WO2011058502A2

WO2011058502A2 - Gas sensing device having a photonic structure operating by means of bloch surface waves and corresponding manufacturing process

Info

Publication number: WO2011058502A2
Application number: PCT/IB2010/055089
Authority: WO
Inventors: Francesco Geobaldo; Emiliano Descrovi; Fabrizio Giorgis; Francesco Michelotti
Original assignee: Politecnico Di Torino; Universita' Degli Studi Di Roma "La Sapienza"
Priority date: 2009-11-11
Filing date: 2010-11-09
Publication date: 2011-05-19
Also published as: WO2011058502A3; IT1396777B1; ITTO20090866A1

Abstract

A gas-sensing device having a photonic structure, which comprises a multilayer structure (39) made of porous material comprising a periodic multilayer (22; 32) configured for generating Bloch surface waves, and means (23; 33) for directing a light beam (25; 36) incident upon said device (30) towards said multilayer structure (22; 32) in attenuated-total-reflection (ATR) conditions in order to generate said Bloch surface waves, said periodic multilayer (22; 32) comprising a first sensitive face (32e) that is free so as to be exposed to the gas, and a second face (32f) opposite to said first sensitive face (32e). According to the invention, said second face (32f) of said periodic multilayer (32) is set facing said means (33) for directing the incident light beam (36) in ATR conditions, and said multilayer structure (32) made of porous material is fixed to said means (33) for directing the incident light beam (36) in ATR conditions via a bonding layer (34) and a transparent substrate layer (31).

Description

"Gas sensing device having a photonic structure operating by means of Bloch surface waves and

corresponding manufacturing process" * * *

TEXT OF THE DESCRIPTION

The present invention relates to a gas-sensing device having a photonic structure of the type that comprises a multilayer made of porous material including a periodic multilayer configured for generating Bloch surface waves, means for directing an incident light beam onto said device towards said multilayer structure in attenuated-total-reflection (ATR) conditions in order to generate said Bloch surface waves, said periodic multilayer comprising a first sensitive face that is free so as to be exposed to the gas and a second face opposite to said first sensitive face.

In the framework of the present description, for reasons of simplicity reference will be made to the detection of gases, there being, however, included in the definition vapours and other aeriforms of organic and inorganic chemical species.

Known to the state of the art are various techniques for detecting gases or vapours using solid- state sensors. In particular, techniques of an optical type are known that exploit the variation of optical properties of optical multilayers, i.e., stacks of alternating thin-film layers with different properties, coupled for example to Bragg mirrors or micro-cavities.

A recently developed technique envisages exploitation of the generation in said optical multilayers of Bloch surface waves. Alongside the surface waves that propagate on metallic-dielectric interfaces (surface plasmons), Bloch surface waves or electromagnetic surface waves are electromagnetic waves that propagate, in a non-radiative manner, at the interface between a homogeneous dielectric material and a periodic structure known as photonic crystal. Excitation of said electromagnetic waves requires use of a prism or a grating in order to couple the incident light to the surface mode and facilitate resonant coupling between the light and the electromagnetic surface modes. The surface waves most studied are those that exist on the surface of metals, but it is possible to operate also with structures that exhibit a photonic bandgap, i.e., periodic structures that can be designed for exhibiting optical properties, such as an efficient inhibition of the radiative propagation in given frequency bands. Said photonic-bandgap structures usually comprise two components with different dielectric constants that are alternated periodically, for example in a multilayer structure, in order to determine coherent effects of scattering and interference that determine the appearance of a photonic bandgap in the composite material, as well as a negative effective dielectric constant that enables electromagnetic surface waves to be sustained at frequencies falling within the transmission bandgap.

Structures with optical gap that exploit the aforesaid principles for detecting chemical species are known, for example, from the U.S. patent application No. US 2009/0010589. Described in said document are multilayer structures for the creation of optical gaps, obtained via multilayers of homogeneous materials. The sensor devices thus obtained operate by coupling the light to the multilayer via a prism or a grating.

More recently, there has been proposed the use of multilayers made of porous material. The publication by E. Guillermain, V. Lysenko, R. Orobtchouk, T. Benyattou, S. Roux, A. Pillonnet, and P. Perriat, Appl . Phys. Lett., 2007, 90, 241116, describes optical biosensors that exploit the electromagnetic surface waves, or Bloch surface waves, which are_. generated at the truncated end of a porous-silicon one-dimension photonic crystal (ID-PC).

