Abstract:
The invention relates to a molecular electronics arrangement comprising a substrate, at least one first strip conductor having a surface and being arranged in or on the substrate, a spacer which is arranged on the surface of the at least one first strip conductor and which partially covers the surface of the at least one first strip conductor, and at least one second strip conductor which is arranged on the spacer and comprises a surface which faces the surface of the at least one first strip conductor in a plane manner. The spacer partially covers the surface of the at least one second strip conductor, and defines a pre-determined distance between the at least one first strip conductor and the at least one second strip conductor. The inventive molecular electronics arrangement also comprises molecular electronics molecules which are arranged between a free region of the surface of the at least one first strip conductor and a free region of the surface of the at least one second strip conductor, the length of said molecules being essentially equal to the distance between the at least one first strip conductor and the at least one second strip conductor. The invention also relates to a method for producing a molecular electronics arrangement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National-Stage entry under 35 U.S.C. § 371 based on International Application No. PCT/DE02/02379, filed 1 Jul. 2002, which was published under PCT Article 21 (2); this application also claims priority under 35 U.S.C. § 119 to German Application No. DE 101 32 640.8, filed Jul. 5, 2001. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a molecular electronics arrangement and a method for fabricating a molecular electronics arrangement. 
     Conventional silicon microelectronics will reach its limits as miniaturization advances further. In particular the development of increasingly smaller and more densely arranged transistors of hundreds of millions of transistors per chip will be subject to fundamental physical problems in the next ten years. If structural dimensions fall below 80 nanometers, the components are influenced by quantum effects in a disturbing manner, and they are dominated by quantum effects at dimensions of below about 30 nanometers. The increasing integration density of the components on a chip also leads to a dramatic increase in the waste heat. 
     One known possible successor technology to follow conventional semiconductor electronics is molecular electronics, in which both electron conduction and fundamental component functions are performed by suitable molecules. An overview of the field of molecular electronics is given by [1], for example. There are predominantly two types of molecules suitable for realizing electronic component functions. They are firstly molecules based on carbon nanotubes and secondly molecules based on polyphenylene. 
     Ellenbogen, J C, Love, J C (1999) “Architectures for molecular electronic computers; 1. Logic structures and an adder built from molecular electronic diodes”, MITRE Nanosystems Group, McLean, Va. discloses examples of molecular electronic molecules which, on account of their atomic structure, have the capability of fulfilling the function of an electrical conductor, an electrical insulator or a diode. Furthermore, the same document discloses molecules which perform the function of logic gates. 
     Accordingly, many component functions that are usually realized by conventional silicon microelectronics can also be realized by molecular electronic molecules, molecular electronics having the advantage of a considerably smaller dimension compared with conventional electronics. Whereas the smallest semiconductor structures that can be realized nowadays are of the order of magnitude of a few nanometers, structural dimensions down to the angstrom range are conceivable in the case of molecular electronic molecules. This means that significantly more compact arrangements of circuits are possible and the waste heat that occurs is a small amount in the case of molecular electronic circuits. Furthermore, Gittins, D I, Bethell, D, Schiffrin, D J, Nichols, R J (2000) “A nanometer-scale electronic switch consisting of a metal-cluster and redox-addressable groups” Nature 408:67–69 discloses that bispyridinium molecules have the property of occurring in two redox states the molecule having a greatly different electrical conductivity in the two redox states. Bispyridinium molecules have a low electrical conductivity in the oxidized state; the electrical conductivity is significantly higher in the reduced state. If a sufficiently high electrical voltage is applied to the molecule, then it is thereby possible to bring about the transition of the molecule from one state to the other. The redox transitions are reversible. On the basis of these properties, Gittins et al. describes the use of bispyridinium molecules as molecular electronic nanoswitches. 
     On account of their electrical properties and their small dimension, recently there has been much discussion concerning the use of organic, in particular biological, molecules for electronic applications. In this case, the problem arises that it is necessary to bind the molecular electronic molecules to suitable contacts, for example metallic conductors. This is necessary for the coupling to conventional silicon microelectronics. 
     Molecular electronic molecules are provided with functional groups which are suitable for chemical binding to a conductor material. The binding of thiol groups (SH), which occur at the end sections of many biomolecules, to conductor surfaces made of gold material has proved to be particularly suitable. The metallic conductors have to be provided in a suitable arrangement at a distance from one another which lies in the nanometers range, in order that the interspaces between two such conductor surfaces can be bridged by the molecular electronic molecules. The linear extents of molecular electronic molecules are typically of the order of magnitude of between a few angstroms and up to hundreds of nanometers. 
     The prior art discloses techniques for enabling a molecular electronic molecular to be coupled to conductor surfaces at two of its end sections. Thus, often a gold interconnect is fabricated by means of electron beam lithography, for example, said interconnect having a constriction of approximately 10 nanometers wide at its narrowest point. Such an interconnect is then torn away in a controlled manner at the narrowest point in a bending device, the distance between the two torn-away ends lying in the nanometers range and being adjustable by means of the bending mechanism. If a solution comprising molecular electronic molecules whose extent essentially corresponds to the distance between the two torn-away ends is then added, these molecules may bind for example via two thiol groups to the two torn-away ends, for example made of gold material, and electrically bridge the latter. 
     Even though this procedure is suitable for laboratory experiments, it is unsuitable for the integration of many molecular bridges. Applications of molecular electronics that are of economic interest aim, however, precisely to form a multiplicity of molecular electronic couplings between metallic electrodes. Therefore, the method described is too time-consuming and too expensive to be considered for the mass production of arrangements with a large multiplicity of molecular electronic components. Furthermore, the degree of reproducibility is low in the case of the described method for coupling metallic conductors by means of a molecular electronic molecule. It is difficult to carry out the tearing-away of the constricted gold interconnect by the bending device under controlled conditions. Furthermore, slightly different torn-away ends are obtained in each tearing-away operation, so that it is not possible to exactly set the distance between the torn-away ends. However, in order that molecular electronic molecules can immobilize on two conductor surfaces, a very precise matching of the linear extent of the molecule, on the one hand, and the distance between the conductor planes, on the other hand, is necessary. 
     As an alternative, there are proposals firstly to fabricate a contact electrode, then to apply the organic material (for example in the form of a monomolecular layer) and then to deposit the second electrode thereon. In this case, the process for fabricating the second electrode is to be chosen in such a way that the organic material is not damaged, which is a considerable restriction. Semiconductor technological method steps are often performed at high temperatures. Biomolecules are often unstable compounds which react extremely sensitively to excessively high temperatures. By way of example, it is known that many proteins denature irreversibly, and can thus be destroyed, at temperatures as low as slightly above room temperature. What is more, many semiconductor technological methods require the presence of aggressive chemicals (e.g. etching chemicals or deposition gases). Sensitive molecules can thereby easily be destroyed. By way of example, proteins and DNA strands can be decomposed if the pH value deviates too far from physiological pH values. For arrangements of economic interest, however, no consideration can be shown for the sensitivity of the molecules during the formation of the second electrode if the arrangements fabricated in accordance with the method are intended to be economically competitive. On the other hand, consideration must be shown for the intactness of the biomolecules, since biomolecules are frequently present only in low concentrations and are often expensive. Therefore, the method described is suitable only to a very limited extent for introducing sensitive, in particular temperature-sensitive, biomolecules. 
     To summarize, it can be stated that the known techniques for forming a coupling of two metal electrodes by means of a molecular electronic module have a series of disadvantages. Thus, structurally adequately defined arrangements cannot be fabricated in accordance with the known techniques and the methods are complicated, expensive and not suitable for economic mass production. 
     DE 100 13 013 A1 and U.S. Pat. No. 6,128,214 in each case describe a chemically synthesized electronic component comprising two crossing conductors between which an electrically addressable molecular species is arranged. 
     Furthermore, WO 01/27972 A2 describes a molecular electronic device which has two conductive contacts and also a conductive bridge connection between the two contacts. The conductive bridge connection can be brought, by means of a voltage pulse, to an arbitrary one of two predetermined states, to a first state of higher electrical conductivity or to a second state, in which the conductive bridge connection has a lower electrical conductivity by comparison with the first state. The conductive bridge connection has organic molecules. 
     U.S. Pat. No. 5,270,965 describes a method for driving a component having a pair of electrodes and, embedded between the latter, an organic insulation layer. 
