Yoann Guilhem

Stéphanie Basseville

Henry Proudhon

Georges Cailletaud

Effects of surface roughness on plastic strain localization in polycrystalline aggregates

come

Effects of surface roughness on plastic strain localization in polycrystalline aggregates

Introduction

Fatigue crack initiation usually takes place at the component surface and is provoked by plastic strain localization along slip bands [1]. This is strongly driven by microstructural features, such as crystallographic orientation and grain boundaries. It has been observed at the macro scale that the surface state of a specimen can affect the fatigue life of a component [2]. This phenomenon has been recently investigated at the mesoscopic scale, using Finite Element computations of polycrystalline aggregates [3]. The aim of our numerical study is to quantify the impact of surface roughness on local mechanical fields, apart from other surface state parameters.

a Corresponding author: yoann.guilhem@ens-cachan.fr This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Article available at http://www.matec-conferences.org or http://dx.doi.org/10.1051/matecconf/20141206009

Numerical model

Méric-Cailletaud's crystal plasticity model is introduced as the local behaviour of each grain, considered as FCC single crystals. The material parameters have been identified on a 316L stainless steel [4]. A rough surface has been generated based on a 1D roughness profile of a brushed finished component and then applied on a 291 grains aggregate build by means of Voronoi tessellation (Fig. 1a). To investigate the effect of roughness intensity, various meshes have been produced by multiplying the roughness map by a factor k r between 0.1 and 1. A 2 cycles loading is applied of 0.2% strain in uniaxial or biaxial mode. The results will be analysed on both the rough aggregate and the flat one, used as reference case.

Results and discussion

The sum of the accumulated plastic slip over all the slip systems is used to characterize plastic flow. Figure 1b (respectively 1c) shows the plastic strain contour map at the free surface of the aggregate for the flat (respectively rough) case. This reveals strong changes in the plastic strain localization patterns. In the reference case, it initially appears as 45 • bands induced by crystallographic orientation whereas in the rough case, localization moves to the valleys of the rough surface. Grain orientation remains a major parameter because multiple zones still show strong discontinuities at grain boundaries. This observation highlights the competition between the localization due to crystallographic orientation and the one induced by the surface topology. As the roughness intensity factor decreases, these modifications are progressively vanishing, thus a critical roughness state exists from which the cause plastic strain localization switch from crystallography to roughness.

However, this effect is getting gradually weaker deep under the surface. Indeed after the first layer of grains, the difference becomes negligible. A statistical analysis based on each Gauss point values is used to determine the depth of the affected zone depending on the roughness intensity. The results provide a linear correlation between the two.

Conclusion

A crystal plasticity finite element analysis of flat and rough polycrystalline aggregates has been carried out to quantify the effect of the roughness on the local mechanical fields. A competition between crystallography and roughness in the localization process has been identified. The depth of the affected zone decreased linearly with roughness intensity. The next step is to study the influence of the grain size compared to the roughness parameters.

06009-p.2

FDMDII -JIP 2014

Figure 1 .

 1 Figure 1. (a) Rough mesh. Plastic strain map for (b) flat case and (c) rough case. (d) Roughness map.


L Pichon

A-C Salaün

G Wenga

R Rogel

E Jacques

Ammonia sensors based on suspended silicon nanowires

Keywords: suspended silicon nanowires, CMOS compatible, ammonia sensor, sensitivity

Resistors based on parallel suspended polycrystalline silicon nanowires are fabricated using the classical top down CMOS silicon technologies. Results show potential use as sensitive units of silicon nanowires for charged chemical species (ammonia) detection. Resistors are promising as low-cost manufacturing gas sensors, for very high sensitive ammonia detection over a large concentration range.