Illustrated schematically in Figure 1 is a multilayer device made of porous silicon according to said technology. Said device, designated as a whole by the reference number 20, comprises a substrate 21 of crystalline silicon with p+ doping, grown by electrochemical etching on which is a multilayer with optical gap 22, or photonic crystal, comprising layers of porous silicon with high refractive index 22H alternating with layers of porous silicon with low refractive index 22L. The crystalline substrate 21 is, for example, 300 μπι thick. The multilayer 22 comprises a sensitive face 22e that is free, i.e., exposed to the air or to the gas to be detected, which is surmounted, in the optical configuration referred to as Otto configuration, by a prism 23 that is separated therefrom via an air gap 24. Said air gap 24 remains in the case where the prism 23 is mechanically constrained, for example by means of clamps, to the multilayer 22.

The wide effective surface, due to the porosity, of the top layer, i.e., the sensitive face 22e of the multilayer 22 in contact with the air gap 24, associated to the extremely restricted regions of resonance of the Bloch surface waves, leads to obtaining a high sensitivity to gaseous chemical species, such as for example ethanol vapours. Said Bloch surface waves are excited by the incident light in an excitation beam 25 at an angle of incidence Θ, with respect to the perpendicular to the plane of the multilayer 22, such as to excite the Bloch waves via the ATR phenomenon. In the framework of the ATR phenomenon, the exponential tail of the excitation beam 25, totally reflected at the base of the prism 23, couples in a resonant way with the Bloch waves. The use of the Otto configuration is determined by the fact that the substrate 21 of silicon with p+ doping presents a marked absorption in the visible and near infrared and does not enable propagation of light. Hence, the surface of the multilayer 22 is set in the proximity of the surface of the prism 24.

This presents at least two drawbacks. In the first place, the efficiency of coupling with the Bloch waves depends to a critical extent upon the width of the air gap between the prism and the multilayer made of porous silicon and must be optimized by controlling accurately the mechanical force applied to the prism. In second place, the prism tends to shield the surface of the multilayer so that the temporal response of the sensor is affected significantly (detection times of around 10 minutes) in so far as the vapours must flow through the air gap before penetrating into the pores of the multilayer .

The object of the present invention is to provide a gas-sensing device having a photonic structure that will overcome the drawbacks of the known art, enabling in particular provision of a gas sensor having a photonic structure with a faster response.

According to the present invention, said object is achieved thanks to a gas-sensing device having a photonic structure presenting the characteristics recalled specifically in the ensuing claims. The invention also regards a corresponding manufacturing process, and a method for detecting gases.

The gas-sensing device having a photonic structure according to the invention enables faster penetration of the gas or vapour to be detected into the porous structure, thus obtaining a faster sensor.

Further characteristics and advantages of the invention will emerge from the ensuing description with reference to the annexed drawings, which are provided purely by way of non-limiting example and in which:

- Figure 1 is a schematic cross-sectional view of a device according to the known art;

- Figure 2 is a schematic cross-sectional view of a device according to the invention;

Figure 3 is a top plan view of a detection system, which uses the device according to the invention ;

- Figures 4a and 4b show diagrams illustrating quantities detected by said sensor device according to the invention;

Figure 5 shows a further diagram illustrating quantities detected by said sensor device according to the invention; and

Figure 6 is a table containing parameters representing operation of the sensor device according to the invention.

In brief, the device according to the invention is a device for detecting gases or vapours that has a photonic structure, i.e., a layered structure, or multilayer structure, of nanometric dimensions, which determines an optical gap (said structure being also referred to as photonic crystal). In particular, said photonic structure comprises a periodic multilayer. It is envisaged to send an excitation light beam, preferably a laser light beam, on a surface of said photonic structure, using means for directing or conveying said incident light beam onto said photonic structure in conditions suited to giving rise to an ATR phenomenon to excite Bloch surface waves in the photonic structure. The photonic structure is made of porous material, in particular via a multilayer of porous material. The porous material is preferably porous silicon (p-Si), the refractive index of which is modulated along the multilayer by modulating the porosity thereof. The surface of the photonic structure made of porous material upon which the light beam impinges is associated via a bonding layer, in conditions of optical continuity, to a first surface of a transparent substrate, whilst associated to a second surface of said transparent substrate are the means for directing the incident light in ATR conditions. A device of this sort obtains responses, i.e., shifts or deformations of the resonance peak, in particular a shift of said peak towards longer wavelengths, with very short rise and fall times.

