Jean-Marie Prival

Andrew Harris

Claudio Robustelli Test

Elena Zanella

Jonas Biren

Oryaëlle Chevrel

Unravelling the emplacement dynamics of silicic lava flows: the case of the Grande Cascade

The breccia

Introduction

Pockets of breccia intermittently crop out at the base of the flow. The contact with the ground is never visible, covered by rubble, but some outcrops expose up to ~3 m of thickness. The breccia is composed by centimetric to decimetric blocks surrounded by a dominant, powdery, grey matrix. Its particle size distribution, particle morphology and componentry all converge towards the hypothesis that the breccia was formed by grinding of the lava under its own weight.

The shear zone

The surface

Problematic and objective Silicic lava flows are a rarely observed style of volcanism for which emplacement models remain poorly constrained. Yet they represent a hazard where oversteeping flow fronts can collapse and generate block-and-ash flows as at Santiaguito (Guatemala) in 1986, Unzen (Japan) in 1991, and Sinabung (Indonesia) in 2014. Our objective is to build new emplacement models for silicic lava flows.

Context

Conclusion

We analyzed the four facies of a thick trachyte flow to infer its emplacement dynamics. We found that its emplacement was driven by deformation, with a solid plug "sliding" over a shear zone and basal breccia, which accommodated most of the stress. The speed and timescale of emplacement, a crucial parameter for hazard assessment, remains unknown.

This study stresses the need to build new emplacement models for silicic lava flows.

Parallel, subvertical foliation planes on the flow's surface.

The plug

The plug is the dominant facies, making ~85 % of the flow's thickness. There is chemical variation from the bottom (62.5 wt% silica) to the top (66.5 %) of the unit. This might be the result of a mixing process between two magmas, as shown by basic enclaves, banded structures and emulsified textures [4]. Lava deformation has created a foliation within the plug, with 5-20 cm thick slabs detaching from the cliff due to weathering.

  Unravelling the emplacement dynamics of silicic lava flows: the case of the Grande Cascade trachyte flow (Monts Dore, France) Jean-Marie Prival, Andrew J.L. J.L. Harris, Claudio Robustelli Test, Elena Zanella, Jonas Biren, Magdalena Oryaëlle Chevrel To cite this version: Jean-Marie Prival, Andrew J.L. J.L. Harris, Claudio Robustelli Test, Elena Zanella, Jonas Biren, et al.. Unravelling the emplacement dynamics of silicic lava flows: the case of the Grande Cascade trachyte flow (Monts Dore, France). Cities on Volcanoes 10, Sep 2018, Naples, Italy. hal-01871387

  The Grande Cascade lava flow is located in the Monts Dore massif, the youngest stratovolcano of the French Massif Central, active from 3.09 to 0.23 Ma. The flow has been dated at 0.38 ± 0.02 Ma [1]. It is 20 to 40 m thick and made of trachyte with up to ~40 % sanidine phenocrysts. The flow can be divided vertically into four facies: (1) a basal breccia; (2) a shear zone; (3) a massive plug; and (4) a foliated surface. Abstract n°892 [1] Cantagrel and Baubron (1983) Chronologie K-Ar des éruptions dans le massif volcanique des Monts Dore : implications volcanologiques. Géologie de la France (2), I, 1-2. [2] Latutrie et al. (2017) Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France). Bulletin of Volcanology 79: 4. [3] Hall (1978) The stratigraphy of Northern Lipari and the structure of the Rocche Rosse rhyolite lava flow and its implications. Unpubl. B.Sc. thesis, University of Leeds. [4] Gourgaud and Maury (1984) Magma mixing in alkaline series: an example from Sancy volcano (Mont-Dore, Massif Central, France). Bulletin of Volcanology 47: 4. of 1019 particles in the 1.5 Φ bin (350-500 µm).

  of France (NASA/NCEI). b: the four volcanic massifs of northwest Massif Central (CRAIG). c: orthophotograph showing the location of the Grande Cascade in the Dore valley (IGN). Methods -Structural analysis: mapping and measuring structures. -Textural analysis: density, vesicularity, crystal orientation… -Anisotropy of Magnetic Susceptibility (AMS) to know flow directions at a local scale.

  the flow's surface showing sub-vertical foliation planes. Model after Hall (1978) showing foliation patterns in the Rocche Rosse rhyolite flow (Lipari).

  Typical outcrop of the shear zone, showing ~2.5 m of subparallel shear planes under the undeformed plug.Measurements of shear planes frequency showing a decrease going upward: one plane every ~7 cm in the lower half, one every ~14 cm in the upper half.Outcrop of breccia lying directly under the massive plug. The blocks are surrounded by a dominant, powdery, grey matrix.Plug outcrop showing a vertical foliation. particles in the 5 Φ bin (31-44 µm).Particle size distribution showing a large amount of fines (> 6 wt% below 32 µm).Detail of the vertical foliation.Determination of the phenocrysts orientation in a slab from the shear zone showing a crude alignment.

