Plasma Enhanced Atomic Layer Deposition of Iridium Oxide for Application in Miniaturized Neural Implants

Abstract

High quality recording of neuronal activities and electrical stimulation require neurotechnical implants with appropriate electrode material. Iridium oxide (IrOx) is an excellent choice of material due to its biocompatibility, low electrochemical impedance, superior charge injection capacity, corrosion resistance, longevity, and electrochemical stability. Plasma enhanced atomic layer deposition (PE-ALD) and a suitable precursor, like (Methylcyclopentadienyl) (1,5- cyclooctadiene) iridium, could be a promising technique to produce highly conformal and performant IrOx-films at low temperatures and low costs. Various studies have reported the deposition of iridium oxide, but usually at very high temperatures. These processes are not suitable for polymer substrates and limit the use of such post-processing together with active implants. In this work the (Methylcyclopentadienyl) (1,5-cyclooctadiene) iridium(I) ((MeCp)Ir(COD)) precursor was used as a promising approach for depositing IrOx-films using low temperature PE-ALD. This precursor is normally used for chemical vapour deposition processes. First experiments were carried out on silicon substrates at deposition temperatures of 110 C°. The precursor was heated up to 75 °C and oxygen plasma was used as coreactant. The deposited films were analysed with EDX and AFM, showing a smooth surface and a promising ratio between the elements iridium and oxygen.

Plasma-Assisted Atomic Layer Deposition of IrO2 for Neuroelectronics

“…From the fitting of the experimental data, the growth-per-cycle (GPC) of the process is 0.28 ± 0.01 Å at 150 °C. The value obtained is lower than the value of 0.66 Å reported recently by Simon et al [ 12 ] for PA-ALD of IrO 2 . Nonetheless, the different GPC could be explained by the use of different precursors, i.e., (EtCp)Ir(CHD) in this work and (MeCp)Ir(COD) in the work of Simon et al [ 12 ].…”

“…The value obtained is lower than the value of 0.66 Å reported recently by Simon et al [ 12 ] for PA-ALD of IrO 2 . Nonetheless, the different GPC could be explained by the use of different precursors, i.e., (EtCp)Ir(CHD) in this work and (MeCp)Ir(COD) in the work of Simon et al [ 12 ]. On the other hand, the reports of Choi et al [ 25 ] and Kim et al [ 26 ] on PA-ALD of IrO 2 focus on the deposition of nanodots; therefore, the GPC value of the process is not specified.…”

“…Only a few reports on the application of IrO 2 grown by ALD for neuroelectronics have been published so far [ 5 , 12 ]. In addition, the electrochemical characterization of the electrode/electrolyte interface in terms of charge injection capacity, charge storage capacity, and double-layer capacitance for IrO 2 grown by ALD is still missing.…”

“…It is worth mentioning that (EtCp)Ir(CHD) was originally designed and synthesized by Kawano et al [ 27 ] in 2004 as a precursor for the metal–organic chemical vapor deposition (MOCVD) of Ir. Recently, Simon et al [ 12 ] have reported a new PA-ALD of IrO 2 (on silicon with its native oxide) using O 2 plasma as a reactant and (Methylcyclopentadienyl) (1,5-cyclooctadiene) Iridium(I) [(MeCp)Ir(COD)].…”

Semiconducting electrodes for neural interfacing: a review

“…Beyond these, novel forms of conventional metallic electrodes with enhanced performance attributes such as impedance have been demonstrated-for example, nanostructured Pt electrodes or IrOx electrodes. 25,26 Parallel to these efforts, semiconductors have shown several unique advantages as electrode materials for neural interfacing in specific applications.…”