Patent Publication Number: US-2003235921-A1

Title: Forming electrical contacts to a molecular layer

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001] The present invention is directed, in general, to forming reliable contacts in nanoscale devices. Specifically, the invention is directed to a process for forming electrical contacts to a molecular layer in a nanoscale electrical device, to the device so formed, and to a method of manufacturing an integrated circuit comprising the nanoscale device.  
       BACKGROUND OF THE INVENTION  
       [0002] There is currently great interest in the development of molecular or nanoscale electrical devices. To this end, much effort has been devoted to developing partial or all-polymer nanoscale electronic devices. In addition to providing higher device densities in integrated circuits, polymeric electronic devices may be more physically flexible and more cost and processing-efficient than conventional inorganic semiconductor devices.  
       [0003] In such nanoscale electronic devices, single molecular layers may form active elements in the device. The efficient formation of reliable electrical contacts to the molecular layer is therefore an important aspect in the commercial production of nanoscale devices. The molecules are typically fixed at one end to a conducive substrate that forms one electrical contact for the device, and to a metal layer on the other end to form a second electrical contact. Conventional processes for depositing the metal onto the molecules include treatment with a metal containing solution, to produce a colloidal metal layer, or evaporation of the metal onto the molecules, to produce an evaporated metal layer.  
       [0004] Do to the sparseness between the molecules, however, there are often gaps between the molecules. In addition, the molecules are typically able to rotate about the first electrical contact. Both the presence of gaps, and the ability of molecules to rotate, impede the attachment of the deposited metal layer to form the second electrical contact. Some of the deposited metal, for example, goes between the gaps between molecules, resulting in an electrical short circuit between the conductive substrate and metal layer.  
       [0005] Furthermore, methods based on treatments with solutions of colloidal metal particles do not produce connections to all the molecules because solution-transported metal particles may attach to randomly distributed single molecules rather than to substantially all of the molecules. Methods based on the direct evaporation of metal onto the molecules are also problematic, because the high kinetic energy of the metal atoms striking the molecules may destroy or alter the structure of the molecular layer. Efforts to reduce the deleterious effects of direct evaporation, such as low temperature evaporation, or shallow angle evaporation, have not improved the production of non-defective devices to satisfactory levels. As a result, conventional processes for the deposition of the metal layer continue to produce a large number of nonfunctional devices, as indicated, for example, by the devices having an undesirably low resistance across the molecular layer. Of all devices produced in a typical conventional process, for instance, only 2% may be functional.  
       [0006] Therefore, previously proposed methods of attaching electrical contacts to a layer of molecules lack the desired reliability demanded by today&#39;s electronics industry. Accordingly, what is needed in the art is a method of forming such contacts, thereby increasing the efficient production of nanoscale electrical devices, while not experiencing the problems associated with previous methods.  
       SUMMARY OF THE INVENTION  
       [0007] To address the above-discussed deficiencies, one embodiment of the present invention provides a process for forming electrical contacts to a molecular layer. The process comprises  
       [0008] coating a surface of a stamp with a metal layer and forming an attached layer of anchored molecules by covalently bonding first ends of the anchored molecules to one of either a conductive or semiconductive substrate or the metal layer. The process further comprise placing the other of the conductive or semiconductive substrate or the metal layer in contact with the attached layer of anchored molecules, the conductive or semiconductive substrate or the metal layer covalently bonding to free ends of the anchored molecules.  
       [0009] In another embodiment, the invention further provides a nanoscale electronic device, comprising a conductive substrate, a layer of anchored molecules and a printed metal layer. The layer of anchored molecules has first and second ends, the first ends of the molecules covalently anchored to the conductive or semiconductive substrate, the second ends able to rotate about the anchored first ends. The printed metal layer is covalently coupled to the second ends of the layer of anchored molecules.  
