Abstract:
A method for coating for a substrate, comprising applying an underlayer of a self assembling monolayer well ordered array of long chain molecules on the substrate; and applying a top layer, over the underlayer, wherein the self-assembling monolayer well ordered array serves as a molecular template organizing formation of said top layer, comprising at least one of a thermally-resistant polymer layer over said self assembling monolayer selected from the group consisting of epoxies, and phosphorus-based polyimides; and a metal oxide, metal nitride, or a ceramic. The self assembling monolayer may be selectively applied to a portion of the substrate, leaving an uncoated region, and the top layer formed only over the areas of the substrate coated with the self-assembling monolayer, resulting in at least one region of the substrate which is not coated with the top layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation of U.S. patent application Ser. No. 11/622,964, filed Mar. 5, 2007, now U.S. Pat. No. 8,158,201, issued Apr. 17, 2012, which is a divisional of U.S. patent application Ser. No. 10/785,887, filed Feb. 23, 2004, now U.S. Pat. No. 7,282,254, issued Oct. 6, 2007, the entirety of which is expressly incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to surface coatings for electronic hardware and, more particularly, to a multi-layered surface coating comprising an upper ceramic layer and a lower, self-assembled monolayer (SAM) for protecting micro-electromechanical systems (MEMS). 
     BACKGROUND OF THE INVENTION 
     Recent developments in silicon micro-electromechanical systems (MEMS) have dramatically reduced length scales of these components, resulting in an increase in surface area-to-volume ratios of these structures. The new, small dimensions tend to cause the MEMS structures to adhere (stiction) to the substrate or adjacent structures in the presence of moisture. In addition, since the structures have surfaces in normal or sliding contact, friction and wear are also crucially affected. Therefore, surface modifications are necessary to improve anti-stiction and wear resistance of these silicon-based structures. 
     Self-assembled monolayer (SAM) coatings and thin vapor-deposited organic coatings have frequently been utilized to improve anti-stiction. Organic coatings, however, are not only prone to rapid wear over long-term use, they also significantly degrade in harsh environments or at elevated temperatures. 
     The present invention uses ceramic coatings for MEMS devices. Ceramics provide the protection necessary for MEMS surfaces, but they impose a fabricating challenge. One difficulty is that conventional ceramic processing cannot be easily implemented for surface coatings because it involves high processing temperatures. This is so even when sol-gel processing is used. 
     Many MEMS devices cannot tolerate the ceramic processing temperature range. Spalling will take place from the surface of the MEMS devices due to thermo-mechanical stresses, thermal shock, and high temperature fluctuation. The protection layer will then become ineffective. 
     In addition, some IC and MEMS devices require hermetic packaging for long-term reliability. Hermetic packaging, in general, utilizes ceramic materials that are deposited upon the surface in an inert atmosphere or vacuum. 
     Ceramic hermetic packages effectively protect the device from oxygen and water permeation, but manufacturing and material costs are high. The packaging content can be as high as 75% to 95% of the overall cost of a MEMS device. 
     Therefore, it would be a significant advance in MEMS technology to be able to use conventional packaging processes with ceramic coatings that may provide hermetic packages without losing any benefits of hermetic encapsulation. 
     The present invention provides a bi- or multi-layer coating for MEMS surfaces. The multi-layer consists of at least one layer comprising a top, hard coating (e.g., ceramics such as BN, TiN, Al 2 O 3 , and ZrO 2 ), and an underlying, compliant SAM coating (organic materials) for silicon and organic materials. This multi-layer coating provides both environmental and hermetical protection without employing expensive packaging materials and processes. 
     The inventive ceramic coatings are fabricated using a low-temperature synthesis having a minimum of shrinkage during processing. The application can use a solution precursor method. In fact, inorganic films can be grown as well upon organic substances, as is routinely observed in nature (this is known as “biomimetic” processing). The self-assembled monolayer (SAM) can be used for this organic template layer and for the subsequent ceramic growth. This fabrication is unique because ceramic film, which usually does not grow well upon bare silicon surfaces, can do so on a SAM-coated surface. The coating may consist of a single bi-layer SAM/ceramic or, in alternate embodiments, multiple layers may be used. 
