Enhanced features of fabricating 3-dimensional Si microstructures via metal-assisted chemical etching

다국어 초록 (Multilingual Abstract)

The demand for high-frequency electronic and photonic devices is continuously increasing. To meet these demands, advanced three-dimensional (3D) semiconductor architectures such as through-Si via (TSV) and trench gate structures have emerged, which require 3D structures with high etch rates. The fabrication of 3D semiconductors has often been realized through conventional dry and wet etching processes such as deep reactive ion etching (DRIE). However, these conventional etching techniques have limited applications in the continuously developing semiconductor technology field. Although conventional dry etching results in the desired vertically etched structures, the required vacuum environment increases the cost of the process, and the bombardment of high-energy plasma ions causes damage to the semiconductor surface. In contrast, although the conventional wet etching process is a simple, low-cost, and low-damage method, it cannot be used to manufacture vertically etched profiles because of its isotropic properties.

To overcome these limitations of former etching techniques, a novel method—metal-assisted chemical etching (MacEtch)—has now become a widely applied technology for fabricating Si nano/microstructures. The MacEtch method is a solution-based etching process based on the redox reactions that occur when a metal-catalyst-deposited semiconductor substrate is immersed in an etching solution. This unconventional etching process overcomes the limitations of both conventional dry and wet etching while still successfully producing high-etch-rate nano/microstructures. Therefore, novel etching techniques are continuously developed based on the fundamental mechanism of the MacEtch process. Moreover, the MacEtch process has also been utilized to successfully produce 3D structures on substrates other than the most commonly used Si, such as compound semiconductors, hence broadening its applicability in photoelectric conversion devices and other applications as well.

The MacEtch mechanism is governed by two main parts: the carrier generation and mass transport processes. Both the carrier generation and mass transport processes must be enhanced to obtain high etch rates and vertical etch profiles in MacEtch. Nonetheless, the distinct charge and mass transport mechanisms of Si microfabrication in MacEtch remains obscure, and the resulting etch rates are often less than 1 µm/min. Thus, this poses a challenge for future industrial implementations of the technology.

In this thesis, the combination of catalyst thickness and etchant temperature were identified to have the greatest effect on the activation of the mass transport and carrier generation rate for Si microstructures, respectively. The aim was to enhance the mass transport and carrier generation rate by controlling these two parameters to achieve high etch rates in the microscale regime. During this process, the effect of controlling the etchant temperature by varying the parameter over a wide range of temperatures was thoroughly investigated. By analyzing the results, the trends and underlying mechanisms of systematically varying the etchant temperature during the MacEtch process were clarified. The MacEtch mechanisms behind the elevation of the etchant temperature and etching in the microscale regime were determined, which have remained vague thus far.

Moreover, the intrinsically required aspects of the catalyst layer used in the MacEtch process are also thoroughly covered. The fundamental reason behind the specific selection of metal catalysts is covered, followed by the demonstration of a novel non-noble metal catalyst used in MacEtch for the first time. The findings from this work are expected to widen the scope of applications of MacEtch and contribute to higher efficiency for future semiconductor technologies.