The responses of ion-sensitive FETs (ISFETs) with thermally grown SiO2 gate regions and of electrolyte-SiO2-Si (EOS) structures to stepwise changes in the pH were studied.\ud
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In addition to a change in the boundary potential at the electrolyte-SiO2 interface which accounts for the observed initial response of ISFETs, a mechanism is also proposed in which one or other hydrogen-bearing species interacts with the surface states at the SiO2-Si interface.\ud
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This proposed mechanism is based on the observed time drift in the response of ISFETs and on the changes in the shape of the quasi-static C-V curves of the EOS structures

Bergveld, P.

Rooij, N.F. de

English

The responses of ion-sensitive FETs (ISFETs) with thermally grown SiO2 gate regions and of electrolyte-SiO2-Si (EOS) structures to stepwise changes in the pH were studied. In addition to a change in the boundary potential at the electrolyte-SiO2 interface which accounts for the observed initial response of ISFETs, a mechanism is also proposed in which one or other hydrogen-bearing species interacts with the surface states at the SiO2-Si interface. This proposed mechanism is based on the observed time drift in the response of ISFETs and on the changes in the shape of the quasi-static C-V curves of the EOS structures

de Rooij, N.F.

Bergveld, Piet

NARCIS 

The influence of the pH on the electrolyte-SiO2-Si system studied by ion-sensitive fet measurements and quasi-static C-V measurements

