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A MICROFABRICATED ATMOSPHERIC MERCURY SENSOR AS A COMPONENT OF A µtas B. MAZZOLAI, V. MATTOLI, V. RAFFA, G. TRIPOLI, D. ACCOTO, A. MENCIASSI AND P. DARIO Scuola Superiore Sant Anna P.zza Martiri della Libertà 33, Pisa - Italy Abstract The design and development of a microfabricated sensor for atmospheric mercury determination is described. The sensor is based on the technique of resistivity variation of thin gold film. Properties, advantages and drawback of two different substrates (glass and PCB) have been presented. Experiments have been carried out to test the linearity of the sensors in function of mercury adsorption. The sensors work in a large range of linearity and need a low power during the regeneration process. Other tests were carried out in order to evaluate the sensor endurance to regeneration. Introduction Mercury represents one of the main environmental pollutants of our planet. A significant amount of research indicates that natural and anthropogenic sources can redistribute this element in the atmosphere, soil and water systems through a complex combination of transport and transformation. Studies on risk assessment and effects caused by mercury on human health, drive the scientists and decision makers to consider this metal as a global pollutant 1,2,3. The work of the Authors has, as a final goal, the development of a low cost µ-tas 4 able to work in cooperative way, thus forming monitoring networks, which produce an enhanced knowledge of the dispersion of a pollutant, and which can be ultimately integrated in the health-oriented monitoring system. In particular, in the present work a new microfabricated sensor, based on the resistivity variation of thin gold film, is described. This sensor is designed to measure elemental gaseous mercury (Hg 0 ) concentrations, which represents 90 to 99% of atmospheric mercury forms 3. Design Choice of the analytical technique The characteristic of mercury of being present in air in atomic state is particularly useful in the detection process, since the metal can be easily excited by UV light (λ = nm) to perform high quality spectroscopic measurements. Most of the instruments dedicated to the atmospheric mercury detection exploit this property 5,6,7. Atomic Absorption Spectroscopy (AAS) is a well established technique for mercury analysis, characterised by a good selectivity and a high sensitivity (with a pre-concentration step). The main drawback is that the lamp requires high power (in the order of some Watts) and the optical cell is subjected to pollution. Moreover the lamp and optical cell are usually quite expensive and are dimensions that limit the miniaturization feasibility. Atomic Fluorescence Spectroscopy (AFS) 8,9, as AAS, is characterised by high selectivity and sensitivity, but it requires high power. A pre-concentration system, usually a gold trap, is used to collect mercury from air 10. The need for an inert gas supply system and a very powerful UV lamps in order to increase the sensitivity dramatically reduces the feasibility of an AFS-based portable analyser. The variable frequency resonator sensor technique 11,12 and the bending cantilever technique 13 have a moderate selectivity and offer good chances of miniaturization because the sensors are typically microfabricated, optical parts are not necessary and the power requirement is quite low. On the other hand these methods have, at the moment, quite poor sensitivity and the detection limit is very high (more than 10 µg/mq). In addition, the regeneration of the coated gold sensor is not a trivial task. The same problems are presented also by the Surface Plasmon-Polaritor Resonance technique 14,15. The technique of resistivity variation of thin gold film 16,17,18 is characterised by good selectivity and absence of optical parts; moreover the power requirements are quite low. As a consequence, an analyser based on this technique can be easily miniaturised. Resistivity variation working principle The gaseous mercury sensor presented in this paper is based on the resistivity changes of thin gold films. The sensor consists of four identical thin gold film resistors mounted in Wheatstone bridge configuration. Two resistors work as sensitive elements, while the others work as reference, in order to minimize the influence of temperature variation. The absorption of mercury on the gold film produces a change in the resistivity of the film itself. The resistance increase due to a partial mercury overlayer on the gold is produced by the change in the surface scattering of the electrons 19. Far from the saturation, this change is proportional to the amount of gold absorbed. After calibration, the signal outgoing from Wheatstone bridge can be directly related to the concentration of mercury in air, provided that the air flow-rate is constant and the sampling time is recorded. For high mercury concentration on gold, the resistance of the sensitive elements approaches the