Electron beam induced deposition of rhodium nanostructures

Electron Beam Induced Deposition (EBID) allows deposition of three-dimensional micro- and nano-structures of conductive and insulating materials on a wide range of substrates. The process is based on the decomposition of molecules of a pre-selected precursor by a focused electron beam. In recent decades EBID of several metals, namely Au, W, Cu and Pt, from different families of precursors, has been achieved and the technique has found some application for small-scale production of laboratory devices and for repair of masks and micro-optoelectronic devices. The weak point of the technique is at present the low purity of the deposited material, caused by metal-organic precursors and by the lack of selectivity of the electron-induced decomposition process. This work is dedicated to EBID of Rh nanostructures from the precursor [RhCl(PF3)2]2. High metal content deposits are expected because the precursor does not contain C atoms and because Rh is one of the less reactive metals. [RhCl(PF3)2]2 as EBID precursor has been characterized by vapor pressure, mass spectrometry and surface residence time measurements. The vapor pressure of 7.5 Pa at room temperature reveals that the precursor is sufficiently volatile for room temperature EBID. The knowledge of the vapor pressure allows also to estimate the number of precursor molecules delivered to the reaction area per unit time. Mass spectrometry measurements allow to know the decomposition path of the precursor under electron impact in the gas phase. The measured spectrum indicates that the molecule decomposes by successive loss of PF3 groups, as confirmed by density functional theory calculations. This is compatible with a high metal content deposit. Residence time measurements show that [RhCl(PF3)2]2 does not decompose on stainless steel surfaces. The measured residence time of 2 ms allows to estimate that the activation energy for desorption of [RhCl(PF3)2]2 on stainless steel is about 0.6 eV and that precursor molecules can travel distances in the micrometer range before being desorbed. EBID structures obtained from [RhCl(PF3)2]2 have been characterized with a wide range of techniques for a better knowledge of the material properties and the deposition process. The deposit morphology has been studied by Transmisssion Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) to characterize the different steps of the deposition process. Crystallographic analysis is carried out by TEM in diffraction mode. Chemical analysis is carried out by Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS). Morphological analysis of deposits carried out at different exposure times reveals that the first phase of the growth process, in close proximity to the substrate, is characterized by an increase of the deposit height and the deposit diameter. On the other hand the second phase of the growth process is characterized by increasing height and constant diameter. TEM contrast profiles of dots and Atomic Force Microscopy (AFM) sections of lines have clearly shown that the EBID rate is highest in the center of the beam and decreases in the peripheral regions. Deposition at variable distances from the precursor source allowed to obtain hollow structures, whose morphology reveals that the precursor reaches the reaction area mainly by direct gas phase transport. Structural analysis and TEM revealed that, independently of the deposition conditions, the deposited material is made up of Rh nanocrystals immersed in a lighter amorphous matrix. Chemical analysis by Auger Electron Spectroscopy revealed that, after removal of the C rich contamination layer by Ar ion sputtering, the average composition of the deposits is about: 60 at.% Rh, 20 at.% P, 5 at.% Cl, 7 at.% N, 8 at.% O. The absence of C and the presence of N and O in the deposit bulk have been confirmed by Electron Energy Loss Spectroscopy. This technique allowed also to prove that Rh is dominant also in deposits of sub-micrometer size (not analyzable with AES) and to determine the elemental distribution in the deposit with nanometer resolution. Comparison of the deposit composition and the positive ions detected by mass spectrometry revealed that EBID, compared to low pressure gas phase ionization, involves a higher number of events, i.e. multi-electron decomposition and rearrangements of partially decomposed species.