LIFT printing using thin donor foil

Abstract

The invention relates to LIFT printing using a thin donor foil. A printing apparatus includes a donor supply assembly that positions a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface such that the donor film approaches a target area on a recipient substrate. An optical assembly directs one or more beams of laser radiation through the first surface of the donor substrate and illuminates the donor film so as to induce ejection of material from the donor film onto the acceptor substrate. Means are provided for mitigating or compensating for variations in reflection of laser radiation across a region of the donor substrate in order to equalize a flux of the laser radiation absorbed in the donor film across the region of the donor substrate.

Description

[0001] Information about divisional applications

[0002] This application is a divisional application. The parent application is the national phase entry of international patent application PCT / IL2020 / 050491, filed on May 4, 2020, with application number 202080032698.9, entitled "LIFT Printing Using Thin Donor Foil." This application claims the benefit of U.S. Provisional Application Serial No. 62 / 844,150, filed on May 7, 2019, under Article 8 of the Patent Cooperation Treaty. Technical Field

[0003] The present invention relates generally to laser induced material transfer, and in particular to methods and apparatus for printing material from a donor substrate onto a receptor substrate. Background Art

[0004] In laser direct writing (LDW), a laser beam is passed through a material to produce a patterned surface with spatially resolved three-dimensional structures through controlled ablation or deposition. Laser-induced forward transfer (LIFT) is an LDW technique that can be applied to deposit micropatterns on surfaces.

[0005] In LIFT, laser photons provide the driving force to eject a small amount of material from a donor film toward an acceptor substrate. Typically, the laser beam interacts with the inner side of the donor film, which is coated onto a non-absorbing carrier substrate. In other words, the incident laser beam propagates through the transparent carrier substrate before the photons are absorbed by the film's inner surface. Above a certain energy threshold, material is ejected from the donor film toward the surface of the acceptor substrate. With appropriate selection of the donor film and laser beam pulse parameters, the laser pulse ejects molten droplets of the donor material from the film, which then land and harden on the acceptor substrate.

[0006] Some LIFT printing systems utilize thin, flexible donor substrates. For example, U.S. Patent No. 9,925,797, the disclosure of which is incorporated herein by reference, describes a printing apparatus comprising a donor supply assembly configured to provide a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface, such that the donor film is positioned proximate to a target area on a receptor substrate. An optical assembly is configured to simultaneously direct multiple laser radiation output beams in a predetermined spatial pattern through the first surface of the donor substrate and irradiate the donor film, thereby inducing ejection of material from the donor film onto a receptor substrate according to the present invention, thereby writing a predetermined pattern onto the target area of ​​the receptor substrate.

[0007] In most LIFT systems, the donor film is held parallel to the receptor substrate. PCT International Publication No. WO 2016 / 116921, the disclosure of which is incorporated herein by reference, describes an apparatus for depositing material on a receptor surface that differs from this common model. The apparatus includes a transparent donor substrate having first and second opposing surfaces, such that at least a portion of the second surface is not parallel to the receptor surface, and a donor film on the second surface. The apparatus further includes an optical assembly configured to direct a radiation beam through the first surface of the donor substrate and illuminate the donor film at a location on a portion of the second surface that is not parallel to the receptor surface, thereby inducing ejection of droplets of molten material from the donor film onto the receptor surface. Summary of the Invention

[0008] Embodiments of the present invention described below provide LIFT printing systems and methods with enhanced print quality.

[0009] Thus, according to an embodiment of the present invention, a printing apparatus is provided that includes a donor supply assembly configured to position a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface such that the donor film is proximate to a target area on a receptor substrate. An optical assembly is configured to direct one or more beams of laser radiation through the first surface of the donor substrate and illuminate the donor film to induce ejection of material from the donor film onto the receptor substrate. A monitoring assembly is configured to measure a change in reflectance of the laser radiation across an area of ​​the donor substrate. A controller is configured to adjust the intensity of the laser radiation in response to the measured change to equalize the flux of the laser radiation absorbed in the donor film across the area of ​​the donor substrate.