Illustrated schematically in Figure 2 is a device 30 according to the invention, which comprises a multilayer with optical gap 32, comprising a periodic arrangement of layers of porous silicon 32H with high refractive index d_h alternating with layers of porous silicon 32L with low refractive index di . Said multilayer 32 comprises a number N of pairs of said layers 32H and 32L. The two outer end layers between the layers of porous silicon 32H with high refractive index d_h alternating with layers of porous silicon 32L with low refractive index di of said multilayer 32 identify a first sensitive face 32e, i.e., the layer that is exposed to the air or to the gas to be detected, and a second face 32f, on the side of the multilayer 32 opposite to the sensitive face 32, associated to which is a buffer layer 38, which is made of porous silicon and is thicker, having a thickness of 20-30 μιτι, which has the purpose of rendering the multilayer more robust in regard to mechanical stresses and forms, together with the periodic multilayer 32, a membrane 39. Said membrane 39, which is independent of and detached from the crystalline-silicon substrate from which it originated, as described more fully in what follows, represents in turn a multilayer structure that is made up of a periodic portion, the multilayer 32, and a further thicker porous layer with functions of support, the buffer layer 38.

The multilayer 32 rests on a first surface 31a of a transparent substrate 31, which functions as substrate layer, preferably constrained to which, via a bonding layer 37, in particular a layer of polymer (elastomer or resin) , is the buffer layer 38 so as to leave the sensitive face 32e on the side opposite to the substrate 31 free, i.e., exposed to the air or to the gas. In the context of the present description, by "transparent substrate" is meant a substrate that will enable a fraction of light, sufficient to be detected by the photodetector used, to reach the photonic structure and consequently excite the Bloch surface wave. The transmittance, in the range of wavelengths associated to an incident light beam 36, for example 1430-1590 nm, and falling within which are the resonance wavelengths, is preferably higher than 10%. The working principle of the device is not affected by the transparency of the substrate, it being possible for the transmittance in principle to tend also towards much lower values, whereas the performance on the effective use of the device is. Hence preferable are thin and high-transparency substrates. Resting on the opposite face 31b of the transparent substrate 31 is a prism 33. Set between the substrate 31 and the prism 33 is a layer of oil 34 with a refractive index compatible with, in particular not lower than, that of the prism 33 and of the transparent substrate 31, to enable optical matching and obtain optical continuity, thus preventing total internal reflection, due for example to an air gap. It is also possible to provide a polymeric bond between the transparent substrate and the prism. In this case, it is important to ensure optical continuity. Alternatively, the membrane can be bonded directly on the prism. The function of the bonding layer is not only to hold together parts of the device according to the invention, but also to keep the membrane 39 and the substrate 32 lying flat. In fact, such thin membranes tend to curl and roll up if they are not bonded.

The membrane 39 comprising the multilayer 32, associated to the substrate 31, is mounted on the prism 33, which is a coupling prism made of material BK7 with refractive index of the prism n_p of 1.501, in a configuration such as to obtain ATR phenomenon in the so-called Kretschmann configuration, which is represented in Figure 2 and will be described in what follows. In said configuration, the second face 32f of the multilayer 32 faces the prism 33. As has been said, the face of the prism 33 contacts the surface 31a of the transparent substrate 3 through a layer of oil 34 with refractive index of the oil n_Q equal, for example, to 1.66. The transparency of the transparent glass substrate 31, the limited thickness of the buffer layer 38 and its porosity determine losses due to low absorption so that the incident light beam 36 can propagate as far as the multilayer 32 and excite efficiently Bloch surface waves at the surface constituted by the interface of truncation of the multilayer with the air.

It should be noted that Figure 2 (as likewise Figure 1) represents the device 30 in an upside-down position with respect to the normal position in use, which envisages that the prism 33 is set at the bottom and the multilayer 32 is set on top of the prism 33 and of the substrate 31.