saturation, i.e. the variation of resistivity becomes asymptotically zero. In order to reuse a saturated mercury sensor, a regeneration step is required. The regeneration can be obtained by direct heating of gold resistors over 100 C. Fabrication The deposition of thin gold films is usually made by Physical Vapour Deposition (PVD), which comprises both sputtering and deposition. The sensor presented in this paper has been fabricated by sputtering because of the following advantages 20 : i. thickness is easily controlled through the deposition time, once the other operative parameters are tuned; ii. film properties, such as step coverage, grain structure, stress and adhesion, can be controlled by altering some process variables, such as negative bias, substrate warming, power and pressure; iii. the short distance between target and substrate reduces the target consumption. The used sputtering machine is a radio frequency model (Sputtering Sistec, model DCC 150) operating at a constant pressure of 1 Pa, using a gold target (purity %). Pure Argon (purity %) was used as sputtering gas. The thickness of the gold film (200 nm) was measured using a sputtering crystal sensor and the film surface morphology of the substrates was studied with Atomic Force Microscopy (AFM). After deposition, the sensor was annealed for 1 h at 80 C. The footprint of the four resistances deposited is represented in Figure 1. Figure 1. Footprint of the four resistances linked in a Wheatstone bridge configuration. Each resistance is a 50 mm long, 0.2 mm wide stripe of gold film, folded in a 9.3 mm x 3 mm rectangle in order to reduce the overall dimensions. Each film is about 200 nm thick. The films were sputtered changing the operative conditions (bias, deposition time and pressure), in order to point out their influence on the film properties. Attention has been paid to the choice of the substrate. Since the sensor is designed to operate in a cheap, miniaturized device, the substrate must assure quality of adhesion but also low cost and possibility of batch fabrication. The four resistances have been deposited onto two types of substrate, thus fabricating two series of sensors. The first substrate is glass (the thickness is 0.1 mm) with a superficial roughness of about 80 nm, while the second one is a piece of Printed Circuit Board (PCB) (the thickness is 0.8 mm) with a superficial roughness of about 300 nm. The overall dimensions of the PCB sensor are: mm x mm x 0.80 mm. While glass is a material commonly used as a substrate, thanks to its high thermal stability, surface smoothness and low cost, the choice of PCB has been mainly suggested by the possibility to easily integrate the electronic contacts, thus allowing an easy handling of the fragile resistors. In addition, the use of PCB substrate offers other important advantages, such as low cost and the possibility to integrate the conditioning electronics on board. The main drawback of PCB substrate, if compared to glass, is the higher thermal inertia. To solve this problem a thinner PCB board is currently under fabrication and a new gold film design with a small exposed surface is in progress. At present, the PCB sensors show a better adhesion of the gold film. Experimental results Fluidic system The set-up used in the experimental activities is schematically shown in Figure 2. The fluidic circuit includes an air filter, a chamber containing the mercury sensor, a miniaturised solid-state flowmeter (Honeywell, AWM2300V Series Microbridge Mass Airflow Sensors, ± 1000 sccm) and an air pump (KNF, NMP 015M; 1.6 l/min, 400mbar in aspiration). The circuit is in aspiration in order to reduce the pollution of mechanical components continuously exposed to air. The components are connected by silicon tubes. A microprocessor allows to monitor the experimental parameters: the excitation current, the differential signal generated by the sensor after amplification, the signal from the flowmeter. A 500 µl sample of mercury saturated air is sucked from a thermos kept at controlled temperature and injected into the aspiration air using a precision manual syringe. The air flow rate is set around to 1 l/min. The injection time is 30 seconds and the readout is made after 30 seconds from the turn-off of the pump in order to allow the recovery of thermal equilibrium, perturbed by massive air flow rate. The average sensitivity of the glass and PCB sensors is respectively of about 48 µv/vex and 20 µv/vex per ng of mercury injected. On the basis of this data, the detection limit can be estimate around 100 pg of mercury; the detection limit in term of concentration clearly depends from the quantity of sampled air. The change in sensitivity between glass and PCB sensors is related to the following factors: a) the different effects of the annealing treatment on sensors to produce a stable film morphology; b) the film response to mercury depends strongly on the film properties such as the surface roughness, crystallite structure, grain