A vailable online at www.sciencedirect.com Sensors and Actuators B 130 (2008) 70–76 TiO 2 nanowires array fabrication and gas sensing properties L. Francioso ∗ , A.M. Taurino, A. Forleo, P. Siciliano CNR-IMM Institute for Microelectronics and Microsystems, S.P. per Monteroni, Lecce University Campus, CNR Area, 73100 Lecce, Italy Available online 25 July 2007 Abstract A cheap nanofabrication process for titania (TiO 2 ) polycrystalline nanowire array for gas sensing applications with lateral size ranging from 90 to 180 nm, and gas sensing characterizations are presented. Alternatively to typical pattern transfer techniques for submicron fabrication, authors focused on a standard 365 nm UV photolithographic process able to fabricate sol–gel nanostructured titania nanowires from a solid thin film. Main aim of present work is the experimental validation of enhanced gas sensing response of nanopatterned metal oxide thin film sensors. Two different kinds of gas sensor with nanopatterned sensitive area have been realized onto silicon substrates and tested towards different EtOH concentrations; experimental tests have been carried out with a contemporary output signals collection from a nanowires-based gas sensor and a second device with solid sensitive film without patterning, in order to validate effects of nanomachining on sensitive material response. © 2007 Elsevier B.V. All rights reserved. Keywords: Metal oxide nanowires; TiO 2 ; Nanometric patterning; Response enhancement 1. Introduction During last years, nanostructures like nanowires and nanobelts (i.e., one-dimensional structures) constitute a novel class of functional materials that have recently gained consid- erable attention from R&D community due to their potential about development of innovative smart devices and systems. Impressive and promising results regarding the synthesis, fabri- cation, and physical properties of these nanostructureshave been just achieved [1–3]. Electrical properties of such nanostructures dependent on high aspect ratio of the structure may be easily modified by addition of small amounts of dopants. This topic is well illustrated, for example, by diffusion of boron or phos- phorous in silicon nanowire in order to modulate the electron or hole concentration, respectively [4]. Among semiconductors, also functional metal oxides can be synthesized in controlled conditions as 1-D nanostructures that, showing electrical trans- port properties characterized by a strong carrier confinement, gain an high significance in several scientific and technologi- cal applications [5–8]. Metal oxides 1-D nanostructures, may be promising gas-sensing materials because their very high surface- to-volume ratio; they are single crystalline (so expected to be ∗ Corresponding author. E-mail address: luca.francioso@le.imm.cnr.it (L. Francioso). more stable), identical crystalline faces exposed to gases, and the nanosize is likely to allow a complete depletion from charge carriers [9–13]. Hence, they can be used for miniaturized highly sensitive chemical sensors [14–18]. The development of tech- niques for rapid electrical testing and reproducible integration of these materials into working sensors may result an enabler for a wide variety of nanotechnology research. The scientific community actually follows different approaches in order to synthesize functional oxides nanostructures, and mainly chemi- cal route techniques or nanoporous templates-based techniques seem to be best candidates [19–21]. Present work applies a cheap and custom nanopatterning process to fabrication of nanomachined metal oxide thin film gas sensors, looking for an experimental validation of enhanced gas sensing response towards different concentrations of EtOH in comparison to standard thin film sensors made of identical TiO 2 polycrystalline sensitive thin film. A preliminary prototype device has been fabricated with a platinum gap microelec- trodes, deposited over about 3500 titania nanowires patterned onto oxidized silicon substrate. Subsequently, an enhanced lay- out of silicon miniaturized gas sensor, with embedded heater and thermometer have been also realized and tested in two dif- ferent typologies: a former typology presents standard solid sensitive titania film and the latter one characterized by tita- nia nanowire array as sensitive area. Sensitive metal oxide films of all tested devices has been synthesized in a single process 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.07.074 L. Francioso et al. / Sensors and Actuators B 130 (2008) 70–76 71 and deposited onto same substrate. A custom photolithographic mask set allows the fabrication of solid and nanopatterned area gas sensors in a single photolithographic step; performance of tested devices and effects of patterned sensitive area will be dis- cussed. In the next sections, fabrication details and controlled environment gas sensing test will be presented for both devices (prototype and enhanced layout sensor). 