Patent Description:
The ability to form electrical devices with components such as electrode traces and electrode pads, on softening polymers opens the opportunity to fabricate flexible electrical devices. Flexible electrical devices have many possible biomedical applications, e.g., as implantable or non-implanted devices. There is a continuing need to improve the processes to fabricate such devices, such that the devices are more sensitive, less prone to delamination of the electrical components from the polymers, and/or, connectable to recording or stimulating control devices.

The invention is as defined in independent claims <NUM> and <NUM>. One embodiment is a method of manufacturing an electrical device. Embodiments of the method comprise forming a patterned inorganic liftoff layer to expose a target electrode site on a softening polymer layer, depositing an electrode layer on the inorganic liftoff layer and on the exposed target electrode site and removing the inorganic liftoff layer by a horizontal liftoff etch to leave the electrode layer on the exposed target electrode site.

In some embodiments, the inorganic liftoff layer can have a thin film stress of less than about <NUM> MPa. In some embodiments, forming the patterned inorganic liftoff layer can include removing portions of the inorganic liftoff layer exposed through openings in a patterned photoresist layer on the inorganic liftoff layer by a fluorine plasma dry etch process. In some embodiments, removing the inorganic liftoff layer by the horizontal liftoff etch can include etchant solvent exposure of the: inorganic liftoff layer, a photoresist layer on the inorganic liftoff layer and portions of the electrode layer on the photoresist layer. The etchant solvent exposure can be for at least about <NUM> hours at about <NUM> and the etchant solvent can include an ester-based photoresist removing solvent or a potassium borate photo developing solution.

Another embodiment is an electrical device comprising a softening polymer layer and an electrode layer on the softening polymer layer. At least a portion of the electrode layer has less than about <NUM> ppm Carbon/ µm<NUM> (micron<NUM>) of organic material thereon.

In some embodiments, the portion of the electrode layer can have a root mean squared surface roughness of about <NUM> or greater. In some embodiments, the portion of the electrode layer has a surface area that is at least about <NUM> percent greater than a surface area of a same sized planar two-dimensional perimeter.

Another embodiment is another method of manufacturing an electrical device. The method comprises forming a softening polymer layer with an electrode layer on a surface of the softening polymer layer and forming a second softening polymer layer on the surface of the softening polymer layer. A mechanical neutral plane of the electrical device is located substantially at or above the surface of the softening polymer layer. An opening in the second softening polymer cover layer exposes a portion of a surface of the electrode layer.

Some embodiments can further include forming an inorganic hardmask layer on the second softening polymer cover layer. Some embodiments can further include removing, by a reactive ion etch process, a portion of the inorganic hardmask layer exposed through an opening in a patterned photoresist layer located on the inorganic hardmask layer to form an opening in the inorganic hardmask layer. Some embodiments can further include removing, by the reactive ion etch process, exposed portions of the cover layer underlying the opening in the inorganic hardmask layer to form the opening in the second softening polymer cover layer. Some embodiments can further include removing the patterned photoresist layer and the inorganic hardmask layer. Some embodiments can further include depositing a second electrode layer on the inorganic liftoff layer and the portion of the surface of the electrode layer. Some embodiments further include removing the inorganic hardmask layer by a horizontal liftoff etch that includes exposing the inorganic hardmask layer to an ester-based photoresist removing solvent or a potassium borate photo developing solution.

Some embodiments further include exposing the surface of the softening polymer layer to a reactive ion etch treatment before forming the second softening polymer layer on the surface of the softening polymer layer. In some such embodiments, the surface of the softening polymer layer after the reactive ion etch treatment can have an at least double a root mean squared surface roughness as compared to the surface before the reactive ion etch treatment. In some such embodiments, a thickness of the softening polymer layer after the reactive ion etch treatment can be within about ± <NUM> percent of the thickness of the softening polymer layer before the reactive ion etch treatment.

In some embodiments, the removing, by the reactive ion etch process, of the exposed portions of the cover layer can include forming a plasma formed from a feed gas of oxygen at about <NUM> W biasing power and a pressure of about <NUM> mTorr.

Another embodiment is another electrical device. The electrical device comprises a softening polymer layer, an electrode layer on a surface of the softening polymer layer and a cover layer composed of a second softening polymer on the surface of the softening polymer layer. A mechanical neutral plane of the device is located substantially at or above the surface of the softening polymer layer. An opening in the second softening polymer cover layer exposes a portion of a surface of the electrode layer.

In some embodiments, the cover layer has a Young's modulus that can be within about ± <NUM> percent of a Young's modulus of the softening polymer layer. In some embodiments the cover layer has a thickness can be within about ± <NUM> percent of a thickness of the softening polymer layer.

Another embodiment can be another method of manufacturing an electrical device. The method comprises placing a stencil mask on a polymer cover layer of the electrical device. Apertures in the stencil mask can each align with one of a plurality of openings in the polymer cover layer, each of the openings exposing contact pad site portions of electrode layers on a softening polymer layer of the electrical device. The method comprises stencil printing to provide a layer of solder paste that fills the apertures and the openings and contacts the exposed contact pad site portions of the electrode layers. The method comprises removing the stencil mask to form discrete solder contacts located on the exposed contact pad site portions of the electrode layers. The method comprises aligning electrical connector electrodes with each of the solder contacts such that each one of connector electrodes lay on the top surface of one of the solder contacts. The method comprises reflowing the solder contacts to couple the connector electrodes to the contact pad site portions of the electrode layers.

In some embodiments the reflowing of the solder contacts includes a reflow cycle that can include a pre-heat phase followed by a soaking phase followed by a reflow phase followed by a cooling phase. The pre-heat phase can include heating the electrical connector electrodes to a temperature of about <NUM>. The soaking phase can include increasing the temperature of the solder contacts to about <NUM>. The reflow phase can include increasing the temperature of the solder contacts and the electrical connector electrodes to a temperature of about <NUM> to form a solder joint. The cooling phase can include decreasing the temperature of the solder joint at a rate of about <NUM>/s.

Another embodiment is another electrical device. The electrical device comprises a softening polymer layer, an electrode layer on a surface of the softening polymer layer and a cover polymer layer on the surface of the softening polymer layer. An opening in the polymer cover layer is filled with a reflowed solder. One end of the reflowed solder, located inside the opening, contacts a contact pad site portion of the electrode layer. Another end of the reflowed solder contacts an electrical connector electrode of the device.

In some embodiments the solder is composed of an indium silver solder.

The embodiments of the disclosure are best understood from the following detailed description, when read with the accompanying FIGUREs. Some features in the figures may be described as, for example, "top," "bottom," "vertical" or "lateral" for convenience in referring to those features. Such descriptions do not limit the orientation of such features with respect to the natural horizon or gravity. Various features may not be drawn to scale and may be arbitrarily increased or reduced in size for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:.

The different fabrication processes disclosed herein are each designed to improve the utility of the resulting flexible electrical devices. In some embodiments, these processes can be performed independently of each other in separate process flows to provide different types of improvements in the respective resulting devices.

While the disclosed embodiments refer to electrode layers such as electrode traces and electrode pads, any of these embodiments could further include transistors, diodes, switches, circuits, or other electrical components, located on the softening polymer layer and, e.g., connected to some such electrode layers.

Embodiments of present disclosure benefit from the recognition that for some embodiments of flexible electrical devices, certain conventional lithographic and patterning processes for forming one or more electrode layers on a softening polymer includes forming an opening in a polymer cover layer to uncover the electrode layer.

