NL8501761A - FLUORION SENSITIVE FIELD-EFFECT TRANSISTOR. - Google Patents

Description

- 1 -

Fluorine ion sensitive field effect transistor.

The invention relates to a fluorine ion-sensitive field effect transistor for detecting and quantifying fluorine ions in solutions.

For determining the activity or the concentration of ions in solutions, ion-sensitive electrodes have found wide application as simple reliable sensors. In particular, the glass electrode for measuring the pH value occupies an important place in the electroanalytical measuring technique.

For the determination of fluorine ions in solution, the use of a LaF2 membrane was proposed in U.S. Patent 515,197.

When using a doped LaF-mono-15 crystal, such membranes achieve a sensitivity corresponding to the Nernst equation, as well as a very high selectivity and good stability.

Disadvantages of this electrode, however, are the high cost of the single crystal, the use of an internal reference solution, which prevents production with a modern mass-scale technology and, if the density is too low, affects the function of the electrode, as well as the high resistance of the membrane , which requires the use of expensive, high-impedance signal conditioning instruments.

It has been proposed by Bergveld (JEEE Trans. BME - 19, 342 1972) to use a field effect transistor as a chemical sensor for concentration determinations, because the interaction of ions with the gate insulator creates a potential difference which influences the supply current (ion-sensitive field effect transistor ISFET). This potential difference arises, as with classic ion-sensitive electrodes, from the Nernst equation. Furthermore, numerous substances have been proposed as a sensitive layer applied to the gate region, thereby making a larger number of ions accessible for the measurement. However, this did not include an ISFET for measuring the fluorine ion 3 / O «

* v J

- 2 - activity.

Examples of sensitive layers for ISFETs are:

Ion Sensitive layer on ISFET

5 H + SiO2

Si3N4

Al2 ° 3

Ta2 ° 5

Na ^ Na-aluminosilicate 10 K + Valinomycin in PVC membrane + 2+

Ca Orion Ca - ion exchanger 92-20-02

Attempts to make SiC> 2 sensitive to fluorine ions by anodic polarization in fluoride-containing solution only increased about 30 mV per concentration decade, which was also highly dependent on pretreatment (Vlasow, Ju. G., Zh. Prikl.

Chim. 55, 1310, 1982).

The combination of a LaF 2 mono crystal with a normal field effect transistor device (TA Fjeldly, - 20 K. Nagy, J. Electrochem. Soc. 127, 1299 1980) gives a fluorine ion-sensitive sensor with a low-ohmic output signal, but by applying the mono -Crystal are expensive to manufacture.

A further drawback is that such a sensor has poor stability over a longer period due to contact problems.

In recent times, experiments have become known, circumventing the disadvantages described by depositing polysilicon conductor tracks connected to the gate region of a field effect transistor 30 with LaF 2, covering the entire structure with photoresist to the fluoride sensitive area (J van der Spiegel et al., sensors, and actuators, 4, 291 1983).

A drawback of this technical solution is the extremely large potential drift, which excludes a practical application, as well as an insufficient detection sensitivity.

The object of the invention is now to provide a device for measuring the fluorine ion activity, β Ja * ^ Cat 3 ƒ i> J 1 - 3 - which allows an economical production, shows a low-ohmic output signal, and thereby a high sensitivity, selectivity , as well as high long-term stability.

The object of the invention is to provide an ion-sensitive semiconductor element which is sensitive to fluorine ions with high selectivity and which can be manufactured in a method compatible with an Si planar technology.

According to the invention, this object is achieved in that a per se known field effect transistor is covered at least in the gate region with a sparingly soluble fluoride, in particular a fluoride of the rare earth metals.

When this fluoride is exposed to a fluoride-containing solution, while protecting all other parts of the device from contact with the electrolyte, using a conventional measurement technique for ISFETs, for example source follower or 20 "constant charge modes", the activity respectively, concentration of the fluorine ions are determined.

Particularly good results in terms of sensitivity, detection limits and stability were obtained when using LaF 2 in the gate region.

The sensitive layer of LaF ^ must thereby have a thickness in the range from 20 nm to 1 µm. The desired sensitivity is achieved both with direct contact of the fluoride layer according to the invention with an insulator and with the use of one or more intermediate layers for applying the fluoride layer according to the invention.