       [0010] Yet another embodiment of the present invention provides a method for manufacturing an integrated circuit. The method comprises forming active device and interconnecting the device to form an operative integrated circuit. Forming the active devices includes forming conductive electrodes on or in a substrate and forming a conductive or semiconductive layer over the conductive electrode and the substrate. A layer of molecules is formed by covalently anchoring a layer of the molecules having first and second ends, the first ends of the molecules being anchored to the conductive or semiconductive substrate and the second ends able to rotate about the anchored first ends.  
       [0011] The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the scope of the invention.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] The invention is best understood from the following detailed description, when read with the accompanying FIGUREs. It is emphasized that in accordance with the standard practice in the optoelectronic industry, various features may not be drawn to scale. The dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
     [0013]FIGS. 1A to  1 D illustrate a process for forming electrical contacts to a molecular layer according to the present invention;  
     [0014]FIG. 2 illustrates components in a nanoscale electronic device of the present invention;  
     [0015]FIG. 3 illustrate, a method for forming an integrated circuit, which may form one environment where a device similar to that shown in FIG. 2, is included;  
     [0016]FIG. 4 illustrates the relationship between current density and voltage for devices made according to the present invention having various contact areas;  
     [0017]FIG. 5 illustrates selected results for: (A and B) reference devices; (C and D) conventionally made devices; or (E and F) devices of the present invention; and  
     [0018]FIG. 6 illustrate the reliability of the process of the present invention to produce devices having a certain contact resistance.  
    
    
     DETAILED DESCRIPTION  
     [0019] The present invention recognizes the advantageous use of using a nanotransfer printing procedure for forming electrical contacts to a molecular layer. The procedure for forming nanoscale patterned thin film metal layers, is disclosed in U.S. patent application Ser. No. __/___,___ to Loo et al., incorporated herein by reference. It has been discovered that this procedure as adapted to the present invention allows for the reliable production of nanoscale devices, which in turn may be incorporated into an integrated circuit.  
     [0020] Referring initially to FIG. 1A to  1 D, illustrated are selected views of the process for forming electrical contacts to a molecular layer. Turning first to FIG. 1A, illustrated is a stamp  100  and the coating of a surface  110  of the stamp  100  with a metal layer  120 . The process for forming the stamp  100  has been disclosed in Loo et al as incorporated above. Briefly, the process may include forming a pattern on a template, the pattern comprising raised and relief portions. The template is coated with a prepolymer and a catalytic agent. The prepolymer is then cured to form an elastomeric rubber. The elastomeric rubber is peeled away from the template to form the stamp  100 . At least one surface  110  of the stamp  100  comprises raised portions  103 , corresponding to relief portions of the template, and relief portions  107 , corresponding to raised portions of the template. In certain embodiments, to facilitate handling, the stamp  100  may be attached to a polymer substrate  130  such as poly (ethylene terephthalate), as disclosed in Loo et al.  
     [0021] The coating of the stamp  100  with the metal layer  120  may be conducted using any conventional process well know to those of ordinary skill in the art. For example, coating may be achieved by treating the surface  110  of the stamp  100  with a solution containing ions corresponding to the metal layer  120 . Alternatively, coating may be accomplished by evaporating metal vapors onto to the surface  110  of the stamp  100 , using conventional thermal evaporation techniques. In certain preferred embodiments, the coating is performed for a sufficient period to form a metal layer  120  about 200 to about 300 Angstroms thickness  125 .  
     [0022] Turning to FIG. 1B illustrated is the formation of an attached layer of anchored molecules  140  by coupling first ends of the anchored molecules  143  to a conductive or semiconductive substrate  150 . The coupling may be accomplishing using any conventional process that will result in substantially all sites on the surface  155  of the conductive or semiconductive substrate  150  being coupled to the first ends  143  of the molecule  140 . For example, coupling may be achieved by placing a surface  155  of the conductive or semiconductive substrate  150  in contact with a solution containing the molecules  140 . In certain embodiments, to facilitate coverage of the substrate  150 , the molecules  140  may be dissolved in a solvent, such as ethanol or similar organic solvent.  
