Method of producing an ISFET and same ISFET

Details Method of producing an ISFET and same ISFET Method of producing an ISFET and same ISFET

1985-11-19

1988-04-05

Lusignan, Michael R.

Chemistry: electrical and wave energy

Apparatus

Electrolytic

437 42, 437235, 204418, 357 25, G01N 2700, H01L 2962, H01L 21312

Patent

active

047357029

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a method of modifying an oxide surface of a semi-conductor material, incorporated for example in an ISFET, in which a polymer coating is applied to the oxide surface.
In analytical chemistry ion sensitive electrodes are used to continuously or otherwise determine positive ions. An important requirement to be met by these electrodes is that they have a long life time and are as chemically inert as possible.
In 1970 Bergveld of the Technical University Twente (TH Twente) described the principle of the so-called "Ion Selective Field Effect Transistor", hereafter referred to as ISFET. The ISFET comprises semi-conductor material, for example p-type silicon, which is provided with an oxide surface, such as SiO.sub.2, Al.sub.2 O.sub.3, Ta.sub.2 O.sub.5 and TiO.sub.2. In the case of a SiO.sub.2 oxide surface, for example, the working of this pH sensitive ISFET is based on the fact that Si-OH groups can be protonised and deprotonised respectively, depending on the concentration of protons in the environment surrounding the ISFET. In this way a potential is built up at the SiO.sub.2 /environment interface, resulting in the occurence of a corresponding charge variance in the silicon. As the potential built up is dependent on the proton concentration in the environment, the proton concentration can be relatively determined in this way.
If the oxide surface is modified as such that another positive ion preferably interacts with the modified oxide surface, an ISFET is made which is suitable for the qualitative and quantitative determination of this other positive ion.
For this purpose, the oxide surface is coated with a layer of a polymer or paste containing suitable (macro) cyclic compounds, which preferably possess a high affinity for a particular metal ion, especially in relation to protons.
By bonding the metal ion in the macrocyclic organic compounds a potential is again created with which the concentrations of this particular positive ion can be determined relatively, and if necessary, in a quantitative manner. The stability of the coating of a polymer applied in this way is, however, not great, because the coating is only physically adhered to the oxide surface, with the result that, in an aqueous environment, water can infiltrate between the coating and the oxide surface, because this oxide surface, in the case of SiO.sub.2 for example, has a polarity which more or less corresponds to that of water. In view of the fact that the coating of the polymer is generally apolar, the polymer will detach after the infiltration of water between it and the oxide surface, and the intended selectivity of the ISFET is lost. A second disadvantage is that such a known ISFET often has an excessive pH sensitivity.
Much research has already been directed at modifying for example the SiO.sub.2 surface by means of a chemical reaction. A disadvantage of all known techniques is that on the SiO.sub.2 surface a number of silanol groups that are present are not always transformed, so that an ISFET manufactured in this way remains pH sensitive.
The British patent application No. 2.017.400 describes the coating of the surface of a gate area of an ISFET with a hydrophobic organic polymer membrane. In this method, prior to the application of the hydrophobic membrane, the oxide surface of the semi-conductor material is treated with a silane coupling agent which couples with the silane groups present on the oxide surface. There is, however, absolutely no question of a chemical bond of the hydrophobic organic polymer membrane subsequently to be applied to the treated oxide surface. On the one hand this is because, according to example 3 of this British publication, the polymer membrane is formed by radical polymerization in the vapour phase, as a result of which the polymer is deposited on the oxide surface of the semi-conductor material physically adheres to it by adsorption. The physically adhered membrane could have been removed, according to the state of the art (Journal Polymer Science, part C, 16, p. 2341, 1967), i

REFERENCES:
patent: 3788895 (1974-01-01), Schimmer
patent: 4302530 (1981-11-01), Zemel
patent: 4476003 (1984-10-01), Frank
Berg et al., "Sensitivity Control of ISFET's by Chemical Surface Modification", Sensors & Actuators, vol. 8, (1985), pp. 129-148.

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