The field effect transistor according to the invention will be further elucidated on the basis of two exemplary embodiments. In the accompanying drawing a schematic representation is given in cross section of a possible embodiment of the ISFET to be used therewith.

EXAMPLE I

As the substrate 1, p-silicon was used, which was provided with oppositely doped drain and supply regions 2 and 3, which are connected to electric conductors 4 and 5, the contacting taking place via: * \ · »J * 7 Λ * 5 'rC i "' -1 v * - 4 - holes in the insulator layer 6 from SiC ^ ·

Over the conductors 4 and 5 there is a further insulator layer 7. Above the gate area, between the discharge and supply areas 2 and 3, the insulator layer 7 is optionally covered with the intermediate layer 8, for example silver, and the LaF 2 is then covered. layer 9 applied.

In another embodiment of the invention, the LaF 2 layer is applied directly to the insulator layer 7. The area outside the gate is covered with a 10 impermeable resin 10 for the liquid to be tested. The field effect transistor according to the invention can be completely produced with conventional semiconductor fabrication technologies and thus represents a very favorable form of sensor in terms of cost, achieving sensitivity, selectivity and stability comparable over time with the LaF 2 mono crystal. .

—5

The lower limit of detection is below 10 moles of fluoride and long-term stability is expressed in a very low potential drift.

To test the sensitive layer of LaF 2, a structure was produced which corresponds in its structure to the layer sequence in the gate region of Fig. 1.

First, 100 nm SiC> 2 as an insulator layer and subsequently a 100 nm thick Si 3 N 2 layer were produced on a Si disk (110-}. On these layers, 50 nm Ag was evaporated, which was again covered with a 150 nm thick layer LaF3.

The back was provided with an ohmic contact and the entire device, down to the LaF 2 layer, was cast in epoxy resin,

The electrode thus obtained was used in solutions of different fluoride content. The samples were characterized by recording the capacitance voltage curves using conventional electrochemical measuring techniques.

The sensitivity is evidenced by the shift of the curve on the voltage axis.

40 The following values were found: 3501 76 f - 5 -

Fluoride Concentration Voltage

mol / liter_ in mV

1. 10 ** 1 101 1. 10 "2 157 5 1. 10" 3 215 1. 10 ”4 272 1. 10 “5 328

The potential drift over 9 months was extremely small and was 0.1 mV per day.

EXAMPLE II

An ion-sensitive field effect transistor corresponding to FIG. 1 was introduced into solutions of different fluoride content.

The gate voltage, which was printed over a standard Kalomel-15 electrode of the solution, was always corrected in such a way that a constant supply current was produced at constant discharge voltage. The necessary gate voltage change appeared to depend on the fluoride concentration.

20 The following readings were included here;

Fluoride concentration Gate voltage change mol / liter. _ in mV_ 1. ΙΟ ** 1 0 1. IO-2 58 25 1. 10 “3 115 1. 10-4. 173 1. 10 "5 230 Λ ij :: ƒ Λ 3 - 6 -

List of reference figures 1. Substrate 2. Drainage area 3. Supply area 4. Electrical conductor 5. Electrical conductor 6. Insulator layer of SiC ^ 7. Insulator layer 8. Intermediate layer 9. LaF ^ layer 10. Resin layer% - conclusions - ss ^^ 7 μ iv -3 I ·· j ^ a

Claims (5)

1. Fluorine-sensitive field effect transistor for detecting and quantifying fluorine ions in solutions, characterized in that on a field effect transistor known per se, at least the gate region is covered with a sparingly soluble fluoride, in particular a fluoride of the rare earth metals, and can be exposed to a solution to be tested, while all other parts of the device are reliably protected from contact with the electrolyte. 2. Fluorine ion-sensitive field effect transistor according to claim 1, characterized in that LaF 2 is used as sparingly soluble fluoride. Fluorine ion-sensitive field effect transistor according to claim 1 or 2, characterized in that the sparingly soluble fluoride has a layer thickness of 20 nm to 1 µm. Fluorine ion sensitive field effect transistor according to claim 1, 2 or 3, characterized in that the sparingly soluble fluoride is applied directly to the insulator layer of the field effect structure. 5. Fluorine ion sensitive field effect transistor according to any one of claims 1 to 3, characterized in that the sparingly soluble fluoride is applied to one or more metallic, semiconducting or ion conducting intermediate layers above the insulator layer. λ 'Ί 7: ¾ 1 r * “S-j”: -J' a