GB2162997A - A fluoride ion sensitive field effect transistor - Google Patents

Abstract

A fluoride ion sensitive field effect transistor comprises a planar P type substrate (1) with source (3) and drain (2) regions between which is formed an insulated gate region (7, 8). Optionally a metallised (Au) layer (8) is formed in the gate. To obtain fluoride ion sensitivity a rare earth fluoride containing layer (9) typically LaF3, overlies the gate. A solution under test in use contacts the layer (9) to modify the conductivity of the transistor. A resin layer (10) protects the transistor, except in the region of the gate, from the solution under test. <IMAGE>

Description

SPECIFICATION A fluoride ion sensitive field effect transistor The invention relates to a fluoride ion sensitive field effect transistor for the detection of fluoride ions in solutions.

lon-sensitive electrodes have been widely used as simple and reliable sensors for the determination of the activity or the concentration of ions in solutions. Glass electrodes for measuring the pH-value in particular play an important role in electroanalytical measurement technology. The use of an LaF3 membrane has been proposed in the US Patent Specification 515 197 for the determination of fluoride ions in solutions. A sensitivity corresponding to the Nernst equation is obtained, as well as a high selectivity and good stability, with the use of a doped LaF3 monocrystal.These electrodes are however disadvantageous as a result of the high cost of the monocrystal, the use of an inner reference solution, which does not facilitate manufacture using modern mass production methods and in the case of permeability impairs the operation of the electrodes, and the high resistance of the membrane which requires the use of expensive, high-resistance measurement amplifiers.

Bergveld (JEEE Trans. BME-1 9, 342, 1 972) has proposed the use of a field effect transistor as a chemical sensor for determining concentrations in solutions, in which a potential difference is produced as a result of the interaction of ions with the gate insulator, which difference acts on the drain current (ion-sensitive field effect transistor ISFET).

This potential difference is a result, as in the case of conventional ion-sensitive electrodes, of the Nernst equation. Moreover a number of substances have been proposed as the sensitive layer applied to the gate area and consequently a relatively large number of ions may be measured. However, there is no ISFET for measuring fluoride ion activity.

Examples of sensitive layers for ISFETs: lon ISFET sensitive layer H + SiO2 5i3N4 Al203 Tacos Na+ Na-alumosilicate K+ Valinomycin in PVC membrane Ca+ Orion Ca2±ion exchanger 92-20-02 Attempts to make SiO2 sensitive to fluoride ions by anodic polarisation in solutions containing fluoride, only gave an increase of approximately 30 mV per concentration decade, and was, moreover, heavily dependent on the preliminary treatment (Vlasow, Ju.G. Zh. print. Chim 55, 1310, 1982).

Although the combination of an LaF3 monocrystal with a normal field effect transistor arrangement (T.A. Fjeldly, K. Nagy, J. electrochem. Soc. 127,1299,1980) gives a fluoride ion sensitive with a low-resistance output signal, this is expensive to manufacture as a result of the use of the monocrystal. A further drawback is that a sensor of this type has a low stability over time as a result of contact problems.

Attempts have recently been made to counter the above drawbacks by vapour deposition using LaF3 on the polysilicon conducting tracks connected with the gate area of a field effect transistor, the entire structure being covered with photoresist up to the fluoride sensitive region (J. van der Spiegel et al., Sensors and Actuators, 4,291, 1983).

The drawbacks of this solution are the extremely large potential drift, which makes it impossible to use the solution in practice, and an unsatisfactory sensitivity.

An object of the invention is to provide a device for the measurement of fluoride ion activity which enables economic manufacture, has a low-resistance output signal and consequently has a high sensitivity and selectivity as well as a high stability with time.

Another object of the invention is to provide an ion-sensitive semiconductor component which is sensitive, with a high degree of selectivity, with respect to fluoride ions and which may be manufactured using processes compatible with Si planar techniques.

These objects are solved in accordance with the invention by a fluoride ion sensitive field effect transistor for detecting fluoride ions in solutions, including a gate electrode covered by a fluoride containing layer which is difficult to dissolve in a fluoride containing solution to be tested, and means for permitting the solution to be tested to influence conductivity of the transistor substantially only by coming into contact with said layer.

If the fluoride containing layer is subjected to a fluoride containing solution, whilst all the other active components of the transistor are protected against contact with the electrolytes, it is possible to determine the activity or concentration of the fluoride ions using a conventional measurement technique for ISFETs, for example a source trace or the "constant charge mode".

