Silicon nanowires (SiNWs) have been reported as a promising piezoresistive material, due to its giant piezoresistive effect under deformation and ultra-compactable dimension. By leveraging SiNWs with MEMS mechanical sensors, the device dimension could be significantly reduced without compromising the sensor’s sensitivity. In addition, due to its high piezoresistance and stable performance even under ultra low supply voltage, i.e, 0.1V, a total power consumption below 1 μW can be achieved for future ultra low power system..

With this in mind, various types of mechanical sensors such as pressure sensor and air flow sensor based on SiNWs have been demonstrated by our group. The mechanical force can be induced by pressure differences, variations of air flow rate or changes of momentum. To maximize the piezoresistive effect, the SiNWs are fabricated on (100) wafer plane along <110> crystal oriented direction and located at the conjunction point between deformable structure and rigid substrate. Therefore, the mechanical force induced deformable structure movement will cause maximum change in strain of the SiNWs. The SiNWs experiences either tensile stress or compressive stress depending on the direction difference of the applied mechanical force. When the tensile stress is applied, the SiNWs get elongated, hence the increment of piezoresistance occurs. However, when compressive stress is applied, opposite piezoresistive effect happens. In the pressure sensors developed, the SiNWs is embedded between the top passivation and bottom box layers. The 2.5μm SiNx layer is coated on top of the sensor diaphragm as passivation, resulting in almost zero initial deflection of sensor diaphragm. In addition, with such embedded structure, the strain changes on SiNWs can be as high as 1.7 %, which are almost 28 times larger compared with reported strain changes of only 0.06% based on traditional 4-probe bending setup. Therefore, SiNWs with extended deformation range can be realized by our unique design.

In the case of MEMS air flow sensor, the sensing areas of current reported piezoresistive air flow sensors are still of millimeter dimension as they are limited by the size of the piezoresistors. With SiWNs as piezoresistive sensing material, the effective sensing area of our new reported flow sensor is now only 20 μm x 90 μm. After external amplification and effective sensing area normalization effects, our reported flow sensor has improved the sensitivity by 3 orders of magnitude.

Selected Publications:

1. Liang Lou, Hongkang Yan, Woo-Tae Park, Dim-Lee Kwong and Chengkuo Lee, Characterization of Piezoresistive Si Nanowires Based Pressure Sensors by Dynamic Cycling Test with Extra-Large Compressive Strain, IEEE Trans. Electron Devices, vol. 59, no. 11, pp. 3097-3103, 2012. [PDF] [DOI]
2. Songsong Zhang, Liang Lou, Woo-Tae Park and Chengkuo Lee, Characterization of silicon nanowire based cantilever air flow sensor, J. Micromech. Microeng., vol. 22, no. 9, 095008, 2012. [PDF] [DOI]
3. Liang Lou, Songsong Zhang, Woo-Tae Park, Julius Ming-Lin Tsai, Dim-Lee Kwong and Chengkuo Lee, Optimization of NEMS pressure sensors with multilayered diaphragm using silicon nanowires as piezoresistive sensing elements, J. Micromech. Microeng, vol. 22, no. 5, 055012, 2012. [PDF] [DOI]

 

<<< BACK TO RESEARCH AREAS