Introduction

Owing to their physical and electrical properties, semiconducting nanowires are the subject of intense research activities. In particular, silicon nanowires (SiNWs) are fully compatible with the high reliable well known CMOS (Complementary Metal Oxide Semiconductor) silicon technology. In addition, thanks to their high surface to volume ratio, SiNWs are good candidates for fabrication of high sensitive chemical sensors [START_REF] Gao | Silicon Nanowire Arrays for Label-Free Detection of DNA Anal[END_REF][START_REF] Cui | Nanowire nanosensors for highly sensitive and selective detection of biological Q and chemical species[END_REF]. SiNWs can be prepared by one of two approaches, "top-down" or "bottom up". In a bottom up strategy the individual base elements (atoms, molecules…) of the system are linked together to form larger subsystems. The main drawbacks of these synthesis methods for a 3D integration are the difficulty in control of size and positioning of the nanowires. In this case, nanowires need to be selectively collected and manipulated to be assembled in a planar layout. The "top down" approach starts from bulk materials and scales down the patterned areas. In this way, several advanced lithographic tools with nanometer size resolution rest on the high cost generated, and more generally the low throughput capability is unsuitable with mass production. In this work polycrystalline silicon (polysilicon) nanowires based resistors used as gas (ammonia) sensors are fabricated. Nanowires are synthesized following the top down approach, using a classical fabrication method commonly used in microelectronic industry: the sidewall spacer formation technique. Assets of this technological process rest on low cost lithographic tools, classical silicon planar technology compatibility, and the possibility to get numerous parallel nanowires with precise location on the substrate.

Devices fabrication

Polysilicon nanowires fabrication is illustrated in the figure 1. A (100) oriented silicon substrate is first capped with 1.5μm thick SiO 2 layer deposited by APCVD (Atmospheric Pressure Chemical Deposition) technique at 420°C, patterned and dry-etched to realize a 100nm thick step. Then, a polysilicon layer is deposited by CVD technique at low pressure (LPCVD) at 550°C and crystallized at 600°C. Dry etching control of the polysilicon layer allows the nanowires formation made of residual polysilicon (spacer) in the step side. Two types of resistors (fig. 2) based on grounded or suspended SiNWs are studied as gas (ammonia) sensors. Resistors process fabrication is compatible with a mass production planar layout. In this case, a polysilicon layer with two differently doped stack regions is deposited on the SiO 2 step: the lower part is undoped and the upper part is highly in-situ doped with phosphorus. A patterning by plasma etching of this polysilicon layer is achieved to both create spacers (undoped polysilicon nanowires) and electrodes (highly doped polysilicon) of the resistors. A wet etching of the SiO 2 layer allows the suspended SiNWs formation anchored between two highly doped Si electrodes (fig. 2. b). Figure 3 shows the SEM image of 100nm radius curvature suspended polysilicon NWs based resistor. Electrical characteristics of these devices were previously reported [START_REF] Demami | Fabrication of silicon nanowires using conventional UV lithography[END_REF]. Because SiNWs act as sensitive units, suspended SiNWs based device offers larger sensing area.

Results

Methodology

Gas (ammonia) detection is carried out measuring the current whereas the voltage of the DC source is fixed at 1V. The deduced R-values are then reported as function of time for devices under exposure to ammonia. Prior to measurements, a highly diluted hydrofluoric acid (2 %) aqueous solution based wet etching is used to remove the native oxide on the SiNWs surface in order to promote chemical species adsorption. Then, SiNWs based devices are put into a vacuum chamber. Electrical measurements are carried out at room temperature and the pressure is monitored at 500 mbar regardless of the gas (ammonia) flow injection. Before beginning the sensing measurement, a degassing process is carried out at 200°C for one hour, which permits to eliminate the impurities adsorbed at the nanowires surface that will influence the conductivity. The sample is then cooled down to room temperature. At first, a flow of nitrogen gas is injected continuously into the chamber during few minutes to guarantee current baseline levelled off. Then, a flow of ammonia is injected with the chosen dilution of NH 3 /N 2 during a few minutes. The gas injection is cut off and the gas mixture of NH 3 /N 2 is evacuated by pumping afterwards. During the whole period of measurement, gas mixture is exchanged all the time whereas the concentration of the gases is kept constant. The potential use of the SiNWs based resistors as ammonia sensors was checked by measuring the detection response, S g , defined as:

g g g R R R S - = (%)
where R and R g are the resistance values for devices under nitrogen and reactive ambience, respectively.

