SENSOR USING A FIELD EFFECT TRANSISTOR AND METHOD OF FABRICATING THE SAME

Abstract: Disclosed is a sensor using a field effect transistor. The sensor includes a field effect transistor having a gate elec­ trode, a reactive monomolecular film formed on the surface of the gate electrode, and a sensing material fixed on the gate electrode through the reactive monomolecular film by the chemical bond. The sensing material is strongly bonded to the reactive monomolecular film in such manner that the sensing material is kept alive. Thereby, sensitivity of the sensor is im­ proved.

Description: This invention is concerned with a sensor comprising a field effect transistor having a gate electrode and a sensing material fixed to the gate electrode. Sensors comprising field effect transistors (FET) are used as pH sensors, sensors of various ions, bio-sensors, and for other purposes. Known bio-sensors, for example, typically comprise glass electrodes having proteins or enzymes fixed to their surface or FETs having gate electrodes with proteins or enzymes fixed thereto. The known methods of bonding proteins or enzymes to such surfaces comprise directly applying the protein or enzyme to the surface; or mixing the protein or enzyme with a resin and applying the mixture to the surface; or bonding the protein or enzyme to resin particles which are then applied to the surface; or bonding the protein or resin by means of an organic film. These methods are not generally satisfactory and the sensitivity of the resulting sensors is not high. Thus, in the first method referred to, the direct application method, the protein or enzyme tends to become detached during use of the sensor and the durability of the sensor is not adequate. In the second, third and fourth methods, the protein or enzyme becomes partially deactivated so that the sensitivity of the sensor is inadequate. Known sensors of these various kinds are described in U.S. Patents 3,831,432 and 4,020,830 and Japanese Patent 59210356. It is an object of this invention, therefore, to provide a sensor comprising an FET having a gate electrode and a sensing material fixed to the gate electrode in which the sensing material is firmly bonded and retains its activity. According to one aspect of the present invention, there is provided a sensor comprising a field effect transistor having a gate electrode and a sensing material fixed to the gate electrode, characterised in that the gate electrode surface has a reactive monomolecular film or a thin film of a photosensitive hydrophilic polymer directly thereon and the sensing material is bonded to the film by means of Schiffs base or CNBr induced bonding whereby the sensing material retains its activity. According to another aspect of the invention, there is provided a method of making a sensor which comprises forming a field effect transistor having a gate electrode on a semiconductor substrate and fixing a sensing material to the gate electrode, characterised by the steps of: forming a photoresist layer on said semiconductor substrate except on the portion of said gate electrode; forming a reactive monomolecular film on the surface of said gate electrode, said reactive monomolecular film being formed by means of a silane surface active agent comprising a reactive group; converting the reactive groups of said silane surface active agent into hydroxyl groups; converting the hydroxyl groups into aldehyde groups; fixing a sensing material which is a protein or enzyme to the aldehyde groups by means of Schiffs base or CNBr induced bonding; and removing said photoresist layer. In a modification of this method, instead of using a photoresist layer to form the monomolecular film on the gate electrode, the monomolecular film is formed all over the substrate of said semiconductor surface and is exposed, except that part on the gate electrode area, to an energy beam which deactivates said reactive monomolecular film. In a further modification of the method according to the invention, instead of forming a monomolecular film on the gate electrode, a thin film of a photosensitive hydrophilic polymer is formed on said semiconductor substrate; said thin film of a photosensitive hydrophilic polymer is removed from said semiconductor substrate except at the portion on said gate electrode by using a mask having an opening which corresponds to said gate electrode; the hydrophilic groups of said polymer are converted into aldehyde groups and the latter are reacted with the sensing material, or the hydrophilic groups of said polymer are reacted with cyanobromide and then with the sensing material. In order that the invention may be more fully understood, preferred embodiments of sensors and their production will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 to Fig. 7 are diagrammatic sections, on an enlarged scale, through a portion of a sensor showing various stages in its production, Fig. 3 to Fig. 6 showing on a molecular scale the disposition and chemical conversions of a monomolecular film forming part of the sensor; Fig. 8 is a diagrammatic representation, on a molecular scale, of a monomolecular film according to the second embodiment of the invention; and Fig. 9 to Fig. 12 are diagrammatic sections, on an enlarged scale, through a portion of a third embodiment of sensor showing various stages in its production. Referring to Figs. 1-7, a first embodiment of sensor is made by first forming an FET 12 on a p-type semiconductor substrate 10. The FET 12 is formed in a conventional manner and it comprises n⁺ type source, drain regions 14, 16, source, drain electrodes 18, 20, gate oxide film 22, gate electrode 24, and insulation film 26. The semiconductor substrate 10 on which the FET 12 is formed is covered with a resist layer 28 and only the resist on gate electrode 24 is exposed, developed and removed to form an opening 30 (Fig. 1). A reactive monomolecular film is then formed