An increasing number of technologies require integration of disparate classes of separately fabricated objects into spatially organized, functional systems. Examples of systems that rely critically on heterogeneous integration range from optoelectronic systems that integrate lasers, lenses and optical fibers with control electronics, to tools for neurological study that involve cells interfaced to arrays of inorganic sensors, to flexible circuits and actuators that combine inorganic device components with thin plastic substrates. The most significant challenges associated with realizing these types of systems derive from the disparate nature of the materials and the often vastly different techniques needed to process them into devices. As a result, all broadly useful integration strategies begin with independent fabrication of components followed by assembly onto a device substrate.
As one example of an integration strategy, Laser Direct-Write (LDW) processing techniques have been succinctly categorized by Arnold and Pique [1]. Some of the present methods fall within the LDW category referred to as Laser Direct-Write Addition (or LDW+) and, more specifically, Laser-Induced Forward Transfer (LIFT) or Laser-Driven Release. This type of a transfer process was first reported by Bohandy et al [2]. LIFT-type processes have been used, for example, to assemble or print fabricated microstructures, and Holmes and Saidam [3], calling the approach Laser-Driven Release, used it for batch assembly in microelectromechanical system (MEMS) fabrication.
Most LDW processes involve ablation of a sacrificial layer that holds an object to a transfer surface. During transfer, the sacrificial layer is vaporized to form a gas that expels the object from the transfer surface to a receiving substrate. However, these processes suffer from time- and material-related expenses resulting from the necessity of forming and then destroying the sacrificial layer. They also risk contamination of the final product due to the ubiquitous presence of the ablated sacrificial material.
A number of patent and non-patent documents describe methods and systems for transfer printing, including U.S. Pat. Pub. No. 2009/0217517; U.S. Pat. Nos. 7,998,528; 7,932,123; and U.S. Pat. No. 7,622,367; Holmes et al., “Sacrificial layer process with laser-driven release for batch assembly operations,” J. MEMS, 7(4), 416-422, (1998); and Germain et al., “Electrodes for microfluidic devices produced by laser induced forward transfer,” Applied Surface Science, 253, 8328-8333, (2007), each of which is hereby incorporated by reference to the extent not inconsistent herewith.
The present invention encompasses a non-contact approach for manipulation and heterogeneous integration that uses controlled release of an object from a transfer device, or stamp, to transfer print objects from one substrate to another. Upon actuation of a transfer device, a physical force, such as a pressure change, a thermal change, an electrostatic change, and/or a mechanical change, leads to release of ink disposed on the transfer surface. The physics of the delamination process that govern this non-contact transfer and methods of printing objects with a wide range of sizes and shapes onto a variety of substrates are described.
In contrast with prior art printing processes that build devices on a receiving substrate, the present invention provides a facile, non-contact transfer printing process that transfers objects, such as prefabricated micro- and/or nano-devices, from a growth/fabrication substrate to a functional receiving substrate that is incapable of supporting device growth and/or fabrication processes. Thus, the present invention may not only be used in place of existing printing processes to fabricated devices, it may also be used in conjunction with existing printing processes for downstream transfer of devices fabricated by existing printing processes onto unique substrates.
In one embodiment, the present invention exploits a mismatched thermo-mechanical response of the prefabricated device (ink) and a transfer surface (stamp) to a force incident on the ink-stamp interface to cause delamination of the ink from the stamp and its transfer to the target/receiving substrate. This process operates at lower temperatures than ablation processes, thus avoiding damage to the functional devices. More importantly, because the transfer does not substantially damage the stamp material, the same area of the stamp can be used multiple times, enabling a pick-print-repeat cycle. This non-contact “pick-and-place” technique provides an important combination of capabilities that is not offered by other assembly methods, such as those based on ablation techniques, wafer bonding, or directed self-assembly.
Besides providing the desired mismatch in thermo-mechanical response with commonly-used semiconductor materials, stamps of the present invention make it possible to directly and selectively pick-up micro- or nano-devices from growth or donor substrates by using well-developed techniques [4-8], such as that described in U.S. Pat. No. 7,622,367, which is hereby incorporated by reference in its entirety. These techniques overcome one of the major limitations of using LIFT-type printing processes for assembling devices, i.e., the transfer of the micro- or nano-devices from the growth/fabrication substrate to the stamp [9]. The present invention therefore combines the facile elegance of transfer-printing processes in taking prefabricated devices directly from their growth substrates to functional substrates with the flexibility of non-contact LIFT processes that are relatively independent of surface properties of the receiving substrate onto which the devices are transferred. The ability to transfer the prefabricated devices enables, for example, the embedding of high-performance electronic and optoelectronic components into polymeric substrates to realize new capabilities in emerging areas such as flexible and large-area electronics, displays and photovoltaics.
The methods presented herein allow manipulation of arrays of objects based on mechanically or thermo-mechanically controllable release from a stamp in a massively parallel and deterministic manner. The mechanics suggest paths for optimizing the material properties of the stamps in ways that have not been explored in soft lithography or related areas. Even with existing materials, the printing procedure provides robust capabilities for generating microstructured hybrid materials systems and device arrays with applications in optoelectronics, photonics, non-planar fabrication and biotechnology. The non-contact, stamp-based methods of the present invention are invaluable tools for printing microelectromechanical (MEM) and nanoelectromechanical (NEM) devices.
In an aspect, a method of transferring ink from a donor substrate to a receiving substrate comprises: providing a transfer device having a transfer surface; providing the donor substrate having a donor surface, the donor surface having ink thereon; contacting at least a portion of the transfer surface with at least a portion of the ink; separating the transfer surface from the donor surface, wherein at least a portion of the ink is transferred from the donor surface to the transfer surface; positioning the transfer surface having the ink disposed thereon into alignment with a receiving surface of the receiving substrate, wherein a gap remains between the ink disposed on the transfer surface and the receiving surface; and actuating the transfer device, the ink, or both of the transfer device and the ink by generating a force that releases at least a portion of the ink from the transfer surface while maintaining at least a portion of said gap, thereby transferring at least a portion of the ink to the receiving surface.
In a method of the invention, for example, the transfer device does not make physical contact with the receiving surface during the entire process resulting in the transfer of the ink to the receiving surface. In a method of the invention, for example, the ink does not make physical contact with the receiving surface while it is disposed on the transfer surface of the transfer device. In a method of the invention, for example, the ink is transferred to the receiving surface by a process not including contact printing, such as dry transfer contact printing. In an embodiment, the gap is at least partially maintained during the entire process. The invention includes methods wherein at least 50% of the gap is maintained during the entire process, and optionally for some applications at least 90% of the gap is maintained during the entire process.
The force applied to the transfer surface generates a mechanical or thermomechanical response. For example, in one embodiment, the step of actuating comprises mechanically actuating, optically actuating, electrically actuating, magnetically actuating, thermally actuating, or a combination thereof. In one embodiment, the step of actuating comprises mechanically stressing an interface between the transfer surface and the ink so as to cause delamination, thereby resulting in release of the ink. In one embodiment, the step of actuating the transfer device uses a laser, a piezoelectric actuator, a gas source, a vacuum source, an electromagnetic source, an electrostatic source, an electronic source, a heat source, or a combination thereof.
When the step of actuating uses a gas source, the gas may be selected from the group consisting of nitrogen, argon, krypton, xenon, and combinations thereof. In one embodiment, the gas source directs a flow or burst of gas onto the transfer device or the ink disposed on the transfer surface of the transfer device, thereby mechanically actuating the transfer device, the ink or both. In one embodiment, the gas source directs the flow or burst of gas through one or more channels or reservoirs in the transfer device onto the ink, thereby generating the force that releases at least a portion of the ink from the transfer surface. The gas source produces gas having a pressure selected from the range of 5 psi to 100 psi, which is, in one embodiment, produced for a period selected from the range of 1 millisecond to 10 milliseconds.
