RU2097755C1 - Ion-selective field-effect transistor - Google Patents

Abstract

FIELD: semiconductor transducers for determination of various chemical substances in solutions by potentiometric methods of analysis in medicine, biology, agriculture, and in environment monitoring systems. SUBSTANCE: ion-selective field-effect transistor, including a single-crystal substrate carrying a gate containing successively applied layers of silicon dioxide and tantalum pentoxide, and an ion-sensitive membrane plasticized in a polyvinyl-chloride matrix additionally uses an intermediate layer of polymeric compound, 1 to 3 mcm thick, selected from the class of comb-shaped polymers based on polyalkylmethacrylates or polyalkylacrylates, with the length of the alkyl chain within C₁₀-C₂₂; the intermediate layer is positioned between the layer of tantalum pentoxide of the gate and ion-selective membrane. EFFECT: improved design. 1 dwg, 1 tbl

Description

 The invention relates to semiconductor sensors for determining various chemicals in solutions by potentiometric methods of analysis and can be used in medicine, biology, agriculture, as well as in environmental control systems.

, полученного термическим окислением кремния при 1150^oC в течение 10 мин. В качестве инкапсулирующего материала используют эпоксидную смолу "Hysol". Наличие промежуточного слоя из поли-2-гидроксиэтилметакрилата обеспечивает хорошую адгезию ионочувствительной гидрофобной мембраны к предварительно силанизированной поверхности затвора ПТ.Known ion-selective field effect transistor (ISPT), including a single crystal substrate with a gate located on it, containing a layer of silicon dioxide, a layer of silanization, an intermediate layer of poly-2-hydroxyethyl methacrylate (hydrogel) and an ion-sensitive membrane based on polyvinyl chloride [1]
The field effect transistor gate consists of a thick silicon dioxide layer

obtained by thermal oxidation of silicon at 1150 ^o C for 10 minutes As the encapsulating material, Hysol epoxy is used. The presence of an intermediate layer of poly-2-hydroxyethyl methacrylate provides good adhesion of the ion-sensitive hydrophobic membrane to the pre-silanized surface of the PT gate.

 However, the transistor has several disadvantages.

The disadvantages of the described ISPT include:
the need for preliminary silanization of the surface of the silicon dioxide gate before applying the intermediate layer. For this purpose, the surface of ISPT is maintained in a solution of methacryloxypropyltrimethoxysilane in toluene containing a certain amount of water or diethylamine, at 90 ^° C. for 45 minutes 4 hours, washed, dried in argon atmosphere;
the duration and complexity of the process of applying an intermediate layer, including the accumulation of a solution of 2-hydroxyethyl methacrylate and 2,2-dimethoxy-2-phenylacetophenone in tetrahydrofuran on the surface, evaporation of the solvent at room temperature (30 min), photopolymerization in an inert atmosphere;
the need for additional treatment of ISPT in a 0.1 M NaCl solution (pH 4) for several (3 -6) hours before applying the ion-sensitive membrane;
additional time to establish the potential before the start of each series of measurements, which is 12 15 hours or more, which makes the described ISPT inconvenient in operation.

The closest technical solution is an ion-selective field-effect transistor, including a single-crystal substrate with a gate located on it, containing successively deposited layers of silicon dioxide and tantalum pentoxide, and an ion-sensitive membrane plasticized in a polyvinyl chloride matrix (PVC) [2]
The ion-selective field effect transistor is manufactured on p-type silicon. External dielectric layers were deposited as follows: SiO _{2 by} thermal oxidation of silicon, Ta ₂ O _{5 by} thermal oxidation of a thin film of metallic tantalum sprayed on top of silicon dioxide.

 An ion-selective membrane was applied to the gate region sealed with an epoxy compound ISPT in the form of a solution of the starting components of PVC, DOP and valinomycin in a ratio of 1 3 0.01 in tetrahydrofuran (THF). The evaporation of THF at room temperature led to the formation on the surface of the dielectric of an elastic, tight-fitting film with a thickness of 100-200 microns.

 The disadvantages of the known ion-selective field effect transistor are the short service life, due primarily to poor adhesion of the membrane to the dielectric coating of the gate zone, which is a serious problem that impedes the mass production of ISPTs with polymer membranes.