The electrode layer may be a thin electrode trace, e.g., configured to conduct electrical signals to and from an external recording or stimulating device, and/or, an electrode interaction pad, e.g., a portion of the electrode layer configured to send blocking or stimulation signals and/or to record electrical signals.

We have found that conventional lithographic and patterning processes can leave residual organic material (e.g., residual photoresist material or residual polymer cover layer material) on the surface of the electrode layer. The presence of such residual organic material, in turn, can reduce the sensitivity of the electrode layer. For instance, not as much electrical charge may be generated at the electrode interaction pad as compared to a same-area pristine pad with no organic material thereon. For instance, the electrode interaction pad may not detect electrical signals generated in nerves as sensitively as compared to a pristine pad with no organic material thereon.

Although aggressive clean-up procedures (e.g., reactive ion etching) may remove some of the residual organic material from the interaction pads, for certain types of electrode materials having rough surfaces, such clean-up procedures may require extended periods, thereby lengthening device fabrication times and complexity, and/or, the clean-up procedures may not remove all of the residual organic material. Moreover, such aggressive clean-up procedures may decrease the surface roughness of the electrode interaction pad which, in turn, may detrimentally decrease the charge injection capacity of the interaction pad. Some aggressive clean-up procedures using oxygen plasmas may oxidize the electrode interaction pad, which in turn, may cause an undesirable decrease of charge injection capacity.

To address these issues, we have developed a liftoff process that allows for the patterning of electrodes on a softening polymer layer. Unlike liftoff processes that use organic photoresist to pattern electrodes on a softening polymer layer, we use an organic photoresist to pattern openings in an inorganic liftoff layer on a softening polymer layer.

We discovered that using an organic photoresist directly as a lift-off material was problematic because the solvents used to lift off the photoresist also can degrade the softening polymer layer and/or delaminate the softening polymer layer from other layers of the device (e.g., a polymer cover layer). In contrast, the etchants used to lift off the inorganic liftoff layer, as disclosed herein, do not substantially degrade or delaminate the softening polymer layer.

One embodiment is a method of manufacturing a flexible electrical device. <FIG> show an example flexible electrical device <NUM> of the disclosure at intermediate stages of a manufacturing process which includes an embodiment of the lift-off process of the disclosure.

<FIG> shows the device <NUM> after forming a softening polymer layer <NUM>.

In some embodiments, the softening polymer layer <NUM> can be formed by polymerizing monomers of the polymer on a sacrificial substrate (e.g. a glass substrate) and then the softening polymer layer <NUM> can be peeled away from the substrate.

The term softening polymer as used herein refers to a polymer that softens by more than <NUM> order of magnitude (i.e., Young's modulus decrease by <NUM> order of magnitude) within an about <NUM> temperature increase (from room temperature, about <NUM>, to about <NUM> or less or in some embodiments to about <NUM>).

Non-limiting example softening polymers that the layer <NUM> can be composed of include hydrogels or shape memory polymers.

As understood by those skilled in the pertinent arts, shape memory polymers are self-adjusting, smart polymer materials whose shape change can be controlled at specific, tailored temperature. Examples of shape memory polymers include polymers formed from a combination of the monomers: <NUM>,<NUM>,<NUM>-triallyl-<NUM>,<NUM>,<NUM>-triazine-<NUM>,<NUM>,<NUM> (<NUM>,<NUM>,<NUM>)-trione (TATATO), tris[<NUM>-(<NUM>-mercaptopropionyloxy)ethyl] isocyanurate (TMICN), trimethylolpropane tris(<NUM>-mercaptopropionate) (TMTMP) and tricyclo[<NUM>. <NUM><NUM>,<NUM>]decanedimethanol diacrylate (TCMDA). Other example shape memory polymers include polymers formed from a combination of thiol- and ene- functionalized monomers.

Other examples of shape memory polymers include polymers formed from a combination of thiol-, ene- and acrylate- functionalized monomer. Non-limiting examples of thiol-ene softening polymer compositions are TATATO/TMICN (<NUM>/<NUM>), and TATATO/TMTMP (<NUM>/<NUM>). Examples of thiol-ene-acrylate softening polymer compositions are TATATO/TMICN/TCMDA (<NUM>/<NUM>/<NUM>), and TATATO/TMTMP/TCMDA (<NUM>/<NUM>/<NUM>), such as disclosed in <NPL>).

Other examples of shape memory polymers include polymers formed from a combination of thiol and isocyanate functionalized monomers. Non-limiting examples include the polymer formed via the base or thermally catalyzed polymerization reaction of thiol and isocyanate monomers or oligomers. The monomers or oligomers may have at least one functional group of thiol or isocyanate present on the molecule, such as but not limited to <NUM>,<NUM>'-(ethylenedioxy)diethanethiol, isophorone diisocyanate, hexamethylene diisocyanate, or tris(<NUM>-isocyanatohexyl)isocyanurate.

Other example shape memory polymers include polymers formed from a combination of thiol- and epoxy- functionalized monomers. Non-limiting examples include the polymers formed from combinations of the thiol- and epoxy- functionalized monomers disclosed by <NPL>).

<FIG> shows the device <NUM> after forming an inorganic liftoff layer <NUM> on the softening polymer layer <NUM>.

Some embodiments of inorganic liftoff layer <NUM> are low stress layers, e.g., layers having a thin film stress of less than about <NUM> MPa, or less than about <NUM> MPa, or less than about <NUM> MPa, or less than about <NUM> MPa for various different embodiments of the layer. While not limiting the scope of the disclosure by theoretical considerations, it is thought that for at least some embodiments, such low stress layers maintain device integrity e.g., by mitigating cracking or wrinkling after and/or during a device fabrication process step, when the softening polymer layer may transition to a softened state.

In some embodiments, forming the inorganic liftoff layer <NUM> includes depositing silicon nitride, silicon oxynitride or silicon oxide by a low stress plasma enhanced chemical vapor deposition process (PECVD). While not limiting the scope of the disclosure by theoretical considerations, it is thought that for at least some embodiments, the use of a PECVD silicon nitride process can advantageously produce a very low stress inorganic liftoff layer <NUM> (e.g., a thin film stress of less than about <NUM> MPa). While not limiting the scope of the disclosure by theoretical considerations, it is thought that for at least some embodiments, the use of a PECVD silicon oxide process can advantageous produce an inorganic liftoff layer <NUM> that may be faster to dissolve via a horizontal etch as further dissolved herein, e.g., as compared to dissolving an inorganic liftoff layer <NUM> formed via PECVD silicon oxynitride.

A non-limiting example of a low stress PECVD silicon oxynitride process to form the inorganic liftoff layer <NUM> includes using feed gases of SiH<NUM>, NH4, He, and N<NUM>, at a pressure of about <NUM> milliTorr and deposition temperature of about <NUM>. A non-limiting example of a low stress PECVD silicon oxide process to form the inorganic liftoff layer <NUM> includes feed gases of SiH<NUM>, NH4, He, and N<NUM>, at a pressure of about <NUM> milliTorr and deposition temperature of about <NUM>.

In some embodiments, forming the inorganic liftoff layer <NUM> includes sputter depositing or evaporating metals onto the softening polymer layer. Non-limiting example of suitable evaporated metals for use as materials of the inorganic liftoff layer <NUM> include gold, titanium, platinum, iridium, and some reactively sputtered materials, such as titanium nitride, Platinum, gold and iridium oxide. Non-limiting example of suitable sputter deposited metals for use as materials of the inorganic liftoff layer <NUM> include porous TiN thin film formed by RF magnetron sputtering using a base pressure of about <NUM>-<NUM> torr with Ar and N<NUM> gas flow of <NUM> sccm and <NUM> sccm, respectively to provide a deposition rate of <NUM>/min.

