CIP (cleaning-in-place) suitable “non-glass” pH sensor based on a Ta2O5-gate EIS structure

doi:10.1016/j.snb.2005.03.053

Sensors and Actuators B: Chemical

Volumes 111–112, 11 November 2005, Pages 423-429

https://doi.org/10.1016/j.snb.2005.03.053 Get rights and content

Abstract

“Non-glass” pH-sensitive capacitive EIS (electrolyte–insulator–semiconductor) sensors consisting of a Ta₂O₅-gate have been investigated in terms of their CIP (cleaning-in-place) suitability. After optimisation of the process parameters, even after 30 CIP cycles, the sensors show a clear pH response and nearly linear calibration curve. The video-microscopic and scanning electron microscopy investigations do not show any visible degradation or destruction of the Ta₂O₅ films.

Introduction

In-line continuous measurement of pH in many batch processes has been still a critical task, because of the inability to use conventional glass pH electrodes in products and the damage of the electrodes from the CIP (cleaning-in-place) or SIP (sterilise-in-place) procedures. Recent progress in the ISFET (ion-sensitive field-effect transistor) design and technology, especially in encapsulation and packaging technology allowing their reliable and cost-effective mass production, made ISFETs very attractive for industrial production and for widespread application as an alternative method to conventional pH glass electrodes [1]. Nowadays, in many in-line process-monitoring systems in biotechnology, food, pharmaceutical and cosmetic industries as well as in water purification and wastewater fields, the breakable glass pH electrodes are gradually replaced by non-glass, unbreakable pH sensors based on ISFET devices [2], [3], [4], [5], [6], [7], [8]. Thus, an in-line pH measurement is now possible in numerous areas of food industry technology for the first time, including applications involving the production of dairy products, mustard, tomato sauce, ketchup, jam, etc., where measurements with the pH glass electrode were formerly restricted or prohibited due to the risk of broken glass getting into the process. Some of these applications require frequent cleaning and sterilization and are characterised by fast temperature changes and high flow velocities [1]. In addition, for applications in foodstuffs, the sensor materials should meet the requirements of the Food and Drug Administration (FDA).

According to estimations, more than 10% of all glass electrodes are replaced due to accidental glass breakage during handling [5]. Resistance to breakage is the most obvious feature of the ISFET compared to the glass electrode. The virtually indestructible and unbreakable ISFET sensors eliminate the problems of delicate glass sensors. No matter how small, glass fragments can bring the batch process to a halt. In addition, by allowing a direct installation into the process, costs for laboratory analysis of sample lines are greatly reduced. Further advantages of ISFET sensors in comparison with the glass electrode are the short response time, the possibility of use in solutions with lower conductivity, the possibility of application at low temperatures and thus, suitability for pH monitoring in cold processes (e.g., monitoring of brines or coolants), less frequent calibration, and possibility of horizontal installation, which is preferred in all applications in food industry [3], [4]. Such non-glass ISFET-based pH electrodes are commercially available from some leading producers of electrochemical sensors, e.g., Mettler-Toledo, Endress + Hauser, Honeywell, IQ Scientific Instruments, Rosemount Analytical Inc., etc. [3], [4], [5], [6], [7], [8], which use different pH-sensitive materials (the exact compositions of the pH-sensitive materials and details of the fabrication have not been reported by the companies), ISFET layouts, encapsulation methods and electrode constructions [1].

The food industry is mainly using CIP agents to clean their process vessels: therefore, not only the encapsulation and packaging materials but also the pH-sensitive material used should be CIP-suitable. The ISFET-based pH electrodes have a limited lifetime in CIP solutions, because the pH-sensitive gate insulator can be irreversibly destroyed during the CIP-cleaning using highly caustic media and high temperatures. For instance, in 2% NaOH solution at 80 °C, the lifetime of an ISFET-based TopHit CPS 401 pH electrode (Endress + Hauser) drops to approximately 10–15 h [4]. Therefore, these sensors should only be used in combination with an additional automatic, retractable holder or chamber. Before the actual CIP cycle, the pH-ISFET sensor is automatically retracted out of the medium and positioned in a chamber, where it can be also calibrated and/or cleaned [3], [4]. The consequence is an increasing sensor price.

Due to the simplicity of the layout, the absence of a complicated encapsulation procedure (in many cases, sufficient electrical isolation of the sensor chip from the surrounding electrolyte solution can be achieved by simply using an O-ring) and thus, easier and cost-effective fabrication [9], [10], non-glass, unbreakable capacitive pH sensors based on EIS (electrolyte–insulator–semiconductor) structures can be a very attractive alternative for in-line process-monitoring applications, e.g., in food and pharmaceutical industries. On the other hand, it can be expected that EIS sensors should have a comparable response behaviour as ISFETs due to their common detection principle and common transducer material.

In this work, we present results of CIP-suitability investigations of EIS sensors with Ta₂O₅ films as pH-sensitive gate material. As it is known, Ta₂O₅ combines both the high pH-sensitivity (see e.g., [11], [12], [13], [14], [15]) and high corrosion-resistant properties in a wide pH range [16], [17]. However, it has been only a very little known about the Ta₂O₅ behaviour in highly caustic media at very low and very high pH values in combination with high temperatures, so far.

