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Significance Statement
Anodic etching of silicon wafers to produce porous silicon (PSi) layers is a commonly used method. Etching in most cases was performed by applying an anodic current to a slice of silicon wafer, dipped together with a counter electrode in an ethanolic solution that contained a certain concentration of Hydrogen Fluoride (HF). It was reported in the literature that the pore-size is affected mainly by the anodic current-density and the HF concentration.
Researchers from Ben-Gurion University of the Negev and Nuclear Research Center-Negev in Israel applied an image-processing method for analyzing top-view Scanning Electron Microscopy (SEM) images of porous silicon samples in order to measure the average diameters of pores and the porosity of the layers.
Their study, published in the Journal Microporous and Mesoporous Materials, measured the porosity of porous silicon layers based on the assumption that ratio between the total areas of pore-openings, which appear as dark spots in the micrographs, to the entire scanned area represents the ratio between the combined volume of the pores to the volume of the entire layer. This volume-ratio is defined as porosity, which is one major term used to describe the structure of porous silicon layers. So far, two main methods were reported in the literature for determining of the porosity of porous silicon: gravitational method and analysis of adsorption isotherms of gases at low temperatures.
For the experimental procedure, Elia et al. (2016) used a modified version of the conventional single-tank cell which is less complicated and forms uniform porous silicon layers. Etching solutions of hydrofluoric acid at various concentrations (8-24%) in ethanol were prepared in order to form layers of different porosities and pore-sizes. Si wafers were of polished p-type, B-doped of typical resistivity 0.002-0.005 Ώcm and thickness of 500-550 μm. Electrochemical etching was performed at various current densities using a programmable dc power supply. Images were obtained with JOEL field emission SEM model JSM 7400F, operated at 3KeV. Top-view SEM images of porous silicon were used to measure quantitative characteristics such as pore radii, size distribution and pore density. Processing was done in three steps using JMIcroVision program Version 1.2.7 which calculated the average pore-size and the total porosity.
Results on porous silicon layers in a resistive HF solution showed that when higher voltage was developed mainly due to insufficient electrolyte concentration for the applied higher currents, the layer starts to crack and peel off leaving a honeycomb-like pattern on surface. The initial measured cell voltage was 2.6V but it rose to 3.5V after few minutes.
SEM images of cross-sections of the porous silicon layers showed uniform parallel longitudinal pores organized in a dense array which formed a layer of uniform thickness growing with etching duration while diameters where found to be a function of current density and HF concentrations. The uniformity of the layer validated the assumption that the appearance of the surface represented the entire volume of the layer.
Effect of etching conditions on pore-size showed reasonable agreement between modes of measurements and calculations (area, perimeter or dimensions). Pore sizes increases with increase in etching current and decrease with increase in HF concentration, in agreement with the trends reported in the literature. Likewise, agreement between pore-size results obtained by image-processing to those of direct measurement verified the validity of the method used to analyze porous silicon samples.
To assess the validity of image processing for porosity measurements, Elia and co-workers compared their porosity results to literature results obtained by two different physical methods. The comparison exhibited good agreement between the three methods of measurements and a clear trend of increasing porosity with increase in current density. This demonstrated the validity of image-processing method for the determination of the porosity in spite of limitations and approximations that are associated with this method.
Figure Legend: SEM images of top view of the surface and of cross-section of a sample of a porous silicon layer.
Journal Reference
Paz Elia1, Einat Nativ-Roth2, Yehuda Zeiri1.3, Ze’ev Porat1,3. Determination of the Average Pore-Size and Total Porosity in Porous Silicon Layers by Processing of SEM Micrographs. Microporous and Mesoporous Materials, Volume 225, 2016, Pages 465–471.
[expand title=”Show Affiliations”]- Department of Biomedical Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Be’er Sheva 84105, Israel.
- Ilse Katz Institute of Nanoscale Science and Technology, Ben-Gurion University of the Negev, P.O. Box 653, Be’er Sheva 84105, Israel.
- Division of Chemistry, Nuclear Research Center–Negev, P.O. Box 9001, Be’er Sheva 84190, Israel[/expand]
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