     The thermal stability of the underlying SAM coating will also be improved by the top ceramic coating, which will prevent SAM oxidation at elevated temperatures. The ceramic layer will also offer ideal thermal protection. Furthermore, the SAM coating will act as a buffer layer for the top ceramic layer to prevent it from cracking or debonding during processing, as well as during device operation. It has been shown through computer simulation that the stress in the ceramic film is in fact relieved when the thickness of a buffer layer is increased. 
     The multi-layer approach maximizes the strengths of both the organic and inorganic coating functions, while self-compensating for the inherent weaknesses of each. The resultant coating provides a synergy by functioning better than the two layers alone. 
     Another advantage of the SAM coating resides in the ceramic/SAM bi-layer(s). This layer can be selectively deposited only on the surfaces to be protected, thus leaving the electrical components intact for the subsequent interconnections. An additional SAM coating on metallic bonding pads is grown for this purpose using a solution route prior to the deposition of ceramic coatings. The second SAM coatings are inert, keeping the ceramic layer from growing on the metal surface. In this way, the ceramic/SAM layers will be deposited selectively on the silicon surfaces. More importantly, the protective coating with hard top overlay will improve a variety of MEMS applications, including bio-MEMS, which are used in medical and biological applications. 
     As aforementioned, the ceramic/SAM coatings applied to organic (plastic) packaging components will protect the device from adverse environments, which normally require ceramic hermetic packaging. It will also provide enhanced thermal stability for the organic packages, which are normally and rapidly degraded at elevated temperatures in air and harsh environments. Owing to the fact that the coating process only adds additional solution dipping processes (i.e., steps), the manufacturing costs will be much cheaper than the normally used hermetic packaging processes, which typically require expensive vacuum equipment and raw materials. 
     In the present inventive coating processes, the entire package is simply dipped into a precursor solution, dried, and pyrolyzed for each SAM and ceramic coating. The processing temperature is low (&lt;300° C.). By using a selective coating process, only the organic surfaces will be coated with a SAM and ceramic layer. This will not cover metal surfaces. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a bi- or multi-layer coating for MEMS surfaces is disclosed. The multi-layer consists of a top, hard coating (e.g., ceramics such as BN, TiN, Al 2 O 3 , and ZrO 2 ), and an underlying, compliant, self-assembly monolayer (SAM) coating for silicon and organic materials. These bi- or multi-layer coatings provide both environmental and hermetical protection without employing expensive packaging materials and processes. The MEMS packages are simply dipped into a precursor solution, dried, and pyrolyzed for each SAM and ceramic coating layer. The processing temperature is low, typically &lt;300° C. By using a selective coating process, only the organic surfaces will be coated with a SAM and ceramic layer, leaving metallic areas free from coating. 
     It is an object of the present invention to provide improved coatings for electronic hardware. 
     It is another object of this invention to provide a bi- or multi-layer coating for MEMS surfaces that provides both environmental and hermetical protection without employing expensive packaging materials and processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which: 
         FIG. 1  illustrates a schematic view of a bi-layer coating of the invention; 
         FIG. 2  depicts a graphic view of a stress plot for the cooling bi-layer coating shown in  FIG. 1 ; 
         FIG. 3  illustrates a schematic flow chart for a method of applying SAM and ceramic bi-layers on a silicon minor using a solution technique; 
         FIG. 4  depicts three separate steps depicting phosphonate- or zirconium-terminated SAM reactions used in subsequent YSZ growth; 
         FIGS. 5A-5D  are sectional schematic diagrams of four multi-layer configurations; 
         FIG. 6  shows a chemical process for growing an azide-terminated SAM; and 
         FIGS. 7A and 7B  show tri-layer coatings of organic-ceramic-organic layers and ceramic-organic-ceramic constructions, respectively. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Generally speaking, a bi-layer coating is deposited upon a substrate. The bi-layer comprises a lower layer of a SAM coating, which is overlaid with a hard ceramic coating. The hard ceramic coating can comprise materials such as: BN, TiN, Al 2 O 3 , and ZrO 2 . The underlying compliant SAM coating can comprise substances containing long chain molecules that chemisorb onto a substrate. This bi-layer provides both environmental and hermetical protection to the electronic hardware without employing expensive packaging materials and processes. 
