Recent Developments in R.F. Magnetron Sputtered Thin Films for pH Sensing Applications—An Overview

pH sensors are widely used in chemical and biological applications. Metal oxides-based pH sensors have many attractive features including insolubility, stability, mechanical strength, electrocatalyst and manufacturing technology. Various metal oxide thin films prepared by radio frequency (R.F.) magnetron sputtering have attractive features, including high pH sensitivity, fast response, high resolution, good stability and reversibility as well as potential for measuring pH under conditions that are not favourable for the commonly used glass electrodes-based pH sensors. In addition, thin film pH sensors prepared by R.F. magnetron sputtering offer many advantages, such as ease of packaging, low cost through the use of standard microfabrication processes, miniaturisation, capability of measuring pH at high temperatures, ruggedness and disposability. In this paper, recent development of R.F. magnetron sputtered thin films for pH sensing applications are reviewed.


Introduction
Nearly all processes involving water require accurate pH monitoring. Most living things rely on a proper pH level to sustain life. All human beings and animals rely on internal mechanisms to maintain OPEN ACCESS the pH level of their blood, which must be between 7. 35 and 7.45. Exceeding this range could prove fatal. Additionally, dysregulated pH in human tumours has been shown to be correlated with tumour progression and malignancy [1]. Optimal soil pH provides best growth condition and high crop yields for corn, wheat and other plants as well as food products. Appropriate pH control retains milk from turning sour, prevents shampoo from hurting our eyes and makes strawberry jelly gel. The pH of wastewater leaving manufacturing plants, purification plants as well as potable water from municipal drinking water plants, must be within a specific pH range that is set forth by regulatory agencies. Other pH applications include neutralization of effluent in steel, pulp and paper, chemical, pharmaceutical manufacturing, cyanide destruction, odour scrubbers, reverse osmosis, pharmaceutical manufacturing, chemical and petrochemical manufacturing. Simply, pH is an integral part of our everyday life [2][3][4].
The well-known glass electrode is the most commonly used pH sensor because of desirable characteristics, such as good sensitivity, stability, and long lifetime [5,6]. However, traditional glass electrodes have many disadvantages, such as high cost, mechanical fragility, difficult to miniaturize, need for wet storage, large size and limited shape. To overcome these drawbacks and further expand the application fields, alternative pH sensor structures based on ion-sensitive field effect transistor (ISFET) [7][8][9], solid state [10,11], hydrogel [12,13], and microelectrode [14,15], have been reported.
In this paper, we review the recent developments in magnetron sputtered thin films for pH sensing applications with particular emphasis on the pH sensitivity, response time, drift rate, stability as well as potential for measuring pH under conditions that are not favourable for the commonly used glass electrodes-based pH sensors.

R.F. Magnetron Sputtering
In a basic sputtering process, a target (or source) material, to be deposited onto a substrate, is bombarded by energetic ions, typically inert gas ions, such as Argon (Ar + ). The forceful collision of these inert gas ions onto the target causes the removal (known as sputtering) of target atoms, which condense on the substrate as a thin film of stoichiometry similar to that of the target material. Though, the basic idea of operation is apparently simple, the actual mechanisms involved in sputtering are quite complex. Nonetheless, the process is limited by low deposition rates, high substrate heating and low ionization efficiencies. These limitations have been overcome by the development of magnetron sputtering system.
Magnetron sputtering is now considered as the most effective process for the deposition of a wide range of thin film materials [36][37][38][39][40]. The main driving force behind this development has been the increasing demand for high quality functional thin films in many diverse market sectors. In many cases, magnetron sputtered films now outperform films deposited by other physical vapour deposition processes, and can offer the same functionality as much thicker films produced by other surface coating techniques. Magnetron sputtering systems generate a strong magnetic field near the target area, which causes the travelling electrons to spiral along magnetic flux lines near the target. This arrangement confines the plasma near the target area without causing the damage to the thin films being formed on the substrate, and maintains the stoichiometry and thickness uniformity of the deposited thin film. Moreover, in an R.F. magnetron sputtering systems, the generated electrons travel a longer distance, hence increasing the probability of further ionizing the inert gas atoms (Ar + ) and generating stable high-density plasma that improves the sputtering process efficiency. Figure 1 illustrates the basic components of an R.F. magnetron sputtering system. Briefly, ionised Ar atoms bombard a sputtering target, thus releasing the molecules/atoms that form thin layers on a substrate. R.F. magnetron sputtering offers additional advantages, including the use of non-conductive targets, charge-up effects and reduced arching due to the use of alternating electric field (R.F. frequency). It is important to note that R.F. magnetron sputtering is especially advantageous in the deposition of thin films using non-conductive target materials. This is at the cost of R.F. power supplies and an impedance matching network between the R.F. generator and the sputtering target. It should also be noticed that without the use of a magnetron, the non-conducting materials may have more difficulty forming into a thin film as they become positively charged. Reactive sputtering can also be used, whereby the deposited film is formed by chemical reaction between the target material and the gas introduced into the vacuum chamber. Oxides and nitrides are examples of thin films developed through reactive sputtering, where the compositions of these films can be controlled by adjusting the relative pressures of the inert and reactive gases.

