STRETCHABLE ELECTRONIC DEVICE AND METHOD OF MAKING THE SAME

A method of manufacturing a stretchable electronic device includes the steps of:          a) depositing a release layer on a temporary rigid substrate;     b) depositing a sacrificial layer on the release layer;     c) fabricating conductive traces and electrical contacts on top of the sacrificial layer;     d) mounting surface mount components (SMCs) to the electrical contacts;     e) depositing a first stretchable polymer layer over the conductive traces and SMCs to form a stretchable substrate;     f) separating the rigid substrate and release layer from the sacrificial layer and the stretchable substrate at an interface between the release layer and the sacrificial layer;     g) removing the sacrificial layer from the stretchable substrate; and     h) depositing a second stretchable polymer layer on a surface of the stretchable substrate exposed by removing the sacrificial layer.

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.

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.1athrough1ris a diagram illustrating the microfabrication steps of fabricating a stretchable and flexible electronic device. The method begins at the step shown inFIG.1awith a temporary rigid substrate100such as a silicon substrate or glass substrate. In the step shown inFIG.1b, a release layer102is deposited on the surface of the temporary rigid substrate100. In some embodiments, the release layer102is polymethylmethacrylate (PMMA). In some embodiments, the release layer102is deposited onto the surface of the temporary rigid substrate100by 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 inFIG.1c, a sacrificial layer104is deposited on top of the release layer102. In some embodiments, the sacrificial layer104is polyimide (PI). In some embodiments, the sacrificial layer104is deposited onto the surface of the release layer102by 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 layer102and the sacrificial layer104is 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 layer102and the sacrificial layer104allows the temporary substrate to act as a rigid carrier through the rest of the processing steps.

In the step shown inFIG.1d, an initial metal layer106is deposited on top of the sacrificial layer104through a metallization process. In some embodiments, the initial metal layer106is deposited using electron beam evaporation. In some embodiments, the initial metal layer106is a gold (Au). In some embodiments, the initial metal layer106is 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 layer106is utilized to increase the surface adhesion of the initial metal layer106to the sacrificial layer104.

In the step shown inFIG.1e, photolithography techniques are utilized to pattern the initial metal layer106. In some embodiments, this includes deposition of a mask layer108on the initial metal layer106.

In the step shown inFIG.1f, a thicker metal layer110is electrodeposited using the initial metal layer106as a seed layer.

In the step shown inFIG.1g, the mask layer108is removed from the initial metal layer106.

In the step shown inFIG.1h, the initial metal layer106is removed (e.g., chemically etched) to expose metal contacts112a,112band traces (not visible) in the desired pattern. In some embodiments, rather than a single layer of electrical traces and metal contacts112a,112b, 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 inFIGS.1mthrough1qand 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 contacts112a,112b. 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 contacts112a,112b. In some embodiments, as shown inFIG.1i, lithography techniques are utilized to deposit a mask layer114. As shown inFIG.1i, the mask layer114exposes a small portion of the metal contacts112a,112b.

In the step shown inFIG.1j, the substrate is dip coated in a molten solder bath, which results in solder layers116being deposited onto exposed portions of the metal contacts112a,112b.

In the step shown inFIG.1k, the mask layer114is removed by a stripping process, leaving the solder layer116positioned on the metal contacts112a,112bas desired.

In the step shown inFIG.1l, a surface mounted device (SMC)118is placed onto the metal contacts112a,112band the solder is heated causing it to reflow and allow self-assembly of the SMC118on the metal contacts112a,112b. 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 SMCs118on the desired contact pads. In addition, optical registration techniques may be utilized to verify placement of metal contacts112a,112band/or SMCs118.

In the steps shown inFIGS.1mthrough1qand inFIG.3, the temporary rigid substrate is removed and replaced with a soft, compliant substrate120. In some embodiments, the soft, compliant substrate120is clear, i.e., transparent, to allow optical signals to be transmitted to and from the SMCs118. In some embodiments, in the step shown inFIG.1m, 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.,1A:2B). As shown inFIG.1m, over molding of the polymer resin results in the SMCs118as well as the other sacrificial layer104being 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 substrate120. In some embodiments, the compliant substrate120is cured. In some embodiments, the polymer resin is flexible and stretchable. At this stage, the temporary rigid substrate—which includes the temporary rigid substrate100, release layer102and sacrificial layer104—is still affixed to the electronic components on a bottom side. The compliant substrate120is located opposite the temporary rigid substrate100, 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 inFIG.1n, the temporary rigid substrate100is separated from the electrical device (e.g., SMCs118and conductive traces) at the interface between the release layer102and the sacrificial layer104, wherein the sacrificial layer104remains affixed to the compliant substrate120. The process of separating the compliant substrate120from the temporary rigid substrate100is 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 layer102and the silicon material forming the temporary rigid substrate100is stronger than a second adhesion force of the PI material forming the sacrificial layer104to the PMMA material forming the release layer102. When a removal force greater than the second adhesion force is applied to the compliant substrate120it detaches from the temporary rigid substrate100at the PMMA-PI interface. A single transfer step for the entire device/circuit is also scalable.

