Patent Publication Number: US-2023155090-A1

Title: Stretchable electronic device and method of making the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a National Stage Application of International Patent Application No. PCT/US2021/031280 filed May 7, 2021, which claims benefit of priority to U.S. Provisional Patent Application No. 63/022,064 filed on May 8, 2020, the entire disclosure of each of which is hereby incorporated by reference. 
    
    
     STATEMENT OF GOVERNMENT RIGHTS 
     This invention was made with United States Government support under Grant No. 1751348, awarded by the National Science Foundation (NSF). The United States Government has certain rights in this invention. 
    
    
     BACKGROUND 
     The present invention is directed to flexible and stretchable electronic devices and in particular to methods of making flexible and stretchable electronic devices. 
     Wearable monitoring devices are utilized in a growing number of applications. The electronics associated with these monitoring devices must be flexible and stretchable to comply with and conform to the body of the wearer. Typical electronic devices fabricated on a printed circuit board are too rigid for this type of application. Many flexible circuits cannot be stretched, preventing them from conforming to the body of the wearer. It would therefore be desirable to fabricate flexible and stretchable electronic devices that can be utilized on wearable devices. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS.  1   a - 1   r    are a diagram illustrating the microfabrication steps implemented to fabricate a flexible and stretchable electronic device according to some embodiments; 
         FIG.  2   a    is a top view of electrical traces fabricated on a rigid substrate according to some embodiments; 
         FIG.  2   b    is a side view of the rigid substrate, detachment layer, sacrificial layer, and electrical trace layer according to some embodiments; 
         FIG.  2   c    is a top view of the rigid substrate that includes one or more surface mounted components (SMCs); 
         FIG.  2   d    is a side view of the rigid substrate shown in  FIG.  2   c    that includes at least one SMC; 
         FIG.  2   e    is a perspective view of a flexible and stretchable electronic device; 
         FIG.  2   f    is a side view of the flexible and stretchable electronic device according to some embodiments; 
         FIG.  2   g    is a perspective view of a transparent, flexible, and stretchable electronic device according to some embodiments; 
         FIG.  2   h    is a perspective view of flexible conductive traces in transparent, flexible, and stretchable electronic device according to some embodiments; 
         FIG.  3    is a perspective view of a flexible and stretchable electronic device being removed from a rigid substrate according to some embodiments; 
         FIG.  4   a    is a perspective view of a flexible and stretchable electronic device being worn on a finger and used in a reflective mode according to some embodiments; and 
         FIG.  4   b    is a perspective view of a flexible and stretchable electronic device being worn on a finger and used in a transmissive mode according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     According to some aspects, this disclosure is directed to electronic devices that are stretchable and flexible and to methods of making the same. In some embodiments, a microfabrication method is utilized to fabricate the flexible and stretchable electronic device, hereinafter referred to as the device. As used herein, the term “stretchable” means that the device is capable of being elongated along at least one axis, and preferably multiple axes. Further, as used herein, the term “flexible” means that the device is capable of bending along multiple axes, in contrast with typical flexible printed circuit boards (PCBs) which are capable of flexing along only a single axis. The method utilizes a substrate formed of a rigid material (e.g., silicon, glass, etc.) on which the device is temporarily disposed to fabricate conductive traces and contact pads on which surface mounted electrical components are mounted to be included in the device. In some embodiments, a release layer, e.g., polymethyl methacrylate (PMMA), is deposited on the substrate, followed by a sacrificial layer, e.g., polyimide (PI), which is then cured to provide a rigid carrier on which the remaining microfabrication steps can be carried out. Following fabrication of the electronic components on the temporary rigid substrate, the electronic components are coated with a flexible and stretchable polymeric material that serves as a flexible and stretchable substrate for the device. This is followed by detachment of the electronics and flexible and stretchable substrate from the temporary rigid substrate at the interface between the release layer and the sacrificial layer. The sacrificial layer is subsequently removed from the flexible and stretchable substrate by an etching process and another layer of flexible and stretchable polymer material is provided over the bottom of the device. The end product is an electronic device in a flexible and stretchable substrate that is well adapted for use as a wearable device. The method of fabricating the electronic device, in particular the temporary use of the rigid substrate, allows for the use of high-precision microfabrication techniques capable of providing the desired integration density while providing a flexible and stretchable substrate that is well-suited for applications such as health monitoring of a person wearing the device. The device has the ability to conform to curved surfaces and is therefore useful in a number of applications, including as a wearable device capable of conforming to the body of a medical patient or other user. 