[0010] In some embodiments, the monitoring assembly is configured to capture an image of an interference pattern formed by the laser radiation, wherein the interference pattern is indicative of the change in the reflection.

[0011] In disclosed embodiments, the donor film comprises a metal, and the donor substrate has a thickness between the first and second surfaces of no greater than 200 μm.

[0012] Additionally or alternatively, the donor substrate is comprised in a continuous flexible foil, and the donor supply assembly comprises a feed roller configured to feed the foil across the target area. In disclosed embodiments, the donor supply assembly is configured to successively position different donor areas of the foil proximate to the acceptor substrate, and the monitoring assembly is configured to separately measure changes in reflectance in each of the donor areas, and the controller is configured to adjust the intensity of the laser radiation irradiating each of the donor areas in response to the separately measured changes.

[0013] According to an embodiment of the present invention, a printing apparatus is also provided, comprising a donor supply assembly configured to position a transparent donor substrate having opposing first and second surfaces with a thickness of no more than 200 μm between the first and second surfaces and having a donor film formed on the second surface, such that the donor film is proximate to a target area on an acceptor substrate. An optical assembly is configured to direct one or more beams of pulsed visible laser radiation having a bandwidth of at least 0.8 nm through the first surface of the donor substrate and irradiate the donor film to induce ejection of material from the donor film onto the acceptor substrate.

[0014] In some embodiments, the pulsed laser radiation comprises pulses having a pulse duration of at least 0.5 ns. Additionally or alternatively, the bandwidth of the visible laser radiation directed through the first surface of the donor substrate is no greater than 1.0 nm.

[0015] In some embodiments, the optical assembly includes a laser that generates an input beam of laser radiation having an initial bandwidth of less than 0.4 nm and a nonlinear optical element configured to receive the input beam and widen the bandwidth of the input beam to at least 0.8 nm. The nonlinear optical element may be configured to widen the bandwidth using an optical Kerr effect. Additionally or alternatively, the nonlinear optical element includes a doped optical fiber.

[0016] According to an embodiment of the present invention, a printing apparatus is additionally provided that includes a donor supply assembly configured to position a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface so that the donor film is proximate to a target area on a receptor substrate. An optical assembly includes a laser that generates an input beam of laser radiation having an initial bandwidth and a nonlinear optical element that receives the input beam and generates an output beam having an output bandwidth that is at least twice the initial bandwidth. The optical device is configured to direct the output beam through the first surface of the donor substrate and illuminate the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0017] According to an embodiment of the present invention, there is further provided a printing apparatus comprising a donor foil, the donor foil comprising a transparent donor substrate having opposing first and second surfaces, a donor film formed on the second surface, and an antireflective coating formed on the first surface. A donor supply assembly is configured to position the donor foil so that the donor film is proximate to a target area on a receptor substrate. An optical assembly is configured to direct one or more beams of laser radiation through the donor foil, through the antireflective coating on the first surface of the donor substrate, and onto the donor film, thereby inducing ejection of material from the donor film onto the receptor substrate.

[0018] In disclosed embodiments, the antireflective coating comprises a plurality of thin film layers.

[0019] According to an embodiment of the present invention, a printing apparatus is further provided that includes a donor foil, the donor foil including a transparent donor substrate comprising a birefringent material, the transparent donor substrate having opposing first and second surfaces and a thickness between the first and second surfaces selected such that the donor substrate behaves as a quarter-wave plate at a selected wavelength, and a donor film formed on the second surface. A donor supply assembly is configured to position the donor foil so that the donor film is proximate to a target area on a receptor substrate. An optical assembly is configured to direct one or more beams of laser radiation of the selected wavelength through the donor foil and onto the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0020] According to an embodiment of the present invention, a printing apparatus is further provided that includes a film carrier comprising a block of transparent material having opposing first and second sides, the block of transparent material positioned so that the second side is proximate to a target area on a receptor substrate. A donor supply assembly is configured to position a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface so that the second surface of the donor substrate contacts the second side of the block of transparent material and the donor film is proximate to the target area on the receptor substrate. An optical assembly is configured to direct one or more beams of pulsed laser radiation through the first side of the block of transparent material and through the donor substrate, and to illuminate the donor film so as to induce ejection of material from the donor film onto the receptor substrate.