When the step of actuating uses a vacuum source, the vacuum source is provided in fluid communication with the transfer device, the ink or both such that the vacuum source produces a pressure on the transfer device, the ink or both, thereby generating the force that releases at least a portion of the ink from the transfer surface. The vacuum source produces a pressure selected from the range of 10−3 torr to 10−5 torr.
When the step of actuating uses an electromagnetic source, the electromagnetic source is provided in optical communication with the transfer device, the ink or both and provides electromagnetic radiation onto the transfer device, the ink disposed on the transfer device or both. In one embodiment, the electromagnetic source provides the electromagnetic radiation onto the transfer surface of the transfer device, the ink disposed on the transfer surface or both. The electromagnetic source may produce radiation in the radio, microwave, infrared, visible, or ultraviolet region of the electromagnetic spectrum having a wavelength selected from the range of 300 μm to 5 μm and/or a power selected from the range of 10 W to 100 W for printing inks with lateral dimensions in the range of 100 microns to 600 microns. For example, the electromagnetic radiation may be characterized by a pulse width selected over the range of 100 μs and 10 milliseconds and/or a focused beam spot having an area selected from the range of 150 μm2 to 1 mm2. In one embodiment, the electromagnetic radiation delivers less than 0.5 mJ of energy to the ink. In one embodiment, the electromagnetic radiation is spatially translated on the transfer surface of the transfer device, for example, at a rate of at least 50 mm/sec, or a rate of at least 100 mm/sec, or a rate selected from the range of 50 mm/sec to 500 mm/sec, or a range of 50 mm/sec to 250 mm/sec, or a range of 50 mm/sec to 150 mm/sec. In an embodiment, the electromagnetic radiation has a wavelength in the near infrared region of the electromagnetic spectrum selected from the range of 800 nm to 1000 nm. In an embodiment, the electromagnetic radiation is absorbed by the ink disposed on the transfer surface of the transfer device. In one embodiment, a laser delivering the electromagnetic radiation may be operated at an electric potential between 0.5 volts and 2.5 volts and/or a current selected from a range of 10 amperes to 25 amperes and/or a power less than or equal to 30 watts.
When the step of actuating uses an electrostatic source, the electrostatic source generates an applied electric field on the transfer surface, the ink disposed on the transfer surface, or both.
When the step of actuating uses a heat source, the heat source heats the transfer device, the ink, or both of the transfer device and the ink, thereby thermally actuating the transfer device, the ink, or both of the transfer device and the ink. The heat source may produce a temperature of the transfer surface selected from the range of 275° C. to 325° C. and/or may produce a temperature gradient in the transfer device selected from the range of 104° C. cm−1 to 105° C. cm−1.
When the step of actuating uses a piezoelectric actuator, the piezoelectric actuator physically contacts the transfer surface of the transfer device, thereby electrically actuating the ink.
In general, the step of actuating induces a thermomechanical force at an interface between the ink and the transfer surface resulting in delamination of the ink from the transfer surface, thereby resulting in release of the ink from the transfer surface. For example, the magnitude and spatial distribution of the force may be selected so as to generate a separation energy between ink and the transfer surface equal to or greater than 1 J/meter2. Typically, delamination begins at a corner of the ink and propagates toward a center of the ink, thereby resulting in release of the ink from the transfer surface. Delamination results, for example, when the transfer device and the ink have a ratio of coefficients of thermal expansion selected from the range of 500 to 2, or 100 to 2, or 50 to 2, or 25 to 2, or 10 to 2 and/or when the transfer device and the ink have a ratio of Young's moduli selected from the range of 10 and 100. For example, the ink may have a coefficient of thermal expansion selected from the range of 1 ppm ° C.−1 to 10 ppm ° C.−1 and the transfer device may have a coefficient of thermal expansion selected from the range of 100 ppm ° C.−1 to 500 ppm ° C.−1 and/or the ink may have a Young's modulus selected from the range of 10 GPa and 500 GPa and the transfer device may comprise at least one elastomer layer having a Young's modulus selected over the range of 1 MPa and 10 GPa. In some embodiments, the force applied to the transfer surface is a non-ablative force.
In one embodiment, the gap is characterized by a distance between the ink disposed on the transfer surface and the receiving surface equal to or greater than 1 micron, or equal to or greater than 5 microns, or greater than or equal to 10 microns, or greater than or equal to 20 microns, or greater than or equal to 30 microns, or greater than or equal to 50 microns. In theory, the gap is characterized by a distance between the ink disposed on the transfer surface and the receiving surface that is infinite. In practice, the accuracy of the process is improved when the gap is equal to or less than 50 microns, or equal to or less than 30 microns, or equal to or less than 20 microns, or equal to or less than 10 microns, or equal to or less than 5 microns, or equal to or less than 1 micron. In one embodiment, the gap is characterized by a distance between the ink disposed on the transfer surface and the receiving surface selected from the range of 1 micron to 50 microns, or selected from the range of 1 micron to 30 microns, or selected from the range of 1 micron to 20 microns, or selected from the range of 1 micron to 10 microns, or selected from the range of 1 micron to 5 microns.
The laser may be spatially translated to release ink having one or more dimensions significantly larger than the focused beam spot diameter. For example, the ink may have a length selected over the range of 100 nanometers to 1000 microns, a width selected over the range of 100 nanometers to 1000 microns and a thickness selected over the range of 1 nanometer to 1000 microns.
In one embodiment, a contact surface of the ink is provided in physical contact with the transfer device, wherein the contact surface has a surface area selected over the range of 106 nm2 to 1 mm2. The ink may, for example, be a material selected from the group consisting of a semiconductor, a metal, a dielectric, a ceramic, a polymer, a glass, a biological material or any combination of these. In one embodiment, the ink is a micro-sized or nano-sized prefabricated device or component thereof. The prefabricated device may be a printable semiconductor element, a single crystalline semiconductor structure, or a single crystalline semiconductor device. For example, the prefabricated device may have a shape selected from the group consisting of a ribbon, a disc, a platelet, a block, a column, a cylinder, and any combination thereof. The prefabricated device may comprise an electronic, optical or electro-optic device or a component of an electronic, optical or electro-optic device selected from the group consisting of: a P-N junction, a thin film transistor, a single junction solar cell, a multi-junction solar cell, a photodiode, a light emitting diode, a laser, a CMOS device, a MOSFET device, a MESFET device, a HEMT device, a photovoltaic device, a sensor, a memory device, a microelectromechanical device, a nanoelectromechanical device, a complementary logic circuit, and a wire.
In some methods, a plurality of prefabricated devices may be provided on the receiving substrate. Substantially all of the prefabricated devices may be transferred from the donor surface to the transfer surface simultaneously and substantially all of the prefabricated devices in contact with the transfer surface may be transferred to the receiving surface simultaneously or one at a time (individually).
In an aspect, at least a portion of the steps of the method of transferring ink from a donor substrate to a receiving substrate may be repeated so as to generate multi-layered ink structures on the receiving surface. For example, multi-layered ink structures may be three-dimensional and at least some of the ink may be deposited onto previously deposited ink.
In some methods of the present invention, the force applied to the transfer device, the ink, or both of the transfer device and the ink does not substantially degrade the transfer device. For example, in one embodiment, the steps may be repeated using a single transfer device between 20-25 times before substantial degradation of the transfer device is detectable.
In one embodiment, the transfer device comprises at least one elastomer layer having a thickness selected over the range of 1 micron to 1000 microns and/or a Young's Modulus selected over the range of 1 MPa to 10 GPa. The transfer device may, for example, comprise an elastomeric stamp, elastomeric mold, or elastomeric mask. In one embodiment, the transfer device comprises at least one elastomer layer operably connected to one or more polymer, glass or metal layers. In some embodiments, the transfer device is at least partially transparent to electromagnetic radiation having wavelengths in ultraviolet, visible or infrared regions of the electromagnetic spectrum. In one embodiment, the transfer device comprises a material selected from the group consisting of glass and silica. In one embodiment, the transfer device is an elastomeric transfer device. For example, the transfer device may comprise polydimethylsiloxane.
