High-Performance Low-Range Differential Pressure Sensors Formed With a Thin-Film Under Bulk Micromachining Technology

In the microelectromechanical systems (MEMS) field, low-range (e.g., kPa or sub-kPa) differential pressure sensors have remained difficult for decades. Generally, there are two unsolved problems that hinder volume production and wide applications in consumer-electronics. First, high sensitivity required very thin and large sensing diaphragm normally fabricated with low yield, and thus, high selling price. Second, high sensitivity required large deflection of the diaphragm easily causeing severe output of non-linearity that degrades the detecting precision. This paper reports a so-called thin-film under bulk (TUB) MEMS technique to high-yield fabricate high-performance low-range pressure sensors for smart central air-conditioning systems. With the single-wafer-based TUB structure, pressure-induced stress is highly concentrated in a bulk-silicon beam-island structure for piezoresistive readout, where a very thin but uniform poly-silicon diaphragm can be formed beneath the bulk-silicon structure to sustain the pressure. The beam-island reinforced structure brings about decreased deflection and highly linear output. Micro-columns are also formed under the bulk-silicon islands as over-range protection stoppers that are important for low-range pressure sensor. After the description about the 1.2 mm×1.2 mm tiny-sized sensor-chip design and its single-wafer fabrication process, a test is conducted that results in 22-mV output for 1.2-kPa measure range and very good linearity of ±0.05% (FS). About 100× over-range protection capability of the sensor is also achieved. The novel high-yield TUB process can be used for low-cost production of high-performance pressure sensors for the applications of new-generation smart central air-conditional systems and spirometers, etc, and the bulk/surface combined TUB micromachining technique promises to be widely used for the formation of various complex MEMS structures.