The increasing demand for complex devices that utilize three-dimensional nanostructures has incentivized the development of adaptable and versatile semiconductor nanofabrication strategies. Without the introduction and refinement of methodologies to overcome traditional processing constraints, nanofabrication sequences risk becoming obstacles that impede device evolution. Crystallographic wet-chemical etching (e.g., Si in KOH) has historically been sufficient to produce textured Si surfaces with smooth sidewalls, though it lacks the ability to yield high aspect-ratio features. Physical and chemical plasma etching (e.g., reactive-ion etching) evolved to allow for the creation of vertical structures within integrated circuits; however, the high energy ion bombardment associated with dry etching can cause lattice and sidewall damage that is detrimental to device performance, particularly as structures progress within the micro- and nano-scale regimes.
Metal-assisted chemical etching (MacEtch) provides an alternative processing scheme that is both solution-based and highly anisotropic. This fabrication method relies on a suitable catalyst (e.g., Au, Ag, Pt, or Pd) to induce semiconductor etching in a solution containing an oxidant and an etchant. The etching would otherwise be inert without the presence of the catalyst. The MacEtch process is modelled after a galvanic cell, with cathodic and anodic half reactions occurring at the solution/catalyst and catalyst/semiconductor interfaces, respectively. The metal catalyzes the reduction of oxidant species at the cathode, thereby generating charge carriers (i.e., holes) that are locally injected into the semiconductor at the anode. The solution interacts with the ionized substrate, which creates an oxide that is preferentially attacked by the etchant. Thus, MacEtch offers a nanofabrication alternative that combines the advantages of both wet- and dry-etching, while also overcoming many of their accompanying limitations. This provides a tunable semiconductor processing platform using controlled top-down catalytic etching, affording engineers greater processing control and versatility over conventional methodologies.
Here, Au-enhanced MacEtch of the ternary alloys InGaP and AlGaAs is demonstrated for the first time, and processes are detailed for the formation of suspended III-V nanofoils and ordered nanopillar arrays. Next, a lithography-free and entirely solution-based method is outlined for the fabrication of black GaAs with solar-weighted reflectance of ~4%. Finally, a comparison between Au- and CNT-enhanced Si MacEtch is presented towards CMOS-compatibility using catalysts that do not introduce deep level traps. Sample preparation and etching conditions are shown to be adaptable to yield an a priori structural design, through a modification of injected hole distributions. Critical process parameters that guide the MacEtch mechanisms are considered at length, including heteroepitaxial effects, ternary material composition, etching temperature, and catalyst type, size, and deposition technique. This work extends the range of MacEtch materials and its fundamental mechanics for fabrication of micro- and nano-structures with applications in optoelectronics, photovoltaics, and nanoelectronics.
Microsystems Engineering (Ph.D.)
Department, Program, or Center
Microsystems Engineering (KGCOE)
Parsian K. Mohseni
Karl D. Hirschman
Seth M. Hubbard
Wilhelm, Thomas S., "Semiconductor Nanofabrication via Metal-Assisted Chemical Etching: Ternary III-V Alloys and Alternative Catalysts" (2019). Thesis. Rochester Institute of Technology. Accessed from
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