The transfer device may be substantially planar or microstructured or nanostructured. A microstructured or nanostructured transfer device comprises at least one relief feature having a surface for contacting ink. The relief feature extends, for example, at least 5 micrometers, or at least 10 micrometers, from the transfer surface. In some embodiments, the relief feature has a cross-sectional area perpendicular to a longitudinal axis of the relief feature, and the cross-sectional area has a major dimension that is less than or equal to 1000 micrometers. The transfer device may comprise a plurality of relief features forming an array and having surfaces for contacting ink. Each relief feature in the array is separated from any other relief feature in the array by a distance of 3 micrometers to 100 millimeters, or 5 micrometers to 1 millimeter, or 10 micrometers to 50 micrometers.
In one embodiment, a layer of absorbing material is encapsulated within the relief feature. The layer may be positioned between 1 micrometer and 100 micrometers, or between 1 micrometer and 10 micrometers, from a distal end of the relief feature and substantially equidistant from the surface of the relief feature. The absorbing material may be selected from the group consisting of silicon, graphite, carbon black, and any metal. Generally, surface preparations (such as nanopatterning) are used to reduce reflection losses and the absorbing material and the incident radiation should be matched to achieve the highest absorption of the incident radiation.
In one embodiment, the receiving substrate is a material selected from the group consisting of: a polymer, a semiconductor wafer, a ceramic material, a glass, a metal, paper, a dielectric material, a liquid, a biological cell, a hydrogel and any combination of these. The receiving surface may be planar, rough, charged, neutral, non-planar, or contoured because the placement accuracy of the transfer method is independent of the shape, composition and surface contour of the receiving substrate.
In some methods of the present invention, the ink adheres directly to the transfer surface. In an alternate embodiment, an absorbing material is provided between the ink and the transfer surface. The absorbing material may be applied to the ink or the transfer surface prior to the step of contacting at least a portion of the transfer surface with at least a portion of the ink, and the absorbing material may be removed after the step of applying a force to the transfer surface. In an embodiment, the absorbing material is a thermal adhesive or a photoactivated adhesive. In an embodiment, the absorbing material has a coefficient of thermal expansion selected from the range of 300 ppm ° C.−1 to 1 ppm ° C.−1, a Young's modulus selected from the range of 100 MPa to 500 GPa, a thickness selected from the range of 2 microns to 10 microns, and/or is selected from the group consisting of materials that absorb at the wavelength of irradiation, such as silicon, graphite, carbon black, metals with nanostructured surfaces, and combinations thereof.
In some methods, the steps of: contacting at least a portion of the transfer surface with at least a portion of the ink, separating the transfer surface from the donor surface, positioning the transfer surface, or any combination of these steps is carried out via an actuator operationally connected to the transfer device and/or by an actuator operationally connected to one or more xyz-positionable stages supporting donor and/or receiving substrates.
In one embodiment, the step of positioning the transfer surface having the ink disposed thereon into alignment with the receiving surface provides the transfer surface in proximity to selected regions of the receiving surface and/or provides registration between the ink and selected regions of the receiving surface. The selected regions of the receiving surface may correspond to devices or device components prepositioned on the receiving surface of the receiving substrate. Generally, the ink is transferred to the receiving surface with a placement accuracy greater than or equal to 25 microns over a receiving surface area equal to 5 cm2 and the proximity is to within 2-5 μm or less.
Without wishing to be bound by any particular theory, there may be discussion herein of beliefs or understandings of underlying principles relating to the devices and methods disclosed herein. It is recognized that regardless of the ultimate correctness of any mechanistic explanation or hypothesis, an embodiment of the invention can nonetheless be operative and useful.
In general, the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art. The following definitions are provided to clarify their specific use in the context of the invention.
“Delamination” refers to separation at an interface between substantially parallel, contacting layers when energy at the interface becomes greater than the energy of adhesion holding the layers in contact with one another.
“Ink” refers to a discrete unit of material capable of being transferred from a donor substrate to a receiving substrate. Ink may be solid, liquid or a combination thereof. “Ink” may, for example, be an atomic or molecular precursor to a device component, a device component, or a prefabricated device.
A “device” is a combination of components operably connected to produce one or more desired functions. A “prefabricated device” is a device that is fabricated on a donor substrate, but destined for a receiving substrate that is less capable than the donor substrate of supporting the fabrication process or incapable of supporting the fabrication process.
A “component” is used broadly to refer to an individual part of a device. An “interconnect” is one example of a component, and refers to an electrically conducting structure capable of establishing an electrical connection with another component or between components. Other components include, but are not limited to, thin film transistors (TFTs), transistors, electrodes, integrated circuits, circuit elements, control elements, microprocessors, transducers, islands, bridges and combinations thereof.
“Actuating” broadly refers to a process wherein a device, device component, structure, or material is acted upon, for example, so as to cause a change in one or more physical, chemical, optical or electronic properties. In an embodiment, for example, actuating comprises one or more of mechanically actuating, optically actuating, electrically actuating, electrostatically actuating, magnetically actuating, and thermally actuating. In some methods and systems of the invention, actuating involves a process in which energy is provided to, or taken away from, a device, device component, structure, or material, such as a transfer device and/or ink. In some embodiments, for example, the energy provided, or taken away, is thermal energy, mechanical energy, optical energy, electronic energy, electrostatic energy or any combination of these. In some methods and systems of the invention, actuating involves activating a transfer device and/or ink so as to generate a force that releases at least a portion of the ink from the transfer surface. In some methods and systems of the invention, actuating involves exposing a transfer device and/or ink to electromagnetic radiation, such as laser radiation, so as to generate a force that releases at least a portion of the ink from a transfer surface of the transfer device. In some methods and systems of the invention, actuating involves exposing a transfer device and/or ink to thermal energy, such as heat, so as to generate a force that releases at least a portion of the ink from a transfer surface of the transfer device. In some methods and systems of the invention, actuating involves exposing a transfer device and/or ink to an electromagnetic field, so as to generate a force that releases at least a portion of the ink from a transfer surface of the transfer device. In some methods and systems of the invention, actuating involves exposing a transfer device and/or ink to a magnetic field, so as to generate a force that releases at least a portion of the ink from a transfer surface of the transfer device. In some methods and systems of the invention, actuating involves physically contacting and/or moving a transfer device and/or ink so as to generate a force that releases at least a portion of the ink from a transfer surface of the transfer device, for example, using a piezoelectric actuator, source of a fluid (e.g., gas source) or a vacuum source. In an embodiment, for example, actuating involves a process wherein a transfer device or ink disposed on the surface of the transfer device does not physically contact the receiving surface of a substrate.
“Alignment” is used herein to refer to the relative arrangement or position of surfaces or objects. For example, the transfer surface of the transfer device and receiving surface of the receiving substrate are in alignment when a gap between the surfaces is a consistent, predetermined separation distance along a vertical axis perpendicular to the planes of the surfaces.
“Registration” is used in accordance with its meaning in the art of microfabrication. Registration refers to the precise positioning of ink, components and/or devices on a selected region of a substrate or relative to ink, components and/or devices that preexist on a substrate. For example, alignment of the transfer surface and receiving surface brings ink disposed on the transfer surface into registration with selected regions of the receiving surface. In some embodiments, the selected regions correspond to ink, devices or device components prepositioned on the receiving surface of the receiving substrate.
