Nanostructured Cr(N,O) based thin films for relative humidity sensing
Graphical abstract
Introduction
Relative humidity measurement is one of the most significant issues in various areas of applications such as instrumentation, automated systems, agriculture and climatology [1,2]. Indeed, relative humidity plays a significant role in a daily basis in activities ranging from biological processes [3,4] to many industrial processes [5,6], being a relevant parameter in corrosion [7,8], among others. To provide the specific atmosphere required in many industrial or biological processes, it is essential to monitor, detect and control humidity under specific conditions ranging from low to high temperatures or in mixtures with other gases by precise and provident sensors [9]. Several studies have demonstrated that the performance of a relative humidity sensor based on thin films is dependent on the surface chemistry and morphological properties of the sensing medium including porosity, surface area, pore size distribution, which can be tuned with specific fabrication technologies [[10], [11], [12], [13], [14]].
The majority of the thin films have been prepared mainly by Physical Vapor Deposition (PVD) methods using the conventional columnar growth (perpendicular to the substrate). Depositions at Oblique Angles, or sculptured thin films, is an original way of modifying many physical-chemical properties of thin films because a wide variety of morphologies are tailored only with a change in direction of the incident particle flux which enables to modify/control the columnar characteristics of thin films, for instance, by inducing zigzag columnar structures in the samples [15,16]. This feature also leads to variations of the physical-chemical responses and induces/improves specific functional properties [16,17] such as a change of the crystallographic growth orientation, material density [18,19], hardness [20], as well as the electrical resistive response, which is characterized by a large electrical noise in the columnar thin films [21,22]. This point can become a concern and demonstrates its inadequacy for high precision sensor applications. In the earliest efforts on oblique angle deposition (OAD), researchers observed polarization sensitive properties and anisotropy originating from the microstructure of the deposited films [23]. When the incident flow is made oblique, the asymmetry in the vapor distribution produces anisotropic shadowing, leading to the development of anisotropic properties in the grown films [24]. The anisotropy produced in OAD thin films appears in the form of anisotropic film properties such as stress, optical birefringence and dichroism [23], magnetic susceptibility [25] and fluid transport [23].
In this scope, it is to notice that few studies report on the relation between the relative humidity sensibility of chromium oxynitride (Cr(N,O)), the PVD-OA process, and the possible morphologies to obtain relative humidity sensors. Besides, Cr(N,O) is an interesting materials system since it can combine the properties of chromium nitride and chromium oxide, being harder than CrN [26,27] and corrosion resistant like Cr2O3 [28].
Thus, the present work focuses on the investigation of the relationship between the microstructure and the relative humidity sensing response of Cr(N,O) thin films deposited by DC-reactive magnetron sputtering at different reactive gas supplies of 15% O2 and 85% N2. To avoid the non-homogeneity associated with the PVD-OA deposition for thick films [18], here we growth films with periodically reversed flux angle (zigzag structure) [16,18]. It is to notice that a nanostructured material with high porosity and large surface area will facilitate the adsorption of water vapor on its surface giving great scope for enhancement in the sensitivity of a relative humidity sensor. The relative humidity response of the Cr(N,O) thin films prepared under different processing conditions was correlated with the microstructural morphology variations. The present results allow us to explore the chromium nitride and chromium oxide properties as humidity sensor with high sensibility/performance for distinct technological applications.
