Extremely stretchable electronics

[0001] This application claims the benefit of U.S. Provisional Application No. 61/113,622 entitled “Extremely Stretchable Interconnects” filed on Nov. 12, 2008, the entirety of which is incorporated herein by reference. Also, this application is a continuation-in-part of, and claims the benefit of U.S. Non-Provisional application Ser. No. 12/575,008, entitled “Catheter Balloon Having Stretchable Integrated Circuitry and Sensor Array” filed on Oct. 7, 2009, the entirety of which is incorporated herein by reference. Application Ser. No. 12/575,008 claimed the priority of U.S. Provisional Application Nos. 61/103,361, filed Oct. 7, 2008 and 61/113,007, filed Nov. 10, 2008 the entirety of each of which is incorporated herein by reference.

[0002] The present invention relates to systems, apparatuses, and methods utilizing expandable or stretchable integrated circuitry, and more particularly to extremely stretchable integrated circuitry.

[0003] The field of stretchable electronics continues to grow due to the demand of high performance and mechanically unconstrained applications of the future. However, stretchable electronics have been thus far limited in stretchability. This has limited the ability of stretchable electronics to accommodate applications that require more extreme stretchability. Therefore a need exists for extremely stretchable electronics.

[0004] This invention is for extremely stretchable electrical interconnects and methods of making the same. In embodiments, the invention comprises a method of making stretchable electronics, which in some embodiments can be out of high quality single crystal semiconductor materials or other semiconductor materials, that are typically rigid. For example, single crystal semiconductor materials are brittle and cannot typically withstand strains of greater than about +/−2%. This invention describes a method of electronics that are capable of stretching and compressing while withstanding high translational strains, such as in the range of −100,000% to +100,000%, and/or high rotational strains, such as to an extent greater than 180°, while maintaining electrical performance found in their unstrained state.

[0005] In embodiments, the stretching and compressing may be accomplished by fabricating integrated circuits (ICs) out of thin membrane single crystal semiconductors, which are formed into “islands” that are mechanically and electrically connected by “interconnects,” and transferring said ICs onto an elastomeric substrate capable of stretching and compressing. The islands are regions of non-stretchable/compressible ICs, while the interconnects are regions of material formed in a way to be highly stretchable/compressible. The underlying elastomeric substrate is much more compliant than the islands, so that minimal strain is transferred into the islands while the majority of the strain is transferred to the interconnects, which only contain electrical connections and not ICs. Each interconnect attaches one island to another island, and is capable of accommodating strain between the two aforementioned islands, including translation, rotation, or a combination of translation with rotation of one island relative to another. Even though the interconnects may be made of a rigid material, they act like weak springs rather than rigid plates or beams. This configuration thereby allows for the making of extremely stretchable electronics.

[0006] These and other systems, methods, objects, features, and advantages of the present invention will be apparent to those skilled in the art from the following detailed description of the preferred embodiment and the drawings. All documents mentioned herein are hereby incorporated in their entirety by reference.

[0017] While the invention has been described in connection with certain preferred embodiments, other embodiments would be understood by one of ordinary skill in the art and are encompassed herein.

[0018] All documents referenced herein are hereby incorporated by reference.

[0019] The present invention accomplishes extremely stretchable electronics by forming the electronics on discrete islands 102 of silicon.

[0020] With reference to the present invention, the term “stretchable”, and roots and derivations thereof, when used to modify circuitry or components thereof is meant to encompass circuitry that comprises components having soft or elastic properties capable of being made longer or wider without tearing or breaking, and it is also meant to encompass circuitry having components (whether or not the components themselves are individually stretchable as stated above) that are configured in such a way so as to accommodate and remain functional when applied to a stretchable, inflatable, or otherwise expandable surface. The term “expandable”, and roots and derivations thereof, when used to modify circuitry or components thereof is also meant to have the meaning ascribed above. Thus, “stretch” and “expand”, and all derivations thereof, may be used interchangeably when referring to the present invention.