     Now referring to  FIG. 1 , a bi-layer coating  10  is shown deposited upon a substrate  12 . The bi-layer comprises a lower layer of a SAM coating  14 , which is overlaid with a hard, ceramic coating  16 . The hard top coating can be an oxide, a nitride, or an insulating polymer. The oxide and nitride coating can be selected from a group of materials consisting of yttrium stabilized zirconia (YSZ), ZrO.sub.2, Al 2 O 3 , SiO 2 , AlMgO, NiFeO, indium oxide, tin oxide, indium tin oxide, YBaCuO, BiSrCaCuO, LaSrMnO, NIO, CuO, FeAlO, BN, TiN, AlN, FeAlN, and combinations thereof. The underlying, compliant SAM coating can comprise substances containing long chain molecules that chemisorb onto a substrate. It will be recognized by those of skill in the materials coating arts that the thickness of each layer may be controlled (i.e., tuned) to meet a particular operating circumstance or environment. 
     Now referring to  FIG. 2 , a bi-layer stress simulation plot is illustrated, as the coating is cooled from 200° C., to 20° C., using a low temperature synthesis, where the SAM coating  14  is used as an organic template for the ceramic layer  16 , as shown in  FIG. 1 . Thermal stability of the underlying SAM coating  14  is improved by the top ceramic layer  16 . The ceramic layer  16  will prevent oxidation of the SAM layer  14 , and the SAM layer  14  will act as a buffer layer for the ceramic layer  16  to prevent it from cracking or debonding. 
     Referring to  FIG. 3 , a schematic flow chart is depicted of the method of applying SAM and ceramic bi-layers on a silicon mirror, using a solution technique. The entire packages are simply dipped into a precursor solution, dried, and pyrolyzed for each SAM and ceramic coating. The thickness of the coating may be varied or tuned to achieve the desired properties by repeating one or more of the steps shown in  FIG. 3 . Alternately, dipping time may be adjusted to obtain a desired thickness for a particular layer. The processing temperature is low (&lt;300° C.). It will be observed that none of the metal surfaces are coated since the temporary, inert “SAM2” surfaces protect them from coating. 
     Synthesis of Bi-Layer Coatings 
     Self-assembled monolayers (SAMs) are well-ordered arrays of long-chain molecules that chemisorb onto a substrate one molecule thick as shown in  FIG. 4 , Step a (A. R. Bishop, R. G. Nuzzo, “Self-assembled monolayers: Recent developments and applications,” Curr. Opin. Col. Int. Sci. 1996, 1, 127-136; A. Ulman, “Formation and Structure of Self-Assembled Monolayers,” Chem. Rev. 1996, 96, 1533-1554). 
     One requirement of SAM formation is a chemical interaction between the substrate and surface-active head-group of the adsorbate. The monolayer is deposited by immersion of the substrate into a dilute, milli-molar solution of these molecules. Under ambient conditions, the molecules arrange themselves into densely packed monolayers usually within several minutes to hours. One of the most extensively studied SAM systems is alkyltricholorosilanes on glass. Under a dry atmosphere, the controlled hydrolysis and condensation of the Si—Cl or Si—O—R groups occurs, with surface hydroxyls leading to complete substrate coverage (Bishop, ibid.; Ulman, ibid.; T. E. Mallouk, H.-N. Kim, P. J. Ollivier, S. W. Keller, “Ultrathin Films Based on Layered Materials,” Compr. Supra. Chem. 1996, 7, 189-217; D. M. Sarno, D. Grosfeld, J. Snyder, B. Jiang, W. E. Jones, Jr., “Multi-Layer Metalloporphyrin Polymers Separated by Bidentate Lewis Base Derivatives on Glass Substrates,” Polymer Pre-prints 1998, 1101; D. M. Sarno, B. Jiang, D. Grosfeld, J. Afriyie, L. J. Matienzo, W. E. Jones, Jr., “Self-Assembled Molecular Architectures: New Strategies Involving Metal-Organic Co-Polymers,” Langmuir 2000, 16, 6191-6199; D. M. Sarno, L. J. Matienzo, W. E. Jones Jr., “Self-Assembly of Ruthenium Porphyrins into Monolayer and Multi-layer Architectures via Heterogeneous Coordination Chemistry,” Mat. Res. Soc. Proc. 2001, 6281; D. M. Sarno, L. J. Matienzo, W. E. Jones Jr., “X-Ray Photoelectron Spectroscopy as a Probe of Intermolecular Interactions in Porphyrin Polymer Thin Films,” Inorg. Chem. 2001, 40, 6308-6315; D. Hohnholz, A. G. MacDiamid, D. M. Sarno, W. E. Jones, Jr., “Uniform thin films of poly-3,4-ethylenedioxythiophene (PEDOT) prepared by in-situ deposition,” Chem. Commun. 2001, 2444-2445). 
     An alkyltricholorosilane or alkyltrimethoxysilane SAM is deposited ( FIG. 4 , Step a) on the native oxide layer of silicon wafers. The organic monolayer is then coated with an inorganic ceramic material, through a low-temperature aqueous solution method or prepolymer method. There is now extensive literature on the chemical choice of the SAM terminus (Bishop, ibid.; Ulman, ibid.; Mallouk, ibid., Sarno, Grosfeld, ibid.; Sarno, Jiang, ibid.; Sarno, Matienzo, ibid.; Sarno, Matienzo, Jones, ibid.; Hohnholz, ibid.; S. Yitzchail, T. J. Marks, “Chromophoric Self-Assembled Superlattices,” Acc. Chem. Res. 1996, 29, 197-202). 
     The terminus can be chosen prior to SAM deposition (via a suitable amphiphile), or subsequent to SAM deposition (terminus conversion). Therefore, there are a wide variety of possible pathways by which one can create a SAM-ceramic bi-layer coating. Several examples follow. 
     Yttrium-Stabilized Zirconia (YSZ) on a Phosphonate SAM 