through the opening 30 with a compound comprising a straight hydrocarbon chain with a vinyl or acetylene group at one end and a chlorosilane group at the other end (the chlorosilane derivative). A compound of this kind may, for example, be represented by the formula: CH₂=CH-(CH₂) n -SiCl₃ where n is an integer of up to 25, preferably of from 10 to 20, and the vinyl groups may be replaced by a CH≡C- group. The monomolecular film 32 of the chlorosilane derivative is formed by a chemical adsorption process (Fig.2). The metal oxide surface of the gate electrode 24 and the chlorosilane groups of the chlorosilane derivative selectively react with each other to form a monomolecular film of the formula: The film may be formed, for example, by dipping the FET 12 for 2 or 3 minutes into a solution of the chlorosilane derivative in a mixture of 80% n-hexane, 12% carbon tetrachloride and 8% chloroform, the solution containing from 2.0 x 10⁻³ to 5.0 x 10⁻² mol/litre of the chlorosilane derivative. The spatial arrangement of the monomolecular film 32 is shown diagrammatically in Fig. 3; it will be seen that the film is bonded to the metal oxide of the gate electrode 24 by a bond 34 of the formula The vinyl groups 36 of the chlorosilane derivative are arranged in an orderly manner on the free surface of the monomolecular film 32. The resist pattern 28 (Figs. 1 and 2) is then removed and the substrate bearing the monomolecular film 32 is dipped into a 1 mol/litre solution of dibolan in tetrahydrofuran at room temperature and then into an aqueous solution containing 0.1 mol/litre of NaOH and 30% H₂O₂ in order to add hydroxyl groups 38 to the vinyl groups 36 (or acetylene groups) on the surface of the monomolecular film 32. The substrate is then dipped into an aqueous periodate solution which oxidises the -OH groups to aldehyde groups 40 (Fig. 5) in accordance with equation (1): A protein or enzyme having a desired specific activity is then fixed to the film 32 by the reaction shown in equation (2): where R₂-NH₂ is a protein or enzyme. The resulting product is shown diagrammatically in Figs. 6 and 7. Fig. 6 shows the protein or enzyme 42 chemically linked to monomolecular film 32 and Fig. 7 shows the sensor having the protein or enzyme sensing material 44 bonded thereto. The coated substrate is then diced and the appropriate electrodes attached thereto to complete the production of the bio-sensor. In the above-described process, the aldehyde method of linking the protein or enzyme was used, but the latter may also be fixed by a cyanobromide method as shown in equation (3): The monomolecular film can also be formed on the gate electrode by the Langmuir-Blodgett (LB) method using CH₂=CH-(CH₂) COOH, CH≡C-(CH₂) COOH or the like. In the above example the monomolecular film was formed only on the gate electrode by using the resist 28, but the same effect can be obtained by forming a chlorosilane derivative film 34,36 all over the surface of the substrate (see Fig. 8) and then polymerising the vinyl or acetylene groups in all areas except that of the gate electrode by exposure to an energy beam 46 (which is such as to cause polymerisation of said groups). This has the effect of deactivating the vinyl or acetylene groups in all areas 48 other than the gate electrode area. The vinyl or acetylene groups in the gate electrode are then converted into the carbinol groups as previously described (Fig.4). The procedures described lead to firm binding of the protein or enzyme to the substrate with no loss of activity. Bio-sensors of good sensitivity and reliability are, therefore, obtained. The procedure can be carried out easily so that the production costs are low. A third embodiment of sensor and its method of production are shown in Figs. 9 to 12. Referring to Fig. 9, a substrate 10 bearing an FET 12 having the same features as that of Fig. 1 is coated with a thin film 50 of a photosensitive hydrophilic polymer. The film 50 is formed by spin coating the substrate with an aqueous solution of the polymer, for example a solution of 10g of pullulan having a molecular weight of 200,000 and 1g of ammonium dichromate in 100cc of water. The film 50 has a thickness of about 0.5 µm. Using a photo mask having a desired pattern, for example a photomask 54 having an opening 52 corresponding to and aligned with the gate electrode 24, the film is exposed to light 56 (Fig.10) and developed to leave a hydrophilic polymer film pattern 58 selectively on the gate electrode (Fig.11). The coated substrate is then dipped in an aqueous periodate solution to oxidise the surface -OH groups to aldehyde groups in accordance with equation (4): A protein or enzyme having a desired specific activity is then reacted with the CHO group- containing film in accordance with equation (2) above. The coated substrate is then diced and appropriate electrodes are attached thereto to complete the production of the bio-sensor. In the process just described, chemical bonding of the protein or enzyme was by the aldehyde reaction, but the protein or enzyme can be equally bound by using a cyanobromide procedure as shown in equation (6): Instead of pullulan, other polysaccharides such as pectin, proteins such as gelatin or casein, or water-soluble polymers, such as polyvinyl alcohol and polyvinyl pyrrolidone may be used to form the thin film of hydrophilic polymer. Any other film-forming material which contains hydroxyl groups in its molecule, such as novolak resins, can also be used for this purpose. Pullulan which is partially acetylated (degree of replacement 1.5 to 2.5) can also be used in place of pullulan; in this case the film is formed using acetone as solvent. The photosensitive thin film can also be made using optical cross-linking agents other than ammonium dichromate, such as other dichromates, diazo compounds, such as diazo resin, or diazide compounds, such as 4,4'-diazide-stilbene-2,2'-disulphonic acid sodium salt.

