A preliminary study on chemical micro-machining of complex three-dimensional patterns on silicon substrates

Confined Chemical Etching for Electrochemical Machining with Nanoscale Accuracy

In the past several decades, electrochemical machining (ECM) has enjoyed the reputation of a powerful technique in the manufacturing industry. Conventional ECM methods can be classified as electrolytic machining and electroforming: the former is based on anodic dissolution and the latter is based on cathodic deposition of metallic materials. Strikingly, ECM possesses several advantages over mechanical machining, such as high removal rate, the capability of making complex three-dimensional structures, and the practicability for difficult-to-cut materials. Additionally, ECM avoids tool wear and thermal or mechanical stress on machining surfaces. Thus, ECM is widely used for various industrial applications in the fields of aerospace, automobiles, electronics, etc. Nowadays, miniaturization and integration of functional components are becoming significant in ultralarge scale integration (ULSI) circuits, microelectromechanical systems (MEMS), and miniaturized total analysis systems (μ-TAS). As predicted by Moore's law, the feature size of interconnectors in ULSI circuits are down to several nanometers. In this Account, we present our perseverant research in the last two decades on how to "confine" the ECM processes to occur at micrometer or even nanometer scale, that is, to ensure ECM with nanoscale accuracy. We have been developing the confined etchant layer technique (CELT) to fabricate three-dimensional micro- and nanostructures (3D-MNS) on different metals and semiconductor materials since 1992. In general, there are three procedures in CELT: (1) generating the etchant on the surface of the tool electrode by electrochemical or photoelectrochemical reactions; (2) confining the etchant in a depleted layer with a thickness of micro- or nanometer scale; (3) feeding the tool electrode to etch the workpiece. Scavengers, which can react with the etchant, are usually adopted to form a confined etchant layer. Through the subsequent homogeneous reaction between the scavenger and the photo- or electrogenerated etchant in the electrolyte solution, the diffusion distance of the etchant is confined to micro- or nanometer scale, which ensures the nanoscale accuracy of electrochemical machining. To focus on the "confinement" of chemical etching reactions, external physical-field modulations have recently been introduced into CELT by introducing various factors such as light field, force field, hydrodynamics, and so on. Meanwhile, kinetic investigations of the confined chemical etching (CCE) systems are established based on the finite element analysis and simulations. Based on the obtained kinetic parameters, the machining accuracy is tunable and well controlled. CELT is now applicable for 1D milling, 2D polishing, and 3D microfabrication with an accuracy at nanometer scale. CELT not only inherits all the advantages of electrochemical machining but also provides advantages over photolithography and nanoimprint for its applicability to different functional materials without involving any photocuring and thermoplastic resists. Although there are some technical problems, for example, mass transfer and balance, which need to be solved, CELT has shown its prospective competitiveness in electrochemical micromachining, especially in the semiconductor industry.

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

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.