A Comparative Study on Electrochemical Micromachining of n-GaAs and p-Si by Using Confined Etchant Layer Technique

Abstract

The confined etchant layer technique has been applied to achieve effective three-dimensional (3D) micromachining on n-GaAs and p-Si. This technique operates via an indirect electrochemical process and is a maskless, low-cost technique for microfabrication of arbitrary 3D structures in a single step. Br(2) was electrogenerated at the mold surface and used as an efficient etchant for n-GaAs and p-Si; l-cystine was used as a scavenger, for both substrates. The resolution of the fabricated microstructure depended strongly on the composition of the electrolyte, and especially on the concentration ratio of l-cystine to Br(-). A well-defined, polished Pt microcylindrical electrode was employed to examine the deviation of the size of the etched spots from the real diameter of the microelectrode. The thickness of the confined etchant layer can be estimated, and thus the composition of the electrolyte can be optimized for better etching precision. The etched patterns were approximately negative copies of the mold, and the precision of duplication could reach the micrometer level for p-Si and the submicrometer level for n-GaAs. Although the same etchant (Br(2)) and scavenger (l-cystine) were used in the etching solutions for GaAs and Si, the etching process, or mechanism, is completely different in the two cases. Compared with the fast etching process on GaAs in an etching solution with a concentration ratio of 3:1 of l-cystine to Br(-), the concentration ratio needs to be 50:1 for etching of Si. For the micromachining of Si, the addition of a cationic surfactant (cetyltrimethylammonium chloride, CTACl) is necessary to reduce the surface tension of the substrate and hence reduce the influence of evolution of the byproduct H(2). The function of the surfactant CTACl in comparison with an anionic surfactant (sodium dodecyl sulfate) was studied in contact-angle experiments and micromachining experiments and then is discussed in detail.

Microfabrication of a Diffractive Microlens Array on n‐GaAs by an Efficient Electrochemical Method

“…Previous studies using a microelectrode as the mold have shown the best etching resolution is around 500 nm, when the concentration of scavenger is much larger than that of the etchant. [35] Moreover, when the concentration of HBr is increased and the generated Br 2 at 1.0 V cannot be consumed by the inadequate scavenger near the mold, this causes lateral diffusion of the etchant according to the principle of CELT. Surprisingly, when the concentration of HBr in the etching solution is increased, etched microstructures are still obtained with the same etching precision and a better uniformity is achieved.…”

“…Previous studies have shown that the etching resolution by CELT is always better for micromachining on GaAs than on Si due to the faster reaction rate of etching on GaAs. [35] Thus, the heterogeneous reaction between the substrate and the etchant influences the etching resolution. When the influence of the heterogeneous reaction on the etching resolution or the thickness of CEL needs to be taken into account, Equation 5 can account for the concentration of species O (Br 2 ) as a function of distance x from the plate-like working electrode and separation d. Equations 6 and 7 only give the concentration distribution of the confined etchant layer, regardless of the reaction between the GaAs and the etchant.…”

Parallel Imaging and Template‐Free Patterning of Self‐Assembled Monolayers with Soft Linear Microelectrode Arrays

“…[25] Parallel electrode arrays have also been used with fixed arrangements of the individual electrodes but without lateral repositioning of the array. [26,27] The necessity of leveling samples relative to the horizontal (x,y) plane of the positioning system adds another technical difficulty that is especially evident for bent and rough samples.…”

Modification and Patterning of Self‐Assembled Monolayers Using Electrogenerated Etchants and Homogeneous Scavenging Reactions in a Scanning Electrochemical Microscope

“…In order to confine the lateral extension of etched areas, the confined etchant layer techniques was introduced and refined (Scheme 1c). [27,28,29] In the ME-substrate gap, Br 2 is also consumed by a homogeneous reaction with a scavenger, for instance L-cystine. [29] ME-sample gap:…”