[0021] In disclosed embodiments, the block of transparent material and the donor substrate have respective refractive indices that differ from each other by no more than 10% at the wavelength of the laser radiation.Additionally or alternatively, the second side of the block of transparent material has a convex shape.

[0022] According to an embodiment of the present invention, there is also provided a printing apparatus comprising a donor supply assembly configured to position a transparent donor substrate having opposing, mutually parallel first and second surfaces and a donor film formed on the second surface, wherein the donor film is proximate to and parallel to a target area on a receptor substrate. An optical assembly is configured to direct one or more beams of laser radiation through the first surface of the donor substrate at an angle of at least 1° from a normal to the first surface and illuminate the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0023] In some embodiments, the optical assembly is configured such that the angle at which the one or more beams of laser radiation pass through the first surface of the donor substrate is at least 5°, or possibly 10°, off normal to the first surface.

[0024] In disclosed embodiments, the optical assembly includes a lens having an optical axis, and the one or more beams of laser radiation are incident on the lens offset from the optical axis, thereby offsetting the one or more beams from a normal to the first surface.

[0025] According to an embodiment of the present invention, a printing apparatus is additionally provided that includes a donor sheet comprising a transparent donor substrate having first and second non-parallel opposing planar surfaces and a donor film formed on the second planar surface. A donor supply assembly is configured to position the donor sheet so that the donor film is proximate to a target area on a receptor substrate. An optical assembly is configured to direct one or more beams of laser radiation through the donor sheet and irradiate the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0026] In disclosed embodiments, the first planar surface is wedged at a wedge angle of at least 1° relative to the second planar surface.

[0027] In one embodiment, the donor supply is configured to position the donor sheet such that the second planar surface is tilted relative to the acceptor substrate. Alternatively, the donor supply is configured to position the donor sheet such that the second planar surface is parallel to the acceptor substrate. In one such embodiment, the optical assembly is configured to direct the one or more beams of laser radiation to impinge on the first planar surface at an angle that is not normal to the first planar surface.

[0028] According to an embodiment of the present invention, there is further provided a printing apparatus comprising a donor foil, the donor foil comprising a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface, at least one of which deviates sufficiently from flatness such that within a printable area of ​​the donor foil, the thickness of the donor substrate varies by at least 10% over any given segment having a lateral dimension of at least 1 mm. A donor supply assembly is configured to position the donor foil so that the donor film is proximate to a target area on a receptor substrate. An optical assembly is configured to direct one or more beams of laser radiation through the printable area of ​​the donor foil and irradiate the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0029] According to an embodiment of the present invention, there is further provided a printing apparatus comprising a donor foil, the donor foil comprising a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface, wherein the second surface is sufficiently roughened to scatter at least 50% of radiation reflected from the second surface at an angle greater than 5° relative to a normal to the second surface. A donor supply assembly is configured to position the donor foil so that the donor film is proximate to a target area on a receptor substrate. An optical assembly is configured to direct one or more beams of laser radiation through the printable area of ​​the donor foil and illuminate the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0030] In some embodiments, the optical assembly is configured to focus the one or more beams of laser radiation to irradiate the donor film with a selected spot size, and the lateral extent of the roughening of the second surface is less than half the spot size. Additionally or alternatively, the donor film has a predetermined thickness, and the root mean square (RMS) roughness of the second surface is less than half the predetermined thickness.