     Other molecular electronic components are described in E. Emberly et. al, Principles for the design and operation of a molecular wire transistor, Journal of Applied Physics, 88 (2000) 9, pp. 5280–5282, 2000, J. Chen et. al., Large On-Off Ratios and Negative Differential Resistance in a Molecular Electronic Device, Science, 286, pp. 1550–1552, 1999, C. P. Collier et. al., Electronically Configurable Molecular-Based Logic Gates, Science, 285, pp. 391–394, 1999 and R. M. Metzger et. al., Unimolecular Electrical Rectification in Hexadecylquinolinium Tricyanoquinodimethanide, Journal Am. Chem. Soc., 119, pp. 10455–10466, 1997. 
     SUMMARY OF THE INVENTION 
     The invention is based on the problem of providing a molecular electronics arrangement in which molecular electronic molecules can be precisely coupled to electrodes with a reduced outlay, and in which the molecular electronic molecules are better protected against destruction. 
     The problem is solved by means of a molecular electronics arrangement and a method for fabricating a molecular electronics arrangement having the features in accordance with the independent patent claims. 
     The molecular electronics arrangement of the invention has a substrate, at least one first interconnect, which has a surface and which is arranged in or on the substrate, a spacer arranged on the surface of the at least one first interconnect, which spacer partly covers the surface of the at least one first interconnect, at least one second interconnect arranged on the spacer, which second interconnect has a surface areally opposite the surface of the at least one first interconnect, the spacer partly covering the surface of the at least one second interconnect, and a predetermined distance between the at least one first interconnect and the at least one second interconnect being defined by means of the spacer. The molecular electronics arrangement furthermore has molecular electronic molecules arranged between an uncovered region of the surface of the at least one first interconnect and an uncovered region of the surface of the at least one second interconnect, the length of which molecules is essentially equal to the distance between the at least one first interconnect and the at least one second interconnect. 
     The invention provides a molecular electronics arrangement in which the distance between the at least one first interconnect and the at least one second interconnect is accurately adjustable by means of the spacer or spacers. An accuracy of the order of magnitude of one atomic layer, i.e. of a few angstroms, can be achieved by applying a defined number of atomic layers of a material to the at least one first interconnect by means of a suitable semiconductor technological method, for example the atomic layer deposition method (ALD method), thereby defining the thickness of the spacer. Since the distance between the first interconnects and the second interconnects should essentially be equal to the length of the molecules to be introduced between them, the distance between the two interconnects can be adapted, according to the invention, to a specific molecule to be incorporated. It should be noted that the length of the respective molecular electronic molecule is dependent on the geometrical arrangement thereof in the interspace between the two interconnects. To put it another way, this means that the distance between the two interconnects may vary by up to a factor of 2 with regard to the length of the respective molecular electronic molecule. In this case, the distance between the two interconnects is always less than the length of the respective molecular electronic molecule. 
     The molecular electronic molecules may be introduced into the molecular electronics arrangement if the at least one interconnect, the at least one second interconnect and the spacer arranged between them have already been formed. This enables a careful incorporation of the often sensitive molecular electronic molecules, for example biomolecules such as DNA strands or proteins. The molecular electronic molecules are thereby protected against negative influences (as described above with reference to the prior art). 
     In the molecular electronics arrangement, furthermore, at least one of the first interconnects and/or at least one of the second interconnects may be coupled to an electrically conductive coupling element, by means of which the interconnects can be coupled to an external circuit. Preferably, each of the at least one electrically conductive coupling elements is a contact hole filled with an electrically conductive material, which contact hole is introduced into the substrate. The electrically conductive coupling element may be fabricated for example from polysilicon material, aluminum material or tungsten material. 
     The molecular electronics arrangement may furthermore have an external circuit, which is coupled to at least one of the first interconnects and/or of the second interconnects by means of the electrically conductive coupling elements. The external circuit is preferably an integrated circuit integrated into the substrate. The external circuit may be fabricated using CMOS technology. The substrate may be a silicon wafer. 
     By virtue of the fact that, according to the invention, electrically conductive coupling elements are provided by means of which the first and/or second interconnects can be coupled to an external electrical circuit, the molecular electronic molecules can also indirectly be coupled to the external circuit. Consequently, the invention realizes a coupling of molecular electronic molecules to conventional silicon microelectronics. The invention clearly creates an interface between the microscopic dimension of molecular electronic molecules and the macroscopic dimension of conventional silicon microelectronics. By way of example, if the molecular electronic molecules are used as microscopic sensors and/or as microscopic electronic components (e.g. electrical conductors, electrical insulators, diodes, transistors, logic gates), then the molecular electronic molecules can be driven or read out by means of the external circuit and by means of the electrically conductive coupling elements. 
     The coupling of the molecular electronic molecules to an external circuit is advantageous since this opens up diverse possibilities of application of the molecular electronics arrangement. By way of example, the molecular electronic molecules may be used as a sensor (for example for the presence of an ambient atmosphere or the presence of an ambient temperature). 
     Furthermore, the molecular electronics arrangement of the invention may also be used as a molecular memory. For this purpose, molecular electronic molecules in the case of which, by way of example, an electrical signal effects a change in a, for example, electrical parameter (e.g. the electrical resistance) may be introduced into the molecular electronics arrangement of the invention. If the electrical resistance of a molecule can optionally be set to a high value or a low value, for example, by application of an external voltage signal, then a molecular electronic molecule may serve as a data memory of one bit. If the molecular electronic molecule is in the state with a high electrical resistance, then this may be interpreted as a logic value “1”; by contrast, if the molecular electronic molecule is in the state with a low electrical resistance, then this may be interpreted as a logic value “0”. The bit stored in a molecular electronic molecule may be read out for example by application of a voltage to the molecular electronic molecule, the current flow through the molecular electronic molecule being low if the electrical resistance thereof is large (logic value “1”), and the current flow through the molecular electronic molecule being high if the electrical resistance thereof is small (logic value “0”). 
     The molecular electronics arrangement of the invention affords numerous further possibilities of application. Thus, by way of example, a biomolecule (for example a DNA half strand, a protein, a biopolymer, a low molecular weight compound) coupled to a first interconnect and to a second interconnect may be examined, i.e. characterized, by means of the molecular electronics arrangement. Furthermore, all biological detection methods which are based on making electrical contact with biomolecules can be carried out using the molecular electronics arrangement of the invention. However, a molecular electronic molecule arranged between a first interconnect and a second interconnect may perform an electrical component function, for example be used as an electrical conductor, as an electrical nonconductor, as a diode, as a transistor or as a logic gate. 
     The diverse possibilities of application of the molecular electronics arrangement of the invention and the associated economic attractiveness of the molecular electronics arrangement are advantageous. 
     Furthermore, the molecular electronics arrangement of the invention may have at least one protective layer on at least one portion of the uncovered surfaces of the first and/or of the second interconnects. The at least one protective layer is preferably fabricated from one or a combination of the materials silicon dioxide and/or silicon nitride. 
     Preferably, the entire surface of the first and/or of the second interconnect which cannot be utilized as an active coupling surface for molecular electronic molecules is covered with the protective layer. In other words, preferably the entire surface of the at least one interconnect, which is configured such that it is essentially parallelepipedal, for example, is provided with the protective layer, with the exception of that surface region to which are coupled molecular electronic molecules which are simultaneously coupled to one of the at least one second interconnect. Analogously, there are covered with the protective layer preferably those surface regions of the at least one second interconnect, which is configured such that it is essentially parallelepipedal, for example, which are free of such molecular electronic molecules, which molecular electronic molecules are not only coupled to the second interconnect under consideration by one end section but are furthermore coupled to one of the first interconnects by another end section. As an alternative, however, it is also possible for only individual regions of the surface of the at least one first interconnects or of the at least one second interconnect to be covered with the protective layer. 
     By virtue of the uncovered surfaces of the at least one first and/or of the at least one second interconnect being at least partly covered with a protective layer preferably fabricated from silicon dioxide and/or from silicon nitride, the invention avoids the situation in which biomolecules couple at such surfaces of the first (and/or respectively the second) interconnects at which the molecular electronic molecules cannot couple to one of the second (and/or respectively the first) interconnects by their other end for geometrical reasons. This is because molecular electronic molecules which are not coupled to one of the first interconnects by one end section and are coupled to one of the second interconnects by their other end section cannot be used as intended. Therefore, such molecules are needlessly lost. By virtue of the free and unutilized surfaces of the interconnects being partly covered with the protective layer, the biomolecules are prevented from binding to these unutilized areas. Costs for molecular electronic molecules coupled to only one interconnect surface are thereby saved. This is advantageous particularly when expensive molecular electronic molecules are intended to be introduced into the molecular electronics arrangement. Biological molecules are often obtainable only in small concentrations and/or volumes, so that in these cases it is a matter of crucial importance to minimize the number of needlessly bound molecular electronic molecules. This is realized by means of the protective layer according to the invention. 
