Improving sensor performance by characterising the selectivity in vapour etching
The vapour etching of sacrificial layers is often a critical process in the fabrication of micro/nano electromechanical systems (MEMS/NEMS) sensors. Compared to wet etch methods, it has several advantages. Smaller devices can be fabricated because stiction does not occur, sample cross-contamination can be avoided, and it is safer to operate. However, in contrast to wet etching, signi cantly lower etch selectivities are reported in the literature and observed by industry practitioners, limiting both this release method’s and MEMS/NEMS sensors potential. This work aims to improve the etch selectivity for the most commonly used vapour etch processes, the silicon etching with xenon di uoride (XeF2) and hydrogen uoride (HF) etching of silicon dioxide. A novel test structure and measurement methodology that allows the accurate selectivity determination for a number of materials and resembles MEMS fabrication conditions was developed, fabricated and characterised. The selectivity of XeF2 vapour etch processes were characterised with this methodology. It was observed that materials such as silicon nitride, which are commonly inert to XeF2 etched when located close to the sacri cial layer, and methods to improve the selectivity were evaluated. Firstly, it was observed that reducing the processing temperature from 25 to 10 °C increases the silicon (Si) to silicon nitride (SiN) selectivity by 68 %. Secondly, the Si: PECVD SiN selectivity improved by an order of magnitude and the Si: LPCVD SiN selectivity between 200 % and 600 % when moderate amounts of hydrogen were added to the processing gas mixture. In contrast to xenon di uoride vapour etching, a catalyst (water or alcohol) and the formation of a thin liquid layer on the sample is required to facilitate hydrogen uoride vapour etching. To improve the limited process control resulting from the complex condensation phenomena, a novel model, which calculates the partial pressures of the individual gas components to establish vapour pressure within the gas phase, was developed and characterised. It was observed that vapour HF etching behaves similar to wet HF etching under these controlled conditions. The silicon dioxide to silicon nitride selectivity was demonstrated to improve by 150 % when reducing the processing temperature from 20 to 5 °C and by 166 % when increasing the liquid lm’s HF concentration from 20 – 90 %. The methods developed in this work substantially improve the vapour etch selectivity and enable the development of smaller, more sensitive and more robust micro and nanosensors.