DD227801A1 - FLUORIDION SENSITIVE FIELD EFFECT TRANSISTOR - Google Patents

Abstract

The invention relates to a fluoride ion-sensitive field effect transistor for the detection and quantitative determination of fluorine ions in solutions. The aim of the invention is to provide a device for measuring the Fluoridionenaktivitaet, which has a high sensitivity, selectivity and a high long-term stability and which allows a cost-effective production. The invention has for its object to provide an ion-sensitive semiconductor device, which is sensitive to high selectivity for fluoride ions and can be produced with a Si-planar technology compatible procedure. The technical problem is solved by the invention by a known field effect transistor is coated at least in the gate region with a heavy fluoride, in particular with a rare earth fluoride, preferably LaF3, wherein for the sensitive layer of LaF3 a layer thickness in the range of 20 nm to 1 mm is possible. The required sensitivity is achieved both with direct contact of the fluoride layer according to the invention with the insulator, as well as with the use of one or more intermediate layers prior to the application of the fluoride layer. The invention is applicable to the quantitative detection of fluorine ions z. As in environmental protection, in the quality control of drinking water, in chemical analysis and in process control and monitoring. Fig. 1

Description

Title of the invention

Fluoride ion sensitive field effect transistor

Field of application of the invention;

The invention relates to a fluoride ion-sensitive field effect transistor for the detection and quantitative determination of fluoride ions in solutions.

Characteristic of the hitherto known technical solutions

To determine the activity or 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 electroanalytical MeJ3 technology.

For the determination of fluoride ions in solutions, US Pat. No. 5,151,197 proposed the use of a LaF.sub.3 membrane

 When using a doped LaFy Sinkristalls with such membranes corresponding to the sixth equation sensitivity and high selectivity and good stability is achieved.

Disadvantages of this electrode, however, are the high price of the single crystal, the use of an inner reference solution that impedes manufacture with modern mass technology and compromises the function of the electrode in the event of leakage and the high resistance of the membrane which requires the use of expensive, high-impedance amplifiers.

Von Bergveld (JEEE'Trans BME - 19.34-2 1972) has

1, 1 0 0 /, - Ρ, A Q -j 1 '"' O

suggest to use a field effect transistor as a chemical sensor for concentration determinations in solutions by a potential difference arises by interaction of ions with the gate insulator, which affects the drain current (ion-sensitive field effect transistor ISFET). This potential difference does not derive from the lunchtime equation for classical ion-sensitive electrodes. In addition, numerous substances have been proposed as a sensitive layer deposited on the gate region, thus making a larger number of ions accessible to measurement. However, this did not include an ISFET for measuring fluoride ion activity.

Examples of sensitive layers for ISFST ¹ S:

Ion sensitive layer on

ISFET

H ⁺ SiO ₂

Ua-Alumosilikat Pj. K ⁺ valinomycin in PVC membrane

Ca ⁺ Orion Ca ion exchanger

92-20-02

Attempts to make SiO ₂ sensitive to fluoride ions by anodic polarization in fluoride-containing solutions gave only an increase of about 30% concentration decade, which was also heavily dependent on pretreatment. (Vlasow, J.G., Zh.prikl. Chim. 55, I310 1982) The combination of a LaF.sup.-single crystal with a normal field-effect transistor arrangement (TA Fjeldly, K. Nagy, J. Electrochem., Soc., 127, 1299 1980). Although one gives

Fluoridionensensitiven sensor with a niederobmigen output signal, however, result from the use of the single crystal high cost in the production. Another disadvantage is that such a sensor due to contact problems has a poor long-term stability.

Recently, attempts have been made to circumvent the described disadvantages in that polysilicon interconnects connected to the gate region of a field-effect transistor have been vapor-deposited with LaF, wherein the entire structure is covered with photoresist except for the fluoride-sensitive region (J. van der Spiegel et al. Sensors and Actuators, 4, 291 1983)

 A disadvantage of this technical solution is the extraordinary

1.5 low potential drift, which excludes a practical application and insufficient detection sensitivity.

Aim of the invention;

The object of the invention is to provide a device for measuring the fluoride ion activity, which allows cost-effective production, has a low-impedance output signal and thereby has a high sensitivity, selectivity and high long-term stability.

Explanation of the essence of the invention ..

The invention has the object of providing a ionensensiti ^v ves semiconductor device which is sensitive to fluoride ions with high selectivity and with a Si-planar technology compatible procedure is prepared to provide.

Erfindungsgamäß the object is achieved in that a known field effect transistor is coated at least in the gate region with a poorly soluble fluoride, in particular a fluoride rare earths.

