Due to increasing demand on the fabrication yield and throughput in micro/nanoscale manufacturing, virtual metrology (VM) has emerged as an effective data-based approach for real-time process monitoring. In this work, a novel automated methodology, without the need for domain knowledge and experience, for extracting useful features from raw optical emission spectroscopy (OES) data is presented. Newly proposed OES features are combined with other types of data, which include tool settings, sensor readings, physical measurements, non-numerical data, and process control parameters. Using partial least squares and support vector regression, VM models for predicting the critical dimension after reactive ion etching are built. The results from the VM model indicate that the coefficient of determination of up to 0.65 and the root mean square Error of 0.08 can be achieved. Compared to the traditional features obtained by the current solution in industry, the performances of VM models via the proposed methodology can enhance the coefficient of determination by 62.5% and reduce the root mean square error by 23.1%.
Silicon nanostructures with high aspect-ratio (AR) features have played an important role in many fields. In this study, we report the fabrication of high AR silicon nanostructures using an inductively coupled plasma reactive ion etching (ICP-RIE) process by controlling the voltage bias at the substrate. The results show that by reducing the radio frequency (RF) bias power to 10 W, the etch selectivity of silicon to photoresist can be enhanced up to 36 times. Using the photoresist as a mask, this process can fabricate 300 nm-period one-dimensional (1D) grating structures with a height up to 807 nm, an improvement of 3.75-fold compared with structures fabricated by normal bias power. Furthermore, the analysis of the etch rate shows that the etch rate decreases over time in 1D gratings but remains constant in 2D pillar arrays, which can be attributed to the removal of the sidewall passivation. By including an O2 ICP-RIE step to remove the remaining polymer mask, the highest AR of 2D pillar structures that can be achieved is 8.8. The optical characterization of the fabricated structures demonstrates effective antireflection properties, where the measurements show that the reflectivity can be suppressed from 35% to 0.01% near normal incidence and 35% to 2.4% at 65° incident angle. The demonstrated low-RF power ICP-RIE process can create high AR nanostructures without the need for an inorganic mask and can find applications in integrated circuits, photonics, and functional nanostructures.
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