Ammonia detection

Previous results [START_REF] Ni | Pichon Au-catalyst silicon nanowires synthesized by vapour-liquid-solid technique in a V-shaped groove : Application for gas sensors[END_REF][START_REF] Rogel | Polycristalline silicon nanowires synthesis compatible with CMOS technology for integrated gas sensing applications[END_REF] showed that for dynamic measurements, that consist in exposing SiNWs based resistors under various ratio of ammonia (from 2 ppm to 700 ppm) at room temperature, resistance decreases (current increases) under exposure. The mechanism of such sensors is explained by two main theories, charge exchanging effect and chemical gating effect. This effect is explained by charges exchange between the adsorbed molecules and the SiNWs. It means that the NH 3 molecules adsorbed on the surface of the SiNWs could transfer charges following the equation: NH 3 → NH 3

+ + e -due to the reducing effect (electron donor) of ammonia. This phenomenon could directly inject electron carriers into the SiNWs, thus decreasing the resistance [START_REF] Talin | Large area, dense silicon nanowire array chemical sensors[END_REF]7]. Moreover, as SiNWs conductance can be modulated by an applied voltage, positively charged gas molecules (electron donors) bound on SiNWs surface can modulate their conductance by changing the volume of the conductive layer. In this case, the adsorbed gas molecules, NH 3 + may act as chemical gates which shift the Fermi level of the undoped SiNWs in the upper part of the band gap and reduces the resistance of the VLS SiNWs based device. For each NH 3 /N 2 gas mixture, maximum values of S g are calculated and reported in figure 4. Results show that S g increases as the NH 3 concentration increases following a linear behavior over a wide range of gas (ammonia) concentration. In this case, gas sensor relative sensitivities, defined as S= S g / NH 3 are estimated and reported in to table 1. Results shows that S is higher for suspended based SiNWs based devices because of a higher surface exchange.

Conclusion

In conclusion, we present the process fabrication of two different configurations of resistors based on SiNWs using classical silicon planar technology. We demonstrate that their performances under exposure show a linear response over a wide range of ammonia concentration. The study highlights that the relative sensitivity is higher for suspended SiNWs based sensors due to a higher exchange surface.These results show potential use of these SiNWs based devices for charged chemical species (ammonia) detection in a fully compatible silicon CMOS technology.

Fig. 1 :

 1 Fig. 1: Schematic view of the fabrication of the (sidewall spacer ) polycrystalline silicon nanowires

Fig. 2 :Fig. 3 :

 23 Fig. 2: Schematic view of grounded (a) and suspended (b) sidewall spacer polycrystalline SiNWs based resistors

Fig. 4 :

 4 Fig. 4: Relative response of de sensors vs the ammonia concentration for suspended, grounded SiNWs resistors

  Lett, 89 (2006) 153102.[7] Yu J.Y., Chung S.W., Heath J.R., Silicon nanowires: preparation, device fabrication, and transport properties, Journal of Physical Chemistry B, 104 (2000) 11864.

Table 1 :

 1 Values of the relative sensitivity to ammonia for grounded and suspended SiNWs based resistors

		Grounded SiNWs based sensors	Suspended SiNWs based sensors
	Relative sensitivity (%/ppm)	4	15.1

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014


Motivation and results

Thanks to their high surface to volume ratio, silicon nanowires (SiNWs) are good candidates as sensitive units for fabrication of high sensitive chemical sensors. SiNWs can be prepared by one of two approaches, "top down" or "bottom up". In a bottom up strategy the individual base elements (atoms, molecules…) of the system are linked together to form larger subsystems, without using high cost lithographic tools. The main drawbacks of these synthesis methods are the difficulty in control of size and positioning of the nanowires. In this case, nanowires need to be selectively collected and manipulated to be assembled in a planar layout. The "top down" approach starts from bulk materials and scales down the patterned areas. In this way, several advanced high cost lithographic tools with nanometer size resolution are needed, and the low throughput capability is unsuitable with mass production. In this work, polycrystalline silicon nanowires are synthesized following the top down approach, using a classical fabrication method commonly used in microelectronic industry: the sidewall spacer formation technique [START_REF] Demami | Fabrication of polycrystalline silicon nanowires using conventional UV lithography[END_REF]. Assets of this technological process rest on low cost lithographic tools, classical silicon planar technology compatibility, and the possibility to get numerous parallel nanowires with precise location on the substrate. Because SiNWs act as sensitive units, specific designs are developed to allow large sensing areas. Two types of resistors (fig. 1 and 2) based on grounded or suspended SiNWs are studied as gas (ammonia) sensors. Resistors process fabrication is compatible with a mass production planar layout. Ammonia molecules adsorbed on the surface of undoped SiNWs act as electron donors (reducing agents) resulting to an increase of the current into the nanowires. The ammonia detection is checked by measuring the relative response of the sensors defined by: S g =(I-I g )/I