[0031] Furthermore, according to an embodiment of the present invention, a method for printing is provided, comprising positioning a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface such that the donor film is proximate to a target area on a receptor substrate. One or more beams of laser radiation are directed through the first surface of the donor substrate and illuminate the donor film to induce ejection of material from the donor film onto the receptor substrate. A change in reflectance of the laser radiation across an area of ​​the donor substrate is measured. In response to the measured change, the intensity of the laser radiation is adjusted to equalize the flux of the laser radiation absorbed in the donor film across the area of ​​the donor substrate.

[0032] According to an embodiment of the present invention, a method for printing is also provided, comprising positioning a transparent donor substrate having first and second opposing surfaces and a thickness of no more than 200 μm between the first and second surfaces, and having a donor film formed on the second surface, such that the donor film is proximate to a target area on an acceptor substrate. One or more beams of pulsed visible laser radiation having a bandwidth of at least 0.8 nm are directed through the first surface of the donor substrate and irradiate the donor film to induce ejection of material from the donor film onto the acceptor substrate.

[0033] According to an embodiment of the present invention, a method for printing is further provided, comprising positioning a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface such that the donor film is proximate to a target area on a receptor substrate. An input beam of laser radiation having an initial bandwidth is directed into a nonlinear optical element, which produces an output beam having an output bandwidth at least twice the initial bandwidth. The output beam is directed through the first surface of the donor substrate and irradiates the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0034] According to an embodiment of the present invention, a method for printing is further provided, comprising providing a donor foil comprising a transparent donor substrate having first and second opposing surfaces, a donor film formed on the second surface, and an antireflective coating formed on the first surface. The donor foil is positioned so that the donor film is proximate to a target area on a receptor substrate. One or more beams of laser radiation are directed through the donor foil via the antireflective coating on the first surface of the donor substrate and illuminate the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0035] According to an embodiment of the present invention, a method for printing is further provided, comprising providing a donor foil comprising a transparent donor substrate comprising a birefringent material, the transparent donor substrate having first and second opposing surfaces and a thickness between the first and second surfaces, and a donor film formed on the second surface, the thickness being selected such that the donor substrate behaves as a quarter-wave plate at a selected wavelength. The donor foil is positioned such that the donor film is proximate to a target area on a receptor substrate. One or more beams of laser radiation at the selected wavelength are directed through the donor foil and onto the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0036] According to an embodiment of the present invention, a method for printing is further provided, comprising positioning a film carrier comprising a block of transparent material having opposing first and second sides such that the second side is proximate to a target area on a receptor substrate. A transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface is positioned such that the second surface of the donor substrate contacts the second side of the block of transparent material and the donor film is proximate to the target area on the receptor substrate. One or more beams of pulsed laser radiation are directed through the first side of the block of transparent material and through the donor substrate, and irradiate the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0037] According to an embodiment of the present invention, a method for printing is also provided, comprising positioning a transparent donor substrate having opposed, mutually parallel first and second surfaces and a donor film formed on the second surface, the donor film being proximate to and parallel to a target area on a receptor substrate. One or more beams of laser radiation are directed through the first surface of the donor substrate at an angle of at least 1° from a normal to the first surface and irradiating the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0038] According to an embodiment of the present invention, a method for printing is further provided, comprising providing a donor sheet comprising a transparent donor substrate having first and second non-parallel opposing planar surfaces and a donor film formed on the second planar surface. The donor sheet is positioned such that the donor film is proximate to a target area on a receptor substrate. One or more beams of laser radiation are directed through the donor sheet and onto the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0039] According to an embodiment of the present invention, there is further provided a method for printing, comprising providing a donor foil comprising a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface, at least one of which deviates sufficiently from flatness such that within a printable area of ​​the donor foil, the thickness of the donor substrate varies by at least 10% over any given segment having a lateral dimension of at least 1 mm. The donor foil is positioned such that the donor film is proximate to a target area on a receptor substrate. One or more beams of laser radiation are directed through the printable area of ​​the donor foil and irradiate the donor film to induce ejection of material from the donor film onto the receptor substrate.

