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The electro-thermal properties of integrated circuit microbolometers

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  1. T & M 2010 - 8 Nov 2010 The electro-thermal properties of integrated circuit microbolometers * M. du Plessis, J. Schoeman, W. Maclean ** C. Schutte * Carl and Emily Fuchs Institute for Microelectronics, University of Pretoria * * Detek, DenelAerospace Systems CEFIM Carl and Emily Fuchs Institute for Microelectronics

  2. T & M 2010 - 8 Nov 2010 Carl and Emily Fuchs Institute for Microelectronics - CEFIM CEFIM Carl and Emily Fuchs Institute for Microelectronics

  3. T & M 2010 - 8 Nov 2010 Infrared thermal images CEFIM Carl and Emily Fuchs Institute for Microelectronics

  4. T & M 2010 - 8 Nov 2010 Principal types of infrared (IR) detectors 1) Photon detectors - [ Cooled ] In photon detectors the absorbed photons directly produce free electrons and holes, to generate a photon-induced current or voltage, either in a photoconductive or photovoltaic mode. 2) Thermal detectors - [ Uncooled ] In thermal detectors the absorbed photons produce a temperature change, which is then indirectly detected by measuring a temperature dependent property of the detector material. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  5. T & M 2010 - 8 Nov 2010 Two classifications of thermal sensors 1)   Direct sensorsDirect sensors convert thermal signals (temperature or heat) to electrical signals. 2)Indirect sensorsIndirect sensors are based on thermal actuation effects, such as thermo-mechanical (thermal expansion) effects. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  6. T & M 2010 - 8 Nov 2010 Three types of direct thermal detectors 1) Bolometers A bolometer changes its resistance according to the change of the temperature, and thus a high temperature coefficient of resistance (TCR) is needed for high sensitivity. 2) Pyroelectriceffects The pyroelectric effect is exhibited by ferro-electric crystals that exhibit electric polarization. They have no direct current (DC) response and therefore must employ radiation modulators. 3) Thermoelectric effects Two junctions made of two different materials are at different temperatures, and the magnitude of the voltage generated across the thermopile junction depends on the type of materials and the temperature difference between the junctions. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  7. T & M 2010 - 8 Nov 2010 CMOS integration of microbolometers • Thermal sensors based on CMOS technology became feasible when CMOS micromachining(MEMS) was established. • Micromachining makes it possible to remove thermally conducting material for the thermal isolation of heated microstructures. • While thermal effects are intuitively considered to be slow, the small size of CMOS microsensors brings about thermal time constants in the millisecond range. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  8. T & M 2010 - 8 Nov 2010 Principle of bolometer IR detection Infrared radiation Sensor absorbs radiation Temperature increases Circuit to measure resistance Temperature sensitive resistive material thermally isolated from ambient ROIC Readout integrated circuit CEFIM Carl and Emily Fuchs Institute for Microelectronics

  9. T & M 2010 - 8 Nov 2010 Micro-machined silicon sensors Two approaches used to thermally isolatethe sensor structure or part of it from the bulk silicon: Bulk-machined sensors Machining the silicon substrate, either from the front or the back. Membranes can be released by surface etching of the silicon Surface machined sensors Using stacked thin films on the front surface. The mechanical structure is released by removing a sacrificial layer underneath it. A. Hierlemann,O. Brand, C. Hagleitner and H. Baltes, “Microfabrication techniques for chemical/biosensors”, Proceedings of the IEEE, Vol. 91, No. 6, pp. 839-863, June 2003. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  10. T & M 2010 - 8 Nov 2010 Bulk machined device - CEFIM, UP CEFIM Carl and Emily Fuchs Institute for Microelectronics

  11. T & M 2010 - 8 Nov 2010 Surface machined bolometer - IMEC, Belgium First ever submicron support beams Stiffness enhancement techniques U-profile – 100 nm sensor thickness – 2.5 ms thermal time constant SEM picture of 50 x 50 micron polySiGe bolometer CEFIM Carl and Emily Fuchs Institute for Microelectronics

