Abstract
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 (size) to (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.
Highlights
A comprehensive review of these non-thermal flow sensors is presented by Wang et al [21]
Thermal conductivity of polymers is typically lower than the ceramics and these are good candidate substrate materials, as summarized in Table 2, for reduced conduction losses in MEMS thermal flow sensors
It is pertinent to highlight that silicon oxide has very low thermal conductivity (k = 1.4W/(m-K) [118]) and there are hardly any studies, excluding a few reported by our group [5,6,83,125,126,130,131], that utilize silicon oxide membranes for thermal isolation of MEMS
Summary
Flow sensors are extensively used for flow measurements in diverse applications in different fields including aerospace [1,2,3,4,5,6], automotive [7], biomedical [8,9,10,11], environmental [12,13,14,15,16,17], hydrodynamics [18]. Thermal conductivity of polymers is typically lower than the ceramics and these are good candidate substrate materials, as summarized, for reduced conduction losses in MEMS thermal flow sensors. It is pertinent to highlight that silicon oxide has very low thermal conductivity (k = 1.4W/(m-K) [118]) and there are hardly any studies, excluding a few reported by our group [5,6,83,125,126,130,131], that utilize silicon oxide membranes for thermal isolation of MEMS thermal hot-film flow sensors produced through a commercial SOI CMOS process Eight such sensors, with two different membrane shapes (i.e., square and circular) and four.
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