[0040] According to an embodiment of the present invention, there is further provided a method for printing, comprising providing a donor foil comprising a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface, wherein the second surface is sufficiently roughened to scatter at least 50% of radiation reflected from the second surface at an angle greater than 5° relative to a normal to the second surface. The donor foil is positioned so that the donor film is proximate to a target area on a receptor substrate. One or more beams of laser radiation are directed through the printable area of ​​the donor foil and irradiate the donor film to induce ejection of material from the donor film onto the receptor substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The present invention will be more fully understood from the following detailed description of embodiments of the invention taken in conjunction with the accompanying drawings, in which:

[0042] Figure 1 is a schematic illustration of a LIFT printing system according to an embodiment of the present invention;

[0043] Figure 2 is a schematic illustration of an optical assembly used in a LIFT printing system according to an embodiment of the present invention;

[0044] Figure 3 is a schematic side view of a monitoring assembly for use in a LIFT printing system according to an embodiment of the present invention;

[0045] Figure 4 is a schematic side view of a laser assembly with increased spectral bandwidth for use in a LIFT printing system according to an embodiment of the present invention;

[0046] Figure 5 is a schematic cross-sectional view of a LIFT donor foil having an antireflective coating according to an embodiment of the present invention;

[0047] Figure 6 is a schematic detailed view of a foil carrier in a LIFT printing system according to an embodiment of the present invention;

[0048] Figure 7A is a schematic detailed view of a LIFT donor foil on which a laser beam is incident at an oblique angle according to an embodiment of the present invention;

[0049] Figure 7B and 7C is a plot schematically illustrating an incident angle of a laser beam on a donor substrate, which is set according to a laser spot size to avoid interference effects, according to an embodiment of the present invention;

[0050] Figure 8A and 8B is a schematic cross-sectional view of a donor foil having a non-uniform surface according to a further embodiment of the present invention; and

[0051] Figure 9A 、 9B 9C are schematic cross-sectional views of a wedge-shaped donor substrate with a laser beam incident thereon at different angles according to further embodiments of the present invention. DETAILED DESCRIPTION

[0052] Overview

[0053] The LIFT-based system described in the aforementioned U.S. Patent No. 9,925,797 is capable of printing patterns with a wide range of donor materials on a variety of different substrates at high throughput. In the disclosed embodiment, the donor substrate comprises a continuous, flexible donor foil that is fed onto a feed roller across a target area and includes a film, such as a metal or rheological material, coated on one side of the transparent donor substrate. The optical assembly in the system includes a laser that emits a single input beam and optics that split the single input beam into multiple output beams and direct (or "steer") the output beams toward the target area according to a predefined spatial pattern.

[0054] In this type of LIFT printing system, the volume of the molten droplets ejected from the donor film is determined by the energy, duration and focused spot size of the laser pulse incident on the donor film. In general, it is desirable for the droplet volume to be as uniform as possible to ensure that the desired pattern is written on the receptor substrate with well-controlled line width and thickness. However, when using this type of system, the inventors discovered substantial variations in the droplet volume, which they were able to trace back to variations in the absorption of the laser energy in the donor film due to interference effects within the donor foil. More specifically, variations in the interference between reflections of the laser beam from different surfaces within the donor substrate—due to varying substrate thickness and internal inhomogeneities—result in large variations in the fraction of the laser energy absorbed at different locations in the film, accompanied by variations in the droplet temperature and volume. Such variations in droplet temperature can have critical effects, especially in applications with high sensitivity to the electrical behavior of the printed pattern.