University of Twente Research Information

Thin Solid Films, 71 (1980) 327-331 © Elsevier Sequoia S.A., Lausanne---Printed in the Netherlands 327 THE INFLUENCE OF THE pH ON THE ELECTROLYTE-SiO2-Si SYSTEM STUDIED BY ION-SENSITIVE FET MEASUREMENTS AND QUASI- STATIC C-V MEASUREMENTS N. F. DE ROOIJ AND P. BERGVELD Department of Electrical Engineering, Twente University of Technology, P.O. Box 217, Enschede (The Netherlands) (Received December 12, 1979; accepted April 14, 1980) The responses of ion-sensitive FETs (ISFETs) with thermally grown SiO2 gate regions and of electrolyte--SiO2-Si (EOS) structures to stepwise changes in the pH were studied. In addition to a change in the boundary potential at the electrolyte-SiO 2 interface which accounts for the observed initial response of ISFETs, a mechanism is also proposed in which one or other hydrogen-bearing species interacts with the surface states at the SiO2-Si interface. This proposed mechanism is based on the observed time drift in the response of ISFETs and on the changes in the shape of the quasi-static C-V curves of the EOS structures. 1. INTRODUCTION Since the ion-sensitive FET (ISFET) was announced in 1970 as a new tool in electrophysiology 1, there have been a number of both theoretical and experimental studies which deal with the influence of variations in the electrolyte composition on the drain current of an ISFET 2-4. In addition to an electric-potential-generating (polarization) mechanism at the electrolyte-insulator (SiO2) interface, a further polarization at the SiO:-Si interface has also been discussed 4. This latter polarization is caused by a change in the density of surface trapping states at the SiO2-Si interface. In this paper results are presented of ISFET measurements in comparison with capacitance-voltage (C-V) measurements on electrolyte-SiO2-Si systems, in view of the discussion mentioned. 2. EXPERIMENTAL 2.1. ISFET measurements The devices used were fabricated according to standard MOS technology as described previously 2. The gate insulator consists of a layer of oxide 500 A thick grown by thermal oxidation of p-type (111)silicon in dry 02 at a temperature of 1200 °C. Aluminium was evaporated onto the oxide prior to heat treatment at 450 °C in wet N 2 for 10 min. This aluminium was etched away before the ISFETs'were tested. 328 N.F .  DE ROOLI, P. BERGVELD The measurement of the drain current was performed with a bridge circuit. The electrolyte was brought into contact with a saturated calomel electrode (SCE) whose potential was adjustable in order to balance the bridge after a change in the pH of the electrolyte. The negative value at the adjusted potential, in fact the liquid gate potential V~, is shown as a function of the pH of the electrolyte in Fig. 1. Here, the pH of the solution was changed by the dropwise addition of an acid or a base. The acid- base combination used was acetic acid-triethanolamine. In Fig. 1 the solid line corresponds to the response of the ISFET immediately after changing the pH and the dotted line to the response after a certain period of time (in this case 4 min) in which the device drifts to a stable value. As can be seen, the change with time is positive for a pH below about 6 and is negative above this value. Typical curves for the change Vs(t ) in V s as a function of time in the two pH regions mentioned are shown in Fig. 2. 250 \ %-0 200 ~ ~ \  % 150 Vg/mV l 100 50 0 4 5 6 7 8 g 10 > pH Fig. 1. The liquid gate potential V, immediately after changing the pH (full line, t = 0 )  and after a period of time t = 4 min (broken line); Va = 100 mV. vQ(~) vg(t) I vg{o~ ti me :> (a) ~ H  iQ (t~g(O) t (b) ApH time > Fig. 2. Transient behaviour of V,(t) after a stepwise change in pH in two pH ranges: (a) pH < 6; (b) pH > 6 .  ~n~ I~rFLt~NCn oF ~ pH oN Tm~ Eos SYSTEM 329 If the initial fast response value of V~ is denoted by VB(0 ), the final equilibrium value is denoted by Vg(~) and z is the time constant, then the drift effect can be described by ~ ( t ) -  ~(0) A~(t) = 1 - e  -'/~ r,(oo)- r (o) - ar,(oo) as can be seen from Fig. 3, where In{1 AVs(t) A V , ( o o ) j  is plotted as a function of time at a pH lower than 6. For relatively short times this equation is not applicable, owing to the process with the initial fast response. The conclusion is that probably two processes take place, one with a small (of the order of milliseconds) and the other with a large (of the order of 5 min) time constant. • > <1 0 . 1  i i i i i 0 5 10 time/min Fig. 3. Plot of ln{1 -(AVg(t)/AV~(oo))} vs. time for the pH range below pH = 6; ~ = 5.7 rain. The process with the small time constant is regarded as the ion exchange process at the electrolyte-SiO2 interface which gives the ISFET the initial fast response. The process with the large time constant may be a process of diffusion through the 500 A thick SiO2 layer s. It is striking that the measured time constant of 5 min is of the same order as that found for the diffusion of molecular hydrogen in SiO2 s. The diffusion of charged species will directly influence the drain current, while the diffusion of neutral species may influence the drain current by means of an interaction with the silicon surface. Experiments with various voltages applied between the reference electrode and the ISFET did not result in changes in the observed time constants. As is known from the literature, hydrogen-bearing species in particular may interact with surface states at the SiO2-Si interface 6' 7. However, the concentration of hydrogen-bearing species depends on the pH of the electrolyte. Therefore a model is suggested in which the observed time drift of an ISFET is a result of the formation of hydr0gen-bearing species at the electrolyte-SiO2 interface which diffuse through the oxide layer to the SiO2-Si interface, where they react in a reversible way with the interface states. In order to investigate whether and how variations in the pH of the electrolyte may influence the surface state density at the SiO2-Si interface, quasi-static C - V  measurements were carried out on electrolyte-SiO2-Si structures, as is described in the following section. 330 N.F. DE ROOD, P. BERGVELD 2.2. C-V measurements In order to obtain quasi-static C - V  curves, the displacement current of the electrolyte-SiO2-Si structure was measured in response to a linear voltage ramp (the voltage sweep rate was about 35 mV s-1). The electrolyte was brought into contact with an SCE. The SiO 2 layer used was a thermally grown oxide 500 A thick pretreated with aluminium, just as in the case of ISFETs described in the previous section. The aluminium was etched away before the start of the experiment. This method has the advantage that any series resistances in the cell, such as those of the reference electrode and the solution, do not influence the displacement current. We have shown in a previous publication that the measured C - V  curves for an electrolyte-SiO2-Si (p-type) capacitor shift along the voltage axis in a pronounced fashion depending on the pH. The shift, a first order effect, can be explained as being due to a change in the electric potential difference across the electrolyte-SiO 2 interface. The shift observed previously between the curves for pH 13 and pH 1 was about 330 mV a. If the change in the electrolyte-SiO 2 potential difference is linear across the whole pH range this change would correspond to about 30 mV per pH unit, which is in agreement with the results in Fig. 1. Furthermore it was found that the minimum in the capacitance ratio (C/Cox)ma n for the C - V  data depends on the pH. The change in (C/Cox)rain is the result of a change in the surface state density at the SiO2-Si interface. This conclusion supports the model suggested earlier with respect to the pH-dependent drift observed in ISFETs. 3. SUMMARIZING REMARKS AND CONCLUSIONS The pH responses of both an ISFET and an EOS structure, each with a thermally grown SiO2 layer in contact with the solution, were studied as a function of the pH. The initial response of the ISFET to a change in pH is ascribed to a change in the boundary potential between the SiO2 and the solution, whereas the observed time drift is correlated with a process of diffusion of one or other hydrogen-bearing species through the SiO2 and the chemical interaction of these species with surface states at the SiO2-Si interface. This view is supported by the results of quasi-static C - V  measurements on EOS structures. In addition to a shift along the voltage axis of the curves as a result of a change in the boundary potential across the electrolyte-SiO2 interface, a change in the shape of the curves as a function of pH was also observed. This effect most probably corresponds to a change in the surface state density at the SiO2-Si interface. REFERENCES ! P. Bergveld, Development of an ion-sensitive solid-state device for neurophysiological measure- ments, 1EEE Trans. Biomed. Eng., 17 (1970) 70. 2 P. Cheung, D. G. Fleming, W. H. Ko and M. R. Neuman (eds.), Workshop on Theory, Design, and Biomedical Application of  Solid State Chemical Sensors, Chemical Rubber Co., Cleveland, Ohio, 1978. 3 J. Janata and R. J. Huber, Ion-sensitive field effect transistors, Ion-Selective Electrode Revs., 1 (1979) 31. THE INFLUENCE OF THE pH oN THE EOS SYSTEM 331 4 P. Bergveld, N. F. de Rooij and J. N. Zemel, Physical mechanisms for chemically sensitive semiconductor devices, Nature (London), 273 (1978) 438. 5 A.G. Revesz, On the mechanism of the ion sensitive field effect transistor, Thin Solid Films, 41 (1977) IA3. 6 P. Balk, Meet. of the Electrochemical Society, San Francisco, 1969. paper 111. 7 E. Kooi, Philips Res. Rep., 21 (1966) 477. 8 N.F.  de Rooij and P. Bergveld, in S. T. Pantelides (ed.), Proc. Int. Topical Conf. on the Physics ofSiO z and its Interfaces, Pergamon, New York, 1978, p. 433. 

The influence of the pH on the electrolyte-SiO2-Si system studied by ion-sensitive fet measurements and quasi-static C-V measurements

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