boundaries and film thickness; these proprieties are closely related to substrates morphology 21. Wheatstone bidge tension output (uv; Vex = 1V) Glass Sensor PCB Sensor ng of Hg injected Figure 3: Signals obtained from a glass and a PCB sensor plotted as a function of the amount of mercury saturated air injected in air circuit. The n potential used for the Wheatstone bridge excitation is 1V. Figure 2: The experimental set-up. Atomic mercury adsorption Experiments have been carried out to test the linearity of the sensor. Figure 3 shows the results of one comparative experiment carried out on a glass based sensor and on PCB sensor. The outcome tension from the Wheatstone bridge was plotted as a function of the volume of the transferred air saturated with mercury. The experimental data and the relative graph obtained show a high readout variation, and a manifestly linear trend. A dedicated series of comparative saturation test is also performed in order to esteem the linear response of sensors and their saturation limits. In these tests an increased amount of mercury saturated air was injected in the air circuit until saturation. The result of the experiments showed that the sensor linear response limit (similar in both sensors) is around 300 ng of injected mercury and the saturation limit is around 5000 ng of injected mercury. Regeneration Different techniques can be exploited to heat the resistors. In order to reduce the power consumption, the most efficient way is to use Joule effect self-heating, by short-circuiting the two passive resistances and supplying a regeneration voltage to the bridge. Tests have been performed in order to evaluate the sensor thermal profile during the regeneration process for different values of regeneration current. Only one of the four resistances was regenerated. During the tests the sensor temperature was monitored by a k- thermocouple (15 µm diameter) located on the film surface. Tests were performed on the same glass and PCB sensors with a resistance of approximately 400 ohm. As first step, the sensor was contaminated by mercury until the achievement of non linear adsorption limit area. Table 1 reports the asymptotic temperature of the resistance, the corresponding current and the time necessary to complete the regeneration of the polluted sensor. As shown in Table 1, it is necessary to maintain the resistance temperature above 130 C to obtain the complete sensor regeneration with a time down 5 minutes. Even if the regeneration current necessary to maintain the resistance at the suitable temperature is strictly related to the sensor resistance value, data reported in Table 1 point out that it is possible to regenerate the sensor with a relative low power consumption. The PCB sensor requires a higher regeneration current to obtain the same temperature used for the glass sensor, because of its higher thermal inertia. Regeneration current Glass ma PCB 33.8 ma Glass ma PCB 35,6 ma Glass ma PCB 37 ma Highest Temperature Regeneration time 110 C 20 min 123 C 12 min 132 C 5 min Table 1. The asymptotic temperature reached by the resistance (about 400 ohm), the corresponding current and the regeneration time are reported. Other tests were carried out in order to evaluate the sensor endurance to regeneration, i.e. the minimum number of regeneration cycles which can be performed without inflicting any mechanical or electrical damage to the resistors. Unused sensors on glass and PCB substrates underwent numerous regeneration cycles after each of which the resistance variation was measured. Figure 4 shows the results obtained: after more than 1300 regeneration cycles no damage was observed. The electrical resistance decreases quickly after the first regeneration but very slowly in the following regeneration steps. In order to evaluate whether reiterated regeneration steps can perturb the capacity of the gold films to collect atmospheric mercury, absorption measurements were performed after some regeneration cycles. The sensor unused and undergone to 100, 200, and 1300 regeneration cycles was subjected to an adsorption tests in order to evaluate an eventual change in the sensor adsorption capacity. The results of the tests performed on PCB sensors in the same operative conditions, are shown in Figure 5. Resistances variation (%) 7% 6% 5% 4% 3% 2% 1% 0% Sensor on glass Sensor on PCB Regeneration cycle number Figure 4. Resistance variation (%) as a function of the number of reiterated regeneration cycles for both glass and PCB substrate. Regeneration temperature 130 C. Wheatstone bidge tension output (mv/100; Vex = 1V) Virgin sensor Sensor after 100 reg. Sensor after 200 reg. Sensor after 1300 reg ng of injected mercury Figure 5. Atomic mercury adsorption on a unused and afterwards regenerated sensor (PCB substrate, 300 Ω). The x-axis reports the total nanograms of injected mercury, the y-axis reports the Wheatstone bridge equilibrium voltage (proportional to the resistance variation). As shown in Figure 5, the