Preconcentrator-based sensor µ-system for low-level benzene detectione

P. Ivanov*a, I. Gràciaa, F. Blancob, J.-P. Raskinc, R.Cumerasa, N. Sabatéa, X. Vilanovab, X. Correigb,

L. Fonsecaa, E. Figuerasa, J. Santandera, C. Canéa aMicrosystems Group, National Centre of Microelectronics, CNM-CSIC, Bellaterra, SPAIN

bDepartment of Electrical Engineering, University Rovira i Virgili, Tarragona, SPAIN cMicrowave Laboratory, Université Catholique de Louvain, Louvain-la-Neuve, BELGIUM

ABSTRACT

In this paper, a preconcentrator-based sensor µ-system for low level benzene detection is presented. It consists of a spiral-shaped µ-reconcentrator with dimensions of 10cm × 300µm × 300µm, followed by a µ-hotplate sensor matrix. The µ-preconcentrator was fabricated on a silicon wafer by means of DRIE and anodic bonding techniques. To obtain the concentration factor of the fabricated devices, a GC/MS: Shimadzu-QP5000 equipment was used. The results obtained showed excellent repeatability and preconcentration factors up to 286. A considerable improvement (1500%) in the sensor responses was achieved with Pd doped SnO2 sensors. The small size of the manufactured devices enables their incorporation in an integrated GC/MS gas sensor system.

Keywords: µ-reconcentrator, preconcentration factor, µ-hotplate gas sensor

1. INTRODUCTION Real time quantitative analysis of volatile chemical organic compounds is essential for environmental monitoring. Generally, detecting chemical vapors relies on intensive and expensive sample collection followed by transport to a remote laboratory for analysis. This procedure limits the frequency and reduces the overall quality of the measurements. Currently available portable instruments (e.g., miniature mass spectrometers) lack the sensitivity for real time air quality monitoring [1]. This deficiency can be solved by developing miniaturized preconcentrators that can be used as front-ends for portable instruments. A specific application of such portable devices concerns the in-field monitoring of parameters related to hazardous compounds and environmental contamination.

It has been shown that the presence of benzene in the soil may be an indicator of environmental contamination [2]. Researchers and engineers use gas chromatographic (GC) devices, sensitive enough to detect benzene in the near surface layers of the earth, to predict the presence of oil reservoirs. Unfortunately, the traditional measurement procedure takes considerable time to detect tens of ppb of benzene, which limits the area inspected per day. If the sampling time and the detection threshold could be reduced, larger areas could be analyzed with more accuracy in a shorted time.

In some cases, as stated above, the concentration of the target gas is too small and below the detection levels of the existing gas sensors. As the detection limit of the currently available gas sensors is difficult to be improved, a preconcentration unit connected to their entrance may be needed [3].

A preconcentrator consists of an absorbent material placed on a heating support. When a preconcentrator is used, the gas mixture to be analyzed flows through it and is accumulated during a certain time into the absorbent. Then the mixture is desorbed by a temperature pulse and driven to the separation column of the gas chromatograph. The thermally released compounds provide narrow desorption peaks with relatively high concentration. These peaks increase the separation efficiency of the GC column, and the high concentration peak increases the sensitivity of the analysis [4].

Conventional stainless-tube preconcentrators are large in size and consume high power during thermal desorption. They also experience problems related with the large dead volume and restricted heating efficiency because of their large thermal mass, and therefore require an additional liquid nitrogen focusing stage [5]. Up to date studies have shown that the fabrication of such devices with silicon technology offers a lot of advantages. The miniaturization leads to improvements of heating rates and power consumption, and allows its integration in a portable monolithic micro-gas

Smart Structures, Devices, and Systems IV, edited by Said Fares Al-Sarawi,Vijay K. Varadan, Neil Weste, Kourosh Kalantar-Zadeh, Proc. of SPIE Vol. 7268, 72680S

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chromatograph [6], which makes these systems suitable for a great number of new applications. The advantages of this approach were shown for the first time in the work of Tian et al. where a µ-preconcentrator unit was presented [7]. Lu and Zeller developed and tested a miniature dual-adsorbent preconcentrator for a micro-sensor-based analytical system, designed to determine complex VOC mixtures in indoor working environments at low part-per-billion levels [8]. A novelty in this field was the development of multiple-stage micro-fabricated preconcentrator-focuser for micro-gas chromatographic system for the analysis of ppb of VOCs in air [9]. The system was capable to analyze up to 50 compounds in approximately 10 min, with a power consumption of approximately 1.5 W.

