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Phys. Status Solidi A 206, No. 6, 1343–1347 (2009) / DOI 10.1002/pssa.200881106 p s sapplications and materials science

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Investigation of humidity adsorption in porous silicon layers

Andras Kovacs*, 1, Dirk Meister**, 2, and Ulrich Mescheder1

1 Institute for Applied Research and Faculty Computer & Electrical Engineering, Hochschule Furtwangen University, Robert-Gerwig-Platz 1, 78120 Furtwangen, Germany

2 Rubotherm Präzisionsmesstechnik GmbH, Universitätsstraße 142, 44799 Bochum, Germany

Received 28 March 2008, revised 23 January 2009, accepted 3 February 2009 Published online 27 March 2009

PACS 68.08.–p, 81.05.Rm, 81.07.–b, 81.65.Mq, 85.85.+j ** Corresponding author: e-mail kov@hs-furtwangen.de, Phone: +49 7723 920 2516, Fax: +49 7723 920 2633 ** e-mail Dirk.Meister@rubotherm.de, Phone: +49 234 70996 23, Fax: +49 234 70996 22

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Tight control of the sensitive layer properties and adjustment of these properties to specific demands are in general key questions for successful sensor developments. In the case of humidity sensing with a po-rous material (capillary condensation) the most important properties are porosity, pore size and pore size distribution, pore morphology and wetting behavior. However, during fabrication of a sensor device the sensitive layer will often undergo a significant change of these properties by struc-tural, material and functional modifications within the po-rous material. For the investigation of humidity adsorption in PS layers different measurement techniques are avail-able. Physical gas adsorption at 77 K (using N2) and hu-midity adsorption are widely used for the characterization of micro- and mesoporous materials [1–5]. Impedance spectroscopy has been used for the investigation of PS

properties for humidity sensor applications [6–8]. Main is-sue of this work is to investigate the correlation between the different measurement methods and material properties. Thus, combining for the first time very different charac-terization techniques, a deeper understanding of the ad-sorption mechanisms in microporous materials is achieved. The results provide valuable information for the develop-ment of PS based humidity sensors as shown in the ab-stract figure [9–11]. 2 Experimental results Porous silicon samples were prepared by anodizing p-Si wafers (10–16 Ω cm) in HF (49 wt%):C2H5OH = 1:1 solution. Current density was varied to achieve structural changes in porous silicon layers. The range of the used current density was 2.5 mA/cm2–30 mA/cm2. The thickness of most of sam-

The dependence of humidity adsorption in porous silicon(PS) layers on the fabrication conditions (current density andoxidation) has been investigated. As-prepared and addition-ally oxidized PS layers have been investigated using physicalgas (N2) adsorption measurement at 77 K, water vapor ad-sorption measurements at room temperature, impedance spec-troscopy and contact angle measurement. Pore size distribu-tions and specific surface areas have been calculated usingthe BJH and BET model. Additional oxidation of porouslayer promotes the humidity adsorption and improves thewetting compared to as-fabricated porous Si layers. The cor-relation between adsorption isotherms, electrical parametersand surface effects are presented and discussed in respect tothe use of PS for humidity sensing. Oxidation of PS layers

improves the sensitivity of PS based humidity sensors con-siderably.

Porous silicon based humidity sensor integrated in an airquality measurement system with signal processing.