These vertical structures could be the result of a ramping process due to a compressive regime behind the flow front, as

	The surface of the flow is covered by
	vegetation, but a few outcrops allow to see
	parallel foliation planes, with an average
	inclination of the magnetic foliation of 88.5°.
	The shear zone is 2-3
	m thick on average,
	but it almost disappear
	locally, where the plug
	is thinner, thus the
	vertical stress less
	intense. The frequency
	of shear planes tends
	to decrease from the
	base to the top of the
	zone.

observed at the nearby puy de Cliergue trachyte flow [2], or at the Rocche Rosse rhyolite flow [3].


Ahmet Lale

Auriane Grappin

David Bourrier

Laurent Mazenq

Aurélie Lecestre

Jérôme Launay

Pierre Temple-Boyer

DESIGN, REALIZATION AND CHARACTERIZATION OF ALL-AROUND SiO2/Al2O3 GATE, SUSPENDED SILICON NANOWIRE CHEMICAL FIELD EFFECT TRANSISTORS

Keywords: ChemFET, finFET, MOSFET, ISFET, silicon nanowire, biosensor, potentiometric sensor, nanosensor, microsensor, pH measurement, microfluidics, gate all-around

We present a sensor platform associated to siliconnanowire chemical field effect transistors (Si-nw-ChemFET). Innovations concern the use of networks of suspended silicon N + /P/N + nanowires as conducting channel, the realization by thermal oxidation and Atomic-Layer Deposition (ALD) of a SiO2/Al2O3 gate insulator allaround the silicon nanowires, and their final integration into covered SU8-based microfluidic channels. The Si-nw-MOSFET/ChemFET fabrication process and electrical/electrochemical characterizations are presented. The fabrication process did not need an expensive and time-consuming e-beam lithography, but only fast and "low cost" standard photolithography protocols. Such microdevice will provide new opportunities for biochemical analysis at the micro/nanoscale.

INTRODUCTION

Chemically sensitive Field Effect Transistors (ChemFET) are microsensors derived by Metal Oxide Semiconductor Field Effect Transistors [1]. These are electronic microsensors designed to measure the pH. ChemFETs have shown interesting properties for the detection in liquid phase [1] [2] and have been functionalized for many applications such as the measurement of potassium [3], sodium [3], ammonium [4], urea [5], creatinine [6], lactate [7], glutamate [8]…

Nevertheless, to open the door to new innovative applications for ChemFET-based microsensors, it is necessary to increase their sensitivity, to decrease response times and their detection limit. To meet this challenge, the development of nanotransistor based on silicon nanowire Si-nw-ChemFET was proposed [9] [10]. In this field, surrounding gate seems the ideal candidate: indeed, low channel dimensions provide new detection mechanisms as well as analysis of micro/nano-volumes, allowing to work at the single cell level [10]. Nevertheless, the development of suspended silicon nanowires should still improve Si-nw-ChemFET detection properties in liquid phase, providing the integration of well-controlled chemically-sensitive gate insulators all-around the Si-nw-based conductive channel.

EXPERIMENTAL Device fabrication

6-inch SOI wafers (Silicon On Insulator, P type, 10 15 atoms/cm 3 ) were used.

These devices consist in single and networked silicon nanowires field effect transistors. Different wire lengths, gate lengths and number of parallel wires were fabricated.

First, source and drain areas were N doped (10 20 atoms/cm 3 ) with arsenic ion implantation. Two different gate length were realized, 0.9µm and 3,9µm.

To fabricate silicon nanowires, projection photolithography with a Canon FPA-3000i4 stepper and an AZ ECI 3012 photoresist were employed. After optimizations, were achieved on a single chip, with the same insolation parameters, patterns of transistors with a single nanowire as channel, next to patterns of transistors with 100 nanowires parallel network (figure 1). The goal is to compare in the future, the pros and cons of each structure, in terms of limit of detection and/or sensitivity for biochemical applications.

Once photoresist nanowires were optimized, nanopatterns were transferred in silicon thanks to a reactive ion etching. After optimization, 200nm x 170nm rectangle section nanowires were obtained with a perfect reproducibility. Two different densities of network were realized: 20 and 100 nanowires over a width of 80µm. Thus, using SOI wafers, networks of suspended silicon nanowires were successfully integrated.

