Abstract:
An adhesion bond between a metallic surface layer and a second surface is formed by treating the layers with a material comprising sulphur-containing molecules. The sulphur-containing molecules are applied as a surface treatment of the surfaces, so that the sulphur-containing molecules act as a coupling agent to bond chemically to both substrates form nanometer-sized structures on the surfaces. The nanometer-sized structures are incorporated into a self-assembly interlayer in between the surfaces, with the interlayer forming a bond to both surfaces.

Description:
RELATED APPLICATIONS 
       [0001]    The present patent application claims priority to Provisional Patent Application No. 61/214,097 filed Apr. 21, 2009, which is assigned to the assignee hereof and filed by the inventors hereof and which is incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The present disclosure relates to a metal interface including microstructures in which metal substrates are modified in order to improve the integrity of the metal interface, such as a metal-polymer interface. 
         [0004]    2. Background 
         [0005]    The present disclosure is concerned with interfacial problems of metal-polymer interfaces. Due to the lack of intrinsic chemical bonding at the interfaces, the integrity of these interfaces is generally poor. Delamination and small molecule penetrations are common along these interfaces. The poor integrity induces failure, e.g., interfacial debonding, corrosion, electromigration, etc., especially under extreme and harsh conditions. 
       SUMMARY 
       [0006]    An adhesion bond is formed between a metallic substrate having at least a metallic surface layer and a second substrate having a second surface. The substrate is subjected to a surface treatment by applying a self-assembly interlayer between the surface, the treatment comprising applying a material comprising a sulphur-containing molecules as a coupling agent to bond chemically to both substrates and form nanometer-sized structures on the substrates. The surfaces are brought into sufficiently close proximity to each other for the interlayer to bond to both surfaces. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawings will be provided by the Office upon request and payment of the necessary fee. 
           [0008]      FIG. 1  is a schematic diagram depicting a cross-sectional view of a metal-polymer interface according to the current disclosure. 
           [0009]      FIGS. 2A-2D  are Scanning Electron Microscope (SEM) photomicrographs of a Cu surface.  FIGS. 2A and 2B  show a control surface and  FIGS. 2C and 2D  show the surface treated with thiol solution. 
           [0010]      FIGS. 3A and 3B  are spectrographs illustrating the effect root-mean-square roughness of a control copper substrate ( FIG. 3A ) and a copper substrate modified with the nanometer-sized features ( FIG. 3B ). 
           [0011]      FIG. 4  is a diagram depicting a test sample configuration including a tapered double cantilever test jig used to perform load testing to determine critical load for stable crack propagation. 
           [0012]      FIG. 5  is a graph depicting demonstrates the typical load-displacement curves obtained from tapered double cantilever tests for a sample having the nanometer-sized structures and a control (untreated) sample, using the test jig of  FIG. 4 . 
           [0013]      FIG. 6  is a graph showing an evaluation of interfacial adhesion improvement of a copper-epoxy interface under the surface treatment described in this disclosure. 
           [0014]      FIG. 7  is a morphological image of an epoxy surface after a fracture test of a sample treated according to the disclosed techniques. 
       
    
    
     DETAILED DESCRIPTION 
     Overview 
       [0015]    Nanometer-sized structures are formed between interfaces of metal with another material to improve the interfacial integrity. A surface treatment of sulphur-containing molecules with metal substrates results in the formation of the nanometer-sized structures on the metal substrates. The nanometer-sized structures are bonded chemically to both metals and non-metallic coatings. With the formation of this strong and high-density interlayer at the interface, micro-crack formation and small molecule transportation is reduced. This can readily improve the integrity of a metal-polymer joint, in an interface of a metal substrate joined to a polymer coating, for example of a metal joined with another material. The resulting interface has increased reliability during operation. The disclosure discloses a surface treatment method and the materials used in modifying metal substrates. This solves the interfacial problems of a metal substrate joined to another material. 
         [0016]    The interface comprises a metal substrate, a polymer coating and a nanometer-sized structure in between the two. The treatment process in this example comprises depositing at least one type of sulphur-containing molecules onto metal substrates. 
         [0017]    In a particular configuration, the nanometer-sized structures are formed by use of sulphur-containing molecules which can be, but not limited to, thiol, sulfide, disulfide, polysulfide or combinations of these materials. 
         [0018]    In a further example, a one-step room temperature surface treatment method is used to modify metal substrates with sulphur-containing molecules. The surface treatment method involves the introduction of nanometer-sized structures on metal substrates. 
         [0019]    Another aspect of the disclosure is the nanometer-sized structures can provide direct chemical bonding to
       1) metal substrates through a metal reactive group, and   2) polymer coating providing bonding through a polymer reactive group.       
