Optical characterization of pore filling in mesoporous multilayers by ultrathin atomic layer deposited hafnium dioxide

Abstract

An optical technique for the characterization of pore filling in mesoporous silicon (Si) layers is presented. The technique is applied to analyze pore filling by ultrathin hafnium dioxide (HfO2) fabricated by thermal atomic layer deposition (ALD). This technique uses mesoporous multilayers generated by electrochemical etching of crystalline Si to form rugate filters with small bandwidth of the corresponding reflectance spectra. The pore filling and penetration depth into the porous material of the ultrathin hafnium dioxide ALD layers are derived from the measured spectral peak shift of the filter characteristic by using an effective medium approximation. Optical modeling of the multilayer filters was used to simulate the influence of the penetration depth of the ultrathin hafnium dioxide layer into the mesoporous Si on the spectral peak shift. Simulation results show that oxidation of porous Si causes blue shift, whereas pore filling with HfO2 causes red shift of the used rugate filters spectra. These spectral shifts are a result of the corresponding refractive index change of the individual mesoporous layers. Experimental results comparing freshly prepared rugate filters with those coated by thermal ALD show that the blue shift of the peak and thus the transformation of the pores' surfaces by oxidation is dominant for the first few ALD process cycles and that surface oxidation is completed after approximately eight ALD cycles. A red shift of spectral peak position, an indicator of an effective HfO2 coating of the mesopores, started to occur after eight cycles. Further ALD cycles increase the red shift continuously up to saturation, where further ALD will not change the optical spectra of the rugate filters any more. This saturation is related to complete pore sealing of the smallest pores. Pore size distribution and saturation of pore coating thicknesses are compared to evaluate the pore filling. The results obtained with ALD HfO2 in mesoporous Si with pore diameter in the range of 4–8 nm demonstrate that the proposed optical characterization method is sensitive enough to study ALD steps in mesoporous materials.