Abstract:
The invention relates to an organic field effect transistor, a method for structuring an OFET and an integrated circuit with improved structuring of the functional polymer layers. Structuring is achieved by scraping the functional polymer into a mold layer in which recesses are initially produced by exposure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is the 35 USC 371 national stage application PCT/DE01/03318 filed on Aug. 29, 2002, which designated the United States of America. 
     FIELD OF THE INVENTION 
     The invention relates to an organic field effect transistor, a method for structuring an OFET and an integrated circuit with improved structuring of the functional polymer layers. 
     BACKGROUND OF THE INVENTION 
     Integrated plastic circuits based on OFETs are used for microelectronic mass applications and disposable products such as identification and product tags. A tag is, for example, an electronic bar code of the kind provided on merchandise or on luggage. OFETs have a wide range of uses as RFID tags: radio frequency identification tags, which need not be arranged only on the surface. In the case of OFETs for these applications, the excellent operating behavior of silicon technology can be dispensed with, but by the same token low production costs and mechanical flexibility should be ensured. The components, such as, for example, electronic bar codes, are typically disposable products and of interest economically only if they are produced in economical processes. 
     To date, owing to the production costs, only the conductor layer of the OFET has been structured. Structuring can only be realized using a two-stage process (“lithography method”, cf. in this respect Applied Physics Letters 73(1), 1998, p. 108.110 and Mol. Cryst. Liq. Cryst. 189, 1990, p. 221-225) with firstly uniform exposure and subsequent structuring, which moreover is material-specific. “Material specificity” means that the process described, with the photochemical components mentioned, only function on the conductive organic material polyaniline. A different conductive organic material, e.g. polypyrrole, cannot readily be structured in this way. 
     The lack of structuring of the other layers, such as at least the structuring of semiconducting and of the insulating layer comprising functional polymers (which may be present in polymeric or oligomeric form), leads to a substantial reduction in performance of the OFETs obtained, but it is dispensed with for cost reasons. The structured layer can be structured by other known methods (such as, for example, printing) only in such a way that the length l , which denotes the distance between source and drain electrodes and is therefore a measure of the power density of the OFET, is at least 30 to 50 μm. However, lengths l of less than 10 μm are desirable, and hence at present no structuring method apart from the complicated lithography method appears reasonable. 
     SUMMARY OF THE INVENTION 
     It was therefore an object of the invention to provide a cost-effective method for structuring OFETs with high resolution which is suitable for mass-production. Furthermore, it is an object of the invention to provide an OFET which is more powerful, because it is equipped with more structured layers, and which is more compact and can be produced with a smaller distance l. 
     The invention relates to an organic field effect transistor (OFET), comprising at least the following layers on a substrate:
         an organic semiconductor layer between and above at least one source electrode and at least one drain electrode which comprise a conductive organic material,   an organic insulation layer above the semiconducting layer and   an organic conductor layer,
 
the conductor layer and at least one of the other two layers being structured. Moreover, the invention relates to a method for structuring an OFET by scraping at least one functional polymer into a negative mold.
       

     Finally, the invention relates to an integrated circuit which comprises at least one OFET which has at least one structured conductor layer and a further structured layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a side elevation sectional view of a substrate over which is a negative mold layer; 
         FIG. 2  is a side elevation sectional view of the substrate and mold layer of  FIG. 1  with the mold layer being exposed through a shadow mask; 
         FIG. 3  is a side elevation sectional view of the substrate and mold layer of  FIG. 2  with recesses formed in the mold layer; 
         FIGS. 4 and 5  are side elevation sectional views of the substrate and mold layer of  FIG. 3  wherein the recesses are shown being filled in with a functional polymer; 
         FIG. 6  is a side elevation sectional view that shows the filled recesses; 
         FIG. 7  is a side elevation sectional view of the substrate and formed product on the substrate; and 
         FIG. 8  is a diagrammatic side elevation view of a process using a moving belt for forming the product on the substrate. 
     
    
    
     A negative mold is designed as a structured layer or a part of a structured layer which contains recesses into which functional polymers, which form, for example, an electrode of an OFET or a semiconductor layer or an insulator layer, is introduced by scraping. 
     The length l which describes the distance between source and drain electrode can be reduced to the order of magnitude of λ (wavelength) of the incident light if the negative mold is structured by exposure to radiation. A preferred OFET is one which has a length l of less than 20 μm, in particular of less than 10 μm and very particularly of 2 to 5 μm or less. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The method comprises the following operations:
         a) an optionally whole-area mold layer, which need not be restricted to the region which is to be structured, is applied to a substrate or a lower layer; this mold layer is not the functional polymer (i.e. semiconducting, conductive or insulating layer) but another organic material which serves as a mold or block for the conductive organic electrode layer; this other organic material should have insulating properties;   b) by exposure through a mask, the mold layer acquires recesses which correspond to the structures;   c) the functional polymer is then scraped in liquid form, as a solution and/or as a melt into these recesses.       