pH Sensing Mechanism
pH measurements have conventionally relied on either optical [41][42][43][44][45] or electrochemical techniques [46][47][48][49][50]. Optical methods offer high sensitivity and non-invasiveness, however, they often require the use of dyes, which can change their optical properties due to interactions with specific analytes. In addition, quantitatively precise optical measurements are typically not straightforward due to the drift of optoelectronic components, such as light source, detector and dye concentration variations. Therefore, it is often necessary to also utilize reference dyes in optical pH measurement methods. On the other hand, electrochemical methods are typically straightforward and inherently quantitative, and require simpler operation and setups. Electrochemical methods, however, have downsides, such as long-term instability and vulnerability to electrode fouling, but nevertheless, they are the most commonly used methods for pH detection, whereby the potential (potentiometric) [20,21,[51][52][53][54][55][56], current (amperometric) [57][58][59][60][61][62][63], or charge in an electrochemical cell serve as the analytical signals. Indeed, the glass electrode is currently the most commonly used electrochemical sensor for potentiometric pH measurements.
Potentiometry is the most preferred technique for monitoring the pH value of aqueous solutions in process control and continuous in-line measurements. In this method, the potential difference between a sensing electrode and a reference electrode immersed in the test solutions is measured. The reference electrode provides a stable constant reference potential while the ion selective electrode (sensing electrode) responds to the change of the hydrogen ion (H + ) concentration in the solution. Potentiometry-based pH sensors developed using R.F. magnetron sputtering mainly consist a metal-oxide or metal-nitride sensing electrode and a silver/silver chloride (Ag/AgCl) reference electrode. The Ag/AgCl reference electrode either consists of a silver-chloride-coated silver wire immersed in an electrolyte solution or screen-printed using Ag/AgCl paste. The equilibrium reaction governing the Ag/AgCl electrode potential response is given by [6,12]: Therefore the equilibrium potential of the electrode can be expressed by the Nernstian equation: where E 0 is the reference electrode potential in Volt, R is the universal gas constant, T is the absolute temperature in Kelvin, n is the number of electrons involved in the reaction, F is the Faraday constant, ax is the activity of species x (AgCl, Ag or Cl − ). The Nernstian equation for a cell is often articulated in terms of base 10 logarithms as follows: where k is the ratio of AgCl to Ag concentration. Nernstian behaviour is demonstrated by the electrode when the measured electrode potential decreases by 59 mV for every decade change in Cl − concentration at room temperature of 25 °C. Fog and Buck proposed five possible interpretations for the pH response mechanism of metal oxides, with the most recognized theory being oxygen intercalation [25]. The following equilibrium reaction represents the oxygen intercalation mechanism: where MOx is a higher metal oxide, MOx−δ is a lower metal oxide and δ is the oxygen intercalation. The Nernst's mathematical equation predicting the potential between the sensing and reference electrodes versus the pH value is given by [21]: The term 2.303 (RT/F) is called the Nernst slope, which is 59.16 mV/pH at room temperature (25 °C).