     As previously discussed, YSZ is an ideal ceramic material for wafer coating. An extensively studied class of self-assembled multilayers is based on .alpha.-zirconium phosphonate, a previously well-known layered inorganic material. One attractive feature of this sequence of self-assembly reactions ( FIG. 4 , Step a) is that one could, in principle, use any one of the steps to initiate growth of YSZ: the phosphonate-(Step b) or the zirconium-terminated film (Step c) could be used as the SAM-YSZ interface. The former would perhaps be better in terms of allowing the correct spacing of zirconiums for the growing YSZ phase. 
     The YSZ film may be grown by sol-gel methods at temperatures below 300° C. The standard precursors are ziriconium sulfate, zirconium acetate (or zirconyl chloride) and yttrium nitrate hydrate. Another appealing aspect of this system is that SAM thickness may be controlled by carrying out additional steps b and c, resulting in multilayers, as shown in  FIG. 4 . The ellipsometry determined thickness scales linearly with the number of layers added, and hundreds of layers can be deposited. Alkyl chain length is another degree of freedom. 
     Both of these features would be expected to influence the stability and expansion properties of the SAM coating. It is also worth noting that while the solid particle of bulk phase .alpha.-zirconium phosphonate contains many defects, this ordered film—based on a 2D, layered inorganic material—may be more forgiving of substrate, SAM, and YSZ defects. 
     Nitride Ceramics on an Azide SAM 
     A second proposed bilayer combination begins with a bromide-terminated SAM ( FIG. 6 ). The surface is easily converted to an azide (L. M. Lander, W. J. Brittain, V. A. DePalma, S. R. Girolmo, “Friction and Wear of Surface-Immobilized C60 Monolayers,” Chem. Mater. 1995, 7, 1437-1439), which is used to anchor C.sub.60, In the present case, the azide surface should be ready to initiate nitride ceramic growth. For example, one can use the polymer precursor method described by Kho et al. for the deposition of smooth, crack-free boron nitride films (J.-G. Kho, K.-T. Moon, G. Nouet, P. Ruterana, D.-P. Kim, “Boron-rich boron nitride (BN) films prepared by a single spin-coating process of a polymeric precursor,” Thin Solid Films 2001, 389, 78-83). 
     The prepolymer is synthesized from ammonium sulfate and sodium tetrahydroborate, dissolved in tetrahydrofuran (THF), spin-coated and pyrolyzed at 900° C. The substrate can then be dip coated into this solution and gently heated (&lt;350° C.) in order to maintain the integrity of the SAM (R. Maboudian, “Surface Processes in MEMS Technology,” Surf. Sci. Reports 1998, 30, 209-268). The resultant quasi-crystalline or amorphous boron nitride films will still have the desired properties of thermal and electrical insulation. One could also study other nitride ceramics, such as those of titanium or silicon. 
     Thermally Insulating Polymer Coatings on Hydroxy SAMs 
     Another strategy is to replace the ceramic with a thermally resistant polymer, such an epoxy or a phosphorus-based polyimide. These materials are low cost, highly processable, and may be polymerized in-situ on suitably modified substrate surfaces (D. Hohnholz, ibid.). A hydroxy-terminated SAM, for instance, is a likely candidate for epoxy deposition. This type of monolayer may be synthesized by way of a methyl ester terminated alkyltrichlorosilane (CH 3 CO 2 CH 2 ) n SiCl 3 , n=8 to 18). The ester is reduced to an alcohol with LiAlH 4 (Ulman, ibid.). 
     Again, self-assembled multilayers may be grown prior to the polymer layer. The hydroxy-terminated SAM surface resembles glass, or the native oxide layer of a silicon wafer, allowing assembly of additional monolayers of the methyl ester alkyltrichlorosilane (Ulman, ibid.). An added benefit of this reaction is that each monolayer is connected by a highly stable 2D siloxane network. 
     Mixed SAMs for Area-Specific Bilayer Growth 
     Finally, it is worth noting that one can use basic SAM chemistry to deposit the bi-layers only on desired areas. The metallic bonding pads that connect the MEMS components can easily be protected by an inert alkanethiol SAM (Bishop, ibid.; Ulman, ibid.; Mallouk, ibid). This layer can be grown either before or after the alkyltricholorosilane SAM, since each is specific for metallic pad and Si substrate, respectively. The ceramic should preferentially grow on the alkyltrichlorosilane areas, where the correct SAM functionality exists, while the methyl caps of the alkanethiol SAM deter ceramic growth. The alkanethiol SAM of the metallic layer could easily be chemically or photochemically etched after the process, since it would not possess a ceramic protective coating. 
     The properties of the bi-layer coatings of this invention are compared with only SAM and ceramic coating, respectively, in Table 1. Note that bio stability is provided by using selected ceramic materials. The choice of appropriate ceramic materials would minimize film formation in devices used in bio-medical applications. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Property comparison among different coating schemes 
               