       FIGS.  1   a  through  1   r    is a diagram illustrating the microfabrication steps of fabricating a stretchable and flexible electronic device. The method begins at the step shown in  FIG.  1   a    with a temporary rigid substrate  100  such as a silicon substrate or glass substrate. In the step shown in  FIG.  1   b   , a release layer  102  is deposited on the surface of the temporary rigid substrate  100 . In some embodiments, the release layer  102  is polymethylmethacrylate (PMMA). In some embodiments, the release layer  102  is deposited onto the surface of the temporary rigid substrate  100  by a spin coat process and then heated at a desired temperature for a length of time (e.g., 180° C. for two minutes). 
     In the step shown in  FIG.  1   c   , a sacrificial layer  104  is deposited on top of the release layer  102 . In some embodiments, the sacrificial layer  104  is polyimide (PI). In some embodiments, the sacrificial layer  104  is deposited onto the surface of the release layer  102  by spin coating and then prebaking at 180° C. and subsequently fully curing in a convection oven at 300° C. under a flow of nitrogen. 
     In some embodiments, the adhesion between the release layer  102  and the sacrificial layer  104  is controlled to ensure the bond is sufficient to be maintained during subsequent fabrication steps but can be detached from each other to separate the rigid substrate from the device. Curing the release layer  102  and the sacrificial layer  104  allows the temporary substrate to act as a rigid carrier through the rest of the processing steps. 
     In the step shown in  FIG.  1   d   , an initial metal layer  106  is deposited on top of the sacrificial layer  104  through a metallization process. In some embodiments, the initial metal layer  106  is deposited using electron beam evaporation. In some embodiments, the initial metal layer  106  is a gold (Au). In some embodiments, the initial metal layer  106  is utilized as a seed layer for subsequent electrodeposition techniques. In this embodiment, the seed layer may be relatively thin (e.g., approximately 400 nm thick). In some embodiments, plasma activation of the initial metal layer  106  is utilized to increase the surface adhesion of the initial metal layer  106  to the sacrificial layer  104 . 
     In the step shown in  FIG.  1   e   , photolithography techniques are utilized to pattern the initial metal layer  106 . In some embodiments, this includes deposition of a mask layer  108  on the initial metal layer  106 . 
     In the step shown in  FIG.  1   f   , a thicker metal layer  110  is electrodeposited using the initial metal layer  106  as a seed layer. 
     In the step shown in  FIG.  1   g   , the mask layer  108  is removed from the initial metal layer  106 . 
     In the step shown in  FIG.  1   h   , the initial metal layer  106  is removed (e.g., chemically etched) to expose metal contacts  112   a ,  112   b  and traces (not visible) in the desired pattern. In some embodiments, rather than a single layer of electrical traces and metal contacts  112   a ,  112   b , the process may be repeated to fabricate a plurality of layers of electrical traces and/or contacts, which conductive layer (including electrical traces and/or contacts) separated by a flexible and stretchable insulative layer. In some embodiments, conductive vias are fabricated in the insulative layer located between the one or more conductive layers to provide a multi-layer flexible and stretchable circuit. In some embodiments the flexible and stretchable insulative layer is the same as the flexible and stretchable substrate described in more detail with respect the steps shown in  FIGS.  1   m  through  1   q    and the deposition process may be essentially the same. In some embodiments, the flexible and stretchable insulative layer may also be patterned according to the desired via interconnects. In other embodiments, other material may be utilized between the layers of electrical traces and/or contacts but the material utilized should be both non-conductive (insulative) as well as flexible and stretchable. In some embodiments, one of the plurality of layers may include a conductive grounding layer. In some embodiments, the conductive grounding layer is fabricated utilizing a conductive polymer to provide the desired conductivity in combination with the desired flexibility and stretchability. At this stage in the process, the substrate is ready for mounting of surface mounted electrical components (SMCs) onto the metal contacts  112   a ,  112   b . Depending on the application, a plurality of different types of SMCs may be utilized, including optical devices, electro-optical devices, electro-mechanical devices, etc. For example, in one application the SMCs include one or more active optical devices (e.g., light emitting diodes, lasers, etc.) and one or more optical sensors for measuring backscattered light. 
     In some embodiments, mounting of SMCs utilizes a lithography and dip coating process to deposit the solder at the desired location on the metal contacts  112   a ,  112   b . In some embodiments, as shown in  FIG.  1   i   , lithography techniques are utilized to deposit a mask layer  114 . As shown in  FIG.  1   i   , the mask layer  114  exposes a small portion of the metal contacts  112   a ,  112   b.    
     In the step shown in  FIG.  1   j   , the substrate is dip coated in a molten solder bath, which results in solder layers  116  being deposited onto exposed portions of the metal contacts  112   a ,  112   b.    
     In the step shown in  FIG.  1   k   , the mask layer  114  is removed by a stripping process, leaving the solder layer  116  positioned on the metal contacts  112   a ,  112   b  as desired. 