Non-limiting example sputter deposition parameters include about <NUM> milliTorr argon pressure and about 200Watt biasing power. Non-limiting example evaporation parameters include about <NUM> milliTorr base pressure, and an about <NUM> per second evaporation rate.

<FIG> shows the device <NUM> after forming an organic photoresist layer <NUM> on the inorganic liftoff layer <NUM> and after patterning the organic photoresist layer <NUM> to form an opening <NUM> therein. The opening <NUM> exposes a portion <NUM> of the inorganic liftoff layer <NUM>, the opening <NUM> lying directly over a target electrode trace site portion <NUM> of the softening polymer layer <NUM>.

In some embodiments the photoresist can be a negative resist such as AZ-nLOF-<NUM> (Microchem Corporation). In other embodiments, negative photoresist materials or positive resist materials such as S-<NUM>, S-<NUM>, S-<NUM> (Rohm and Haas Company) may be used.

Patterning the organic photoresist layer <NUM> to form the opening <NUM> therein can be accomplished by conventional photolithographic patterning and etching procedures familiar to those skilled in the pertinent art.

<FIG> shows the device <NUM> after forming a patterned inorganic liftoff layer <NUM> by removing the portion <NUM> of the inorganic liftoff layer <NUM> exposed by the patterned photoresist layer <NUM> to form an opening <NUM> therein to thereby expose the target electrode trace site portion <NUM> of the softening polymer layer <NUM>.

In some embodiments, removing the exposed portion <NUM> of the inorganic liftoff layer <NUM>, which transfers the patterns formed in the photoresist layer <NUM> to the inorganic lift-off layer <NUM>, can include a fluorine plasma dry etch process using SF<NUM> gas at about <NUM> milliTorr pressure.

<FIG> shows the device <NUM> after depositing an electrode material layer <NUM> on the inorganic liftoff layer <NUM> and on the exposed target electrode trace site portion <NUM> of the softening polymer layer <NUM>.

In some embodiments, the electrode material layer <NUM> can be deposited by a sputtering, an evaporation or an electro-deposition process. Non-limiting example electron deposition parameters include no sample rotation with accelerating voltage of about 10KV. Non-limiting example evaporation parameters include <NUM> milliTorr base pressure, and <NUM>-<NUM> Angstroms per second evaporation rate. Non-limiting example sputtering parameters include RF magnetron sputtering with base pressure of about <NUM>-<NUM> torr with Ar and N<NUM> gas flow of about <NUM> sccm and about <NUM> sccm, respectively to provide a deposition rate of about <NUM>/min.

While not limiting the scope of the disclosure by theoretical considerations, for some embodiments, it is thought that sputter depositing may be preferred because the electrode material layer <NUM> formed has a smaller grain size e.g., because the layer is formed with less stress. While not limiting the scope of the disclosure by theoretical considerations, it is thought that least for some embodiments, the grain size and continuity of the deposited metal can affect the quality of the resulting structure. Also, for some embodiments, a sputter depositing process may mitigate the formation of rough edges which may result, e.g., if the electrode material layer <NUM> coats the sidewalls of the resist and then rips during liftoff.

In some embodiments, the electrode material layer <NUM> can have a thickness of about <NUM>, <NUM>, <NUM>, <NUM> <NUM> or <NUM> (microns).

In some embodiments, the electrode material layer <NUM> can be composed of a metal that is flexible, that can covalently bond to thiols (e.g., present in some embodiments of the softening polymer layer <NUM>), has a low electrical impedance and does not readily oxidize. Non-limiting examples include gold and silver.

In other embodiments, the electrode material layer <NUM> can be composed of titanium nitride, platinum, titanium, iridium, or iridium Oxide (e.g., sputtered Iridium Oxide or electrodeposited Iridium Oxide) copper, aluminum, or other metals that can be deposited by sputtering or evaporation and are compatible with the inorganic liftoff layer <NUM>, e.g., such that subsequent photolithographic processing steps do not etch any underlying layers including the inorganic liftoff layer <NUM>.

In some embodiments, the electrode material layer <NUM> can be composed of carbon nanotubes, such as poly(<NUM>,<NUM>-ethylenedioxythiophene) nanotubes, deposited via a sputtering process.

<FIG> shows the coverless device <NUM> after removing: the inorganic liftoff layer <NUM>, the overlying organic photoresist layer <NUM>, and the overlying electrode material layer <NUM>, by a horizontal liftoff etch process, to leave a thin film electrode trace <NUM> on the target electrode trace site portion <NUM> of the softening polymer layer <NUM>.

In some embodiments, the horizontal liftoff etch process includes exposing the partially constructed device <NUM>, e.g., as shown in <FIG>, to an etchant solvent to slowly remove the layers <NUM>, <NUM>, <NUM> over at least several hours of time, e.g., which helps to avoid damaging the softening polymer layer <NUM>. In some embodiments, the period of exposure to the solvent can range from about <NUM> hours to <NUM> hours at about <NUM>. In some such embodiments, the device <NUM> is exposed to an etchant solvent including or composed of larger molecules which help make the etchant solvent compatible with, e.g., not dissolve, the softening polymer layer <NUM>. Non-limiting examples of the etchant solvent includes an ester-based photoresist removing solvent, such as RR5 (Futurrex, Franklin, NJ), or, a photo developing solution such as a potassium borate solution (potassium borate:water <NUM>:<NUM>) for about <NUM> hours.

In some embodiments, after exposure to the etchant solvent, the device <NUM> can be transferred to an oven (e.g., about <NUM> to <NUM> for about <NUM> to <NUM> hours) to remove the etchant solvent to result in the coverless flexible device shown in <FIG>, having an thin film electrode trace that is pristine, that is, has never been directly exposed to photoresist or residual organic materials from the above described process steps.

As illustrated in <FIG>, in some embodiments, a thickness <NUM> of the inorganic liftoff layer <NUM> is deposited so as to be sufficiently large to facilitate getting the horizontal etchant solvent under the metal layer <NUM> and thereby facilitate an efficient rapid liftoff process. Having too thin of the liftoff layer <NUM> may result in the etchant solvent not getting under all of the metal layer <NUM> and consequently, the metal layer <NUM> may not be lifted off properly and metal flecks can be left on the surface of the softening polymer layer <NUM>.

The target thickness <NUM> of the liftoff layer will depend on the size, and the thickness, of the metal layer <NUM> to be removed by the lift-off process. For example, in some embodiments, e.g., when the metal layer <NUM> has a horizontal dimension in the plane of the softening polymer layer <NUM> of at least about <NUM>, the inorganic liftoff layer <NUM> can have a thickness <NUM> in a range from about <NUM> to <NUM>, and, in some embodiments, at least about <NUM> thick.

<FIG> show an example flexible electrical device <NUM> of the disclosure at intermediate stages of an example manufacturing process of the disclosure with another embodiment of the lift-off process of the disclosure.

<FIG> shows the device <NUM> after forming a thin film electrode trace <NUM> on a softening polymer layer <NUM>.

In some embodiments, the thin film electrode trace <NUM> on the softening polymer layer <NUM> can be formed by an embodiment of the process disclosed in the context of <FIG>. This may be preferred, e.g., for some device embodiments where the electrode material that the electrode trace <NUM> is composed of can form a relatively rough outer surface (e.g., a root mean squared, rms, surface roughness of about <NUM> or greater or a surface area that is at least about <NUM> percent greater than a surface area of perfectly planar electrode having a same two-dimensional perimeter), which as disclosed elsewhere herein can make the complete removal of photoresist material difficult.

