Titanium Nitride Microelectrodes Deposited by Ion Beam Assisted E-beam Evaporation
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1
Tampere University of Technology, Department of Automation Science and Engineering, BioMediTech, Finland
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2
University of Tampere, NeuroGroup, BioMediTech, Finland
Abstract
An alternative method for fabricating titanium nitride (TiN) microelectrodes is presented. In order to decrease the impedance and noise levels of microelectrodes, one of the most common methods is to coat the electrodes with TiN. Usually that has required the use of a sputtering device, but we have demonstrated that also an e-beam coater can be used for TiN deposition, if equipped with an ion source. Our first 30 µm microelectrodes fabricated by ion beam assisted deposition (IBAD) have impedances around 75 kΩ, which is close to the impedances reported for sputter deposited TiN microelectrodes.
Motivation
The simplest microelectrode array (MEA) structure consists of a glass substrate, a metal layer for contact pads, tracks and microelectrodes, and an insulator layer on top. However, all-metal electrodes, e.g. Pt, Au, or Ti, tend to have rather high noise and impedance levels (~1000 kΩ or more for 30 µm electrodes). Because of that, the microelectrodes are typically coated with an additional layer of some porous material for increasing the effective surface area and thus decreasing impedance and noise levels. Of the common low impedance materials platinum black suffers from mechanical stability issues, activated iridium oxide tends to deactivate too fast for long term measurements and carbon nanotube (CNT) coatings can be considered to be still in an experimental phase, even though already commercially available. On the contrary, as far as we know, titanium nitride coating [1] has not been reported having any major drawbacks. However, if the instance interested in fabricating its own MEAs does not have access to a reactive sputtering device nor capital to buy one, a major drawback does exist. A common alternative method for sputtering, e-beam evaporation, does not suit as such for depositing two compound materials of TiN kind as nitrogen tends to escape during the evaporation and only titanium ends up on the substrate. However, if the e-beam coater can be equipped with an ion source, whose capital costs are only a fraction of the ones of a new sputtering device, TiN deposition is possible [2]. Schematics of such a system is presented in Figure 1. Titanium pellets can be used as evaporation source material and when the beam of nitrogen ions reacts with the titanium vapor, the end result is titanium nitride.
Materials and Methods
Microelectrode arrays with glass substrate, 400 nm of titanium as track material, and 500 nm of silicon nitride as insulator layer were fabricated using common microfabrication processes. The photoresist used as an etching mask for making openings in the insulator layer was not removed after the dry etching step, but was re-used as the lift-off mask for TiN deposition. After a short Ar etching to remove the native oxide on existing titanium electrodes, 400 nm of TiN was deposited using ion beam assisted deposition (Saintech ST-55 gridd-less ion source) and lift-offed with acetone in ultrasound bath. Major deposition parameters were: E-beam evaporation rate 2 Å/s, ion current density 11 µA/cm2, anode power 650 W, and gas ratio (N2/Ar) 80 % / 20 % with the flow rates of 13.2 and 3.2 sccm. AFM measurements were used to verify the surface roughness increase and the impedance measurements with Multichannel Systems MEA-IT impedance tester were used to verify the decrease in impedance, compared to all-titanium micro-electrodes.
Results
Based on AFM measurements, IBAD TiN microelectrodes have the surface-area-ratio of 18-31 % compared with 4 % of pure Ti. For 30 µm IBAD TiN microelectrodes the average impedance at 1 kHz was measured to be 75 kΩ, which is close to the impedance of 45 kΩ measured at the same time for commercial Multichannel Systems 60MEA200/30iR-Ti MEA with sputtered TiN microelectrodes, and much less than >1800 kΩ measured earlier for uncoated all-titanium microelectrodes [3].
Discussion and Conclusions
Ion beam assisted deposition has been demonstrated to be a feasible alternative to sputtering in fabricating TiN microelectrodes. Preliminary noise measurements and the first experiments with neural cells indicate that IBAD TiN microelectrodes have a competitive noise level and the material seems to be biocompatible. The future cell experiments will show whether the slightly higher impedance compared to commercial sputter deposited microelectrodes has any practical impact on signal-to-noise ratio. For exact comparison between sputtered and IBAD TiN MEAs, both coatings should be fabricated for otherwise 100 % identical MEAs.
References
[1] Janders et al., 1996. Proc. 18th Annu. Int. Conf. IEEE Eng. Med. Biol. Soc.
[2] López et al., 2001. Appl. Surf. Sci. 173.
[3] Ryynänen et al., 2012. J. Vac. Sci. & Tech. A, vol. 30, num. 4.
Figure Legend
Figure 1. Schematics of an ion beam assisted deposition (IBAD) system.
Acknowledgements
This work was funded by Finnish Funding Agency for Technology and Innovation (TEKES), Council of Tampere Region, and the Finnish Culture Foundation
Keywords:
microelectrode,
Titanium nitride,
e-Beam evaporation,
Ion beam assisted deposition,
MEA
Conference:
MEA Meeting 2016 |
10th International Meeting on Substrate-Integrated Electrode Arrays, Reutlingen, Germany, 28 Jun - 1 Jul, 2016.
Presentation Type:
Poster Presentation
Topic:
MEA Meeting 2016
Citation:
Ryynänen
T,
Toivanen
M,
Narkilahti
S and
Lekkala
J
(2016). Titanium Nitride Microelectrodes Deposited by Ion Beam Assisted E-beam Evaporation.
Front. Neurosci.
Conference Abstract:
MEA Meeting 2016 |
10th International Meeting on Substrate-Integrated Electrode Arrays.
doi: 10.3389/conf.fnins.2016.93.00123
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Received:
22 Jun 2016;
Published Online:
24 Jun 2016.
*
Correspondence:
Mr. Tomi Ryynänen, Tampere University of Technology, Department of Automation Science and Engineering, BioMediTech, Tampere, Finland, tomi.ryynanen@tuni.fi