 
         [0022]    With formation of nanometer-sized features on the metal surface, strong joints are introduced at the interface. 
         [0023]    The nanometer-sized structures are attached to both metal substrate and polymer coating with chemical bonds. With the formation of those nanometer-sized structures within the interface, strong interfacial adhesion has been achieved. The densely packed nanometer-sized structures at the interface can also hinder small molecules penetration along the interface. This reduces the chance of undesirable chemical reaction at the interface, which can significantly improve the integrity of metal-polymer interfaces. The techniques can be applied to any areas where the integrity of metal-polymer interfaces is concerned, including adhesion promotion, corrosion inhibition and diffusion hindrance. The surface treatments can improve the interfacial performance of a metal-polymer joint in terms of adhesion, corrosion, diffusion, and other properties related to the integrity of the metal-polymer interface. 
         [0024]    Formation of a self-assembly interlayer within a metal-polymer interface is performed so as to improve the integrity of the interface. The interlayer is formed when sulphur-containing molecules are chemically attached to both metal substrates and polymer coatings. Nanometer-sized structures are used in the interlayer, and are used as an adhesion promoter, corrosion inhibitor and diffusion barrier to improve integrity of the interface. 
         [0025]    Self-Assembly Interlayer 
         [0026]      FIG. 1  illustrates a schematic diagram for an interface structure  111  and includes a metal substrate  113 , a polymer coating  115  and a self-assembly interlayer  121  in between the metal substrate  113  and the polymer coating  115 . The self-assembly interlayer  121  has nanometer-sized structures  123  according to the example. The nanometer-sized structures  123  are attached to both the metal substrate  113  and the polymer coating  115  and comprise sulphur-containing molecules which chemically bond to both the metal substrate  113  and the polymer coating  115 . 
         [0027]    The sulphur-containing molecules are introduced to the interface  111  to form the nanometer-sized structures  123 . The sulphur-containing molecules are chosen to chemically bond with the metal substrate  113  and to promote an adhesion bond with the polymer coating  115  by modifying the surface of the metal substrate  113  with sulphur-containing molecules at the surface of the metal substrate  113 . 
         [0028]    The metal substrate  113  is selected from the group consisting of pure metal or an alloy containing copper, silver, gold, iron, aluminum, mercury, nickel, zinc, lead, tin or combinations of those. The metal substrate  113  can be manufactured from any suitable process including vapor deposition, electroplating, electroless plating, sputtering, evaporation, electrode position, roll annealing, etc. The metal substrate  113  may be of any thickness, and may have with any suitable surface roughness. The roughness may vary from one angstrom (Å) to 10,000 micrometers (μm), and varies with the particular application. The affinity of the sulphur-containing molecules is particularly effective with gold, silver and copper but also applies to other metals, for example, aluminum, aluminum alloys and the other metals described above. 
         [0029]    The polymer coating  115  can be a thermoset, which can include, but is not limited to, epoxies, cyanoesters, polyimides, maleimide, bismaleimide triazine, silicones, and combinations thereof. Alternatively, the polymer coating  115  can be a thermoplastic, which can include, but is not limited to, polymethylmethacrylate, polyacrylonitrile, polyamide, polybutylene, polyurethane and combinations thereof. The polymer coating  115  can be applied on the metal substrates in any of the following types: sheet type, solvent type, emulsion type or aqueous solution type. 
         [0030]    The surface treatment, in adsorbing the sulphur-containing molecules onto metal substrate  113 , can be solution deposition, vapor deposition, thermal spraying, electrochemical deposition or other suitable processes under any possible conditions that forms nanometer-size structures  123  on metal substrates  113 . When the self-assembly interlayer  121  is prepared from solution deposition, the sulphur-containing molecules can be dissolved in a solvent selected from alcohol, chloroform, tetrahydrofuran, acetone, actonitrile, water or other solvents that can dissolve the sulphur-containing molecules. The solution concentration for the surface treatment is prepared in range from 0.01 millimoles to 100 millimoles with pH between 3 to 14. The surface treatment involves immersion of the metal substrate  113  to the solution with an immersion time ranging from one second to one month under any agitation conditions according to reaction rate. 
         [0031]    The self-assembly interlayer  121  is made of sulphur-containing molecules which comprises at least a metal reactive portion  127  and a polymer reactive portion  129 . The metal reactive portion  127  is chemically bonded to metal substrates  113  through sulphur atoms. The metal reactive portion  127  is derived from sulphur-containing molecules which can be, but not limited to thiol, sulfide, disulfide, polysulfide or combinations of these materials. A criterion in selection of the sulphur-containing molecules is that they can form a nanometer-sized structure on the metal surface so as to ensure strong chemical bonds at the interface. 