In other embodiments, the thin film electrode trace <NUM> on a softening polymer layer <NUM> can be formed by other conventional lithography and patterning processes. For example, as disclosed in <CIT>, the electrode material layer can be deposited (e.g., physical vapor deposition) on a sacrificial substrate (e.g., glass, silicon or a polymer) and then the softening polymer layer can be formed on the electrode material layer by a transfer-by-polymerization process. For example, a photoresist layer can be deposited on the electrode material layer on the softening polymer layer, the photoresist layer can be patterned to form opening therein to expose a negative or reverse target electrode trace site portion of the softening polymer layer, and the exposed portions of the electrode material layer can be removed, and then the photoresist layer can be removed, to uncover the thin film electrode trace <NUM> on the softening polymer layer <NUM>.

This process may be used for some device embodiments where the electrode material that the electrode trace <NUM> is composed of can form a relatively smooth outer surface (e.g., a rms roughness of less than about <NUM> or a surface area that is within about <NUM> percent of a surface area of perfectly planar electrode having a same two-dimensional perimeter or a surface area of a same sized planar two-dimensional perimeter), in which case, the complete removal of photoresist material using conventional procedures is not expected to be challenging.

<FIG> shows the device <NUM> after forming a polymer cover layer <NUM> over the electrode trace on the softening polymer layer. In some embodiments, the polymer cover layer <NUM> can be composed of a second softening polymer and deposited by an embodiment of the processes such described in the context of <FIG> as disclosed elsewhere herein. In some embodiments, the polymer cover layer can be composed of a polymer such as Parylene-C or other parylene derivatives, e.g., deposited by a chemical vapor deposition process. In some embodiments the polymer cover layer can be composed of polyethylene terephthalate (PET), or other PET derivatives.

<FIG> shows the device <NUM> after forming an inorganic hardmask layer <NUM> on the polymer cover layer <NUM>. The hardmask layer <NUM> can be composed of any of the materials and formed by any of the processes used to form the inorganic liftoff layer <NUM> such as disclosed elsewhere herein in the context of <FIG>. Unlike the lift-off layer <NUM>, however, embodiments of the hardmask layer <NUM> can have a smaller thickness <NUM> than the thickness <NUM> of the liftoff layer <NUM>. This follows because the hardmask layer <NUM> is not used as a solvent horizontally etched lift-off material, but rather, is used for defining a pattern and protecting the underlying polymer cover layer <NUM> and the softening polymer layer <NUM> from being damaged by a reactive ion etch patterning step as further disclosed elsewhere herein in the context of <FIG>. For example, in some embodiments, the inorganic hardmask layer <NUM> can have a thickness <NUM> in a range from about <NUM> to about <NUM>.

<FIG> shows the device <NUM> after forming an organic photoresist layer <NUM> on the inorganic hardmask layer <NUM> and after patterning the organic photoresist layer <NUM> to form an opening <NUM> therein to thereby expose a portion <NUM> of the inorganic hardmask layer <NUM>, the opening <NUM> lying directly over a target electrode pad site portion <NUM> of the electrode trace <NUM>. Any of the same photoresist materials and photolithography procedures as described in the context of <FIG> can be used to deposit and pattern the photoresist layer <NUM>.

<FIG> shows the device <NUM> after removing the exposed portion <NUM> (<FIG>) of the inorganic hardmask layer <NUM> to form an opening <NUM> therein to thereby expose a portion <NUM> of the polymer cover layer <NUM>.

In some embodiments, removing the exposed portion <NUM> of the inorganic hardmask layer <NUM> includes a reactive ion etch process using a CF<NUM>/O<NUM> plasma at about <NUM> mtorr pressure and biasing power of about <NUM> W for about <NUM> with a base pressure of about <NUM> mtorr.

<FIG> shows the device <NUM> after removing the exposed portion of the polymer cover layer <NUM> to form an opening <NUM> therein to thereby expose the target electrode pad site portion <NUM> of the electrode trace <NUM>.

In some embodiments, removing the exposed portion <NUM> of the polymer cover layer <NUM> includes a reactive ion etch process. For example, when the polymer cover layer <NUM> is composed of a second softening polymer then the deep reactive ion etch process described elsewhere herein in the context of <FIG> can be used. For example, when the polymer cover layer <NUM> is composed of parylene then an oxygen plasma reactive ion etch process can be used.

<FIG> shows the device <NUM> after removing the inorganic hardmask layer <NUM> and the overlaying photoresist layer <NUM>.

In some embodiments, removing the inorganic hardmask layer <NUM> and the photoresist layer <NUM> includes soaking the device <NUM> in a photoresist removing solvent such as RR5 for about for about <NUM> to <NUM> hours to remove the photoresist layer <NUM>, followed by transferring the device to an oven (e.g., about <NUM> to <NUM> for about <NUM> to <NUM> hrs. ) to remove the solvent and then soaking the device <NUM> in hydrofluoric acid (e.g. concentrated <NUM> or <NUM>% HF diluted in water H<NUM>O:HF <NUM>:<NUM> or <NUM>:<NUM>) for about <NUM> minute.

<FIG> shows the device <NUM> after forming an inorganic liftoff layer <NUM> on the polymer cover layer <NUM> and on the exposed target electrode interaction pad site portion <NUM> of the electrode trace <NUM>.

The inorganic liftoff layer <NUM> can be composed of any of the same materials and be deposited by the same procedures as for the inorganic liftoff layer <NUM> disclosed elsewhere herein in the context of <FIG>. Embodiments of the inorganic liftoff layer <NUM> can be adjusted to have a same thickness <NUM> as the thickness <NUM> of the inorganic liftoff layer <NUM> such as disclosed elsewhere herein in the context of <FIG>.

<FIG> shows the device <NUM> after forming an organic photoresist layer <NUM> on the inorganic liftoff layer <NUM>, and, after patterning the organic photoresist layer <NUM> to form an opening <NUM> therein to thereby expose a portion <NUM> of the inorganic liftoff layer <NUM>, the opening <NUM> lying directly over the target electrode pad site portion <NUM> of the electrode trace <NUM>. Any of the same photoresist materials and photolithography procedures, such as described in the context of <FIG>, can be used to deposit and pattern the photoresist layer <NUM>.

<FIG> shows the device <NUM>, after removing the photoresist layer <NUM> and the exposed portion <NUM> of the inorganic liftoff layer <NUM> to thereby form a patterned inorganic liftoff layer <NUM> having an opening <NUM> therein to thereby expose the target electrode pad site portion <NUM> of the electrode trace <NUM>.

Any of the liftoff layer removal procedures, such as disclosed elsewhere herein in the context of <FIG>, can be used to remove the photoresist layer <NUM> and the exposed portion <NUM> of the inorganic liftoff layer <NUM>.

<FIG> shows the device <NUM> after depositing a second electrode material layer <NUM> on the inorganic liftoff layer <NUM> and the exposed target electrode pad site portion <NUM> of the electrode trace <NUM>.

Any of the electrode materials and deposition procedures such as described elsewhere herein in the context of <FIG> can be used to deposit the second electrode material layer <NUM>.

In some embodiments, the second electrode material is selected to provide the layer <NUM> with a high surface roughness that is conducive to providing a high charge injection capacity interaction pad (e.g., about <NUM> or greater mC/cm<NUM> in some embodiments). Non-limiting examples of such electrode materials include titanium nitride, sputtered iridium oxide or carbon nanotubes.

