Micromachined pressure sensors based on micro-electro-mechanical-systems (MEMS) technology are commonly used in a range of applications including medicine, aviation, and electronics. A capacitive transducer is a major form of the micro-electro-mechanical-systems and has several benefits over the piezo-resistive sensors, which include small temperature drift, high resistance to packaging stress, high sensitivity, and low power consumption.
MEMS sensors have been developed by implementing stainless steel for applications in biomedical and harsh environment applications. In the latter application, the silicon die of the sensor is normally encapsulated in a robust housing and sealed by a protective diaphragm, which are both made of corrosion resistant as well as high fracture strength stainless steel. This packaging comes with increased manufacturing cost and alteration in the sensing behavior after packaging.
There have been developed studies to construct capacitive pressure sensors with the sensing diaphragm and the substrate directly out of stainless steel in a bid to eliminate the need for additional packaging, therefore, avoiding the stated problems. Stainless steel-based sensors exhibit superior robustness against high pressures as well as corrosive media. However, this comes at a cost of pressure sensitivity that is caused by the high stiffness of the diaphragms. Therefore, to address this concern, there is a need to use other stainless steel-based capacitive pressure sensors that encompass non-stainless steel diaphragm.
Researchers led by Professor Kenichi Takahata at The University of British Columbia in Vancouver developed a capacitive gauge pressure sensor that was built on the bulk-micro machined medical-grade chip with a gold-covered sensing diaphragm. The diaphragm was to offer chemical robustness as well as biocompatibility. For the sensor design, the authors analyzed the incorporation of dead-end hole structures in the reference cavity for its advantage in improving sensors sensitivity. Their work is published in Sensors and Actuators A: Physical.
The authors developed a 1.5×1.5 mm2 stainless-steel chip of capacitive pressure sensor as well as its integration method with an aim on smart implant applications. They entirely micro fabricated the sensors with biocompatible materials via thermal bonding of gold-polyimide diaphragm film to the chip dies, which were made of medical-grade stainless steel. They verified the reference cavity design with dead-end holes made in the stainless steel die both experimentally and theoretically in order to improve pressure sensitivity. The authors also investigated laser micro welding as a substitute path for this form of stainless steel-based sensors. This was in a bid to establish a permanent and reliable bond onto stainless steel platform gadgets.
As expected from theoretical analysis, the presence of dead-end holes inside the air-sealed reference cavity was confirmed experimentally to improve the sensor’s sensitivity. The authors observed that the fabricated sensors exhibited an average sensitivity of about 100 ppm/mmHg over an average gauge pressure of 250 mmHg. The sensors were laser micro welded to the stainless steel substrates and indicated no considerable alteration of their sensing capability after welding. The welding process was observed to provide mechanical as well as electrical advantage over the typical conductive epoxy bonding.
The sensor incorporation through micro welding was as well shown on stainless steel stents and hypodermic needle, as an initial path towards its implementation to developing smart devices with a pressure monitoring capability. Therefore, laser micro welding is a promising method for packaging developed sensors and a number of micro-devices based on stainless steel materials.
Xing Chen, Daniel Brox, Babak Assadsangabi, Mohamed Sultan Mohamed Ali, and Kenichi Takahata. A stainless-steel-based implantable pressure sensor chip and its integration by micro welding. Sensors and Actuators A, volume 257 (2017), pages 134–144.Go To Sensors and Actuators A