The method for pH detection

Traditionally, the methods for the measurement of pH values include indicator reagent, pH test strips, metal electrode and glass electrode. There are some drawbacks on the other methods, except for glass electrode. Such as, indicator reagent can show different colors at different solvent, but it only exhibit a range of pH not the accuracy value; As pH test strips immersed in the test liquid, they show a particular color

corresponding to the pH of the solution. These are similar to indicator reagent; The hydrogen electrode method is a golden standard for all methods of pH measure. The activity of the hydrogen ions is determined by potentiometric measurement using a standard hydrogen electrode and a reference electrode. In order to ensure a saturated layer of hydrogen adsorbed at the platinum surface, hydrogen gas is continuously bubbled around the platinum electrode. However, this method is not suitable for daily use due to the inconvenience of handling hydrogen gas [2]. Because of some limitations in practical applications of the first three methods, the glass electrode becomes the most widely used method for the pH measurement, and it is considered to be the standard measuring method.

The glass electrode is most widely used for pH measurement due to idea Nernstian response independent of redox interferences, short balancing time of electrical potential, high reproducibility and long lifetime. However, glass electrode has several drawbacks for many industrial applications. Firstly, they are unstable in alkaline or HF solutions or at temperatures higher than 100°C. Also, they exhibit a sluggish response and are difficult to miniaturize. Moreover, they cannot be used in food or in in vivo applications due to their brittle nature [1]. There is an increasing need for alternative pH electrodes [2].

New trends of pH measurements such as optical-fiber-based pH sensor, mass-sensitive pH sensor, metal oxide sensor, conducting polymer pH sensor, nano-constructed cantilever-based pH sensor, ISFET-based pH sensor and pH-imaging sensor. In this study, we will discuss what problems in practical applications of ISFETs and how to improve them.

2.5 The theory of ISFET

Since ISFET was the first reported by Bergveld, research on new material of sensing thin and fabrication process to improve the sensitivity and stability has been continuously proposed [5-7]. At the same time, the mechanism of the pH response of pH ISFET has also been studied extensively [6-12]. Electrochemical measurement of pH utilizes devices that transduce the chemical activity of the hydrogen ion into an electronic signal, such as an electrical potential difference or a change in electrical conductance. The followings are the theoretical foundations which are mostly adopted to characterize the ISFET.

2.5.1 From MOSFET to ISFET

The ISFET is a new approach of electrochemical measurement of pH, which is similar to MOSFET except that the metal/poly gate is replaced by sensing layers, and the sensing layer is immersed in aqueous solution. Because of it can not directly supply on the aqueous, therefore the reference electrode is adopted to connect with sensing layer. The reference electrode not only supply stable voltage but also can connect the circuit with sensing layer to make a loop. It can trace back to the history of the development of ISFET, it is not difficult to find out the similarities between ISFET and MOSFET. In general MOSFET is Metal-Insulator-Semiconductor structure, ISEFT is Electrolyte-Insulator-Semiconductor structure. The most obvious characteristic is the similarity between their structures. For this reason, the best way to comprehend the ISFET is to understand the operating principle of a MOSFET first.

When MOSFET is operated in the so-called ohmic or non-saturated region, the drain current ID is given by:

where COX is the gate insulator capacitance per unit area; μ is the electron mobility in the channel; W/L is the width-to-length ratio of the channel; VGS is gate to source voltage; VDS is drain to source voltage and VT is the threshold voltage. VT can be described by following expression:

F where VFB is the flat-band voltage; QB is the depletion charge in the silicon substrate, andψF is the potential difference between the Fermi level and intrinsic Fermi level.

The degree ofψF is dependent on the doped concentration. VFB can be described by following expression:

QOX is the charge in the oxide and QSS is the surface state density at the oxide-silicon interface. Substitution of Eq. (2-4) in Eq (2-5), the general form of the threshold voltage of a MOSFET can be described by following expression:

F

are no longer to considered on the ISFET. At Figure 2-1 illustrates, it can observe the similarities and differences between MOSFET and ISFET. When immersed in a aqueous solution, it must occur surface potential at the oxide-solution interface. The surface potential must take in into account. Hence the threshold voltage become the following expression: where Eref is the constant potential of the reference electrode; χ^sol is the surface dipole potential of the solution which also has a constant value. The surface dipole

potential is different from aqueous solution, even though a little variation of surface dipole potential at disparity aqueous solution. The value compare to other term is too small to take as a constant. All terms are constant except Ψ0, it is the kernel of ISFET sensitivity to the electrolyte pH which is controlling the dissociation of the oxide surface. In order to obtain an accuracy pH value, to investigate a high pH sensitivity ISFET on the electrode-electrolyte interface is necessary.