If this fluoride is exposed to a fluoride-containing solution, while all other parts of the assembly are protected against contact with the electrolyte, then using

tion of a measuring technique customary for ISFSTs, e.g. Source follower or "constant-charge mode" determines the activity or concentration of fluoride ions «

Particularly good results in terms of sensitivity, detection limit and stability were achieved when using LaFo in the gate area.

The sensitive layer of LaF 1 must have a thickness in the range of 20 nm to 1 μm. The desired sensitivity is achieved both with direct contact of the fluoride layer according to the invention with the insulator, as well as with the use of one or more intermediate layers before applying the fluoride layer according to the invention.

embodiment

The field effect transistor according to the invention will be explained in more detail with reference to two exemplary embodiments. The accompanying drawing shows a sectional view through a possible embodiment of the ISFBT's. Embodiment 1

As substrate 1, p-type silicon was used, which was provided with oppositely doped drain and source regions 2 and 3, which are connected to electrical conductors 4 and 5, the contacting being made through holes in the insulator layer 6 of SiO ₂ . __

above the conductors 4 * and 5 there is another insulator layer 7 above the gate region, between the drain and source regions 2 and 3 the insulator layer 7 is optionally provided with an intermediate layer 8, e.g. Silver, coated and then applied to the LaFo layer 9.

In another embodiment of the invention, the LaF 2 layer is applied directly to the insulator layer 7. The area outside the gate is coated with a resin 10 impermeable to the liquid to be tested.

The field effect transistor according to the invention can be produced completely with conventional technologies for the production of semiconductor structures and thus represents a very cost-effective

favorable type of a sensor, wherein with the LaF., - A crystal comparable sensitivity, selectivity and long-term stability is achieved.

The lower Hachweisgrenze is below 10 ^J mol fluoride and the long-term stability is expressed in a very low potential drift.

In order to check the sensitive LaFo layer, a structure was created whose structure corresponds to the layer sequence in the gate region of FIG.

On a Si wafer (110), first 100 nm of SiO.sub.p as an insulator layer and then a 100 nm thick Si.sub.IT.sub.IT layer were produced. 50 nm Ag were deposited on these layers, which in turn was coated with a 150 nm thick LaFo layer.

The reverse side was provided with an ohmic contact and cast the entire assembly, except for the LaF ^ layer in epoxy resin

 The thus-obtained electrode was used in solutions of various fluoride content.

The characterization of the samples was carried out by recording the capacitance-voltage curves using the usual electrochemical measurement technique

 The sensitivity results from the displacement of the curves on the stress axis.

The following values were found:

fluoride concentration tension Mol / liter in mV 1 . 1O ~ ¹ 1. 10 ™ ² 101 157 1 . 10 ~ ³ 215 1 · 10 "^ 272 323

The potential drift was extremely low over a period of 9 months and amounted to 0.1 mY per day.

Embodiment 2:

Thus, an ion-sensitive field-effect transistor corresponding to FIG. 1 was placed in solutions with different fluoride content.

The gate voltage applied across a standard quiescent electrode of the solution was corrected so as to give a constant drain current at constant drain voltage. The required gate voltage change proved to be dependent on the fluoride concentration.

The following measured values were recorded here:

Fluoride concentration Gates voltage change (mol / liter) mV

1 . 10 " ¹ 0

1 . 10 " ² 53

1 . io ~ ³ ₁₁₅

1 . 10 " ⁴ 173

1 . 10 ~ ³ 230

Claims (5)

Inventive Attention h 1. Fluoride ion sensitive field effect transistor, for the detection and quantitative determination of fluoride ions in solutions, characterized in that on a known field effect transistor at least the gate region with a poorly soluble fluoride, in particular a rare earth fluoride, is covered and exposed to a solution to be examined can be, while all other parts of the arrangement are reliably protected against contact with the electrolyte. 2. Fluoridionensensitiver field effect transistor according to item 1, characterized in that a preferred "variant in .15 the use of LaF ^ .. as sparingly soluble., Fluoride consists. 3. Fluoride ion-sensitive Feldeffektransistor according to item 1, characterized in that the sparingly soluble fluoride has a layer thickness of 20 nm to 1 pm. 4. Fluoride ion-sensitive field effect transistor according to item 1, characterized in that the sparingly soluble fluoride is applied directly to the insulator layer of the field effect structure. 5. Fluoride ion-sensitive field effect transistor according to item 1, characterized in that the sparingly soluble fluoride is applied to one or more metallic, semiconductive or ion-conducting intermediate layers above the insulator layer. Hierau 1 sheet drawing.