Elsevier

Vacuum

Volume 191, September 2021, 110333
Vacuum

Nanostructured Cr(N,O) based thin films for relative humidity sensing

https://doi.org/10.1016/j.vacuum.2021.110333Get rights and content

Highlights

  • Cr(N,O) thin films produced by Oblique Angle Deposition (PVD-OA).

  • ZigZag like structure induced by PVD-OA deposition.

  • Multifunctional high-performance Cr(N,O) coatings with humidity sensing characteristics.

  • The maximum sensibility of the Cr(N,O) coatings was found to be 1.87 × 10−2 ± 7.00 × 10−5 (RH%)−1.

Abstract

This work presents multifunctional coatings prepared by Physical Vapor Deposition with Oblique Angle deposition with relative humidity sensing capability. Thin films based on chromium oxynitride (Cr(N,O)) were prepared with zigzag nanostructures and varying the amount of nitrogen and oxygen in order to enhance the humidity response. The obtained Cr(N,O) thin films show an increasing amount of oxygen as the O2/N2 mixture flow increases, leading to a transition from chromium nitride to chromium oxide. Further, a change from a bcc-Cr to a fcc-CrN phase is obtained as the O2/N2 mixture flow increases, which is attributed to a Cr2N ditrigonal scalenohedral crystalline structure (β-Cr2N) with the presence of a CrO crystalline structure. The amount of oxygen present in the composition of the coating leads to high electrical resistivity, ranging from 1.27 × 10−6 ± 5.02 × 10−8 Ω m for the sample with 16 at. % of O to 4.50 × 109 ± 4.20 × 107 Ω m for the sample with 60 at. % of O. The highest sensibility to the relative humidity (RH) variation from 40% to 80% was 1.87 × 10−2 ± 7.00 × 10−5 (RH %)−1, being suitable for relative humidity sensing applications.

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)

  • T. Wierzchoń et al.

    Corrosion resistance of chromium nitride and oxynitride layers produced under glow discharge conditions

    Surf. Coating. Technol.

    (2000)
  • S. Wang et al.

    Characterization of chromium thin films by sputter deposition

    J. Alloys Compd.

    (2011)
  • R.P. Domingues et al.

    Thin films composed of Au nanoparticles embedded in AlN: influence of metal concentration and thermal annealing on the LSPR band

    Vacuum

    (2018)
  • R. Mientus et al.

    Optical and electronic properties of CrOxNy films, deposited by reactive DC magnetron sputtering in Ar/N2/O2(N2O) atmospheres

    Surf. Coating. Technol.

    (2005)
  • R. Arvinte et al.

    Preparation and characterization of CrN x O y thin films: the effect of composition and structural features on the electrical behavior

    Appl. Surf. Sci.

    (2011)
  • J. Lin et al.

    A comparative study of CrN x coatings Synthesized by dc and pulsed dc magnetron sputtering

    Thin Solid Films

    (2009)
  • J. Inoue et al.

    Oxidation properties of Cr(N,O) thin films synthesized by pulsed laser deposition

    Nov. Mater. Process. by Adv. Electromagn. Energy Sources

    (2005)
  • T. Suzuki et al.

    Preparation of Cr(Nx,Oy) thin films by pulsed laser deposition

    Thin Solid Films

    (2002)
  • C.Y. Lee et al.

    Humidity sensors: a review

    Sens. Lett.

    (2005)
  • H. Farahani et al.

    Humidity Sensors Principle, Mechanism, and Fabrication Technologies: A Comprehensive Review

    (2014)
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