# High aspect ratio diamond microelectrode array for neuronal activity measurements

M. Bonnauron, Samuel Saada, Lionel Rousseau, Gaelle Bazin Lissorgues,
Christine Mer, Philippe Bergonzo

► To cite this version:

M. Bonnauron, Samuel Saada, Lionel Rousseau, Gaelle Bazin Lissorgues, Christine Mer, et al. High aspect ratio diamond microelectrode array for neuronal activity measurements. Diamond 2007, the 18th European Conference on Diamond, Diamond-Like Materials, Carbon Nanotubes, Nitrides and Silicon Carbide, Sep 2007, Berlin, Germany. pp.1399-1404, 10.1016/j.diamond.2007.12.065. hal-02434396

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High aspect ratio diamond microelectrode array for neuronal activity measurements

M. Bonnauron<sup>a</sup>, S. Saada<sup>a</sup>, L. Rousseau<sup>b</sup>, G. Lissorgues<sup>b</sup>, C. Mer<sup>a</sup>, P. Bergonzo<sup>a,*</sup>

a CEA-LIST, Laboratoire Capteurs Diamant, Saclay, France

b Université Paris-EST ESIEE, Esycom, Noisy-le-Grand, France

**Abstract** Diamond exhibits several attractive properties for bio-sensing applications. In particular its high bio inertness, high electrochemical stability, and optical transparency provide diamond with high interests for neural activity study. The purpose of this study is the realisation of microelectrodes arrays (MEA) in diamond for neurons slices study. Due to a cellular lysis on the edge of the tissue slices studied, electrodes have to be at least 60 µm in height even though the electrode surface has to be minimised in order to achieve good signal noise rate. Silicon MEA with metal contacts were realised using (Deep Reactive Ion Etching) DRIE and coated with Nanocrystalline Diamond (NCD) using (Bias Enhanced Nucleation) BEN technique. We focus the study on the understanding of the BEN nucleation process on such high aspect ratio electrodes. Several parameters such as slope of the substrate, conductivity and chemical nature of the substrate were studied in order to enable selective nucleation necessary to fabricate diamond MEAs. The study leads to the optimised development of a processing route enabling the selective coating of the active tips of high aspect ratio MEAs without altering the electrical insulation between probes.

Keywords: CVD diamond; Nucleation; BEN; Microelectrode arrays

# 1. Introduction

In the past few years, novel systems for neuroelectronic interfacing have been developed, and several are widely commercialised such as e.g. Cochlear implants. They play an important role for several medical palliative treatments where electrical stimulation is required, such as for the Parkinson diseases [1]. More prospective projects are now aiming at the possibility to alleviate disabilities with the possibility to provide disabled people with a computer driven motricity [2], or also to provide shape vision to blind people [3,4]. In a more fundamental aspect, microelectrode arrays (MEA) allow to study neural communication at a meso scale, and for example between Magnetic Resonance Imaging (MRI) or Electroencephalography (EEG) and single cell patch clamps. Better understandings on the architecture of neural networks are developing and will help the design of future in-vivo prosthesis.

The present work is focussing on the recent development of novel microelectrode array prototypes combining a silicon array of high density and shape ratio (up to 1024 tips of 80 µm in height, separated by 50 µm spacing) with complete software interface and embedding signal treatment [5]. Such devices are now industrially developed, and provide the ability to record neural activity e.g. from spinal cords deposited on the high aspect ratio electrodes.

However, when used for the stimulation of neurons, the voltage bursts used often have destructive effects on the quality of the electrodes, especially after intensive use. Our work here therefore focuses on the improvement of the electrode stability, by replacing the noble metal (eg: platinum, gold...) termination of the electrodes that seemed to be damaged after several stimulations, and we propose to use the same structure with a conductive Boron doped NanoCrystaline diamond (BNCD) layer on top of the high aspect ratio electrodes. The main objectives of the current project are to with 80 µm in height are required, that also exhibit a high spatial density since several neural signals are to be recorded at the same time. Finally, since stimulation with electrical action potentials is often detrimental to live cells, we prefer to exploit the electrochemical properties of the diamond interface, that would enable to drive electrons to the neurons directly for a more biomimetic and less aggressive approach to the neurons. This therefore also implies that the material exhibits remarkable electrode properties, and only diamond can combine these features. The BNCD electrode material grown in our laboratory has been shown to exhibit remarkable electrochemical properties, with Pt reactivity and long term stability [6]. It constitutes the ideal material for the current application.

