(Semi-)transparent ALD TiN microelectrodes
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1
Tampere University of Technology, BioMediTech Institute and Faculty of Biomedical Sciences and Engineering, Finland
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2
University of Tampere, BioMediTech Institute and Faculty of Medicine and Life Sciences, Finland
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3
VTT Technical Research Centre of Finland Ltd, Finland
Motivation
In order to avoid challenging imaging through the cell culturing medium an inverted microscope is, typically, preferred in imaging of the cells growing on a microelectrode array (MEA). However, one major optical obstacle remains – the tracks and especially the electrodes are often opaque, which hinders the visibility of the cells just there where the visibility would be needed the most. In some MEA types transparent conducting material, indium tin oxide (ITO), is used as the track material, but due to its rather high impedance and noise levels ITO is rarely used as the electrode material. Impedance and noise of the electrode can be decreased by increasing the effective surface area of the electrode. Such porous or columnar structure is typical, for example, for a common MEA electrode material titanium nitride (TiN). That material, however, cannot generally be considered transparent, unless the layer is very thin, a few tens of nanometers. At such low thickness special attention has to be taken to guarantee that the thin film is continuous and homogenous. For that, in this study we have taken the benefit of atomic layer deposition (ALD) method’s capability to deposit high quality TiN thin films already at the thicknesses where the layer can still be considered as transparent.
Materials and Methods
Microscope slide grade glass (49 × 49 × 1 mm) was used as the substrate. TiN layers were prepared by ALD at 450 °C, using TiCl4 and ammonia as precursors and nitrogen as a carrier gas [1]. Performing 1000, 2000 and 3000 ALD cycles resulted in 21, 42 and 63 nm thick TiN layers on both sides of the glass substrate, respectively.
Before continuing the MEA fabrication process, the transparency of the ALD TiN thin films was measured with Ocean Optics JAZ spectrometer. In order to keep the MEA structure as simple as possible, there were no separate layers or materials for the electrodes and tracks, but they all were wet etched with HF and H2O2 to the same ALD TiN layer. The etching also removed the TiN layer from the bottom side of the substrate. The MEA layout used in this experiment was the standard 8 × 8 layout with 30 µm electrodes. Next the 100 + 500 + 100 nm stack of PECVD SiO2:SiN:SiO2 was deposited as the insulator layer and openings for the electrodes and the contact pads were etched with RIE. Finally, for some of the samples 400 nm of TiN was deposited on the contact pads and grounding electrodes with ion beam assisted e-beam deposition (IBAD) [2] to increase the mechanical durability. Impedances of ready-made electrodes were measured with MEA-IT (Multi Channel Systems MCS GmbH).
In cell experiments, human induced pluripotent stem cell (hiPSC) –derived cortical neurons (cell line 10212.EURCCs) were cultured for 14 days on the ALD TiN MEAs and commercial reference MEAs with opaque TiN electrodes (MCS 60-6wellMEA200/30iR-Ti-w/o). Spontaneous activity of the cells was recorded on days 3, 7, 10 and 14 after plating with MCS MEA2100 system. Sampling rate was 25 kHz and neuronal spikes were detected from 200 Hz high-pass filtered data when their amplitude crossed the threshold of -5 × standard deviation of noise. In conjunction with the MEA measurements, the MEAs were imaged using an Axio Observer.A1 inverted microscope connected to an Axiocam 506 color camera (both from ZEISS). Finally, the cells on selected MEAs were fixed with 4 % paraformaldehyde and stained for nuclei (4',6-diamidino-2-phenylindole) as well as neuronal markers β-tubulinIII (BTUB3, 1:500; T8660, Sigma-Aldrich) and neurofilament heavy chain (NF-H, 1:250; A00136, GenScript). For secondary antibody labeling, Alexa Fluor 488 (1:400; A21202, Thermo-Fisher) and Alexa Fluor 568 (1:400; A11041, Thermo-Fisher) were used. Fluorescence imaging was done using an Olympus IX51 microscope equipped with an Olympus DP30BW camera (Olympus Corporation).
Results
The transparency of ALD TiN was highest around 550-600 nm wavelength and varied there from ~18 % of 63 nm layer to ~45 % of 21 nm layer when measured from samples having ALD TiN coating still on both sides of the glass. In the final electrodes the coating, however, is left only on one side making the electrodes more transparent than the values above indicate. Impedances were from 600 to 1100 kΩ. The cells and the fluorescence signal after neuronal staining were visible through the electrodes and the tracks of all three thickness, except 63 nm thickness of ALD TiN blocked the fluorescence signal from all stainings. Despite higher impedance compared with commercial opaque TiN electrodes all the ALD TiN MEAs recorded signals comparable to commercial reference MEAs. No biocompatibility nor mechanical stability issues concerning ALD TiN were observed during the cell experiments, except the additional coating was found useful for the contact pads.
Discussion and Conclusions
The proposed concept offers a promising transparent alternative for ITO electrodes. The impedances of thin ALD TiN MEA electrodes were lower or comparable with values typically reported for ITO electrodes in the literature. Especially the 21 nm and 42 nm thick versions can be considered transparent and also in 63 nm version the cells can be seen at least partly through the electrodes. As ALD is capable of producing highly columnar TiN morphology, it may be an interesting choice for sputtering and IBAD also in fabricating thicker opaque low impedance and low noise electrodes.
Acknowledgements
This work was supported by Business Finland (formerly known as The Finnish Funding Agency for Technology and Innovation (TEKES)), the Academy of Finland (the Finnish Centre of Excellence in Atomic Layer Deposition (ALDCoE) and the Centre of Excellence in Body-on-Chip Research) and the Finnish Culture Foundation. The authors thank Juha Heikkilä for assistance with the cell experiments, and associate professor Riikka Puurunen for additional contribution with ALD TiN.
References
[1] Grigoras, K. et al. (2016). Nano Energy 26, 340–345.
[2] Ryynänen, T. et al. (2016). Front. Neurosci. Conf. Abstr. MEA Meet. 2016.
Keywords:
Titanium nitride,
ALD,
Transparent,
MEA,
Neurons
Conference:
MEA Meeting 2018 | 11th International Meeting on Substrate Integrated Microelectrode Arrays, Reutlingen, Germany, 4 Jul - 6 Jul, 2018.
Presentation Type:
Poster Presentation
Topic:
Microelectrode Array Technology
Citation:
Ryynänen
T,
Pelkonen
A,
Grigoras
K,
Ylivaara
O,
Ahopelto
J,
Prunnila
M,
Narkilahti
S and
Lekkala
J
(2019). (Semi-)transparent ALD TiN microelectrodes.
Conference Abstract:
MEA Meeting 2018 | 11th International Meeting on Substrate Integrated Microelectrode Arrays.
doi: 10.3389/conf.fncel.2018.38.00111
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Received:
16 Mar 2018;
Published Online:
17 Jan 2019.
*
Correspondence:
Mr. Tomi Ryynänen, Tampere University of Technology, BioMediTech Institute and Faculty of Biomedical Sciences and Engineering, Tampere, Finland, tomi.ryynanen@tuni.fi