Si3N4, ISFET

Fig. 6.4 Titration curves of 20 ml of (3 mM phthalic acid (H2Ph) + 2 mM CF3S03H) in various AN-H20 mixtures with 1.0 M Bu4NOH (in MeOH). Water content in (v/v)% is shown on each curve. Obtained using Si3N4-ISFET (Shindengen Indus. Co.) as pH sensor and titrating at 0.005 ml min-1 [15].

Fig. 6.5 Titration curves of 5 mM picric acid in AN with 1 M BU4NON (in MeOH), recorded simultaneously with four pH sensors, but at different titration speeds for (a) to (c). In (a), curve 1 is for Si3N4-ISFET, 2 for Ta205-ISFET, 3 for Ir02 pH-sensor, and 4 for glass electrode. The pH-ISFETs were obtained from Shindengen Indus. Co. and the Ir02 pH-sensor from TOA Electronics Ltd [17c].

Recently, Ta2Os- and Si3N4-type pH-ISFETs have been used in non-aqueous systems, by preparing them to be solvent-resistant [17]. In various polar non-aqueous solvents, they responded with Nernstian or near-Nernstian slopes and much faster than the glass electrode. The titration curves in Fig. 6.5 demonstrate the fast (almost instantaneous) response of the Si3N4-ISFET and the slow response of the glass electrode. Some applications of pH-ISFETs are discussed in Section 6.3.1. 

The hydrolysed surface of Ihe Si3N4 insulator functions as a pH-sensitive membrane [90, 105, 116, 179]. A penicillin-sensitive ISFET is based on this membrane that is covered by an immobilized layer of penicillinase, converting penicillin into the penicillanic acid anion with liberation of hydrogen ions [24]. Another version of pH-sensitive ISFETs has membrane gates made of TajOs [3] or of a suitable glass [39]. The latter ISFET with a gate made of alumino- or borosilicate glass is sensitive to sodium ions. Other ISFETs are sensitive to halide ions [22, 153, 178], [105, 115, 130] and Ca [90, 105]. 

The glass electrode responds especially slowly in protophilic aprotic solvents like DMSO and DMF, sometimes taking 1 h to reach a steady potential. In such cases, the use of Si3N4- and Ta205-type pH-ISFETs is very promising, because they almost respond instantaneously and with Nernstian or near-Nemstian slopes [19]. 

The pH window is very wide in solvents that are weak both in acidity and basicity. The widths of the pH window are well over 30 in such solvents, compared to about 14 in water (Table 6.6). The usefulness of these expanded pH regions is discussed in Section 3.2.2. In particular, potentiometric acid-base titrations in such solvents are highly useful in practical chemical analyses as well as physicochemical studies [22]. Acid-base titrations in lion-aqueous solvents were popular until the 1980s, but now most have been replaced by chromatographic methods. However, the pH-ISFETs are promising to realize simple, rapid and miniature-scale acid-base titrations in lion-aqueous solvents. For example, by use of an Si3N4-type pH-ISFET, we can get an almost complete titration curve in less than 20 s in a solution containing several different acids [17d]. 

Fig. 6.8 Response of (a) glass electrode, (b) Si3N4-ISFETand (c) Ta205-ISFET to the proton transfer activity coefficient [1 7a].

ETH 227, Fluka AG), 63.9 wt. % of 2-nitrophenyloctylether (NPOE Dojin Research Laboratories Co. Ltd.), 0.4 wt. % of sodium tetraphenylborate (NaBPh4, Dojin Research Laboratories Co. Ltd.) and 28.6 wt. % of PVC (Dojin Research Laboratories Co. Ltd.) onto the Si3N4 gate of the ISFET devices (0.5 mm x 5.5 mm x 0.2 mm catheter type ISFET donated by Shindengen Electric Mfg. Co. Ltd.) at several times to avoid pin-holes. The resulting Na+ PVC/ISFETs were allowed to dry overnight. The thickness of the membrane was approximately 0.1 mm. 

Design and fabrication of ISFET was described in Ref. [88] The interest in ISFET arises chiefly from their application as pH and ion sensors. A graphical procedure to find PZC from capacitance-voltage characteristics of electrolyte-insulator-semiconductor and metal-insulator-semiconductor structures was discussed [89]. Due to the choice of electrolyte (2 mol dm Na2S04) the PZC values reported in this study (2.5 for Si02, 2.8 for Ta20s and 3-3.4 for Si3N4) are not likely to be the pristine values due to specific adsorption of anions. 

Since the ISFET is based on the field-effect transistor, let us recall briefly how the latter operates (see, e.g.. Ref. 98). The field-effect transistor (Fig. 19a) represents the so-called MIS (metal-insulator-semiconductor) structure (hence the abbreviation MIS-FET), i.e., a semiconductor base, onto which an insulating layer and a metal electrode (gate) are deposited. The base usually is a p-type silicon plate and the insulator, a Si02 or Si3N4 layer. With a thickness of 100-200 nm, the resistance of this layer is of the order of 10 fl. Two regions are produced in the base by local... 

For the purposes of pH measurement, the hydrated silicon oxide dielectric of a MOSFET can be used like a glass membrane. Better response to pH, however, can be achieved by MNSFETs with a silicon-nitride dielectric, which are commonly used for this purpose (but not for in vivo measurement in blood because of the high thrombogenicity of Si3N4). Only the surface of the insulator may be in contact with a liquid sample other parts of the ISFET have to be electrically extremely isolated, e.g., by epoxy encapsulation. 

Another approach to eliminate the inner filling solution of conventional ISEs was introduced also in the 1970s and is based on the use of field effect transistors (FETs). These devices are referred to as ISFETs, that is, ion-sensitive FETs, and belong together with enzyme FETs (EnFETs) and gas sensitive FETs to the larger category of ChemFETs (chemically sensitive FETs). In the case of the ISFET, the ISE membrane is applied to the Si3N4... 

As basic sensor a pH-l FET chip was developed which can be used for different ISM depositions. It contains a dual ISFET structure (n-channel FET, LP-CVD Si3N4 gate membrane, gate size 16 x 400 pm) and a temperature sensitive diode. The chip is fabricated in a standard MOS>production line. For using the chip in a ISFET difference measuring mode without a conventional reference electrode an integrated pseudo reference electrode has been designed too. 

In order to improve performance, inorganic oxides other than Si02 for pH sensors such as AI2O3, Si3N4 and Ta20s have been deposited on top of the Si02 by chemical vapour deposition, in order to increase sensitivity and decrease the hysteresis and drift of Si02-based ISFETs. 

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