     This exploits the effect that molecular electronic molecules couple highly selectively only to very specific surfaces. By way of example, it known that thiol groups (SH radicals) couple particularly well to gold surfaces. Furthermore, thiol groups, as occur at the end sections of many molecular electronic molecules, also couple to platinum or silver surfaces. Furthermore, trichlorosilane radicals (SiCl 3  groups) adhere well to silicon, aluminum and titanium. Trichlorosilane radicals also adhere well to various oxides, so that molecular electronic molecules with trichlorosilane end groups can also be coupled to conductors whose surface has a thin oxide layer. This consideration shows that the material of the protective layer is to be chosen such that the molecular electronic molecules do not bind, or bind only poorly, to said material. Silicon dioxide and/or silicon nitride are suitable as materials for the protective layer. 
     The at least one first interconnect or the at least one second interconnect is preferably fabricated from one or a combination of the materials gold, platinum, silver, silicon, aluminum and titanium. These materials are suitable for binding to end groups (in particular thiol groups or trichlorosilane radicals) which often occur in molecular electronic molecules. 
     The spacer is preferably fabricated from one or a combination of the materials silicon dioxide and/or silicon nitride. By using these materials for the spacer, with reference to the explanations above, the invention avoids the situation in which molecular electronic molecules couple parasitically to the surface of the spacer. Therefore, the spacer, if it is fabricated from a suitable material, may support the functionality of the protective layer or serve as a protective layer. 
     As explained above, the two-sided coupling of the molecular electronic molecules to the interconnects is for example by means of a thiol-gold coupling and/or a coupling of a trichlorosilane radical to one of the materials silicon, aluminum and titanium. 
     Preferably, in the molecular electronics arrangement of the invention, at least a portion of the molecular electronic molecules are redox-active bispyridinium molecules and/or alkyltrichlorosilane molecules. 
     The method according to the invention for fabricating a molecular electronics arrangement is described below. In the method according to the invention for fabricating a molecular electronics arrangement of the invention, in a first step, at least one first interconnect, which has a surface, is introduced into or applied to the surface of a substrate. In a second step, a spacer layer is applied to the surface of the at least one first interconnect, which spacer layer at least partly covers the surface of the at least one first interconnect. In a third step, at least one second interconnect, which has a surface areally opposite the surface of the at least one first interconnect, is applied to the spacer layer, a distance between the at least one first interconnect and the at least one second interconnect being prescribed by means of the spacer layer. In a fourth step, the spacer is formed by the spacer layer being partly etched back, in such a way that an uncovered region of the surface of the at least one first interconnect and an uncovered region of the surface of the at least one second interconnect remains. In a fifth step, molecular electronic molecules are arranged between the uncovered region of the surface of the at least one first interconnect and the uncovered region of the surface of the at least one second interconnect, the length of which molecules is equal to the distance between the at least one first interconnect and the at least one second interconnect. 
     As described above, in the first step, at least one first interconnect, which has a surface, is introduced into or applied to the surface of a substrate. 
     Preferably, the at least one first interconnect is introduced into the surface of the substrate using damascene technology. The damascene technique is a method which makes it possible to produce a trench filled with a metal material in a substrate. In this case, firstly the substrate into which the metal material is intended to be introduced is planarized for example using the CMP technique (“Chemical Mechanical Polishing”). At least one trench is then etched into the substrate into which the metal material is intended to be introduced. Afterward, metal material is deposited into the at least one trench. The conductor material projecting beyond the trenches is then removed by polishing once again using the CMP method (“Chemical Mechanical Polishing”), so that only the metallized trench filling remains. In this way, only the top side of the metallic interconnect is uncovered. It should be added that the metallic material can be introduced into the trench for example by using the sputtering method or by CVD deposition (“Chemical Vapor Deposition”). 
     As described above, the at least one first interconnect is preferably introduced, i.e. integrated, into the surface of a substrate. This is advantageous since this enables the coupling to a conventional microelectronic silicon circuit integrated for example into the substrate. 
     In the second method step according to the invention, a spacer layer is applied to the surface of the at least one first interconnect, which spacer layer at least partly covers the surface of the at least one first interconnect. 
     Preferably, the spacer layer is applied to the surface of the layer arrangement by means of the atomic layer deposition method (ALD method). Clearly, the distance between the at least one first interconnect and the at least one second interconnect is defined by means of the thickness of the spacer layer, the thickness of the spacer layer essentially corresponding to the distance between the two molecular docking sites. In order to be able to precisely set this distance with an accuracy of the order of magnitude of atomic distances (i.e. distances in the range of a few angstroms), the method of atomic layer deposition (ALD method) is preferably used for depositing the spacer layer onto the at least one interconnect. The ALD method is a technique for depositing thin layers onto a surface and is related to the CVD method (“Chemical Vapor Deposition”). The ALD method is clearly a CVD method in which an atomic layer is in each case deposited onto a surface by alternately introducing two reaction gases into a process space. Firstly, a first reaction gas is introduced into a process space, so that firstly an atomic layer of a material which is determined by the first reaction gas is deposited on the surface of the substrate that is to be covered with a layer. Afterward, a second reaction gas is introduced into the process space, so that there is deposited on the monoatomic layer determined by the material of the first reaction gas a further monoatomic layer determined by the material of the second reaction gas. Monoatomic layers defined by the materials of the first reaction gas and of the second reaction gas, respectively, are gradually deposited on the surface of the layer arrangement. A layer having a very uniform thickness is produced in this way, it being possible for the thickness to be set to an accuracy of one atomic layer, i.e. to the accuracy of a few angstroms, by the number of process steps. The exact setting of the thickness of the layer can be realized by simple counting of the number of process steps. 
     By virtue of the spacer layer being applied to the at least one first interconnect using the ALD method, the distance between the at least one first interconnect and the at least one second interconnect can be adjusted exactly and can thus be adapted flexibly to specific molecular electronic molecules that are to be introduced. This is just as advantageous as the high degree of reproducibility which is achieved for example by counting the process steps, i.e. the alternation from a first reaction gas to a second reaction gas. Preferably, the spacer layer is fabricated from one or a combination of the materials silicon dioxide and silicon nitride. 
     In a third method step, at least one second interconnect, which has a surface areally opposite the surface of the at least one first interconnect, is applied to the spacer layer, the distance between the at least one first interconnect and the at least one second interconnect being prescribed by means of the spacer layer. 
     This step may be realized in practice by depositing and patterning an electrically conductive layer on the surface of the layer arrangement obtained. This may be done for example by means of a suitable photolithography and etching method. 
     The at least one first interconnect and/or the at least one second interconnect may be fabricated from one or a combination of the materials gold, platinum, silver, silicon, aluminum and titanium. 
     In a fourth step, the spacer is formed by the spacer layer being partly etched back, in such a way that an uncovered region of the surface of the at least one first interconnect and an uncovered region of the surface of the at least one second interconnect remains. 
     This method step may be realized for example by the spacer layer being partly etched away using a suitable etching method, for example by means of wet-chemical etching, so that at least one spacer remains. Preferably, the spacer is undercut, i.e. the spacer situated beneath the at least one second interconnect is laterally etched back to an extent such that the at least one second interconnect projects at least partly laterally above the spacer, and that the at least one first interconnect arranged beneath the spacer projects laterally at least partly above the spacer. A region of the surface of the at least one first interconnect and a region of the surface of the at least one second interconnect are thereby uncovered. Clearly, the two areas of the at least one first interconnect and at least one second interconnect to which a molecular electronic molecule can be coupled on both sides lie approximately parallel opposite one another, at a distance from one another which essentially corresponds to the length of a molecular electronic molecule to be introduced. 
     In a fifth method step, molecular electronic molecules are arranged between the uncovered region of the surface of the at least one first interconnect and the uncovered region of the surface of the at least one second interconnect, the length of which molecules is equal to the distance between the at least one first interconnect and the at least one second interconnect. 
     As explained above, there are specific material pairs of conductor surfaces and end sections of molecular electronic molecules which bind particularly well with one another, for example the thiol-gold bond. If the surface of the at least one first interconnect and, respectively, at least one second interconnect uncovered in accordance with the method is fabricated from a material (for example gold) which is sufficiently well suited to the binding of specific molecular electronic molecules (for example those which have thiol end groups), then the molecular electronic molecules will spontaneously couple to one of the first interconnects by one end section and to one of the second interconnects by the other end section. For this purpose, by way of example, the molecular electronic molecules are introduced in dissolved form into the molecular electronics arrangement, and a bond forms automatically (so-called self-assembly technique). 
     The molecular electronic molecules are preferably redox-active bispyridinium molecules and/or alkyltrichlorosilane molecules. 