<FIG> shows the device <NUM> after removing the inorganic liftoff layer <NUM> and the overlying second electrode material layer <NUM> by a horizontal liftoff etch to leave a thin film electrode interaction pad <NUM> on the target electrode pad site portion <NUM> of the electrode trace <NUM>.

Any of the liftoff layer removal procedures, such as described elsewhere herein in the context of <FIG>, can be used to remove the inorganic liftoff layer <NUM> and the overlying second electrode material layer <NUM>.

Another embodiment is a flexible electrical device. <FIG> shows an example embodiment of the flexible electrical device <NUM>, e.g., formed by the processes disclosed elsewhere herein in the context of <FIG>. The device <NUM> comprises a softening polymer layer <NUM> and a thin film electrode trace <NUM> on the softening polymer layer <NUM>. The outer surface <NUM> of the electrode trace <NUM> has less than <NUM> ppm Carbon/ µm<NUM> (micron<NUM>) of organic material thereon.

Some embodiments of the outer surface <NUM> of the electrode trace <NUM> can have a rms surface roughness of about <NUM> or greater. Some embodiments of the outer surface <NUM> of the electrode trace <NUM> can have a surface area that is at least about <NUM> percent greater than a surface area of perfectly planar electrode trace having a same two-dimensional perimeter. As illustrated in <FIG> the device <NUM> can be coverless, that is, there can be no overlaying polymer cover layer on the softening polymer layer <NUM> or the thin film electrode trace <NUM>.

<FIG> shows another example embodiment of a flexible electrical device <NUM>, e.g., formed by the processes disclosed elsewhere herein in the context of <FIG>. The device <NUM> comprises a softening polymer layer <NUM> and a thin film electrode trace <NUM> on the softening polymer layer <NUM>. The device <NUM> also comprises an electrode interaction pad <NUM> located on a portion <NUM> of the electrode trace <NUM>. The device <NUM> also comprises a polymer cover layer <NUM> covering the electrode trace <NUM> and the softening polymer layer <NUM>. An opening <NUM> in the polymer cover layer <NUM> exposes an outer surface <NUM> of the electrode interaction pad <NUM>. The outer surface <NUM> of the electrode interaction pad <NUM> has less than <NUM> ppm Carbon/ µm<NUM> (micron<NUM>) of organic material thereon.

Some embodiments of the outer surface <NUM> of the electrode interaction pad <NUM> can have a rms surface roughness of about <NUM> or greater. Some embodiments of the outer surface <NUM> of the electrode interaction pad <NUM> can have a surface area that is at least about <NUM> percent greater than a surface area of perfectly planar electrode interaction pad having a same two-dimensional perimeter. As illustrated in <FIG> the device <NUM> can be covered, that is, there can be a polymer cover layer <NUM> on the softening polymer layer <NUM> and portions of the thin film electrode trace <NUM>.

Embodiments of present disclosure benefit from the recognition that embodiments of flexible electrical devices, when bent, are prone to electrode layer delamination when the electrode layer is located away from the mechanical neutral plane of the device.

As familiar to those skilled in the art, the mechanical neutral plane of a plate is defined in bending theory as the plane at which the normal stress is null. Its position is of importance to determine the best location for the electrode components embodiments of the flexible electrical device. When a homogeneous device is subjected to external (pure) bending only, this neutral plane is coincident with the bending axis. However when a multilayered device is considered, with stress-free strains mismatches (with or without external bending applied), the neutral plane shifts from the bending axis, and there can be one, several or even no neutral planes in the device. The mechanical neutral plane's location can often be obtained after solving the system's stress distribution. In the case of films on a substrate, solving the system's stress distribution would require the nondestructive removal of each film from the substrate. The procedure includes constructing the composite from the freely standing layers subject to the assumptions of no resultant edge forces or bending moments.

Solving such stress distribution systems for example flexible electrical device embodiments of the disclosure revealed that for certain embodiments of the flexible electrical device, the mechanical neutral plane can be shifted substantially away from the location of the electrode layer. In particular certain combinations of the softening polymer layer and the polymer cover layer, e.g., having substantially different thicknesses and/or stiffness, can result in a large shift in the mechanical neutral plane away from the surface of the softening polymer layer, e.g., where an electrode layer could be located on.

As an example, consider a flexible electrical device comprising an about <NUM> (micron) thick polymer cover layer composed of the more rigid polymer parylene and an <NUM> (micron) thick of a less rigid softening polymer layer composed of a shape memory polymer, referred to herein as SMP6, formed from the polymerization of a stoichiometric combination of the monomers TMICN and TATATO and <NUM> mol% TCMDA. The mechanical neutral plane of such a device is predicted to be several micrometres (microns) away from the surface of the softening polymer layer. We expect that an electrode layer formed on the softening polymer layer of such a device would be more prone to delamination than desired, e.g., when the device is bent.

Sometimes such large shifts in the neutral plane can be partially mitigated by, e.g., increasing the thickness of the more rigid polymer cover layer and or decreasing the thickness of the softening polymer layer. However, these mitigation measures may be insufficient for providing flexible devices that are not too rigid so as to not be able to bend enough, e.g., to form tightly curled structures, and/or, not be too fragile so as to rip, e.g., during surgical implantation, and/or, still have sufficient softening to facilitate long term functionality (e.g., months) after device implantation in living tissue.

As disclosed herein, such large shifts in the neutral plane can be substantially eliminated by manufacturing a device using a polymer cover layer composed of a second softening polymer that can have substantially a same stiffness and/or thickness as the softening polymer layer. Consequently, the electrode layer of such a double softening polymer layer-containing device can be located substantially at or near the mechanical neutral plane of the device.

The ability to use a cover layer composed of a second softening polymer, however, can be problematic. For instance, the use of conventional photolithography materials and methods to define openings in the polymer cover layer to thereby expose interaction pad portions of the electrode layer can damage or dissolve the second softening polymer or etch the metal due to bombardment of plasma on metal electrodes.

As disclosed herein a reactive ion etch process is used to etch through the second softening polymer layer. A patterned hardmask layer is used to define openings to expose portions of the second softening polymer layer to the reactive ion etch process while other portions of the second softening polymer layer are protected by the hardmask from being damaged by the reactive ion etch process.

<FIG> show an example flexible electrical device <NUM> of the disclosure at intermediate stages of an example manufacturing process of the disclosure with double softening polymer layer-containing embodiments.

<FIG> shows the device <NUM> after forming a softening polymer layer <NUM> with a thin film electrode trace <NUM> thereon. As further illustrated, some embodiments of the device <NUM> can optionally further include a thin film electrode interaction pad <NUM> on the electrode trace <NUM>.

The softening polymer layer <NUM>, electrode trace <NUM> and the optional electrode interaction pad <NUM> can be composed of any of the materials, have any of the thicknesses, and, be formed by any of the processes, as disclosed elsewhere herein in the context of <FIG> and <FIG>.

<FIG> shows the device <NUM> during an optional reactive ion etch surface treatment of an exposed surface <NUM> of the softening polymer layer <NUM> to form a treated softening polymer layer surface <NUM>. We discovered that for some embodiments, a subsequently formed cover layer composed of the second softening polymer may not adhere as well to the softening polymer layer <NUM> as desired, and consequently, the layer may be prone to delamination. In some embodiments, the formation of a treated softening polymer layer surface <NUM> facilitates adhesion of second softening polymer cover layer to the softening polymer layer <NUM>.

While not limiting the disclosure by theoretical considerations, it is thought that, for at least some embodiments, the reactive ion etch surface treatment facilitates adhesion by increasing the surface area and/or increasing the number of hydroxyl groups on the treated softening polymer layer surface <NUM> which, e.g., can form covalent bonds to the cover layer composed of the second softening polymer.