Particularly good results with respect to sensitivity, detection limits and stability have been obtained using LaF3 in the gate area.

The sensitive layer of LaF3 preferably has a thickness in the range of 20 nm to 1 nm. The required sensitivity is achieved both in the case of direct contact of the fluoride layer of the invention with the insulator, and by the use of one or a plurality of intermediate layers before the application of the fluoride layer of the invention.

The field effect transistor of the invention is described in further detail with reference to two embodiments thereof, given by way of example with reference to the accompanying drawing which shows a section through a possible embodiment of the ISFET.

Embodiment 1.

P-type silicon was used as the substrate 1 and was provided with oppositely doped drain and source regions 2 and 3 which were connected with electrical conductors 4 and 5, the contact taking place by means of openings in the insulating layer 6 of SiO2 A further insulating layer 7 was disposed above the conductors 4 and 5. The insulating layer 7 was coated, if necessary, with an intermediate layer 8, for example of silver, and the LaF3 layer 9 was applied thereto as by coating, above the gate area and between the source and drain regions 2 and 3.

In a further embodiment of the invention, the LaF3 layer was applied directly to the insulating layer 7. The area outside of the gate region was coated with a resin 10 which could not be permeated by the fluid to be tested. The field effect transistor of the invention may be completely produced using conventional technologies for the manufacture of semiconductor structures and is consequently a very economic type of sensor, in which a degree of sensitivity, selectivity and stability over time comparable to that of an LaF3 monocrystal is achieved.

The low detection limit is below 10-5 moles fluoride and the stability over time has a very low potential drift.

In order to test experimentally the sensitive layer of LaF3 a structure was produced corresponding to the sequence of layers in the gate area of Fig. 1, as follows: 100 nm of SiO2 was firstly placed on a silicon chip 110 as the insulating layer and a Si3N4 layer having a thickness of 100 nm was then produced. 50 nm of Ag were then disposed by vapour deposition on these layers, and this layer was then coated with a layer of LaF having a thickness of 1 50 nm.

The near side was provided with an ohmic contact and the entire structure was embedded in epoxy resin bounding the periphery of the LaF3 layer.

The transistor obtained in this way was inserted in solutions having different fluoride contents.

The samples were characterised by plotting of the capacitance-voltage curves using conventional electrochemical measurement techniques.

The sensitivity was determined from the displacement of the curves on the voltage axis.

The following values were obtained: Fluoride concentration Voltage Mol/litre in mV 1.10-' 101 1.10-2 157 1.10-3 215 1.10-4 272 1.10-5 328 The potential drift over a period of 9 months was extremely low and amounted to 0.1 mV per day.

Embodiment 2 An ion-sensitive field effect transistor corresponding to Fig. 1 was inserted in solutions with different fluoride contents.

The gate voltage which was superimposed on the solution via a standard calomel electrode was corrected in each case in such a way that a constant drain voltage. The required gate voltage modification proved to be dependent on the fluoride concentration.

The following measured values were obtained in this case: Fluoride concentration Gate voltage Mol/litre modification mV 1.10-' o 1.10-2 58 1.10-3 115 1.10-4 173 1.10-5 230 The invention may be used in the quantitative analysis of fluoride ions, for example in environmental protection, in the quality control of drinking water, in chemical analysis and in process control and monitoring.

From the foregoing it will be seen that the invention has the advantage that it is simple to produce by processes compatible with Si planar fabrication technology.

Claims (7)

1. A fluoride ion sensitive field effect transistor for detecting fluoride ions in solutions, including a gate electrode covered by a fluoride containing layer which is difficult to dissolve in a fluoride containing solution to be tested, and means for permitting the solution to be tested to influence conductivity of the transistor substantially only by coming into contact with said layer.

2. A transistor according to claim 1, wherein said layer comprises a rare earth fluoride.

3. A transistor according to claim 2 wherein said rare earth fluoride comprises LaF3

4. A transistor according to any preceding claim wherein said layer has a thickness of 20 nm to 1.,us.

5. A transistor according to any preceding claim wherein said fluoride containing layer is applied directly to an insulating layer of said field effect structure.

6. A transistor according to any of claims 1 to 4 wherein said fluoride containing layer is applied to one or a plurality of metal, semiconducting or ionconducting intermediate layers disposed over an insulating layer of the field effect structure.

7. A fluoride ion sensitive field effect transistor substantially as hereinbefore described with reference to the accompanying drawings and either of the examples.