Uniform Metal-assisted Chemical Etching and the Stability of Catalysts
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Uniform Metal-assisted Chemical Etching and the Stability of Catalysts 1
Liyi Li , Colin M. Holmes1, Jinho Hah1, Owen J. Hildreth2 and Ching P. Wong1 1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A. 2 Arizona State University, Tempe AZ 85281, U.S.A. ABSTRACT Recently, metal-assisted chemical etching (MaCE) has been demonstrated as a promising technology in fabrication of uniform high-aspect-ratio (HAR) micro- and nanostructures on silicon substrates. In this work, MaCE experiments on 2 μm-wide line patterns were conducted using Au or Ag as catalysts. The performance of the two catalysts show sharp contrast. In MaCE with Au, a HAR trench was formed with uniform geometry and vertical sidewall. In MaCE with Ag, shallow and tapered etching profiles were observed, which resembled the results from isotropic etching. The sidewall tapering phenomena can be explained by the dissolution and redeposition of the Ag catalyst in the etchant solution. The existence of Ag that was redeposited on the sidewall was further confirmed by energy dispersive spectrum. Also, etchant composition is found to play a profound role in influencing the etching profile by the Ag catalysts. INTRODUCTION Fabrication of high-aspect-ratio (HAR) micro- and nanostructures on silicon substrates on silicon (Si) substrates is the key process in a variety of applications, such as microelectronics [1], microelectromechanical system [2], microfluidic devices [3] and optoelectronic devices [4]. Until now, these HAR structures are mainly made by reactive ion etching, which requires high investment for tools and maintenance. On the other hand, several wet chemical etching methods, such as the KOH-based basic etching and the HF-based electrochemical etching, have been used for fabrication of grooves and pores on Si in micro- and nanoscale. However, their performance are highly limited in terms of the applicable geometry of the etching profiles. Metal-assisted chemical etching (MaCE) is a novel wet etching technology that shows distinguished performance in fabricating HAR structures with versatile 3D geometry [5, 6]. In MaCE, a layer of noble metal is deposited on the Si surface as catalyst. The etching occurs when the metal-loaded sample is immersed in a wet solution, which typically consists of hydrofluoric acid (HF) and hydrogen peroxide (H2O2). The etching profile in MaCE can be controlled by the geometry of the catalyst through lithography and the composition of the etching solution. A variety of 3D structures have been successfully fabricated by MaCE, such as nanowires [7-9], nanopores [10] and nanogratings [4]. Especially, by design of the catalyst’s geometry, structures with 3D complexity can be obtained, such as spiral cavities [11, 12] and scooped-out channels [13-16]. The unique performance as well as the low-cost nature make MaCE attractive to both academia and industry. Conceivably, the uniformity of the etching profile is essential to any etching technology that is applicable in the HAR structu
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