2. Experimental The engineering of a cheap nanometric structures fabrica- tion run onto silicon substrates has been defined considering fabrication challenge of submicron structures of metal oxide gas sensitive materials only implementing standard 365 nm UV lithography and dry plasma etching. The process, successfully completed, yields fabrication of TiO 2 nanowires large array over silica mesa, characterized by wires’ width ranging from 90 to 180 nm. The nanopatterning process has been applied for fabrication of both prototype device and enhanced layout one, described below in detail. Hard control of sensitive film thick- ness and resist mask uniformity onto silicon substrate represents main points responsible for a successful fabrication. The pro- cess starts with the deposition by sol–gel route of a thin layer of undoped titania metal oxide following experimental procedure described elsewhere [22]; previous structural characterizations of these metal oxide films by X-ray diffraction (XRD) showed that, after the calcination step at 500 ◦ C for 1 h, the lattice sta- bilizes to anatase phase. The spinning process was carried out onto a full 3 inch (1 0 0)-oriented silicon wafer, thermally oxi- dized up to 400 nm of grown oxide; the sol–gel solution was spun statically onto wafer before spinnerrotation at 2000 rpm for 30 min. Sensitive film before final high temperature firing is still amorphous and characterized by poor sensitiveness to gaseous environment; so a final calcination step was carried out at 500 ◦ C to obtain fully crystallized film to anatase phase. Calcined films become inert to strong acid and the deposition of a polymeric matrix (photoresist) onto this calcinated films does not affect its gas sensing properties as well. At this point the fabrication step is described by first picture of Fig. 1, depicting silicon wafer with annealed film on top. Subsequently, a thin layer of positive photoresist (S1805 from Shipley) was spun onto film surface to define a resist mask with typical strip array structures 500 nm width and 800 nm of pitch between two strips. Spinner rotation was set to 4500 rpm and the resist thickness obtained was about 400 nm after soft-bake step onto hotplate at 115 ◦ C for 120 s. Defined the suitable resist mask described above, high pres- sure plasma in a Oxford Plasmalab 80 RIE reactor has been adopted to perform micromachining of titania thin films; the identification of process parameters for heavy isotropic etching, was oriented towards a total process pressure of 200 mTorr and SF 6 chemistry; to limit photoresist damage and increase selec- tivity, only SF 6 was introduced in the chamber during discharge, 40 sccm total flow, and a RF power density applied to aluminium reactor plate of about 1.5 W/cm 2 . Preliminary etching rate cal- ibration, showed that titania thin films are fastly etched and a suitable etching time of about 390 s gave optimal results. Typical SEM pictures of titania nanowires array is reported in Fig. 2. Afterwards realization of titania nanowire array, became challenging electrical properties investigation of patterned mate- rial as nanowires conducting paths, by means of electrical contacts deposited between ends of nanowires connected in parallel, about 100 ␮m spaced and deposited over about 3500 nanowires (metal paths across three different patterned areas). Fig. 3 shows prototype layout adopted for preliminary gas sens- ing tests for this innovative structure; silicon substrate was connected with gold wires bonded on platinum paths deposited Fig. 1. Nanowire array fabrication process. 