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Fundamentals of Microfabrication and MEMS Fabrication Technologies
Published in Sergey Edward Lyshevski, Nano- and Micro-Electromechanical Systems, 2018
In bulk micromachining, wet- and dry-etching processes are widely used. Wet etching is the process of removing material by immersing the wafer in a liquid bath of the chemical etchant. Wet etchants are categorized as isotropic etchants (which attack the material being etched at the same rate in all directions) and anisotropic etchants (which attack the material or silicon wafer at different rates in different directions, and therefore shapes/geometry can be precisely controlled). In other words, isotropic etching has a uniform etch rate at all orientations, while for anisotropic etching, the etch rate depends on crystal orientation. Some etchants attack silicon at different rates depending on the concentration of the impurities in the silicon (concentration-dependent etching). Isotropic etchants are available for silicon, silicon oxide, silicon nitride, polysilicon, gold, aluminum, and other commonly used materials. Since isotropic etchants attack the material at the same rate in all directions, they remove material horizontally under the etch mask (undercutting) at the same rate as they etch through the material. Hydrofluoric acid etches the silicon oxide faster than silicon. Anisotropic etchants, which etch different crystal planes at different rates, are widely used, and the most popular anisotropic etchant is potassium hydroxide (KOH) because it is the safest one to use. The application of concentration-dependent etching is illustrated below.
Wet Chemical and Plasma Etching
Published in Andrew Sarangan, Nanofabrication, 2016
Wet chemical etching is conceptually very simple and does not require a significant infrastructure. The process basically consists of immersing a photoresist-patterned substrate in a chemical bath to allow the liquid chemicals to work their way through the openings in the photoresist pattern and dissolve the underlying film. It is a highly scalable process—a large number of wafers can be loaded into a carrier and immersed in the wet bath at the same time. The main drawback of wet etching is its isotropic nature, especially on amorphous or polycrystalline films. The isotropic nature results in an etch rate that is equal along all directions. This produces an undercut below the photoresist pattern, as illustrated in Figure 7.1. As discussed in Chapter 6, isotropic etching results in a significant narrowing of the etched line and a loss of resolution, especially when the thickness of the film being etched is on the same order of magnitude as the lithographically patterned dimensions. Wet etching was widely used in electronic circuit manufacturing until the 1980s until the device geometries became too small for this process. Nevertheless, it is still used in research laboratories when the device dimensions are large and also in the manufacturing process for micro-electro-mechanical systems (MEMS).
Materials for Microelectromechanical Systems
Published in Mohamed Gad-el-Hak, MEMS, 2005
Christian A. Zorman, Mehran Mehregany
Isotropic etching of a semiconductor in liquid reagents is commonly used for removal of work-damaged surfaces, creation of structures in single-crystal slices, and patterning single-crystal or polycrystalline semiconductor films. For isotropic etching of Si, the most commonly used etchants are mixtures of hydrofluoric (HF) and nitric (HNO3) acid in water or acetic acid (CH3COOH), usually called the HNA etching system.
Nanoprotection from SARS-COV-2: would nanotechnology help in Personal Protection Equipment (PPE) to control the transmission of COVID-19?
Published in International Journal of Environmental Health Research, 2023
Zhi Xin Phuna, Bibhu Prasad Panda, Naveen Kumar Hawala Shivashekaregowda, Priya Madhavan
El-Atab et al. (2020) has developed a reusable nanoporous silico-based membrane filter using combination of lithography and potassium hydroxide (KOH)-based isotropic etching steps (El-Atab et al. 2020). Normal membrane filter uses straining, where the pore size is smaller than the particle size. Filtration resistance and membrane fouling may occur due to the deposition of cake where the filtered particles accumulate on the surface of membrane that require cleaning (Okamoto et al. 2001; Sabia et al. 2014). Thus, to overcome the said problem, the nanoporous membrane filters are intrinsically hydrophobic allowing the droplets to roll and slide on the inclined area of the mask. This can result in antifouling and self-cleaning, which can overcome the cake formation in normal membrane filter (El-Atab et al. 2020). Patterning and KOH etching of silico-on-insulator wafer was first used to develop a silico-based nanoporous template. Reactive ion etching was then used to transfer the pattern on the nanoporous template onto an ultrathin and hydrophobic polymeric film (Figure 4). The nanoporous membrane is made to be attached on N95 facemask that can be removed and replaced with a new one while using the same mask. This can reduce the pore size down to 5 nm to achieve enhanced filtration efficiency. This can solve the reusability problems but at the same time maintain the breathability as well as effective filtration (El-Atab et al. 2020).
