Journal of Porous Materials 7, 33–36 (2000) c  2000 Kluwer Academic Publishers. Manufactured in The Netherlands. Potential, Temperature and Doping Dependence for Macropore Formation on n-Si with Backside-Illumination M. HEJJO AL RIFAI, M. CHRISTOPHERSEN, S. OTTOW, J. CARSTENSEN AND H. F ¨ OLL Faculty of Engineering, Kaiserstr. 2, D-24143 Kiel, Germany Abstract. Applying an anodic bias on a silicon HF contact and illuminating the backside of a n-type silicon wafer allows to create macropores. The formation of “random macropores” is studied in this paper by determination of the influences of the potential, the temperature and the doping level. A statistical approach is used to evaluate the micrographs. The formation of the macroporous layer consists of two phases. Beginning with a plane surface and homogeneous dissolution of silicon, first pores occur after some time. In this nucleation phase the thickness of the homogeneously dissolved Si depends strongly on the doping level and the temperature but only weakly on the applied bias. In a second phase of stable pore growth the density of pores is investigated as a function of temperature and anodic potential. For low doped material we find a strong stabilisation influence of the deep space charge region (SCR) in the nucleation as well as in the stable pore growth phase. Thus an increased anodic bias decreases the density of pores. For highly doped silicon no stabilisation influence of the SCR is found. The pore growth is dominated by the electrochemical dissolution rate, i.e. increasing the potential increases the density of the macropores. Keywords: macropore formation, illumination, temperature dependence, anodic potential, doping dependence 1. Introduction Applying an anodic bias on a silicon HF contact allows to create pores which differ radically in size, morphology and physical properties depending on the experimental setup [1, 2]. In this paper we discuss the formation of macropores in n-type silicon with back- side illumination [1–4]. By changing the light inten- sity the generation rate of minority carriers within the silicon and in consequence the etching current may be controlled independently of the anodic potential. In contrast to porous silicon layers (PSL) the macro- pores diameters are in the micrometer scale. Using a standard photolithography for generating etch pits as nucleation centers for pore growth, a very regular array of pores can be generated, which serves as a starting point for a Si- microstructuring technique [5] or to cre- ate a “photonic crystal” [6]. This paper investigates the dependence on the anodic potential, electrolyte tem- perature and doping for the macropore formation of “random pores”, i.e. we do not prestructure etch pits as nucleation centers for the pore growth. 2. Experimental Setup Two series of experiments were performed on n-Si (100) with 4 doping concentrations (0.5, 2, 5 and 20  cm): The first series at constant anodic bias U an , varying the electrolyte temperature between T = 5 ◦ C and T = 30 ◦ C and the second with fixed tempera- ture T = 20 ◦ C, varying the anodic potential between U an = 1 V and U an = 6 V. Taking a 4 wt% HF-solution the silicon was etched with a current density j = 5 mA/cm 2 . SEM analysis was used to yield the informa- tion on the pore lengths, pore diameters and distances between the pores. 3. Results and Discussion Analyzing the growth of random pores, one must dis- tinguish between the nucleation phase and the phase of stable pore growth. Starting from a plane surface, first a roughening of the surface occurs, followed by formation of shallow pores, often with a higher density