2.5.2 The Response of pH at the Oxide-Electrolyte Interface

The surface of any metal oxide always contains hydroxyl groups, in the case of silicon dioxide SiOH groups [13]. These groups consist of donate and accept a proton from the solution. Therefore, as ISFET sensing layer like SiO2 contact an aqueous solution, the change of pH will change the SiO2 surface potential. These reactions can be expressed by:

The potential between the gate insulator surface and the electrolyte solution causes a proton concentration difference between bulk and surface that is according to Boltzmann:

constant and T is the absolute temperature. The subscripts B and S refer to the bulk and the surface, respectively.

There are two important parameters which are related to ISFET sensitivity, β_S and C . _S β_S is the symbol of the surface buffer capacity, the ability of β_S as the oxide surface to deliver or take up protons, and C_S is the differential double-layer capacitance, of which the value is mainly determined by the ion concentration of the bulk solution via the corresponding Debije length.

0 S where σ0 is the surface charge per unit area. β_S is called the intrinsic buffer capacity because it is the capability to buffer small changes in the surface pH (pH ),but not in _s the bulk pH (pH ). _B

Because of charge neutrality, an equal but opposite charge is built up in the electrolyte solution side of the double layer σDL , shown in Figure 2-2 illustrates . This charge can be described as a function of the integral double layer capacitance, Ci, and the electrostatic potential:

σDL=-CiΨ0=-σ0 (2-13) The integral capacitance will be used later to calculate the total response of the ISFET on changes in pH. The ability of the electrolyte solution to adjust the amount the of stored charge as result of a small change in the electrostatic potential is the differential capacitance, C : _S As a result, combination of Eq.(2-12) to (2-14) lead to an expression for the sensitivity of the the electrostatic potential change in +

HS

a :

0

Rearrange Eq. 2-15 gives a general expression for the sensitivity of the electrostatic potential to changes in the bulk pH [13]:

0 2.3 kT _B

The parameter α is a dimensionless sensitivity parameter that varies between 0 and 1, depending on the intrinsic buffer capacity, β_S , of the oxide surface and the differential capacitance C . We can get the maximum value α so that the _S sensitivity become -59.2 mV/pH at 298K which is called Nernstian sensitivity.

Therefore, the intrinsic buffer capacity β_S need to be the more higher or the double layer capacity C to be the more lower. In ideal, the intrinsic buffer capacity _S β_S=∞

or the double layer capacity

C =0 would be the best. It appears that the usual SiOS 2 from MOSFET does not fulfil the requirements of a high vale of β_S. The pH sensitivity is low depending also on the electrolyte concentration through C . Therefore other films such as ZrO_S 2 were introduced to increase the values of β_S. The higher the intrinsic buffer capacity so that the less important of the value of C which means that independent of the _S electrolyte concentration a Nernstian sensitivity can be achieved over a pH range from 1 to 13.

2.6 Drift Phenomenon

Drift phenomenon is while ISFET expose to an aqueous solution for a long time, shift of ISFET gate voltage after a proper time from the response of the ISFET device.

It has been reported by Dun et al. [18]. According to Hein and Egger [19], two types of drift have to be distinguished, the storage drift (irreversible shift without any applied voltages) and the long-term drift (irreversible shift under operating conditions). The initial drift means the drift after 3 h from the response starting [20].

The former’s influence on drift is generally smaller than the latter. This result can be found in the previous work [1, 14-16] and also in the measurement data of this research. The phenomenon called drift is a slow, continuous, change of the threshold voltage of an ISFET in the same direction. It is difficult to identify the cause of this phenomenon, which could be either a surface or a bulk effect, or both. There are some possible reasons causes of drift [17].

(1) Variation of the surface state density (Dit) at the Si/SiO2 interface which means the drift dependence of diffusion mechanism.

(2) Some surface effects, such as the rehydration of a surface that is partially dehydrated and ion exchange involving OH^- ions.

(3) Drift of sodium ion under the influence of the insulator field. Given an effective diffusion coefficient Deff, it is clear that a bulk redistribution of sodium which has left a trap near the edge of the SiO2.