In some embodiments, for example, the reactive ion etch surface treatment at least doubles the rms surface roughness of the surface <NUM>. In some embodiments, the rms surface roughness of the pretreated surface <NUM> increases from about <NUM> to about <NUM> (micron) for the reactive ion etch surface treated surface <NUM>.

The conditions of reactive ion etch surface treatment are selected to form the treated surface <NUM> without substantially etching away the softening polymer layer <NUM>, e.g., so as to substantially reduce the thickness of the layer <NUM>. For instance, in some embodiments, a thickness <NUM> of the pretreated layer <NUM> and a thickness <NUM> of the reactive ion etch surface-treated layer <NUM> are the same within about ±<NUM> percent and in some embodiments within about ±<NUM> percent.

In some embodiments, the reactive ion etch etchants <NUM> are formed from argon and oxygen gases feeds and the duration of the reactive ion etch can be in a range from about <NUM> seconds to about <NUM> seconds. In some embodiments, the ratio of argon to oxygen (Ar/O<NUM>) in in a range of about <NUM>:<NUM> to about <NUM>:<NUM>. One skilled in the pertinent arts would understand how these conditions could be adjusted to roughen any of the embodiments of softening polymer layers disclosed herein.

For example, when the softening polymer layer <NUM> is composed of a <NUM> (micron) thick layer <NUM> of SMP6, some embodiments of the reactive ion etch surface treatment includes an Ar/O<NUM> ratio of about <NUM>:<NUM> applied for about <NUM>.

<FIG> shows the device <NUM> after forming a cover layer <NUM> composed of a second softening polymer on the surface (e.g., untreated surface <NUM> or treated surface <NUM>) of the softening polymer layer <NUM>. As illustrated, the electrode trace layer <NUM> and the optional electrode interaction pad <NUM> are also covered by the second softening polymer cover layer <NUM>.

The second softening polymer cover layer <NUM> can be composed of any of the softening polymer materials as disclosed elsewhere herein in the context of <FIG>.

The second softening polymer cover layer <NUM> can be formed by a spin coating process. In some embodiments for example, an appropriate amount of the second softening polymers can be dispensed on the untreated surface <NUM> or treated surface <NUM> of the softening polymer layer <NUM> and then spun at about <NUM> rpm for about two minutes to achieve an about <NUM> (micron) thickness of the second softening polymer cover layer <NUM> on the softening polymer layer <NUM>. One skilled in the pertinent art would understand how different thicknesses of second softening polymer cover layer <NUM> could be achieved though different combinations of adjusting the amounts of second softening polymers dispensed and/or adjusting the speed or duration of spinning.

In some embodiments, to facilitate providing a mechanical neutral plane <NUM> substantially at the surface (e.g., at surface <NUM> or <NUM>) of the softening polymer layer <NUM>, the second softening polymer cover layer <NUM> can be configured to have a substantially same stiffness as a stiffness of the softening polymer layer <NUM>. For instance, in some embodiments the second softening polymer cover layer <NUM> selected to have a Young's modulus that is substantially equal to (e.g., at least within about ±<NUM> percent, or, about ±<NUM> percent, in some embodiments) a Young's Modulus of the softening polymer layer <NUM>. In some embodiments the Young's modulus is substantially the same over a range of target operating temperatures of the device <NUM> (e.g., about <NUM>° to <NUM>° or about <NUM>° to <NUM> for some embodiments).

In some embodiments, to facilitate having a substantially same stiffness as the stiffness of the softening polymer layer <NUM>, the second softening polymer cover layer <NUM> can be composed of the same softening polymer that the softening polymer layer <NUM> is composed of.

In some embodiments, to facilitate having a substantially same stiffness as the stiffness of the softening polymer layer <NUM>, the second softening polymer cover layer <NUM> can have a substantially same thickness <NUM> (e.g., within about ±<NUM> percent, or about ± <NUM> percent) as the softening polymer layer's <NUM> thickness (e.g., thickness <NUM> or <NUM>). For example, in some embodiments, when the softening polymer layer's <NUM> thickness e.g., thickness <NUM> or <NUM>) equals <NUM> (microns) or <NUM> (microns) then the second softening polymer cover layer's thickness <NUM> can equal about <NUM> ± <NUM> (microns) and <NUM>±<NUM> (microns), respectively.

In some embodiments, to mitigate electrode layer delamination, the stiffness of the second softening polymer cover layer <NUM> can be adjusted so that the mechanical neutral plane <NUM> is located above the surface (e.g., above surface <NUM> or <NUM>) of the softening polymer layer <NUM>. For example, the stiffness of the second softening polymer cover layer <NUM> can be adjusted such that the mechanical neutral plane <NUM> is located above the surface (e.g., above surface <NUM> or <NUM>) of the softening polymer layer <NUM> and at a surface <NUM> of the thin film electrode trace <NUM> or at a surface <NUM> of the optional electrode interaction pad <NUM>.

Adjusting the stiffness of the second softening polymer cover layer <NUM> can including adjusting one or both of the composition or thickness <NUM> of the layer <NUM>.

For example, consider a device <NUM> embodiment where the softening polymer layer <NUM> is composed of SMP6 has a Young's modulus at <NUM> equal to about <NUM> GPA when dry, the layer <NUM> has a thickness (e.g., thickness <NUM> or <NUM>) of <NUM> (microns) and the thin film electrode trace <NUM> composed of gold having a thickness of <NUM> (microns) and the electrode interaction pad <NUM> composed of sputtered iridium oxide has a thickness of <NUM> (microns).

To provide a mechanical neutral plane <NUM> at the surface <NUM> of the thin film electrode trace <NUM>, the second softening polymer cover layer <NUM> can be adjusted so as to have a thickness of about <NUM> (microns).

To provide a mechanical neutral plane <NUM> at the surface <NUM> of the optional electrode interaction pad <NUM>, the second softening polymer cover layer <NUM> can be adjusted so as to have a thickness of about <NUM> (microns).

Alternatively, the neutral plane <NUM> can be adjusted by changing the monomer composition so as to change the Youngs modulus of the resulting underlying softening polymer <NUM>. For example, for the above example, changing the composition from TATATO/TMICN/TCMDA (<NUM>/<NUM>/<NUM>) to (<NUM>/<NUM>/<NUM>), can shift the neutral plane <NUM> to the surface <NUM> and by further reducing the amount of TCMDA to <NUM>%,(<NUM>/<NUM>/<NUM>), the neutral plane <NUM> can be shifted to surface <NUM> under biological implantation conditions (e.g., about <NUM> in a wet aqueous environment). For example, as disclosed in Ware et al. (<NUM>), the polymer formed from TATATO/TMICN/TCMDA (<NUM>/<NUM>/<NUM>), TATATO/TMICN/TCMDA (<NUM>/<NUM>/<NUM>) or TATATO/TMICN/TCMDA (<NUM>/<NUM>/<NUM>) can have a Storage Modulus (G', related to Young's Modulus by Poisson's ratio as understood by those skilled in the pertinent art) of about <NUM> MPa in a dry state at <NUM>, and, a G' in a range from about <NUM> MPa (<NUM>/<NUM>/<NUM>) to about <NUM> MPa (<NUM>/<NUM>/<NUM>) at <NUM>.

<FIG> shows the device <NUM> after forming an inorganic hardmask layer <NUM> on the second softening polymer cover layer <NUM>. The hardmask layer <NUM> can be composed of any of the materials and can be formed by any of the processes to form the inorganic liftoff layer <NUM> such as disclosed elsewhere herein in the context of <FIG>. The hardmask layer <NUM> can have any of the thicknesses as disclosed elsewhere herein in the context of <FIG>.