     The negative mold of the structure on the mold layer can be produced by exposing a photoresist layer on the substrate or a lower layer. The material of the negative mold may be a photoresist which has recesses after exposure through a mask, such as, for example, a shadow mask, or another structuring method already described and subsequent development. 
     Resists suitable for this purpose are all commercially available and the methods for structuring them, for example, by exposure, are known from the literature. 
     The advantage of the doctor blade method consists in the fact that the difficult structuring of the functional polymer can be implemented by the established and proven photoresist method. It is thus possible to rely on the rich technical background, and extremely fine structures can be achieved. The doctor blade method is moreover not material-specific. Rather, polyaniline, but also any other conductive organic layer, e.g. polypyrrole, can be used for the production of electrodes by the doctor blade method. Any other organic material, such as, for example, polythiophene as a semiconductor and/or polyvinylphenol as an insulator, can likewise be structured thereby, i.e. the whole OFET. 
     In the multilayer structure of an OFET, one or more layers can be produced by the doctor blade method. In the case of a plurality of layers, the photoresist technique in the formation of the negative mold is preferred because, for example, the imprint method does not structure the mold layer over the whole layer thickness but leaves in the recesses a certain base which prevents electrical contact with the layer underneath. This plays no role for the first layer, e.g. source/drain electrodes, but does for all further layers. 
     According to a preferred embodiment of the method,  FIG. 8 , the method is operated continuously, i.e., a belt  9  with the mold layer  1 . is transported past different stations in succession, where first recesses  8  are formed in the mold layer  1 , by for example, exposure using a mask  10  applied to the mold layer  1 . The recesses  8  are then filled at least once with functional polymer  7  by means of a doctor blade  6  at a doctor blade station further down the course. The mold layer portions and mask are then removed at the final station. 
     Another approach for avoiding a possible height difference between negative mold and functional polymer is to repeat the scraping process, with the result that the volume of the negative mold is simply further filled. 
     As a rule, the functional polymers can be substantially left in their optimum consistency. Thus, for example, polyaniline, a conductive organic material, has a certain viscosity at optimum conductivity. If polyaniline is to be printed, its viscosity must be adjusted to the value adapted to the printing method. This generally means adverse conductivity effects. The viscosity range for scraping is greater than that for printing, so that as a rule no viscosity changes need be made to the organic material. 
     Finally, an advantage of the scraping method is the ability to obtain thick layers. Thus, for example, the conductivity of 1 μm thick polmer electrodes is effectively higher than in the case of the usual 0.2 μm layer thickness. An OFET having a layer thickness in the range up to 1 μm, in particular in the range from 0.3 to 0.7 μm, is therefore advantageous. 
     According to a preferred embodiment of the method, it is operated continuously, i.e. a belt with the mold layer is transported past different stations in succession, where first recesses are formed in the mold layer by, for example, exposure using a mask and said recesses are then filled at least once with functional polymer by means of a doctor blade station in the further course. 
     Here, a “functional polymer” is defined as any organic, organometallic and/or inorganic material which is functionally involved in the production of an OFET and/or of an integrated circuit comprising a plurality of OFETs. These include, for example, a conductive component (e.g. polyaniline), which forms an electrode, the semiconducting component, which forms the layer between the electrodes, and the insulating component. It is expressly pointed out that the term “functional polymer” accordingly also includes nonpolymeric components, such as, for example, oligomeric compounds. 
     Here, “organic” summarizes everything which “is based on organic material”, the term “organic material” including all types of organic, organometallic and/or inorganic synthetic materials which are referred to in English, for example, as “plastics”. These comprise all types of substances with the exception of the classical semiconductors (germanium, silicon) and the typical metallic conductors. A restriction in the dogmatic sense to organic materials as carbon-containing material is accordingly not intended, but the broad use of, for example, silicones is also envisaged. Furthermore, the term is not intended to be restricted to polymeric or oligomeric materials, but the use of “small molecules” is also entirely conceivable. 
     Here, “lower layer” is defined as any OFET layer to which a layer to be structured is applied. The mold layer comprising the mold polymer is adjacent to the “lower layer” or the substrate. Furthermore, the term “polymer” here does not specify a polymeric state of aggregation for the mold polymer, but this substance may also be any plastic which can be used in practice for the formation of a negative mold. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the method is explained in more detail below with reference to schematic figures. 
       FIG. 1  shows the substrate or the lower layer  2  to which the mold layer of the negative mold  1 , for example comprising a mold polymer, such as a photoresist, is applied over the whole surface. As shown in  FIG. 2 , the mold layer is exposed through a shadow mask  4  to, for example, UV radiation  3 . This results in the formation of recesses S in the mold layer  1 , as shown in FIG.  3 . The functional polymer  7  is then scraped into these recesses by means of a doctor blade  6  (FIGS.  4  and  5 ).  FIG. 6  shows how the functional polymer  7  fills the recesses  8  of the mold layer  1  in the prepared OFET.  FIG. 7  shows the final product on the substrate.  FIG. 8  shows an implementing conveyor apparatus for forming the product. Here the substrate lower layer  2  is on a moving belt  9  that moves in the direction of the arrow. The mask  4  is applied, the mold lower layer  2  is then exposed through the mask to form recesses  8 , a doctor blade  6  then fills the recesses  8  with the polymer  7 , the mold layer and mask portions are then removed leaving the product  7 ′ on the belt.