     As shown by the description above, the molecular electronics arrangement of the invention can be fabricated by a method whose method steps are standard semiconductor-technological method steps. By virtue of the fact that in accordance with the method, exclusively standard techniques can be used which can be carried out on standardized machines as are available in many semiconductor-technological laboratories and factories, there is no need for a complicated development of machines for realizing the method for fabricating a molecular electronics arrangement. This saves time and development costs. 
     Furthermore, it is possible in accordance with the method to bring about a large multiplicity of couplings between a first interconnect and a second interconnect by means of molecular electronic molecules with the result that, in contrast to the molecular electronics arrangements known from the prior art, the molecular electronics arrangement of the invention is also suitable for industrial scale integration. Thus, the molecular electronics arrangement of the invention is not only of interest for laboratory experiments in pure research, but also opens up areas of application that are of economic interest for molecular electronics. 
     The method for fabricating a molecular electronics arrangement may have the further method step that at least one of the first interconnects and/or at least one of the second interconnects are coupled to at least one electrically conductive coupling element, so that the interconnects can be coupled to an external circuit. This may be realized in particular by a procedure in which the electrically conductive coupling elements are formed by introducing contact holes into the substrate and filling the contact holes with an electrically conductive material. Preferably, the electrically conductive coupling elements are fabricated from polysilicon material, aluminum material or tungsten material. 
     Furthermore, at least one portion of the first and/or of the second interconnects may be coupled to an electrical circuit by means of the electrically conductive coupling elements. The external circuit is preferably integrated into the substrate. By way of example, the external circuit may be integrated into the substrate by means of CMOS technology. By way of example, a silicon wafer may be used as the substrate. 
     This realizes a coupling of the molecular electronic molecules to external conventional silicon microelectronics, which opens up many areas of application for the molecular electronics arrangement fabricated in accordance with the method. 
     The method for fabricating a molecular electronics arrangement may furthermore have, before the introduction of the molecular electronic molecules into the arrangement, the further method step that at least one protective layer is applied to at least one portion of the uncovered regions of the surfaces of the first and/or of the second interconnects. The protective layer is preferably fabricated from one or a combination of the materials silicon dioxide and/or silicon nitride. 
     As explained in detail above, the formation of a protective layer on a portion of the uncovered surfaces of the first interconnects to which are bound no molecular electronic molecules which are furthermore also bound to one of the second interconnects reduces the number of those molecular electronic molecules which cannot be used as intended. Analogously, the formation of the protective layer at surface regions of the at least one second interconnect which are suitable therefor reduces the number of those molecular electronic modules which couple to the uncovered surfaces of the at least one second interconnects without at the same time being coupled to one of the at least one first interconnect by another end section. Clearly, the formation of the protective layer has the effect that a higher proportion of the molecular electronic molecules introduced into the molecular electronics arrangement couple as intended on both sides to one of the first and, respectively, one of the second interconnects. As a result, fewer molecular electronic modules are required, which reduces the costs and which enables the defined introduction of such molecular electronic molecules which are available only in very low concentration. 
     As an alternative, in accordance with the method according to the invention for fabricating a molecular electronics arrangement, instead of the application of the at least one second interconnect to the spacer layer, a continuous conductor layer and also a further protective layer are applied to the arrangement of the surface, and the continuous conductor layer and the further protective layer are patterned in such a way that an at least one second interconnect at least partly covered with a further protective layer is thereby formed. The further protective layer is preferably fabricated from one or a combination of the materials silicon dioxide and silicon nitride. 
     A further refinement of the method according to the invention for fabricating the molecular electronics arrangement of the invention consists in the fact that, during the introduction of the molecular electronic molecules into the arrangement, electrical voltages are applied at least to a portion of the interconnects, in such a way that the molecular electronic molecules are coupled with a preferred geometrical orientation to one of the at least one first interconnects and to one of the at least one second interconnects. 
     The electrical signals necessary for this are preferably provided by the external circuit which is coupled to the interconnects via electrically conductive coupling elements. Clearly, an electric field is formed by electrical voltages applied between the first interconnects and the second interconnects. If molecular electronic molecules are now introduced into the arrangement, these molecular electronic molecules are influenced by the electric field provided that the molecular electronic molecules have an electrical multipole moment, that is to say are electrically charged, for example. Thus, by way of example, many proteins at physiological pH values have an electrical charge which originates from charged amino acids predominantly at the surface of the protein molecules. DNA molecules may also have an electrical charge; DNA half strands often have a negative electrical charge. On account of the electric field generated by the electrical voltages applied to the interconnect, an electrical force acts on the electrically charged molecular electronic molecules. These bring the electrically charged molecular electronic molecules preferably into a geometrical orientation in which the potential energy of the molecular electronic molecules in the electric field is minimal. As a result, a preferred direction is prescribed for the molecular electronic molecules when coupling on both sides to one of the at least one first interconnect and one of the at least one second interconnects, which increases the degree of order of the arrangement. This has the advantage of a particularly high reproducibility. 
     Exemplary embodiments of the invention are illustrated in the figures and are explained in more detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are shown in the figures and are described in more detail in the following, in which: 
         FIG. 1A  shows the chemical structure of a bispyridinium molecule, 
         FIG. 1B  shows the chemical structure of an alkyltrichlorosilane molecule, 
         FIG. 2A  shows a cross-sectional view of a molecular electronics arrangement in accordance with a first exemplary embodiment of the invention, 
         FIG. 2B  shows a cross-sectional view of a molecular electronics arrangement in accordance with a second exemplary embodiment of the invention, 
         FIG. 2C  shows a cross-sectional view of a molecular electronics arrangement in accordance with a third exemplary embodiment of the invention, 
         FIG. 3A  shows a plan view of a molecular electronics arrangement in accordance with an exemplary embodiment of the molecular electronics arrangement according to the invention for use as a molecular memory, 
         FIG. 3B  shows a cross section along the section line I–I′ from  FIG. 3A  of a molecular electronics arrangement in accordance with the exemplary embodiment of the molecular electronics arrangement according to the invention for use as a molecular memory, 
         FIG. 4A  shows a cross-sectional view of a layer arrangement after a first method section in accordance with a preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 4B  shows a cross-sectional view of a layer arrangement after a second method section in accordance with a preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 4C  shows a cross-sectional view of a layer arrangement after a third method section in accordance with a preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 4D  shows a cross-sectional view of a layer arrangement after a fourth method section in accordance with a preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 4E  shows a cross-sectional view of a layer arrangement after a fifth method section in accordance with a preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 5A  shows a cross-sectional view of a layer arrangement after a fifth method section in accordance with another preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 5B  shows a cross-sectional view of a layer arrangement after a sixth method section in accordance with the other preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 5C  shows a cross-sectional view of a layer arrangement after a seventh method section in accordance with the other preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 5D  shows a cross-sectional view of a layer arrangement after an eighth method section in accordance with the other preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 5E  shows a cross-sectional view of a layer arrangement after a ninth method section in accordance with the other preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 6A  shows a cross-sectional view of a layer arrangement after a third method section in accordance with a further preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 6B  shows a cross-sectional view of a layer arrangement after a fourth method section in accordance with the further preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, 
         FIG. 6C  shows a cross-sectional view of a layer arrangement after a fifth method section in accordance with the further preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, and 
         FIG. 6D  shows a cross-sectional view of a layer arrangement after a sixth method section in accordance with the further preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement. 
     
    
    
     DETAILED DESCRIPTION 
     Firstly, a description is given, with reference to  FIGS. 1A–1B , of exemplary embodiments of molecular electronic molecules as are included in the molecular electronics arrangement of the invention. 
     The structure of redox-active bispyridinium molecules  100  is described below with reference to  FIG. 1A . As disclosed in [2], the electrical resistance of redox-active bispyridinium molecules  100  can be switched back and forth between two values, the electrical resistance being low in the reduced redox state and the electrical resistance being high in the oxidized redox state. 
     The redox-active bispyridinium molecule  100  shown in  FIG. 1A  has a few substituents (designated by X, Y, Z) which serve as spacers for a chemical element or a chemical group or for a molecule radical. Furthermore, variables p, q, n, m are shown in the bispyridinium molecule  100  shown in  FIG. 1A , each of which variables assumes an integral positive value (1, 2, 3, . . . ) independently of one another. 