Illustrated in top plan view in Figure 3 is a detection system, which comprises the sensor device according to the invention. The laser light beam with a TE polarization 36 is emitted by a tunable laser diode 57 associated to an optical fibre 58, comprising a fibre collimator 58a, which conveys the laser beam 36 through a polarizer 59 and is used for illuminating the multilayer 32 through the prism 33. Designated by Θ is the angle between the normal to the base of the prism 33 and the direction of the laser beam 36 incident upon the system formed by the substrate 31 and by the multilayer 32 underneath it. A lens 41 focuses the light reflected by the system made up of the substrate 31 and the multilayer 32 on a photodiode 60. The system comprises a motor-driven rotation stage 42, which enables profiles of reflectance to be obtained operating at a fixed wavelength and rotating, via said motor-driven rotation stage 42, the sensor 30 and the photodiode 60 with respect to the incident beam 36. A processor 45 is in communication with the laser diode 57 to control operation and the wavelength of emission thereof, as well as with a controller 44 for controlling the motor-driven rotation stage 42. Said processor 45 also acquires the signal of the photodiode 60 for processing the reflectance values. The multilayer 30 is surmounted by a flow cell 43, for example with a volume V₀ of 1.4 cm³, used for detection of vapours and set in contact in a air-tight way with the surface by means of an O-ring.

Figure 3, which is a top plan view, in effect illustrates a measuring configuration that envisages resting of the device 30 on the side wall, for convenience in the use of the measuring set-up. Of course, the orientation of the device 30 in space does not have any effect on the measurement.

Appearing in Figure 4a is a diagram representing a map of reflectance R(Q,X) , measured as a function of the angle of incidence Θ varied through the rotation stage 42 and of the wavelength λ in air of the laser 57, for a sensor 30 having a multilayer 32 with a number N of periods, i.e., alternating pairs of layers 32H and 32L, equal to 12. The reflectance is measured in the window Be [32°, 67°] and λε[1.45 μπι, 1.59 μπι] . The dark regions correspond to low reflectance values. A vertical line of transition LL at θ~42° indicates the angle of total internal reflection. The dispersion due to the Bloch surface waves appears as a thin line (designated by BS ) for values greater than the angle of total internal reflection. Also highlighted in Figure 4a is an area GM linked to the propagation of a guided mode in the bulk of the multilayer 32.

Figure 4a also contains an insert plot 4a' , which represents the reflectance Κ(λ) at a fixed angle Θ = 58.37°, at which the excitation of surface waves appears as a resonance with Lorentzian form. It may be noted that at the minimum the reflectance Κ(λ) decreases by 46% and a width of the resonance Δλρ¾ΗΜ is 7.3 nm. Said value of width of the resonance

is larger than what is noted for a multilayer in Otto configuration, like that of Figure 1, with N = 25 ( Δλ ΜΗ = 5-1 nm) . This is due mainly to the fact that in this case the coupling, which in a Kretschmann configuration is controlled by the number N of periods, is weaker and determines broadening in the ATR.

Illustrated in Figure 4b are measurements of time- resolved spectral reflectance Κ(θ₀,λ), which are obtained in the wavelength window e[1.41 μπι, 1.59 μπι] and regard a multilayer with a greater number N of periods, in particular N = 25, in order to restrict the line width, thus increasing the sensitivity to the detection of vapours. Also in this case the darker areas indicate the areas with lower reflectance, i.e., the areas of the resonance peak.

The diagram of Figure 4b shows the response of the sensor 30 following upon exposure to saturated vapours of different alcohols in equilibrium at room temperature T_amb = 25°C and pressure vapour p₀. The first column of the table of Figure 6 indicates the vapours of the three different chemical species used, i.e., vapours of methanol, ethanol, and 2-propanol, whilst the other columns of the table of Figure 6, as detailed more fully in what follows, give the values of different quantities measured for each of said chemical species .

The sensor 30 operates at a fixed angle θο = 50.66°. Said value of fixed angle θο is such as to enable, in the absence of vapours, observation of Bloch surface waves at an original wavelength λ₀ of 1480 nm. The reflectance as a function of the wavelength is acquired at each time interval At in a spectral range including the original wavelength λ₀. The values of time interval At in which the measurement is made can be varied, as illustrated in the table of Figure 6. The insert 4b' of Figure 4b represents the spectrum of reflectance around the original wavelength λ₀ at the initial time. On account of the large number of periods (N = 25) of the multilayer, the resonance is only 14% deep, but the width of the resonance peak is Δλρ_ΜΗΜ = 4.9 nm, indicating a higher sensitivity as compared to the case illustrated in Figure 4a (N = 12) .

