Abstract:
A measurement sensor for analyzing a nonpolar liquid contains a field effect transistor that has an exposed gate contact for wetting with the nonpolar liquid, and an electrical shield that surrounds the gate contact and has openings for inflow and outflow of the nonpolar liquid.

Description:
BACKGROUND INFORMATION 
     U.S. Pat. No. 4,882,292 describes a ChemFET for the analysis of polar liquids. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to a measurement sensor for analyzing a nonpolar liquid, having a field effect transistor that has an exposed gate contact for wetting with the nonpolar liquid, and an electrical shield that surrounds the gate contact and has openings for inflow and outflow of the nonpolar liquid. 
     In nonpolar liquids, the use of a reference electrode in the conventional sense is not advisable, since no electrical or even ionic currents flow in order to compensate for potential differences in the liquid. A reference potential is introduced according to the present invention by the fact that the entire environment of the ChemFET is brought to a defined potential by means of the electrical shield. 
     A further aspect relates to a method for analyzing a nonpolar liquid by sensing the conductivity of a current channel of a field effect transistor whose gate dielectric is wetted by the nonpolar liquid. 
     A further aspect relates to a method for manufacturing a measurement sensor, having the steps of: patterning drain and source regions in a semiconductor substrate, depositing a gate dielectric above a gate channel defined by the drain and source regions, making contact to the drain and source regions, burying the gate dielectric in a sacrificial material, depositing a conductive semiconductor layer on the sacrificial material above the gate dielectric, introducing openings into the conductive semiconductor layer in order to form the electrical shield, and under-etching the conductive semiconductor layer in order to expose the gate dielectric. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross section of a measurement sensor. 
         FIG. 2  is a plan view of the measurement sensor of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of a measurement sensor  1  will be explained with reference to a partial cross section in  FIG. 1 .  FIG. 2  shows measurement sensor  1  in a plan view. Measurement sensor  1  has a ChemFET 2 and an electrical shield  3 . 
     ChemFET 2 is patterned into a semiconductor substrate  10 . Semiconductor substrate  10  is doped with a dopant of a first conductivity type. Two regions for source  11  and drain  12 , doped with a dopant of a second conductivity type, are introduced into semiconductor substrate  10 . Source  11  and drain  12  are contacted via electrodes  13 ,  14 . Source  11 , drain  12 , and electrodes  13 ,  14  are encapsulated by an insulating protective layer  15 . 
     A gate dielectric  21  is applied above a channel region  20  between source  11  and drain  12 . Gate dielectric  21  influences the conductivity of a gate channel  22  that can form in substrate  10  adjacently to gate dielectric  21  and between source  11  and drain  12 . 
     Gate dielectric  21  can be grown or deposited onto semiconductor substrate  10 . 
     Examples of gate dielectric  21  encompass Al 2 O 3 , Si 3 N 4 , SiO 2 , diamond, polycrystalline or amorphous SiC, and polymers having a high chemical resistance to fuels (e.g. stabilized polyamides, polyether ether ketone, polyether sulfone, polyphenylene sulfide, partly or entirely halogenated or fluorinated olefins), and layer combinations thereof. The gate dielectric can furthermore be additionally coated with swellable plastics or porous materials. In the embodiment depicted, gate dielectric  21  is not covered with a further layer, but is exposed and can be brought into contact with a liquid. 
     Gate dielectric  21  is at a floating potential, since it is not coupled via an electrode to a reference potential. Gate dielectric  21  assumes the potential of the environment. The electric fields at gate dielectric  21  are thus defined by the environment. 
     The nonpolar liquid, and substances dissolved or emulsified in the nonpolar liquid, adsorb at the exposed surface of gate dielectric  21 . There is a characteristic adsorption rate for each combination of a substance and the material selected for gate dielectric  21 . Adsorption results in modified electrostatic fields, and influences the dielectric properties of gate dielectric  21 . The change in gate dielectric  21  has an effect on the conductivity of gate channel  22 , which conductivity can be evaluated by an external circuit (not described here). The evaluation can be supported by tables from which the nature and quantity of the substances can be ascertained based on an absolute change in conductivity, a rate of the change in conductivity, etc. 
     An electrical shield  3  is arranged on protective layer  15 . Electrical shield  3  surrounds gate dielectric  21 . A constant electrical potential that is predefined by the potential of electrical shield  3  exists inside electrical shield  3 . The nonpolar liquid, like a vacuum, exerts no influence on the electrical fields and potentials inside the shield. Electrical shield  3  can be set to a defined electrical potential. 
     Electrical shield  3  can be constituted from doped semiconductor material or from a metal, in particular of the platinum group or gold. 
     Electrical shield  3  has openings  31  through which the nonpolar liquid can flow through electrical shield  3 . Openings  31  can be introduced into electrical shield  3  by way of an etching method. 
     A method for manufacturing measurement sensor  1  can make use of the following steps: Firstly a FET having a source  11 , drain  12 , a gate channel  22 , and a gate dielectric  21  is produced. The method steps necessary for this are sufficiently known and will therefore not be discussed further. 
     A sacrificial material is applied locally onto gate dielectric  21 . The sacrificial material is selected from materials that can be selectively etched with respect to the gate dielectric. A layer of conductive semiconductor material is deposited on the sacrificial material. The layer can be supported mechanically on an insulating layer above source  11  and drain  12 . The conductive semiconductor material can encompass, for example, porous silicon carbide. Openings  31  can be formed through the pores of the porous silicon carbide. With other conductive semiconductor materials, openings  31  can be etched into the layer in the vicinity of gate dielectric  21  using masking steps. The layer, patterned in this fashion, forms electrical shield  3 . The sacrificial material is removed selectively with respect to electrical shield  3  and gate dielectric  21 , or can remain as a porous protective layer on the gate at an unmodified or reduced layer thickness. 
     Gate dielectric  21  can be coated with a thin metal layer, e.g. 1 nm to 3 nm, for example gold or a platinum metal. The interface of the metal with gate dielectric  21  influences the conductivity of gate channel  22 . The interface, and consequently also the conductivity of gate channel  22 , are influenced in the context of adsorption onto the thin metal layer. Adsorption rates onto the metal layers differ from those onto dielectric materials, as does their influence on gate channel  22 . 
     Measurement sensor  1  can have two or more ChemFETs whose gate dielectrics  21  have different material compositions or are coated with other metals. The different correlations with the adsorbed substances allow their quantities to be sensed separately from one another. 
     Gate dielectric  21  can have a swelling polymer. In particular, a polymer that swells in gasoline, diesel fuel, kerosene, or oil can be used. 
     Electrical shield  3  surrounding the field effect transistor can be set to a varying, e.g. oscillating, electrical potential. Functional monitoring of the sensor can be carried out by way of the variation in potential. Further information regarding the analytes, e.g. dielectric properties, can likewise be determined by way of a varying potential.