The technological innovations are remarkable in artificial intelligence, internet of things, and automatic driving. These technologies are supported by semiconductor devices, that are required to be more highly integrated with a higher performance. Recent semiconductor devices have three-dimensional (3D) structures due to the scaling limits, and the ratio of vertical to horizontal sizes, or an aspect ratio, tends to increase. In order to manufacture those semiconductor devices for a practice use, cleaning processes are becoming much more important. While there are many reports on cleaning processes and their evaluations for a flat sample surface, research on 3D devices is limited.
Our final goal is to develop a non-destructive method to evaluate cleaning characteristics at the bottoms of 3D structures of semiconductor devices, which are probably the most difficult areas to be cleaned. As a test sample, in this study, we focused on 3D nanostructures on a Si surface. Our proposed method is as follows. (i) We fabricate 3D structures with different materials only at their bottoms. (ⅱ) In Angle-Resolved X-ray Photoelectron Spectroscopy (ARXPS) measurements, we control the take-off angles of photoelectrons strictly so that signal from the heterogeneous “landmark” element is detected. This guarantees the collection of photoelectrons not from the sidewalls but from the bottoms. (ⅲ) XPS spectra in (ii) also include signal from the top surface of the 3D structure, which needs to be removed. Thus, as a next step, we set a very shallow take-off angle to collect photoelectrons emitted only from the top surface. By subtracting this signal from that in (ii), we expect to extract only the “bottom condition”. The most important point in this scheme is to embed a heterogeneous element only at the bottoms of 3D nanostructures on a Si surface without introducing mechanical damages. In order to achieve this, we used Metal-Assisted Chemical Etching (MACE).
MACE is a solution process to fabricate various 3D nano- and micro-structures, including nanoporous layers, nanowires, 3D objects, micro-electromechanical systems, x-ray optics. [1]. In typical MACE of Si, a metal-loaded Si surface is immersed into a mixed solution of HF and H2O2 to cause local electrochemical reactions at the metal/Si interface. The metal serves as a catalyst for the reduction of H2O2, which injects holes into the Si. Because the hole concentration in Si becomes higher around the metal catalyst, a Si surface is readily oxidized underneath the metals. This is followed by the prompt dissolution of the SiO2 layer in HF as a silicon fluoride compound, leaving nanostructures possessing a high aspect ratio with the metal catalyst at their bottoms. Figure 1(a) shows a schematic drawing for this etching mechanism. MACE is a low-cost and anisotropic etching method without leaving any crystallographic defects on the fabricated Si structures.
Figure 1(b) depicts the process flow of a trench formation. First, thin metal films composed of Au /Ti was deposited on a Si substrate by electron beam (EB) evaporation. Second, the photoresist was patterned using conventional photolithography. Au stripes were then formed by the use of an etchant and acetone to remove unnecessary Au and the resist, respectively. The Au/Ti/Si sample was immersed into a mixed solution of HF, H2O2, and H2O. A resultant surface structure was imaged by Scanning Electron Microscope (SEM) as shown in Fig. 1 (c). It demonstrates the formation of Si nanotrenches with a width and a depth of around 200 nm and 1.2 µm, respectively. And their aspect ratio is approximately 6. It was also confirmed that Au catalysts reside at the bottoms of the trench structures in Figure 1(c) as we intended.
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