 3 Influence of Dopants on Electrical Conductivity of SrTiO 3 log(σ / (Ωcm) -1 ) 1 0-1 -2-3 λ 0.995 1,005 acceptor doped donor doped T = 800 C Sr 2+ Ti 4+ O 2-3 -4 acceptor donor -5-20 -16-12 -8-4 0 log(po 2 / bar) Acceptors: Al, Ni, Fe Donors: Nb, Ta, Sb, Y, La, Ce, Pr, Nd, Pm, Sm, Gd 4 2
Temperature Independence: High Acceptor Concentration in SrTiO 3 electrical conductivity / (Ω cm) -1 1 0,1 950 C 900 C 850 C 800 C 750 C Sr(Ti 0,65 Fe 0,35 )O 3 m = 0,2 Response times T / C t 90 / ms 900 6.5 800 26 750 83 700 185 10-20 10-15 10-10 10-5 10 0 po 2 / bar [1] Menesklou et al, MRS fall meeting, Vol. 604, p. 305-10 (1999). 5 Oxygen Sensor in Thick Film Technology 6 3
Transient Behavior of Porous Sr 1-x La x TiO 3 for x=0.005 and x=0.03 0,1 T = 850 C 0,01 X=0.03 σ / S/cm 1E-3 X=0.03 1E-4 Sr 1-x La x TiO 3 porous ceramic T = 850 C 1E-5 0 5 10 15 20 25 30 35 t / h 7 Mechanisms in Semiconducting Gas Sensor Interface - Gas adsorption 2e + O 2 (g) = 2O(s) Induce space charge barrier modulate 1. Surface conduction Grain boundary barrier 2. Grain boundary barrier 8 4
Sensor Configuration A single 9 mm 2 chip sensor array with: four sensing elements with interdigitated structure electrodes heater temperature sensor 9 Schematic Cross Section of Mounted Sensor 10 5
Resistance Response to Gas Environment Schematic of Gas Sensor Structure 100k ZnO(Ar:O 2 =7:3) 1 M 9710746 20V Datum: 23.02.2001-27.02.2001 Steuerdatei: allgas_h2.stg Meßprotokoll: 273 S1219a S1219b S1219c S1219d Electrical Measurement Pt electrode SiO 2 layer H 2 H 2 H 2 H 2 ZnO film Si wafer resistance / Ohm ZnO film (150 nm) Electrode: Pt(200 nm)/ta(25 nm) film Insulation layer: SiO 2 layer (1 µm) Substrate: Si wafer Gas flow / sccm 0 20 40 60 80 110 100 90 80 70 60 50 40 30 20 10 0-10 0 20 40 60 80 NO2 time / h MFC2 Temp NH3 Feuchte CO NO2kl H2 Temp:360C, H 2, CO, NH 3 (10, 50 and 100 ppm), NO 2 (0.2, 0.4, and 2 ppm) 110 100 90 80 70 60 50 40 30 20 10 0-10 Pt-100 resistance / Ω [ Pfad: \ alpha missy Messungen messplatz_1 ] M. Jägle / 27.02.2001 11 MicroElectroMechanical Systems - MEMS Bulk Micromachining Surface Micromachining Micromachining - Application of microfabrication tools, e.g. lithography, thin film deposition, etching (dry, wet), bonding 12 6
Gas Sensors and MEMS Miniaturization Reduced power consumption Improved sensitivity Decreased response time Reduced cost Arrays Improved selectivity Integration Smart sensors 13 Microhotplate 14 7
Microhotplate Sensor Platform NIST Microhotplate Design 15 Microhotplate Characteristics Milli-second thermal rise and fall times programmed thermal cycling low duty cycle Low thermal mass low power dissipation Arrays enhanced selectivity 16 8
Harsh Environment MEMS High temperatures Oxidation resistant Chemically inert Abrasion resistant Wide band gap semiconductor/insulator 17 Photo Electro-chemical Etching - PEC Features: materials versatility e.g. Si, SiC, Ge, GaAs, GaN, etc. precise dimensional control down to 0.1 mm through the use of highly selective p-n junction etch-stops fabrication of structures with negligible internal stresses fabrication of structures not constrained by specific crystallographic orientations 18 9