 The objective of the proposed technical solution is to create an ion-selective field effect transistor with improved analytical characteristics, i.e. with increased life time and almost complete elimination of the effects of pH changes on its work.

This object is achieved in that the ion-selective field effect transistor, including a single crystal substrate with a gate located on it, containing successively deposited layers of silicon dioxide and tantalum pentoxide, and an ion-sensitive membrane plasticized in a polyvinyl chloride matrix, additionally contains an intermediate layer of a polymer compound with a thickness of 1 3 μm, selected from the class of comb-like polymers based on polyalkyl methacrylates or polyalkyl acrylates with an alkyl chain length of C ₁₀ -C ₂₂ , p In this case, the intermediate layer is located between the gate tantalum pentoxide layer and the ion-selective membrane.

Гребнеобразные полимеры получают путем термосополимеризации исходного мономера с использованием в качестве инициатора перекиси бензоила.To comb polymers are polymers of the following structure

Comb polymers are obtained by thermopolymerization of the starting monomer using benzoyl peroxide as an initiator.

 Comb-like polymers are usually models for studying the structure of mesophase and liquid crystalline polymers, and in some cases are used as thickeners for oils and modifiers in the processing of polymers.

Our tests of comb-like polymers based on polyalkyl methacrylates and polyalkyl acrylates with an alkyl chain length of C ₁₀ -C ₂₂ showed the ability of side chains in macromolecules of comb-like polymers to intra- and intermolecular ordering, as well as crystallization regardless of the structure of the main polymer chain, which provides the necessary combination of properties namely, sufficient adhesive strength, determined primarily by the chemical structure of the main chain, and good protective properties due to hydrophobic side alkyl groups. This polymer film has good adhesion properties. Using it as an adhesive when applying ion-selective polyvinyl chloride membranes to the surface of the gate tantalum pentoxide in the manufacture of ISPT increases the duration of the ISPT by at least two months. At the same time, peeling of the membrane ceases to be a factor limiting the lifetime of ISPT. Such analytical characteristics of ISPT as sensitivity and lifetime are improved.

 Specified in the claims, a thickness of 1 μm of the intermediate layer is sufficient to obtain a continuous coating without tears and through holes. At the same time, a thickness of 3 μm does not exceed those limiting values at which a decrease in sensitivity to potential changes on the membrane surface is observed and stability in the operation of ISPT cannot be guaranteed.

 The drawing shows a General view of an ion-selective field effect transistor in section.

 The ion-selective field effect transistor contains: a single-crystal sapphire substrate 1, an n-type silicon epitaxial layer 2 oriented in the crystallographic plane (100), a source region 3 and a drain 4, a gate from sequentially deposited silicon dioxide layer 5 and tantalum pentoxide layer 6, a collector 7 from aluminum, an ion-sensitive membrane 8, an intermediate layer 9 of a polymer compound located between the gate layer 6 and the ion-selective membrane 8, encapsulating material 10. The transistor is placed in the test Op 11 together with the reference electrode 12. Electrode 12, the collector 7, the substrate 1 are connected with a measuring instrument 13.

To create an ion-sensitive sensor, a silicon-based field effect transistor on a sapphire substrate 1 is used. The transistor contains an epitaxial layer 2 of n-type silicon, oriented in the crystallographic plane (100); source 3 and drain 4 are created by ion implantation of phosphorus ions, followed by distillation of the impurity by annealing at 850 ^o C in an argon atmosphere. The gate coating of the field effect transistor consists of a layer 5 of silicon dioxide obtained by thermal oxidation, and a layer 6 of tantalum pentoxide deposited on silicon dioxide; both layers have a thickness of 0.1 μm. The source-drain channel resistance at zero charge at the gate is about 700 Ohm • V. As the encapsulating material 10, polyimide (thickness up to 40 μm) is used.

For the preparation of ion-selective membranes based on polyvinyl chloride, a generally accepted standard technique is currently used. Currently, there is every reason to believe that the analytical characteristics of both ISPT and ion-selective electrodes (ISE) are determined primarily by the ion-selective membrane. Thus, the proposed version of the protective film can be used to prepare ISPT for all those ions for which the corresponding ISEs are known, of which there are only about thirty commercially available. In addition to the nitrate-selective ISPT, taken as an example, the authors have developed calcium ISPT, and currently, ISPs sensitive to BF ions are being developed $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$ , Au (CN) $\genfrac{}{}{0ex}{}{\text{-}}{\text{2}}$ and Li ⁺ . To prepare a nitrate-sensitive membrane, tetradecylphosphonium nitrate (5 wt.%) Is used as an electrode-active compound, dibutyl phthalate (68 wt.) Is a plasticizer, and the polymer base is S-70 polyvinyl chloride (27 wt.).

