Structural design and characteristics of a thermally isolated, sensitivity-enhanced, bulk-micromachined, silicon flow sensor

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Abstract
A thermal flow sensor which has been bulk micromachined from a high-resistivity (, n-type) silicon wafer is herein reported. A structurally and functionally complex sensing element has been fabricated, using standard etching and deposition techniques, which demonstrates a very high degree of thermal isolation, as well as uniquely enhanced sensitivity characteristics. This device is explored primarily from a structural point of view to illustrate the formation of silicon nitride bridges, in the form of `chevrons', which provide its thermal isolation. Results of extensive testing of the electrical and mechanical characteristics are presented which demonstrate the thermal sensitivity effects. The current - voltage characteristic curve is shown to be S shaped (i.e. with a negative differential resistance regime), and results in both positive- and negative-temperature-coefficient modes of operation, as well as high sensitivity. The voltage - flow velocity curve is found to be similar to that of other `hot-wire' type devices and yields a flow sensitivity of about in air, depending on range and bias. The transient response of these sensors was obtained by using single pulses of current, to validate the effectiveness of the thermal isolation structure. The time response depends on the varied size (mass) of the device, but response times into the millisecond range have been obtained. Vibration tests on the silicon nitride thermal isolation bridges showed 95% mechanical yield after processing and virtually no degradation after vibration up to 50 g at 100 Hz.