“Semiconductor” refers to any material that is an insulator at a very low temperature, but which has an appreciable electrical conductivity at a temperature of about 300 Kelvin. In the present description, use of the term semiconductor is intended to be consistent with use of this term in the art of microelectronics and electronic devices. Useful semiconductors include those comprising elemental semiconductors, such as silicon, germanium and diamond, and compound semiconductors, such as group IV compound semiconductors such as SiC and SiGe, group III-V semiconductors such as AlSb, AlAs, AlN, AlP, BN, BP, BAs, GaSb, GaAs, GaN, GaP, InSb, InAs, InN, and InP, group III-V ternary semiconductors alloys such as AlxGa1-xAs, group II-VI semiconductors such as CsSe, CdS, CdTe, ZnO, ZnSe, ZnS, and ZnTe, group I-VII semiconductors such as CuCl, group IV-VI semiconductors such as PbS, PbTe, and SnS, layer semiconductors such as PbI2, MoS2, and GaSe, oxide semiconductors such as CuO and Cu2O. The term semiconductor includes intrinsic semiconductors and extrinsic semiconductors that are doped with one or more selected materials, including semiconductors having p-type doping materials and n-type doping materials, to provide beneficial electronic properties useful for a given application or device. The term semiconductor includes composite materials comprising a mixture of semiconductors and/or dopants. Specific semiconductor materials useful for some embodiments include, but are not limited to, Si, Ge, Se, diamond, fullerenes, SiC, SiGe, SiO, SiO2, SiN, AlSb, AlAs, AlIn, AlN, AlP, AlS, BN, BP, BAs, As2S3, GaSb, GaAs, GaN, GaP, GaSe, InSb, InAs, InN, InP, CsSe, CdS, CdSe, CdTe, Cd3P2, Cd3As2, Cd3Sb2, ZnO, ZnSe, ZnS, ZnTe, Zn3P2, Zn3As2, Zn3Sb2, ZnSiP2, CuCl, PbS, PbSe, PbTe, FeO, FeS2, NiO, EuO, EuS, PtSi, TIBr, CrBr3, SnS, SnTe, PbI2, MoS2, GaSe, CuO, Cu2O, HgS, HgSe, HgTe, HgI2, MgS, MgSe, MgTe, CaS, CaSe, SrS, SrTe, BaS, BaSe, BaTe, SnO2, TiO, TiO2, Bi2S3, Bi2O3, Bi2Te3, BiIa, UO2, UO3, AgGaS2, PbMnTe, BaTiO3, SrTiO3, LiNbO3, La2CuO4, La0.7Ca0.3MnO3, CdZnTe, CdMnTe, CuInSe2, copper indium gallium selenide (CIGS), HgCdTe, HgZnTe, HgZnSe, PbSnTe, Tl2SnTe5, Tl2GeTe5, AlGaAs, AlGaN, AlGaP, AlInAs, AlInSb, AlInP, AlInAsP, AlGaAsN, GaAsP, GaAsN, GaMnAs, GaAsSbN, GaInAs, GaInP, AlGaAsSb, AlGaAsP, AlGaInP, GaInAsP, InGaAs, InGaP, InGaN, InAsSb, InGaSb, InMnAs, InGaAsP, InGaAsN, InAlAsN, GaInNAsSb, GaInAsSbP, and any combination of these. Porous silicon semiconductor materials are useful for aspects described herein. Impurities of semiconductor materials are atoms, elements, ions and/or molecules other than the semiconductor material(s) themselves or any dopants provided to the semiconductor material. Impurities are undesirable materials present in semiconductor materials which may negatively impact the electronic properties of semiconductor materials, and include but are not limited to oxygen, carbon, and metals including heavy metals. Heavy metal impurities include, but are not limited to, the group of elements between copper and lead on the periodic table, calcium, sodium, and all ions, compounds and/or complexes thereof.
A “semiconductor component” broadly refers to any semiconductor material, composition or structure, and expressly includes high quality single crystalline and polycrystalline semiconductors, semiconductor materials fabricated via high temperature processing, doped semiconductor materials, inorganic semiconductors, and composite semiconductor materials.
“Substrate” refers to a material, layer or other structure having a surface, such as a receiving surface, that is capable of supporting one or more components or electronic devices. A component that is “bonded” to the substrate refers to a component that is in physical contact with the substrate and unable to substantially move relative to the substrate surface to which it is bonded. Unbonded components or portions of a component, in contrast, are capable of substantial movement relative to the substrate.
“Functional layer” refers to a layer that imparts some functionality to a device. For example, a functional layer may contain semiconductor components. Alternatively, the functional layer may comprise multiple layers, such as multiple semiconductor layers separated by support layers. The functional layer may comprise a plurality of patterned elements, such as interconnects running between electrodes or islands.
“Structural layer” refers to a layer that imparts structural functionality, for example by supporting and/or encapsulating device components.
“Polymer” refers to a macromolecule composed of repeating structural units connected by covalent chemical bonds or the polymerization product of one or more monomers, often characterized by a high molecular weight. The term polymer includes homopolymers, or polymers consisting essentially of a single repeating monomer subunit. The term polymer also includes copolymers, or polymers consisting essentially of two or more monomer subunits, such as random, block, alternating, segmented, grafted, tapered and other copolymers. Useful polymers include organic polymers or inorganic polymers that may be in amorphous, semi-amorphous, crystalline or partially crystalline states. Crosslinked polymers having linked monomer chains are particularly useful for some applications. Polymers useable in the methods, devices and components described herein include, but are not limited to, plastics, elastomers, thermoplastic elastomers, elastoplastics, thermoplastics and acrylates. Exemplary polymers include, but are not limited to, acetal polymers, biodegradable polymers, cellulosic polymers, fluoropolymers, nylons, polyacrylonitrile polymers, polyamide-imide polymers, polyimides, polyarylates, polybenzimidazole, polybutylene, polycarbonate, polyesters, polyetherimide, polyethylene, polyethylene copolymers and modified polyethylenes, polyketones, poly(methyl methacrylate), polymethylpentene, polyphenylene oxides and polyphenylene sulfides, polyphthalamide, polypropylene, polyurethanes, styrenic resins, sulfone-based resins, vinyl-based resins, rubber (including natural rubber, styrene-butadiene, polybutadiene, neoprene, ethylene-propylene, butyl, nitrile, silicones), acrylic, nylon, polycarbonate, polyester, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyolefin or any combinations of these.
“Elastomeric stamp” and “elastomeric transfer device” are used interchangeably and refer to an elastomeric material having a surface that can receive as well as transfer a material. Exemplary elastomeric transfer devices include stamps, molds and masks. The transfer device affects and/or facilitates material transfer from a donor material to a receiver material. The methods of the present invention do not “substantially degrade” the elastomeric transfer device. As used herein, “substantial degradation” refers to chemical/physical decomposition or material removal occurring within at least 50 nm or within at least 100 nm of the transfer surface of the elastomeric transfer device.
“Elastomer” refers to a polymeric material which can be stretched or deformed and returned to its original shape without substantial permanent deformation. Elastomers commonly undergo substantially elastic deformations. Useful elastomers include those comprising polymers, copolymers, composite materials or mixtures of polymers and copolymers. Elastomeric layer refers to a layer comprising at least one elastomer. Elastomeric layers may also include dopants and other non-elastomeric materials. Useful elastomers include, but are not limited to, thermoplastic elastomers, styrenic materials, olefinic materials, polyolefin, polyurethane thermoplastic elastomers, polyamides, synthetic rubbers, PDMS, polybutadiene, polyisobutylene, poly(styrene-butadiene-styrene), polyurethanes, polychloroprene and silicones. In some embodiments, an elastomeric stamp comprises an elastomer. Exemplary elastomers include, but are not limited to silicon containing polymers such as polysiloxanes including poly(dimethyl siloxane) (i.e. PDMS and h-PDMS), poly(methyl siloxane), partially alkylated poly(methyl siloxane), poly(alkyl methyl siloxane) and poly(phenyl methyl siloxane), silicon modified elastomers, thermoplastic elastomers, styrenic materials, olefinic materials, polyolefin, polyurethane thermoplastic elastomers, polyamides, synthetic rubbers, polyisobutylene, poly(styrene-butadiene-styrene), polyurethanes, polychloroprene and silicones. In an embodiment, a polymer is an elastomer.
“Conformable” refers to a device, material or substrate which has a bending stiffness that is sufficiently low to allow the device, material or substrate to adopt any desired contour profile, for example a contour profile allowing for conformal contact with a surface having a pattern of relief features.