 To prepare ISPT with an intermediate layer, an aliquot of a comb-shaped polymer benzene solution is applied onto the surface of tantalum pentoxide, which provides a coating thickness of 2 μm and, after benzene evaporation, twice 1 μm of a 10% solution of membrane material in tetrahydrofuran. The transistors thus prepared are held in a desiccator for 15 hours.

 Measurements using the claimed ion-selective field effect transistor is carried out as follows.

При использовании полимеров II m 15 III m 17 наблюдается существенное снижение pH-чувствительности поверхности оксида тантала для 0,4 и 0,8 мВ/pH соответственно. Будучи промежуточным слоем, полученная полимерная пленка не только устраняет практически полностью воздействие изменения pH на работу ИСПТ, но и улучшает адгезию ионоселективных поливинилхлоридных пластифицированных мембран к поверхности затвора.Before measurements, ISPT is kept in a 0.1 M NaNO ₃ solution for 1 h, monitoring the change in membrane potential over time. The measurement of the potential of nitrate-selective PT is carried out on a computerized device at a given current strength in the source-drain circuit of 100 μA. The design provides special protection against breakdown of the dielectric layer on the gate when changing samples. To eliminate the influence of light and static electricity on the value of the measured potential, the electrochemical cell is placed in a dark grounded metal box. As the reference electrode, a silver chloride electrode is used. The duration of the measurements is determined on the basis of the shape of the recorded curves, usually it is 5 minutes The analysis is carried out on a series of standard solutions in series with concentrations of 1 • 10 ^-5 and 0.1 M NO $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ and measure the values of a stable equilibrium analytical signal. The relative mean square deviation of the measurement results does not exceed 0.03 and is limited by the noise level of the measuring system. ISPT with an intermediate layer of the following comb-like polymers were investigated:

When using polymers II m 15 III m 17, a significant decrease in the pH sensitivity of the tantalum oxide surface is observed for 0.4 and 0.8 mV / pH, respectively. Being an intermediate layer, the obtained polymer film not only eliminates almost completely the effect of pH changes on the operation of ISPTs, but also improves the adhesion of ion-selective polyvinyl chloride plasticized membranes to the gate surface.

The change in the angular coefficient of the calibration curve (S) of nitrate-selective transistors with an intermediate layer of comb-shaped polymer with C ₁₈ in time depending on storage conditions is presented in the table.

As can be seen from the table, there is an improvement in the electrode function of the nitrate-selective PT when stored in air: the angular coefficient of the linear portion of the calibration graph increases to 59 mV / pNO ₃ . The linearity of calibration graphs is maintained in the range of 10 ^-4 -0.1M NO $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$
In addition, as can be seen from the table, when stored in a solution of 0.1 M NaNO ₃ for 2 months, the sensitivity exceeds 46 mV / pNO ₃ . At the same time, when applying a nitrate-selective polyvinyl chloride membrane directly to the surface of the PT gate, the detection sensitivity at the initial time of use does not exceed 40 mV / pNO ₃ , and the lifetime of such ISPT due to peeling of the membrane is less than 5 days.

 Thus, the obtained results allow us to talk about the creation of ISPT with a long lifetime while maintaining sensitivity.

Claims (1)

An ion-selective field-effect transistor, comprising a single crystal substrate with a gate located on it, containing sequentially deposited layers of silicon dioxide and tantalum pentoxide, and an ion-sensitive membrane, plasticized in a polyvinyl chloride matrix, characterized in that the transistor further comprises an intermediate layer of a polymer compound with a thickness of 1 3 μm selected from class of comb polymers based on polyalkyl acrylates or polyalkyl methacrylates having an alkyl chain length of C _{1 0} C _{2 2,} et m intermediate layer is disposed between the layer of tantalum pentoxide gate and ion-selective membrane.

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