The diagram in Figure 4b shows a situation in which at the start the flow cell 43 is full of air. At a given time, indicated by the dashed line GI, the cell 43 is filled with vapour and its inlet 43a and outlet 43b are consequently closed, thus determining non- stationary conditions of measurement. There may hence be noted a marked shift towards longer wavelengths of the resonance peak, followed by a slow decay towards the original wavelength λο . When the vapour is completely evacuated (indicated in Figure 4b by the dashed line GO) and the environmental air fills the flow cell 43 again, there may be noted a sharp shift towards the original position of the wavelength λ₀ corresponding to the resonance of the Bloch surface wave in the absence of gas. The same behaviour may be noted after any number of measuring cycles, this demonstrating the complete reversibility of the response of the sensor 30.

Illustrated in Figure 5 is a plot that represents the dynamic behaviour of the wavelength of excitation of the Bloch waves _BSWR as a function of time for methanol (circles), ethanol (squares), and 2-propanol (triangles), deriving from measurements similar to those described with reference to Figure 4b. In Figure 5, there may be noted a very wide and sharp initial shift of the excitation wavelength _BSWR, which occurs on a time scale shorter than the time interval At, indicating that the detection is affected by the capillary condensation of the vapours at the surface of the multilayer 32 in very short times starting from exposure to the vapours. The response time hence appears much shorter than the response times observed in porous-silicon optical microcavities , where the sensitive layer is buried within the multilayer.

Figures 4b and 5 hence show slow decay of the shift, indicating onset of a second phenomenon, after rapid condensation and setting-up of an equilibrium between the liquid in the structure made of porous silicon and the vapour in the cell 43 at a pressure p<Po- Said phenomenon is identified as a possible diffusion of the liquid condensed in the porous structure, starting from the initial conditions, in which the liquid is located only in the surface layer of the multilayer 32. In fact, since the detection mechanism is based upon a transduction via Bloch surface waves, when the liquid leaves the surface to penetrate deeply in the multilayer 32, the perturbation of the Bloch waves due to the liquid decreases, and hence the shift decreases. The phenomenon is noted with measurement carried out in non-stationary conditions and pre-set amounts of alcohol in the flow cell. The temporal decay of Figure 5 can be interpolated by a double-exponential curve with a short time constant τ₃ and a long time constant x_L the values of which appear in the table of Figure 6.

Even assuming that all the vapour in the flow cell condenses uniformly in the multilayer in the proximity of the surface exposed to the vapours, it is estimated that the liquid can occupy a volume in the sensor 30 up to a maximum thickness h_MAX, which can be estimated on the basis of the vapour pressure po, the density p of each chemical species, and a weighted-average porosity P_AV = (PHd_H+ P_LCIL) / ( d_H+d_L) , which in the case of the example is 54%. Appearing in the table of Figure 6 are the corresponding values of p₀, p, mass (in terms of molar content) in the vapour phase within the flow cell before condensation m_v (n_v) , and the maximum thicknesses h_MAX for each of the three chemical species of the example. In all the cases the maximum thickness h_MAX is smaller than the overall thickness L, so that a diffusion of the liquid in the initially empty regions of the multilayer 32 is to be expected.

Table of Figure 6 also gives a value of maximum shift of the resonance wavelength Δλ_ΜΑΧ;ΤΗ calculated for a multilayer 32 filled with each of the three chemical species of the example up to the maximum thickness h_MAX, as well as values of measured shift of the resonance wavelength Δλ_{ΜΑΧίΕΧΡ}, which are much smaller than said maximum shift Δλ_ΜΑΧ,_ΤΗ and indicate the fact that probably the liquid does not fill completely the pores of the multilayer 32, but reaches a filling thickness ho- Designated by k in the table of Figure 6 are average diffusion coefficients of the liquid in the porous structure. Said diffusion coefficients k can be calculated on the basis of the time constants, in particular of the long time constant T_l. Figure 5 comprises in this regard an insert plot 5' , which represents the dependence of the concentration c(xo,t) calculated analytically at three fixed depths Xo : xo = 0, xo = h₀, and Xo = 2h₀; in particular the diagram represents the quantity c(x₀,t)/co, where c₀ is the concentration for a homogeneous infiltration as far as the critical thickness h₀, to represent the decay in logarithmic scale and highlight that after a certain time the decay becomes a single-exponential decay, irrespective of the values of filling thickness ho and of the corresponding concentration c₀. The time constant of the decay is referred to the lowest-order harmo