Then the realization of the gate structure was performed. After RCA cleaning, a dry thermal oxidation of 22nm was achieved and an alumina film of 26nm was deposited by ALD. The advantage of this technique is to deposit extremely conformal films all around the nanowires, while providing well-controlled thicknesses [11]. Thus, a SiO2/Al2O3 gate was integrated all-around the 500 nm

Photoresist nanowires

Figure 1: SEM view of a network of photoresist nanowires, before reactive ion etching (tilt 52°).

silicon nanowire (figure 2). This Al2O3 layer was used as a gate chemically-sensitive layer as well as the microdevice passivation layer in liquid phase [9][12].

Figure 2: SEM view of silicon nanowires after gate insulator completion (tilt 52°).

Once the gate/source/drain metallic contacts and interconnections were ended, Si-nw-MOSFET and Si-nw-ChemFET were fabricated (figure 3). Then, using a specific SU8-3D technique [13], covered 5µm width microfluidic channels were finally realized in order to deal with liquid phase analysis at the microscale (figure 4).

DEVICE CHARACTERIZATIONS

Si-nw-MOSFET/ChemFET were characterized electrically and electrochemically using standard I-V experiments (figure 5). The MOSFET works without liquid, so it makes easier some characterizations. It allows to check the quality of the manufacturing process, and to discriminate problems due to the transistor from those related to the microfluidics.

Si-nw-MOSFET characterizations

The threshold voltage of 42 transistors was measured and the mean value was 0.62V with a standard deviation of only 0.26V (figure 6). That shows the quality of the SiO2/ALD-Al2O3 gate dielectric and the Si/SiO2 interface.

The influence of 2 parameters on electric characteristics was studied: the number of parallel nanowires (1, 20 and 100) and the gate length L G (0.9µm "short" and 3.9µm "long"), the length of the nanowires is 10µm.

Firstly, very low subthreshold (IOFF) currents (7.10 -14 A for VGS = -2V, VDS = 0.1V) were obtained for a parallel network of 100 nanowires (figure 7). This leakage current is the same for both gate lengths and proportional to the number of parallel nanowires. Therefore, it allows to estimate the leakage current of X nanowires. The leakage current through the gate dielectric (IG) is lower than 10 -15 A (limits of the measuring instrument) for 100 parallel nanowires under 3V. So, the IG current is lower than 10 -17 A for one nanowire and shows the insulation quality of the SiO2/Al2O3 double layer.

The number of parallel nanowires and the gate length does not influence the subthreshold slope, which is 6 decades/V.

The ratio ION/IOFF is about 9 decades, thanks to the very low IOFF current.

The transconductance is 4.3 times higher for nanowires with the short gate length in comparison with the long gate length, exactly the length ratio between the long and short gate lengths. The transconductance is also proportional to the number of parallel nanowires (figure 8).

All these characterizations have confirmed the well operation of Si-nw-MOSFET. The main characteristics are grouped in the table 1.

Table 1: evolution of the main characteristics with the gate length and the number of parallel nanowires for the Si-nw-MOSFET.

Si-nw-ChemFET characterizations

The Si-nw-ChemFET is like a Si-nw-MOSFET which gate metallization is replaced by a microfluidic channel. Sinw-ChemFETs were characterized in liquid phase with a specific setup. The liquid was polarized with an Ag/AgCl/KClsat double junction glass reference electrode.

Like for the study of Si-nw-MOSFET, the influence of 2 parameters on electric characteristics was studied: the number of parallel nanowires (1 and 100) and the gate length LG (0.9µm "short" and 3.9µm "long"), the length of the nanowires is 10µm.

The subthreshold leakage current (IOFF) and the leakage current through the gate dielectric (IG), for 100 parallel nanowires, were not measurable because they were lower than the limit of the measuring instrument used (5.10 - 11 A) (figure 9). That means an IOFF current and an IG current lower than 5.10 -13 A for one nanowire. These very low leakage currents allow a high ION/IOFF ratio reaching at least 10 7 for one nanowire and 10 8 for 100 parallel nanowires.

Concerning the subthreshold slope, excellent characteristics were obtained, with 8 decades/V. Like for the Si-nw-MOSFET, this value is independent of the number of parallel nanowires and gate length. As for the Si-nw-ChemFET, the transconductance is inversely proportional to the gate length. The Nernstian response to pH variations of 58mV/pH is in agreement with theory (figure 10). There is no sensitivity differences when the gate length or the number of parallel nanowires changes, no hysteresis in the pH responses and no sensitivity to interfering ions Na + and K + (results not shown).

All these characterizations have shown the well behavior of Si-nw-ChemFET in liquid phase, and demonstrated the performances of the bilayer SiO 2 /ALD-Al2O3 for ChemFET fabrication.