     The bispyridinium molecule  100  shown in  FIG. 1A  has a first substituent  101 , which is designated by Z and is coupled to a first alkyl group  102 . The first alkyl group  102  is furthermore coupled to a second substituent  103  designated by Y, and the second substituent  103  is furthermore coupled to a second alkyl group  104 . The second alkyl group  104  is furthermore coupled to a first nitrogen ion  106 , which first nitrogen ion  106  is part of the first six-membered ring  105 . The first six-membered ring  105  has a honeycomb-like structure of six components that are closed to form a ring; inter alia, the first six-membered ring  105  has four third substituents  107   a ,  107   b ,  107   c ,  107   d . The positively charged nitrogen ion  106  is coupled to the first of the third substituents  107   a , the first of the third substituents  107   a  is coupled to the second of the third substituents  107   b , the second of the third substituents  107   b  is coupled to a first carbon atom  108 , the first carbon atom  108  is coupled to the third of the third substituents  107   c , the third of the third substituents  107   c  is coupled to the fourth of the third substituents  107   d  and the fourth of the third substituents  107   d  is coupled to the nitrogen ion  106 , as a result of which the annular arrangement of the atoms or molecules  106 ,  107   a ,  107   b ,  108 ,  107   c ,  107   d  is closed. The first of the third substituents  107   a , the second of the third substituents  107   b , the third of the third substituents  107   c  and the fourth of the third substituents  107   d  are in each case designated by the spacer X. As is further shown in  FIG. 1A , the first carbon atom  108  of the first six-membered ring  105  is coupled to a second carbon atom  110  of a second six-membered ring  109 . The second six-membered ring  109 , like the first six-membered ring  105 , has an annular structure of six atoms or molecules; inter alia, the second six-membered ring  109  has four fourth substituents  111   a ,  111   b ,  111   c ,  111   d . The second carbon atom  110  is coupled to the first of the fourth substituents  111   a , the first of the fourth substituents  111   a  is coupled to the second of the fourth substituents  111   b , the second of the fourth substituents  111   b  is coupled to a second nitrogen ion  112 , the second nitrogen ion  112  is coupled to the third of the fourth substituents  111   c , the third of the fourth substituent  111   c  is coupled to the fourth of the fourth substituents  111   d  and the fourth of the fourth substituents  111   d  is coupled to the second carbon atom  110  in order to close the annular structure of the second six-membered ring  109 . The first of the fourth substituents  111   a , the second of the fourth substituents  111   b , the third of the fourth substituents  111   c  and the fourth of the fourth substituents  111   d  are in each case designated by the spacer X. The second nitrogen ion  112  is furthermore coupled to a third alkyl group  113 , and the third alkyl group  113  is furthermore coupled to a fifth substituent  114 , which is designated by the spacer Y in accordance with  FIG. 1A . The fifth substituent  114  is coupled to a fourth alkyl group  115  and the fourth alkyl group  115  is coupled to a sixth substituent  116 . The sixth substituent  116  is designated by the spacer Z. The first substituent  101  and the sixth substituent  116  represent two end sections of the bispyridinium molecule  100 . 
     Preferably, each of the spacers X denotes a CH group, a CR group, a nitrogen atom, an oxygen atom or a sulfur atom. The letter R in the abovementioned CR group itself again represents a spacer, which preferably represents an alkyl group, an aryl group, a benzyl group, a CN group, a COOH group, a COOR′ group, a CONHR′ group, an NO 2  group, an OH group, an OR′ group, an NH 2  group, an NHR′ group, an NR′R″ group, an SH group, an SR′ group or another chemically suitable element or another chemically suitable radical. R′ and R″ in turn designate any desired chemically suitable radicals or groups, for example alkyl groups. 
     Furthermore, each of the spacers Y designates a CH 2  group, an oxygen atom, an NH group, an NR′ group, a COO group, a CONH group, a CH═CH group, a C≡C group, an aryl group or another chemically suitable element or another chemically suitable radical. R′ in turn designates any desired chemically suitable radicals or groups, for example alkyl groups. 
     Furthermore, each of the spacers Z denotes a CH 3  group, a CH═CH 2  group, an SH group, a sulfur—sulfur group, an SiCl 3  group, an Si(OR′) 3  group, an SiR 2 (OR′) group, a PO 3 H group or another chemically suitable element or another chemically suitable radical. R′ in turn designates any desired chemically suitable radicals or groups, for example alkyl groups. 
     It should again be pointed out that the variables m, n, p, q may be any positive integer independently of one another. 
       FIG. 1A  shows a bispyridinium molecule  100  having a series of double bonds (e.g. within the first six-membered ring  105 ). For specific combinations of substituents, single bonds may result instead of said double bonds, e.g. if divalent oxygen is chosen for the spacer X. 
     In accordance with a preferred exemplary embodiment of the invention, the bispyridinium molecule  100  has end sections Z  101 ,  116  which are thiol groups (SH radicals). It is known that thiol groups immobilize particularly well on gold surfaces. 
     As a second example of a molecular electronic molecule, an alkyltrichlorosilane molecule  117  is shown in  FIG. 1B . This molecule has an alkyl group  118  and a trichlorosilane radical (SiCl 3  group)  119 . The alkyl group  118  has a CH 3  group in an end section of the alkyltrichlorosilane molecule  117  and furthermore has p CH 2  groups arranged in series. In this case, the variable p may again be any integral positive number. The alkyl group  118  is coupled to a trichlorosilane radical  119 , the trichlorosilane radical  119  being situated at the other end section of the alkyltrichlorosilane molecule  117 . It is known that trichlorosilane radicals  119  can be coupled particularly well to silicon, aluminum and titanium. Furthermore, trichlorosilane radicals  119  can also be coupled to various oxides. In order to realize such a coupling, it is possible to use for example a thin surface layer of an oxide on a conductor. 
     It must be emphasized that the molecules  100 ,  117  which are shown in  FIG. 1A ,  FIG. 1B  and are described above are mentioned only as examples of possible molecular electronic molecules which the molecular electronics arrangement of the invention may have. The molecular electronics arrangement of the invention may furthermore have molecular electronic molecules which are, by way of example, proteins, DNA half strands, DNA molecules, low molecular weight compounds, polypeptides, polymers or other suitable chemical compounds. Nanotubes, in particular carbon nanotubes, may also be used as molecular electronic molecules. 
       FIG. 2A  shows a molecular electronics arrangement  200  in accordance with a first preferred exemplary embodiment of the invention. 
     The molecular electronics arrangement  200  has a substrate  201 , at least one first interconnect  202 , which has a surface  202   a  and which is arranged in the substrate  201 , a spacer  203  arranged on the surface  202   a  of the at least one first interconnect  202 , which spacer partly covers the surface  202   a  of the at least one first interconnect  202 , at least one second interconnect  204  arranged on the spacer  203 , which interconnect has a surface  204   a  areally opposite the surface  202   a  of the at least one first interconnect  202 , the spacer  203  partly covering the surface  204   a  of the at least one second interconnect  204 , and a predetermined distance between the at least one first interconnect  202  and the at least one second interconnect  204  being defined by means of the spacer  203 , and molecular electronic molecules  205  arranged between an uncovered region of the surface  202   a  of the at least one first interconnect  202  and an uncovered region of the surface  204   a  of the at least one second interconnect  204 , the length of which molecules is equal to the distance between the at least one first interconnect  202  and the at least one second interconnect  204 . 
     Furthermore,  FIG. 2A  shows an enlarged illustration of a molecular electronic molecule  206 . The length of the molecular electronic molecule is designated by “l” in this enlarged illustration of a molecular electronic molecule  206 . Furthermore,  FIG. 2A  shows the thickness “d” of the spacer  203 , which defines a predetermined distance “d” between the at least one first interconnect  202  and the at least one second interconnect  204 . 
     In accordance with the exemplary embodiment shown in  FIG. 2A , the substrate  201  is a preprocessed silicon wafer, the at least one first interconnect  202  and the at least one second interconnect  204  are fabricated from gold material, and the spacer  203  is fabricated from silicon dioxide material. In accordance with the exemplary embodiment of the molecular electronics arrangement  200  as shown in  FIG. 2A , the molecular electronic molecules  205  are bispyridinium molecules. The two-sided coupling of the bispyridinium molecules to the interconnects  202 ,  204  is realized as thiol-gold coupling. As is further shown in  FIG. 2A , the molecular electronic molecules  205  are coupled to one of the first interconnects and to one of the second interconnects at two of their end sections, which is made possible by virtue of the fact that the thickness “d” of the spacer  203  is essentially equal to the length “l” of the molecular electronic molecules  205 . 
       FIG. 2B  shows a molecular electronic arrangement  210  in accordance with a second preferred exemplary embodiment of the invention. 