     [0023] In certain preferred embodiments, coupling is performed by placing the conductive or semiconductive substrate  150  in a chamber  160  and placing a source  165  of the molecules  140  in the chamber  160 . The source  165 , may be for example, a petri dish containing a sufficient amount of molecules  140  to ensure substantially complete coverage of the conductive or semiconductive substrate  150 . The chamber  160  is then maintained at a temperature and pressure sufficient to allowing coupling between the first ends of the molecule  143  and the substrate  150 . In certain preferred embodiments, for example, the chamber  160  is maintained at room temperature (i.e., about 23° C.), and a pressure of less than about 0.001 Torr for at least about 15 minutes.  
     [0024] Turning to FIG. 1C illustrated is placing the metal layer  120  in contact with the attached layer of anchored molecules  140 , the metal layer  120  chemically bonding to free ends of the anchored molecules  147 . Chemically bonding between the free ends  147  and the metal layer  120  occurs rapidly and without further processing steps. For example, contacting the anchored layer of molecules  140  and the metal layer  120  may be done at room temperature (−23° C.) in room air. Similarly, no additional force need be applied other than the inherent adhesion between the stamp  100  and the substrate  150 .  
     [0025] Contact is maintained for a period sufficient to ensure substantially complete chemical bonding of the metal layer  120  to free ends of the anchored molecules  147 . In certain preferred embodiments, for example, placing the metal layer  120  in contact with the attached layer of anchored molecules  140  occurs for less than about 15 seconds, and more preferably less than about 3 seconds.  
     [0026] After the contact period, the stamp  100  is peeled away from the substrate  150  to yield a substrate  150  having metal layers  170  covalently bonded to the anchored molecules  140  in discrete locations corresponding to raised portions  103  on the stamp  100  (FIG. 1D).  
     [0027] In other preferred embodiments, the stamp  100  bearing the metal layer  110  may be placed in chamber  160 , and the first ends  143  of the molecules  140  coupled to the metal layer  110 . The stamp  100  bearing metal layer  110  and molecules  140  attached thereto, are then contacted to the conductive or semiconductive substrate  150 . Contact is for a sufficient period to ensure complete chemical bonding of the conductive or semiconductive substrate  150  to free ends of the anchored molecules  147 . After the contact period, the stamp  100  is peeled away from the substrate  150  to yield a substrate  150  having metal layers  170  covalently bonded to the anchored molecules  140  in discrete locations corresponding to raised portions  103  on the stamp  100 , similar to that depicted in FIG. 1D, with the exception that there are substantially no anchored molecules  141  attached to the conductive or semiconductive substrate  150  that are also not attached to the metal  170 .  
     [0028] Another embodiment of the present invention, illustrated in FIG. 2, is a nanoscale electronic device  200 . For clarity, components analogous to that shown to FIGS. 1A to  1 D, retain analogous numbering. The device  200  may include a conductive or semiconductive substrate  250  and a layer of anchored molecules  240  having first  243  and second ends  247 , the first ends  243  of the molecules  240  being anchored to the conductive or semiconductive substrate  250 . The second ends  247  are able to rotate about the anchored first ends  243 .  
     [0029] The device  200  further includes a printed metal layer  270  coupled to the second ends  247  of the layer of anchored molecules  240 . The term printed metal layer  270  refers to a metal layer covalently associated with the anchored molecules and forming a substantially uniform blanket coverage over the anchored molecules  240 , at discrete locations on the substrate  250 , as defined by the raised pattern on the stamp  100 , as discussed elsewhere herein.  
     [0030] The anchored molecules  240  of the device  200  may be comprised of one or more compounds characterized by the chemical formula:  
     F′−(R) n −F″ 
     [0031] F′ comprises the first end  243  wherein the first end  243  comprises a first functional moiety capable of chemically bonding to the conductive or semiconductive substrate  250 . F″ comprises the second end  247  wherein the second end  247  comprises a second functional moiety capable of chemically bonding to the metal layer  270 . R comprises a bridge  245  covalently linking the first  243  and second ends  247 , where R  245  comprises individually substituted or unsubstituted nonmetal atoms, and 0≦n≦20.  