DISCUSSION

These good results are due to the excellent control of the nw-Si/insulator/electrolyte interface using an all-around gate with high dielectric capacity. Junctions on the nanowires make possible to increase the ION/IOFF ratio by decreasing strongly the current when the transistors are in their blocked mode. The reduction of the gate length make possible to increase the transconductance, it is interesting for the amplification of biochemical signals like action potentials [14]. The bilayer SiO2/Al2O3 is advantageous because the Si/SiO2 allows a very good interface, which is highly important for a MOSFET based component. The second layer, the ALD-Al2O3 is a good (high-k) insulator in liquid phase. That is why gate leakage currents are so low. Thanks to Al2O3, Si-nw-ChemFET shows near Nernstian responses to pH changes without hysteresis and with selectivity against K + and Na + ions, which means more accuracy and sensitivity. This allows to hope for a much better signal-to-noise ratio for biological applications than planar ChemFET.

CONCLUSION

Silicon nanowire based MOSFET and ChemFET (Si-nw-MOSFET and Si-nw-ChemFET) were designed, realized and presented in this paper. Electrical and electrochemical characterizations have shown good results. In short-term perspective, thanks to the small dimensions of sensing area and to improved detection performances, the accurate and reproducible monitoring of biological metabolisms is expected on living cells using suspended Sinw-ChemFET.

CONTACT

Pierre Temple-Boyer: temple@laas.fr 100 wires, LG 3.9µm

Figure 3 :

 3 Figure 3: a) Schematic view and b) SEM view of the Sinw-ChemFET with interconnections.

Figure 4 :

 4 Figure 4: a) and b) schematic views of the Si-nw-ChemFET and the microfluidic channel. c) SEM view of suspended silicon nanowires inside a covered microfluidic channel (tilt 52°).

Figure 5 :

 5 Figure 5: schematic view of the polarization for Si-nw-ChemFET characterizations.

Figure 6 :

 6 Figure 6 : distribution of the threshold voltage of the Si-nw-MOSFET.

Figure 7 :

 7 Figure 7: Id(Vgs) characterizations of Si-nw-MOSFETs, semi-logarithmic scale. (Vds = 0.1V)

Figure 8 :

 8 Figure 8: Id(Vgs) characterizations of Si-nw-MOSFETs, linear scale. (Vds = 0.1V)

Figure 9 :

 9 Figure 9 : Id(Vgs) characterizations in liquid phase of Sinw-ChemFETs, with a semi-logarithmic scale.(Vds = 1V)

Figure 10 :

 10 Figure 10: Id(Vgs) characterizations in liquid phase of Si-nw-ChemFET, with a semi-logarithmic and a linear scale, for three different pH.(Vds = 1V)

ACKNOWLEDGEMENTS

This work was partly supported by LAAS-CNRS micro and nanotechnologies platform, member of the French RENATECH network.


Matthieu Joly

Laurent Mazenq

Maurane Marlet

Pierre Temple-Boyer

Céline Durieu

Jérôme Launay

All-solid-state multimodal probe based on ISFET electrochemical microsensors for in-situ soil nutrients monitoring in agriculture

Keywords: ISFET sensor, In-situ soil analysis, Multisensor probe, Ion-selective membrane, Nutrients levels determination, Agriculture

INTRODUCTION

Feeding the world's population has huge climate and environmental impacts. New farming methods should be developed where fertilizers doses are determined over time at small scale according to soils variability. Conventional soil analysis methods are performed on soil samples by off-site laboratories which make them expensive and time consuming. Thus, our project aims to provide a low-cost, robust and real-time multimodal tool based on ISFET microsensors for in-situ monitoring of soil nutrients.

Since the 2000s, several studies showed the promising future of potentiometric sensors in precision agriculture. Most of them focus on the development of flow injection analysis (FIA) systems in which ionselective electrode (ISE) or ISFET are used for nutrients measurements in soil extracts performed on-site or onthe-go [START_REF] Kim | Soil macronutrient sensing for precision agriculture[END_REF][START_REF] Lehmann | Miniature Multisensor Probe for Soil Nutrient Monitoring[END_REF]. Futagawa et al. worked on the integration of pH, temperature and electrical conductivity (EC) microsensors on a silicon chip for hydroponic applications [START_REF] Futagawa | A Miniature Integrated Multimodal Sensor for Measuring pH, EC and Temperature for Precision Agriculture[END_REF]. Only Artigas et al. reported the fabrication of pH-ISFET, pK-ISFET, pCa-ISFET and pNO3-ISFET used directly in soil [START_REF] Artigas | Application of ion sensitive field effect transistor based sensors to soil analysis[END_REF].

Nevertheless, more studies are needed to fully assess some important parameters of ISFET's detection capability in soil: influence of soil moisture, ISFET drift and lifetimes and the in-situ detection of other important nutrients like NH4 + ammonium ions.