Sensitivity
The sensitivity of a pH electrode is determined by the linear slope response of the pH electrode as defined by the Nernst equation. It is represented in the unit of mV/pH [64].

Response Time
The response time is defined as the time at which the pH concentration in a solution is changed on contact with a pH sensor and a reference electrode has reached 95% (or 90%) of the final value. The response time is reported as t95% or t90% in seconds or minutes [5,64].

Drift Rate
The potential drift is defined as the difference between the peak potential value and the 90% value of the saturated potential. The potential drift measured over time is drift rate and generally represented in mV/h [64].

Hysteresis
Hysteresis is defined as the difference in the electrochemical potentials measured at same pH level. The electrochemical potentials at the same pH level may be different due to various factors, such as different oxidation states and the degree of hydration on the film surfaces, which may establish a new equilibrium of ions every time in the redox reactions at different times of testing [5,64]. In potentiometric measurements, it is represented in mV.

Temperature Coefficient of Sensitivity
The temperature coefficient of sensitivity (TCS) is an important parameter that is typically used to compensate for temperature variations, and evaluate the effect of temperature on the output voltage of the pH sensor. The TCS of the pH sensor is represented in mV/pH °C [65,66].

Reversibility
Reversibility is an essential property in pH measurements for practical applications. To evaluate the reversibility, the pH sensor potential is measured in different pH solutions in the acid-to-base direction and vice versa [5,21].

Resolution
The resolution of a pH sensor is defined as the minimum change in pH above the noise floor that can be detected by the pH-sensor [64,67]. Table 1 shows the summary of pH sensing materials prepared by R.F. magnetron sputtering in recent years. pH sensors are generally characterised using the buffer solutions of known pH values before applying the sensor for real time pH monitoring. Recently, we have developed pH sensor consisting R.F. magnetron sputtered ruthenium oxide (RuO2) sensing electrode in conjunction with an on-chip integrated Ag/AgCl reference electrode. A 99.95% pure RuO2 sputtering target was used to sputter 50 nm, 175 nm, 300 nm and 425 nm RuO2 thin-films in 90% Ar + 10% O2 environment with R.F. power 100 W at 1 mTorr process pressure. Potentiometric measurement has been used to test the sensor in buffer solutions of pH 4, pH 7 and pH 10. Figure 2 shows the schematic representation of potentiometric measurement setup. Briefly, in the experiments, a unity-gain amplifier was developed and used as a buffer amplifier to provide impedance matching, therefore preventing signal loss. An Agilent 34410A high performance digital multimeter was used for real-time potential recording, which has a built-in web interface that makes it easy to setup and transfer readings using only a PC with web browser. Figure 3 shows measured sensor potential versus pH value for different RuO2 thin-film thicknesses [21]. Furthermore, effect of RuO2 sensing electrode thickness on the performance of pH sensing was investigated. The thin film RuO2 sensing electrode with thickness ranging from 50 nm to 425 nm were prepared using 99.95% pure RuO2 sputtering target with R.F. power 100 W in 90% Ar + 10% O2 environment at 1 mTorr process pressure. The highest sensitivity of 68.63 mV/pH was reported for RuO2 film thickness of 300 nm when tested in buffer solutions of pH 4, pH 7 and pH 10. Figure 4 shows the response time versus the RuO2 thin-film thickness for different pH test solutions. For 50 nm thick RuO2 film, the measured response times were 3 s, 10 s and 73.5 s for pH 4, pH 7 and pH 10, respectively. For 175 nm thick RuO2 film, the response times for pH 4, pH 7 and pH 10 were from 3 s, 4 s and 22.5 s, respectively. On the other hand, when the thickness of the RuO2 film was increased to 300 nm, the response times for pH 4, pH 7 and pH 10 were 3 s, 8 s and 3 s, respectively. The results in Figure 4 demonstrate that a faster response time can be attained by increasing the RuO2 thin-film thickness up to 300 nm [21]. The pH sensor stability measured in pH 4, pH 7 and pH 10 solutions is assessed through the measured potential-versus-time curves shown in Figure 5. It is evident from Figure 5 that the sensing film thickness of 300 nm shows stable potential outputs for all pH values. The pH sensor reversibility was also investigated by switching the pH sequentially in pH loop (4-7-10-7-4). Figure 6 shows the average measured potentials for different RuO2 thin film thicknesses. The experimental results in Figure 6 demonstrate an excellent reversibility and stable pH sensor's response for a sensing film thickness of 300 nm.      Various materials deposited using magnetron sputtering are listed in Table 1. These materials have been widely studied for pH sensing application including water quality, engine oil oxidation detection, Coca-Cola, base water, lemon, wine, vinegar, milk, seawater and hard clam (meretrix lusoria) cultivated solutions, glucose concentration, total cholesterol concentration and biosensor applications.

