FORMATION OF ARBITRARILY SHAPED 3D-FORMS IN SILICON BY ELECTROCHEMICAL WET-ETCHING A. Ivanov + , U. Mescheder + *, M. Kröner ^ , P. Woias ^ + Institute of Applied Sciences Furtwangen, *Department Computer and Electrical Engineering, Hochschule Furtwangen University, Robert-Gerwig-Platz 1, 78120 Furtwangen, Germany ^ Department of Microsystems Engineering, IMTEK, University Freiburg, Georges-Koehler-Allee 102, 79110 Freiburg, Germany alexey.ivanov@hs-furtwangen.de Abstract – Application of the anodization process for structuring silicon wafers to create arbitrarily shaped forms of optical quality is discussed. The quality of the surfaces anodized by current densities in the range from 1.0 A/cm² to 3.0 A/cm² is investigated with AFM. Formation of convex and concave forms by anodization at different current densities is reported. A new approach to form 3D-profiles by anodization of pre-structured surfaces is introduced. Key Words: silicon structuring, anodization, electropolishing I INTRODUCTION In the last decade electrochemical wet-etching (anodization) of silicon has attracted much attention since many applications of porous silicon - a possible product of the process - have been introduced. This article focuses on the creation of arbitrarily shaped 3D-forms by local anodization of silicon. Local formation of the sacrificial porous silicon layer and direct etching (electropolishing) will be discussed, with high emphasis on shape and surface quality of the fabricated structures. Fig. 1: Schematic of 3D anodization process to control the shape of the anodized form by masks on both sides of the wafer. Arrows represent the current flow lines [2]. The fundamentals of the anodization of silicon are described in [1], its application for a 3D-structuring of silicon in [2, 3]. Since positive charge carriers (holes) are needed to start the dissolution of silicon in an HF-water electrolyte, the process can be controlled by the current density of an electric current. For example, in [2] both sides of the wafer were structured to control the local current flow through the wafer, and thus locally anodized 3D forms have been achieved (Fig. 1). In this paper a new approach for 3D-structuring is proposed which does not need substructures in the masking layers and which works with only one side masking layer. II EXPERIMENTAL CONDITIONS In our experiments an anodization of circular silicon samples with a diameter of 30 mm (p-Si, 10-20 Ohm-cm, thickness 525 µm) was conducted in a double-tank cell configuration. For all experiments described in this paper an electrolyte based on 29.8 wt.% hydrofluoric acid (HF(50 wt.%):C 2 H 5 OH 1:1) was used. The front side of the samples was covered with a stress-free silicon nitride mask (thickness 250 nm) with local openings. To provide an ohmic contact of the samples’ backside to the electrolyte, this side was implanted to achieve a sheet resistance of 30- 40 Ohm/□. The experiments were conducted in constant current mode, with current density being calculated for the area of the openings in the silicon nitride mask before anodization (initial active area). In general, to produce structures with good surface quality by anodization, the following process flows can be applied (Fig. 2): (a) Porous silicon as sacrificial layer To form porous silicon, anodization is performed at low current densities. As was shown before by experiments and simulations [2], an insulating mask at the front side provides a concentration of current flow near the edges of the mask. At low current densities, the etch rate near the edges of the windows in the mask is therefore higher than in their centers, which results in the formation of so- called edge effect (convex) structures. The edge effect limits the range of practically achievable structures. In general the surface remaining after removal of the porous silicon is rough. Therefore, a short electropolishing step is needed to enhance the surface quality. (b) Direct electropolishing: Electropolishing of silicon is achieved by anodization at high current densities. Here, the p + p − Si SiN HF:Ethanol porous silicon