The first demonstration of a micro preconcentrator with a micro gas chromatograph and micro detector was presented in the work of Sandia National Laboratories [10] and [11]. They have developed micro-fabricated preconcentrators with a thin layer of adsorbent film on a heated-membrane which were designed to detect specific chemical toxic agents. Nevertheless, the devices were not able to provide neither large adsorbent capacity for preconcentration nor uniform heating. An extensive study which justifies the operation of the micro fabricated preconcentrators was presented R. P. Manginell, et al. [12]. Another micro-preconcentrator unit based on 3D-microheater surrounded by an insulating membrane was presented by A.M. Ruiz et al. [13].

In this study we present a micro-preconcentrator, which consists of a spiral-shaped channel with an integrated heater, housing the porous adsorbent powder. The proposed structure is extremely versatile and permits to connect various spirals in series in order to increase the operating volume if needed. The temperature pulses are provided by a Aluminum heater patterned on the bottom of the channel. The device is capable to house enough absorbent material enabling the ppb detection of benzene in air.

2. DEVICE FABRICATION Two different devices were fabricated using the silicon technology. The µ-preconcentrator was fabricated on a silicon wafer by means of Deep Reactive Ion Etching (DRIE) and anodic bonding techniques. To evaluate the preconcentration capacity of the fabricated µ-preconcentrator, thick film gas sensors based on pure and doped SnO2 and WO3 sensitive layers were fabricated on micro-hotplate substrates.

2.1 Fabrication of the µ-preconcentrator

The µ-preconcentrator presented in this work was fabricated on silicon by means of DRIE and anodic bonding techniques, as follows: (a) Silicon wafers <100>, p-type, 500 µm thick, was used; (b) Spiral-like column configuration was defined on the silicon by DRIE; (c) Aluminum metallization and heater patterning was performed; (d) The wafer containing the µ preconcentrator was sealed to a Pyrex cover plate by anodic bonding. Fig. 1 shows a schematic view of the fabricated structure.

Fig. 1. Schematic diagram of the spiral µ-preconcentrator. The device dimensions are 10 cm × 300 µm × 300 µm (LxHxW).

Once the structure which was going to house the absorbent material was fabricated, an exhaustive study related to different absorbents was made. There is a wide range of commercially available materials to be used in the field of

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thermal desorption. Choosing the most appropriate one for the desired application is a difficult task. Differences between adsorbents are based on their capability to efficiently retain and release the test compounds. In the strong adsorbents, compounds are retained to the point that they are not efficiently released during the first desorption process and a portion of them can be observed when a subsequent desorption is carried out. Recoveries of 80% or greater are typically considered acceptable in most of the thermal desorption methods [14].

In the miniaturized pre-concentrator presented in this work the adsorbent selected is Carbopack X. (60/80 mesh, 0.18-0.25 mm particle diameters) with a specific surface area of 240 m2/g. This area is very high if compared to other graphitic carbons and is due to the introduction of small pores in the adsorbent [15]. This adsorbent efficiently releases benzene, toluene or xylene when it is heated above 200°C. Another advantage is that this graphitic solid adsorbent is hydrophobic, which guarantees more reliable results in the detection of the released compounds by using semiconductor metal oxide gas sensors, normally sensitive to humidity. In order to provide the temperature pulses needed for the desorption process the Al heater patterned on the bottom of the column was used. The heater is able to provide quick temperature pulses of 200ºC, needed for releasing the compounds absorbed by the Carbopack X. By patterning the heater inside the column, the power consumption was reduced 3 times in comparison to patterning the heater on the Pyrex cover [16]. The carrier gas is driven along the spiral preconcentrator by means of µ -needles placed at the entrance and output of the device. These needles were glued inside the channel by using Ag conductive paste, which provides electrical contact to the Al heater placed in the bottom of the column.

The pre-concentrator was filled with 400 µg of Carbopack X which was previously activated by an annealing in nitrogen atmosphere at 300ºC for 5 hours. As the channel width/depth is 300 µm, the particles of Carbopack X are distributed uniformly in the channel, avoiding the stack formation. The particle size of approximately 200 µm evades restriction of the airflow through the channel and reduces the possibility of clogging. The final structure of the fabricated preconcentrator together with the flow/electrical connections is presented in Fig. 2.

a)

b)

c)

Fig. 2. a) Front view of the spiral µ-preconcentrator full with Carbopack X with the details of inlet/outlet and micro-needles

connected. b) Close view of the channels filled with absorbent c) Active material.

