Abstract:
An apparatus, method, and system, the apparatus and system including a flexible microsystems enabled microelectronic device package including a microelectronic device positioned on a substrate; an encapsulation layer encapsulating the microelectronic device and the substrate; a protective layer positioned around the encapsulating layer; and a reinforcing layer coupled to the protective layer, wherein the substrate, encapsulation layer, protective layer and reinforcing layer form a flexible and optically transparent package around the microelectronic device. The method including encapsulating a microelectronic device positioned on a substrate within an encapsulation layer; sealing the encapsulated microelectronic device within a protective layer; and coupling the protective layer to a reinforcing layer, wherein the substrate, encapsulation layer, protective layer and reinforcing layer form a flexible and optically transparent package around the microelectronic device.

Description:
GOVERNMENT RIGHTS 
       [0001]    This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The U.S. Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to flexible packaging for electronic devices, in particular, flexible packaging for microelectronic devices such as photovoltaic cells. Other embodiments are also described and claimed. 
       BACKGROUND 
       [0003]    Photovoltaic solar cells have the potential to provide power well beyond the needs of the power grid and other fixed facility power needs. There is a need for power in remote locations where it is not feasible to transport batteries or fuel for generators such as in space or in remote terrestrial areas where there is not a power grid and there is not access to fuel. 
         [0004]    For photovoltaic solar cells to be useful for many of these remote power applications requires performance characteristics that are not provided by the rigid rectangular modules used for residential, commercial, and utility scale solar power installations. The performance characteristics required for remote power applications include low mass per area or (more specifically) a high power output per unit of mass, high efficiency, the ability to tightly pack or roll the photovoltaic module into a small volume for enhanced portability (provided by creating a solar module that is highly flexible), and ruggedness to rough treatment that may occur during transport. 
         [0005]    One method to provide a system with these characteristics is to reduce the size of solar cells to allow unique behaviors that are enhanced with very small cells. In this aspect, small and thin photovoltaic cells have been developed. These cells can be formed and then assembled by various means onto a receiving substrate providing electrical interconnects. The receiving substrate with the cells can then be packaged to provide a module with the desirable characteristics for providing remote power. 
       SUMMARY 
       [0006]    An apparatus, system and method for forming a flexible packaging for electronic devices, for example, microelectronic devices such as photovoltaic solar cells. The packaging is designed to completely encapsulate the electronic devices and provide mechanical robustness, moisture resistance and a high degree of flexibility to the assembly of electronic devices. 
         [0007]    In one embodiment, the apparatus includes a flexible microsystems enabled microelectronic device package including a microelectronic device positioned on a substrate. The apparatus further includes an encapsulation layer encapsulating the microelectronic device and the substrate. A protective layer may be positioned around the encapsulating layer and a reinforcing layer may be coupled to the protective layer. The substrate, encapsulation layer, protective layer and reinforcing layer may form a flexible and optically transparent package around the microelectronic device. 
         [0008]    In one embodiment, the method may include encapsulating a microelectronic device positioned on a substrate within an encapsulation layer. The encapsulated microelectronic device may then be sealed within a protective layer and coupled to a reinforcing layer. The substrate, encapsulation layer, protective layer and reinforcing layer may form a flexible and optically transparent package around the microelectronic device. 
         [0009]    In one embodiment, the system may include a microelectronic device module comprising a plurality of photovoltaic cells electrically coupled to a substrate. An optically transparent and moisture resistant encapsulation module may encapsulate the plurality of photovoltaic cells and the substrate. A reinforcing layer may be coupled to the encapsulation module. The reinforcing layer and the encapsulation module may form a flexible package around the plurality of photovoltaic cells. 
         [0010]    The above summary does not include an exhaustive list of all aspects of the present invention. It is contemplated that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the Detailed Description below and particularly pointed out in the claims filed with the application. Such combinations have particular advantages not specifically recited in the above summary. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings: 
           [0012]      FIG. 1  schematically illustrates a cross-sectional side view of one embodiment of a plurality of microelectronic devices. 
           [0013]      FIG. 2  schematically illustrates a cross-sectional side view of the microelectronic devices of  FIG. 1  assembled on a flexible substrate layer. 
           [0014]      FIG. 3  schematically illustrates a cross-sectional side view of an encapsulation layer formed over the assembly of  FIG. 2 . 
           [0015]      FIG. 4  schematically illustrates a cross-sectional side view of a protective layer formed over the assembly of  FIG. 3 . 
           [0016]      FIG. 5  schematically illustrates a cross-sectional side view of a perimeter protective layer formed around the assembly of  FIG. 4 . 