[0055] The embodiments of the invention described herein address this problem using a variety of different approaches. These approaches are applicable to a variety of donor sheets, including both flexible donor foils and those based on more rigid substrates. The disclosed approaches include modifications to both the donor sheet itself and the application of the laser beam to the donor sheet, and they can be used individually or in virtually any combination. They share the property of mitigating interference, resulting in a substantially uniform amount of laser energy absorbed at all locations on the donor film, and a uniform temperature and volume of the ejected droplets.

[0056] System Description

[0057] Figure 1 FIG2 is a schematic illustration of a LIFT printing system 20 according to an embodiment of the present invention. System 20 includes an optical assembly 24 that writes a predetermined pattern onto a receptor substrate 22 by laser-induced material transfer from a continuous transparent donor foil 26. As described in more detail below, donor foil 26 comprises a thin, flexible sheet of a transparent donor substrate coated with a donor film on the side facing the receptor substrate. Alternatively, the donor substrate can comprise a rigid or semi-rigid material. Receptor substrate 22 can comprise any suitable material, such as glass, ceramic, or plastic, as well as other dielectric, semi-conductive, or even conductive materials.

[0058] The donor supply assembly 30 positions the printable area of ​​the donor foil 26 proximate to the target area 28 on the acceptor substrate 22. A donor film (as shown in the figure below) is formed on at least the lower surface of the printable area of ​​the donor foil. In the depicted example, a feed roller 32 advances and holds the donor foil 26 in the desired position. Alternatively, the donor supply assembly may include other types of positioning mechanisms, such as a linear positioning stage.

[0059] Optical assembly 24 directs one or more output beams of laser radiation in a predefined spatial pattern across the upper surface of donor foil 26, thereby irradiating the donor film on the lower surface. The laser is typically controlled to output a series of pulses having a suitable wavelength, duration, and energy to induce ejection of material from the donor film onto receptor substrate 22. The choice of laser beam parameters depends, among other things, on the composition and thickness of the donor film. By sweeping one or more beams across target area 28 and controlling which beam is pulsed at each location, optical assembly 24 is capable of writing essentially any suitable pattern onto the target area of ​​the receptor substrate.

[0060] The system 20 also includes a positioning assembly, which may include an XY stage 34, for example, on which the acceptor substrate 22 is mounted. The stage 34 shifts the acceptor substrate 22 relative to the optical assembly 24 and the donor supply assembly 30 to enable the system 20 to write spatial patterns on different target areas 28 on the surface of the acceptor substrate. Additionally or alternatively, the positioning assembly may include a motion component (not shown) that shifts the optical assembly 24 and, where appropriate, the donor supply assembly 30 on the surface of the acceptor substrate. In the depicted embodiment, a feed roller 32 advances the foil 26 to provide a fresh area of ​​donor film on the target area 28 at each location where a pattern is to be written. The optical assembly 24 is typically programmed and controlled to write different spatial patterns on different target areas.

[0061] The system 20 can be used to print a wide variety of different donor materials from the foil 26 onto the substrate 22. For example, the donor material can include a suitable metal, such as copper or aluminum, and alloys of these and other metals, which can be deposited on the surface of the foil 26 using any suitable deposition process known in the art. Additionally or alternatively, the donor material can include a rheological material, such as a metal paste (silver, copper, or nickel, among other metals) or a ceramic paste. Further, additionally or alternatively, the donor material on the film 26 can include a metal or dielectric ink, including modified metal and dielectric inks with additives to provide controlled viscosity, as well as a viscous adhesive, a conductive adhesive, or other paste (e.g., solder paste). The metal paste or ink can include a pure metal or a metal alloy. Non-conductive solids, such as polymers, oligomers, or monomer solutions, can also be incorporated into the donor material.

[0062] These donor materials can be coated onto a variety of donor foils for use in system 20. For example, foil 26 can comprise a polymer material such as PET, PEN, polyimide, or PEEK. Alternatively, donor foil 26 can comprise a thin, flexible glass. The foil can be smooth or structured (e.g., having indentations in the foil), and can intentionally deviate from being flat, as described below with reference to Figure 8A / B is further explained. The foil may be coated with a single layer or multiple layers, for example with a thin transparent dielectric layer, or a thin metal layer, or a combination of such layers.

