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For realization of a highly sensitive thermal microflow sensor with small active area of 100 X 100 micrometers 2, a main interest is focused on decreasing thermal loss due to a substrate and air medium. A basic microstructure that was composed of a vacuum cavity with 6.2 micrometers depth and a stacked membrane was formed by the DECTOR (deep cavity using trench oxidation and release) process using silicon surface micromachining. On the vacuum platform, a n+-doped heater and two n+/p+-doped thermopiles with 1.0-micrometers -width poly-Si lines and Pt RTDs as major parts of the flow sensor were subsequently implemented by CMOS processing. The completed sensor had a microfluidics with a microchannel of 500(w) X 200(d) micrometers 2. The thermopiles as main temperature sensors showed fast thermal response time of 68 microsecond(s) and maximum thermoelectric responsivity of 25 mV/mW. Flow measurements up to 430 sccm for air and 80 (mu) l/min for water revealed that the sensor outputs were significantly enhanced with the increase of the heater power and decrease of the distance between the heater and thermopile hot junction. Irrespective of a much smaller active area compared to bulk-micromachined thermal flow sensors, a high sensitivity of about 3.08 X 10-2 mV/mW/sccm and 2.3 X 10-2 mV/mW/((mu) l/min) was achieved for air and water as working fluid, respectively. Thanks to the vacuum platform and optimized sensor configuration, it is possible to improve flow sensitivity and to extend a linear flow range, accompanying with a reduced sensor size and low power consumption.
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