EXPERIMENTAL Microsensor fabrication

Our microsystem is fabricated on 4-inch silicon substrates and composed of an ISFET, a miniature solidstate reference electrode, two platinum electrodes for EC measurements and a temperature sensor in MOSFET technology (fig. 1).

N-channel, normally-off pH-ISFET microsensors were fabricated using a 50nm silicon nitride Si3N4 pHsensitive layer deposited by Low Pressure Chemical Vapor Deposition (LPCVD) on top of a 50nm thermal oxide SiO2 layer. Such ISFET was already studied in previous work, evidencing a quasi-Nernstian sensitivity around 50mV/pH [START_REF] Temple-Boyer | Study of front-side connected chemical field effect transistor for water analysis[END_REF].

A silver/silver chloride quasi-reference electrode dedicated to quick on-site analysis was made using electrodeposition methods. We are currently working on a reference membrane with a good potential stability and small chloride concentration dependence. Our results, though, will not be shown in the present paper.

However, long-term measurement (at least a few months) with fully integrated microsystems in such a complex environment as soils is still a scientific challenge. One of the main reasons comes from the fact that no solid-state reference electrode displaying potential stability equivalent to macroscale and conventional reference electrode is so far available [START_REF] Shinwari | Microfabricated Reference Electrodes and their Biosensing Applications[END_REF]. So, we also decided to assess the stability of a WE200 Silvion electrode, a macroscale reference electrode dedicated to permanent installation in soils. The detections of ammonium and nitrate ions were allowed by casting an ion-sensitive membrane on top of the pH-ISFET Si3N4 layer. It is composed of a fluoropolysiloxane (FPSX) polymer matrix purchased from Dow Corning (730 FS) and sonicated in tetrahydrofuran with ionophores and ionic additives (Table 1). The FPSX matrix was chosen for its good

A B

adhesion properties on Si3N4 surface leading to an improved sensor's lifetime [START_REF] Cazalé | Study of field effect transistors for the sodium ion detection using fluoropolysiloxanebased sensitive layers[END_REF].

Electrochemical measurements

The performances of the ISFETs were examined using specific potentiometric ISFET-meter electronic interface with drain voltage Vds and current Id fixed to 2.0 V and 0.1 mA respectively. A multi-channel potentiostat, VMP3 from BioLogic Science Instruments, was used to measure the open-circuit potential of Silvion reference electrode. A silver silver chloride double junction electrode purchased from Metrohm was used as a reference electrode in aqueous solution and in soils. Sensitivity and selectivity tests were performed by increasing the activity of the primary ions using a 0.1M ammonium nitrate NH4NO3 aqueous solution. Selectivity coefficients were obtained with the Fixed Interference Method (FIM). We made sure that ISFETs exhibited quasi-Nernstian behavior in order to fulfil the IUPAC recommendations [START_REF] Buck | Recommendations for nomenclature of Ion-Selective Electrodes[END_REF]. Measurements in soils were done by inserting the ISFET's PCB directly into the soil in a way that the sensitive part of the sensor was in close contact with the soil. All measurements were carried out at room temperature.

RESULTS AND DISCUSSION

In-situ detection of soil pH

We first studied the detection capability of pH-ISFET microsensors inserted directly into the soil in order to prove the feasibility of a direct measurement method as opposed to soil extract analysis.

Two silty-clay soils were first analyzed according to the standard method where a glass pH electrode is used in a 20vol% suspension of soil in water. It led to pH values of 4.7 and 8.4. After a three points-calibration in pH buffer solutions (pH 4, 7 and 10), quick in-situ measurements of pH-ISFET were recorded for both soils at different soil moisture levels (fig. 2). The results are in great accordance with the standard method. Indeed, the maximum error was only 0.5pH unit and there was no correlation with soil moisture. In-situ measurements were even possible at soil relative water content of 40% which is below typical wheat growths conditions. With its small measurement errors relative to an agriculture application and its robustness, pH-ISFET is fully compatible for quick on-field pH determination. In comparison, the more classical glass pH electrode used in field conditions can easily lead to measurements issues due to a clogged reference junction or a break of the glass bulb. This study aimed at assessing the pH-ISFET lifetime in soil to know if a monitoring during all wheat growth stages could be possible. Relative water content (75%) was kept constant thanks to a daily subirrigation. Both microsensors displayed a linear (R²≥ 0.87) and relatively small drift (<0.6mV/day) (fig. 3). The ageing of pH-ISFET caused by soil environment was evaluated by a comparison of their sensitivity before and after the experiment. Both ISFET kept their initial sensitivity (51mV/pH).

