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Through-chip connections
Published in David Crawley, Konstantin Nikolić, Michael Forshaw, 3D Nanoelectronic Computer Architecture and Implementation, 2020
As device dimensions shrink, the connecting pad sizes are also reduced. This requires a large number of anisotropic through-wafer conductive connections with a very small pitch (≤5 μm). A suitable technology for realizing through-wafer holes with an opening size of only a few micrometres is needed. This is very difficult with DRIE as it is very sensitive to area distribution and microloading effects. In this section, we present a novel approach to realize very large arrays of closely spaced through-wafer interconnections using a process based on macroporous silicon formation, wafer thinning and metal electroplating. Macroporous silicon formation by electrochemical etching has been investigated for micromachined components and optical devices [11, 12]. For our investigation very deep pores (100–200 μm) only a few micrometres in size are needed. This means that the process for macroporous silicon formation needs to be tuned to achieve these very-high-aspect-ratio structures. Furthermore, the large number of through-wafer holes in a very small area demands a very good uniformity, i.e. very high anisotropy on a large area. Finally, the metal filling of the pores by Cu electroplating to form the through-wafer plugs is investigated.
Advanced Biotechnology
Published in Lawrence S. Chan, William C. Tang, Engineering-Medicine, 2019
In the mid-1990s, a method to create high-aspect-ratio silicon microstructures by vertically etching into the bulk of the silicon substrate was introduced (Lärmer and Schilp 1996). Unlike conventional reactive-ion assisted plasma etching, the process described in Lärmer and Schilp 1996, commonly known as Deep Reactive Ion Etching (DRIE), actually alternates frequently between plasma etching and plasma deposition of etch inhibitor within the same chamber. An inductively coupled RF magnetic field is also added to enrich the density of ions and neutrals within the plasma to increase the etch rate (inductively coupled plasma, or ICP). During the etch cycles, SF6 is introduced into the chamber long enough to etch silicon materials for up to one or a few μm deep. Then during the deposition steps, C4F8 is introduced instead, which results in the deposition of about 50 nm-thick of fluorocarbon polymer, coating both the sidewalls and the bottom of the etch pits. In the following etch step with SF6, the polymer is removed far more readily from the bottom of the etch pits than the sidewalls due to the directionality of the accelerating ions bombardment, allowing silicon etch to proceed downward while the sidewalls are protected by the fluorocarbon polymer. Common etch masks for this process are photoresist, with an etch selectivity of 100:1 or more, and SiO2, with selectivity up to 300:1. Since its invention, ICP-DRIE has gained widespread popularity and has become one of the most used micromachining processes.
Nanofabrication
Published in Zhigang Li, Nanofluidics, 2018
Compared with RIE, DRIE is highly anisotropic. It is capable of creating deep trenches with steep sidewalls and of high aspect ratio in a substrate. In DRIE, two RF power sources are used, which ensure the generation of high-density plasma (>1011 cm−3) at low pressures (<1Pa), leading to a high etching anisotropy. Another reason that DRIE can achieve steep sidewalls is the Bosch etching technique, which is also known as the pulsed or time-multiplexed etching. The Bosch etching includes two alternating processes: one is the standard etching using plasma and the other is the protection of sidewalls through passivation deposition, which coats the sidewalls by a layer of Teflon-like substance using octafluorocyclobutane (C4F8). These two processes are performed for a short time alternatively. During etching, after the sidewalls and the bottom surface of a channel are deposited with a passivation layer, the standard etching process resumes, which removes the passivation layer on the bottom surface first through ion bombardments and then etches the substrate vertically downward, as shown in Figure 4.4. Channels of a desired depth can be fabricated by controlling the number of cycles of the two processes. Since DRIE is capable of offering fast and anisotropic etching, it can be used for the fabrication of microchannels and fluid reservoirs, which are essential in a nanofluidic system. For nanochannel fabrication, DRIE is less common because the roughness of the sidewalls can be a few to tens of nanometers (Quévy et al., 2002).
Investigation of Various Commonly Associated Imperfections in Radiofrequency Micro-Electro-Mechanical System Devices and its Empirical Modeling
Published in IETE Journal of Research, 2023
A. Karmakar, B. Biswas, A.K. Chauhan
Bulk micromachining is a widely used fabrication process for the realization of multiple kinds of RF devices in MEMS technology. It is achieved either by dry or wet methods of etching process with anisotropic means. DRIE is a well-known dry etching high-aspect-ratio process (HARP). Whereas, anisotropic wet etching method of the silicon wafer is done with the help of TMAH (Tetra Methyl Amino Hydroxide) and KOH (Potassium Hydroxide). Out of these, KOH has become more popular because of its inherent merit of less non-toxic nature and high etch-rate (1.1–1.2 µm/min), as compared to 0.8 µm/min for TMAH solution. The standard process of 40% KOH solution at 800C gives the optimum solution for silicon micromachining. However, it requires a continuous stirring method to maintain the homogeneous property. Etch-stop techniques are implemented for timely terminating this etching process. Although precautions [22-23] are taken during the processing time, there may be a high chance of non-uniformity in case membrane thickness maintaining at the last stage of the whole process, as the whole solution doesn’t remain homogeneous within the entire volume of etch-bath and throughout the whole process duration. This non-uniformity may be of two types: within wafer or wafer-to-wafer. It can be a fatal error in the case of a micro-machined antenna array problem. Though during designing antenna was optimized by assuming uniform membrane thickness for each element, however, due to process variation, each of the array elements gets non-uniform silicon substrate beneath it. Ultimately the resonant frequency will be changed along with its other radiation characteristics. The whole phenomenon can be explained with the help of the proposed electrical model, as shown in Figure 11.