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A novel TUB (thin-film under bulk) process for high-performance pressure sensors of sub-kPa measure-range

For the first time, the study proposes and develops a MEMS process of TUB (thin-film under bulk) to develop a novel MEMS configuration, where a surface-micromachined poly-Si thin-film is constructed under a bulk-micromachined single-crystalline silicon (SC-Si) layer. The TUB process can be performed in standard IC-foundry to volume fabricate complex-structured MEMS devices in single (111)-silicon wafer. As the first application example of the TUB technology, piezoresistive pressure sensors for low range measurement are developed. The low-range pressure sensors have been high-yield fabricated, with the tiny chip size of 1.2mm by 1.2mm. Thanks to the proposed TUB process where etching stop is realized for removing CS-Si but retaining poly-Si, the complex sensor structure of CS-Si beam-island reinforced poly-Si thin-diaphragm can be uniformly fabricated (see Figure 1). The CS-Si beam-mass reinforced poly-Si diaphragm well restricts large deflection induced nonlinear output signal, while the stress concentration effect on the CS-Si secures piezoresistive sensitivity to pressure. With the piezoresistive Wheatstone-bridge integrated in the CS-Si beam, high-performance piezoresistive output signal is achieved. The fabricated pressure sensor exhibits sensitivity of 13.36mV/kPa before amplification and low nonlinearity of 0.3%, thereby providing sub-kPa range applications like distributed outlet monitoring of central air-conditional system. The bulk/surface combined micromachining process is promising for various complex MEMS structures.