         [0032]    The polymer reactive portion  129  is chemically bonded to polymer coating  115 . The polymer reactive portion  129  can be selected according to the target polymer adhesive. It should be reactive to the functional group of the polymer coating  115 . The polymer reactive portion  129  can be derived from groups including halogen, epoxide, glycol, N, O, S, haloalkyl, hydroxy, aryloxy, benzloxy, alkoxy, haloalkoxy, amino, monoalkylamino, dialkylamino, heteroalkylamino, amide, alcohol, carboxylic acid, ester, ether, anhydride, acyloxy, acyl, ketone, quinone, aldehyde, carbohydrate, alkyl, aryl, and combinations thereof. The sulphur-containing molecules according to the current disclosure can contain alkyl, cycloalkyl, heterocyclic, alkadienyl, alkyne, aryl, heteroaryl, alkylaryl or arylalkyl or combinations thereof; and it may be singly or multiply substituted in singular or multiply bonded fashion. 
         [0033]    The sulphur-containing molecules are generally applied as bi-functional molecules. The bi-functional characteristic may be achieved by including one sulphur or one thiol component (reactive to metal) and the other with organic reactive group (reactive to epoxy). The advantage of this technology is that the sulphur-containing molecules can be chemically bonded with both the metal substrates and the polymer. The densely formed bi-functional sulphur-containing molecules used in this configuration are advantageous in that the molecules enhance the chemical bonds along the interface. This avoids adding sulphur-containing molecules directly to epoxy formulation, which could hinder chemical bond formation and possibly reduce the adhesion. 
         [0034]    Formation of the Nanometer-Sized Features 
         [0035]      FIGS. 2A-2D  are Scanning Electron Microscope (SEM) photomicrographs of a Cu surface.  FIGS. 2A and 2B  show a control surface and  FIGS. 2C and 2D  show the surface treated with thiol solution.  FIGS. 2C and 2D  are Scanning Electron Microscope (SEM) photomicrographs of a copper surface treated with thiol solution.  FIG. 2C  depicts the Cu surface at 20,000 magnification and  FIG. 2D  depicts the Cu surface at 50,000 magnification. These figures show surface morphology of copper substrates after solution deposition of sulphur-containing molecules, including formations of the nanometer-sized features typically range in size between approximately 100 nm and 10,000 nm. The size of the nanometer-sized features can be in the range of one angstrom to ten thousands angstroms. The thickness of self-assembly interlayer  121  can be in the range of 1 Å-100 k Å and the size of the nanometer-sized structures in the self-assembly interlayer  121  can be in the range of one 1 Å-10 k Å. These nanometer-sized features provide bonding sites to the polymer coating  115 . 
         [0036]    In comparing the control surface ( FIGS. 2A and 2B ) with the thiol treated surface ( FIGS. 2C and 2D ), nanometer-sized molecular network was observed on the thiol treated surface ( FIGS. 2C and 2D ). On the treated surface, the network structure of nanometer-size fibrils was observed from the thiol modified Cu surface and the nanometer-sized fibril network covered the entire substrate surface. 
         [0037]      FIGS. 3A and 3B  are spectrographs illustrating surface roughness, obtained by use of an optical three-dimensional profiler, of a control copper substrate ( FIG. 3A ) and a copper substrate modified with the nanometer-sized features ( FIG. 3B ). The average root-mean-square roughness for both the control and the substrate with the self-assembly interlayer was around hundreds of nanometers, with the data indicating no significant roughness change is induced with the surface treatment. In the example, the root-mean-square roughness of the control and the thiol-treated substrate is 126±31 and 119±13 nm, respectively. 
         [0038]    Tapered Double Cantilever Beam Test 
         [0039]      FIG. 4  is a diagram depicting a test sample configuration including a tapered double cantilever test jig  403 . Jig  403  is used to perform load testing to determine critical load for stable crack propagation. The test jig  403  is made of copper and comprises bottom  407  and top  409  halves. The test jig  403  provides an interface structure which includes an epoxy adhesive  421 , a testing interface layer  415  and the top metal substrate  409 . To ensure failure take place along the interface structure, the testing interface layer  415  comprises a precrack region  431  and a self-assembly interlayer  433 . Bottom half  407  forms a metal substrate and includes a non-testing layer  413  provided as an adhesion promoter layer to prevent foul debonding along the bottom substrate  407 . Epoxy adhesive  421  is positioned between the non-testing layer  413  and the testing interface layer  415 . A non-limiting example of an epoxy adhesive material is Henkel Loctite Hysol FP4526, which is an epoxy underfill (epoxy adhesive) material. The test sample was mounted on a MTS 858 Universal Testing Machine equipped with a 25 kN±5N axial force load cell. The applied load was controlled by setting the crosshead velocity to 30 μm/min in tension. The fracture load was recorded when the crack was stably propagating. 