pH Sensing Materials and Applications
Y. H. Liao et al. [22] have used RuO2 thin film as the sensing layer of hydrogen ion selective electrodes (ISE) for pH array sensors. The RuO2 pH-sensing membrane was prepared using the R.F. sputtering system at a process pressure of 10 mTorr in Ar:O2 = 4:1 gas ratio at an R.F. power of 100 W for 1 h. The pH sensitivity of the RuO2 sensing membrane for the developed ISE devices was about 55.64 mV/pH and the drift rate was 0.38 mV/h for a pH 7 buffer solution. The hysteresis widths of the RuO2 ISE device were 4.36 mV and 2.2 mV in pH 7-4-7-10-7 and pH 7-10-7-4-7 loop cycles, respectively. J. Chou et al. [23] have investigated reactively sputtered ruthenium oxide (RuOx) and ruthenium nitride (RuN) sensing membrane deposited on silicon substrate for pH sensing applications. The RuOx film was prepared in an Ar/O2 process gas ratio of 40 sccm/15 sccm at an R.F. power of 100 W with a 10 mTorr pressure for 60 min. A linear pH sensitivity of 55.52 mV/pH was reported for pH values between 1 and 12 at room temperature. Moreover, in order to increase the sensing range of the pH sensor, the RuN was deposited in an Ar/O2 process gas ratio of 15 sccm/30 sccm at 10 mTorr for 1 h with an R.F. power of 100 W. The RuN membrane exhibited a linear pH sensitivity of 57.05 mV/pH over a pH range of 1-13 at room temperature. Additionally, RuOx/Si sensing membrane tested for the detection of the penicillin G showed a rapid response time of 35 s in comparison to 120 s response time of SnO2/ITO. M. Chen et al. [28] have developed a Ta2O5 based capacitive electrolyte-ion sensitive membrane-oxidesemiconductor (EIOS) pH sensor and investigated its performance by means of electrochemical impedance spectroscopy (EIS) and capacitance-voltage (C-V) measurements. A 60 nm Ta film was R.F. sputtered onto oxidized silicon substrate in an Ar atmosphere at 3.3 mTorr working pressure with an R.F. power of 300 W for 5 min. Subsequently, the Ta2O5 film was obtained by the oxidation of the Ta layer in oxygen atmosphere for about 3 h at 525 °C, which resulted in a Ta2O5 film thickness of about 155 nm. Electrochemistry measurements were carried out on an advanced two-electrode electrochemical system, with a conventional Ag/AgCl (3 M KCl) electrode being the reference electrode. A Nernstian response of ~56.19 mV/pH in the pH range 1 to 10 was attained. The EIS measurement revealed that the space charge capacitance can easily distinguish pH changes.
C. Tsai et al. [68] have studied the pH sensing characteristics and hysteresis effect of the R.F. sputtered tin oxide (SnO2) electrode. The hysteresis experiments were carried out with short pH loop (7-4-7-10-7) and long pH loop (7-2-7-12-6). The hysteresis value was less than 7 mV in short pH loop and 9 mV for pH 7 in acid side (HA) and 11 mV in alkali side (HB). The reported pH sensitivity of the SnO2 was about 59 mV/pH for pH range of 2-12. J. C. Chou et al. [69] have carried out long-term monitoring of sea water using solid-state TiO2:Ru sensing electrodes for hard clam cultivation. In this study, a co-sputtering system was used to fabricate the TiO2:Ru sensing film deposited on a p-type silicon substrate. The co-sputtering parameters were: R.F. power of 100 W, D.C. power of 30 W, process pressure 30 mTorr and Ar/O2 ratio of 40 sccm/2 sccm, and a sputtering time of 60 min. The mean sensitivity and linearity of the TiO2:Ru sensing electrode were 55.20 mV/pH and 0.999, respectively, over a pH range of 1-13. Particularly, the TiO2:Ru sensing electrode was able to measure the pH levels for two sample solutions of pH 8 and 