Effect of fabrication tolerances in macroporous silicon photonic crystals

- 700 nm pitch 3d photonic crystals of macroporous silicon structures are characterized.
- Statistic models for the modulation period and amplitude are described.
- Fabrication errors have a 5 % standard deviation from z period.
- Pore profile amplitude has negligible effect on optical performance.
- Optical properties show small photonic bandgap reduction and resonance shift.

Macroporous silicon photonic crystals show a great potential for a range of applications such as optical sensing or signal processing. These applications require tight fabrication tolerances. In particular, the effect of process variability in 3-d photonic crystals and out of plane propagation has been seldom studied in literature. In this paper we report the effect fabrication imperfections on the spectral response of macroporous silicon photonic crystals. To quantify fabrication disorder and its influence, several 3-d macroporous silicon structures were fabricated consisting of modulated pores arranged in a square lattice. The pore modulation is similar to a stretched sinusoidal waveform. Lattice pitch is 700 nm, pore diameter is in the range from 250 nm to 520 nm, and modulation period is 1.2 μm. The samples were characterized by SEM inspection and the actual etched pore profiles extracted. The statistical analysis of the profiles reveals two main sources of randomness: radius variability and modulation period irregularity. Surface roughness and asymmetry do not seem to play a major role. Several FDTD simulations have been performed based on the statistical parameters extracted, and the results are compared to actual FT-IR measurements of the fabricated samples. The obtained results show that, in general, the dispersion in z period has the most severe effect in the structure's optical response.

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