  12. T & M 2010 - 8 Nov 2010 Bolometer thermosensitive materials • Important considerations in choosing bolometer material • A high TCR, • Low noise, especially 1/f noise, • Not too high resistivity, and • Compatibility with post processing IC fabrication. Many materials have been used for bolometers, such as • Metals ( Pt, Ti), and • Semiconductors (VOx, amorphous silicon). The semiconductor materials exhibits a TCR of approximately –2 %/K, which is 10 times that of metals. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  13. T & M 2010 - 8 Nov 2010 Bolometer thermal analysis  = fraction IR power absorbed PO = incident IR power, W G = thermal conductance, W/K = GGAS + GSOL f = frequency of modulation, Hz  = thermal time constant = H/G, sec H = thermal capacitance, J/K Gaseous thermal Heat conductance GGAS of suspended plate PO Membrane or Plate GSOL TB Solid thermal H conductance T of supporting leg Thermal capacity TS of suspended plate • TB = Plate (bolometer) temperature • TS = Substrate (ambient) temperature CEFIM Carl and Emily Fuchs Institute for Microelectronics

  14. T & M 2010 - 8 Nov 2010 Bolometer electrical analysis IB VB Voltage sensitivity RV : IB With noise voltage vn in system, we can define the system performance parameter NETD, the Noise Equivalent Temperature Difference. t ton<<  with Ad = bolometer area, m2 Trade-off For RV and NETD , we need  ,  , G , Ad CEFIM Carl and Emily Fuchs Institute for Microelectronics

  15. T & M 2010 - 8 Nov 2010 Thermal conductivity vs. fill factor trade-off Single level surface machining A. Rogalski, “Optical detectors for focal plane arrays”, Opto-Electronics Review, Vol. 12, No. 2, pp. 221-245, 2004. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  16. T & M 2010 - 8 Nov 2010 Advanced detector structures Surface machining Higher fill factors than bulk machining Double level machining Higher fill factors than single level machining Single level Double level Surface micromachining Surface micromachining D. Murphy et al, “Resolution and sensitivity improvements for VOxmicrobolometer FPAs”, Proceedings of SPIE Vol. 5074, Infrared Technology and Applications XXIX, pp. 402-413, 2003. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  17. T & M 2010 - 8 Nov 2010 Comparing single and double level devices Single level , 50 m pixel Double level , 25 m pixel D. Murphy et al, “Resolution and sensitivity improvements for VOxmicrobolometer FPAs”, Proceedings of SPIE Vol. 5074, Infrared Technology and Applications XXIX, pp. 402-413, 2003. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  18. T & M 2010 - 8 Nov 2010 Bolometer performance vs. pixel dimension Constant thickness, TCR, absorption and ROIC design 1000 Double level 1 m design rules Double level 2 m design rules Single level 1 m design rules 100 Single level 2 m design rules Relative performance 10 1 10 20 30 40 50 60 Pixel dimension (m) D. Murphy et al, “Resolution and sensitivity improvements for VOxmicrobolometer FPAs”, Proceedings of SPIE Vol. 5074, Infrared Technology and Applications XXIX, pp. 402-413, 2003. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  19. T & M 2010 - 8 Nov 2010 Surface machined bolometer at L-3 Communications, USA C.M. Hanson et al, “Small pixel a-Si/a-SiGe bolometer focal plane array technology at L-3 Communications”, Proc. of SPIE Vol. 7660, 76600R-2, 19 May 2010. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  20. T & M 2010 - 8 Nov 2010 L-3 Communications, USA TCR = 3.9 % / K 17μm pixel technology 1024  768 NETD8-12μm ~ 35mK Thermal time constant ~10ms Gth~ 5 nW/K 0.35μm photolith IR absorptance ~ 90% CEFIM Carl and Emily Fuchs Institute for Microelectronics

  21. T & M 2010 - 8 Nov 2010 Our experimental device 600 nm Au 5 µm 60 nm Ti Cavity 100 nm SiO2 Membrane 900 nm Si 3N4 Si 3N4 200 nm SiO2 • 2μm Cavity • 2μm Al 1 μm SiO2 Ti Si bulk 9 μm 73 μm Wi= width of supporting leg di = thickness of supporting leg li = length of supporting leg i = thermal conductivity of supporting leg material Ad= device area ds = cavity separation gas = gas thermal conductivity = 0.026 W/mK , 1 atm N2 = 0 in vacuum 29 μm • 97 μm CEFIM Carl and Emily Fuchs Institute for Microelectronics