72 L. Francioso et al. / Sensors and Actuators B 130 (2008) 70–76 Fig. 2. SEM images of TiO 2 nanowires onto silica mesa (×4500); inset (×90,000). over thermal oxide, outside sensitive areas; this device has been heated at operative temperature (500 ◦ C) onto a resistive ceramic plate and exposed to EtOH injection on dry air carrier. Further physical details of this prototype are reported in Table 1. About fabrication of second kind of sensor, the layout of enhanced gas sensor device, with embedded platinum heater and ther- Table 1 Physical parameters of prototype sensor nanowires array Parameters Values Single nanowire width (nm) 90–180 Single nanowire length (␮m) 1400 Single nanowire thickness (nm) 50 Nanowires length between biased electrodes (␮m) 100 Total measured nanowires (100 ␮m) in parallel ≈3500 mometer is depicted in Fig. 4; the device is characterized by 1.5 mm × 1.5 mm size (right side of picture), 350 ␮m thick sil- icon die, 10 ␮m pitch electrodes, 5 V maximum power supply and 440 ␮m × 440 ␮m active area; it requires only 200 mW at 350 ◦ C with an heater resistance of 20  at room temperature. The fabrications process needs only two mask levels and allows batch fabrication of about 1000 chips for a single 3 min silicon wafer. The fabrication process is identical to previous prototype device: in fact, after spinning and firing of sensitive film, a dry etching performs the patterning of sensors’ sensitive areas, both for nanowires-based sensors and solid film-based ones. For these samples, nanowires structures present maximum width of about 180–200 nm, measured with the support of our JEOL JSM 6500 SEM-FEG software measuring tools. Fabri- cated devices have been diced and packaged on a 10-pin TO-5 socket for controlled environment characterizations (Fig. 4, left Fig. 3. Optical microscope images of metal contacts configuration on prototype sensor: darker area is the nanowire array while electrical platinum contacts are visible as bright paths. Square pads on top are 100 ␮m × 100 ␮m. Fig. 4. Digital pictures of enhanced layout packaged gas sensor (left) and front side view of 1.5 mm × 1.5 mm sensor structure (right). L. Francioso et al. / Sensors and Actuators B 130 (2008) 70–76 73 side), carried out in a 120 cm 3 volume stainless steel cham- ber, 200 sccm dry air carrier constant flow and PC-controlled acquisition bench facility. Further test bench details are reported elsewhere [23]. 3. Results and discussion Working principles of developed chemoresistive gas sensors involve chemiadsorption and charge transfer processes between the gas molecules and metal oxide (MOX) film, which cause a simple electrical resistance variation of the gas sensing element, hence, they are characterized by a real functioning easiness. Basically, the effects of the microstructure, namely, the porosity in the packing of the metal oxide particles, the large interface-to- volume ratio, the grain size and more specifically the ratio of the grain size to the Debye length (L D ) are well recognized param- eters which control the electrical conduction properties and the gas sensing mechanism. If the size d of the nanocrystalline parti- cles is so low (d < L D ) that the grains are completely depleted and the Schottky barriers are so short that a flat band condition can be assumed [24–26].TiO 2 material at 550 ◦ C, presents a mixed conduction mechanism, mainly based on electronic conduction and structure defects-dominated conduction(oxygen deficiency) mainly related to oxygen vacancies and Titanium interstitials [27]. Preliminary gas sensing test were carried out to verify use- fulness of this patterning process to enhance the performance of a solid thin film, making account of gas-interaction-depleted region influence of sensitive film; Fig. 5 shows dynamic responses of solid thin film devices in comparison with nanowires array prototype to EtOH vapours in dry air carrier, with a total flow of 100 sccm and 5 min exposure pulses to 6% EtOH. The concentration of test gas is still to high, but these results were collected as preliminary investigation about usefulness of this approach. Nevertheless, the performance of solid thin film devices based on pure polycrystalline TiO 2 , 50 nm thick, onto a standard 2 mm × 2 mm silicon substrate pro- vided by platinum heater, was compared with micromachined Fig. 5. Dynamic responses comparison between solid thin film device and nanowires array prototype towards EtOH pulses at 500 ◦ C. Fig. 6. Dynamic responses comparison between enhanced layout solid thin film device and nanowires-based towards 3 and 2% EtOH pulses at 550 ◦ C. sensors based on titania nanowires. An increment of about three- order of magnitude by nanopatterned device towards identical gas concentrations and operative parameters was registered for nanopatterned device; also response time is faster compared with traditional thin film device of pure TiO 2 . Experimental results gained with the enhanced sensor layout are reported from Figs. 6–11; all graphs report a comparison in terms of dynamic response and response calculated as saturated current ratio measured during EtOH injections and dry air (S = R EtOH /R air ). Layout of investigated devices is reported in Fig. 5 and each graph reports experimental data from a sensor with solid TiO 2 thin film and a