DE19502862B4 - FET-based sensor - Google Patents

Abstract

Sensor based on a field effect transistor with a gate contact isolated from a substrate by a gate dielectric, characterized in that the gate dielectric is a layer of amorphous hydrogen-containing carbon (aC: H) with an interface state density ≤ 5 · 10 ¹¹ cm ⁻² · eV ⁻¹ , and that sensitive groups for biochemically active substances are bound to the surface of the aC: H layer.

Description

The The invention relates to a sensor based on FET (FET = field effect transistor).

In MOSFET devices (MOSFET = Metal Oxide Semiconductor field effect transistor) the current flow between Drain and source controlled by applying a control voltage to the gate. The potential at the gate contact influences the training or the Lifting a conductive Channel (inversion layer) in the semiconductor base material (silicon) below the gate contact: transistors from the enrichment or Depletion type (see: G. Bohle, E. Hofmeister "Semiconductor Components for the Electronics ", Siemens Aktiengesellschaft, pages 34 and 35, as well as: R. Müller "Components of semiconductor electronics", 3rd edition, Springer-Verlag Berlin, Heidelberg 1987, pages 158 to 161).

In order to avoid an electrical short circuit between the gate contact and the silicon substrate, the gate contact is insulated from the substrate by an insulator layer (gate oxide). This layer is subject to a number of requirements so that the external electrical field can penetrate through the insulator to the semiconductor material. For example, the insulator layer must be thin (approx. 100 nm) and puncture-proof, it must not have any pinholes and it must have a low density of interfaces (approx. 10 ¹¹ cm ^-2 · eV ^–1 ). The gate oxide is generally produced by thermal oxidation of the silicon substrate (see: I. Ruge "Semiconductor Technology", 2nd edition, Springer-Verlag Berlin, Heidelberg 1984, pages 352 to 357).

The Principle of current control by applying a corresponding control potential at the gate can also be used to implement sensors. For this will be the gate - instead a metal contact - with a conductive liquid brought into contact i.e. contacted, a reference electrode in the liquid is arranged. In this way there is a potential at the gate which essentially depends on the potential difference between the gate insulator surface and the semiconductor material, i.e. the silicon substrate (see in addition: P. Bergveld, A. Sibbald "Analytical and Biomedical Applications of Ion-Sensitive Field-Effect Transistors ", Elsevier Science Publishers B.V., Amsterdam 1988, pages 39 to 49).

The Drain-source current of an ion-selective realized in this way Field effect transistor, ISFET for short, depends on the surface potential the gate insulator surface and thus, for example, of the ion concentration in the liquid. According to this principle, both ion and pH sensitive field effect transistors can be realized. An important point is that the liquid is not directly with the Semiconductor material comes into contact, i.e. that the gate material is tight against is the medium to be examined.

Currently, the gate regions of field-effect transistors manufactured using conventional technology, in which the gate material is a thermal oxide, optionally provided with Si ₃ N ₄ , are provided with suitable electroactive membrane materials (see P. Bergveld et al., Op. Cit., Pages 46 to 48). , Layer structures of the type SiO ₂ / membrane or SiO ₂ / parylene / membrane (parylene is a plasma-polymerized aromatic polymer) are used for this purpose. An organic parylene layer can also be used to fix ion-selective molecules covalently to the surface; However, cumbersome multilayer structures, such as SiO ₂ / Si ₃ N ₄ / parylene, are necessary to achieve the required tightness against the analytical solution (see P. Bergveld et al., op. cit., page 43). However, the inevitably associated relatively large layer thickness affects the sensitivity of the sensor element.

task the invention is to design a sensor based on FET in such a way that this Gate material - too in thinner Layer - one covalent coupling of sensor molecules allowed and at the same time the electrical or electronic function with the requirements Fulfills, So that the operability of the sensor guaranteed is.

This is achieved according to the invention in that the gate dielectric is a layer of amorphous hydrogen-containing carbon (aC: H) which has an interface state density ≤ 5 · 10 ¹¹ cm ⁻² · eV ⁻¹ , and that on the surface of the aC: H layer Groups are bound which are sensitive to biochemically active substances.

At the Sensor according to the invention is by the biochemically active substances affects the gate potential, and this enables detection of these substances. It is important that the reaction between the biochemically active substances and the sensitive groups of the sensor is reversible.

The sensitive groups are in particular enzymes, such as glucose oxidase, and complexing agents, for example in the form of suitable cage compounds. The biochemically active substances to be detected are, for example, glucose, fructose, lactate, urea and potassium ions (K ⁺ ).

The sensitive groups are preferred coupled to the aC: H layer via adhesion promoters; however, a direct connection to the aC: H surface can also take place, ie without using an adhesion promoter. Aminoalkyl alkoxysilanes, for example, serve as adhesion promoters and are treated with aliphatic dialdehydes to bind the sensitive groups. Other suitable adhesion promoters are glycidoxypropyl alkoxysilanes and reactive aliphatic and aromatic carboxylic acid derivatives, such as acid chlorides and anhydrides.

A special feature of the amorphous hydrogen-containing carbon used in the sensor according to the invention is its low interface state density, which is 5 5 · 10 ¹¹ cm ⁻² · eV ⁻¹ . Such a material is known and is obtained by hydrogen treatment of aC: H at elevated temperature and pressure (see also EP 0 617 462 A2 ).

The a-C: H layer, which is advantageously structured, generally has an optical bandgap between 0.8 and 2.7 eV. To achieve good insulation properties the optical bandgap preferably 0.9 to 1.7 eV. The a-C: H layer instructs their surface advantageous oxygen-containing groups, the oxygen concentration is preferably between 1 and 20 atomic%.