           [0017]      FIG. 6  schematically illustrates a cross-sectional side view of an adhesive layer formed on the assembly of  FIG. 5 . 
           [0018]      FIG. 7  schematically illustrates a cross-sectional side view of a further protective layer formed on the adhesive layer of the assembly of  FIG. 6 . 
           [0019]      FIG. 8  schematically illustrates a cross-sectional side view of a further adhesive layer formed on the protective layer of the assembly of  FIG. 7 . 
           [0020]      FIG. 9  schematically illustrates a cross-sectional side view of a flexible reinforcing layer connected to the adhesive layer of the assembly of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 1  schematically illustrates a cross-sectional side view of one embodiment of a plurality of microelectronic devices. Representatively, in one embodiment, the microelectronic devices  102 A- 102 F may be microsystems enabled photovoltaic (MEPV) cells. It should be understood that the terms “photovoltaic solar cell”, “photovoltaic cell”, “solar cell” and “cell” may be used interchangeably herein to refer to any of microelectronic devices  102 A- 102 F. In addition, it should be understood that although microelectronic devices  102 A- 102 F are described as solar cells herein, they may be any type of microscale component or macroscale component that could benefit from any of the flexible packaging embodiments disclosed herein. Representatively, microelectronic devices  102 A- 102 F could be light emitting diode devices, integrated circuit devices, or other semiconductor devices or the like. In addition, the term “flexible” as used herein should be understood as referring to the ability of any package, module, assembly, layer or material described herein of being bent and returning to its original non-bent configuration without breaking. For example, the package, module, assembly, layer or material described herein may be considered “flexible” where it has a bend radius of from about 0.75 mm to about 1 cm, for example, from about 2 mm to about 8 mm, or from about 3 mm to about 5 mm and can be returned to a non-bent configuration relatively easily. 
         [0022]    Microelectronic devices  102 A- 102 F may be, in some embodiments, as small as 10 micrometers across and 1 micrometer thick to 100s of micrometers across and 40-50 micrometers thick devices which may be fabricated on a wafer according to any standard microprocessing techniques. Once fabricated, microelectronic devices  102 A- 102 F may be separated from the wafer by, for example, a chemical or mechanical separating technique (e.g. application of an HF solution which chemically separates the devices from the wafer). For example, the devices may be individually detached from the wafer by, for example, an etching process using a hydrofluoric acid (HF) solution to undercut the cells. These “free floating” cells may then be assembled into sheets by attracting the individual cells to a desired position on a substrate using self-assembly techniques. 
         [0023]      FIG. 2  schematically illustrates a cross-sectional side view of the microelectronic devices of  FIG. 1  assembled on a flexible substrate. To form the flexible packaging disclosed herein, microelectronic devices  102 A- 102 F are connected to a substrate  204  to form microelectronic device module  200 . Substrate  204  may be, in some embodiments, a flexible substrate having circuitry or wiring formed therein. In such embodiments, metal interconnections may be formed between substrate  204  and microelectronic device  102 A- 102 F such that microelectronic devices  102 A- 102 F may be electrically connected to substrate  204  and other assemblies within which they may be integrated (e.g. a concentrated photovoltaic module). Substrate  204  may be made of any material capable of forming a flexible substrate. Representative materials may include plastic polymeric materials, including, but not limited to polyimide, polyethersulfone, polyether ether ketone (PEEK) or a transparent conductive polyester film. 
         [0024]    Microelectronic devices  102 A- 102 F may be bonded to substrate  204 . In some embodiments, microelectronic devices  102 A- 102 F are bonded to substrate  204  with an adhesive layer  202 . Adhesive layer  202  may, in some embodiments, be made of an adhesive material such that bonding is achieved by adhering microelectronic devices  102 A- 102 F to substrate  204 . A representative adhesive material may be a high temperature adhesive such as cyanate ester. In such embodiments, the cyanate ester adhesive is applied to substrate  204  followed by placement of microelectronic devices  102 A- 102 F on top of the adhesive. Once in position, the assembly is heated to a high temperature to cure the adhesive. In other embodiments, adhesive layer  202  may be formed by any type of bonding material, for example, solder bumps which can be deposited on substrate  204  at locations where a connection to microelectronic devices  102 A- 102 F is desired and then heated to bond devices  102 A- 102 F to substrate  204 . Alternatively, adhesive layer  202  may be made of an epoxy, bismalimide, or bismalimide-triazine material. In any case, it is important that any material used for adhesive layer  202  be a material which is compatible with microelectronic devices  102 A- 102 F and any electrical connections (e.g. metal interconnections or wiring) formed between microelectronic devices  102 A- 102 F and substrate  204 . It is also important that a material for adhesive layer  202  not substantially impact or reduce a flexibility of the packaging. 