     In the step shown in  FIG.  1   l   , a surface mounted device (SMC)  118  is placed onto the metal contacts  112   a ,  112   b  and the solder is heated causing it to reflow and allow self-assembly of the SMC  118  on the metal contacts  112   a ,  112   b . At this point in the microfabrication process, the device can be electrically and mechanically tested. In some embodiments, these steps may utilize automated microfabrication techniques. For example, pick and place machines may be utilized to place the SMCs  118  on the desired contact pads. In addition, optical registration techniques may be utilized to verify placement of metal contacts  112   a ,  112   b  and/or SMCs  118 . 
     In the steps shown in  FIGS.  1   m  through  1   q    and in  FIG.  3   , the temporary rigid substrate is removed and replaced with a soft, compliant substrate  120 . In some embodiments, the soft, compliant substrate  120  is clear, i.e., transparent, to allow optical signals to be transmitted to and from the SMCs  118 . In some embodiments, in the step shown in  FIG.  1   m   , a polymer resin is applied to the substrate in an over-molding step. In some embodiments the polymer resin is a transparent urethane rubber material, e.g., CLEAR FLEX™ manufactured by Smooth-On Inc. of Macungie, Pa., that is comprised of two component polymers mixed together in a predetermined ratio (e.g.,  1 A: 2 B). As shown in  FIG.  1   m   , over molding of the polymer resin results in the SMCs  118  as well as the other sacrificial layer  104  being covered by the polymer resin. As described above, in some embodiments the polymer resin is transparent. In some embodiments, the assembled components are treated with ozone for a length of time to increase the bonding strength between the components and the compliant substrate  120 . In some embodiments, the compliant substrate  120  is cured. In some embodiments, the polymer resin is flexible and stretchable. At this stage, the temporary rigid substrate—which includes the temporary rigid substrate  100 , release layer  102  and sacrificial layer  104 —is still affixed to the electronic components on a bottom side. The compliant substrate  120  is located opposite the temporary rigid substrate  100 , affixed to the top side (as discussed in more detail below in the next step, the substrate is flipped wherein the top side becomes the bottom side). 
     In the step shown in  FIG.  1   n   , the temporary rigid substrate  100  is separated from the electrical device (e.g., SMCs  118  and conductive traces) at the interface between the release layer  102  and the sacrificial layer  104 , wherein the sacrificial layer  104  remains affixed to the compliant substrate  120 . The process of separating the compliant substrate  120  from the temporary rigid substrate  100  is dry i.e., no solvent is necessary to make the separation. This separation relies on a differential interfacial adhesion between PMMA and PI materials. A first adhesion force between the PMMA material forming the release layer  102  and the silicon material forming the temporary rigid substrate  100  is stronger than a second adhesion force of the PI material forming the sacrificial layer  104  to the PMMA material forming the release layer  102 . When a removal force greater than the second adhesion force is applied to the compliant substrate  120  it detaches from the temporary rigid substrate  100  at the PMMA-PI interface. A single transfer step for the entire device/circuit is also scalable. 
     In the step shown in  FIG.  1   o   , the sacrificial layer  104  is removed from by a plasma etching process. As shown at in  FIG.  1   o   , the compliant substrate  120  is flipped upside down, wherein the plasma etching process removes the sacrificial layer  104  from the compliant substrate  120 . Following the step shown in  FIG.  1   o   , electrical contacts and traces are exposed on the (now) top side of the compliant substrate  120 . 
     In the step shown in  FIG.  1   p   , electrical contacts are established as a required and in the step shown in  FIG.  1   q   , a polymer layer  122  is deposited on top of the device to fully encapsulate the device. The final device is shown in  FIG.  1     r.    
     In this way, the method described above utilizes a rigid substrate to pattern and place electrical components in the density required utilizing microfabrication techniques. Following fabrication of the device the temporary rigid substrate is removed and the device is encapsulated in a stretchable silicone substrate. 
     Referring now to  FIGS.  2   a - 2   f   , the flexible and stretchable electronic device is illustrated at several different stages of manufacture as well as in operation.  FIG.  2   a    is a top view of electrical traces fabricated on a temporary rigid substrate according to some embodiments. In this view, a temporary rigid substrate  200  supports a plurality of conductive traces  202  as well as a plurality of electrical contacts  204 . As discussed above, utilization of a rigid substrate allows traditional microfabrication techniques to be utilized, allowing for density of conductive traces  202  and density of electrical contacts  204  not possible without microfabrication techniques. In the embodiment shown in  FIG.  2   a   , conductive traces  202  have a geometry designed to accommodate stretching and/or bending of the device. 