The gate dielectric is produced by plasma chemical deposition of aC: H layers. These layers can be deposited on silicon substrates in a thin layer (approx. 100 to 500 nm) from a hydrocarbon plasma (RF excitation, capacitive coupling, self-bias voltage between –200 and –900 V); they are sufficiently impervious to water or aqueous media and can be photostructured (see: H. Reichl (Editor) "Micro Systems Technologies 90", Springer-Verlag Berlin, Heidelberg 1990, pages 307 to 312). Appropriate high-pressure hydrogen action achieves a sufficiently low interface state density (see EP 0 617 462 A2 ).

In a subsequent etching step With an oxygen plasma, the a-C: H layers become superficially oxygen-containing Links created (OH, COOH, CO), to which - if necessary via additional Coupling molecules, such as aminoalkyl alkoxysilanes, glycidoxypropyl alkoxysilanes and reactive ones Carboxylic acid derivatives - the sensor molecules, for example Enzymes that can be coupled. Enzymes coupled to such layers show in the fixed state sufficient enzyme activity, which are used to detect corresponding molecules in a ChemFET can.

Based of embodiments the invention is intended to be closer explained become.

A silicon wafer (p-doped Si; resistance: 60 Ω · cm) is - after pretreatment (sputtering in argon for 2 min at 0.2 mbar and a self-bias voltage of –900 V) - in an RF-excited (RF = Radio frequency, ie 0.1 to 100 MHz), capacitively coupled CH ₄ plasma (see for example DE 37 25 700 A1 respectively. U.S. Patent 5,055,421 ) coated with aC: H at a pressure of 1 mbar and a self-bias voltage of –420 V (layer thickness: 0.45 μm; optical bandgap: 1.06 eV). To improve the density of the interface states, the silicon substrate coated in this way is charged with 200 bar of hydrogen in an autoclave and kept at a temperature of approx. 275 ° C. for 8 hours (internal pressure: approx. 400 bar). At the aC: H obtained in this way, an interface state density of 21011 cm ⁻² · eV ^{−1 is} determined at the C / Si interface (evaluation from the CV characteristic curve according to EH Nicollian, JR Brews "MOS (Metal Oxide Semiconductor) Physics and Technology ", John Wiley & Sons, New York, 1982, pages 325 to 328). After the high-pressure hydrogen treatment, the silicon substrate is treated in an oxygen plasma for 20 s (0.2 mbar, −500 V self-bias voltage), which results in an oxygen content of 16.8 atomic% on the surface (at a depth of 2 to 5 nm) ( Carbon content: 80 atom%; nitrogen content: 1.1 atom%; bulk oxygen content: 0.6 atom%).

A sample of the substrate with the dimensions 2.5 cm × 1 cm produced in the above-mentioned manner is placed in a desiccator and held there. At a pressure of approx. 15 mbar, 0.1 ml of 3-aminopropyltriethoxysilane is injected into the sample through a septum. After 5 minutes of exposure, the desiccator is aerated. The adhesion promoter evaporated onto the sample is then baked at 150 ° C. for 15 minutes. The sample is then shaken in a 5% glutardialdehyde solution for 2 hours at room temperature on a shaking table (50 min ^-1 ). After thorough rinsing with a buffer (pH 7.2), the sample is introduced into 40 ml of a glucose oxidase solution (120 mg glucose oxidase with an activity of 252 U / mg in 40 ml of a buffer with pH 7.2) and applied for 24 hours at room temperature shaken the shaking table (50 min ^–1 ). The mixture is then extracted for 5 min with a 1 M NaCl solution and rinsed thoroughly with a buffer (pH 7.2); the sample is then stored in this buffer. To determine the enzyme activity, the sample is placed in 4 ml of a 1.25% D - (+) - glucose solution. At 25 ° C, gassing with purified compressed air is vigorous for 30 min. The residual sugar content is then determined using an enzymatic glucose test (Granutest 100, Diagnostica Merck No. 12193) certainly. From the measured D - (+) - glucose breakdown, an enzyme activity of 0.4 μmol / min is calculated for the sample; after three days the enzyme activity is 0.22 μmol / min.

Claims (7)

Sensor based on a field effect transistor with a gate contact isolated from a substrate by a gate dielectric, characterized in that the gate dielectric is a layer of amorphous hydrogen-containing carbon (aC: H) with an interface state density ≤ 5 · 10 ¹¹ cm ⁻² · eV ⁻¹ , and that sensitive groups for biochemically active substances are bound to the surface of the aC: H layer. Sensor according to claim 1, characterized in that the a-C: H layer has an optical bandgap between 0.9 and 1.7 eV. Sensor according to claim 1 or 2, characterized in that the a-C: H layer is structured. Sensor according to one of claims 1 to 3, characterized in that that the a-C: H layer on the surface oxygen-containing groups with a concentration between 1 and Has 20 atomic%. Sensor according to one or more of claims 1 to 4, characterized in that the sensitive Groups are enzymes or complexing agents. Sensor according to one or more of claims 1 to 5, characterized in that the sensitive Groups over Adhesion promoters are coupled to the a-C: H layer. Sensor according to claim 6, characterized in that the adhesion promoter aminoalkyl alkoxysilanes treated with aliphatic dialdehydes, Glycidoxy propyl alkoxysilanes and reactive aliphatic or aromatic Carboxylic acid derivatives are.