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

Manipulating etch selectivities in XeF2 vapour etching

The vapour etching of silicon sacrificial layers is often a critical process in the fabrication of micro/nanosystems. This method has a number of attractive features, in particular, high etch rates of sacrificial silicon layers and good selectivities associated with photoresist, SiO₂, stoichiometric Si₃N₄ and a number of regularly used metal films. However, materials that are commonly inert to XeF₂ are etched when located in the proximity of a silicon sacrificial layer. This proximity is a common situation in the fabrication of such systems and can become a critical issue affecting process control and device reliability. This work uses test structures that have been designed to be very sensitive, thereby delivering much lower selectivities then are typically reported in the literature. This sensitive quantification of the proximity effect is used to evaluate methods designed to improve the selectivity. This work suggests that a reduction in the processing temperature from 25°C to 10°C increases the Si: PECVD SiN selectivity by 68%. However, a more easily implemented modification is to flow hydrogen into the reaction chamber. This method improves the Si: PECVD SiN selectivity by an order of magnitude and the Si: LPCVD SiN selectivity between 200% and 600%. [2020-0346]

Test Structure for Measuring the selectivity in XeF2 and HF Vapour Etch Processes

Etch selectivity between layers is an important parameter in the fabrication of microelectronic and microsystems. This is particularly true in the case of isotropic gas/vapour etching methods used to release free standing structures through the selective etching of sacrificial layers. Commonly used structural materials have been reported to be largely inert when exposed to a given vapour etchant, indicating high selectivity when measured against typical sacrificial layers. However, there is growing evidence that these structural layers are actually etched at an enhanced rate if they are located in the proximity of the sacrificial layer being removed. Hence, removal rates given in the literature that have resulted from measurements of layers that have been etched in isolation can no longer be trusted to characterize critical etch processes in device fabrication. In this paper, a test structure is reported that enables a far more accurate determination of the etch selectivity between sacrificial and structural materials. The method is demonstrated with the two most common vapour etch processes. Firstly, the XeF2 vapour etch of a polysilicon sacrificial layer located above a silicon nitride structural layer, and secondly, the HF vapour etch of silicon dioxide placed above a silicon nitride structural layer. Both datasets are presented. The polysilicon and silicon nitride layers, etched with XeF2 show a selectivity of 5:4. The silicon dioxide and silicon nitride layers etched with HF, show a selectivity of 6: 1 to 8: 1. This file includes the data underlying the figures 7 and 8 in this work.Ronde, Markus. (2020). Test Structure for Measuring the selectivity in XeF2 and HF Vapour Etch Processes, [dataset]. University of Edinburgh. School of Engineering. Institute of Integrated Micro and Nanosystems

Test structure for measuring the selectivity in vapour etch processes

Etch selectivity between layers is critical in the fabrication of microelectronics and microsystems. This is particularly true in the case of isotropic gas/vapour etching methods used to release free standing structures through the selective etching of sacrificial layers. Commonly used structural materials have been reported to be largely inert when exposed to a given vapour etchant, indicating high selectivity when measured against typical sacrificial layers. However, there is growing evidence that these structural layers are actually etched at an enhanced rate if they are located in the proximity of the sacrificial layer being removed. Hence, removal rates given in the literature that have resulted from measurements of layers that have been etched in isolation can no longer be trusted to characterize critical etch processes in device fabrication. In this paper, a test structure is reported that enables a far more accurate determination of the etch selectivity between sacrificial and structural materials. The method is demonstrated by a XeF2 vapour etch of a polysilicon sacrificial layer located above a silicon nitride structural layer. A dataset is presented for the polysilicon and silicon nitride layers, which shows a selectivity of 5:4. The raw dataset for Fig. 6 is provided.Ronde, Markus; Walton, Anthony J; Terry, Jonathan G. (2020). Test structure for measuring the selectivity in vapour etch processes. [dataset]. University of Edinburgh. School of Engineering

Manipulating Etch Selectivities in XeF2 Vapour Etching

This workbook contains the raw data for figures 2 - 8, some datasets were used to generate multiple figures. In this case, they are summarised in one datasheet. You can switch between them using the tabs at the bottom of the spread sheet. The experimental method was previously presented at the ICMTS conference in 2020, more details are available in: M. Rondé, A. J. Walton and J. G. Terry, "Test Structure for Measuring the Selectivity in Vapour Etch Processes," 2020 IEEE 33rd International Conference on Microelectronic Test Structures (ICMTS), Edinburgh, United Kingdom, 2020, pp. 1-5, doi: 10.1109/ICMTS48187.2020.9107934. It is recommended to read this publication in order to fully understand how the data presented here was generatedRondé, Markus; Terry, Jonathan; Walton, Anthony. (2020). Manipulating Etch Selectivities in XeF2 Vapour Etching, [dataset]. The University of Edinburgh. School of Engineering. Institute of Integrated Micro and Nanosystems.https://doi.org/10.7488/ds/2929