[0021] In embodiments, the discrete islands mention above are discrete operative (in embodiments, arranged in a “device island” arrangement) and are themselves capable of performing the functionality described herein, or portions thereof. In embodiments, such functionality of the operative devices can include integrated circuits, physical sensors (e.g. temperature, pH, light, radiation etc), biological and/or chemical sensors, amplifiers, A/D and D/A converters, optical collectors, electromechanical transducers, piezo-electric actuators, light emitting electronics which include LEDs, and combinations thereof. The purpose and advantage of using standard ICs (in embodiments, CMOS, on single crystal silicon) is to have and use high quality, high performance, and high functioning circuit components that are also already commonly mass-produced with well known processes, and which provide a range of functionality and generation of data far superior to that produced by a passive means.

[0022] In an example, the discrete islands 102 may range from about, but not limited to, 10-100 μm in size measured on an edge or by diameter, and connecting said islands 102A-B with one or more extremely stretchable interconnects 104. The novel geometry of the interconnects 104 is what makes them extremely compliant. Each interconnect 104 is patterned and etched so that its structural form has width and thickness dimensions that may be of comparable size (such as their ratio or inverse ratio not exceeding about a factor of 10); and may be preferably equal in size. In embodiments, the dimensions may not be greater than about Sum (e.g. where both dimensions are about 1 μm or less). The interconnect 104 may be formed in a boustrophedonic style such that it effectively comprises long bars 108 and short bars 110 as shown in FIG. 1. This unique geometry minimizes the stresses that are produced in the interconnect 104 when subsequently stretched because it has the effective form of a wire, and behaves very differently than interconnect form factors having one dimension greatly exceeding the other two (for example plates). Plate type structures primarily relieve stress only about a single axis via buckling, and withstand only a slight amount of shear stress before cracking. This invention may relieve stress about all three axes, including shears and any other stress.

[0023] In addition, because the interconnect 104 may be formed out of rigid materials, after being stretched it may have a restorative force which helps prevent its wire-like form from getting tangled or knotted when re-compressing to the unstretched state. Another advantage of the boustrophedonic geometry is that it minimizes the initial separation distance between the islands 102A-B. This is illustrated in FIG. 1. One or more interconnects 104 may be formed in various ways, as shown in FIGS. 2-4. The parts of the interconnect 104 where the majority of stresses build up during stretching may be the short linking bars. To minimize cracking here, the short linking bars 110A may be made several micrometers wider than the longer bars 108, as shown in FIG. 5.

[0024] In embodiments, the connection point of the interconnect 104 to the device island 102 may be anywhere along the device island edge, or may be at a point on the surface of the device island 102 (in which case the interconnect may be located just above the plane of the device island).

[0025] In embodiments, device islands 102 may be made on any suitable material substrate, provided that a top membrane layer of said substrate that contains the ICs can be freed from the bulk of the substrate and transfer printed onto an elastomeric substrate.

