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High-density out-of-plane microneedle arrays were fabricated with a single photolithography and two deep reactive ion etching (DRIE) steps in anisotropic and isotropic modes, respectively.Microneedles in various heights were monolithically created by the identical DRIE processes and scanning electron microscopy images showed extremely sharp sub-micron (~145-nm-wide) tip.Diverse cross-sectional shapes of microneedles were implemented by altering photomask patterns., Out-of-plane microneedle structures are widely used in various applications such as transcutaneous drug delivery and neural signal recording for brain machine interface. This work presents a novel but simple method to fabricate high-density silicon (Si) microneedle arrays with various heights and diverse cross-sectional shapes depending on photomask pattern designs. The proposed fabrication method is composed of a single photolithography and two subsequent deep reactive ion etching (DRIE) steps. First, a photoresist layer was patterned on a Si substrate to define areas to be etched, which will eventually determine the final location and shape of each individual microneedle. Then, the 1st DRIE step created deep trenches with a highly anisotropic etching of the Si substrate. Subsequently, the photoresist was removed for more isotropic etching; the 2nd DRIE isolated and sharpened microneedles from the predefined trench structures. Depending on diverse photomask designs, the 2nd DRIE formed arrays of microneedles that have various height distributions, as well as diverse cross-sectional shapes across the substrate. With these simple steps, high-aspect ratio microneedles were created in the high density of up to 625 microneedles mm−2 on a Si wafer. Insertion tests showed a small force as low as ~ 172 µN/microneedle is required for microneedle arrays to penetrate the dura mater of a mouse brain. To demonstrate a feasibility of drug delivery application, we also implemented silk microneedle arrays using molding processes. The fabrication method of the present study is expected to be broadly applicable to create microneedle structures for drug delivery, neuroprosthetic devices, and so on.

We investigated the formation mechanism of a rounded silicon-germanium (SiGe)-etch front (rounding) in gate-all-around field-effect transistor (GAA-FET) manufacturing. This rounding is created by the isotropic etching of the SiGe layer after anisotropic etching of the SiGe/Si stack, which degrades device characteristics. The etch-time dependence of the rounding amount during isotropic SiGe etching with nitrogen trifluoride plasma indicates that rounding is mainly formed in an initial stage of SiGe etching, namely, etch time less than 15 s. Cross-sectional scanning transmission electron microscopy and energy dispersive x-ray spectroscopy (STEM EDX) measurement indicated that a Ge-containing layer formed on the sidewall of SiGe/Si patterns before isotropic SiGe etching. From these results, we propose a formation model of SiGe rounding below. The Ge composition in the Ge-containing layer has a gradient due to ion-assisted Ge diffusion during anisotropic etching of the SiGe/Si stack. This gradient induces rounding during isotropic SiGe etching because the etch rate of the SiGe layer decreases as the Ge composition decreases. To validate our model, the Ge-containing layer after anisotropic etching was removed by post-etch treatment and the Ge spectra on the sidewall was reduced to the detection limit of STEM EDX. As a result, the rounding amount after isotropic SiGe etching improved from 2.7 to 1.8 nm. This reduction indicates that the formation of the Ge-containing layer during anisotropic etching of the SiGe/Si stack is one of the main causes of rounding after isotropic SiGe etching.