Copper Oxide Thin Films for Sub-Ppm Acetone Detection Obtained By Glancing Angle Magnetron Sputtering Deposition
Authors: Rydosz Artur 
Published: July 2021

Introduction
The number of gas-sensing applications increases year by year including industrial application as well as new approaches such as medical diagnostics, where gas sensors are used for measurement of volatile organic compounds present in the exhaled human breath. Over 3500 various VOCs have been already detected in the exhaled human breath, and a single breath consists of 500 various VOCs [1]. Apart from the conventional breath alcohol detectors, the novel applications appeared such as the detection of biomarkers [2]. One of the promising compounds that can be used as a biomarker is acetone, which is considered to be the biomarker of diabetes since people with diabetes tend to have higher acetone detection that healthy people [3,4]. The exhaled acetone is usually in the range of 0.2 - 0.8 ppm and over 2.5 ppm for people without and with diabetes, respectively. In order to measure such low, sub-ppm, concentrations, laboratory systems are applied including GC-MS [5], PTR-MS [6], and recently e-noses [7] as well as single sensors [8].
In this paper, the investigation results on the copper oxide (CuO) deposited by utilization of magnetron sputtering technology with glancing angle technique are presented, where CuO was deposited at various parameters such as deposition angle, argon/oxygen ratio. The responses to various sub-ppm acetone concentrations are presented.
CuO deposition
The gas-sensing applications of CuO have been previously confirmed and described [9]. Here, the CuO thin films were deposited by the UHV magnetron sputtering system from Kurt J. Lesker with a glancing angle deposition technique realized by utilization of a commercial ECR manipulator, which allows us to change the deposition angle, substrate rotation, and it has integrated heater that can be heated up to 850oC. The base vacuum pressure and deposition vacuum pressure were 0.001 Pa and 0.3 Pa, respectively. The temperature was fixed at 200oC, the power at 100W, the argon/oxygen ratios were 33%, 50%, 66%, 80% and 100% (pure oxygen without argon). After deposition, the samples were annealed at 400oC for 4 hours in the air.

Gas-sensing measurements

The developed sensors were tested in the dedicated measurement setup based on the quartz-tube oven with gas-dosing lines, where various target gas concentrations are obtained by utilization of mass flow controllers 1179B (MKS Instruments, USA) and acetone canisters with 5 ppm of acetone (Air Products, UK). The gas sensor response (S) was defined as resistance ratio S=(Rgas-Rair)/Rair, where Rgas and Rair are electrical resistances in gas and air, respectively.

Results and Conclusions

The sensors have been measured under exposure to acetone in the 0.25–2.5 ppm range, which covers the exhaled acetone range. The results have confirmed the influence of the glancing angle deposition for gas detection, in this case for acetone detection. The highest responses have been obtained for samples deposited at 85o (Fig.1b), however, the SEM verification will be carried out to compare the morphology and nanorods distribution. Moreover, the deposition system allows us to deposit the copper oxides at various argon/oxygen concentrations including deposition in pure oxygen. The results presented in Fig.1a shows, that the highest responses have been obtained for samples deposited at 20%/80% Ar/O2 ratio. It has been underlined, that the samples were annealed after deposition and preheated for 4h at 300oC, 350oC, and 400oC before the measurements. Further investigations are focused on choosing the optimal deposition conditions for acetone-sensors as well as long-term stability tests are performing.

Fig.1. The sensors’ responses under exposure to acetone in the 0.25–2.5 ppm range at various operating temperatures for gas-sensing layer deposited at various (a) oxygen concentrations; (b) deposition angles.

References

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Figure 1