Photo Electro-chemical Etching - PEC Electro-chemical etching p-type h + h + h + h + semiconductor + - Photo electrochemical etching Light source electrolyte n-type h+ semiconductor h + + - + - 19 Examples... Arrays of stress free 4.2 µm thick cantilever beams. Photoelectrochemically micromachined cantilevers are not constrained to specific crystal planes or directions. Similar structures successfully micromachined from SiC by Boston MicroSystems personnel 20 10
Smart Gas Sensor A Self Activated Microcantilever-based Gas Sensor 1. A device capable of sensing a change in environment and responding without need for a microprocessor 2. A device has both gas sensing and actuating function by integration of semiconducting oxide and piezoelectric thin films. Sensor Chemical Environment Micro- Processor Actuator Microfluidic structure 21 Smart Gas Sensor 1. Semiconducting oxide thin films for high gas sensitivity : Microstructure (Nano-Structure) and Composition 2. Piezoelectric thin films for providing actuating function 3. Thin film electroceramic deposition methods for integrating with silicon microcantilever beam : Compatibility with Si micromachining technology 4. Microcantilever structures for the self activated gas sensor : High performance in chemical environment 22 11
Resonant Gas Sensor Resonant Frequency: f R = 1/2l (µ o /ρ o ) 1/2 where l = resonator thickness, µ o = effective shear modulus and ρ o = density Mass change causes shift in resonant frequency : (m 0 - m) / m o (f + f) / f Gas Sensor elements : (I) Active layer interacts with environment - stoichiometry change translates into mass change (II) Resonator transduces mass change into resonance frequency change m f Active layer Electrode Resonator Electrode 23 Choice of Piezoelectric Materials Temperature limitations of piezoelectric materials Material Max Operating Temperature ( o C) Limitations Quartz 450 High loss LiNbO3 300 Decomposition Li2B4O7 500 Phase transformation GaPO4 933? Phase transformation La2Ga5SiO4 (Langasite) 1470? Melting point 24 12
Design Considerations Bulk conductivity dependent on temperature and PO 2 contributes to resonator electrical losses Modify bulk conductivity - how? Stability to oxidation and reduction process limited oxygen non-stoichiometry slow oxygen diffusion kinetics Defect chemistry and diffusion kinetics study f R (T) : Temperature dependence of resonant frequency need to differentiate from mass dependence Minimize @ intrinsic and device-levels 25 Langasite : Bulk Electrical Properties Single activation energy in the temperature range 500-900 C Extrapolated room temperature conductivity: σ = 4.4 10-18 S cm -1 T [ C] -4 900 800 700 600 500 10 Y-cut 10-5 σ [S cm -1 ] 10-6 σ 0 = 2.1 S cm -1 E A = 105 kj mol -1 10-7 8 9 10 11 12 13 10 4 /T [1/K] 26 13
Langasite : f R (T) Temperature dependence of the resonance frequency (f R ) of a resonator device with difference mass loads. 1,77 1,76 1,75 f A [MHz] 1,74 1,73 1,72 1,71 Contact 1 (f Co1 ) Contact 2 (f Co2 ) Calculation f Co1 + f( m Co2 ) 0 100 200 300 400 500 600 700 800 T [ C] 27 Ongoing Activities Resonator (Langasite) -- H.Seh & H. Fritze Defect chemistry Oxygen diffusion/exchange studies Bulk conductivity dependence on T and PO 2 Active Layer (PCO) -- T. Stefanik Transport-Defect chemistry correlations Gas Sensor -- Add active layer (PCO) using PLD nanocrystalline vs microcrystalline Sensor testing 28 14