(Invited) Scratch Controled Electrochemical Production of Macropores in Silicon

Abstract

Electrochemical etching of macropores in Si can be done either randomly (wild pores) or lithographically defined. When the points to be etched are not defined, the etching process starts at defects, or statistically distributed on the surface, just limited by the space-charge-region created in the semiconductor as effect of the electric field applied [1]. Ordered pores are usually accomplished by lithography plus a preliminary etching step for the formation of inverted pyramids [2]. At the pits of the pyramids the electric field is more intense during the electrochemical etching process, causing that the probability to etch pores is higher at those points. Defining the positions where pores are etched is necessary for different applications, where order is important. However, this makes the process to produce pores, which is not necessarily clean, more expensive with the need clean-room appliances (masking layer deposition and lithography).In order to simplify the production of ordered micropores, in this work it is proposed to take advantage of the increased probability of surface defects to act as nucleation points of growing pores. An electronic grade single side polished p-type Si wafer, with resistivity of 20 Ωcm was used as starting material. The electrolyte was a 10 % (v/v) HF solution in DMF. The wafer was scratched with fine sand paper previous to the etching process. Electrochemical etching was performed aplying a current density of 20 mA/cm^2 for 10 min. Macropores grew mainly along the scratches (see Figure). The spacing between pores depends on the distances between scratches. The regions without scratches have a lower probability to be etched. However, if the defects introduced by the sand paper are cured by a short chemical etching process in KOH, the probability to electrochemically etch those sections is further increased, in comparison with the sections with scratches and no KOH treatment. With the observations, it is straight to think that it is possible to do deffect engingineering to tune the electrochemical etching of micropores in Si.[1] E. Quiroga-González, H. Föll, book chapter "Chapter 2: Fundamentals of Silicon Porosification via Electrochemical Etching", in "Porous Silicon: Formation and Properties, Volume 1", editor G. Korotcenkov, CRC Press. (2015). ISBN: 9781482264548.[2] E. Quiroga-González, E. Ossei-Wusu, J. Carstensen, H. Föll, J. Electrochem. Soc. 158(11) E119 (2011). Figure 1