Fabrication of porous anodic aluminium oxide layers on paper for humidity sensors

Abstract

The fabrication process of anodic aluminium oxide (AAO) on a paper-based substrate is challenging because the use of a highly economical, conformable and ecological substrate. This process is based on a simple, cost-effective, self-ordering anodization of aluminium, which yields a vertically aligned and a highly ordered nanoporous structure. The manufacturing process on a flexible substrate with the use of phosphoric acid is described, and a capacitive sensor produced on this layer is investigated versus the humidity concentration and the frequency.

The AAO morphology and the structure obtained were analysed, and the electrical properties of the sensing layer were investigated, using a 4192A LF network analyser in a climatic chamber. The surface conduction mechanism and the effect of humidity on the dielectric properties of a porous anodic aluminium oxide are discussed.

Introduction

Humidity and temperature are the most frequently measured physical quantities in the measurement sciences [1]. The development of humidity sensors has been researched because of the great attention given by the healthcare industry, meteorology, air conditioning systems, food quality monitoring, and industrial and agricultural applications [2], [3], [4]. The fabrication of ultra-sensitive sensing nanomaterials has received considerable attention in recent years through their unique physical and chemical properties [5], [6]. The porous anodic aluminium oxide material prepared by the self-ordering synthesis based on electrochemical anodization has great potential for bio-electronic sensing devices. Also, this material also has been extensively used as a host or template structure for the fabrication of diverse nanometer devices, such as magnetic, photonic and nano-electronic devices [7], [8], [9]. Since the work of Masuada [10], the porous anodizing process has been extensively studied and widely used. From the use of the three main acids (phosphoric acid, oxalic acid and sulphuric acid) and untilize mixed solutions, such as sulphuric acid–ethanol (H₂SO₄–C₂H₅OH) or oxalic acid–ethanol (H₂C₂O₄–C₂H₅OH), the pore opening can range from 4 nm to 500 nm, and the pore length can range from 10 nm to several microns, with pore density varying from 1.0 × 10⁹ cm⁻² to 1.0 × 10¹¹ cm⁻² [11], [12], [13]. With the two-step anodization process reported first by Masuada, the AAO layer made with a high degree of pore ordering improved the AAO nanostructuring surface and properties. Combining this process with additional wet chemical etching expands the opportunities for controlling these structural parameters in AAO. The structural parameters which define the porous layer are the pore diameter (d_p), the interpore distance (d_int), the pore length (L_p), and the oxide-barrier layer thickness (τ_obl). Fig. 1 shows the AAO structure model (top and cross section), which was first reported by Keller [14].

During these last 30 years, several studies have investigated the performance of the porous AAO humidity sensor and to understand the surface conduction mechanism. The studies reported by Nahar and several researchers mainly note the characteristic in which capacitance increases slowly with relative humidity until the relative humidity reaches 40% to 50% and then shows a sharp surge in the high humidity environment [15], [16], [17], [18], [19], [20]. The capacitance characteristic sensitivity on large humidity ranges depends on the diameter range of the nanoporous AAO layer and the change of capacitance is determined by the high dielectric constant at the interface of the absorbed water to AAO layer. The quality of a thin AAO layer is directly connected to the sensor's behaviour, which depends principally on the thickness of the porous layer, the density and the size of the pores, as well as the pore uniformity and circularity. Other technological manufacturing processes such as the concentration and the temperature of the electrolyte, and the current density can also influence the sensor's characteristics [2], [21]. Previous studies found that the electrolyte anions incorporated into the oxide layer play a decisive part in the carrier-transport mechanism. Nahar et. al. had largely studied the effect of humidity on the variation of dielectric properties of porous anodic aluminium oxides. According to these investigations, the electrical properties of the humidity sensor based on the AAO layer are governed by the surface conductivity of the AAO layer and the conductivity of the water in the pores. The model proposed that, at low humidity, the phonon-induced electron tunneling between donor water sites and, at high humidity, the protonic conduction is responsible of the variation observed.

The growth of the AAO layer has been mostly conducted on aluminium sheets measuring hundreds of micrometers in thickness. Different studies presented aluminium anodization on different substrates, such as silicon, glass and plastic, with evaporated or sputtered aluminium thin film [22], [23], [24], [25], [26], [27]. Recently, the production of scientific papers on flexible electronics is increasingly important. These studies note the intrinsic properties of this type of substrate, such as foldability, flexibility, conformability and easy tearing for electronic systems [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]. Actually, lots of research has been performed using paper substrates [38], [39], [40], [41], [42], which represent a good candidate for flexible electronics, but the techniques used are very large scale and different: screen printing processes, thermal evaporation, flexography, coating techniques, ink printing and many news processes. Whiteside's group developed technologies for rapid prototyping of electronic circuits for applications in consumer electronics, packaging, for uses in the military and homeland security, for applications in medical sensing or low-cost portable diagnostics. With different processing methods, they demonstrated many electronic devices on paper, such as piezoresistive MEMS sensors or microfluidic devices, for analysis of a number of compounds relevant to human health, such as glucose, cholesterol, lactate and alcohol in blood or urine [40], [41], [42], [43], [44]. Thus a paper with multiple benefits offers a set of properties that are completely different from the properties of plastics, such as has presence everywhere (ubiquitous in modern society), has biodegradability, has fast and low cost accessibility and also has easy use (can be trimmed with scissors or perforated for easy tearing). Because of the interest to produce electronic components (sensors, electronics circuits, etc.) that would be easily adaptable to everyday products, the substrate used for our applications are common papers that can be found in typical office or stationary shops.

This article demonstrates that paper would be a good candidate to produce humidity sensors with thin AAO film made by a potentiostatic anodization method. Structural characterizations (SEM and XPS measurement) of the AAO layer fabricated and the electrical parameters (capacitance versus humidity and frequency) of the humidity sensor are presented, before the discussion about these results.