nanowires (NW) patterned one. Fig. 6 shows the dynamic behaviour comparison of both devices exposed to 2 and 3% EtOH injections in dry air carrier; operative sensors temperature was 550 ◦ C; rise times for both devices are comparable but NW-based device shows a longer recovery time. About the response, as reported in Fig. 7,NW- based device performs better performance with a response equal to about 50. The dynamic signal recorded with 3% EtOH injec- tion suffers of a slower and irregular saturation signal, probably Fig. 7. Response analysis of enhanced layout solid thin film device and nanowires-based towards 3 and 2% EtOH pulses at 550 ◦ C. 74 L. Francioso et al. / Sensors and Actuators B 130 (2008) 70–76 Fig. 8. Dynamic responses comparison between enhanced layout solid thin film device and nanowires-based towards 0.3 and 0.6% EtOH pulses at 550 ◦ C. Fig. 9. Response analysis of enhanced layout solid thin film device and nanowires-based towards 0.3 and 0.6% EtOH pulses at 550 ◦ C. Fig. 10. Dynamic responses comparison between enhanced layout solid thin film device and nanowires-based towards 1200 and 1800 ppm EtOH pulses at 600 ◦ C. Fig. 11. Response analysis of enhanced layout solid thin film device and nanowires-based towards 1200 and 1800 ppm EtOH pulses at 600 ◦ C. related to a poor filling and/or mixing of carrier stream before cell injection; also the response chart shows a smallest response for higher concentrations that may be easily explained. Fig. 8 reports tests with lowest gas target concentrations; experimental conditions are unchanged, with 10.0 V applied to interdigitated contacts, 550 ◦ C operative temperature and 0.3 and 0.6% of injected EtOH in dry air carrier. The NW-patterned sensors exhibit a higher response also in this case, but recovery time is longer than solid sensitive film devices. Considering lowest gas concentrations, expected responses are smallest and reported in Fig. 9; brighter columns represent the nanowires sensor and lowest ones the gas response of standard solid film device. Prop- erties of devices at higher sensitive film temperature (600 ◦ C) are reported in Fig. 10, with 1200 and 1800 ppm EtOH injec- tions; the enhanced response of nanowires sensor is confirmed also at higher temperatures, and in this case the recovery times become shorter in comparison with solid film devices, reported as dark plot; the current level of nanopatterned sensor is about three orders of magnitude smallest than solid thin film sensor (10 −10 A versus about 10 −7 A of baseline current). The response at 1200 ppm of EtOH exposure is higher than one order of mag- nitude versus the smaller response of solid thin film (about 2.5) as reported in Fig. 11. It is noticeable that compared sensors for all graphs described above have been fabricated from iden- tical sol–gel titania film synthesis and manufactured on same silicon wafer; the sensitive film patterning step for nanowires fabrication and small 440 ␮m × 440 ␮m active area definition for solid film-based devices has been performed with identi- cal dry etching process; also the platinum layers deposition in UHV sputtering system has been performed at the same time for both kind of sensors. These fabrication details contribute to a rigorous experimental validation of devices properties. The effect of nanowires dimensions on response of this kind of sen- sors to a fixed concentration of EtOH has been investigated keeping the enhanced layout of gas sensor described above and testing nanowires properties with different lateral dimen- sions. Results gained with 3500 ppm of EtOH in a 200 sccm L. Francioso et al. / Sensors and Actuators B 130 (2008) 70–76 75 Fig. 12. Nanowires size response effects of enhanced layout sensor devices, towards 3500 ppm EtOH pulses at 500 ◦ C. dry air carrier are reported in Fig. 12; sensor operative tem- perature was set to 500 ◦ C and contact bias to 3.0 V. Identical gas injections have been performed for a solid thin film devices and a nanowires-based device with typical nanowires size of about 100 and 200 nm, respectively. The response enhancement within solid and 200 nm NW devices is not so evident (left and center column), while a good response increment was recorder for 100 nm NW-based device. It is clear that between solid and 100 nm nanowires sensor, about 100% response enhancement may be highlighted keeping fixed other experimental param- eters like gas concentration, temperature, electrodes bias and device’s layout. 