Claims: A sensor comprising a field effect transistor having a gate electrode and a sensing material fixed to the gate electrode, characterised in that the gate electrode surface has a reactive monomolecular film or a thin film of a photosensitive hydrophilic polymer directly thereon and the sensing material is bonded to the film by means of Schiffs base or CNBr induced bonding whereby the sensing material retains its activity. A sensor according to claim 1, characterised in that the sensing material is a protein or an enzyme. A sensor according to claim 1 or 2, characterised in that the reactive monomolecular film is formed from a compound comprising a straight hydrocarbon chain containing up to 25 carbon atoms with a vinyl or acetylene group at one end and a chlorosilane group at the other end. A sensor according to claim 1 or 2, characterised in that the photosensitive hydrophilic polymer contains hydroxyl or carboxylic groups. A sensor according to any of claims 1,2 and 4, characterised in that the bonding between the photosensitive hydrophilic polymer and a sensing material which is a protein or an enzyme is effected through a bond of one of the following formulae: or -CH=N- . A sensor according to any of claims 1,2,4 and 5, characterised in that the photosensitive hydrophilic polymer is derived from a polysaccharide, a protein, or a water-soluble synthetic polymer. A sensor according to claim 6, characterised in that the photosensitive hydrophilic polymer is derived from pectin, casein, polyvinyl alcohol or polyvinyl pyrrolidone. A sensor according to any of claims 1, 2 and 4 to 7, characterised in that the hydrophilic polymer is rendered photosensitive by cross-linking with an optical cross-linking agent which is a dichromate, a diazo compound or a diazide compound. A sensor according to claim 8, characterised in that the optical cross-linking agent is ammonium dichromate, a diazo resin, or 4,4'-diazidestilbene-2,2'-disulphonic acid sodium salt. A method of making a sensor which comprises forming a field effect transistor having a gate electrode on a semiconductor substrate and fixing a sensing material to the gate electrode, characterised by the steps of: forming a photoresist layer on said semiconductor substrate except on the portion of said gate electrode; forming a reactive monomolecular film on the surface of said gate electrode, said reactive monomolecular film being formed by means of a silane surface active agent comprising a reactive group; converting the reactive groups of said silane surface active agent into hydroxyl groups; converting the hydroxyl groups into aldehyde groups; fixing a sensing material which is a protein or enzyme to the aldehyde groups by means of Schiffs base or CNBr induced bonding; and removing said photoresist layer. A method according to claim 10, characterised in that the silane surface active agent is a compound comprising a straight hydrocarbon chain containing up to 25 carbon atoms with a vinyl or acetylene group at one end and a chlorosilane group at the other end, the chlorosilane groups reacting with the gate electrode surface and the vinyl or acetylene groups being converted into aldehyde groups. A method according to claim 10 or 11, characterised in that the monomolecular film is formed on the gate electrode surface by a chemical adsorption process or by a Langmuir-Blodgett process. A method according to claim 10, characterised in that instead of using a photoresist layer to form the monomolecular film on the gate electrode, the monomolecular film is formed all over the substrate of said semiconductor surface and is exposed, except that part on the gate electrode area, to an energy beam which deactivates said reactive monomolecular film. A method according to claim 10, characterised in that instead of forming a monomolecular film on the gate electrode, a thin film of a photosensitive hydrophilic polymer is formed on said semiconductor substrate; said thin film of a photosensitive hydrophilic polymer is removed from said semiconductor substrate except at the portion on said gate electrode by using a mask having an opening which corresponds to said gate electrode. the hydrophilic groups of said polymer are converted into aldehyde groups and the latter are reacted with the sensing material, or the hydrophilic groups of said polymer are reacted with cyanobromide and then with the sensing material.

Inventors: OGAWA, KAZUFUMI
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1986
Main classification: G01N27
IPC codes: G01N27