         [0040]    The load testing was performed for a control sample and for samples treated with sulphur-containing molecules. During testing of a control (untreated) sample, testing interface layer  415  is monolithic, except for separation from the epoxy adhesive  421 . During testing of a treated sample, testing interface layer  415  comprises a precrack region  431  without sulphur-containing molecules treatment and a treated region, the self-assembly interlayer  433 . 
         [0041]      FIG. 5  is a graph depicting demonstrates the typical load-displacement curves obtained from tapered double cantilever tests for a sample having the nanometer-sized structures and a control (untreated) sample, using the test configuration of  FIG. 4 . This graph demonstrates the typical load-displacement curves obtained from tapered double cantilever tests for a sample having the nanometer-sized structures and a control (untreated) sample. The test produced results in which the critical load for stable crack propagation for the control and the samples treated with sulphur-containing molecules as 39.2 N±3.3 N and 224.6 N±23.6 N respectively. 
         [0042]      FIG. 6  is a graph showing an evaluation of interfacial adhesion improvement of a copper-epoxy interface under the surface treatment described in this disclosure. The figure compares the interfacial adhesion energies (Jm −2 ) of a treated sample under this disclosure with a control (untreated) sample. The interfacial adhesion energies increase from 4.8±0.8 Jm −2  with the control sample to 159±33 Jm −2  with the sample treated with sulphur-containing molecules. 
         [0043]      FIG. 7  is a SEM morphological image of an epoxy surface  705  after a fracture test of a sample treated with the current disclosure. Depicted are an adhesive failure region  712  and a cohesive failure region  714 . The upper view shows the fractured surface of epoxy with thiol treated samples after the tapered double cantilever test. The photos illustrate fractures occur in the sample with both adhesive and cohesive failure. The SEM indicates crack propagation with a large amount of energy dissipated. 
         [0044]    Examples in implementing the nanometer-sized interlayer in between metal substrates and polymer coatings are given a follows: 
       Example 1 
     (1) Surface Treatment of Metal Substrate 
       [0045]    5 g of 4-aminobenzenethiol is dissolved in ethanol and diluted to 5 mM solution for metal substrate treatment. Copper foil samples in the dimensions of 10 mm×70 mm×0.2 mm are used as substrates. Formation of the nanometer-sized structures on metal substrates is achieved by immersing the substrates into the 4-aminobenzenethiol solution at room temperature with stirring overnight. Excessive thiol molecules are eliminated from the substrates by rinsing with sufficient ethanol and blown dry in nitrogen. 
         [0046]    (2) Application and Bonding of Polymer Coating 
         [0047]    10 g of epoxy adhesive (Henkel Loctite Hysol FP4526) is dispensed onto the copper substrates after substrate treatment. The polymer underfill is bonded with the substrates by thermal curing at 80° C. for six hours. 
       Example 2 
       [0048]    The nanometer sized-structures in between copper-epoxy are prepared in the same manner as in example 1, except that the surface modification agent is not 4-aminobenzenethiol but p-tolylmethanethiol. 10 g of p-tolylmethanethiol are dissolved in acetone. The thiols are adsorbed onto copper substrates at room temperature with stirring for two days. The bonding process of the polymer coating is the same as that in Example 1. 
       Example 3 
       [0049]    The nanometer-size structures in between copper-epoxy are prepared in the same manner as in example 1, except that the surface modification agent is not 4-aminobenzenethiol (ATP) but 11-aminoundecane-1-thiol hydrochloride. 2 g of 11-aminoundecane-1-thiol hydrochloride are dissolved in water. The thiols are adsorbed onto copper substrates at room temperature with stirring for two hours. The bonding process of the polymer coating is the same as that in Example 1. 
         [0050]    Example Materials 
         [0051]    Other preferred sulphur-containing molecules as examples of materials adopted in this disclosure are given in the following list:
   3-methylbenzenethiol,   3-aminobenzenethiol,   m-tolylmethanethiol,   4-amino-2-methylbenzenethiol,   4-methylbenzenethiol,   4-tert-butylbenzenethiol,   3-methyl-4-(p-tolyldisulfanyl)aniline,   4-(12-aminododecyl)-3-methylbenzenethiol,   4-((4-amino-2-methylphenyl)disulfanyl)benzoic acid,   4-amino poly(3-methylbenzl)thiol   4-tert-butylbenzenethiol   
 
       CONCLUSION 
       [0063]    It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the subject matter, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.