         [0025]      FIG. 3  schematically illustrates a cross-sectional side view of an encapsulation layer formed over the assembly of  FIG. 2 . Once microelectronic device module  200  is formed, it is encapsulated within encapsulation layer  302  to form an encapsulated or encapsulation module  300 . In some embodiments, for example where microelectronic devices  102 A- 102 F are PV cells, encapsulation layer  302  may be optically transparent such that light waves can be transmitted to the PV cells through encapsulation layer  302 . Representatively, in one embodiment, encapsulation layer  302  is any optically transparent material, which is also flexible so that it does not significantly impact a flexibility of the packaging. In some embodiments, encapsulation layer  302  may be made of an elastomeric material capable of accepting large strain. Representative materials may include, but are not limited to, silicone materials such as polydimethylsiloxane (PDMS) as well as other materials such as ethylene vinyl acetate (EVA), polyurethane, and polyolefin. Other suitable materials, depending upon the type of devices encapsulated within encapsulation layer  302 , may include fire retardant materials, fire retardant treated materials and waterproof materials, including but not limited to, polyesters, nylon, acrylic and other commercial brands such as Marko®, Marlan® and Nomex®. 
         [0026]    Depending upon the material selected for encapsulation layer  302 , encapsulation layer  302  may be formed by a spin coating, doctor blading or a lamination technique. For example, in the case of a silicone encapsulation layer, encapsulation layer  302  may be formed by spin coating the material over microelectronic devices  102 A- 102 F such that it covers all exposed surfaces of microelectronic devices  102 A- 102 F and allowing it to cure. Alternatively, encapsulation layer  302  may be formed by a film of material which can be thermally laminated around microelectronic devices  102 A- 102 F. In some embodiments, encapsulation layer  302  may have a thickness of less than 60 micrometers, for example, 50 micrometers or less, or from about 25 micrometers to about 50 micrometers. In addition to providing a protective transparent layer through which light can be transmitted to microelectronic devices  102 A- 102 F, encapsulation layer  302  may also facilitate bonding of devices  102 A- 102 F to substrate  204  since it can encapsulate each of devices  102 A- 102 F and any exposed surfaces of substrate  204 . 
         [0027]      FIG. 4  schematically illustrates a cross-sectional side view of a protective layer formed over the assembly of  FIG. 3 . Once encapsulation layer  302  is formed, the resulting encapsulated module  300  is covered by a protective layer  402  according to the process shown in  FIG. 4-FIG .  7 . Representatively, in one embodiment, protective layer  402  is applied over an exposed top face  404  (side covering devices  102 A- 102 F) of encapsulation layer  302  and bonded to encapsulation layer  302 . For example, in one embodiment, protective layer  402  may be formed by surface treating the protective layer  402  and applying it to encapsulation layer  302  before encapsulation layer  302  is cured, encapsulation layer  302  is then cured such that the two layers bond together. In other embodiments, protective layer  402  is made of a material that can be spin coated onto protective layer  402  and bonded thereto prior to or after curing of encapsulation layer  302 . Suitable materials for protective layer may include any substantially flexible materials capable of forming a moisture resistant seal around encapsulation module  300 . In addition, suitable materials may be any material, particularly in cases where devices  102 A- 102 F are PV cells, which is optically transparent and can provide mechanical protection to devices  102 A- 102 F. Representative materials may include, but are not limited to, polychlorotrifluoroethylene (PCTFE), polytetrafluoroethylene (PTFE), ethylene chlorotrifluoroethylene (ECTFE), ethylene tetrafluoroethylene (ETFE) or polyvinylidene difluoride (PVDF). In some embodiments, protective layer  402  may have a thickness of less than 60 micrometers, for example, 50 micrometers or less, or from about 25 micrometers to about 50 micrometers. 
         [0028]      FIG. 5  schematically illustrates a cross-sectional side view of a perimeter protective layer formed around the assembly of  FIG. 4 . Perimeter protective layer  502  may be formed around a perimeter of encapsulation module  300 , which remains exposed after application of protective layer  402 . In some embodiments, perimeter protective layer  502  may be formed from a film which has a cut out center dimensioned to fit around encapsulation module  300 . In this aspect, once the film opening is formed, perimeter protective layer  502  may be positioned around encapsulation module  300  and sealed against protective layer  402  by, for example, an ultrasonic welding or thermal welding process. In other embodiments, perimeter protective layer  502  may be a sealant tape or sealant bead applied around the exposed perimeter of encapsulation module  300  and sealed to protective layer  402 . 