Development of microstructured fish scale collagen scaffolds to manufacture a tissue-engineered oral mucosa equivalent
Published in Journal of Biomaterials Science, Polymer Edition, 2020
Ayako Suzuki, Hiroko Kato, Takahiro Kawakami, Yoshihiro Kodama, Mayuko Shiozawa, Hiroyuki Kuwae, Keito Miwa, Emi Hoshikawa, Kenta Haga, Aki Shiomi, Atsushi Uenoyama, Issei Saitoh, Haruaki Hayasaki, Jun Mizuno, Kenji Izumi
Microcontact printing, photolithography, and laser patterning have been introduced as micropatterning techniques for glass or plastic culture substrates [23]. Recently, Yu et al. reported a micromilling technology to create PDMS molds, followed by the fabrication of hydrogel scaffolds with 3 D undulated microtopographies mimicking the dermal papilla in the skin [9]. In the present study, we reported a novel approach for constructing tilapia scale type I collagen scaffolds with 3 D microtopographic structures mainly involving the following three processing steps (Figure 2): (1) manufacturing of a silicon semiconductor substrate with a combination of anisotropic and isotropic etching, (2) fabrication of four different types of negative molds made of PDMS or Si, (3) fabrication of microstructured fish scale collagen scaffolds. This semiconductor process allows any configurations of microstructure fabrication mimicking the connective tissue papilla of the oral mucosa because of anisotropic etching that has shape controllability, such as steep undulation and isotropic etching, which allows fabrication of truncated micropatterns. In addition, this semiconductor process could serve as a high-throughput technique useful in manufacturing off-the-shelf biomaterials in regenerative medicine.
Casimir-Lifshitz quantum state of superhydrophobic black-silicon surfaces manufactured by a metal-assisted hierarchical nano-microtexturing process
Published in Philosophical Magazine, 2019
Bhaskar Parida, Sel Gi Ryu, Keunjoo Kim
A similar hierarchical nanopatterning process over the microstrips was also reported, but using the lithographic implementation of a free-standing nanostencil membrane and the deposition of Cr nanodots as etching masks under the dry etching process [26]. The electron-beam-evaporated Ag particles covered the Si surface and the thin-film deposition obeyed the Volmer–Weber mechanism of nanoisland growth [27]. The annealed sample showed nanodots coalescence, and the catalytic behaviour of the Ag nanodots allowed etching of the Si surface for the formation of nanoporous structures. In the solution, a cathodic reaction occurred at the Ag sites, moving the electrons from the Si to the H2O2, thereby generating Si-based holes. These holes were injected into the Si, thereby allowing the oxidisation of the Si beneath the Ag particles for the formation of silicon dioxide (SiO2); this is known as the anodic reaction. The formed SiO2 was then dissolved in the HF-containing solution, leaving behind nanopits. As the etching time was increased, the charge transfer between the Ag–Si interface continued vertically and deep nanopores were formed. The Si nanopits that formed beneath the Ag nanodots are attributed to the Ag nanoparticles, whose sizes inversely affect the etching rate [28]. The porous structure of the Ag-free surface is related to the isotropic etching of the Si in the HF–H2O2 solution [29]. As the etching time was increased from 10 to 60 sec, the nanopores on the Ag-free surface were strongly developed.