In the step shown inFIG.1o, the sacrificial layer104is removed from by a plasma etching process. As shown at inFIG.1o, the compliant substrate120is flipped upside down, wherein the plasma etching process removes the sacrificial layer104from the compliant substrate120. Following the step shown inFIG.1o, electrical contacts and traces are exposed on the (now) top side of the compliant substrate120.

In the step shown inFIG.1p, electrical contacts are established as a required and in the step shown inFIG.1q, a polymer layer122is deposited on top of the device to fully encapsulate the device. The final device is shown inFIG.1r.

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 toFIGS.2a-2f, the flexible and stretchable electronic device is illustrated at several different stages of manufacture as well as in operation.FIG.2ais a top view of electrical traces fabricated on a temporary rigid substrate according to some embodiments. In this view, a temporary rigid substrate200supports a plurality of conductive traces202as well as a plurality of electrical contacts204. As discussed above, utilization of a rigid substrate allows traditional microfabrication techniques to be utilized, allowing for density of conductive traces202and density of electrical contacts204not possible without microfabrication techniques. In the embodiment shown inFIG.2a, conductive traces202have a geometry designed to accommodate stretching and/or bending of the device.

FIG.2bis a side view of the device shown inFIG.2a, in which a temporary rigid substrate200includes a rigid silicon substrate205, a release layer208(e.g., PMMA) and a sacrificial layer210(e.g., PI). As described above, the release layer208and the sacrificial layer210are cured to ensure that temporary rigid substrate200is sufficiently rigid to fabricate electrical traces and mount surface mounted components (SMCs) as shown inFIG.2c. In this example, conductive traces202and electrical contacts204have been deposited and fabricated on top of the temporary rigid substrate200.

FIG.2cis a top view that illustrates a plurality of surface mounted components (SMCs)234that are mounted to the temporary rigid substrate200according to some embodiments. In this view, the temporary rigid substrate200provides support during the mounting and soldering of the plurality of SMCs234onto the electrical contacts204shown inFIGS.2aand2b. In some embodiments, placement of the plurality of SMCs234may take advantage of pick-and-place commonly utilized with microfabrication techniques.

FIG.2dis a side view of the device shown inFIG.2c, in which SMCs234are mounted on the electrical contacts204. At this time, the temporary rigid substrate200provides the rigidity necessary to mount the SMCs234onto the electrical contacts. In some embodiments, pick and place machines are utilized to mount the plurality of SMCs234onto the desired electrical contacts204. A benefit of utilizing a rigid substrate (i.e., the temporary rigid substrate200) 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 toFIGS.1a-1r, following placement of the surface mounted components on the electrical contacts204, the temporary rigid substrate200is removed and the remaining components are encased in a flexible and stretchable polymer layer as shown inFIGS.2eand2f.

With respect toFIG.2e, an embodiment is shown in which the flexible and stretchable electronic device215includes a plurality of surface mounted optical devices. In particular, in the embodiment shown inFIG.2e, the device includes an infrared (IR) LED220, a red-green-blue (RGB) LED222, an IR phototransistor224, and a visible light phototransistor226. Each of these components represents a type of surface mounted device (SMC) mounted onto the electrical contacts204shown inFIG.2a. In addition, the embodiment shown inFIG.2eillustrates a clear flexible and stretchable polymer substrate228surrounding the conductive traces202and SMR devices, including IR LED220, RGB LED222, IR phototransistor224, and visible light phototransistor226. As illustrated inFIG.2g, the geometry of the conductive traces202(e.g., undulating, meandering, sawtooth, and/or serpentine geometry) allows the conductive traces202to handle bending and/or stretching. Likewise, the flexible and stretchable polymer substrate228is 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 substrate228be transparent or nearly transparent to allow the transmission of light to and from the optical devices.

FIG.2fis a side view of the device shown inFIG.2e. In the embodiment shown inFIG.2f, the entire device is encapsulated in the flexible and stretchable polymer substrate228. Encased within the flexible and stretchable polymer substrate228is a pair of electrical contacts230a,230b, a pair of solder joints232a,232b, and SMCs234(e.g., IR LED220, RGB LED222, IR phototransistor224, or visible light phototransistor226). The geometry of the conductive traces202ensures that the stretchability and flexibility of the device does not prevent electrical conductivity between the electrical contacts230a,230b, solder joints232a,232b, and the SMCs234.

FIG.2gis a perspective view illustrating the transparent and stretchable electrical device fabricated as shown inFIGS.2a-2fin operation. In particular, the embodiment shown inFIG.2gillustrates the use of optoelectronic devices, which can be utilized in a plurality of patient and health monitoring applications.

Although the embodiment shown inFIGS.2a-2eutilizes 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.