         [0029]    Perimeter protective layer  502  may be made of the same material as protective layer  402 . Representatively, perimeter protective layer  502  may be made of any material capable of forming a moisture resistant seal around encapsulation module  300 . Representative materials may include, but are not limited to, polychlorotrifluoroethylene (PCTFE), polytetrafluoroethylene (PTFE), ethylene chlorotrifluoroethylene (ECTFE), ethylene tetrafluoroethylene (ETFE) or polyvinyl idene difluoride (PVDF). 
         [0030]      FIG. 6  schematically illustrates a cross-sectional side view of a further adhesive layer formed on the assembly of  FIG. 5 . Adhesive layer  602  may be applied to substrate  204  to facilitate attachment of a final protective layer  702  along a bottom side  604  of encapsulation module  300  and a bottom side  606  of perimeter protective layer  502 , as shown in  FIG. 7 . In this aspect, although adhesive layer  602  is shown formed along the bottom side  604  of substrate  204 , it may instead or additionally, be formed along a top side  704  of the final protective layer  702  as shown in  FIG. 7 . Adhesive layer  602  may be formed by any material capable of bonding substrate  204  to another protective layer  702 . In preferred embodiments, the material for adhesive layer  602  is an elastomeric material having adhesive properties. Representatively, in some embodiments, adhesive layer  602  is an adhesive material such as thermoplastic polyurethane (TPU). In this embodiment, adhesive layer  602  is formed by a solid pellet resin which is dispersed in a solvent and brush, spray, doctor blade or equivalent applied to substrate  204  or a film that can bond one material to another, such as by a thermal lamination process. In still further embodiments, adhesive layer  602  may be made of the same material as adhesive layer  202 . In any case, it is important that any material used for adhesive layer  602  be a material which is compatible with microelectronic devices  102 A- 102 F and any electrical connections (e.g. metal interconnections or wiring) formed between microelectronic devices  102 A- 102 F and substrate  204 . It is also important that a material for adhesive layer  602  be substantially flexible and/or elastomeric such that it does not significantly impact or reduce a flexibility of the packaging. In some embodiments, adhesive layer  602  may have a thickness of less than 40 micrometers, for example, 30 micrometers or less, or from about 10 micrometers to about 25 micrometers. 
         [0031]    In addition to bonding protective layer  702  to substrate  204  as shown in  FIG. 6 , protective layer  702  should be sealed to the bottom surface  606  of perimeter protective layer  502  as shown in  FIG. 7 . In one embodiment, protective layer  702  is substantially the same material as protective layer  402  and perimeter protective layer  502 . Protective layer  702  may be sealed to perimeter protective layer  502  using ultrasonic welding or thermal welding process to apply heat and pressure to melt and bond the layers together. It is noted, however, that it is important that adhesive layer  602  be confined to an area between protective layer  702  and substrate  204  and not extend into the seal line between protective layer  502  and protective layer  702  when protective layer  502  and protective layer  702  are bonded using a welding process. Rather, protective layer  502  and protective layer  702  are bonded together similar to protective layer  402  and protective layer  502 , using, for example, a thermal welding process or a sealant. Once protective layer  702  is sealed to protective layer  502 , and in turn, protective layer  402 , the layers  702 ,  502  and  402  in combination form a protective barrier which seals encapsulation module  300 . The protective barrier may provide a moisture barrier, diffusion barrier and mechanical barrier around the layers and devices therein. 
         [0032]    Although formation of the protective layer is shown in  FIG. 4-FIG .  7  as a multi-step process in which multiple layers are applied, it is further contemplated that the protective layer may be formed in any manner and using any material capable of completely sealing the encapsulation module  300  shown in  FIG. 3 . For example, the encapsulation module  300  may be spin coated or spray coated with a protective material, initially in a liquid form, which is capable of sealing all exposed surfaces of encapsulation module  300 . 