4. Conclusions Present work focused on application of a standard 365 nm UV lithography for fabrication of a nanowires-patterned fully integrated gas sensor device based on TiO 2 metal oxide thin film. Main aim of the activity was the experimental valida- tion of metal oxide sensors performance enhancement together with the demonstrated integration capability of a nanowires titania array into a single-side silicon substrate as working gas sensor. Investigated devices presented low power con- sumption and integrated platinum heater and thermometer. Experimental gas sensing tests in a controlled environment ver- ified that devices characterized by a nanopatterned sensitive area exhibit higher gas response and a recovery time typically longer at 550 ◦ C. Preliminary investigations at higher temper- atures show a reduced recovery time, keeping the amplified responses. Observed behaviour was confirmed by a preliminary pro- totype sensor without integrated heater and also by enhanced silicon miniaturized devices, characterized in the second sec- tion of the paper. In conclusion, a simple nanomachining of a metal oxide film results in an enhanced performance in terms of responses considering that this patterning process exposes a wider area of single wires structure to gaseous environment, con- tributing to deeper carriers depletion after exposure to oxidizing gases. Further investigation by I–V plots and gas-sensing tests with thinner nanowires array are ongoing, while the application of this patterning procedure to different metal oxide materials is actually under investigation. Acknowledgments Authors kindly acknowledge Mr. Flavio Casino for tech- nical support during gas sensing tests. 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Since 2001, he works in the Institute for Microelectronic and Microsystems of the Italian National of Research Council (C.N.R I.M.M.) in Lecce (Italy) in the field of silicon micromachined systems and thin film gas sen- sor, in charge to develop fabrication processes and new sensors designs. Since February 2002, he is in the position of researcher working on within silicon technology and integration of sol–gel process into silicon devices. At present he works in the field of combustion control sensors with implementation of thin film based gas sensors and development of micromachining process of metal oxide layers. Antonella M. Taurino received her degree in physics from the University of Lecce in April 2000, with a thesis on electronic nose. In 2001, she took an advanced post degree specialization course in electron microscopy. In 2004, she got her PhD in materials engineering with a thesis on nanostructured based gas sensors devices. At present, she works in the field of structural and elec- trical characterization of innovative nanostructured materials for gas sensors application. Angiola Forleo received the degree in physics from the University of Lecce in April 2000 with a thesis on semiconductor gas sensors. In 2000, she was with the Department of Physics, University of Lecce, where she was involved in deposition of thin films making use of the pulsed laser deposition technique. Since 2001, she is working at the IMM-CNR Institute of Lecce. She researches the interactions between gases and mixed oxides and the electrical and optical characterization of thin films for organic and inorganic gas sensors. Dr. Pietro Siciliano, physicist, senior researcher, received his degree in physics in 1985 from the University of Lecce. He took his PhD in physics in 1989 at the University of Bari. During the first years of activities he was involved in research in the field of electrical characterisation of semiconductors devices. He is currently a senior member of the National Council of Research in Lecce, where he has been working from many years in the field of preparation and char- acterisation of thin film for gas sensor and multisensing systems, being in charge of the Sensors and Microsystems Group. He is responsible for several national and international projects at IMM-CNR in field of Sensors and Microsystems, mainly for environmental, automotive and agro-food applications. He has been organiser and Chairman of International Conferences and Director of Interna- tional Schools on Sensors and Microsystems. He is member of the Steering Committee of AISEM, the Italian Association on Sensors and Microsystems. At the moment he is Director of IMM-CNR in the Department of Lecce. . online at www.sciencedirect.com Sensors and Actuators B 130 (2008) 70–76 TiO 2 nanowires array fabrication and gas sensing properties L. Francioso ∗ , A.M. Taurino,. (TiO 2 ) polycrystalline nanowire array for gas sensing applications with lateral size ranging from 90 to 180 nm, and gas sensing characterizations are presented.