where I and I g are the current values for devices in vacuum and reactive ambience respectively. First measurements presented in fig. 3 are carried out at room temperature, in a vacuum chamber, under controlled ammonia/nitrogen (NH 3 :N 2 ) mixture varying from 2 ppm to 700 ppm. For each type of sensors, response to ammonia detection follows a linear behaviour (fig. 3). Results highlight that S g is higher for suspended polycrystalline SiNWs based devices, related to a high surface to volume ratio of the sensitive units. In addition, relative sensitivity, ∆S g /∆[NH 3 ], of such sensors is linearly dependent on the surface to volume ratio of the sensitive units (fig. 4). These results show potential use of these SiNWs based sensors for charged chemical species (ammonia) detection in a fully compatible silicon CMOS technology. The effects of the shape of sensitive silicon nano-objects (nanowires, nanoribbons…), silicon crystal quality, and the low detection limit of the sensors will also be discussed.

Fig. 1 :Fig. 3 :Fig. 4 :

 134 Fig. 1: Schematic view of grounded (a) and suspended (b) sidewall spacer polycrystalline SiNWs based resistors

  L. Pichon, A. C. Salaun, G. Wenga, R. Rogel, E. Jacques Département Microélectronique et Microcapteurs, Institut d'Electronique et de Télécommunications de Rennes (DM2-IETR, UMR 6164), Rennes, FRANCE Resistors based on parallel suspended polycrystalline silicon nanowires are fabricated using the classical top down CMOS silicon technologies. Results show potential use of silicon nanowires for charged chemical species (ammonia) detection. Resistors are promising as low-cost manufacturing gas sensors, for very high sensitive ammonia detection over a large concentration range.

	Summary (maximum 150 words)

Al electrode Suspended Sidewall spacer SiNW


Laurent Pichon

Anne-Claire Salaün

Régis Rogel

Gertrude Wenga

Emmanuel Jacques

Brice Le Borgne

Planar

Planar silicon CMOS technology silicon nanowires based sensors

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

Planar silicon CMOS technology silicon nanowires based sensors

  F. Demami, L. Pichon, R. Rogel., A. C. Salaun, Materials Science and Engineering 2009, 6, 012014.

  Laurent PICHON, Anne Claire SALAÜN, Régis ROGEL, Gertrude WENGA, Emmanuel JACQUES, Brice Le Borgne

	Institut d'Electronique et de Télécommunications de Rennes, Département Microélectronique et
	Microcapteurs, Université de Rennes 1, 263 avenue du général Leclerc, 35042 Rennes, France
	Auteur contact : lpichon@univ-rennes1.fr
	Mots clés : silicon nanowires, sensors, CMOS, silicon planar technology
	Owing to their physical and electrical properties, silicon nanowires (SiNWs) are currently attracting
	much attention as promising components for future nanoelectronic devices such as nanowire field

effect transistors, photovoltaic devices and bio-chemical sensors. The need of a fast and precise detection of early disease symptoms, as well as the need of environment safety, becomes now the main leitmotivs of the societal development. The incorporation of semiconductor nanowires into the chemical and biological sensor application receives a great interest. Thanks to their high surface to volume ratio, SiNWs are the subject of intense research activities because of their potential applications in sensing biological and chemical agents in solutions and gas environment. Resistors and Field Effect Transistors based on polycrystalline SiNWs are fabricated using the classical top down CMOS silicon planar technologies. Nanowires are fabricated following the sidewall spacer formation technique

[1]

. Results show potential use as sensitive units of silicon nanowires for charged chemical species (ammonia, pH) detection. Devices are promising as low-cost manufacturing very high sensitive sensors.