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The etching of high aspect ratio structures in silicon via the Bosch process is essential in modern technologies such as microelectromechanical systems (MEMS) and through‑silicon vias (TSV) fabrication. The process can be very demanding on the mask selectivity due to long etching times, and it has been shown that an Al2O3 hard mask is very suitable in this regard, as it offers significantly higher selectivity compared to the conventional SiO2 or resist masks. In this work, we employ a combination of Scanning Electron Microscopy (SEM), Spectroscopic Ellipsometry (SE) and X-Ray Photoelectron Spectroscopy (XPS) depth profiling to scrutinize the Al2O3 mask etching mechanism and therefore the origin of the extraordinary high selectivity. We demonstrate that by increasing the passivation step time, a thicker fluorocarbon polymer layer is formed on the Al2O3, and Al2O3 is then removed with a minuscule average etch rate of 0.01 nm/min. XPS depth profiling reveals that during Deep Reactive Ion Etching (DRIE) using the Bosch process, an AlFx layer is formed between the polymer and Al2O3. As AlFx is non-volatile, it requires sputtering to be removed. If the polymer layer is thick enough to attenuate the incoming ions such that their energy is not sufficient to lead to desorption of AlFx, such as when using a longer passivation time, the mask is not eroded. By investigating the surface after different amounts of DRIE cycles, we also obtained information about the formation rate of AlFx and the changes in the Al2O3 and polymer thicknesses over the course of a DRIE process. These findings further expand the knowledge of DRIE and can help process engineers to tailor the processes accordingly.
The resistive switching properties of HfO₂ based 1T-1R memristive devices are electrically modified by adding ultra-thin layers of Al₂O₃ into the memristive device. Three different types of memristive stacks are fabricated in the 130 nm CMOS technology of IHP. The switching properties of the memristive devices are discussed with respect to forming voltages, low resistance state and high resistance state characteristics and their variabilities. The experimental I–V characteristics of set and reset operations are evaluated by using the quantum point contact model. The properties of the conduction filament in the on and off states of the memristive devices are discussed with respect to the model parameters obtained from the QPC fit.
The two main features of the memristive devices which makes them the promising candidates for neuromorphic applications are low power consumption and CMOS compatibility. The monolithic integration of memristive devices with CMOS circuitry paves the way for in-memory computing. This chapter focuses on the factors governing the CMOS integration process. Firstly, the influence of CMOS baseline technology selection on the memristor module is briefly discussed. Secondly, the selection of metal level interconnects and their effect on the memristive device performance is explained. Further, the widely used deposition technique for the CMOS compatible memristive switching layers is presented. Finally, the implementation of the optimized process for the fabrication of the memristive module and its influence on the device performance is presented in terms of electrical characterization results.
Plasma-enhanced atomic layer deposition of silicon nitride for front-end-of-line applications
(2023)
SiN deposition at low temperatures using the plasma-enhanced atomic layer deposition (PEALD) process is gaining momentum. A high-quality SiN layer deposited using a lower thermal budget and low wet etch rates are desired for front-end-of-line applications in semiconductor industries. In this study, deposition of PEALD SiN is investigated by utilizing a highly reactive trisilylamine silicon precursor and three different reaction partners for a nitrogen precursor. The quality of PEALD SiN layers is compared with the reference standard, low-pressure chemical vapor deposition SiN layers. The properties of different SiN layers are interpreted using FTIR and XPS material characterization techniques. Furthermore, the wet etch rates of as-deposited and annealed PEALD SiN layers are investigated. Finally, the conformality of PEALD SiN layers is assessed in trench and horizontal high aspect ratio structures.