“Conformal contact” refers to contact established between two or more surfaces. In one aspect, conformal contact involves a macroscopic adaptation of one or more surfaces (e.g., contact surfaces) to the overall shape of another surface. In another aspect, conformal contact involves a microscopic adaptation of one or more surfaces (e.g., contact surfaces) to another surface resulting in an intimate contact substantially free of voids. In an embodiment, conformal contact involves adaptation of an ink surface(s) to a receiving surface(s) such that intimate contact is achieved, for example, wherein less than 20% of the surface area of an ink surface of the device does not physically contact the receiving surface, or optionally less than 10% of an ink surface of the device does not physically contact the receiving surface, or optionally less than 5% of an ink surface of the device does not physically contact the receiving surface.
“Young's modulus” is a mechanical property of a material, device or layer which refers to the ratio of stress to strain for a given substance. Young's modulus may be provided by the expression:
where E is Young's modulus, L0 is the equilibrium length, AL is the length change under the applied stress, F is the force applied, and A is the area over which the force is applied. Young's modulus may also be expressed in terms of Lame constants via the equation:
where λ and μ are Lame constants. High Young's modulus (or “high modulus”) and low Young's modulus (or “low modulus”) are relative descriptors of the magnitude of Young's modulus in a given material, layer or device. In some embodiments, a high Young's modulus is larger than a low Young's modulus, preferably about 10 times larger for some applications, more preferably about 100 times larger for other applications, and even more preferably about 1000 times larger for yet other applications. In an embodiment, a low modulus layer has a Young's modulus less than 100 MPa, optionally less than 10 MPa, and optionally a Young's modulus selected from the range of 0.1 MPa to 50 MPa. In an embodiment, a high modulus layer has a Young's modulus greater than 100 MPa, optionally greater than 10 GPa, and optionally a Young's modulus selected from the range of 1 GPa to 100 GPa.
“Inhomogeneous Young's modulus” refers to a material having a Young's modulus that spatially varies (e.g., changes with surface location). A material having an inhomogeneous Young's modulus may optionally be described in terms of a “bulk” or “average” Young's modulus for the entire material.
“Low modulus” refers to materials having a Young's modulus less than or equal to 10 MPa, less than or equal to 5 MPa or less than or equal to 1 MPa.
“Bending stiffness” is a mechanical property of a material, device or layer describing the resistance of the material, device or layer to an applied bending moment. Generally, bending stiffness is defined as the product of the modulus and area moment of inertia of the material, device or layer. A material having an inhomogeneous bending stiffness may optionally be described in terms of a “bulk” or “average” bending stiffness for the entire layer of material.
Thermomechanically driven, non-contact transfer printing devices and methods will now be described with reference to the figures and the following non-limiting examples.
In one embodiment, absorbing material 3004 forms a contiguous or non-contiguous coating or laminated layer on the surface of transfer device 3000(2), such that ink 3002(2) is in direct contact with absorbing material 3004. The absorbing material may be applied to the ink or the transfer surface prior to the step of contacting at least a portion of the transfer surface with at least a portion of the ink, and the absorbing material may be removed after the step of applying a force to the transfer surface.
In another embodiment, absorbing material 3004 is embedded within transfer device 3000(2) and disposed within 10 micrometers from the transfer surface upon which ink 3002(2) is adhered. In this embodiment, ink 3002(2) may be protected from excessive heating because the relative heating of transfer device 3000(2) and ink 3002(2) may be preselected by determining the placement, concentration and composition of absorbing material 3004. For example, to minimize heating of ink 3002(2), absorbing material 3004 may be positioned farther from the transfer surface than when greater heating of ink 3002(2) is desired.
Mietl [10] describes a transfer printing process involving both the pick-up of microstructures from a donor substrate and their deposition or ‘printing’ onto a receiving substrate using an elastomeric stamp. The present invention also starts with an elastomeric stamp made of PDMS and optionally patterned with posts, to selectively engage the desired nano- or micro-devices on the donor or inking substrate. The mechanism for inking the stamp is similar to previously described mechanisms [4-8], relying on the strong adhesive forces between PDMS and the nano- or micro-devices to extract the ink from the donor or inking substrate. For deposition, however, the inked stamp is brought close (between 3 to 10 microns) to the receiving substrate onto which the devices are to be deposited. A pulsed laser beam is focused on the interface between the stamp and the devices to release and drive the device to the receiving substrate. The wavelength of the laser is chosen so that the stamp material is transparent, while the ink is more absorbing.
To realize this process, a LNTP print head is created by using an electronically pulsed 30 W 805 nm laser diode with a minimum pulse width of 1 ms. The laser is coupled into the system through a 250 μm core optical fiber. At the end of the fiber are a 4 mm diameter collimator and a focusing lens with a 30 mm focal distance to focus the laser beam on a circular area with a diameter of approximately 400-800 μm.
The laser print head is tested by using a 2×2 mm, 1 mm thick PDMS stamp with a 200×200 μm, 100 μm tall post patterned on it. The stamp is affixed to a glass backing. For the ink, a donor substrate is fabricated using conventional fabrication processes to obtain anchored, but undercut, 100×100×3 μm square single crystal silicon chips. An automated printer is constructed by integrating a programmable, computer-controlled xyz positioning stage, with the print head, high-resolution optics and vacuum chucks for the donor and receiving substrates. As depicted in the process schematic of
A second feasibility test is conducted to demonstrate the construction of 3-dimensional assemblies using such a process. Here a 3-layer pyramid, shown in
Transfer printing of an InGaN-based μ-LED onto a CVD-grown polycrystalline diamond on silicon substrate is demonstrated in
LNTP Mechanism and Experimental Observations.
The primary phenomenon driving the LNTP process is not ablation but, instead, the mismatched thermo-mechanical responses of the stamp and the ink which cause the delamination of the ink from the stamp and its transfer to the receiving substrate. The mechanism by which the microstructure is delaminated from the stamp and transferred to the receiving substrate is described herein and high-speed photography evidence in support of this mechanism is provided.
Since a PDMS stamp is transparent in the near IR range, the laser radiation is transmitted through the stamp and is incident on the ink which absorbs some fraction of the incident laser energy and, as a result, heats up. The ink, in turn, acts as a heat source for the PDMS stamp, conducting heat across the stamp-ink interface to raise the temperature of the PDMS stamp in the vicinity of the interface. The rise of temperature in the stamp and ink leads to thermal expansions in both. This, due to the considerable difference in the coefficients of thermal expansion for the two materials (αs=310 ppm/° C. for PDMS [11] and αc=2.6 ppm/° C. for Silicon [12]) and the restriction placed on their free expansion by the contact interface between them, must be accommodated by bending (or the formation of a curvature) in the stamp-ink composite. This stresses the interface and, when the energy release rate due to delamination at the interface exceeds the work of adhesion of the interface, the ink is released from the stamp. The increase in bending strain (and hence bending strain energy difference between the stamp and the ink) from the center of the ink to its boundaries and the stress concentration at the discontinuity caused by the boundary of the ink suggest that the delamination by this proposed mechanism will start at the outside boundary/corner of the ink and progress inwards towards its center. This predicted inward propagation of the delamination front is in remarkable contrast to the outward propagation that is observed when ablation of a sacrificial layer or the stamp materials is the mechanism driving the delamination and ejection of the microstructure (See [13]).
To observe the delamination mechanism, the printer's high-resolution camera was replaced with a high-speed camera (Phantom v7.3). Preliminary tests indicated that the illumination produced by the laser pulse was sufficient to produce adequate contrast in the image frames of the camera at speeds up to around 2500 fps.
A Thermo-Mechanical Fracture Mechanics Model for LNTP.
To verify the plausibility of the mechanism proposed, the amount of radiation absorbed by the ink during a typical laser pulse used for printing was measured. This information was then used as the input for analytic and numerical models to determine the temperature of the ink and the stamp at and around the stamp-ink interface. This leads to a high enough energy release rate at the stamp-ink interface that exceeds the work of adhesion such that the ink delaminates from the stamp. Finally, a scaling law for delamination of the stamp-ink interface is established, which governs the critical time for delamination.