2.2 Fabrication of the micro-hotplate gas sensors

In the last years the interest in micromachined gas sensors has become more and more important. It can be explained by the advantages that micromachining technology offers for gas sensing applications. Principally, low power consumption

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I

II IL:III I

II

L -

(e.g. tens of mW at operating temperatures around 300°C), fast thermal response (in the range of ms), on-chip signal processing electronics, integrated µ-sensor arrays, small size and low cost. The deposition of the gas-sensitive layer is one of the most important aspects in the preparation of gas sensors. Trying to improve the sensor sensitivity, many efforts have been addressed on the addition of different noble metals to the sensing layer and, to enhance the catalytic reactions between target gases and the active area of the device [17].

In order to evaluate the preconcentration capacity of the fabricated µ-preconcentrator, thick film gas sensors based on pure and doped SnO2 sensitive layers were fabricated on µ-hotplate substrates (Fig. 3). The technological process needed to fabricate the sensors had the following steps: (a) Deposition of the membrane layer; The dielectric membranes consist of a 300 nm thick Si3N4 layer deposited by low-pressure chemical vapor deposition (LPCVD) and implanted with boron ion to reduce their inherent stress; (b) Deposition and patterning of a POCl3-doped polysilicon heating meander with a resulting sheet resistance of 18 Ω/sq and a temperature coefficient of resistivity (TCR) of 6.79 x 10-4 C-1. The heater was also used as a temperature sensor; (c) Deposition of 800 nm thick SiO2 layer by plasma enhanced chemical vapor deposition (PECVD) to electrically insulate the heater from the electrodes and the sensing film; (d) Opening of contacts for the heater bonding pads through the insulation layer; (e) A Ti/Pt 20 nm /200 nm thick layer were deposited by sputtering and patterned by lift-off to define the electrodes and heater bonding pads. For the electrodes an interdigited configuration was chosen, width and separation were fixed at 20 and 100 µm respectively; (f) Patterning of the backside etch mask in the nitride layer; (g) Deposition of the sensing layer onto the electrode area; (h) Backside silicon etching with KOH at 70 °C (40% wt.) to create the thermally-insulated membranes; (i) Mounting of the chips in a TO package and wire bonding.

Fig. 3. Sensor matrix composed by 4 sensors (SnO2, SnO2+1wt%Pt, SnO2+1wt%Au, SnO2+1wt%Pd).

For this study, different sensor matrices have been developed. Initial study showed that tin dioxide has better affinity to benzene, than tungsten trioxide. Furthermore, between the different doping used, Pd showed optimal performance.

3. RESULTS AND DISCUSSION The concentration capacity of the fabricated device was obtained by using two different methods - GC/MS equipment (for numeric calculation of the preconcentration factor) and micro hotplate gas sensors (for estimation of the real functionality of the µ-preconcentrator).

3.1 Measurement procedure

As already stated, for numeric calculation of the preconcentration factor (CF) obtained with the preconcentrator presented in this study, a GC/MS Shimadzu-QP5000 equipment with a 6 way valve was used. A schematic view of the measurement system can be observed in Fig. 4.

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CO2 +

C6H6

GC/MS Equipment

µConcentrator

1 He

2

CO2

Injection 6 port valve

Gas Sensors

1 3 port valve

2

Fig. 4. Schematic view of the characterization system.

The experimental procedure consisted of adsorbing benzene and then desorbing it into the entrance of the GC-MS system. At the beginning, a monitoring of the bottle concentration, which consists in five consecutive measurements called blanks, was performed. The values obtained give us the average benzene concentration in the calibrated bottle. In the second step, consecutive series of three adsorption/desorption processes, using the µ-preconcentrator, were made.

A continuous 20 or 35 sccm flow of 150 ppb of benzene diluted in CO2 was passed trough the active material for 10-25 minutes. Afterwards, a 30 seconds electrical pulse was applied to the heater in order to reach the desorption temperature (200ºC). The power consumption at this temperature is less than 1W. The desorption peak was monitored with the GC-MS acquisition software. Fig. 5 shows a typical chromatogram of desorption.

Fig. 5. Typical chromatogram of blank/desorption.