9, confirming the weak alkali range and suitable meretrix lusoria growth. L. M. Kuo et al. [70] have reported a precise pH microsensor developed using R.F. sputtering of iridium oxide (IrO2) and tantalum pentoxide (Ta2O5) films on Pt-electrode. In their study, an easily implemented surface modification scheme was adopted, based on employing a Ta2O5 membrane that covers the IrO2 electrode, making the sensor insensitive to H + ions and hence eliminating redox species interference. The sensitivity of the sensor was in the range of 59.44-59.50 mV/pH for the pH range 2-13. Furthermore, the pH sensor displayed high ion selectivity with respect to K + , Na + , and Li + with log KH,M values (~−12.4) with a working lifetime exceeding one week.
W. Bunjongpru et al. [71] have developed pH-ISFET sensor using CMOS compatible processes. A nanocrystalline-aluminum nitride (AlN) thin film (acted as an ion-sensitive membrane) was prepared by reactive gas-timing R.F. magnetron sputtering without heating the substrate and post annealing. In a conventional reactive gas-timing sputtering system, the feeding gas is on-off controlled periodically in such a way that the deposited AlOxNy film has a quite stable composition of aluminum, nitrogen and oxygen (Al:O:N = 52:18:30%) all over the entire film area. The deposition parameters used for the preparation of AlN films of various thicknesses (20, 40, 80 nm) were 12 sccm Ar and 7 sccm N2 gas flow, gas-timing (Ar:N2 = 10 s:90 s), and an R.F. power of 200 W, without heating the substrate. The pH-sensitivity characteristics showed that the pH sensitivity increases with increasing the film thickness. The highest pH-sensitivity attained was 54.50 mV/pH, for an AlN layer thickness of 80 nm. J. Chou et al. [72] have investigated the current-voltage characteristics of an ion-sensitive field effect transistor (ISFET) to determine the pH value at the point of zero charge (pHpzc) of a pH-ISFET device comprising a 25 nm TiO2 sensing membrane R.F. sputtered for 120 min at a pressure of 30  L. M. Kuo et al. [73] have developed a pH-sensor using an R.F. sputtered WO3/IrO2 diode, which was sealed thoroughly by an Al2O3 encapsulation layer, and investigated the diode-like current-voltage characteristics of the sensor in response to hydrogen-ions. To develop the pH-sensor structure, an RF-sputtering of the WO3 target (99.99% pure) was carried out in Ar + 35% O2 at a pressure of 45 mTorr and a temperature of 100 °C. Subsequently, an annealing process was carried out in a high temperature (350 °C) chamber at a pressure of 15 mTorr for 3.5 h. The IrO2 film was also fabricated by R.F. sputtering using an iridium target (99.95% pure) at a pressure of 60 mTorr using Ar and 50% O2 as process gases. Next, the Al2O3 film was R.F. magnetron sputtered using an Al2O3 target (99.99% pure) at a process pressure of 70 mTorr in Ar and 50% O2 process gases for 1.5 h. The measured thickness of the WO3, IrO2 and Al2O3 films were 300 nm, 120 nm and 25 nm, respectively. The solid-state sensor exhibited good stability, repeatability and reversibility in various pH environments ranging from pH 2 to 12 at room temperature. The pH-sensor sensitivity was 0.168 µA/pH for pH 2-12. It was concluded that the chemically sensitive device would be useful for the construction of durable microsensors for tracing the acidity in environmental and biological applications.