     [0032] The first functional moieties may comprise any functional groups that would facilitate the formation of covalent bonds between the conductive or semiconductive substrate  250  and the first ends  243  of the molecule  240 . In certain preferred embodiments, for example, the first functional moieties are selected from the group consisting of thiols, monocarboxylates, dicarboxylates and alkoxyls. One of ordinary skill in the art would understand that the selection of first functional groups may vary according the chemical composition of the substrate  250 . For example, if the substrate  250  is composed of gallium arsenide, then the first functional moieties preferably comprise thiols, monocarboxylates or dicarboxylates. Alternatively, if the substrate  250  is composed of gold, then the first functional moieties preferably comprise thiols. Or, if the substrate  250  is composed of silicon, then the first functional moieties preferably comprise alkoxyls.  
     [0033] The second functional moieties may comprise any functional groups that would facilitate the formation of covalent bonds between the printed metal layer  270  and the second ends  247  of the molecule  240 . In certain preferred embodiments, for example, the second functional moieties are selected from the group consisting of thiols and disulphides.  
     [0034] As noted (R) n    245 , the bridge  245 , may comprise any chemical composition comprising non metal atoms that covalently links the first  243  and second ends  247 . In certain embodiments R may comprise substituted (e.g., —SiH 2 —, —CH 2 —, —NH—), or non-substituted (e.g., —S—, —O—, —Se—) nonmetals atoms that are repeated n times. In certain preferred embodiments, for example, R comprises an alkane group having the chemical formula: (—CH 2 —) and 1≦n≦10. R comprising aromatics, such as a 4,4′ biphenyl group (i.e., R═—C 6 H 4 —; n=2), or related compounds, are also within the scope of the present invention.  
     [0035] Various processing considerations may guide the selected of molecules  240 . For example, in certain embodiments, the molecule  240  should be sufficiently volatile that when placed in chamber  160  (FIG. 1C) the molecule will enter the gas phase in sufficient concentrations to couple to and coat the entire substrate  240  within an acceptable period. In other embodiments, the molecule  240  should be a liquid or sufficiently soluble in a solvent, so as to couple to and coat the entire substrate  250  when the liquid or solution is contacted with the substrate  250 .  
     [0036] The printed metal layer  270  may comprise any metal that can covalently couple the molecule  240  and provide an electrical contact between the device  200  and other electrical components. In certain preferred embodiments, for example, the printed metal  220  layer is selected from the group consisting of Gold, Silver, Copper, Platinum, Palladium, Tungsten, Aluminum and alloys thereof.  
     [0037] Likewise, the conductive or semiconductive substrate  250  may comprise any material that can covalently couple to the molecule  240  and provide an electrical contact between the device  200  and other electrical components. In certain preferred embodiments, for example, conductive or semiconductive substrate  250  is selected from the group consisting of, Gallium Arsenide, Silicon, Indium Phosphide, Gold, and Tungsten Oxide. One of ordinary skill in the art would understand that certain substrates  250 , such as Silicon, may be further contain a conventional dopant introduced using conventional techniques, to increase its conductivity.  
     [0038] As noted above the printed metal layer  270  and the conductive or semiconductive substrate  250  form electrical contacts for the device  200 . In certain embodiments, for example, the layer of anchored molecules  240  forms a one of a channel and a gate dielectric, the conductive or semiconductive substrate  250  forms the other of a first electrode and a channel, and the printed metal layer  250  forms a second electrode of a field effect transistor.  
     [0039] As further illustrated in the experimental section to follow, devices  200  of the present invention can be efficiently fabricated with fewer defects than previously obtained from conventional devices. For example, the device  200  of the present invention may have a contact resistance between the printed metal layer  220  and the conductive or semiconductive substrate  250  that is at least about 10, more preferably 100, and even more preferably 1000 times higher than a contact resistance for a substantially identical device except having an evaporated metal layer or colloidal metal layer.  
