Design and characterization of low aspectratio nanopore arrays

Alumina nanopores seem to be very promising concerning many applications, such as DNA analysis 1‐2, separation of molecules 3‐4 or fabrication of nanopillars 5‐6. The nanopores are made by anodization, which is a really simple fabrication process requiring only an electrochemical cell. It makes possible to fabricate a large number of highly reproducible nanostructures for a low cost. While much research has been done on high aspect‐ratio nanopores, this project focuses on low aspect‐ratio nanopore arrays, for etch mask applications. The aim is to transfer the nanoporous pattern to silicon, and thus obtain a heterostructure that might be used as a nanofluidic diode. This aim of this semester project has been first to develop an efficient method to characterize the different pore parameters, by analyzing the SEM pictures of the arrays with ImageJ. It is necessary as the automatic analysis results in many errors. Then, the different anodization parameters, such as the choice of the acid, the pores opening time and the anodization time, have been tuned in order to get nanopores with a low aspect ratio, while keeping their diameter under 50nm. Concerning the analysis method, a systematic one has been found, involving a Gaussian filter, which reduces the noise, and a subtraction of the background, to highlight the pores. A comparison of the nanopores array fabricated in sulphuric acid and oxalic acid has been done. It has been shown that anodization in oxalic acid at 40V is a good choice for low aspect‐ratio applications. The pores opening step, which removes the grains on top of the array, is done in H3PO4 6% wt. The anodization time has also to be carefully controlled. Some etching tests finally highlight the importance of having low aspect ratio, as the nanoporous pattern is fully transferred to silicon with oxalic acid, but not with sulphuric, which produces higher aspect ratio nanopores. Concerning the fabrication of the low aspect‐ratio nanopores, some improvements can still be done, e.g. by adding an etching stop.