       FIG.  2   b    is a side view of the device shown in  FIG.  2   a   , in which a temporary rigid substrate  200  includes a rigid silicon substrate  205 , a release layer  208  (e.g., PMMA) and a sacrificial layer  210  (e.g., PI). As described above, the release layer  208  and the sacrificial layer  210  are cured to ensure that temporary rigid substrate  200  is sufficiently rigid to fabricate electrical traces and mount surface mounted components (SMCs) as shown in  FIG.  2   c   . In this example, conductive traces  202  and electrical contacts  204  have been deposited and fabricated on top of the temporary rigid substrate  200 . 
       FIG.  2   c    is a top view that illustrates a plurality of surface mounted components (SMCs)  234  that are mounted to the temporary rigid substrate  200  according to some embodiments. In this view, the temporary rigid substrate  200  provides support during the mounting and soldering of the plurality of SMCs  234  onto the electrical contacts  204  shown in  FIGS.  2   a  and  2   b   . In some embodiments, placement of the plurality of SMCs  234  may take advantage of pick-and-place commonly utilized with microfabrication techniques. 
       FIG.  2   d    is a side view of the device shown in  FIG.  2   c   , in which SMCs  234  are mounted on the electrical contacts  204 . At this time, the temporary rigid substrate  200  provides the rigidity necessary to mount the SMCs  234  onto the electrical contacts. In some embodiments, pick and place machines are utilized to mount the plurality of SMCs  234  onto the desired electrical contacts  204 . A benefit of utilizing a rigid substrate (i.e., the temporary rigid substrate  200 ) during this process is that it allows for standard microfabrication techniques—such as pick and place machines—to be utilized to fabricate the electronic device. As described above with respect to  FIGS.  1   a - 1   r   , following placement of the surface mounted components on the electrical contacts  204 , the temporary rigid substrate  200  is removed and the remaining components are encased in a flexible and stretchable polymer layer as shown in  FIGS.  2   e    and  2   f.    
     With respect to  FIG.  2   e   , an embodiment is shown in which the flexible and stretchable electronic device  215  includes a plurality of surface mounted optical devices. In particular, in the embodiment shown in  FIG.  2   e   , the device includes an infrared (IR) LED  220 , a red-green-blue (RGB) LED  222 , an IR phototransistor  224 , and a visible light phototransistor  226 . Each of these components represents a type of surface mounted device (SMC) mounted onto the electrical contacts  204  shown in  FIG.  2   a   . In addition, the embodiment shown in  FIG.  2   e    illustrates a clear flexible and stretchable polymer substrate  228  surrounding the conductive traces  202  and SMR devices, including IR LED  220 , RGB LED  222 , IR phototransistor  224 , and visible light phototransistor  226 . As illustrated in  FIG.  2   g   , the geometry of the conductive traces  202  (e.g., undulating, meandering, sawtooth, and/or serpentine geometry) allows the conductive traces  202  to handle bending and/or stretching. Likewise, the flexible and stretchable polymer substrate  228  is capable of handling bending and/or stretching. In addition, in applications in which one or more of the SMCs are optical devices requiring to transmit and/or receive transmitted light, it is important that the flexible and stretchable polymer substrate  228  be transparent or nearly transparent to allow the transmission of light to and from the optical devices. 
       FIG.  2   f    is a side view of the device shown in  FIG.  2   e   . In the embodiment shown in  FIG.  2   f   , the entire device is encapsulated in the flexible and stretchable polymer substrate  228 . Encased within the flexible and stretchable polymer substrate  228  is a pair of electrical contacts  230   a ,  230   b , a pair of solder joints  232   a ,  232   b , and SMCs  234  (e.g., IR LED  220 , RGB LED  222 , IR phototransistor  224 , or visible light phototransistor  226 ). The geometry of the conductive traces  202  ensures that the stretchability and flexibility of the device does not prevent electrical conductivity between the electrical contacts  230   a ,  230   b , solder joints  232   a ,  232   b , and the SMCs  234 . 
       FIG.  2   g    is a perspective view illustrating the transparent and stretchable electrical device fabricated as shown in  FIGS.  2   a - 2   f    in operation. In particular, the embodiment shown in  FIG.  2   g    illustrates the use of optoelectronic devices, which can be utilized in a plurality of patient and health monitoring applications. 
     Although the embodiment shown in  FIGS.  2   a - 2   e    utilizes a plurality of optoelectronic devices, in other embodiments various other configurations of optoelectronic devices and/or other electrical devices may be microfabricated on the flexible and stretchable substrate. For those applications that do not require transmission of light, the stretchable substrate is not required to be transparent. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention is not limited to the disclosed embodiment(s), but that the invention will include all embodiments falling within the scope of the appended claims.