             
          
           
               
                   
                 Organic 
                   
                 Organic + 
               
               
                   
                 SAM 
                 Ceramic 
                 Top Ceramic  
               
               
                 Properties 
                 Coatings 
                 Coatings 
                 Coatings 
               
               
                   
               
             
          
           
               
                 Transparency 
                 3 
                 2.5 
                 2 
               
               
                 Protection from 
                 1 
                 3 
                 3 
               
               
                 moisture/ion permeation 
                   
                   
                   
               
               
                 Wear-resistance 
                 1 
                 3 
                 3 
               
               
                 Thermal stability 
                 1 
                 3 
                 3 
               
               
                 Thermal insulation 
                 1 
                 3 
                 2 
               
               
                 Stress-free film 
                 3 
                 1 
                 3 
               
               
                 Strain-tolerance during 
                 3 
                 1 
                 3 
               
               
                 thermal cycling Anti-stiction 
                 3 
                 2 
                 2 
               
               
                 Shelf-life (extended storage) 
                 1 
                 3 
                 3 
               
               
                 Ease of cleaning with 
                 1 
                 3 
                 3 
               
               
                 solvents and chemicals 
                   
                   
                   
               
               
                 Conformal coating 
                 3 
                 1.5 
                 3(?) 
               
               
                 Hermetic packaging 
                 Required 
                 Not Required 
                 Not required 
               
               
                 Bio Stability 
                 1 
                 3 
                 3 
               
               
                 (3: better, 2: good, 1: poor) 
               
               
                   
               
             
          
         
       
     
     Note that the data shown in Table 1 supports using a bi-layer coating methodology to improve properties such as protection from moisture and/or ion formation, wear resistance, thermal stability, mechanical stress reduction, anti-stiction, shelf life extension, ease of cleaning with solvents, conformal coatings, hermetic packaging, and biologically stable films that resist bio-film formation. 
     While a bi-layer configuration has been used extensively herein for purposes of disclosure, the bi-layer concept may be expanded into a generalized multi-layer approach. It will be recognized to those of skill in the art that the inventive approach of selectively forming a SAM layer and then pyrolizing a ceramic overlayer may be repeated any number of times to form multi-layer configurations. Four such configurations are shown in  FIGS. 5A-5C .  FIG. 5A  shows a configuration wherein substrate  12  is coated first with an inorganic layer  18  (typically a ceramic as described hereinabove). Next an organic layer  20  (i.e., a SAM) is coated followed by, in order, another inorganic layer  22  and a final SAM layer  24 . It will be noted that layers  22 ,  24  may be repeated n−1 times where n represents the total number of composite organic/inorganic layers formed on substrate  12 . 
       FIG. 5B  shows a second multi-layer configuration wherein substrate  20  is first coated with an organic layer  20  and which is then overcoated with an inorganic layer  18 . An additional organic layer  24  followed by an inorganic layer  22  is added. As in the configuration of  FIG. 5A , the final layer  22 ,  24  may be repeated n−1 times. 
       FIGS. 5C and 5D  show configurations similar to those of  FIGS. 5A and 5B , respectively. However, after the final layer have been added to the configurations of  FIGS. 5A and 5B , a final inorganic layer  26  is added to the configuration of  FIG. 5C  and a final organic layer  28  is added to the configuration of  FIG. 5D . 
     It will be recognized that many other multi-layer configuration may be formed from pairs of organic/inorganic materials. Consequently, the invention is not considered limited to those configurations chosen for purposes of disclosure. 
     Tri-layer coatings comprising either organic-ceramic-organic layers or ceramic-organic-ceramic layers have also be found useful. These tri-layer coatings provide all protective advantages described hereinabove.  FIGS. 7A and 7B  show schematic representations of a substrate  12  overlaid with organic  20 , ceramic  22 , and organic  24  layers, and ceramic  18 , organic  20 , and ceramic  22  layers, respectively. 
     Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.