We operated the same study on Silvion reference electrode (fig. 3). As expected, the open-circuit voltage evolution in time showed a linear (R²=0.92) and small drift (0.11mV/day).

Thus, our pH-ISFET showed a lifetime in soil of at least six months and we identified a suitable reference electrode making the project of a long-term monitoring of soil nutrients possible.

Figure 3: Time evolution of the potential of two pH-ISFET (left y-axis) and a Silvion reference electrode (right y-axis) inserted for six months in a silty-clay soil. An Ag/AgCl glass double junction reference electrode was used during the measurements.

In-vitro detection of nitrate and ammonium ions

Analytical responses and calibration curves obtained for pNO3-and pNH4-ISFET are shown in fig. 4, 5 and6.

Both sensors exhibited a linear response to the concentration of primary ions with a quasi-Nernstian sensitivity of 59.0mV/pNH4 and 56.2mV/pNO3, respectively. The limits of detection (LOD) calculated as recommended by IUPAC from the intersection of the two extrapolated segments of the calibration curve, were 3.2µM and 17µM, respectively. Both the LOD and the linear range are in accordance with standard values of mineral nitrogen in field conditions. The main detection characteristics are summarized in Table 2 which shows that our probe should meet the requirements for a soil study application. A specific care was taken for the determination of FIM selectivity coefficient (Table 3) as direct measurements in soils could obviously not be done without the presence of interfering ions.

Regarding nitrate detection, we chose to use Nitrate Ionophore V as it displays the best selectivity coefficients reported so far for a commercially available ion exchanger [START_REF] Paczosa-Bator | Potentiometric Sensor Platform Based on a Carbon Black Modified Electrodes[END_REF]. Surprisingly, the relative responses of our pNO3-ISFET towards interfering ions indicate a strong Hofmeister behavior with a poor selectivity towards chloride. This result could be explained by the use of the FPSX matrix instead of the more conventional PVC. Indeed, fluoropolymers display lower polarity which does not facilitate the extraction of more lipophilic anions such as chloride. The use of a non-fluorinated siloxane may increase the selectivity of our sensors towards chloride while ensuring a good adhesion on the gate ISFET. In the other hand, fluorinated siloxanes are used on electrochemical sensors for antifouling strategies, an interesting property given the objective of our system [START_REF] Whelan | Antifouling strategies for marine and riverine sensors[END_REF]. Nevertheless, with exception to coastal soils in which high content in chloride is expected, our pNO3-ISFET should be selective enough for an application to soil monitoring.

Overall our pNH4-ISFET exhibited really good selectivity coefficients towards the main cations likely to be found in soil. As expected with nonactin ionophore, potassium K + interfered the strongest but our ISFET showed a slight improvement of selectivity in comparison with most of the nonactin-based membrane reported [START_REF] Umezawa | Potentiometric Selectivity Coefficients of Ion-Selective Electrodes. Part I. Inorganic Cations (Technical Report)[END_REF].

In-situ detection of ammonium ions

Finally, we tested our pNH4-ISFET in silty-clay soils of varying nitrogen content. Nitrogen mineralization is a process where the soil microbial population converts organic nitrogen to ammonium and nitrate forms. It is a temperature-dependent phenomenon. Thus, starting with two pots of the same soil, we obtained two distinct nitrogen content soil samples by putting them respectively at 38°C and 4°C for one month. Measurements performed into these soils directly after calibration allowed to discriminate the two levels of nitrogen. We successfully recorded an increase in ammonium concentration caused by the stimulation of nitrogen mineralization by soil microorganisms (fig. 7).

CONCLUSION

Our pH-ISFET succeeded in measuring acidic and basic soils, for short and long periods of time, with only an elementary packaging and in avoiding the need for a time-consuming step of soil sampling. Thus, these results validated the feasibility of a direct measurement method in soil. Then, our nitrogen-functionalized ISFET exhibited quasi-Nernstian sensitivity and detection limit in accordance with standard values in field conditions. Using the same new method, our pNH4-ISFET showed promising results in being able to differentiate two soils of different nitrogen content. Overall, this work highlights the promising future of the ISFET technology for precision agriculture. Our work will now focus on multiplying the tests of functionalized ISFET in order to assess, in particular, the lifetime of the ion-sensitive membrane in real soil conditions.

Figure 1 :

 1 Figure 1: (A) Schematic top view of the microsystem. (1) Platinum electrodes for EC measurements; (2) Miniature Ag/AgCl quasi-reference electrode; (3) ISFET microsensor; (4) MOSFET for temperature measurement. (B) View of the chip after electronic packaging on a printedcircuit board (PCB) and encapsulation.