where τ = kn²/L², which substantially corresponds to the long time constant r_L, from which the coefficient of diffusion k of the corresponding chemical species can be derived. There is then the indication that the diffusion coefficient k is proportional to the molar volume Vm of the chemical species. Hence in practice it is possible, from the analysis of the response of the sensor 30 according to the invention to identify an unknown chemical species, estimating the molar volume Vm thereof on the basis of the coefficient of diffusion k measured.

There now follows a description of the process for producing the device 30 and in particular the membrane 39 comprising the multilayer 32.

The multilayer 32 is prepared starting from a single polished wafer of crystalline silicon with orientation (100) with p+ boron doping (resistivity <7mQcm) . It is envisaged to carry out a step of calibrated electrochemical etching in

35%HF/35%H₂O/30%EtOH solution. In said context, the silicon wafer is preliminarily subjected to a step of depassivation, via treatment in air at 300°C for 2 h to depassivate from the hydrogen the boron atoms underlying the surface, which can create a parasitic surface film, worsening the optical quality of the porous silicon. A step of rinsing in HF is then performed, using a 20%HF/20%H₂O/60%EtOH solution for 5 minutes before electrochemical etching in order to remove the oxide layer formed during the previous depassivation step; the wafer is then rinsed again a number of times with ethanol and dried via a flow of argon gas.

The process of electrochemical etching comprises operating in a Teflon cell with a platinum electrode, at the etching temperature of -25°C so as to present smoother interfaces between the layers and a better control of the refractive index and of the thickness of the individual layers. The multilayer 32 is obtained by applying a pseudo-periodic waveform that ranges between 12.7 and 19 mA cm^" , for a number N of cycles (22 s per cycle) . An etch stop is applied, for example for 10 seconds, to ensure the best homogeneity of the layers. The multilayer 32 comprises N periods of layers with high refractive index (subscript H) and low refractive index (subscript L) characterized by the following thicknesses, d_H and d_L respectively, and porosities, p_H and p_L, respectively: d_H = 215 nm, p_H = 49% and d_L = 240 nm, p_L = 58%. At a wavelength λ = 1530 nm the refractive indices are, respectively, n_H = 2.15 and n_L = 1.89. The number N of periods considered herein ranges between 10 and 25, but also values outside these intervals are possible. The first layer, corresponding to the sensitive face 32e of the multilayer 32 has a low refractive index and high porosity, for sustaining Bloch surface waves with TE polarization.

After the process of chemical etching that forms the multilayer of porous silicon, an electrochemical etch is carried out for synthesis of a thick buffer layer 38 in order to reduce the friability of the membrane 39 that is obtained after the detachment step. The buffer layer 38 is subjected to electrochemical etching at 35 mA cm^-2 for 430-550 s (according to the value of N) in the same conditions, to obtain a porosity of the buffer p_B of 68% and a refractive index of the buffer n_B of 1.61.

The membrane 39, which is constituted by the multilayer 32 and the buffer layer 38, for an overall thickness L in this case of 37 μιη, is then detached from the silicon wafer by applying a current of 190 mA cm^"2 for 21 s in a 35%HF/35%H₂O/30%EtOH solution at - 25°C, using an electropolishing reaction. This is followed by a number of steps of rinsing with ethanol and drying in a flow of argon, before the membrane 39 is positioned on a soda lime microscope slide , for example Menzel-Glaser extra white glass, with index of the substrate n_s = 1.517, which functions as transparent substrate 31. The slide is previously coated with a polymeric bonding layer, for example a Δ5214Ε (Clariant) photoresist layer. The slide is positioned in such a way that the layer 32e of the multilayer 32 is free to be exposed to the gas or vapour to be detected. The microscope slide is then set on a hot-plate and heated to 110°C in air for 5 minutes, to cause evaporation of the residual solvent in the photoresist and promote creation of a polymeric bond between the membrane and the glass, i.e., the bonding layer 34. In order to increase the force of the bonding interface generated by heating, a force is applied on the membrane 39 to keep it in optimal contact against the slide that forms the transparent substrate 31. The force is normally applied to the surface of the membrane 39. To render the force uniform and prevent any stress, fracture, or defects in the membrane, during this operation a second slide is set on the membrane 39 to distribute the load uniformly.