To measure the heat flux available in a laser pulse used for delamination, the receiving substrate is replaced with a photodiode power meter (Thorlabs S142C) as depicted in
This experiment is performed in two steps: in the first stage the ink is loaded on the stamp and subjected to a 4 ms long laser pulse with intensity just below that needed to produce delamination. The photodiode power meter measures the energy in the laser pulse that passes around the chip. In the second step of this measurement, the ink is removed from the stamp and the same 4 ms laser pulse is sent to the stamp with the photodiode power meter measuring the energy in the laser pulse that emerges out from the stamp. The difference between these two measurements is the energy in the pulse that is absorbed by the ink.
Finite element method [15] is used in the transient heat transfer analysis. The top surface of the glass backing layer is fixed, and the top surface of the silicon chip is constrained to move with the bottom surface of the post on the PDMS stamp. Other surfaces in this model are free to move. As explained earlier, the silicon chip absorbs part of the incident laser energy and behaves as a heat source. As indicated by the experimental measurements, the heat source here is the silicon chip or ink surface at the stamp-ink interface that inputs 0.151 mJ of energy over a 4 ms interval, that is, 0.0376 W of power. Finite element analysis is performed for a 4 ms interval of time. An axisymmetric model is used and hence the equivalent radius of the silicon chip is 56 μm with a same in-plane area as the 100×100 μm square chip.
An analytical model is developed to establish a scaling law governing the delamination of the silicon chip from the PDMS post. For simplicity, an axisymmetric model is adopted for the system of the PDMS post and silicon chip (
with the initial condition ΔTPDMS|t=0=0, where
cPDMS=1460 J·kg−1·K−1, ρPDMS=970 kg·m−3, and λPDMS=0.15 W·m−1·K−1 are respectively the specific heat, mass density, and heat conductivity of PDMS [11]. The temperature distribution then induces a thermal strain in PDMS, which gives analytically the energy release rate G for the delamination of the stamp-ink interface [17]. For the work of adhesion γ of the stamp-ink interface, the criterion for interface delamination G=γ gives the absorbed laser power P by the silicon chip as a function of critical time t for delamination
where αPDMS=3.1×10−4K−1 and μPDMS=0.67 MPa are respectively the coefficient of thermal expansion and shear modulus of PDMS, csilicon=708 J·kg−1·K−1 and ρsilicon=2300 kg·m−3[11,18] are respectively the specific heat and mass density of the silicon chip. This suggests that the normalized absorbed laser power
depends on the normalized critical time for delamination
via a single non-dimensional combination of the specific heat and mass density of silicon and PDMS, and aspect ratio of silicon chip,
The function, f, involves a number of integrals and is evaluated numerically to produce the curve shown in
for the situation being modeled. For the situation reported in the experiment and used in the FEA model, P=0.0376, gave the critical time for delamination to be 1.8 ms. This is indicated by the circular red dot on the graph, agreeing well with the analytical model's prediction.
To further verify the scaling law, an experiment was conducted in which the pulse time was kept constant and the laser power was gradually increased until delamination occurred. The incident power of the silicon chip corresponding to these conditions was measured as previously described at the beginning of this section (see
Conclusions and Discussions.
A millisecond laser pulse from a near infrared diode laser with power in the tens of watts was focused at the interface between a transparent stamp (of PDMS) and absorbing microdevices (of SCS, GAAS and GAN) ‘ink’, that have about a 2 orders of magnitude difference in the coefficient of thermal expansion. The strain energy release rate generated at the stamp-ink interface is sufficient to overcome the work of adhesion at the interface, and therefore results in the release and transfer of the microdevice from the stamp to a nearby receiving substrate. High-speed photography evidence clearly shows the delamination process is resulting from the elastic mismatch strain when the temperature of the stamp-ink system is raised. Measurements of IR flux incident on the chip, coupled with analytical and numerical models further validate the approach.
Because the stamp is not damaged during this process, it is possible to use this as the basis of a simple, pick-and-place assembly process for assembling 3-D microdevices that cannot easily be fabricated by other processes, as well as for printing functional microdevices into or onto different substrates to enable emerging technologies such as flexible and stretchable electronics. This ability to transfer microdevices from a PDMS stamp to different receiving substrates has been integrated into ‘printer’ by creating a laser print head and installing it into a computer controlled positioning stage. The full printing cycle, i.e. extracting microdevices from the growth/fabrication substrate and assembling them on a receiving substrate has been successfully implemented and successfully demonstrated for a number of cases where such transfer would be difficult, if not impossible.
One challenge in laser-driven transfer printing is to reduce the temperatures at which delamination and transfer occur. Increasing the laser power increases strain energy release rate and facilitates delamination at the stamp-ink interface. But, it also increases the temperatures of the microdevice and the stamp. The analytical and numerical models presented above suggest that effective methods to reduce the stamp temperature include increasing the elastic modulus, coefficients of thermal expansion and thermal conductivity, the specific heat, mass density, and thickness of the ink. Decreasing the specific heat and mass density of the stamp also help to reduce the temperatures reached during the process.
The LNTP process of the present invention can be used to transfer micro- or nano-devices (ink) to receiving substrates having various surface characteristics because the LNTP process is independent of receiving surface characteristics. For example, the receiving surface may be planar, rough, charged, neutral, non-planar, and/or contoured.
The present example demonstrates the applicability of the LNTP methods to liquids, biological cells, and the like. In the present example, a glass-backed transfer stamp having a 100 μm PDMS post was used to transfer a 3 μm thick×100 μm×100 μm silicon chip onto a water droplet disposed on a hydrophobic gold coating. The hydrophobicity of the gold coating causes the water droplet to present a highly spherical surface for receiving the silicon chip. A schematic of the technique is shown in
This Example demonstrates a new mode of automated micro transfer printing called laser micro transfer printing (LμTP). As a process, micro-transfer printing provides a unique and critical manufacturing route to extracting active microstructures from growth substrates and deterministically assembling them into or onto a variety of functional substrates ranging from polymers to glasses and ceramics and metallic foils to support applications such as flexible, large-area electronics, concentrating photovoltaics and displays. Laser transfer printing extends micro-transfer printing technology by providing a non-contact approach that is insensitive to the preparation and properties of the receiving substrate. It does so by exploiting the difference in the thermo-mechanical responses of the microstructure and transfer printing stamp materials to drive the release of the microstructure or ‘ink’ from the stamp and its transfer to substrate. This Example describes the process and the physical phenomena that drive it. It focuses on the use of this knowledge to design and test a print head for the process. The print head is used to demonstrate the new printing capabilities that LμTP enables.
In Micro-Transfer Printing (μTP), a patterned viscoelastic stamp is used to pick up and transfer functional microstructures made by conventional microfabrication techniques in dense arrays on typical growth/handle substrates (such as silicon, germanium, sapphire or quartz) to a broad range of receiving substrates such as transparent, flexible and stretchable polymers, glass, ceramics and metallic foils. This provides an efficient pathway to the manufacture of flexible electronics and photovoltaics, transparent displays, wearable electronics, conformal bio-compatible sensors and many more [1, 2].
While the process is simple and easy to implement, its robustness is dependent on the properties and preparation of the surface of the receiving substrate. For successful printing, the adhesion between the ink and receiving surface must be sufficient to extract the ink from the stamp and, when these conditions are satisfied, the surface must be clean and flat so that good contact is developed with the ink. Thus, printing on low-adhesion surfaces, patterned surfaces or soft gels can be challenging.
The process depicted in
In this Example, a new, non-contact mode for this process is developed that uses a laser to supply the energy required to drive the release of the ink from the stamp and its transfer to the receiving substrate. Since it does not rely on the strength of ink-substrate interface, created by mechanically pressing the ink onto the receiving substrate, to achieve its release from the stamp, the process does not depend on properties or the preparation of the receiving substrate for successful printing. Further, by using a scanned laser beam to address different inks or microstructures on the stamp, high-throughput modes of printing, not susceptible to small wedge errors between the stamp and the substrate, are possible. Thus, this new process mode, called Laser-Driven Micro-Transfer Printing (LμTP), is a highly scalable, robust and versatile printing process.