(4) Injection of electrons from the electrolyte at strong anodic polarizations created negative space charge inside sensitive films.

Dun et al. [18] recognized that the drifts of Si3N4 and Ta2O5 gate ISFET both change toward the output voltage increasing. This condition is the same as that some negative charges (OH^-) rise on the sensitive surface.

The great part of people most supported the drift phenomenon are the cases (1)

and (2). There are two models such as the site-binding model and the gel model, which are classified according to the location where the mechanism of pH-sensitivity is presumed to occur. These models can help us to have a further understanding of the transport of mobile ions. Nonetheless, these two models are only the characteristics of ions transport in the insulator, while the physical model for the gate voltage drift is going to be presented in the next section.

2.6.1 Dispersive Transport

Dispersive transport was brief reviewed in [1] and it is observed in a broad class of disordered materials. In an amorphous material, dispersive transport may arise from hopping motion through localized states (hopping transport), trap-limited transport in the presence of traps possessing an exponential energy distribution (multiple-trap transport), or a combination of the aforementioned transport mechanisms (trap-controlled hopping transport) [21]. Regardless of the specific dispersive mechanism involved, however, dispersive transport leads to a characteristic power-law time decay of diffusivity [22] which can be described by

1 where D00 is a temperature-dependent diffusion coefficient which obeys an Arrhenius relationship, ω0 is the hopping attempt frequency, and β is the dispersion parameter satisfying 0<β<1. Dispersive transport leads to a decay in the density of sites/traps occupied by the species undergoing transport. This decay is described by the stretched-exponential time dependence given by

] where ΔNS/T(t) is the area density (units of cm^-2) of sites/traps occupied, τ is the time

constant associated with structural relaxation, and β is the dispersion parameter.

2.6.2 Expression for Drift

In general, the surface of a sensing film is known to undergo a relatively slow conversion to a hydrated SiO2 layer or contain oxygen atoms during contact with an aqueous solution [23-28], Since hydration leads to a change of the chemical composition of the sensing film surface, it is reasonable to assume that the dielectric constant of the hydrated surface layer differs from that of the sensing film bulk. The overall insulator capacitance, which is determined by the series combination of the surface hydration layer and the underlying sensing film, will exhibit a slow, temporal change. When drift phenomenon occurs at the surface of an actively-biased ISFET, the gate voltage will simultaneously exhibit a change to keep a constant drain current.

The change in the gate voltage can be written as:

) Since the voltage drop inside of the semiconductor is kept constant, ΔVG(t) becomes

)] where VFB is the flatband voltage and Vins is the voltage drop across the insulator. VFB

and Vins are given by the following expression:

OX where Qinv is the inversion charge. If the temperature, pH, and the ionic strength of the solution are held constant, Eref, χ^sol, Ψ0, and ΦSi can be neglected, so the drift can be

In this study, the gate oxide of the fabricated ISFET was composed of two layers, a lower layer of thermally-grown SiO2 of thickness, xL, and an upper layer of sputter-grown ZrO2 of thickness, xU.. CI(0) is the effective insulator capacitance given by the series combination of the thermally-grown SiO2 capacitance, εL/xL, and the sputter-grown ZrO2 capacitance, εU/xU. Ci(t) is analogous to CI(0), but an additional hydrated layer of capacitance make Ci always smaller than CI, εHL/xHL, at the oxide-electrolyte interface must be considered, and the sputter-grown ZrO2

capacitance is now given by εU/[xU－xHL]. The series combinations of the capacitances are illustrated in Figure 2-3. Therefore, the drift is given by

)

From this equation, we observed that drift of gate voltage ΔVG if the substrate type was different, it might to be positive or negative value. Because of the value of ΔVG

is positive or negative, it is depend on the Qinv and QB . Other terms at Eq.(2-25) can be appropriate as constant value no matter what the substrate is. According to this assume it is possible to eliminate the drift or hold the drift to be a constant at any other pH aqueous solution through the CMOS ISFET. By applying dispersive transport theory, an expression for xHL(t) is given by [1]

[ ]

where AD is the cross-sectional area, and Nhydr is the average density of the hydrating species per unit volume of hydration layer. Thus, combination of Eq.(2-20) to (2-27) the gate voltage drift can be expressed by the following formula:

[ ]

From this equation, we can expect that if the time of gate oxide immersing in the test-solution is long enough (determined by the constant τ ), the gate voltage drift will approach a constant value which is greatly dependent on the hydration depth, xHL(∞).