The solution described above presents various advantages as compared to the known art.

The gas-sensing device according to the invention advantageously enables a gas sensor having a photonic structure, based upon Bloch surface waves in a photonic structure of porous material, to be obtained that is faster. Advantageously, an autonomous membrane is obtained comprising the photonic structure, which can be applied in a more effective optical configuration within the sensor, in particular constraining it to a transparent substrate. In this way, it is possible to excite the surface waves and detect the associated perturbations following upon exposure to the vapours or gases with markedly shorter response times, of the order of tens of seconds. A further reduction of the response time can advantageously be obtained using a single-wavelength detection scheme.

The gas-sensing device according to the invention advantageously enables a less critical assemblage of the prism facing the multilayer, in so far as, by suppressing the air gap, the efficiency of coupling with the Bloch waves no longer depends in a critical way upon its width. The use of a thin membrane, consisting in a multilayer associated to a homogeneous buffer layer bonded to a transparent substrate enables the measurement of reflectance in Kretschmann configuration to be carried out exposing the free side to the gaseous analyte.

The gas-sensing device according to the invention moreover enables identification of the chemical species on the basis of the analysis of the long-term decay of the response, linked to the diffusion of the condensed liquid in the porous silicon. In the Otto configuration the gas was blown into in the gap between the multilayer and the prism; consequently, the process of diffusion was considerably slowed down by the geometry of the system.

Advantageously, the device according to the invention is obtained by combining techniques in themselves known and consolidated, such as the production of porous multilayers, and micromachining processes for detachment of nano-structured membranes and bonding on substrates, in a simple and reproducible way.

Of course, without prejudice to the principle of the invention, the details of construction and the embodiments may vary widely with respect to what is described and illustrated herein purely by way of example, without thereby departing from the scope of the present invention.

According to a variant, the membrane comprising the multilayer and the buffer layer can be fixed directly to the means for directing the light beam, in particular to the surface of the prism. In this case, the buffer layer constitutes the transparent substrate layer, its limited thickness determining a sufficient transparency at the operating wavelengths, according to the criteria described previously, to enable operation of the device according to the invention. In this case, the bonding layer is applied between the buffer layer and the prism^' by bonding the membrane directly on said prism.

Possible spheres of application of the system proposed comprise gas-sensor systems, chemical sensor systems, sensor systems for alcohol in the vapour phase, and sensors for applications in the medical, biological, environmental and agricultural-foodstuff sectors .

In a preferred embodiment of the invention, use of porous silicon as porous material is envisaged in so far as the technology of production and machining of porous silicon to obtain the multilayer is consolidated, but it is clear that the porous material may even be different, for example porous-silicon carbide or porous-gallium arsenide.

Claims

1. A gas-sensing device having a photonic structure, which includes a multilayer structure (39) made of porous material comprising a periodic multilayer (22; 32) configured for generating Bloch surface waves, means (23; 33) for directing an incident light beam (25; 36) upon said device (30) towards said multilayer structure (22; 32) in attenuated-total- reflection (ATR) conditions in order to generate said Bloch surface waves, said periodic multilayer (22; 32) comprising a first sensitive face (32e) that is free so as to be exposed to the gas and a second face (32f) opposite to said first sensitive face (32e) , said device being characterized in that said second face (32f) of said periodic multilayer (32) is set facing said means (33) for directing the incident light beam (36) in ATR conditions and in that said multilayer structure (39) made of porous material is fixed to said means (33) for directing the incident light beam (36) in ATR conditions via a bonding layer (37) and a transparent substrate layer (31) .

2 . The device according to Claim 1, characterized in that it uses as porous material porous silicon.

3 . The device according to Claim 1 or Claim 2, characterized in that said means (33) for directing the incident light beam (36) comprise a prism.

4. The device according to Claim 3, characterized in that said device is configured according to a Kretschmann optical configuration.

5 . The device according to one or more of Claims 1 to 4, characterized in that said periodic multilayer (32) comprises a number (N) of pairs of alternating layers (32H, 32L) with two different degrees of porosity, respectively, one high and one low, said layers (32H, 32L) comprising said first free sensitive face (32e) having high porosity, for sustaining a Bloch surface wave, and said second face (32f), on the side of the multilayer (32) opposite to said first sensitive face ( 32e ) .