     The molecular electronics arrangement  210  shown in  FIG. 2B  has the features of the molecular electronics arrangement  200 , and the molecular electronics arrangement  210  furthermore has further features. Each of the first interconnects  202  and each of the second interconnects  204  is in each case coupled to an electrically conductive coupling element  211 , by means of which the interconnects  202 ,  204  can be coupled to an external circuit.  FIG. 2B  shows only the electrically conductive coupling element  211  coupled to the first interconnect  202 . However, each of the second interconnects  204  shown in  FIG. 2B  is also in each case coupled to an electrically conductive coupling element (not shown in  FIG. 2B ). In accordance with the exemplary embodiment of the molecular electronics arrangement  210  as shown in  FIG. 2B , the electrically conductive coupling element  211  is a contact hole  211   a  filled with an electrically conductive material, which contact hole  211   a  is introduced into the substrate  201 . The electrically conductive coupling element  211  is fabricated from tungsten material in accordance with the exemplary embodiment described. 
     Furthermore, the molecular electronics arrangement  210  has an external circuit  212 , which is coupled to the first interconnects  202  and the second interconnects  204  by means of the electrically conductive coupling elements  211 . The coupling of the first interconnect  202  to the external circuit  212  is depicted in  FIG. 2B  and is effected via the electrically conductive coupling element  211 . The three second interconnects  204  shown in  FIG. 2B  are also coupled to the external circuit  212  by means of electrically conductive coupling elements (not shown in  FIG. 2B ). Consequently, according to the invention, the circuit  212 , which is a conventional microelectronic circuit, is coupled to the molecular electronic molecules  205  via the coupling element  211  and the interconnects  202 ,  204 . 
     Furthermore, the molecular electronics arrangement  210  has a protective layer  213  on a portion of the uncovered surfaces of the first interconnects  202  and of the second interconnects  204 . In accordance with the exemplary embodiment of the molecular electronics arrangement  210  as shown in  FIG. 2B , the protective layer  213  is fabricated from silicon dioxide material. 
     In accordance with  FIG. 2B , the protective layer  213  has seven partial sections, three cap-like coverings of the second interconnects  204  and four parallelepipedal coverings of the first interconnect  202 . As shown in  FIG. 2B , those surface regions of the first interconnect  202  and, respectively, of the second interconnect  204  on which the protective layer  213  is applied are not suitable regions for binding molecular electronic molecules  205  thereto in such a way that the molecular electronic molecules  205  are coupled to a first interconnect  202  by one end section and to a second interconnect  204  by the other end section. The surface regions of the first and second interconnects  202 ,  204  which are covered with the protective layer  213  are therefore unsuitable for the two-sided binding of molecular electronic molecules  205  since, proceeding from these surfaces, a further interconnect is not arranged at a distance “d” corresponding to the length “l” of the molecular electronic molecules  205  in order to provide the second coupling site. All the molecular electronic molecules  205  which dock at such surfaces of the first and second interconnects  202 ,  204  cannot be used as intended. The electrical protective layer  213  therefore serves to avoid the binding of molecular electronic molecules  205  to unsuitable surface regions of the first and second interconnect  202 ,  204 . 
       FIG. 2C  shows a molecular electronics arrangement  220  in accordance with a third preferred exemplary embodiment of the invention. The molecular electronics arrangement  220  shown in  FIG. 2C  differs from the molecular electronics arrangement  210  shown in  FIG. 2B  essentially in respect of the following aspects. 
     Firstly, the first interconnect  202  is arranged on the substrate  201  in accordance with the exemplary embodiment of the molecular electronics arrangement  220  as shown in  FIG. 2C , and is not integrated in the substrate  201  as in accordance with the exemplary embodiment of the molecular electronics arrangement  210  as shown in  FIG. 2B . 
     Furthermore, in accordance with the molecular electronics arrangement  220 , the external circuit  212  is provided as an integrated circuit which is introduced into the substrate  201 . The external circuit  212  is fabricated using CMOS technology. 
     The protective layer  213  is fabricated from silicon nitride material in accordance with the exemplary embodiment of the molecular electronics arrangement  220  as shown in  FIG. 2C . The protective layer  213  from  FIG. 2C  has three partial regions, each of these three partial regions of the protective layer  213  being applied on one of the three second interconnects  204  in order to envelop the latter essentially in a U-shaped manner. According to  FIG. 2C , the first interconnect  202  is free of a protective layer  213 . 
       FIG. 3A  shows a plan view and  FIG. 3B  a cross-sectional view along the section line I–I′ of a molecular electronics arrangement  300  in accordance with an exemplary embodiment of the molecular electronics arrangement according to the invention for use as a molecular memory. The molecular electronics arrangement  300  has a substrate  301 , three first interconnects  302 , which have a surface  302   a  and which are arranged in the substrate  301 , a spacer  303  arranged on the surface  302   a  of the three first interconnects  302 , which spacer partly covers the surface  302   a  of the three first interconnects  302 , three second interconnects  304 , which are arranged on the spacer  303  and have a surface  304   a  areally opposite the surface  302   a  of the three first interconnects  302 , the spacer  303  partly covering the surface of the three second interconnects  304 , and a predetermined distance between the three first interconnects  302  and the three second interconnects  304  being defined by means of the spacer  303 , and molecular electronic molecules  305  arranged between an uncovered region of the surface  302   a  of the three first interconnects  302  and an uncovered region of the surface  304   a  of the three second interconnects  304 , the length of which molecules is equal to the distance between the three first interconnects  302  and the three second interconnects  304 . Furthermore, in accordance with the molecular electronics arrangement  300 , the three first interconnects  302  and the three second interconnects  304  are in each case coupled to an electrically conductive coupling element  306 , by means of which the interconnect  302 ,  304  can be coupled to an external circuit  307 . The electrically conductive coupling elements  306  coupled to the three second interconnects  304  are not shown in  FIG. 3A ,  FIG. 3B . One of the three coupling elements  306  by means of which the three first interconnects  302  can be coupled to an external circuit are shown in  FIG. 3B  and are contact holes  306   a  filled with an electrically conductive material, which contact holes  306   a  are introduced into the substrate  301 . Furthermore,  FIG. 3B  shows an external circuit  307 , which is coupled to the three first interconnects  302  and the three second interconnects  304  by means of the electrically conductive coupling elements  306 . 
     The enlarged illustration of a molecular electronic molecule  308  depicted in  FIG. 3B  reveals that the molecular electronic molecules  305  have a length “l”. This length “l” of the molecular electronic molecules  305  is essentially equal to the distance “d” between the first interconnects  302  and the second interconnects  304 , this thickness “d” being prescribed by the thickness of the spacer  303 . 
     In accordance with the exemplary embodiment of the molecular electronics arrangement  300  according to the invention as shown in  FIG. 3A ,  FIG. 3B , which arrangement can be used as a molecular memory, the molecular electronic molecules  305  are bispyridinium molecules. 
     The principle of how the molecular electronics arrangement  300  can be used as molecular memory is described below. As shown in  FIG. 3A , the molecular electronics arrangement  300  is, in plan view, essentially a grid formed by two interconnects  302 ,  304  arranged in crossed fashion one above the other. In each crossover region between one of the first and one of the second interconnects  302 ,  304 , redox-active bispyridinium molecules  305  are introduced, by means of which one of the first interconnects  302  is coupled to one of the second interconnects  304  i.e. the interspace is bridged. As described above, the bispyridinium molecules  305  have the property that they can be present in an oxidized state and in a reduced state. The bispyridinium molecules  305  have a high electrical resistance in the oxidized state, whereas the bispyridinium molecules  305  have a low electrical resistance in the reduced state. By applying a sufficiently large voltage between the two end sections of a bispyridinium molecule  305 , the bispyridinium molecule  305  can be reversibly switched back and forth between the two redox states. Once again with reference to  FIG. 3B , it is possible, by way of example, for a voltage provided by the external circuit  307  to be applied between one of the first interconnects  302  and one of the second interconnects  304 , and a voltage is then also present between the thiol end groups of the bispyridinium molecule  305 . By applying such a suitable voltage, it is therefore possible, in each crossover region between one of the first interconnects  302  and one of the second interconnects  304 , for a bispyridinium molecule  305  arranged in said crossover region to be switched back and forth between its two redox states. In other words, through a suitably chosen voltage, it is possible to establish whether a bispyridinium molecule  305  is present in an oxidized state with a high electrical resistance or in a reduced state with a low electrical resistance. In this way, each crossover region between one of the first interconnect  302  and one of the second interconnect  304  may be regarded as a molecular memory with a data quantity of one bit, which, by applying a sufficiently high voltage, can be allocated a logic value “1” (oxidized, high electrical resistance) or “0” (reduced, low electrical resistance). Thus, a total of nine bits can be stored in accordance with the exemplary embodiment of the molecular electronics arrangement  300  as shown in  FIG. 3A  and  FIG. 3B . Thus, by way of example, in a crossover region between one of the first interconnects  302  and one of the second interconnects  304 , a bispyridinium molecule  305  arranged in between may be brought to the reduced redox state, so that this bispyridinium molecule  305  has a low electrical resistance. As an alternative, in a crossover region between one of the first interconnects  302  and one of the second interconnects  304 , a bispyridinium molecule  305  may be brought to the oxidized redox state, so that the bispyridinium molecule  305  has a high electrical resistance. A bispyridinium molecule  305  in a state with a high electrical resistance may then be interpreted for example as a bit with a logic value “1”, and a bispyridinium molecule  305  in a state with a low electrical resistance may be regarded for example as a bit with a logic value “0”. Therefore, what is realized according to the invention is that a data quantity of one bit can be stored in each crossover region between one of the first interconnects  302  and one of the second interconnects  304 . 