Other studies have also reported the possibility to use diamond electrodes fabricated from single wires [8–10] or with very sharp form for AFM applications [11]. Arrays of 3D electrodes have also been used as Field Effect Array [12] and 2D dimensions arrays have been used for electrochemical detection [13–16]. However, none of those systems propose the combination of high density, high aspect ratio and electrochemical stability and reactivity for neuronal signal monitoring and stimulating. The system we propose here is based on a formerly existing design [5], i.e. a novel silicon microelectrode array, fabricated from DRIE on silicon that is coated with electrode quality BNCD material, with a particularly high aspect ratio. This enables the possibility to directly mechanically insert the electrodes within the tissue cells for the measurement of their action potentials. Their shapes are those of exponentially conic tips of 80 µm in height for a base diameter of 80 µm. Using the 3D etched silicon as a support we avoid the time consuming difficulties associated with diamond etching that would result from a whole diamond tip system, [17,18] and further there is no need for full substrate dissolution as required with AFM high ratio diamond tips [19]. Challenges however are to preserve the silicon high aspect ratio tip shapes without damaging the needles nor smoothing their surface under a thick diamond coating layer. The layer has thus to be as thin as possible in order to fit the shape of the silicon needles. This implies the possibility to nucleate diamond on 3D shaped structures, which constitute the most challenging part of this work [20]. In fact, common micrometer powder scratching using ultrasonic excitation would result in severe damage to the silicon needle tips and the nucleation densities reached are generally not sufficient. Nanoseeding using ultradispersed diamond powder is seen as a very promising novel development, but spreading uniformly seeds on a 3D surface appears challenging, and further the adhesion of the obtained film, here crucial, remains uncertain to-date. The approach we therefore have preferred for the current realisation is based on Bias Enhanced Nucleation (BEN). This allows realising very dense nucleation with little damage to the needles and a good adhesion. At the same time, the
BEN technique allows to create patterns from selective nucleation [21] thus enabling a bottom-up approach for the MEA device fabrication. First results on patterning with BEN have indeed already been reported, using e.g. spontaneous nucleation on Pt with a high selectivity compared to that on silicon dioxide [22]. realise MEA that will allow the study of the formation of the neural network itself [6], in close collaboration with the CNIC laboratory [7] on embryonic mouse spinal cords in the prenatal week. In this particular case it is necessary to reach a cell population which is located from 10 to 80 µm from the uppermost layer of the preparation. In order to avoid damage to the preparation, tips with 80 µm in height are required, that also exhibit a high spatial density since several neural signals are to be recorded at the same time. Finally, since stimulation with electrical action potentials is often detrimental to live cells, we prefer to exploit the electrochemical properties of the diamond interface, that would enable to drive electrons to the neurons directly for a more biomimetic and less aggressive approach to the neurons. This therefore also implies that the material exhibits remarkable electrode properties, and only diamond can combine these features. The BNCD electrode material grown in our laboratory has been shown to exhibit remarkable electrochemical properties, with Pt reactivity and long term stability [6]. It constitutes the ideal material for the current application.

Other studies have also reported the possibility to use diamond electrodes fabricated from single wires [8–10] or with very sharp form for AFM applications [11]. Arrays of 3D electrodes have also been used as Field Effect Array [12] and 2D dimensions arrays have been used for electrochemical detection [13–16]. However, none of those systems propose the combination of high density, high aspect ratio and electrochemical stability and reactivity for neuronal signal monitoring and stimulating. The system we propose here is based on a formerly existing design [5], i.e. a novel silicon microelectrode array, fabricated from DRIE on silicon that is coated with electrode quality BNCD material, with a particularly high aspect ratio. This enables the possibility to directly mechanically insert the electrodes within the tissue cells for the measurement of their action potentials. Their shapes are those of exponentially conic tips of 80 µm in height for a base diameter of 80 µm. Using the 3D etched silicon as a support we avoid the time consuming difficulties associated with diamond etching that would result from a whole diamond tip system, [17,18] and further there is no need for full substrate dissolution as required with AFM high ratio diamond tips [19].

Challenges however are to preserve the silicon high aspect ratio tip shapes without damaging the needles nor smoothing their surface under a thick diamond coating layer. The layer has thus to be as thin as possible in order to fit the shape of the silicon needles. This implies the possibility to nucleate diamond on 3D shaped structures, which constitute the most challenging part of this work [20]. In fact, common micrometer powder scratching using ultrasonic excitation would result in severe damage to the silicon needle tips and the nucleation densities reached are generally not sufficient. Nanoseeding using ultradispersed diamond powder is seen as a very promising novel development, but spreading uniformly seeds on a 3D surface appears challenging, and further the adhesion of the obtained film, here crucial, remains uncertain to-date. The approach we therefore have preferred for the current realisation is based on Bias Enhanced Nucleation (BEN). This allows realising very dense nucleation with little damage to the needles and a good adhesion. At the same time, the BEN technique allows to create patterns from selective nucleation [21] thus enabling a bottom-up approach for the MEA device fabrication. First results on patterning with BEN have indeed already been reported, using e.g. spontaneous nucleation on Pt with a high selectivity compared to that on silicon dioxide [22].