6 . The device according to one or more of Claims 1 to 5, characterized in that said multilayer structure (39) comprises a buffer layer (38) on said second face (32f) of the periodic multilayer (32) .

7 . The device according to Claim 6, characterized in that said buffer layer (38) corresponds to said transparent substrate layer (31), and said bonding layer (34) constrains said buffer layer (38) to said means (33) for directing the incident light beam (36) in ATR conditions.

8 . The device according to one or more of Claims 1 to 6, characterized in that said transparent substrate layer (31) comprises a substrate made of transparent material, in particular a microscope slide, said multilayer structure (39) being fixed via said bonding layer (34) to a first surface (31a) of said substrate layer (31), said means (33) being fixed to a second surface (31b) of said transparent substrate ^■ for directing the incident light beam (36) in ATR conditions .

9 . The device according to Claim 8, characterized in that it comprises an optical-matching layer (34), in particular a layer of oil, set between said transparent substrate layer (31) and said means (33) for directing the incident light beam (36) in ATR conditions.

10 . The device according to one or more of Claims 1 to 9, characterized in that said incident light beam (36) is a laser light beam with TE polarization emitted by a tunable laser diode (37) .

11 . The device according to one or more of Claims 1 to 10, characterized in that it is associated to a motor-driven rotation stage (42) designed to rotate said multilayer (32) to obtain profiles of reflectance at a variable angle of incidence (Θ) .

12 . The device according to one or more of Claims 1 to 11, characterized in that said multilayer (32) is associated in an air-tight way to a flow cell (43) for introduction of said gases or vapours.

13 . A process for producing a gas-sensing device having a photonic structure, comprising the operations of:

preparing, starting from a crystalline substrate, a multilayer (22; 32) made of porous material via electrochemical etching, and

associating said multilayer (22; 32) to means for directing an incident beam (33; 43), in particular to a prism,

said process being characterized by:

detaching from said crystalline substrate a membrane (39) comprising said multilayer (32) of porous material ;

applying said membrane (39) on the first surface (31a) of a transparent substrate (31), which bears on said first surface (31a) a bonding layer (34), in particular a transparent polymeric bonding layer; and associating said directing means (33) to a second surface (31b) of said transparent substrate (31) .

14 . The process according to Claim 13, characterized in that said step of electrochemical etching comprises:

providing said multilayer (32) by applying a periodic current for a number of cycles egual to a number (N) of periods of layers with high refractive index (¾) and low refractive index (njj of said multilayer (32);

providing a buffer layer with refractive index (η ) lower than the high refractive index (nn) and the low refractive index (nj of the layers of said multilayer ( 32 ) ; and

detaching from said crystalline substrate a membrane (39) comprising said multilayer (32) of porous material and said buffer layer (38) .

15 . A method for detecting gases that uses a gas- sensing device according to one or more of Claims 1 to 12, comprising the operations of:

detecting the spectral reflectance of said multilayer (32) as a function of the angle of incidence (Θ) of the incident light beam (36) and/or of the wavelength (λ) of said beam (36);

identifying a resonance peak associated to the excitation of Bloch waves and a corresponding wavelength of excitation of the Bloch waves (^BSWR l and

monitoring the shift of said wavelength of excitation of the Bloch waves ^BSWR) ^or said resonance peak for detecting the presence of gas.

16 . The method for detecting gases that uses a gas- sensing device according to one or more of Claims 1 to 12, comprising the operations of:

detecting the spectral reflectance of said multilayer (32), as a function of the angle of incidence of the incident light beam (Θ) and/or of the wavelength (λ) of said beam (36);

identifying a resonance peak associated to the excitation of Bloch waves and the corresponding wavelength of excitation of the Bloch waves ^BSWR) >' and

monitoring the shift in time of said wavelength of excitation of the Bloch waves ^BSWR) °f said resonance peak to identify the gaseous chemical species, in particular calculating a constant of diffusion (k) of said chemical species as a function of a decay time ( TJJ of said wavelength of excitation of the Bloch waves ^BSWR) from a condition of shift towards longer wavelengths to a an original wavelength ( λο ) to obtain, from the constant of diffusion (k), the molar volume (V_m) in order to identify the gaseous species.