absorption capacity of a previously unused sensor changes after the first regeneration, after that the sensor shows an optimal measurement reproducibility. The different performance of unused sensor may be a consequence of the sputtering stress. The first regeneration has the effect of a stress relieving thermal annealing, which ultimately reduces the electrical resistance. This result agrees with the previous observation on the initial decrease of sensor resistance and with the data reported in literature concerning the resistance variation depending from temperature 22. This phenomenon underlines the necessity to submit the unused sensor to a suitable annealing treatment before the use to obtain a more stable and crystallized film. 4.4 Thermal modelling While the stationary temperature of the sensor can be easily measured using a contact thermocouple, as already described, the experimental analysis of the transient is not at all trivial; this aspect is mainly related to the high thermal capacity of the thermocouple in comparison to the thin film, and the not ideal thermal contact between the flat gold surface and the cylindrical wire of the thermal sensor. Numerical simulations of the electrothermal effects during the transient on the glass and PCB sensors in the regeneration process were carried out. The software chosen for the numerical analysis was the Coventorware In particular, the MemETherm module in the MEMS Solvers Category was used. This module computes the thermal field and the electrical potential resulting from an imposed voltage and/or current flowing through a resistive material. The properties of the materials used in the simulation were first obtained by literature. Initial stationary simulations were carried out in order to match the numerical values of the thermal conductivity of gold as a function of temperature through comparison with experimental results. In particular the Temperature Coefficient of Resistance (TCR) of the sputtered gold film results to be C -1. Figure 6 shows a typical thermal profile of the Wheatstone bridge when a resistor is regenerated. The transient simulations have shown how the different thickness and thermal properties of the two substrates have a deep impact on the transient during the warming up and cooling down steps, thus influencing the minimum time required for a measurement and, most prominently, the power consumption during this demanding process. Figure 6: Stationary thermal profile during a regeneration process. Discussion and future work Although the use of PCB has many advantages from a practical point of view, such as handling ability and the possibility to integrate active electronic components on the board, from a microfabrication point of view some drawbacks have been observed. The glass substrate shows a higher reproducibility respect of the PCB one, which is strictly interrelated to the different superficial roughness. As a consequence, the gold film on the glass substrate is characterized by a more compact grain structure, while the PCB substrate has a more inhomogeneous and porous structure, as confirmed by AFM measurement at a smaller scale (scanning area of 8µm x 8µm). Current research is focused on studying possible solutions, i.e. deposition of an intermediate layer of SiO 2, which assures the same performances of the glass substrate in terms of reproducibility. Experimentally it has been observed that a higher value of film resistances is typically obtained on PCB substrate, under the same operative conditions (about 20% in excess with regards to glass). Since the amount of deposited gold is the same for both substrates, a higher superficial roughness implies a higher superficial atomic density (numbers of atoms per surface unit). The higher is the atomic density, the smaller is the film thickness in accord with the experimental results. Figures 7 (a) and (b) report respectively a scanning of a small area of glass and PCB substrates made by a fiber optical microscopy. performances of the glass substrate in terms of reproducibility. Figure 7: (a) Gold film on glass substrate. (b) Gold film on PCB substrate The pictures show that the morphology of the gold film surface strongly depends to the surface of the substrate. Future work will be mainly focused on two aspects: 1. surface pre-treatment of PCB boards, in order to reduce the substrate roughness to obtain a gold film with the desired properties; 2. integration on the PCB of active electronic components: a differential operational amplifier and an EEPROM will be mounted on the same board of the sensor, in order to include the calibration data and an offset calibration circuit. Conclusion A microfabricated sensor for atmospheric mercury determination has been described and experimentally tested. The sensor is based on the resistivity variation of thin gold film technique, characterised by good selectivity and absence of optical parts. Experiments on the l
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