Synthesis and characterization of sputtered titanium nitride as a nucleation layer for novel neural electrode coatings

Significance Statement

Recording brain activity in e.g. Parkinson’s disease demands electrodes with special requirements to have the capacity to chronically interface with tissues and deliver a stimulus that is adequate to initiate an action potential. In addition, to improve their efficacy, it would be beneficial to minimize the electrode dimensions to a microscale and below in a bid to curtail tissue trauma and invasiveness, therefore realizing facile selectivity in stimulation and recording.

Unfortunately, reducing the electrode dimensions can limit electrical, biological, and mechanical performance. As the dimensions of the electrode are reduced, the charges passing per unit area also increases. This leads to an increase in electrode voltage and impedance, therefore reducing the total charge that can be delivered safely. For this reason, such electrodes call for alternative neutral interface materials which have the capacity to deliver charge densities that are larger than conventional materials which include iridium oxide, platinum and platinum-iridium.

Developing high surface area electrodes can be a good way of compensating for the limited electrode performance initiated by the reduction of the electrode dimensions. Enhancing an electrode surface by coating it with nanostructured materials has been indicated to have potential for improving the efficacy of neural electrodes.

R.A. Sait and R. B. M. Cross at De Montfort University Leicester in the United Kingdom deposited titanium nitride thin films by non-reactive radio frequency sputtering that offered a straightforward method by optimizing radio frequency power and argon flow rate. The main aim of their study was to utilize the sputtered titanium nitride layer as a nucleation substrate for titanium nitride nanowires. Their work is published in Applied Surface Science.

The authors deposited titanium nitride films through non-reactive radio frequency magnetron sputtering towards the preparation of a novel titanium nitride nanowires neutral interface. They varied sputtering parameters of radio frequency power and argon flow rate in a bid to realize their effects on the structural, electromechanical and electrical attributes of the titanium nitride films.

The authors observed that relatively higher kinetic energy species coupled with a low rate of deposition led to crystals oriented in the (111) and (200) planes. Characterization of the titanium nitride films indicated that surface roughness increased significantly with argon flow rate while it decreased slightly with radio frequency power. The authors explained the effects of sputtering parameters on resistivity in terms of defects initiated by re-sputtering effects and stoichiometry variation. They found resistivity increased as the nitrogen to titanium ratio reduced as a consequence of decreasing Argon flow rate. This was due to nitrogen depletion in the films, which led to defects in the material and incorporation of oxygen atoms.

Sait and Cross achieved the highest capacitance and largest water window for the optimized titanium nitride film at high radio frequency power and an argon flow rate where the films indicated a crystalline structure together with a low resistivity. Tuning resistivity and capacitance of the seeding layer may help to mediate the transduction of the action potential from an electrode coated with the optimized film. Titanium nitride films, functioning as a nucleation layer and having attributes of a crystalline structure, may yield high-quality and highly-aligned nanowires.

Titanium nitride nanowires could improve the mechanical, physical and electrochemical performance of neural electrodes and offer safer, more effective stimulation and recording over a longer period while keeping the electrode-neuron interface more robust and efficacious.

Synthesis and characterization of sputtered titanium nitride as a nucleation layer for novel neural electrode coatings. Advances in Engineering

3D AFM images of TiN thin film sputtered at an RF power of 300W and Argon flow rate of a) 10sccm b) 80sccm showing the effect of re-sputtering at low argon flow rate.

Synthesis and characterization of sputtered titanium nitride as a nucleation layer for novel neural electrode coatingsSynthesis and characterization of sputtered titanium nitride as a nucleation layer for novel neural electrode coatings. Advances in Engineering

SEM image of TiN sputtered film

About The Author

Roaa Sait is currently a PhD student at De Montfort University Leicester in the UK. Her research interests lie in the areas of material science and nanotechnology. Her current work involves the study of flexible neural electrode interface materials for improving recording and stimulation of neurons. She is focused on controlling the properties of Titanium Nitride nanostructures to enhance this application.

About The Author

Dr. Richard Cross is a Senior Research Fellow within the School of Engineering and Sustainable Development, Faculty of Technology at De Montfort University Leicester UK, where he also received his PhD in Large Area Electronic Materials. Primary research interests involve the development of novel nanomaterials and their incorporation into useful devices.

His major research focus going forward involves investigations at the interface between biological systems and artificial devices, in particular for healthcare-related applications, bioelectronics and ultimately to facilitate understanding of neuronal processes.

ORCID: 0000-0002-4244-4377

Reference

R.A. Sait and R.B.M. Cross. Synthesis and characterization of sputtered titanium nitride as a nucleation layer for novel neural electrode coatings. Applied Surface Science, volume 424 (2017), pages 290–298.

 

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