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Open AccessArticle

Sensitivity Enhancement of Silicon-on-Insulator CMOS MEMS Thermal Hot-Film Flow Sensors by Minimizing Membrane Conductive Heat Losses

1
Engineering Department, University of Cambridge, Cambridge CB3 0FA, UK
2
Institute of Avionics & Aeronautics, Air University, E-9, Islamabad 44000, Pakistan
3
AMS Sensors UK Ltd., Deanland House, 160 Cowley Road, Cambridge CB4 0DL, UK
*
Author to whom correspondence should be addressed.
Sensors 2019, 19(8), 1860; https://doi.org/10.3390/s19081860
Received: 7 March 2019 / Revised: 11 April 2019 / Accepted: 15 April 2019 / Published: 18 April 2019
(This article belongs to the Special Issue Semiconductor and CMOS-Based Sensors for Environmental Monitoring)
Minimizing conductive heat losses in Micro-Electro-Mechanical-Systems (MEMS) thermal (hot-film) flow sensors is the key to minimize the sensors’ power consumption and maximize their sensitivity. Through a comprehensive review of literature on MEMS thermal (calorimetric, time of flight, hot-film/hot-film) flow sensors published during the last two decades, we establish that for curtailing conductive heat losses in the sensors, researchers have either used low thermal conductivity substrate materials or, as a more effective solution, created low thermal conductivity membranes under the heaters/hot-films. However, no systematic experimental study exists that investigates the effect of membrane shape, membrane size, heater/hot-film length and M e m b r a n e (size) to H e a t e r (hot-film length) Ratio (MHR) on sensors’ conductive heat losses. Therefore, in this paper we have provided experimental evidence of dependence of conductive heat losses in membrane based MEMS hot-film flow sensors on MHR by using eight MEMS hot-film flow sensors, fabricated in a 1 µm silicon-on-insulator (SOI) CMOS foundry, that are thermally isolated by square and circular membranes. Experimental results demonstrate that: (a) thermal resistance of both square and circular membrane hot-film sensors increases with increasing MHR, and (b) conduction losses in square membrane based hot-film flow sensors are lower than the sensors having circular membrane. The difference (or gain) in thermal resistance of square membrane hot-film flow sensors viz-a-viz the sensors on circular membrane, however, decreases with increasing MHR. At MHR = 2, this difference is 5.2%, which reduces to 3.0% and 2.6% at MHR = 3 and MHR = 4, respectively. The study establishes that for membrane based SOI CMOS MEMS hot-film sensors, the optimum MHR is 3.35 for square membranes and 3.30 for circular membranes, beyond which the gain in sensors’ thermal efficiency (thermal resistance) is not economical due to the associated sharp increase in the sensors’ (membrane) size, which makes sensors more expensive as well as fragile. This paper hence, provides a key guideline to MEMS researchers for designing the square and circular membranes-supported micro-machined thermal (hot-film) flow sensors that are thermally most-efficient, mechanically robust and economically viable. View Full-Text
Keywords: MEMS thermal flow sensors; review; conduction losses; heater/hot-film; membrane shape; membrane to heater ratio; silicon-on-insulator (SOI); complementary metal oxide semiconductor (CMOS) MEMS thermal flow sensors; review; conduction losses; heater/hot-film; membrane shape; membrane to heater ratio; silicon-on-insulator (SOI); complementary metal oxide semiconductor (CMOS)
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Mehmood, Z.; Haneef, I.; Ali, S.Z.; Udrea, F. Sensitivity Enhancement of Silicon-on-Insulator CMOS MEMS Thermal Hot-Film Flow Sensors by Minimizing Membrane Conductive Heat Losses. Sensors 2019, 19, 1860.

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