Section snippets

Sensor preparation

Four types of capacitive EIS sensors (EIS-1, -2, -3 and -4) with Si–SiO₂–Ta₂O₅ or Si–SiO₂–Cr–Ta₂O₅ structures (p-Si, ρ = 1–10 Ω cm, 30 nm thermally grown SiO₂) have been prepared using different process parameters (electron-beam evaporation of the Ta layer on the heated and non-heated substrate; structures without and with additional Cr layer (∼5 nm); thermal oxidation of the Ta layer at 450 and 510 °C), which are summarised in Table 1. After oxidation of the 50–60 nm thick Ta layer in dry oxygen

Results

Fig. 2, Fig. 3 show a typical constant-capacitance response (a) and a calibration curve (b) of an EIS sensor (type EIS-4) as prepared (before CIP procedure) and after 30 CIP cycles, respectively. It can be seen, that even after this very rude cleaning procedure (30 CIP cycles), the sensors show a clear pH response in a series measurement in buffer solution from pH 10 to 3 and then, from pH 3 to 10, both resulting in a nearly linear calibration curve. A similar ConCap behaviour has also been

Discussion

Generally, better sensor characteristics in terms of high sensitivity, small drift and low hysteresis have been obtained for EIS sensors with Ta₂O₅ films prepared at an oxidation temperature of 510 °C (EIS-1, -3, -4). These results are in good agreement with previously performed experiments with ISFETs with Ta₂O₅-gates prepared by thermal oxidation of sputtered Ta films [11], [15], [20]. In these experiments, higher pH sensitivities (55–58 mV/pH in the linear range from pH 2 to 12), higher

Conclusions

The obtained results could demonstrate the CIP-suitability of Ta₂O₅ films and thus, a possibility of application of capacitive EIS sensors in in-line pH monitoring systems in food industry. Such a sensor can be placed in direct contact with food for pH measurements without the risk of broken glass fragments.

Acknowledgement

Part of this work was supported by the Ministerium für Bildung und Forschung des Landes Nordrhein-Westfalen, Germany.

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Michael J. Schöning was born in Bruchsal, Germany, in 1962. He received his diploma in electrical engineering in 1989 and the doctoral degree (PhD) in electrical engineering in 1993, both from the Technical University (TH) Karlsruhe. In 1989 he joined the Institute of Radiochemistry at the Research Centre Karlsruhe, Germany. Since 1993, he has been with the Institute of Thin Films and Interfaces at the Research Centre Jülich, and since 1999 he is a professor for applied physics at the University of the Applied Sciences Aachen, Germany. His research subjects concern silicon-based chemical and biological sensors, thin film techniques, solid-state physics, semiconductor devices and microsystem technology.

Diana Brinkman received her diploma degree in biomedical engineering in 2002 from the University of Applied Sciences Aachen, Division Jülich, Germany. Her main interest concern field-effect-based capacitive chemical sensors.

David Rolka received his diploma degree in biomedical engineering in 2003 from the University of Applied Sciences Aachen, Division Jülich, Germany. His main interest concern field-effect-based capacitive chemical sensors and flow-injection analysis systems.

C. Demuth received his diploma in physical chemistry from the University of Zurich. In 1999, he obtained his PhD from the Centre of Chemical Sensors (Federal Institute of Technology, Zurich). Since 2000, he is working in the R&D group of Mettler Toledo, Process Analytics in the field of pH and conductivity sensors.

Arshak Poghossian was born in Yerevan, Armenia in 1949. He received his PhD degree in solid-state physics from the Leningrad Electrotechnic Institute, Russia in 1978, and the Dr. Sci. (Engineering) degree in solid-state electronics and microelectronics from the State Engineering University of Armenia, Yerevan, in 1995. He was both an Associate Professor at State Engineering University of Armenia and a Director of Microsensor Ltd. (Yerevan) from 1991 to 1996. Since 1996, he has been a Professor at the University of Management and Information (Yerevan). Since 1998, he has been with the Institute of Thin Films and Interfaces at the Research Centre Jülich, and since 2004 he is with the University of Applied Sciences Aachen (Division Jülich), Germany. His research interests are solid-state chemical sensors and biosensors, sensor materials, microsystem technology, nanotechnology and nano-devices.

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CIP (cleaning-in-place) suitable “non-glass” pH sensor based on a Ta2O5-gate EIS structure

Abstract

Introduction

Section snippets

Sensor preparation

Results

Discussion

Conclusions

Acknowledgement

Sens. Actuators B

Sens. Actuators B

Sens. Actuators B

Sens. Actuators B

Sens. Actuators B

Mater. Sci. Eng. R

Microelectron. Reliability

Thin Solid Films

Sens. Actuators B

Encapsulation of ISFET sensor chips

Sens. Actuators B

CIP (cleaning-in-place) suitable “non-glass” pH sensor based on a Ta₂O₅-gate EIS structure