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ples was 20 µm. To compensate the influence of current density on anodization rate different anodization times were used. Only the sample anodized at 2.5 mA/cm2 had a thickness of 10 µm due to the very low anodization rate in this case. Adsorption in the porous layer (mean value of pore size 2.1–2.8 nm, pores size distribution typically 1.5–6 nm, porosity 60–70%) depends strongly on the sur-face conditions and on the properties of the adsorbed film. Therefore, as-prepared and oxidized porous layers were prepared to investigate the corresponding adsorption prop-erties. Oxidized PS probes were fabricated using a two step thermal oxidation process at 400 °C (60 min) and 780 °C (30 min) in order to reduce the oxidation-induced stress. On a Si surface such an oxidation process will result in an oxide thickness of 4 nm. However, based on the BJH-analyzes [12] and the derived change of the mean value of pore size by oxidation (0.62 nm) it can be concluded that for microporous Si the oxidation rate is much smaller than for c-Si. Three independent measurement techniques were used to investigate the important material properties for humid-ity adsorption in porous silicon layers. N2 adsorption measurements at 77 K provide detailed information about the structural properties of porous layers such as specific surface area, pore size and pore size distribution and mor-phology. Water adsorption isotherms yield information about the wetting and hydrophobic or hydrophilic proper-ties of porous materials. Impedance spectroscopy was used to measure the sensitivity of electrical parameters (imped-ance) on humidity and to find the correlation between ad-sorption effects and electrical layer properties used in sen-sor applications. The relation between surface and volume wetting can be analyzed by comparing adsorption iso-therms and contact angle measurements. Contact angle measurements provide additional information about hydro-phobic or hydrophilic properties of layer surfaces. The re-sults of these very different characterization techniques are linked together to obtain a deeper understanding of the relevant material properties and their modification during a typical sensor process. 2.1 N2 adsorption measurements at 77 K N2 adsorption measurements at 77 K were performed using Belsorp-mini, co. Rubotherm/Bel. Figure 1 shows the measured N2 adsorption isotherms of as-prepared (as-formed) and of oxidized PS as function of relative pressure (p/p0) for several layers prepared at different anodization current densities. The N2-adsorption isotherms of as-formed and of oxidized PS layers are very similar. As-formed PS layers adsorb slightly more N2 than oxidized PS layers (about 20% more at p = p0). In respect to the BET [13] classification the adsorption isotherms of as-prepared porous Si can be described as type V, for oxidized a behav-iour between type IV and type V is observed. As comple-ment, also desorption isotherms were investigated. In all cases, hysteresis loops of type H2 in the 1985 IUPAC clas-sification [14] has been found. This corresponds to the data

Figure 1 N2 adsorption (full line, filled symbols) and desorption (dotted line, open symbols) isotherms of as-prepared (a) and oxi-dized (b) PS at T = 77 K. Samples were anodized with current densities ranging from 2.5 mA/cm2 to 30 mA/cm2. Va is the ad-sorbed nitrogen volume in cm3 (STP)/g.

reported in [15] for similar porous Si samples, but with pores open at both ends. The hysteresis loops of as-prepared and of oxidized samples are similar. It should be noted that adsorption takes place only in the porous layer of the samples. In Fig. 1 the Va values are normalized to the total sample weight including silicon substrate (thick-ness of PS = 20 µm, thickness of substrate = 380 µm).

2.2 Water adsorption measurements Water va-por adsorption measurements at room temperature were performed using Belsorp-aqua, co. Rubotherm/Bel. Fig-ure 2 shows the measured water adsorption isotherms of as-formed and oxidized PS as function of relative pressure (p/p0) for several layers formed at different current densi-ties. As-prepared PS layers have an about ten times smaller active water adsorption volume than oxidized PS layers. Additionally, the character of the p/p0-dependance is changed drastically by oxidation. Whereas as-prepared po-rous Si is of type III (BET classification), oxidized porous Si shows for water a type V character as for N2. Due to extremely long measurement times, we concentrate on ad-

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Figure 2 Water vapor adsorption isotherms of as-formed (a) and oxidized (b) PS at room temperature. Same samples as in Fig. 1.