<FIG> also shows the device <NUM>, after forming an organic photoresist layer <NUM> on the inorganic hardmask layer <NUM> and after patterning the organic photoresist layer <NUM> to form an opening <NUM> therein, to thereby expose a portion <NUM> of the inorganic hardmask layer <NUM>, the opening <NUM> laying directly over the electrode trace <NUM> or over the optional electrode interaction pad <NUM> or over a target electrode pad site portion <NUM> of the electrode trace <NUM> (see e.g., <FIG>). Any of the same photoresist materials and photolithography procedures, such as described elsewhere herein in the context of <FIG>, can be used to deposit and pattern the photoresist layer <NUM>.

<FIG> shows the device <NUM>, after patterning the inorganic hardmask layer <NUM> by removing the exposed portion <NUM> (<FIG>) of the inorganic hardmask layer <NUM> to form an opening <NUM> in the inorganic hardmask layer <NUM> to thereby expose a portion <NUM> of the second softening polymer cover layer <NUM>.

Removing the exposed portion <NUM> of the inorganic hardmask layer <NUM> can include any of the procedures as disclosed elsewhere herein in the context of <FIG>.

<FIG> shows the device <NUM>, after removing the exposed portion <NUM> the second softening polymer cover layer <NUM> to form an opening <NUM> therein to expose a surface <NUM> of the electrode interaction pad <NUM> (or an analogous surface of the electrode trace <NUM>, or of a target electrode pad site portion <NUM> of the electrode trace <NUM>).

In some embodiments, removing the exposed portion <NUM> of the second softening polymer cover layer <NUM> includes a deep reactive ion etch process.

For example, when the second softening polymer cover layer <NUM> is composed of an about <NUM> (micron) thick layer of SMP6, one embodiment of the deep reactive ion etch process can include plasma formed from a feed gas of oxygen at about <NUM> W biasing power and a pressure of about <NUM> mTorr for about <NUM> hours under a base pressure of about <NUM> mtorr. For example, when the second softening polymer cover layer <NUM> is composed of an about <NUM> (micron) thick layer of SMP6, one embodiment of the deep reactive ion etch process can include plasma formed from a feed gas of oxygen at <NUM> W biasing power and a pressure of about <NUM> mTorr for about <NUM> hours under a base pressure of about <NUM> mtorr.

Based on these examples one skilled in the pertinent art would understand how to adjust the conditions and parameter ranges of the deep reactive ion etch process to form openings in any of the disclosed embodiments of softening polymer layers.

As further illustrated in <FIG>, in some embodiments, the patterning, removing and deep etching procedures can be used to form one or more through-holes in the device <NUM>.

For example, patterning the organic photoresist layer <NUM> can further include forming a second opening <NUM> therein to thereby expose a second portion <NUM> of the inorganic hardmask layer <NUM> that does not directly overlay the electrode trace <NUM> or the optional electrode interaction pad <NUM> or the target electrode pad site portion <NUM>. For instance, the second opening <NUM> may be located remotely from the electrode trace <NUM>, e.g., near a perimeter <NUM> of the softening polymer layers <NUM>, <NUM>.

For example, removing a portion of the inorganic hardmask layer <NUM> can further include removing the second portion <NUM> of the inorganic hardmask layer <NUM> to form an opening <NUM> therein to thereby expose a second portion <NUM> of the second softening polymer cover layer <NUM>.

For example in some embodiments, the deep etch procedure can include removing the exposed second portion <NUM> of the second softening polymer cover layer <NUM> to form an opening <NUM> therein to thereby expose an underlying portion <NUM> of the softening polymer layer <NUM> which is also removed by the deep etch to form an opening therein <NUM>. The openings <NUM>, <NUM> together form a through-hole <NUM> in the double softening polymer layers <NUM>, <NUM> of the device <NUM>.

In some embodiments, the through-holes <NUM> are used to facilitate anchoring or fixing the device <NUM> to a structure present in the target environment of the device <NUM>. For example, in some embodiments, a suture material can be passed through one or more such through-holes <NUM> of the device <NUM> and then the suture material can be tied to a tissue structure that the device is implanted inside of, or, located on.

<FIG> shows the device <NUM>, after removing the inorganic hardmask layer <NUM> and the overlaying photoresist layer <NUM>.

The hardmask layer <NUM> and the overlaying photoresist layer <NUM> can be removed by any of the procedures as disclosed elsewhere herein in the context of <FIG>.

<FIG> illustrates another embodiment of a flexible electrical device <NUM> of the disclosure, e.g., formed by the processes disclosed elsewhere herein in the context of <FIG>. The device <NUM> comprises a softening polymer layer <NUM> and one or more electrode layers (e.g., electrode trace <NUM> and optional electrode interaction pad <NUM>) on the softening polymer layer <NUM>. The device <NUM> further comprises a cover layer <NUM> composed of a second softening polymer. An opening <NUM> in the polymer cover layer <NUM> exposes an outer surface <NUM> of the electrode layer. A mechanical neutral plane <NUM> of the device <NUM> is located substantially at or above the surface <NUM> of the softening polymer layer <NUM>.

Forming electrical contacts on flexible electrical devices that comprise softening polymer layers poses a number of problems. Solder reflow processes can oxidize and destroy the softening polymer layer or can delaminate from thin film electrode traces on the softening polymer layer due to poor adhesion. Elevated temperatures during a solder reflow can cause the softening polymer layer to soften and thereby not provide a solid base to electrical contact connection. Atoms of conventional solders can migrate into the electrode trace layer during solder reflow and thereby undesirably reduce the electrical conductivity of the electrode trace layer. Conventional approaches to prevent such atomic migration and poor adhesion include forming a nickel adhesion and barrier layer between the solder contact and the contact pad site on the electrode trace. However, heat can stress nickel layers formed on electrode trace layers thereby causing the nickel layer to curl up and delaminate from the electrode trace layer. Additionally, the use of lead-based solder and nickel adhesion and barrier layers in flexible electrical devices that can be implanted in living tissue can be undesirable due to the bio-toxicity of such metals.

To address these problems, we have developed a solder reflow process for forming solder contacts for electrical connector electrodes coupled to thin film electrode traces on a softening polymer layer. Embodiments of the solder contact can be formed from a low melting point lead-free solder paste and a nickel adhesion and barrier layer is not required. As further disclosed below, the solder reflow cycle has optimized timing for preheating, soaking, reflow and cooling to facilitate attachment coupling an electrical connector electrode to an electrode trace on the softening polymer layer while minimizing damage to the softening polymer layer.

<FIG> show an example flexible electrical device <NUM> of the disclosure at intermediate stages of an example manufacturing process of the disclosure using the solder reflow process disclosed herein.

<FIG> shows the device <NUM> after forming one or more thin film electrode traces <NUM> on a softening polymer layer <NUM>, and, forming a polymer cover layer <NUM> on the softening polymer layer <NUM> and electrode traces <NUM>. Patterned openings <NUM> in the polymer cover layer <NUM> expose contact pad site portions <NUM> of the electrode traces <NUM>.

The softening polymer layer <NUM>, polymer cover layer <NUM> and thin film trace electrodes can be composed of any of the materials, have any of the physical properties (e.g., thickness, roughness, carbon-free surface) and be formed by any of the procedures such as disclosed in the context of <FIG>, <FIG> and <FIG>.