The small peak corresponds to the blank obtained while the bottle concentration was monitored. Comparing the average peak area of the blank and its corresponding desorption allows us to define the CF according to the following expression:

60ml 20ml Reference level

Intensity

Ret. Time (min).1 2 3 4 5

100e3

200e3

300e3

400e3

35 sccm

20 sccm

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CF = Ad / Ab, where Ad is the integrated area obtained from the desorption measurement, and Ab is the area achieved with the measurement of the concentration in the bottle.

3.2 Results obtained via GCMS characterization

The results obtained showed excellent repeatability. From the results presented in Table 1 it could be seen that higher flows and longer collection times result in improved concentration factors.

Table 1. Measurement results obtained via GCMS characterization of the fabricated preconcentrators.

Collection time (min)

Concentration factor Flow 20 sccm

Concentration factor Flow 35 sccm

10 93 147 15 118 189 20 194 286

The flow rate during the absorption and during desorption was kept unchanged in order to obtain comparable results. Preconcentration factors of up to 286 were obtained during the characterization of the µ-preconcentrator for the benzene flow and times considered. The results suggest that even after charging the device during 20 minutes, the breakthrough volume (BTV) is still not reached. Additional studies will be performed in order to obtain the BTV value for the fabricated structure. These studies will permit us to define the most appropriate combination of time and flow, so that maximum concentration factors can be obtained. The CF’s achieved, allow extending the response of the semiconductor gas sensors to the benzene detection in the ppb range.

3.3 Results obtained via gas-sensor characterization

Once, the concentration factors for each test condition were obtained, measurements using the µ-hotplate gas sensors with thick screen-printed sensitive material were performed. In this case instead of connecting the outlet of the preconcentrator to the GC-MS, we attached it to a chamber containing two micro array sensors. The sensor matrix was composed by sensitive layers of tin oxide (pure and doped with 1% in weight of Au, Pt and Pd) deposited on silicon micromachined substrates with low heat capacity [18]. Ambient temperature was kept constant (30°C) during the measurement phase. A typical sensor response to benzene desorption could the observed in Fig. 6.

2,4

2,5

2,6

2,7

2,8

2,9

3

0 100 200 300 400 500 600 700 800 900

Time, sec

Vol

tage

, V

A

B

Fig. 6. Sample sensor response (without preconcentrator - zone A, with preconcentrator - zone B).

The sensor response was monitored with a home made program. Fig. 6 (Zone A) presents the sensor response to 150 ppb of benzene. It could be observed that the sensor response Ra/Rg (where Ra is the sensor resistance in air and Rg is the resistance in the presence of gas) without preconcentrator is very low. Afterwards, a temperature pulse was applied to the

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preconcentrator for desorption of the benzene and the result obtained is shown in Fig. 6 (zone B). By calculating the Ra/Rg we can observe that the sensor response is clearly improved. Detailed information about the enhancement of the sensor response can be found in table 2. This value depends on the characteristics of the sensitive layer used in the sensor. In order to obtain comparable results of the desorption measurements made with a preconcentrator and sensors, the flow trough the sensors was kept equal to the flow used in the GC-MS (35 sccm).

Table 2. Measurement results obtained via sensors characterization of the fabricated preconcentrators.

Sensor type Sensitivity to 150ppb of C6H6 without preconcentrator

Sensitivity to 150ppb of C6H6 with preconcentrator

SnO2 1,15 7,4 SnO2 + 1% Au 1,25 9,85 SnO2 + 1% Pt 1,75 8,3 SnO2 + 1% Pd 2,85 10,35

4. CONCLUSIONS The proposed spiral µ-preconcentrator represents a important step for the realization of an integrated, portable and low-cost miniaturized device useful for a big number of applications that require high sensitivity and fast response time. The small size of the manufactured device enables its incorporation in an integrated GC/MS-gas sensor system in order to determine the concentration factor for different test conditions.

Although additional studies must be performed to achieve the BTV value of the fabricated structure, the results obtained showed high repeatability and preconcentration factors of up to 286 was found. These values allow the enhancement of the response of the semiconductor gas sensors to the ppb range for benzene detection. Up to 10 times of improvement in the sensors response has been achieved when using the preconcentrator-gas sensor set, best results were obtained with Pd doped tin dioxide sensors.

ACKNOWLEDGMENTS

Part of this work has been financially supported by the Spanish Ministry of Education and Science, project Integration of Intelligent Systems for Security-ISIS (TEC2007- 67962-C04-01/MIC).

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