sorption measurements. However, one sample of as-formed PS was also investigated in respect to desorption behavior. Within the measurement accuracy, no hysteresis was observed for as-prepared samples using water as ad-sorbate. It should be noted that with the sensor set-up de-scribed in Section 3 desorption can be improved electri-cally by using the integrated heater. For the use of porous silicon as sensitive layer in hu-midity sensors hydrophilic properties and a large specific surface area (SSA) for water adsorption are of particular importance. Figure 3 shows the specific surface area ratio for water vapour and nitrogen adsorption derived from the data plotted Figs. 1 and 2. The specific surface area ratio of water/nitrogen is only about 2% for as-prepared PS and about 30–40% for oxidized PS. Oxidized PS shows significantly larger water adsorption. The specific surface area ratio of oxidized PS is increasing slightly with decreasing current density, i.e. with decreasing pore size. Due to the very low specific volume Va the specific surface area ratio could not be evaluated using BET theory in the case of low current densities (<20 mA/cm2) for as-prepared PS. However, the large influence of oxida- tion for more effective water adsorption is clearly demon-strated.

Figure 3 Ratio of specific surface area (SSA) derived from water and nitrogen adsorption isotherms.

2.3 Impedance spectroscopy measurements Im-pedance measurements have been carried out with different samples of PS to determine the sensitivity of electrical layer properties on relative humidity. Different to the sen-sor design described in [11] where an area of interdigitated electrodes with electrode separation of some micrometers were used, here Al surface electrodes with large electrode distance (1 cm) were chosen to reduce the influence of sur-face adsorption effects and to increase volume effects in

Figure 4 Impedance of as-prepared (a) and oxidized (b) PS as function of the humidity; measurement frequency 100 kHz. The samples were prepared with the same current densities as those shown in Figs. 1 and 2.

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the PS layer. Figure 4 shows the dependence of the imped-ance on humidity and on current density used during PS formation. The relative impedance change is significantly larger for the oxidized PS (b) than for the as-formed PS (a). As-formed PS has a very low sensitivity (slope) when us-ing large electrode distances. It should be noted that the sample prepared at 2.5 mA/cm2 was 10 µm thick whereas all other samples had a thickness of 20 µm. It is obvious that the anodization current density has a considerable in-fluence of impedance value and also on slope (oxidized PS) which is a result of dependence of pore size distribu-tion, pore morphology and porosity on current density.

2.4 Contact angle measurements Contact angle measurements were used to measure the surface wetting of PS layers using Millique (double filtrated) water. Figure 5 shows the difference of contact angles immediately after wetting for as-prepared and oxidized PS anodized at 30 mA/cm2. The contact angles are in the range of 75°–105° for as-prepared PS and of 20°–25° for oxidized PS (Fig. 6). As-prepared PS shows strong a hydrophobic propertyes whereas oxidized PS has hydrophilic properties. Contact angle depends slightly on the used anodization current density for as-prepared PS. Contact angle is increasing slightly with increasing current density, i.e. with increasing pore size and corresponding surface roughness. Oxidized PS samples do not show a significant dependence of con-tact angle on current density. 3 Sensor application Porous silicon can be used in a wide range of sensor applications as active material. In [10] an air quality sensor is presented where PS is used for humidity sensing. In this system (see the abstract figure) the sensor array integrates three different types of sensors (gas sensor, humidity sensor and temperature sensor) on a single chip. As a result, the quantitative measurement of gases’ concentrations and compositions, humidity and temperature can be done on a single miniaturized sensor array. For the humidity sensing module a porous silicon based humidity sensor was developed and realized with heating elements (Joule’s heating) integrated on a hotplate. The hotplate technique is a very promising solution for the monolithic integration of porous silicon based sensors with metal oxide gas sensors or other sensor types. Especially it

Figure 5 Contact angle measurement of as-prepared (a) and oxi-dized (b) PS layer with 30 mA/cm2 anodization current.