         [0033]      FIG. 8  schematically illustrates a cross-sectional side view of a further adhesive layer formed on the protective layer of the assembly of  FIG. 7  followed by application of a reinforcing layer in  FIG. 9 . Once the protective layers  702 ,  502  and  402  are sealed around encapsulation module  300 , a reinforcing layer  902  is bonded to protective layer  702 . In one embodiment, reinforcing layer  902  is bonded to protective layer  702  by applying a further adhesive layer  802  between reinforcing layer  902  and protective layer  702 . Representatively, in one embodiment, adhesive layer  802  is applied to one or both of protective layer  702  and reinforcing layer  902 , then the two layers are bonded using a thermal lamination process. In one embodiment, adhesive layer  802  is made of substantially the same material as adhesive layer  602  and applied to the associated protective layer in a similar manner. For example, in one embodiment, adhesive layer  802  is made of an elastomeric adhesive such as TPU. In this embodiment, adhesive layer  802  is formed by a solid pellet resin which is dispersed in a solvent and brush, spray, doctor blade or equivalent applied to protective layer  702  or a film that can bond one material to another, such as by a thermal lamination process. In any case, it is important that any material used for adhesive layer  802  be a material which is compatible with microelectronic devices  102 A- 102 F and any electrical connections (e.g. metal interconnections or wiring) formed through adhesive layer  802 . It is also important that a material for adhesive layer  802  be substantially flexible and/or elastomeric such that it does not significantly impact or reduce a flexibility of the packaging. In some embodiments, adhesive layer  802  may have a thickness of less than 40 micrometers, for example, 30 micrometers or less, or from about 10 micrometers to about 25 micrometers. 
         [0034]    Reinforcing layer  902  is coupled to a side of protective layer  702  opposite microelectronic devices  102 A- 102 F. Reinforcing layer  902  may be any type of material layer which provides a mechanical backer to the above discussed device assembly. Representatively, reinforcing layer  902  may be made of a flexible fabric material having a very high modulus and strength. For example, reinforcing layer  902  may be any material that is abrasion and penetration resistant and can reduce a mechanical stress on the rest of the package. For example, in one embodiment, reinforcing layer  902  may be made of a fiber reinforced material including, but not limited to, a Vectran®, polyester, aramid, twaron, Kevlar®, Spectra®, polyethylene, carbon fiber or a glass woven fabric. Other suitable materials may include fire retardant materials, fire retardant treated materials and waterproof materials, including but not limited to, polyesters, nylon, acrylic and other commercial brands such as Marko®, Marlan® and Nomex®. 
         [0035]    It is further contemplated, that wiring  904 , which provides an electrical connection between microelectronic devices  102 A- 102 F and any assembly within which it may be integrated, may further be provided. For example, wiring  904  may be connected to substrate  204  and extend through protective layer  702  and out the module assembly  900  through a region between protective layer  702  and reinforcing layer  902  as shown. Wiring  904  may, however, extend out of module assembly  900  through other layers or regions of module assembly  900 . Regardless of where wiring  904  exits module assembly  900 , it is important that wiring  904  also be sealed at any exit ports within and/or between layers so as not to allow moisture transmission to microelectronic devices  102 A- 102 F. 
         [0036]    It is further contemplated that in addition to module assembly  900  being a flexible package, it be relatively thin. For example, in some embodiments, an overall thickness of module assembly  900  may be 3 mm or less, for example 500 micrometers or less, for example, less than 400 micrometers, for example, 375 micrometers or less, more specifically, from about 100 micrometers to about 375 micrometers, or from about 200 micrometers to about 300 micrometers. 
         [0037]    While certain embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that the invention is not limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those of ordinary skill in the art. For example, although processes for packaging of microelectronic devices such as PV cells are described herein, it is contemplated that the devices need not be limited to such devices. Rather, electronic devices or components of any size which could benefit from a flexible, and in some cases, optically transparent packaging, are contemplated. For example, other types of devices that may be packaged within a flexible packaging using the techniques described herein may include, but are not limited to, DIACs, diodes (rectifier diode), gunn diodes, IMPATT diodes, laser diodes, light-emitting diodes (LED), photocells, PIN diodes, schottky diodes, tunnel diodes, VCSELs, VECSELs, zener diodes, bipolar transistors, darlington transistors, field-effect transistors, insulated-gate bipolar transistor (IGBT)s, silicon controlled rectifiers, thyristors, TRIACs, unijunction transistors, hall effect sensors (magnetic field sensor), integrated circuits (ICs), charge-coupled devices (CCD), microprocessor devices, random-access memory (RAM) devices, or read-only memory (ROM) devices. The description is thus to be regarded as illustrative instead of limiting. 
         [0038]    In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiments. It will be apparent however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. The particular embodiments described are not provided to limit the invention but to illustrate it. The scope of the invention is not to be determined by the specific examples provided above but only by the claims below. In other instances, well-known structures, devices, and operations have been shown in block diagram form or without detail in order to avoid obscuring the understanding of the description. Where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated in the figure to indicate corresponding or analogous elements, which may optionally have similar characteristics. 
         [0039]    It should also be appreciated that reference throughout this specification to “one embodiment”, “an embodiment”, “one or more embodiments”, or “different embodiments”, for example, means that a particular feature may be included in the practice of the invention. Similarly, it should be appreciated that in the description, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects may lie in less than all features of a single disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of the invention.