[0026] In the present invention, the interconnects 104 (as described herein) may be formed either monolithically (i.e., out of the same semiconductor material as the device islands) or may be formed out of another material. In one non-limiting example embodiment, the stretchable electronics arc fabricated on a silicon-on-insulator (SOI) wafer, having a 1 μm thick top silicon layer and a 1 μm thick buried oxide layer. Devices arc formed on the top silicon wafer, and arranged into a square pattern of islands 102A-D and interconnects 104 of the general form shown in FIG. 4, in which the islands 102 are 100 μm on an edge, and the interconnects 104 are 1 μm wide, and the space between each long bar is 1 μm, and the interconnects 104 comprise 10 long bars 108, all about 100 jam long. The islands 102 and interconnects 104 are formed in an etching step which removes the excess silicon. The islands 102 and interconnects 104 are coated with a 1 μm layer of polyimide that is patterned to only cover the islands 102 and interconnects 104. Next, the islands 102 and interconnects 104 are released in an HF etch which undercuts the underlying buried oxide. After drying, the islands 102 and interconnects 104 are transfer printed with a Polydimethylsiloxane (PDMS) stamp onto an elastomeric substrate 602. After being picked up by the transfer stamp, and prior to being placed onto the elastomeric substrate 602, the backsides of the islands 102 may be coated with a layer of polyimide (patterned to only cover the islands 102 and interconnects 104), and an additional layer of evaporated 3 nm chromium and 30 nm silicon dioxide selectively over the island regions to improve adhesion to the elastomeric substrate 602 at those locations, and not along the interconnects 102. The elastomeric substrate 602 may be PDMS or another highly compliant material. The elastomeric substrate 602 may additionally be molded or etched into the shape shown in FIG. 6A-B, to further increase selective adhesion in the device island region but not the interconnect region, and to reduce the amount of material strain in the elastomeric substrate 602 that is transferred to the device islands 102. In this example, the interconnects may accommodate stretching the device islands apart by approximately up to 800 μm. In addition, the interconnects 104 of this example may be capable of accommodating lateral shear displacements of about 800 μm. In general, they may be capable of accommodating any relative displacement of the two islands such that they remain approximately within 800 μm of each other. In addition, the interconnects 104 may accommodate corkscrew type rotations of one island relative to another about any of the three axes of rotation. This feature may be limited only by the interconnects becoming entangled within each other. In any practical application, the completed stretchable device may not be so severely rotated, and the interconnect may easily accommodate rotations of up to 180°. It is noted that by increasing the number of long bars 108 used in the interconnect 104, or by increasing the length of the long bars 108, the interconnect may be able to accommodate even larger displacement strains. In embodiments, there may be no practical upper limit to the amount of displacement enabled through the present invention.

[0027] In another embodiment the elastomeric substrate 602 may comprise two layers separated by a height. The top “contact” layer contacts the device island 102 as in the embodiment illustrated in FIG. 6. In addition, FIG. 7 shows the bottom layer 702 may be a “wavy” layer containing ripples or square waves molded into the substrate 602 during elastomer fabrication. These square waves enable additional stretching, whose extent depends on the amplitude and wavelength of the waves pattern-molded in the elastomer 602. FIG. 7 shows one non-limiting layout and topology of an elastomeric substrate 602 relative to the position of the interconnects 104 and device islands 102A-B. In an example, a two layer molded substrate can be fabricated using two step process consisting of two types of negative photoresist (SU-8 50 and SU-8 2002; Microchem Corporation). The negative resists can be spin-coated on a transfer silicon wafer with spin speeds of 3000 rpm. The SU-8 50 layer can be spun on the wafer, and subsequently cured with UV radiation. Once the SU-8 50 layer has hardened, the SU-8 2002 can be spun and cured with a photo-mask and an alignment tool. In this example, the thickness of the SU-8 50 and SU-8 2002 are 40-50 μm 708 and 2-10 μm 704, respectively. The 40-50 μm thick regions of SU-8 50 contain ripples 702 of SU-8 2002 (in this instance in the form of square waves) on their surfaces. Upon curing of the SU-8 2002 layer, liquid PDMS can be poured over the SU-8 patterns to form a substrate in the shape of the SU-8 molds 802, as shown in FIG. 8. The amplitude of the ripples in the SU-8 mold 802 can be varied by changing the spin speed used for spinning the thin layer of SU-8 2002. In this configuration, the interconnects 104 are free-standing. The entire substrate-device configuration can be immersed in non-cured elastomer (fluid layer) layer followed by a cured layer of PDMS to encapsulate the fluid and devices.