The next section describes the laser transfer printing process and the phenomena it exploits. It also provides a detailed design of the laser print head for a prototype laser transfer printing tool along with its calibration and testing. The third section demonstrates successful LμTP for situations that would be difficult to achieve with conventional transfer printing. It also explores one important parameter, separation distance of the stamp and receiving substrate on the accuracy of the transfer. Finally, conclusions are discussed.
LμTP builds on micro-transfer printing technology [3, 4]. It uses the same well-developed semiconductor processing technologies for creating donor substrates with dense arrays of printable microstructures, the same materials and techniques for fabricating the transfer stamps, and the stamps are ‘inked’ with microstructures using the same strategies [3,4]. The critical point of departure is the printing or transfer of the ink from the stamp to the receiving substrate. Instead of using contact-based mechanical means, LμTP uses a pulsed laser beam focused on the interface between the stamp and the microstructure to release and drive the microstructure to the receiving substrate. The wavelength of the laser is chosen so that the stamp material is transparent to the laser while the ink is absorbing, e.g., an IR laser with wavelength 805 nm. Additionally, the stamp material is chosen so as to have a large mismatch in the coefficient of thermal expansion (CTE). For example, in the prototype reported here, single crystal silicon is used as the ink and PDMS as the stamp with CTEs of 2.6 ppm/° C. and 310 ppm/° C. respectively, to produce a CTE mismatch of two orders of magnitude.
Bohandy [13] was the first to report a laser-driven deposition process. Holmes and Saidam [14] reported a process called Laser-Driven Release and used it for printing prefabricated metal microstructures from a glass fabrication substrate onto a receiving substrate. Arnold and Pique [15] have reported widely on what they call the Laser-Induced Forward Transfer (LIFT) process. In all these approaches, the driving mechanism is laser ablation at the interface. Much of the reported research uses pico- or femtosecond lasers and sacrificial layers at the microstructure-support structure (stamp) interface with a low vaporization temperature and a high absorptivity at the laser wavelength to enhance the delamination forces produced by ablation. The unique aspects, then, of LμTP, include but are not limited to:
Detailed modeling and analysis of the process are described in [23]. This Example concentrates on the design of the printing tool for the process.
A prototype LμTP was developed by designing a printhead and integrating it with an xyz-positioning stage. A schematic of the print head is shown in
One of the first steps in the realization of the schematic of the prototype print head of
To compute the delamination temperature, the approach originally proposed by Stoney [16] for an infinitely thin film as modified by Freund [17] for finite film thickness was used. Silicon was used as the thin film (thickness, hc=3 μm) and PDMS as the substrate (thickness, hs=100 μm) to model film delamination. As previously mentioned, the PDMS stamp has a higher coefficient of thermal expansion; thus, when heated, the PDMS expands more than the Si ink, although the expansion is constrained due to a common interface shared by the two materials. As a result, strains accrue in both materials. To estimate this strain, a constant, uniform temperature distribution throughout the ink and the immediate vicinity of the post on the stamp was assumed. The strain energy exists solely because of an incompatible elastic mismatch strain that arises when the temperature is increased by an amount ΔT above room temperature (the conditions at which the interface was created) due to heating by laser pulse, as no external applied tractions or stresses exist in the system. Consequently, the Si chip undergoes a biaxial tensile stress; assuming the printing chip is an isotropic, elastic, homogenous material; its strain energy density at the interface is given by, U(z=½hs):
where the elastic modulus (Ec=179.4 GPa) and Poisson ratio (νc=0.28) denote the elastic constants of silicon [3]. Hence, the strain energy density is composed of the mid-plane extensional strain, ∈0, the strain arising from the mismatch in thermal expansion coefficients between the chip and substrate, ∈m, and the curvature, κ, of the chip about a center of curvature equivalent to half of the substrate's thickness, hs/2. The mismatch in thermal expansion coefficients of the stamp and chip produces a strain, ∈m=(αs−αc)ΔT.
The potential energy, V, is found by integrating Equation 1 with respect to the height of the system. By taking the variants of the potential energy and checking for stability of the system (i.e. ∂V/∂∈0=0 and ∂V/∂κ=0), two equations and two unknowns are obtained, the midplane extensional strain (∈0) and the curvature (κ), that can be solved to yield:
In these equations, shorthand notation is used where h (=hc/hs) and m (=Ec*(1−νs)/Es(1−νc)) refer to the ratios of the thicknesses and biaxial moduli of the chip to the substrate, respectively. Also, κst and ∈st refer to the solution of the Stoney equation, where the chip is infinitely thin. From this analysis, the stress in the chip at the interface is given by:
The strain energy accumulation in the system is relieved by deformation, giving rise to a curvature of the microstructure/stamp system, as shown in
where σa is the applied external stress [26], which is zero in this case. When this energy release rate is greater than the adhesion energy of the Si-PDMS interface, one can expect delamination to occur and the ink to be released from the stamp. The above analysis was used to arrive at a relationship between the energy release rate, G (J/m2), and the temperature to which the system is raised above room temperature, ΔT (° C.). This is shown in
A number of investigators have reported values in the range of 0.05 to 0.4 J/m2 for the adhesion energy of Si-PDMS interfaces [4, 10, 18-20]. From
As stated in the description of the process, the laser heats up the Si ink that, in turn, heats up the interface and the PDMS in the vicinity. To achieve this, a COMSOL® finite element model was used with the Si ink acting as the heat source. The strength of the heat source was varied and the corresponding steady state temperatures were computed.
From this value of heat input rate, it is possible to approximate to 150 mJ over 4 ms or 0.0375 W and to calculate the power required in the laser pulse, but one must account for reflective and transmission losses as well as for the intensity distribution in the beam. For 800 nm radiation, the coefficient of absorption for silicon, αc=103 cm−1 or its absorption depth is about 10 μm. The intensity of the radiation emerging from a 3 μm thick sheet of silicon as a fraction of the intensity of the incident radiation, I0, is given by:
which for h=3 μm becomes approximately 0.75. With 75% of the radiation lost to transmission, only 25% of the radiation that enters the silicon is available for heating the ink. Dealing next with the fraction of the beam area that is incident on the silicon ink, one major consideration is to uniformly heat the ink across its lateral dimension. If one considers a Gaussian beam, then too small of a beam diameter will result in a hot spot at the center of the ink. The power, P(r), contained within a radius r of the beam is given by (see, for example, [22]):
where P(∞) is the total power in the beam and ω0 is the beam radius. For r=0.23 ω0, the intensity drop from the beam center to the perimeter of the circle is 0.1 or 10%. This will provide relatively uniform heating, but only 10% of the beam energy is contained in the circle. Finally, one must deal with the reflectivity of polished silicon, which at 800 nm is 0.328. Thus only 67.2% of the radiation incident on the ink is absorbed by, or transmitted through, it.
In summary, to provide the required 0.0375 W of heating, the beam power in the plane of the ink-stamp interface must be:
Thus, it is not only feasible to thermo-mechanically delaminate the model silicon ink from the PDMS stamp by exploiting the mismatch in CTEs, it is possible to do so with a moderately powered diode laser.
The prototype printer along with the laser printing head is calibrated to relate the beam power available at the ink-stamp interface for different current settings of the laser. Also, the validity numbers used in the analysis and design of the printer are verified.
To relate the current settings on the laser and the beam energy as it arrives at the stamp-ink interface, a photodiode power meter with a pre-calibrated reader (Thorlabs PM100D) is used, as shown in the schematic of
To verify the delamination conditions previously stated, a two-step experiment is performed. The model ink (100×100×3 mm silicon square) is loaded onto the stamp using the standard transfer printing pick-up step [3, 4]. Next the printing step is attempted. Here the pulse duration is set to 4 ms and pulses of increasing power (obtained by gradually increasing the current) are used until the power level at which transfer occurs is reached. This gives the minimum energy input settings for a 4 ms pulse at which transfer of the ink takes place. After this, the receiving substrate is replaced with the photodiode power meter and two laser power recordings are made with the same pulse times but a current setting just a little bit lower that that needed to achieve transfer. The first measurement is made with the beam passing through an empty stamp and the second is made with the ink on the stamp. Integrating the power measured across the duration of the pulse gives the total energy arriving at the power meter due to the pulse. The difference between the total energy arriving at the photometer with and without the ink gives the sum of the energy reflected and absorbed by the ink. Knowing the reflectivity, it is possible to obtain the energy absorbed by the ink and available for heating the ink. Also, Equation 7 gives the beam power at the plane of the ink-stamp interface required for delamination and transfer to be around 2.25 W. Examining the power recording allows for verification of the design.