M. Brischwein et al. [74] have demonstrated R.F. sputtered ruthenium oxide sensors for extracellular recording of cell-mediated pH changes in cell culture media. In this study, ruthenium oxide was directly grown with MCF-7 and L 929 cells and found to be fully biocompatible. The measured pH sensitivity range was 52-58 mV/pH and the sensor's response was almost linear between pH 5.5-11. The drift rate of the sensor was typically in the range of 1-2 mV/h. It was also reported that an increase in the thickness of the ruthenium oxide layer increases in response time and decreases in drift rate. In addition, it was observed that the pronounced redox cross-sensitivity of ruthenium oxide is a limiting factor in situations where the concentration of dissolved oxygen is not constant or cannot be directly measured. The observed sensitivity to dissolved oxygen was ~0.2 mV/hPa. Furthermore, ruthenium oxide spots on ceramic sensor chips were used for an exemplary cell based assay with MCF-7 cells, demonstrating an extracellular acidification response to the alkaloid drug cytochalasin B. In comparison with optochemical pH-sensors used for the quantification of extracellular acidification, metal oxides may be directly grown with cells and the active area of sensor spots can be made considerably small. They do not suffer from a gradual decline of performance during operation and they are not affected by interfering fluorescent substances.
J. Her et al. [75] have developed a highly pH sensitive electrolyte-insulator-semiconductor (EIS) device incorporating Gd2O3 and GdTixOy sensing films deposited on silicon substrates through reactive R.F. sputtering for biomedical engineering applications. A 40 nm Gd2O3 film was deposited on a silicon substrate by reactive R.F. sputtering from a gadolinium oxide target in diluted O2 environment (Ar/O2 = 5 sccm/2 sccm) at a substrate temperature of 27 °C, whereas a 40 nm GdTixOy film was deposited by reactive R.F. co-sputtering from both gadolinium and titanium targets in the same diluted O2 environment. All samples were annealed in a rapid thermal annealing oven in an O2 environment at selected reaction temperatures in the range of 700-900 °C for 30 s. Compared with Gd2O3 sensing membranes, the EIS device featuring GdTixOy sensing membrane annealed at 900 °C exhibited the highest sensitivity of 64.13 mV/pH for pH 2-12, a smaller hysteresis voltage of 1 mV for the pH loop 7-4-7-10-7 and a lower drift rate of 0.4 mV/h in a pH 7 buffer solution. The high pH sensitivity was attributed to the high surface roughness of the sensing film. Finally, the detection of glucose in serum was successfully demonstrated using EIS with glucose oxidase-immobilized alginate films. The concentration of serum glucose measured by the GdTixOy EIS biosensor was comparable to that determined by a commercial assay kit. The GdTixOy glucose biosensor can detect glucose with reasonable sensitivity (8.37 mV/mM) in solutions containing glucose of concentrations in the range 0.5-6 mM, making them suitable for general blood glucose monitoring.

Conclusions
Recent developments of magnetron sputtered thin films for pH sensing applications have been reviewed in this paper. Various metal oxide pH-sensing structures prepared by R.F. magnetron sputtering have been used for pH sensing applications. We have discussed the potential of various metal oxide based pH sensing electrodes, and highlighted the unique properties of ruthenium oxide and iridium oxide electrodes for pH-sensing applications, such as high sensitivity, good potential stability, wide temperature range, fast response and outstanding corrosion resistance. Furthermore, various applications of magnetron sputtered pH sensors have been discussed, including fluid quality analysis, glucose and cholesterol concentration monitoring and biosensing.