## 2. Experimental details

In the current work the study was performed on silicon MEAs as fabricated in a clean environment and from the use of Deep Reactive Ion Etching (DRIE) [5]. This technique is used to fabricate a silicon vertical side wall by alternating successive steps of anisotropic and isotropic etching. It enables the fabrication of denser matrixes than using standard RIE or liquid phase etching [23]. On the 3D arrays a 400 nm thin thermal silicon oxide was overgrown in order to avoid electrical losses into the substrate. On top of the silicon oxide a standard Ti/Pt pattern was prepared for electrical connection of the MEAs.

High rate nucleation prior to BNCD growth was performed using a Microwave Plasma enhanced CVD (MPCVD) reactor enabling Bias Enhanced Nucleation (BEN) pretreatment on 2 in. diameter substrates. The MPCVD deposition parameters were previously optimised for high nucleation density onto (111)-Si [20], and here adapted in order to preserve the structure of the needles. Parameters were tuned with a pressure of 30 mBar, a CH4/H2 ratio of 10/90, and an optimised bias voltage of -307 V. The microwave power was adjusted from 0.4 to 1.1 kW with respect to the sample dimension and in order to reach a temperature of 760 °C as measured using a pyrometer. After nucleation, the growth of the BNCD layer was performed using a separate experimental setup, and using conventional nanocrystalline diamond layer growth conditions [24,25]. All samples were probed using a High Resolution Scanning Electron Microscope HR-SEM (HITACHI S-4500 FEG-SEM) that enables the quantification of the crystal densities.

## 3. Aspect ratio selectivity

The study concentrated first on the nucleation on 3D macroscale objects (objects are in the order of several hundreds of micrometer), in order to study the influence of the shape of the substrates and on the nucleation density during the BEN process. This was performed on stairs, fabricated on 300 µm thick silicon, with structures exhibiting heights varying from 0 to 7200 µm (the BEN step used parameters are: 30mBar of pressure during 5min, at 10% of CH4 in H2, with a MW power of 1.1 kW. The BEN current is of 50 mA. After this short nucleation step, a growth step follows during 10min at 30mBar using 10% CH4 in H2, at 775–802°). Then this was adapted at the microscale to high aspect ratio tips, with silicon structures of 16 µm in height and 25 µm in diameter. This study was conducted on 10 × 10 mm² samples grown at the same time using almost identical conditions as above (but a power of 450 W, for a BEN current of 100 mA).

<figure><img src="image_2.png" /><figcaption>Fig. 1. SEM observation of the effect of the height of a tip on the nucleation at the tip pedestal.</figcaption></figure>
The first experiment consists of probing the influence of the shape of the tips since above heights of a few hundredths of microns they are likely to interfere with the plasma sheath. This was performed in situ by attaching vertically on top of the sample tested a slab of silicon of varying height. On Fig. 1 is shown the result on the sample: the white regions on the figure show where nucleation has occurred. The vertical silicon slab is located in the plane that corresponds to the top of the figure, thus corresponding to a tip with zero height on the top left corner of the figure, to a tip with several millimetres in height on the top right corner. We deduce from this experiment that at the macroscale, for small stairs, nucleation occurs down next to its pedestal, whether for stairs of several 100s of microns, seeds seem to appear at the bottom of the pedestal (Fig. 1). Indeed it is likely that the immersion of a stair of several 100s of microns into the plasma alters the plasma sheath and thus the nucleation.

If this is applied onto a microscale cylindrical pillar, as shown in Fig. 2 with a 16 µm height, we observe that although a diamond layer has been grown on its top, there was no growth at all on its side walls, thus confirming the preceding observations. Further, no difference in terms of nucleation density nor of diamond film morphology were apparent between the top of the pillar and the floor next to it, as this could be associated with an insignificant influence of the temperature gradient due to the shape on the nucleation.