Section snippets
Thin films fabrication
The Cr(N,O) thin films were deposited at room temperature from a Cr target (dimensions 200 × 100 × 6 mm3 and a purity of 99.96 at. %) located at 70 mm from the substrate holder operating in a dynamic step mode to guarantee homogeneous depositions and uniformity on film thickness. This procedure was developed by using a customized vacuum reactor with a capacity of 60 dm3 equipped with a direct current power supply, Hüttinger Elektronik, model PFG 2500DC, connected to a rectangular magnetron. The
Morphological and structural characterization
Fig. 3 show the top and fractured cross-section view of the Cr-based thin films growth by PVD-OA with increasing O2/N2 flow, in order to evaluate the zigzag morphology and surface porosity. From the figure we observe that the increasing of O2/N2 flow during the PVD-OA growth process leads to morphological feature variations of the Cr(N,O) thin films, namely the columnar arrangement, including type, density, and surface characteristics. For pristine Cr thin films, well-defined zigzags structures
Conclusions
In summary, coatings based on chromium oxynitrides have been developed for humidity sensing applications by tailoring thin film microstructure and composition. The modification on the structural columnar structure, in which zigzag nanostructures has been induced by OA combined with reactive magnetron sputtering allowed us to reach samples with favorable structural and electrical properties for sensor applications. Increasing the amount of O in the composition allows to increase the sensitivity
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thank the Portuguese Foundation for Science and Technology (FCT) for the strategic funding UID/FIS/04650/2020 and the European Regional Development Fund under the Operational Programme “Competitiveness and Internationalization” - Research and Development (R&D) and Innovation to SMEs – R&D Individuals Projects, grant agreement n° 038397. A. Ferreira thanks the FCT for the contract under the Stimulus of Scientific Employment (CTTI-31/18 – CF (2) junior researcher contract). M.A.C.
References (41)
- et al.
Humidity sensors: principles and applications
Sensor. Actuator.
(1986) - et al.
Principles, methods and applications of the convective drying of foodstuffs
Food Bioprod. Process. Trans. Inst. Chem. Eng. Part C.
(1999) Water relations of food spoilage microorganisms
Adv. Food Res.
(1957)- et al.
The effect of relative humidity on the atmospheric corrosion of defective organic coating materials: an EIS study with a new approach
Corrosion Sci.
(2000) - et al.
Influence of the sputtering pressure on the morphological features and electrical resistivity anisotropy of nanostructured titanium films
Appl. Surf. Sci.
(2017) - et al.
Piezoresistive response of nano-architectured Ti x Cu y thin films for sensor applications
Sensors Actuators A Phys
(2016) - et al.
Glancing angle deposition to modify microstructure and properties of sputter deposited chromium thin films
Surf. Coating. Technol.
(2003) - et al.
Nanostructured functional Ti-Ag electrodes for large deformation sensor applications
Sensors Actuators, A Phys.
(2014) - et al.
Tuning electrical resistivity anisotropy of ZnO thin films for resistive sensor applications
Thin Solid Films
(2018) - et al.
Microstructure of Cr ( N , O ) thin fi lms studied by high resolution transmission electron microscopy
Thin Solid Films
(2017)
Corrosion resistance of chromium nitride and oxynitride layers produced under glow discharge conditions
Surf. Coating. Technol.
Characterization of chromium thin films by sputter deposition
J. Alloys Compd.
Thin films composed of Au nanoparticles embedded in AlN: influence of metal concentration and thermal annealing on the LSPR band
Vacuum
Optical and electronic properties of CrOxNy films, deposited by reactive DC magnetron sputtering in Ar/N2/O2(N2O) atmospheres
Surf. Coating. Technol.
Preparation and characterization of CrN x O y thin films: the effect of composition and structural features on the electrical behavior
Appl. Surf. Sci.
A comparative study of CrN x coatings Synthesized by dc and pulsed dc magnetron sputtering
Thin Solid Films
Oxidation properties of Cr(N,O) thin films synthesized by pulsed laser deposition
Nov. Mater. Process. by Adv. Electromagn. Energy Sources
Preparation of Cr(Nx,Oy) thin films by pulsed laser deposition
Thin Solid Films
Humidity sensors: a review
Sens. Lett.
Humidity Sensors Principle, Mechanism, and Fabrication Technologies: A Comprehensive Review
Cited by (3)
Flexible multifunctional hard coatings based on chromium oxynitride for pressure-sensing applications
2022, Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films