Figure 2 :

 2 Figure 2: Comparison of pH measurements performed by pH-ISFET at different soil moisture levels in an acid and a basic silty-clay soil. The difference with the standard method of soil pH determination (green lines) is indicated above each measurement point. Furthermore, we recorded for six months the signal drift of two pH-ISFET directly inserted in pots of siltyclay soil using the simple packaging showed on fig.1.This study aimed at assessing the pH-ISFET lifetime in soil to know if a monitoring during all wheat growth stages could be possible. Relative water content (75%) was kept constant thanks to a daily subirrigation. Both microsensors displayed a linear (R²≥ 0.87) and relatively small drift (<0.6mV/day) (fig.3). The ageing of pH-ISFET caused by soil environment was evaluated by a comparison of their sensitivity before and after the experiment. Both ISFET kept their initial sensitivity (51mV/pH).We operated the same study on Silvion reference electrode (fig.3). As expected, the open-circuit voltage evolution in time showed a linear (R²=0.92) and small drift (0.11mV/day).Thus, our pH-ISFET showed a lifetime in soil of at least six months and we identified a suitable reference electrode making the project of a long-term monitoring of soil nutrients possible.

Figure 4 :

 4 Figure 4: Analytical response of the pNO3-ISFET to the increase of NH4NO3 concentration.

Figure 5 :

 5 Figure 5: Analytical response of the pNH4-ISFET to the increase of NH4NO3 concentration.

Figure 6 :

 6 Figure 6: Calibration curves of the pNO3-ISFET and pNH4-ISFET in aqueous solution of NH4NO3.

Table 3 :

 3 Selectivity coefficients (log K ij ) obtained with the Fixed Interference Method for pNH 4 -and pNO 3 -ISFET. The constant concentration of interfering ion is indicated in brackets.

Figure 7 :

 7 Figure 7: Plot vs Time of pNH4-ISFET signal. The measurements are first done in two calibration solutions, and then in high and low nitrogen content soils before being checked again in the two same calibration solutions.

Table 1 :

 1 Membrane composition of pNH4-and pNO3-ISFET (% weight).

		Matrix	Ionophore	Ionic additives
	pNH4-	FPSX	Nonactin	K-t-4ClPB
	ISFET	(95.0%)	(3.5%)	(1.5%)
	pNO3-ISFET	FPSX (97.6%)	Nitrate ionophore V (1.5%)	TDMAC (0.9%)

ACKNOWLEDGEMENTS

This work was partly supported by LAAS-CNRS micro and nanotechnologies platform, member of the French RENATECH network.


Ahmet Lale

Auriane Grappin

Emmanuel Scheid

Jérôme Launay

Pierre Temple-Boyer

Design, realization and characterization of silicon nanowire ion sensitive field effect transistors

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Design, realization and characterization of silicon nanowire ion sensitive field effect transistors

Ahmet LALE 1 , Auriane GRAPPIN 1 , Emmanuel SCHEID 1 , Jérôme LAUNAY 1 , Pierre TEMPLE-BOYER 1 1 LAAS-CNRS, Université de Toulouse, F-31031 Toulouse, France

Abstract-The aim of our research project is to achieve a potentiometric multi-sensor platform, consisting of ion sensitive field effect transistors ISFET and MOSFET, which channel is a nanowire or a network of horizontal Silicon nanowires and whose gate insulator is alumina Al2O3 deposed by Atomic Layer deposition ALD. This microdevice will provide chemical and biological analyses in the liquid phase, in microfluidic channels.

I. INTRODUCTION

Ion sensitive field effect transistors are microsensors derived by metal oxide semiconductor field effect transistors [1]. These are electronic microsensors designed to measure the pH. ISFETs have been functionalized for many applications such as the measurement of potassium [2], sodium [2], ammonium [3], urea [4], creatinine [5], lactate [6], glutamate [7] ...

Today, to open the door to new innovative applications, it is necessary to increase the sensitivity, decrease response times and detection limit of ISFET. To meet this challenge, the transistor based on silicon nanowires with surrounding gate seems to be the ideal candidate: low channel dimensions allow to work in micro-volume and thus to reduce the amount of analyte. But the biggest advantage lies in being able to work at the single living cell level. Moreover, the atomic layer deposition (ALD) now allows to achieve, for these nanoISFETs, controlled nanometric thickness gate insulators with adequate quality in terms of defects.