[0028] In another embodiment, the PDMS in the lower layer may be designed with periodic sinusoidal ripples 702B. In embodiments, this ripple configuration may be achieved by bonding Si nanoribbons on the surface of pre-strained PDMS in a uniform parallel pattern. The release of the prestrain in the PDMS substrate generates sinusoidal waves along the thin Si-nanoribbons (caused by buckling) and the surface of the PDMS substrate. The amplitude and wavelength of these waves 702B may depend on the extent of uniaxial pre-strain exerted on the PDMS and on the mechanical properties of the Si-nanoribbons. The wavy surface on the PDMS may be used as a transfer mold. Two-part liquid plastic solution can be poured over the wavy PDMS substrate and cured at room temperature over time (−2 hrs). Once the plastic hardens, the plastic substrate can be peeled away from the PDMS. This new plastic transfer substrate with wavy surface features can be used to produce more PDMS substrates containing wave features. The wavy PDMS may serve as the lower layer of PDMS as in the previous embodiment. To produce a two layer PDMS structure, a top layer of PDMS can be plasma bonded to this lower layer of PDMS using oxygen plasma surface activation to produce the substrate illustrated in FIG. 9.

[0029] In another embodiment, the PDMS transfer stamp is stretched after the islands 102A-B and interconnects 104 are picked up. A subsequent transfer to another elastomeric substrate 602 may place these pre-stretched devices in a configuration, which allows the new elastomeric substrate to undergo compression. The devices may be able to accommodate that compression because the interconnects are pre-stretched.

[0030] In another embodiment, the interconnects 104 are not made out of the same material as the device islands 102. In this case, the islands 102A-B are completely isolated from each other by etching, with no interconnects in between. In an example, a layer of polyimide may then be deposited, contact vias etched to various locations on the surface of the device island 102, and then metal interconnects 104 deposited and patterned into a boustrophedonic pattern, followed by another layer of polyimide. Both layers of polyimide may now be patterned and etched to leave a small border around the interconnects 104 (thereby fully encapsulating the interconnects). These interconnects may have the advantage that they arc already fully encapsulated in polyimide and will not adhere as well to the elastomeric substrate as the device islands will. The other advantage is that these interconnects may not be limited to only connecting along the edge of an island. The contact via may be etched anywhere on the surface of the island 102, including near the center. This may allow for easier connections to devices, more connections than possible only along an edge, increased strain compliance, decreased strain at the contact vias, and multiple layers of interconnects made with polymer passivation layers in between, allowing even more interconnects, or allowing one device island 102A to connect to a non-neighboring device island 102B.

[0031] In another embodiment of the invention, the device islands 102 are fabricated and transfer printed onto the elastomeric substrate 602, or substrate comprising a polymeric release layer and polymeric non-release layer. After transfer printing, the interconnects 104 are formed as described above, which may be possible because they do not require any high temperature processing, and then in the latter case, the release layer is etched and the devices that are on the non-release layer, are transfer printed onto another elastomeric substrate 602. In the former case, the islands 102 may be transferred onto the elastomeric substrate using pick and place technology so that islands 102 that are initially fabricated very close to each other are spread apart when they are transfer printed. This allows the interconnects 104 to be fabricated in a pattern that resembles their stretched configuration (if desired), to allow compression.

[0032] In embodiments, the present invention may comprise a stretchable electrical interconnect 104, including an electrical interconnect 104 for connecting two electrical contacts 102A-B (e.g. device islands 102A-B), where the electrical interconnect 104 may be arranged boustrophedonicially to define rungs 108 (i.e. long bars 108) between the contacts 102A-B, and where the rungs 108 may be substantially parallel with one another and where a plurality of rungs 108 may have substantially the same length and displacement therebetween. In addition, the ratio of the length of the plurality of rungs 108 and the displacement between the plurality of rungs 108 may be large, such as at least 10:1, 100:1, 1000:1, and the like. The electrical integrity of the electrical interconnect 104 may be maintained as stretched, such as to displacements that are increased to 1000%, 10000%, 100000%, and the like during stretching. In embodiments, the rungs 108 may be substantially perpendicular to the contacts 102A-B, the interconnection 104 may have a trace width and/or inter-rung spacing ranging between 0.1-10 microns. In embodiments, the two electrical contacts 102A-B may be located on an elastomeric substrate 602, the electrical contacts 102A-B may be bonded to the substrate 602 and the interconnection 104 not bonded to the substrate 602, the electrical contacts 102A-B may be semiconductor circuits, metal contacts, and the like.