In some embodiments, the softening polymer layer <NUM> and/or the polymer cover layer <NUM> can be formed from the polymerization of monomers that includes at least one type of thiol-functionalized monomer. It is thought that such a polymer layer, when formed on certain electrode materials such as gold, e.g., by the transfer-by-polymerization process, can have improved adhesion to the subsequently patterned thin film electrode traces due to covalent bonding between the thiols and the electrode material. This in turn, is thought to mitigate against delamination of the electrode trace layer <NUM> from the softening polymer layer <NUM> and/or the polymer cover layer <NUM> during the solder reflow process.

<FIG> shows the device <NUM> after placing a stencil mask <NUM> on or over the polymer cover layer <NUM> such that each aperture <NUM> in the stencil mask <NUM> align with one of the openings <NUM> in the polymer cover layer <NUM> that expose a contact pad site portion <NUM>. The stencil mask <NUM> can be made of any conventional stencil material such as stainless steel or other metal materials familiar to those skilled in the pertinent art.

<FIG> also show the device <NUM> after depositing a solder paste <NUM>, e.g., a lead free solder paste, on the stencil mask <NUM> and locating a stencil printer <NUM> adjacent to the solder paste <NUM>. The term lead free solder as used herein refers to solder having less than about <NUM>% lead.

In some embodiments, the lead free solder paste <NUM> can be composed of an indium silver solder. The use of indium-silver-solder has an advantage of eliminating the need for a nickel adhesion and barrier layer because, under the printing and reflow conditions used herein, the indium and silver atoms do not readily migrate into the thin film trace layer <NUM>.

In some embodiments, the indium silver solder has a mole ratio of indium: silver equal to about <NUM>:<NUM>. In some embodiments, the indium silver solder has a melting point of about <NUM>. In other embodiments, the lead-free solder paste <NUM> can include alloys of indium, silver, gold, tin and bismuth.

Those skilled in the pertinent arts would be familiar with how, as part of a stencil printing process, the stencil printer <NUM> could be configured to move the solder paste <NUM> over the stencil mask <NUM> (e.g., along direction <NUM>) so that the solder paste is transferred through the apertures <NUM> and into the openings <NUM>, to thereby transfer the pattern of the stencil mask <NUM> onto the polymer cover layer <NUM> of the device <NUM>.

Those skilled in the pertinent arts would be familiar with stencil printing parameters, e.g., the stencil printer pressure and printing speed and temperature to accomplish stencil printing.

<FIG> shows the device <NUM> after completing the stencil printing process to provide a layer <NUM> of the solder paste that fills the apertures <NUM> and openings <NUM> and contacting the exposed contact pad site portions <NUM> of the electrode traces <NUM>.

<FIG> shows the device <NUM> after removing the stencil mask <NUM> to form discrete solder contacts <NUM> located on the exposed contact pad site portions <NUM> of the electrode traces <NUM>. As illustrated, for some embodiments, to facilitate subsequent contact to an electrical connector electrode, a top surface <NUM> of the solder contact <NUM> is located above an outer surface <NUM> of the polymer cover layer <NUM>.

<FIG> shows the device <NUM> after aligning electrical connector electrodes <NUM> with each of the solder contacts <NUM> such that each one of the connector electrodes <NUM> lays on the top surface <NUM> of one of the solder contacts <NUM>.

In some embodiments the electrical connector electrodes <NUM> are composed of gold film having a thickness of about <NUM> to <NUM>. In some embodiments, the electrical connector electrodes <NUM> are aligned manually with the solder contacts <NUM> with the aid of a microscope and micro-manipulators. In some embodiments, alignment is aided using thin adhesive film such as PDMS or tape adhered to connector electrodes <NUM>.

<FIG> shows the device <NUM> after a solder reflow process to couple the electrical connector electrodes <NUM> to the contact pad site portions <NUM> of the electrode traces <NUM> via the reflowed solder <NUM>.

In some embodiments the solder reflow process includes a reflow cycle conducted in a solder reflow oven. In some embodiments the solder reflow cycle includes a pre-heat phase, to adjust the device <NUM> including the solder contacts <NUM> and the electrical connector electrodes <NUM> (and some embodiments a recording or stimulating control device coupled to the electrical connector electrodes <NUM>) to a desired temperature for subsequent steps and/or volatilize and outgas any solvent present in the solder. Some embodiments of the pre-heat phase includes increasing the temperature by about <NUM>/s until the device <NUM> including the solder contacts <NUM> and the electrical connector electrodes <NUM> reach a temperature of about <NUM>.

In some embodiments, the solder reflow cycle includes a soaking phase during which time any solvent in the solder past continues to outgas and flux present in the solder is activated. Some embodiments of the soaking phase includes increasing the temperature by about <NUM>/s until the solder contacts <NUM> and the electrical connector electrodes <NUM> reach a temperature of about <NUM> at which point solder paste volatiles are removed and fluxes are activated.

In some embodiments, the solder reflow cycle includes a reflow phase where the maximum allowable temperature of the entire solder reflow cycle is reached. During this cycle the activated flux reduces surface tension at the juncture of the metals to accomplish metallurgical bonding, facilitating individual solder powder spheres to enter a liquid state and combine. Some embodiments of the reflow phase includes increasing the temperature by about <NUM>/s until the device <NUM> including the solder contacts <NUM> and the electrical connector electrodes <NUM> reach a temperature of about <NUM>.

In some embodiments, the solder reflow cycle includes a cooling phase to gradually cool the device <NUM> and solidify the solder joints formed between the solder contacts <NUM> and the electrical connector electrodes <NUM> and in some embodiments, cool any recording or stimulating control device coupled to the electrical connector electrodes <NUM>. Gradual cooling help prevent excess intermetallic formation or thermal shock to the components. Some embodiments of the cooling phase includes decreasing the temperature at a rate of <NUM>/s until rate of decrease reaches <NUM>/s and until the device <NUM> including the joined solder contacts <NUM> and the electrical connector electrodes <NUM> is at room temperature.

Based on the above examples, one skilled in the pertinent art would understand how to adjust these various phases of the solder reflow cycle, e.g., to accommodate various compositions of the solder contacts <NUM> or electrical connector electrodes <NUM>.

<FIG> illustrates another embodiment of a flexible electronic device <NUM>, e.g., formed by any embodiments of the processes disclosed in the context of <FIG>.

The device <NUM> comprises a softening polymer layer <NUM> and a thin film electrode trace <NUM> on the softening polymer layer <NUM>. The device <NUM> further comprises a cover polymer layer <NUM>. An opening <NUM> in the polymer cover layer <NUM> is filled with a reflowed lead-free solder <NUM>. One end <NUM> of the reflowed lead-free solder <NUM> inside the opening <NUM> contacts a contact pad site portion <NUM> of the electrode trace <NUM>. Another end <NUM> of the reflowed lead-free solder <NUM> contacts the electrical connector electrode <NUM> of the device <NUM>.

In some embodiments, the electrical connector electrode <NUM> can be electrically connected to recording or stimulating control devices such as an integrated circuit (e.g. an application specific integrated circuit), a nano-Omnetics connector, a wireless chip or a metal wire connected these or other devices.

Claim 1:
A method of manufacturing an electrical device, comprising:
forming a softening polymer layer with an electrode layer on a surface of the softening polymer layer;
forming a second softening polymer cover layer on the surface of the softening polymer layer, wherein:
a mechanical neutral plane of the electrical device is located at or above the surface of the softening polymer layer, and
an opening in the second softening polymer cover layer exposes a portion of a surface of the electrode layer, and
wherein the softening polymer layer and the second softening polymer cover layer are composed of a softening polymer having a Young's modulus that decreases by more than <NUM> order of magnitude from <NUM>° C to <NUM>° C.