     [0040] Yet another embodiment of the present invention is a method for manufacturing an integrated circuit. The method comprises forming active devices and interconnecting said devices to form an operative integrated circuit. One of ordinary skill in the art would understand that such devices could be assembled to form a variety of components in integrated circuits. Such components may include, for example, field effect transistors (FET), Metal Oxide Semiconductor Field-Effect Transistor MOSFET, Complementary Metal Oxide Semiconductor (CMOS), bipolar transistors and similar devices, and therefore the details of such assembly steps are not presented here.  
     [0041]FIG. 3 illustrates a selected view of a process for forming a active devices  300  in the integrated circuit. Any of the embodiments of process and devices discussed herein may be used to form the active devices  300 . One of ordinary skill in the art would understand, that nanoscale devices  200  having a molecular layer  240  may be incorporated into devices  300  where thin internal layers of active or passive material would present an advantage. Forming the active devices  300  includes forming conductive electrodes  385 ,  390  (e.g., source and drain) on or in a substrate  395 . A conductive or semiconductive layer  350  is formed over the conductive electrodes  385 ,  390  and the substrate  395 . A layer of molecules  340  is formed by covalently anchoring a layer of the molecules  340  having first and second ends,  343 ,  347 , the first ends  343  of the molecules being anchored to the conductive or semiconductive substrate  350  and the second ends  347  able to rotate about the anchored first ends  343 . Forming the device further includes imprinting an electrode  370 , such as a gate electrode, by contacting a stamp  100 , such as that shoawn in FIG. 1A, having a metal layer located thereon to the second ends  347  of the layer of molecules  340  to form a covalent bond between the metal layer  370  and the second ends  347 .  
     [0042] As noted elsewhere herein, the present invention allows for the efficient production of integrated circuits with a low number of non functioning nanoscale device components. For example, in certain embodiments, the method results in at least about 99% of nanoscale devices  200 , that may be incorporated into a transistor  300 , have a contact resistance between the printed metal layer  270  and the conductive or semiconductive substrate  250  of greater than about 1×10 5  ohm cm 2 . In other preferred embodiments, the method results in at least about 99% of the formed nanoscale devices  200 , have a contact resistance within about ±2 log units of a median of a logarithm of the contact resistance.  
     [0043] Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the scope of the invention.  
     [0044] Experiments  
     [0045] A first series of experiments was conducted to examine the reliability of using a conventional contact probe to measure the electrical conduction between contacts formed in the nanoscale devices of the present invention. Nanoscale devices having different contact areas were fabricated using the processes described herein. Specifically, the conductive substrate comprised GaAs, the anchored molecules comprised  1 , 8  octane dithiol and the printed metal layer comprised gold.  
     [0046] The rubber elastomeric stamp was fabricated as described elsewhere herein and in Loo et al., using a prepolymer comprising polydimethyl siloxane and platinum catalyst (Sylgard 184 Elastomer Kit, Dow-Corning, Midland, Mich.). The stamp was coated with gold (˜10 Angstrom/s) using conventional thermal evaporation using an electron beam, a pure gold target and pressure of 10 7  Torr, at room temperature for about 20 to about 30 s.  
     [0047] To remove the superficial oxide layer GaAs substrates were etched with either concentrated HCl or NH 3 OH (either at ˜30 wt %) for about 2 min, rinsed with deionized water and dried, prior to forming an attached layer of anchored molecules. To attach the 1,8 octane dithiol molecules, the GaAs substrates were placed in a commercial desiccator, and about 2-3 drops of 1,8 octane dithiol was added to a petri dish located in the desiccator. A vacuum was formed in the desiccator using a house vacuum (˜0.001 Torr) for about 15 minutes.  