[0033] In embodiments, the present invention may comprise a stretchable electrical interconnect 104, including an electrical interconnect 104 for connecting two electrical contacts 102A-B, where the electrical interconnect 104 is arranged boustrophedonicially to define rungs 108 between the contacts 102A-B, and where the interconnect 104 maintains electrical conductivity and electrical integrity when a displacement between the contacts 102A-B is increased, such as by 1000%, 10000%, 100000%, and the like.

[0034] In embodiments, the present invention may electrically interconnect two electrical contacts 102A-B with a stretchable interconnection 104 that has the ability to twist between the two electrical contacts 102A-B by up to approximately 180 degrees while maintaining electrical integrity of the stretchable interconnection 104.

[0035] In embodiments, the present invention may be a device including a body having a stretchable surface (e.g. an elastomeric substrate 602), and a stretchable electronic circuit including (i) a first discrete operative device 102A, (ii) a second discrete operative device 102B, and (iii) a stretchable interconnect 104 connecting the first discrete operative device 102A to the second discrete operative device 102B, where the interconnect 104 may have a substantially boustrophedonic pattern and be able to maintain electrical conductivity when stretched, such as up to 1000%, 10000%, 100000%, and the like. The stretchable electronic circuit may be affixed to the stretchable surface of the body. In embodiments, the connection may be to a metal contact, to a semiconductor device, and the like. The first discrete operative device 102A, the second discrete operative device 102B, and the stretchable interconnect 104 may all be made from the same material, and that material may be a semiconductor material.

[0036] In embodiments, the present invention may attach at least two isolated electronic components (which in embodiments may be discrete operative devices) 102A-B to an elastomeric substrate 602, and arrange an electrical interconnection 104 between the components 102A-B in a boustrophedonic pattern interconnecting the two isolated electronic components 102A-B with the electrical interconnection 104. The elastomeric substrate 602 may then be stretched such that components 102A-B separate relative to one another, where the electrical interconnection 104 maintains substantially identical electrical performance characteristics that the electrical interconnection 104 had in a pre-stretched form. In embodiments, the stretching may be a translational stretching, where the separation between the isolated electronic components 102A-B increases by a percent as a result of the stretching, such as 10%, 100%, 1000%, 10000%, 100000%, and the like. The stretching may be a rotational stretching, where the rotation may be greater than a certain rotation angle, such as 90°, 180°, 270°, 360°, and the like, where the stretching may be in all three axes. In embodiments, the electrical interconnection 104 may be made from semiconductive material. The electrical interconnection 104 may be made from the same semiconductor material as the isolated electronic components 102A-B, fabricated at the same time as the isolated electronic components 102A-B, and the like. The semiconductor material may be a single crystal semiconductor material. The electrical interconnection 104 may made of a different material than the isolated electronic components 102A-B, such as a metal. In embodiments, the interconnect material 104 may be loosely bound to the elastomeric substrate 602, not connected at all, raised above the surface of the elastomeric substrate 602, and the like. In embodiments, the at least two isolated semiconductor circuits may be fabricated on an upper surface 604 of the elastomeric substrate 602 separated by a lower surface 608 of the elastomeric substrate 602, and the electrical interconnection 104 may be fabricated at the level of the upper surface 604 of the elastomeric substrate 602. In this way, the electrical interconnection 104 may have no direct contact with the lower level 608, and thereby be substantially free from adhesion to the lower level 608 during stretching. In addition, the lower surface 608 of the elastomeric substrate 602 may include a wavy form 702, where the wavy form 704 may allow the elastomeric substrate 602 to expand during stretching.

[0037] While the invention has been described in connection with certain preferred embodiments, other embodiments would be understood by one of ordinary skill in the art and are encompassed herein.

[0038] All documents referenced herein are hereby incorporated by reference.