     A description is given below of how the data quantity of one bit stored in each crossover region between one of the first interconnects  302  and one of the second interconnects  304  can be read out. If an electrical voltage is applied between one of the first interconnects  302  and one of the second interconnects  304  by means of the external circuit  307 , then the current flow through the bispyridinium molecules  305  which are introduced between the two interconnects  302 ,  304  depends on the magnitude of the electrical resistance of the bispyridinium molecules  305 . If the bispyridinium molecules  305  have previously been brought to an oxidized state with a high electrical resistance, then the current flow is low. By contrast, if the bispyridinium molecules  305  have previously been put into a reduced state with a low electrical resistance, then, in accordance with Ohm&#39;s law, the current flow through the bispyridinium molecules  305  is high. In accordance with the statements above, a high current flow may be interpreted as a bit with a logic value “0”, and, in accordance with the statements above, a low current flow may be interpreted as a bit with a logic value “1”. The data quantity of one bit that can be stored in each crossover region between one of the first interconnects  302  and one of the second interconnects  304  can be read out in the manner described. 
     The data quantity stored in the crossover regions between the first interconnects  302  and the second interconnects  304  can be erased for example by applying to all the crossover regions between the corresponding first interconnects  302  and the corresponding second interconnects  304  electrical voltages such that all the bispyridinium molecules  305  are brought e.g. to the reduced state with a high electrical conductivity, which corresponds to a logic value “0”. As a result, all the bits are reset to the logic value “0”, i.e. the information is erased. 
     It must be emphasized that one or a plurality of bispyridinium molecules  305 , each of which can be reversibly switched back and forth between an oxidized, electrically poorly conductive state and a reduced, electrically readily conductive state, may be introduced in each crossover region between one of the first interconnects  302  and one of the second interconnects  304 . 
     Furthermore, a protective layer  213 , as are shown for example in the molecular electronics arrangements  210  and  220  from  FIG. 2B  and  FIG. 2C , respectively, is not provided in the molecular electronics arrangement  300  of the invention. As an alternative, in the case of the molecular electronics arrangement  300 , too, which is configured as molecular memory, protective layers may be provided on the unused uncovered surfaces of the first interconnects  302  and of the second interconnects  304 . 
     Exemplary embodiments of the method according to the invention for fabricating a molecular electronics arrangement are described below. 
     A first preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement is described below with reference to  FIG. 4A ,  FIG. 4B ,  FIG. 4C ,  FIG. 4D ,  FIG. 4E . 
     In accordance with the first preferred exemplary embodiment of the method for fabricating a molecular electronics arrangement, in a first step, at least one first interconnect  402 , which has a surface  402   a , is introduced into the surface of a substrate  401 . 
     The layer arrangement  400  shown in  FIG. 4A  is obtained as a result. In accordance with the exemplary embodiment described, the at least one interconnect  402  is fabricated from gold material. The introduction of the at least one interconnect  402  into the substrate  401  is effected using the damascene technique. As described above, in accordance with the damascene technique, firstly a trench is etched into the surface of a substrate  401 . Afterward, a metal layer is deposited onto the surface of the layer structure obtained. By means of CMP (“Chemical Mechanical Polishing”), the applied metal layer is polished away from the surface to an extent such that metal material remains only in the trench and a layer structure with a planar surface  402   a  is obtained. In accordance with the exemplary embodiment described, a preprocessed silicon wafer is used as the substrate  401 . 
     In a second method step of the exemplary embodiment of the method for fabricating a molecular electronics arrangement, a spacer layer  403  is applied to the surface  402   a  of the at least one first interconnect  402 , which spacer layer at least partly covers the surface  402   a  of the at least one first interconnect  402 . 
     In accordance with the exemplary embodiment described, the spacer layer  403  is applied to the layer structure  400  using the ALD method (Atomic Layer Deposition) in order to obtain the layer arrangement  404  shown in  FIG. 4B . Silicon dioxide is used as material for the spacer layer  403 . As described above, the ALD method is a modified CVD method (“Chemical Vapor Deposition”) in which the layer thickness can be set to an accuracy of one atomic layer (i.e. down to the range of a few angstroms) by alternately introducing two reaction gases into a process space, a monoatomic layer in each case being deposited on the surface. Through a suitable choice of the number of method steps, the thickness of the spacer layer  403  can therefore be set very accurately as a consequence of using the ALD method. 
     In a next method step, at least one second interconnect  405 , which has a surface  405   a  areally opposite the surface  402   a  of the at least one first interconnect  402 , is applied to the spacer layer  403 , a distance between the at least one first interconnect  402  and the at least one second interconnect  405  being prescribed by means of the spacer layer  403 . 
     The layer structure  406  shown in  FIG. 4C  is obtained as a result. The method step described has a plurality of substeps. In a first substep, a gold layer is applied to the surface of the layer arrangement  404 . In a second step, using a suitable lithography and etching method, the applied gold layer is patterned in such a way that the at least one second interconnect  405  remains on the surface of the layer structure  406 . In accordance with the exemplary embodiment described, two second interconnects  405  are fabricated in accordance with the method. 
     In a next method step, two spacers  403   a  are formed by the spacer layer  403  being partly etched back in such a way that an uncovered region of the surface of the at least one first interconnect  402  and an uncovered region of the surface of the at least one second interconnect  405  remain. 
     In other words, in this method step, the spacer layer  403  is etched back in such a way that spacers  403   a  remain. In accordance with the described exemplary embodiment of the method for fabricating a molecular electronics arrangement, this is done by means of wet-chemical etching back of the spacer layer  403  fabricated from silicon dioxide. The layer arrangement  407  resulting from this method step is shown in  FIG. 4D . As shown in  FIG. 4D  the spacers  403   a  are undercut, so that the remaining regions of the spacers  403   a  are sufficiently narrow in the horizontal direction in accordance with  FIG. 4D  that uncovered surfaces  402   a ,  405   a  of the first and second interconnects  402 ,  405 , respectively, remain in at least one crossover region between one of the first interconnects  402  and one of the second interconnects  405 . In other words, the first and second interconnects  402 ,  405  projecting on both sides laterally beyond the spacers  403   a  form grooves  408  in which the two interconnect planes are opposite one another in plane-parallel fashion and into which molecular electronic molecules of a suitable thickness can be introduced. 
     In a further method step, molecular electronic molecules  409  are arranged between the uncovered region of the surface of the at least one first interconnect  402  and the uncovered region of the surface of the at least one second interconnect  405 , the length of which molecules is equal to the distance between the at least one first interconnect  402  and the at least one second interconnect  405 . 
     The layer arrangement  410  shown in  FIG. 4E  is obtained as a result. By virtue of the surfaces  402   a ,  405   a  of the first and second interconnects  402 ,  405  being partly uncovered and being provided at a distance from one another which corresponds to the length of the molecular electronic molecules  409  to be introduced, and by virtue of gold material being chosen as material for the first and second interconnects  402 ,  405 , to which gold material molecular electronic molecules  409  can bind well via thiol groups, precautions are taken in order to realize the binding of molecular electronic molecules  409  to the interconnects  402 ,  405 . If the molecular electronic molecules  409  are now passed to the layer arrangement  407 , for example in dissolved form, then the molecular electronic molecules  409  are automatically coupled on both sides to one of the first interconnects  402  and one of the second interconnects  405 . Since the coupling proceeds automatically without external action, the term self-assembly technique is also used. 
     The layer arrangement  410  shown in  FIG. 4E  largely corresponds to the molecular electronics arrangement  200  shown in  FIG. 2A . 
     In a further method step, at least one of the first interconnects  402  and/or at least one of the second interconnects  405  may be coupled to at least one electrically conductive coupling element, so that the interconnects  402 ,  405  can be coupled to an external circuit. Furthermore, in a further method step, a portion of the first and/or of the second interconnects  402 ,  405  may be coupled to an external circuit by means of the electrically conductive coupling element. In order to form the electrically conductive coupling elements, contact holes may be introduced into the substrate  401  and the contact holes may be filled with an electrically conductive material. 