     [0048] The GaAs substrate was then removed from the desiccator rinsed with ethanol and dried over nitrogen gas. After drying, the gold-layered stamp was contacted with the substrate for between about 2 and about 15 seconds. The stamp was then peel off the substrate to yield the nanoscale device. As a routine test to ensure that the gold layer was chemically bonding to free ends of the 1,8 octane dithiol, selected devices were adhered to adhesive tape (Scotch Tape®, 3M Company, St. Paul, Minn.) and the tape was examined for the absence of gold.  
     [0049]FIG. 4 illustrates selected results showing the relationship between current density and voltage for devices made according to the present invention having various contact areas. The relationship between current density and voltage was nearly the same for contact areas ranging from about 62.5 microns by 62.5 microns (i.e., 2.5 mil×2.5 mil) to about 500 microns by 500 microns (i.e., 20 mil×20 mil). This indicates that the method for measuring voltage and current across the nanoscale devices was reproducible.  
     [0050] In a second series of experiments, the relationship between current and voltage was examined for a number of nanoscale devices. FIG. 5 illustrates selected results for: (A and B) reference devices (ref); (C and D) conventionally made devices (prior art); or (E and F) devices of the present invention. The reference devices comprised gold evaporated onto to GaAs substrates, with no intervening molecular layer. The conventionally made devices comprised substantially identical devices as the present invention except having an evaporated metal layer onto the GaAs substrate with 1,8 octane dithiol anchored thereto. The gold was evaporated onto the substrate using the same thermal evaporation methodology as described in the first experiment for coating the stamp. Evaporation was done at either: (C) room temperature (˜23° C.) or (D) about −15° C. The devices of the present invention were prepared substantial the same as described in the first experiment.  
     [0051]FIG. 5 shows that the current passing through the conventionally made devices (C &amp; D) was only about one order of magnitude less than the reference devices (A &amp; B). In contrast, substantially less current (i.e., about 3 orders of magnitude) passes through the devices of the present invention (E &amp; F) as compared to conventionally made devices (C &amp; D).  
     [0052] Contact resistance was calculated from data such as that illustrated in FIG. 5, by determining resistance from the slope of plots of current versus voltage, using data from about −0.1 V to about 0.0 V, and multiplying resistance by the area of the contact (i.e., area of GaAs and gold layer). Representative contact resistances (RA) for the devices depicted in FIG. 5 are summarized in TABLE 1. Standard deviations reported in TABLE 1 are based on the standard deviation of the slope of current versus voltage data, as determined by linear regression analysis.  
                           TABLE 1                                      Device   RA (Ohm · cm 2)             Reference (A)   43.1 ± 5.2           Reference (B)   79.7 ± 8.6           Conventional (C)   140.8 ± 14.9           Conventional (D)   1166 ± 543.8           Present (E &amp; F)   1.67 × 10 7  ± 1.06 × 10 7                        
 
     [0053] As illustrated in TABLE 1, for the conventionally made devices the contact resistance between the evaporated gold layer and the GaAs substrate ranged from about 1.8 to about 27 times higher than the contact resistance of the reference devices. In contrast, the contact resistance of the present invention were at least about five orders of magnitude higher that the contact resistance of the reference device. Moreover, the contact resistance of the present devices were at least about 4 orders of magnitude higher than a contact resistance for the conventionally made devices having an evaporated metal layer.  
     [0054] A third series of experiments was conducted to examine the reliability of the process of the present invention to produce devices having a certain contact resistance. About 100 nanoscale devices were produced in a similar manner as described in the first experiment. A device having a substantial number of shorts is expected to have a contact resistance of less than about 1×10 3  ohm cm 2 .  
     [0055]FIG. 6 show the result of the experiment. Counts refers the number of devices having a Log 10 (RA) value within 0.5 unit ranges depicted horizontal scale in FIG. 6. At least about 99% of the devices have a contact resistance between the printed gold layer and the GaAs substrate of greater than about 1×10 5  ohm cm 2 . And, at least about 99% of the device had a contact resistance within about ±2 log units of a median of a logarithm of the contact resistance (Log 10 (RA)).