  22. T & M 2010 - 8 Nov 2010 Thermal conductance vs. pressure 10-4 Conventional model: G = GSOL + GGAS 10-5 G GSOL Thermal conductance (W/K) 10-6 GGAS 10-7 10-4 10-2 100 102 104 Pressure (Torr) M. Ou-Yang and J. Shie, “Measurement of effective absorbance on microbolometers”, IEEE Tran. Instr. and Meas., Vol. 55, No. 3, pp.1012-1016, June 2006. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  23. T & M 2010 - 8 Nov 2010 Our improved analytical model 1,2 W e 1. Cross section d Sidewall thermal gaseous conduction e Heat flow d s Substrate 2. T T T T T(x) 1 2 3 4 Equivalent thermal length Lth rsolx P exp(-x/Lth) ggasx x x x CEFIM Carl and Emily Fuchs Institute for Microelectronics

  24. T & M 2010 - 8 Nov 2010 Our improved analytical model 3,4 3. T(x) T T T T 0 1 2 3 T(L) rsolx Distrubuted thermal conduction in legs P E P +P S E x p p p p x L P x=L S x 4. Plate and leg Wp Spreading resistance RC We N. Topaloglu, P.M. Nieva, M. Yavuz, J.P. Huissoon, “Modeling of thermal conductance in an uncooledmicrobolometer pixel”, Sensors and Actuators A, Vol. 157, 2010, pages 235 to 245. CEFIM Carl and Emily Fuchs Institute for Microelectronics

  25. T & M 2010 - 8 Nov 2010 Thermal modeling and simulation at atmospheric pressure 50 Conventional model F sw 40 R C K Modified model 30 L th CoventorWare 20 simulation T(x) Short section 10 Long section Plate 0 0 20 40 60 80 100 120 140 Distance x μm CEFIM Carl and Emily Fuchs Institute for Microelectronics

  26. T & M 2010 - 8 Nov 2010 Thermal modelling and simulation under vacuum conditions 50 Conventional model Modified model CoventorWare 40 K 30 20 T(x) Short section 10 Long section Plate 0 0 20 40 60 80 100 120 140 Distance μm CEFIM Carl and Emily Fuchs Institute for Microelectronics

  27. T & M 2010 - 8 Nov 2010 Experimental determination of the thermal conductance at atmospheric pressure 6.1 IB 6.0 VB DC G = 60 μW/K 5.9 RB1 1( 104) 5.8 RB = VB / IB • / G = 17.5 • = 0.1 % / K 5.7 Self heating of device: 5.6 5.5 0 0.5 1.0 1.5 2.0 IB2A2( 106) CEFIM Carl and Emily Fuchs Institute for Microelectronics

  28. T & M 2010 - 8 Nov 2010 Experimental determination of the thermal time constant at atmospheric pressure Bolometer current iB Bolometer voltage vB Time (RH) I Bolometer H VL =  IL RL VT =  IH RL v B V VH =  IH RH i (RL) B I L t f Time V = IH ( RH  RL ) = IHR The bolometer resistance will rise exponentially during tf with the exponential time constant equal to the thermal time constant  CEFIM Carl and Emily Fuchs Institute for Microelectronics

  29. T & M 2010 - 8 Nov 2010 Thermal time constant transient curve vB(t) 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0 100 200 300 400 500 600 Time μs Thermal time constant  = 160 μs Thermal capacitance H =   G = 9.5 nJ/K (Atmospheric pressure) CEFIM Carl and Emily Fuchs Institute for Microelectronics

  30. T & M 2010 - 8 Nov 2010 Predicted thermal parameters under vacuum conditions for our device Atmospheric pressure Vacuum G 60 μW/K 600 nW/K  160 μs 16 ms H 9.5 nJ/K 9.5 nJ/K CEFIM Carl and Emily Fuchs Institute for Microelectronics

  31. T & M 2010 - 8 Nov 2010 “Acid test” for IR room temperature system State of the art microbolometer NETD ≈ 30 mK CEFIM Carl and Emily Fuchs Institute for Microelectronics

  32. T & M 2010 - 8 Nov 2010 Conclusions Theory/modelingand design of IR bolometers well understood Improvements to analytical modeling – atmospheric pressure Experimental determination of thermal parameters THANKS TO AMTS (TIA) CEFIM Carl and Emily Fuchs Institute for Microelectronics