II. STUDY OF ALD ALUMINA

To achieve the gate insulator of our transistors, we have chosen to replace the silicon nitride Si3N4, previously used for the ISFETs in our past studies, by alumina Al2O3 which provides better sensitivity [8], which is a better insulator in liquid phase [8] and allows to be more selective against sodium and potassium than silicon nitride [8]. We chose to deposit the alumina through the ALD. The advantage of this technique is to deposit extremely compliant films and whose thicknesses are controlled close to the molecular monolayer [9]. The principle is to make successively reacting precursors, directly on the substrate's surface. A successive reaction of the precursors creates a molecular monolayer of alumina and this cycle is repeated n times according to the desired thickness. The first work was to make alumina films from 100 ALD cycles (~ 11nm) to 600 ALD cycles (~ 54nm) to study the deposition kinetic. Figure 1 shows the evolution of the average thickness (measured by ellipsometry) deposed by each ALD cycle to measure the deposition velocity. We compared the evolution of the refractive index of these deposits (figure 1) and we have highlighted a correlation between the refractive index and the deposition kinetic. Previously we obtained a 40nm deposit, with a refractive index of 1.65, a bit too low, the refractive index of the massive alumina being 1.78 without extinction coefficient [10]. To improve refractive index of our material, we did a thermal annealing campaign of 5 minutes under N2/H2 and we were significantly closer to the refractive index of the massive alumina (figure 2), on the other hand, we measured an extinction coefficient, that we explain by deficiencies in oxygen that appear in our material with the temperature rise. So, we decided to make our post-deposition annealing under O2 and we have improved the index of refraction as previously (figure 2) while keeping a null extinction coefficient. An X-ray diffraction study of ALD Al2O3 showed that alumina films are amorphous after deposition, but they crystallize when they are annealed up to 750°C. It shows that a structural modification of alumina induces the optical index improvement.

We have made MIS capacitors (metal insulator semiconductor) with the alumina as an insulator, to electrically characterize the ALD deposition. With C(V), we noticed that the flat-band voltage decreases with annealing temperature, which means that it lowers the number of charges in insulator (figure 3). The accumulation capacity increases as well as the dielectric constant from 8.5 for nonannealed layers to 10 for layers annealed at 900°C (figure 4), conform to the literature of massive alumina [11].

III. NANOISFET STRUCTURE

We used, on 6 inch SOI wafers, the projection photolithography with a resolution lower than 250nm, less than the maximum resolution of used Canon FPA-3000i4 stepper (350nm) and photoresist AZ ECI 3012, (400nm). After optimizations, we have managed to achieve on a single chip, with the same insolation parameters, transistors with a single nanowire as channel, next to transistors with 100 nanowires parallel network. The goal is to eventually compare in liquid phase, the pros and cons of each structure. We obtained reproducibly nanowires width less than 200nm. Once photoresist nanowires were optimized, we transferred them into silicon thanks to a reactive ion etching. Source and drain areas were N doped with arsenic. Then comes the realization of the gate insulator with alumina after RCA cleaning process (figure 4).

Once the transistors and interconnections ended (figures 4), we realized covered fluidic channels, using the SU8-3D technique [12], which has been optimized for 5 µm wide channels (figure 5). Firstly, concerning MOSFET characterizations, we obtained very low subthreshold current: 10 -14 A for Vg = 2V and low leakage currents through the gate (figure 6). We have then characterized nanoISFETs in liquid phase, and as for the MOS transistors, we got excellent characteristics, with extremely low leakage currents through gate insulator (10 -12 A for Vg = 2V). Subthreshold Id current is also excellent. In addition, we have a good response to the pH variations (figure 7). We studied alumina deposed by ALD and we have demonstrated that it is possible to improve its physical and dielectric characteristics using thermal annealing under oxygen. We then realized nanoMOSFETs and nanoISFETs using alumina as gate dielectric. The first pH characterizations made in liquid medium are very encouraging for the future, with the perspective of the study of single living cells.

Figure 1 :

 1 Figure 1 : evolution of refractive index and average thickness deposited by each ALD cycle based on the total number of completed ALD cycles.

Figure 2 :

 2 Figure 2 : changes in refractive index of alumina according to the postdeposition annealing temperature.

Figure 3 :

 3 Figure 3 : left, number of charges in the insulator according to the annealing temperature under O2. Right, dielectric constant depending on the annealing temperature under O2.

Figure 4 :

 4 Figure 4 : left, silicon nanowires, after gate insulator completion. right nanoMOSFET and nanoISFET with interconnections.

Figure 5 :

 5 Figure 5 : dense nanowire network in a SU8 microfluidic channel, after removing the cover through a FIB etching.

Figure 6 :

 6 Figure 6 : I(V) characterization of a nanoMOSFET, red with a linear scale and blue with a semi-logarithmic scale. (VDS = 0.1V)

Figure 7 :

 7 Figure 7 : I(V) characterization of a nanoISFET, in black pH 4, pH 7 in blue, red at pH 7 with a semi-logarithmic scale. (VDS = 0.1V)