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Title Ultra-low power microbridge gas sensor
Publication Date
Date Accessioned
Degree MS
Discipline/Department Mechanical Engineering
Degree Level masters
University/Publisher Georgia Tech
Abstract A miniature, ultra-low power, sensitive, microbridge gas sensor has been developed.The heat loss from the bridge is a function of the thermal conductivity of thegas ambient. Miniature thermal conductivity sensors have been developed for gaschromatography systems [1] and microhotplates have been built with MEMS technologywhich operates within the mW range of power [2]. In this work a lower power microbridgewas built which allowed for the amplification of the effect of gas thermalconductivity on heat loss from the heated microbridge due to the increase inthe surface-to-volume ratio of the sensing element. For the bridge fabrication,CMOS compatible technology, nanolithography, and polysilicon surfacemicromachining were employed. Eight microbridges were fabricated on each die,of varying lengths and widths, and with a thickness of 1 μm. A voltagewas applied to the sensor and the resistance was calculated based upon thecurrent flow. The response has been tested with air, carbon dioxide, helium,and nitrogen. The resistance and temperature change for carbon dioxide was thegreatest, while the corresponding change for helium was the least. Thus the selectivity of the sensor todifferent gases was shown, as well as the robustness of the sensor. Another aspect of the sensor is that it hasvery low power consumption. The measuredpower consumption at 4 Volts is that of 11.5 mJ for Nitrogen, and 16.1 mJ forHelium. Thesensor responds to ambient gas very rapidly. The time constant not only showsthe fast response of the sensor, but it also allows for more accuratedetection, given that each different gas produces a different correspondingtime constant from the sensor. The sensor is able to detect differentconcentrations of the same gas as well. Fromthe slopes that were calculated, the resistance change at 5 Volts operation wasfound to be 2.05mΩ/ppm, 1.14 mΩ/ppm at 4.5 Volts, and 0.7 mΩ/ppm at 4 Volts. Thehigher voltages yielded higher resistance changes for all of the gases thatwere tested. Theversatility of the microbridge has been studied as well. Experiments were donein order to research the ability of a deposited film on the microbridge, inthis case tin oxide, to act as a sensing element for specific gases. In thissetup, the microbridge no longer is the sensing element, but instead acts as aheating element, whose sole purpose is to keep a constant temperature at whichit can then activate the SnO film, making it able to sense methane. In conclusion,the microbridge was designed, fabricated, and tested for use as an electrothermalgas sensor. The sensor responds to ambient gas very rapidly with differentlevels of resistance change for different gases, purely due to the differencein thermal conductivity of each of the gases. Not only does it have a fastresponse, but it also operates at low power levels. Further research has beendone in the microbridge's ability to act as a heating element, in which the useof a SnO film as the sensing element, activated by the microbridge, was studied. REFERENCES: 1. D.…
Subjects/Keywords Hydrogen detector; Methane detector; Helium detector; Thermal conductivity detector; Gas sensor; Microbridge; Palladium film; Tin oxide film; Gas detectors
Contributors Peter Hesketh (Committee Chair); Mostafa Ghiaasiaan (Committee Member); Todd Sulchek (Committee Member)
Country of Publication us
Record ID handle:1853/43723
Repository gatech
Date Indexed 2020-01-03
Issued Date 2012-04-06 00:00:00
Note [degree] MS; [advisor] Committee Chair: Peter Hesketh; Committee Member: Mostafa Ghiaasiaan; Committee Member: Todd Sulchek;

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…schematic of thermal conductivity microbridge gas sensor 6 Figure 2.1: Original design; typical die with 100x2 m microbridges 7 Figure 2.2: a) Schematic of a cross section of a typical microbridge; b) Schematic of a close up of a typical…

microbridge 8 Figure 2.3: Final die design for Die 1 10 Figure 2.4: a) Die 1 – 50x1 µm; b) Die 2 – 100x2 µm with 50x1 µm center; c) Die 3 – 50x1 µm with 25x0.5 µm center 11 Figure 2.5: a) Die 4 – 2 point, 50x1 µm; b) Die 5 – 2 point…

…due to RIE plasma 20 Figure 2.9: Schematic of the fabrication procedure for the microbridges 21 Figure 2.10: Shadow mask windows – a) Die 4; b) Die 5; c) Die 6 22 Figure 2.11: SEM image of palladium coated microbridge 23 Figure…

…2.12: a) SEM image of released microbridge; b) Picture of a packaged sensor 24 Figure 2.13: SEM image showing the double exposure method 25 Figure 2.14: UV absorption of ma-N2403 resist 26 Figure 2.15: Desired line width 500 nm; a)…

…dose of 250 mC/cm2, producing a line width of about 434 nm; b) dose of 270 mC/cm2, producing a line width of about 451 nm 27 Figure 2.16: Initial alignment mark after platinum deposition viii 28 Figure 2.17: Microbridge pattern using ma-N2403…

…43 Figure 3.10: Resistance change as a function of different methane concentrations in mixture 44 Figure 3.11: a) Resistance vs. temperature for the 50x1 m microbridge; b) Resistance vs. temperature for the 100x2 m with 50x1 m center…

microbridge 46 Figure 3.12: Resistance vs. temperature linear fit – a) 50x1 µm microbridge; b) 100x2 µm with 50x1 µm center microbridge; c) Separate 50x1 µm microbridge up to 100°C 48 Figure 4.1: Schematic of first experimental setup on the new…

…sensors 50 Figure 4.2: Schematic of life test setup 51 Figure 4.3: Schematic of palladium coated sensor setup 52 Figure 4.4: Schematic of tin oxide coated sensor setup 54 Figure 4.5: Microbridge with platinum runner 55 ix Figure 4.6: Resistance…