To explain such gradient of nucleation density between the vertical and horizontal parts, one has to refer to the mechanisms driven during the BEN step. In fact, BEN effect is strongly depending on the energy of the species impacting the surface. This energy depends on the electric field intensity in the plasma sheath which leads to a bombardment of the substrate surface. Assuming both vertical and horizontal walls at the same temperature have similar electrical conductivities, we deduce that only the field lines can be the origin of the nucleation discrepancies. This was simulated using a finite element electrostatic simulation of the sheath structure over the silicon surface just considering the influence of the geometry on the electric field without considering the gas ionisation. The results are shown on Fig. 3 where is displayed the field intensity gradient around two stairs of differing heights. If the stair is big with respect to the sheath dimension, the plasma is locally surrounding the stair and the electrical field also goes around the stair (Fig. 3b). This way, the BEN field also accelerates ions in the lateral parts of the tips, and nucleation occurs on the vertical walls. On the other hand, if the stair height is small, then the sheath is hardly affected by the stair, and the electrical field remains normal to the whole surface of the wafer, thus vertical, and ions accelerated towards the substrate do not hit the vertical tip planes (Fig. 3a).

In other words, we can say that for small stairs, the electrical field remains normal to the surface and there is no bombardment on the vertical part and thus no nucleation appears on the vertical parts. The absence of perturbation of the sheath was also confirmed by high nucleation densities observation down to the pedestal of the wall. For higher steps, nucleation is observed on vertical parts but decreases close to the vicinity of the pedestal of the tip. The approximate limit between both regimes stands between 200 and 400 µm (deduced from Fig. 1). For much lower heights, and as concerned here where we aim at tips of the order of 80 µm, we see that the tip shape will have a reduced influence on the plasma sheath, and therefore it is likely we will observe almost no nucleation on vertical walls. This was confirmed using silicon tips with 50µm in height silicon as well as on tips terminated by a cylinder at the top. Only the angle of the surface is likely to matter.

<figure><img src="image_3.png" /><figcaption>Fig. 2. 16 µm height cylindrical silicon electrode covered with 200 nm NCD.</figcaption></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Influence of the shape of the sheath.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 4. Influence of the angle of a silicon surface with respect to the sample plane, at the microscale.</figcaption></figure>

4. Geometrical considerations with respect to the surface slope

The influence of the angle of the silicon surface with respect to the sample plane on the nucleation properties was studied on a 300 µm thick silicon wafer with an edge polished manually in order to obtain a progressive angle from 20 to 80°. The images are shown on Fig. 4: at the top part of the cylindrical shape ending needle, the limit between the steep slope shape and vertical shape is clearly visible. On Fig. 4b, an insignificant difference of nucleation between flat and steep slope shapes is visible. Fig. 4c gives a view of the whole needle. We observe very high nucleation selectivity on the vertical parts but it appears interesting to nucleate onto different substrate angle in order to determine the limits of this selectivity. These microscale studies did not reveal any limitation concerning the nucleation onto tilted substrate until it is not vertical: nucleation was observed at the macroscale onto varying slopes with an angle from 20° (nucleation density equal to 9.10¹¹ cm⁻²) to 80° (nucleation density equal to 2.10¹¹ cm⁻²).

The best processing conditions were optimised at a bias step duration of 6 min, followed by a 45 min growth step. This enables high surface coverage while preventing damage of the tip shape (Fig. 5). Further, there was no concentration of the particles bombarding the substrate at the top of the tips, since no localised increase of the temperature was observed. Homogeneous growth was here attained on the tips over the device dimensions (typ. 3 × 10 mm²).

<figure><img src="image_7.png" /><figcaption>Fig. 5. Silicon tip after 6 mn BIAS and 45 mn growth.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. Comparison between Pt and Si at -307 V 550 W 765 °C for a 10×10 mm² sample.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. 5 µm diamond line onto Si patterned with SiO2.</figcaption></figure>

5. Selective nucleation

In a second part of the study, we then focussed on the possibility to use the BEN pretreatment to directly pattern the diamond nucleation, from enhanced selective promotion of the ion bombardment on localised areas. At first, in order to study the nucleation on metals, we studied the case of Pt: 1 mm in diameter Pt pads of 200 nm in thickness were prepared on 10×10 mm² silicon samples. The result after nucleation demonstrated that no visible difference is observed between the nucleation on silicon with respect to that on Pt. This is very
valuable since it may enable from selective nucleation on Pt with respect to that of insulating line to pattern the diamond film layers. To
demonstrate this, we studied the nucleation directly on 2D platinum coated MEA structures as fabricated onto silicon substrates with no
particular insulation between the Pt contacts. However, the Pt lines are themselves insulated from the silicon substrate by a 400 nm silicon
dioxide layer. This test was performed on a large area device with size of 40× 40 mm², and in order to prevent the growth of diamond outside
the MEA, i.e. on the metal routes towards bonding pads, we used a stainless steel plate mask that exhibited a 20× 20 mm² window that
enable partial exposure of the MEA structure to the plasma, and particularly on the active tip region. A short duration growth of 10 min at
99% H2, 1% CH4, 780°C, confirmed the possibility to nucleate diamond on both Pt and Si in a similar manner, as witnessed on Fig. 6 that
compares the grain structure of the thin diamond layer between Pt (left) and Si (right). No visible difference between the two regions, nor
catalytic effect on Pt are observed as seen on 0, this will enable the possibility to reach high density and homogeneity of the nucleation on
the Pt pads.

