STRUCTURAL DESIGN AND CHARACTERISTICS OF A THERMALLY ISOLATED, SENSITIVITY-ENHANCED, BULK-MICROMACHINED, SILICON FLOW SENSOR

A thermal flow sensor which has been bulk micromachined from a high-re sistivity (50 Omega cm, n-type) silicon wafer is herein reported. A st ructurally and functionally complex sensing element has been fabricate d, using standard etching and deposition techniques, which demonstrate s a very high degree of thermal isolation, as well as uniquely enhance d sensitivity characteristics. This device is explored primarily from a structural point of view to illustrate the formation of silicon nitr ide bridges, in the form of 'chevrons', which provide its thermal isol ation. Results of extensive testing of the electrical and mechanical c haracteristics are presented which demonstrate the thermal sensitivity effects. The current-voltage characteristic curve is shown to be S sh aped (i.e. with a negative differential resistance regime), and result s in both positive- and negative-temperature-coefficient modes of oper ation, as well as high sensitivity. The voltage-flow velocity curve is found to be similar to that of other 'hot-wire' type devices and yiel ds a flow sensitivity of about 60 mV (m s(-1))(-1) in air, depending o n range and bias. The transient response of these sensors was obtained by using single pulses of current, to validate the effectiveness of t he thermal isolation structure. The time response depends on the varie d size (mass) of the device, but response times into the millisecond r ange have been obtained. Vibration tests on the silicon nitride therma l isolation bridges showed 95% mechanical yield after processing and v irtually no degradation after vibration up to 50 g at 100 Hz.