LμTP provides new capabilities for transfer printing technology. As previously stated, it is substantially independent of the properties and topography of the receiving surface. Hence, it should be possible to print on surfaces with low adhesion energy, structured surfaces where contact area is a small fraction of the surface, and non-flat surfaces. Each of these cases was tested and demonstrated to be feasible. Additionally, the possibility of printing on liquids and gels is also demonstrated. Finally, positional errors for printing on low adhesion energy surfaces are experimentally characterized. The model ink, 100×100×3 micron Si squares, was used for these demonstrations. Further, the printing for these demonstrations was conducted with the pulse time set to 4 ms, and the power level set to 2.5 W.
Printing silicon inks on silicon surfaces is generally difficult with flat PDMS stamps because of the low adhesion at the Si—Si interfaces. It is easily accomplished by the LμPT process.
Printing of inks on non-flat (e.g. spherical) surfaces, including the surface of a liquid droplet, was performed.
Finally, to demonstrate printing on partial and recessed surfaces, a number of substrates with different features were prepared.
In this Example a new mode of transfer printing has been demonstrated and an automated transfer printing machine to implement the new mode was prototyped. In this mode of micro-transfer printing, a laser supplies the energy to drive a thermo-mechanical delamination process that releases the ink from the stamp and transfers it to the receiving substrate. A procedure for designing the print head is developed and verified. This new printing mode, called Laser Micro-Transfer Printing (LμTP), extends the versatility of micro transfer printing by making the process virtually independent of the properties and preparation of the receiving substrate. Thus, printing on low adhesion surfaces, curved, partial and recessed surfaces—operations that are typically difficult in more conventional modes—are easily performed, as demonstrated on a prototype laser micro-transfer printer.
This Example explores parameters related to laser micro-transfer printing. The setup used for this parametric study directs the beam from the optical cable through the stamp and makes it incident on a photodiode to obtain the incident power/energy. A typical photodiode has two limitations. First, the precalibrated board is slow and cannot be integrated with the set up to be synchronized with the laser pulse. Second, the power range for measurements is limited to about 2.5 W. To overcome these limitations, faster but uncalibrated data-acquisition was used and a 5% optical filter was used to reduce the power. Overlapping measurements were made to relate the pre-calibrated power measurements without the filter to those made with the high-speed data acquisition system with the filter.
To compute the power incident on the chip (ink), for each experiment reported, power measurements were made with and without the ink on the stamp. The difference provides the energy incident on the ink. Knowing the emissivity, the absorbed energy can be estimated.
The power required for delamination decreases with pulse width up to a point and then stays constant. After about 4 ms pulses, the minimum power to delaminate stayed the same. This is possibly because the steady state temperature reached for lower power settings was not high enough to produce the energy release rate to overcome the adhesion energy at the interface.
For these experiments all other factors were kept constant, only the chip (ink) thickness was varied. 100×100 micron chips were subjected to 4 ms laser pulses, where pulse width was shown to be substantially constant. The pulse power was gradually increased until delamination was achieved.
Power measurements were made with and without the chip on the stamp to obtain the energy input into the process (by taking the difference in the area under the power curve). Incident energy may be a misnomer here because transmission losses could be quite high for the thinner chips. Transmitted energy would be captured by the power sensor. Therefore the trend seen must be due to factors other than transmission losses.
The strain energy due to bending that is stored in the chip decreases as the cube of the chip thickness. Therefore the system must be deformed much more to produce the energy release rate needed to overcome the adhesion energy at the interface. Therefore more energy must be input into the system for thinner chips.
For these experiments all other factors were kept constant, only the chip (ink) size was varied. As shown in
As shown in
All references cited throughout this application, for example patent documents including issued or granted patents or equivalents; patent application publications; and non-patent literature documents or other source material; are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in this application (for example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference).
The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the invention has been specifically disclosed by preferred embodiments, exemplary embodiments and optional features, modification and variation of the concepts herein disclosed can be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims. The specific embodiments provided herein are examples of useful embodiments of the invention and it will be apparent to one skilled in the art that the invention can be carried out using a large number of variations of the devices, device components, and method steps set forth in the present description. As will be apparent to one of skill in the art, methods and devices useful for the present methods can include a large number of optional composition and processing elements and steps.
When a group of substituents is disclosed herein, it is understood that all individual members of that group and all subgroups, including any isomers, enantiomers, and diastereomers of the group members, are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure. When a compound is described herein such that a particular isomer, enantiomer or diastereomer of the compound is not specified, for example, in a formula or in a chemical name, that description is intended to include each isomer and enantiomer of the compound described individually or in any combination. Additionally, unless otherwise specified, all isotopic variants of compounds disclosed herein are intended to be encompassed by the disclosure. For example, it will be understood that any one or more hydrogens in a molecule disclosed can be replaced with deuterium or tritium. Isotopic variants of a molecule are generally useful as standards in assays for the molecule and in chemical and biological research related to the molecule or its use. Methods for making such isotopic variants are known in the art. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently.
The following references relate generally to fabrication methods, structures and systems for making electronic devices, and are hereby incorporated by reference to the extent not inconsistent with the disclosure in this application.
It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural reference unless the context clearly dictates otherwise. Thus, for example, reference to “a cell” includes a plurality of such cells and equivalents thereof known to those skilled in the art, and so forth. As well, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. It is also to be noted that the terms “comprising”, “including”, and “having” can be used interchangeably. The expression “of any of claims XX-YY” (wherein XX and YY refer to claim numbers) is intended to provide a multiple dependent claim in the alternative form, and in some embodiments is interchangeable with the expression “as in any one of claims XX-YY.”
Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. Nothing herein is to be construed as an admission that the invention is not entitled to antedate such disclosure by virtue of prior invention.
Whenever a range is given in the specification, for example, a range of integers, a temperature range, a time range, a composition range, or concentration range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure. As used herein, ranges specifically include the values provided as endpoint values of the range. As used herein, ranges specifically include all the integer values of the range. For example, a range of 1 to 100 specifically includes the end point values of 1 and 100. It will be understood that any subranges or individual values in a range or subrange that are included in the description herein can be excluded from the claims herein.
As used herein, “comprising” is synonymous and can be used interchangeably with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, “consisting of” excludes any element, step, or ingredient not specified in the claim element. As used herein, “consisting essentially of” does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. In each instance herein any of the terms “comprising”, “consisting essentially of” and “consisting of” can be replaced with either of the other two terms. The invention illustratively described herein suitably can be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.
One of ordinary skill in the art will appreciate that starting materials, biological materials, reagents, synthetic methods, purification methods, analytical methods, assay methods, and biological methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such materials and methods are intended to be included in this invention. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed can be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.
This application claims the benefit of priority from U.S. Provisional Patent Application Nos. 61/507,784, filed Jul. 14, 2011, and 61/594,652, filed Feb. 3, 2012, each of which is hereby incorporated by reference in its entirety.
This invention was made with United States governmental support awarded by the Center for Nanoscale Chemical-Electrical-Mechanical System (NanoCEMMS), a Nanoscale Science and Engineering Center sponsored by the National Science Foundation under Award No. 0749028 (CMMI). The U.S. government has certain rights in the invention.
Number | Date | Country | |
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61507784 | Jul 2011 | US | |
61594652 | Feb 2012 | US |
Number | Date | Country | |
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Parent | 13549291 | Jul 2012 | US |
Child | 15374926 | US |