Optical interferometry and photoacoustics as in-situ techniques to characterize the porous silicon formation: a review

Abstract

Porous Silicon (PSi) is a groundbreaking material because its physicochemical properties can be customized through its porosity. This means that monitoring and control of the growing parameters allows the fabrication of PSi-based systems with controlled properties. Interferometry and photoacoustics are non -invasive, noncontact, real-time (in-situ) techniques used to characterize the phenomena that takes place during the formation of PSi. This work presents the mathematical and experimental aspects related to the implementation of the techniques mentioned above, which are meant to characterize the PSi growth in fluoride-based electrolyte media. These methods can determine macroscopic parameters of PSi such as thickness, porosity profile trough effective medium approximation (EMA), refractive index, etching rate, and RMS roughness under 100 nm. The monitoring ability of these techniques is strongly dependent on the wavelength of radiation used. However, it is possible to monitor thickness from λ 0 /4 to ∼ 1/α 0 , where α 0 is the optical absorption coefficient at λ 0 . Also, these techniques can be implemented as feedback control on the etching processes for fabrication of PSi.

Design, fabrication, and optical characterization of one-dimensional photonic crystals based on porous silicon assisted by in-situ photoacoustics

“…The etching rate of c-Si in HF-based media was not constant for long etching times 18,20 , and the films presented porosity gradients as a function of depth 10,23 . Also, the electrolyte composition changed because of the chemical species released during the etching 7 .…”

“…S1 and S2 are individual layers of PSi that were obtained upon varying the anodizing current density from 5 to 60 mA/cm 2 . Each etching was monitored using a differential photoacoustic system 18 , and temperature of the cell was kept at 25 °C. These series of individual layers were used as a calibration of the cell to obtain the of porosity, etching rate, photoacoustic cycle time, and interface roughness as a function of current density.…”

“…Thus, Table 2 reports a roughness value of internal faces determined by GA and SEM. The previous works done by this group 18–20 showed that the PA cycle observed during the PSi formation depends on refractive index, porosity, etching rate, and laser wavelength (Eq. 1), and the etching rate is almost constant for short anodizing times.…”

“…However, the physicochemical properties of PSi are critically dependent on the etching parameters, and there are not theoretical models to predict the refractive index, absorption coefficient, thickness, porosity, and interfaces roughness. Nonetheless, in-situ methodologies based on infrared spectroscopy 15 , laser interferometry 16–18 , and photoacoustic 18–20 have been developed as an alternative to monitor the formation in real time of PSi thin films that allows feedback to control the electrochemical reaction.…”

Porosity calibration in a 4-layer porous silicon structure to fabricate a micro-resonator with well-defined refractive indices and dedicated to biosensing applications

“…The slight mismatching could be due to inhomogeneous porosity through each layer and the interface roughness which is around 10 nm [39]. Real improvement of quality and reproducibility of PSi layers could be obtained by performing the multilayers at low temperature [40] and by controlling the PSi formation in situ using for example photoacoustic method [41][42][43]. Moreover, improvement of the calculation of the theoretical reflectance spectra could be done taking into consideration interface roughness in the simulation [44].…”