Optical Applications of Macroporous Silicon
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Optical Applications of Macroporous Silicon
Volker Lehmann Infineon Technologies, Coporate Research , München, Germany
ABSTRACT Two promising optical applications of macroporous silicon are presented. Due to the high contrast in dielectric constant between the air filled pores and the silicon walls the porous structure exhibits a photonic band gap for infrared radiation perpendicular to the pore axis. By photolithograpic patterning waveguides and optical cavities can be realized in this two-dimensional photonic crystal. Along the pore axis a short-pass filter characteristic is observed for ultraviolet and visible light. Such macropore filters are of high optical quality and may replace conventional filters in imaging systems.
ELECTROCHEMICAL ETCHING OF MACROPORES Pores with micrometer radii can be formed in direction in moderately doped ntype silicon if it is anodized in a hydrofluoric acid electrolyte while the backside of the wafer is illuminated. If this process is applied to a polished silicon substrate, the pores will grow in a random pattern. However, if etchpits, for example generated by standard photolithograpy and alkaline etching, are present, pore formation will initiate at these pits. This process has been improved over the last decade and pore diameters between 0.5 µm and 20 µm, pore length of up to the wafer thickness and aspect ratios of up to 1000 are shown to be feasible [1,2]. The pores are very straight even for high aspect ratios, due to the direction being the preferred growth direction. An example of an electrochemically etched macropore array is shown in Fig. 1; a two-dimensional square lattice of pores has been etched in a 350 µm thick, 40 Ωcm (100) silicon sample. The sample has been cleaved and polished under 45° angle for better visibility of the pore-geometry. The crossectional shape of a macropore is intermediate between circle and square, due to facetting along (110) planes. Such shape is undesirable for photonic crystal applications especially for triangular lattices. The as-etched macropores are therefore widened by thermal oxidation and subsequent wet etching. By this process the pore cross section becomes perfectly circular and the pore diameter to pitch ratio can be easily tuned. Defects in the lattice, which constitute waveguides or optical cavities, as shown in Fig. 2, can easily be defined by photolithograpy. Micro-structuring of the photonic crystal material itself, which is required for attachment of glass fibers, as shown in Fig. 2, is performed by a photolithograpically structured aluminum- or polysilicon- mask and standard plasma etching as reported in Ref [3]. The electrochemical etching process forms dead-end pores. For shortpass filter applications, however, light transmission along the pore is required. In order to obtain through-pores the remaining non-porous substrate at the backside of the sample (13µm in Fig. 1) is etched-back in an alkaline solution (50% KOH, 90°C).
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Figure 1. Scanning electron micrograph of the cleaved edge of a macroporous silicon s
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