<figure><img src="image_10.png" /><figcaption>Fig. 8. BEN nucleation onto Pt/SiO2/Si Microelectrode arrays.</figcaption></figure>

On the other hand we then wish to prevent the possibility to grow diamond on the areas that are separating the tips on the MEAs. This is studied using similar structures as presented in the last experiment, i.e. 40× 40 mm² MEA structures, with tips, but the difference here is that the insulating oxide layer is left on the silicon in the areas that are separating the tips. As such, lines of SiO2 patterned down to 5 µm were fabricated, still on silicon, and nucleation images are shown on Fig. 7. It appears that we indeed obtain a remarkable selectivity, as no nucleation occurs on the silicon dioxide, whether it does occur on the nearby Si. In fact, by focussing in Fig. 7 in the transition region between at edge of the SiO2 lines, we observe a very narrow line (<100 nm) of extremely bright luminosity on the SEM image, we attribute to the charge of the oxide on the edge of the oxide during the SEM imaging. Further, clearly we observe that the nucleation is reduced in the shallow region that is close to the oxide edge. This subtle nucleation difference could either be attributed to the local perturbation of the electrical field lines, that could lead to a reduced ion bombardment during the BEN step, or also to the fact that the oxide being etched during the ion bombardment does locally release some oxygen to the gas phase that may perturbate strongly the nucleation. To study this possible influence of the oxygen onto the BEN step, we studied this process while oxygen was added to the gas phase during growth. Indeed, above O2 contaminations of the order of 9000 ppm, it was reproducibly observed that the O precursor was preventing the nucleation during the BEN step, although it was not at lower values. We repeated the same experiment using other insulating layers that do not contain oxygen, and namely LGS and AlN, and reduced effect was observed, although still slightly visible. Those sets of experiments tends to demonstrate that the extremely high selectivity SiO2 offers with respect to other oxides can be associated to both its insulating nature but also the fact that it contains oxygen.

Finally the possibility to selectively nucleate on Pt lines was studied with a Pt MEA fabricated on silicon dioxide structures made on a silicon structure. The difficulty here will be the fact that the Pt lines cannot be biased since they are floating on the SiO2 insulating layer. Indeed the nucleation densities observed (Fig. 8) were approaching $10^9$, thus lower than on the former experiment on Pt (>10$^{10}$). If the reason for this reduced nucleation density remains due to the floating bias of the Pt lines, one way to improve the process would simply be to ground the lines during the bias step.

**6. Conclusion**

We have optimised a process enabling to coat 3D structures with micrometer tips with BNCD thin layers. The possibility to tune the selective growth not only from SiO2 but also from the angle of the surface with respect to the sample plane was demonstrated. In order to develop a new route for the fabrication of diamond coated microelectrode arrays, we therefore propose the following process. (i) At first the silicon tips are fabricated from DRIE in order to reach the design specifications for the tissues to probe. (ii) Then, wet oxidation, leading to a 400 nm thick SiO2 layer, will be used to provide a chemical and physical pattern for bottom-up approach for diamond patterning. (iii) Pt deposition and lift-off photo lithography allows decreasing the resistance between the contact pad and the diamond electrodes and provides favourable field for nucleation. (iv) BEN on grounded platinum and silicon oxide allows high nucleation rates as well as high selectivity of the nucleation. (v) Growth of the BNCD layer creates an electrochemically stable and reactive electrode material for neuronal signal recordings. (vi) An isolated polymer layer (e.g. made of SU8) is finally used to avoid the contact between the metal lines and the physiological conductive media.
## Acknowledgements

Although not exclusively driven by the project, the work benefited from the DREAMS project support within the sixth European framework (FP6-NMP-2006-676033345) for financial support in the CEA-LIST laboratory for the improvement of the diamond electrode material quality. Further, the authors also wish to acknowledge Dr B. Yvert, from CNIC (CNRS Bordeaux) for useful discussions.

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