Figure 6 Contact angles of as-formed and oxidized PS layers directly after wetting as function of current density.

provides the possibility for a dynamic principle of humid-ity sensing: thermal desorption of water out of the pores provide better reproducibility (refresh function) of the sen-sor characteristics and allows to derive the relative humid-ity (rh) level even at high rh where all pores are already completely filled by capillary condensation. High sensor sensitivity can be achieved by optimization of the sensor design and by improvement of adsorption properties of the sensitive layer. Interdigitated electrodes with reduced elec-trode distance (µm instead of cm as in the set-up used for Fig. 4) further increase the sensitivity of impedance on relative humidity. An optimized surface treatment of the PS layer, for example thermal oxidation, and an optimiza-tion of the pore size distribution further increase the spe-cific surface area of the PS layer for humidity adsorption and thus the sensor sensitivity. 4 Discussion and conclusion The performance of humidity adsorption of PS layer was analyzed using ad-sorption isotherms with water vapor, impedance spectros-copy and contact angle measurements. N2-adsorption measurements at 77 K provide structural material proper-ties of the porous silicon layers such as specific surface area, pore size and pore size distribution. The specific sur-face area of the porous materials is calculated using the BET theory in the low pressure range (p/p0 < 0.35). From N2 adsorption isotherms the extracted specific surfaces ranges from 500 m2/cm3 to 700 m2/cm3 and do not depend significantly on oxidation treatments. In contrast, water vapor adsorption strongly depends on surface treatment (oxidation). The form of the water vapor adsorption iso-therms in Fig. 2a (type III in IUPAC classification) shows that the interaction between sample surface and water va-por is weak for as-prepared PS. Complete capillary con-densation is not possible in as-prepared PS and therefore saturation (all pores are filled with water) can not be achieved. On the other hand as-formed PS shows pro-nounced hydrophobic behavior. Figure 2b clearly shows, that for p/p0 > 0.5 capillary condensation takes place in

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oxidized PS layers. Therefore the specific surface area for water is very low for as-formed PS (approximately 10 m2/cm3) and is increased drastically by oxidation (200–250 m2/cm3). Compared to the results of N2 adsorp-tion (500–700 m2/cm3) only 28–43% of the specific sur-face area is activated in the case of water adsorption (Fig. 3). Electrical measurements, mainly impedance spec-troscopy are used in sensor systems to convert the physical effects in electrical signals. Effective adsorption in porous silicon layers results in increasing sensor sensitivity. The relative change of impedance between 0% and 100% rela-tive humidity is as small as 5% for as-prepared PS but as large as 90% for oxidized PS (Fig. 4). These results dem-onstrate the advantage of oxidized PS for humidity sensing [11]. Contact angle measurement is a method to analyze surface wetting of different layers and gives direct infor-mation about the hydrophilic or hydrophobic behavior of surfaces. Contact angles are very different for as-prepared PS and oxidized PS. The contact angles are in the range of 75°–105° for as-prepared PS and 20°–25° for oxidized PS (Fig. 6). The relative strong dependence of the hydropho-bic properties of as-prepared PS (upper curve in Fig. 6) on anodization current density and thus on morphology of the PS layer is almost completely eliminated by oxidation. In the latter case the morphology is of minor importance for the overall hydrophilic properties of the oxidized layer. From our results we can conclude that the macroscopic wetting behavior of the surface is correlated to the micro-scopic wetting behavior within the pores which is a volume effect. The volume effects in the porous layer contribute to its specific surface area and surface properties. All three independent measurement techniques support the assump-tion that thermal oxidation promotes the humidity adsorp-tion in the PS layer. As-prepared PS shows rather hydro-phobic and oxidized PS rather hydrophilic properties. The use of different measurement techniques gives more de-tailed information about the correlation between adsorption properties, electrical parameters and surface effects in PS. The investigation of the humidity adsorption in PS layers

can be applied to the optimization of sensors based on po-rous Si, especially of humidity sensors.

Acknowledgements The authors would thank B. Müller, HFU Furtwangen, J. Kritwattanakhorn, now with Infineon Tech-nologies AG, Regensburg, and J. Viertel, IMTEK, Freiburg for valuable discussions and technological support.

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