     The method steps by which the contact holes are introduced into the substrate  401  and by which the contact holes are filled with an electrically conductive material cannot be seen from  FIG. 4A ,  FIG. 4B ,  FIG. 4C ,  FIG. 4D ,  FIG. 4E . Moreover, the external circuit, which is preferably integrated into the substrate  401 , is not shown in  FIG. 4A ,  FIG. 4B ,  FIG. 4C ,  FIG. 4D ,  FIG. 4E . Preferably, the associated method steps are carried out before the at least one first interconnect  402 , which has a surface  402   a , is introduced into the substrate  401 . In other words, the electrically conductive coupling element and the external circuit, in accordance with  FIG. 4A , are arranged at the layer arrangement from below. 
     A second preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement is described below with reference to  FIG. 4A ,  FIG. 4B ,  FIG. 4C ,  FIG. 4D ,  FIG. 5A ,  FIG. 5B ,  FIG. 5C ,  FIG. 5D ,  FIG. 5E . 
     The first four method steps in accordance with the second preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement are identical to the first four method sections described above with reference to the first preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement. The layer arrangement  407  shown in  FIG. 4D  is obtained after carrying out these method steps. Said layer arrangement is used as a starting point for the description of the further method sequence. 
     In a departure from the above-described first preferred exemplary embodiment of the method for fabricating a molecular electronics, the method step which leads from the layer arrangement  407  shown in  FIG. 4D  to the layer arrangement  410  shown in  FIG. 4E  is not carried out below. 
     Instead, before the introduction of the molecular electronic molecules into the layer arrangement  407 , at least one protective layer  503  is applied to at least one portion of the uncovered regions of the surfaces of the first and/or of the second interconnects  402 ,  405 . 
     After carrying out this method step, the layer arrangement  505  shown in  FIG. 5D  is obtained, which has a plurality of substeps. In a first substep, an auxiliary layer  500  is applied to the layer arrangement  407 , thereby obtaining the layer arrangement  501  shown in  FIG. 5A . In accordance with the described exemplary embodiment of the method for fabricating a molecular electronics arrangement, the deposition of the auxiliary layer  500  is realized by means of a CVD deposition of a silicon nitride layer. 
     In a subsequent substep, the applied auxiliary layer  500  is etched back anisotropically. The layer arrangement  502  shown in  FIG. 5B  is thereby obtained. Anisotropic etching back means that the material of the auxiliary layer  500  is removed from the surface of the layer arrangement  501  in a predeterminable thickness by means of a directed etching method. As shown in  FIG. 5B , after carrying out the anisotropic etching, the material of the auxiliary layer  500  remains exclusively in regions near the sidewalls of the spacers  403   a  and the second interconnects  405 . Partial sections of the surface of the layer structure  502  are uncovered as a result. In particular, partial regions of the surface of the first interconnect  402  and partial regions of the second interconnect  405  are uncovered. 
     In a next substep, a protective layer  503  is applied to the surface of the layer arrangement  502 . The application of the auxiliary layer  503 , which is fabricated from silicon dioxide material in accordance with the second preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, is effected in “directed” fashion. This means that the silicon dioxide layer is applied to the layer arrangement  502  without the sidewalls being covered. The layer arrangement  504  shown in  FIG. 5C  is thereby obtained. In particular, as shown in  FIG. 5C , surface regions of the auxiliary layer  500  remain uncovered. 
     In a next substep, the material of the auxiliary layer  500  is etched away by wet-chemical etching using hot phosphoric acid (H 3 PO 4 ). Hot phosphoric acid has the property of etching silicon nitride material, whereas silicon dioxide material is not etched by hot phosphoric acid. Therefore, only the auxiliary layer  500  is etched away during the wet-chemical etching method, whereas the protective layer  503  is not etched. This results in the layer arrangement SOS shown in  FIG. 5D . 
     The layer arrangement  505  shown in  FIG. 5D  essentially corresponds to the preferred exemplary embodiment of the molecular electronics arrangement  210  as shown in  FIG. 2B . In particular, regions covered with the protective layer  503  made of silicon dioxide material remain on parts of the uncovered surfaces of the first interconnect  402  and on parts of the uncovered surfaces of the second interconnect  405 . This avoids a parasitic binding of molecular electronic molecules to such surface regions of the first and second interconnects  402 ,  405 , at which, for geometrical reasons, molecular electronic molecules cannot simultaneously couple to one of the first interconnects  402  and to one of the second interconnects  405 . 
     In a subsequent method step, molecular electronic molecules  409  are arranged between the uncovered region of the surface of the at least one first interconnect  402  and the uncovered region of the surface of the at least one second interconnect  405 , the length of which molecules is equal to the distance between the at least one first interconnect  402  and the at least one second interconnect  405 . The resulting layer arrangement  506  with the introduced molecular electronic molecules  409  in accordance with the second preferred exemplary embodiment of the method for fabricating a molecular electronics arrangement is shown in  FIG. 5E . 
     A third preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement is described below with reference to  FIG. 4A ,  FIG. 4B ,  FIG. 6A ,  FIG. 6B ,  FIG. 6C ,  FIG. 6D . 
     The first two method steps in accordance with the third preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement are identical to the first two method sections described above with reference to the first preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement. After carrying out these method steps, the layer arrangement  404  shown in  FIG. 4B  is obtained. Said layer arrangement is used as a starting point for the description of the further method sequence. 
     In a next method step, a continuous conductor layer and also a further protective layer  601  are applied to the surface of the layer arrangement  404 . 
     In accordance with the described third preferred exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, the conductor layer made of gold material is deposited on the surface of the layer arrangement  404 , and a further protective layer  601  fabricated from silicon nitride material is subsequently deposited on the surface of the continuous conductor layer. 
     In a subsequent method step, the continuous conductor layer and the further protective layer  601  are patterned in such a way that an at least one second interconnect  602  covered at least partly with the further protective layer  601  is thereby formed. 
     In other words, the conductor layer fabricated from gold material and the further protective layer  601  arranged thereabove are patterned jointly. This can be done for example by means of a suitable photolithography and etching method. After carrying out this method step, the layer arrangement  600  shown in  FIG. 6A  is obtained. 
     In a subsequent method step, before molecular electronic molecules are actually introduced into the arrangement, a protective layer  603  is applied to at least one portion of the uncovered regions of the surfaces of the first and/or of the second interconnects  402 ,  602 . The method step described has a plurality of substeps. 
     In a first substep, a protective layer  603  is applied on the entire surface of the layer arrangement  600 . This is realized according to the invention by a silicon nitride layer being deposited on the surface of the layer arrangement  600  using the CVD method, thereby obtaining the layer arrangement  604  shown in  FIG. 6B . 
     In a further substep, the protective layer  603  is etched back anisotropically, thereby obtaining the layer arrangement  605  shown in  FIG. 6C . In other words, material of the protective layer  603  with a predeterminable thickness is etched away from the entire surface of the layer arrangement  604 , so that, as shown in  FIG. 6C , regions of the spacer layer  403  are uncovered on the surface of the layer arrangement  605 . 
     In a further method step, spacers  403   a  are formed by means of partial etching-back of the spacer layer  403 , in such a way that an uncovered region of the surface of the at least one first interconnect  402  and an uncovered region of the surface of the at least one second interconnect  602  remain. 
     This method step is realized by the spacer layer  403  fabricated from silicon dioxide material being etched back wet-chemically and being undercut. Spacers  403   a  are thereby formed, so that, as shown in  FIG. 6D , the first interconnects  402  and the second interconnects  602  project laterally on both sides beyond the spacers  403   a . Clearly, in accordance with the third exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, as shown in  FIG. 6D , the second interconnects  602  are partly covered with a U-shaped protective layer, which protective layer is formed by the protective layer  603  and the further protective layer  601 . Therefore, that surface region of the second interconnect  602  which is covered with the protective layer  601 ,  603  is protected from a parasitic binding of molecular electronic molecules. 
     In a next method step, molecular electronic molecules  606  are arranged between the uncovered region of the surface of the at least one first interconnect  402  and the uncovered region of the surface of the at least one second interconnect  602 , the length of which molecules is equal to the distance between the at least one first interconnect  402  and the at least one second interconnect  602 . 
     After the introduction of the molecules, which are bispyridinium molecules in accordance with the described exemplary embodiment of the method according to the invention for fabricating a molecular electronics arrangement, the layer arrangement  607  shown in  FIG. 6D  is obtained. The layer arrangement  607  shown in  FIG. 6D  essentially corresponds to the molecular electronics arrangement  220  shown in  FIG. 2C .