[0063] The system 20 can similarly be used to print on a wide range of different receptor types. Typically, the system 20 shown in the figure is used to print on flat receptor substrates, such as glass, polymer foils (such as PET, PEN, polyimide, or PEEK), thermoset materials, or printed circuit substrates (which can be epoxy-based, epoxy composites, or glass / epoxy resins, such as FR4). In addition or alternatively, the system 20 can be used to print on paper using various surface treatments for applications such as packaging, including different types of paper. The paper surface treatment can include a thin coating with an organic layer. Further, in addition or alternatively, the system 20 can be used to print on other types of substrates, such as ceramic substrates, metal foils, or composites. As a further alternative, the system can be modified to print on uneven substrates, including curved substrates made from some of the materials mentioned above (molded plastics, polymer foils, molded ceramics, etc.). Further applications of such systems are described in the above-mentioned U.S. Patent No. 9,925,797.

[0064] The beam parameters of the optical assembly 24 are dictated by the type of material to be printed (rheological or solid, metal, or dielectric) and the thickness of the donor film, and are selected to provide the desired droplet size and good print quality. For example, when printing a metal donor film having a thickness of 0.5 μm to 1 μm on the donor foil 26, the optical assembly 24 can typically be adjusted to provide laser pulses with a pulse duration in the nanosecond range (typically at least 0.5 ns), a spot size on the foil 26 of 20 μm to 30 μm, and a pulse energy of 3 μJ to 10 μJ using a green or ultraviolet laser source. Further details of the desired laser pulse and spot size parameters are discussed with reference to the following figures.

[0065] Interference detection and compensation

[0066] Figure 2 is a schematic illustration showing details of optical assembly 24 according to an embodiment of the present invention. Laser assembly 40 emits a pulsed beam of optical radiation, which may include visible, ultraviolet, or infrared radiation. For example, laser assembly 40 may include an Nd:YAG laser with a frequency doubler or tripler at its output, as is known in the art. Alternatively, other suitable types of lasers may be used. In some embodiments, laser assembly 40 may include other components, such as nonlinear optical elements for extending the laser bandwidth (e.g., as described below with reference to FIG). Figure 4 described).

[0067] exist Figure 2In the embodiment shown in FIG, an acousto-optic deflector 42 splits an input light beam into multiple output light beams. To this end, a multi-frequency drive circuit 48 applies a drive signal to a piezoelectric crystal 49, which drives the deflector 42 to generate an acoustic wave in the deflector that splits the input light beam. Alternatively, the acousto-optic deflector 42 can be driven to output and scan a single light beam. Further alternatively, the acousto-optic deflector 42 can be replaced by a suitable mechanical scanner (e.g., a rotating mirror). The drive circuit 48 typically includes one or more analog or digital oscillators with suitable amplification and interface circuitry for generating appropriate drive signals for the various components of the optical assembly 24.

[0068] At least one scanning mirror 46 scans the light beam over the foil 26 via the scanning lens 44. Figure 2 Only a single mirror 46 is shown in FIG, but alternative embodiments (not shown) may utilize dual-axis mirrors that can be scanned together or independently and / or any other suitable type of beam scanner known in the art.

[0069] The drive circuit 48 can drive the AOD 42 in various modes to generate a plurality of output beams. For example, in the above-mentioned U.S. Patent No. 9,925,797 and U.S. Patent No. 8,395,083, Figure 2 Several suitable drive techniques and auxiliary focusing and scanning optics that may be used in optical assembly 24 are described in the description of A / B, the disclosure of which is incorporated herein by reference. According to one of these techniques, drive circuit 48 generates a multi-frequency drive signal that causes the acousto-optic deflector to diffract the input beam into multiple output beams at different corresponding angles. Drive circuit 48 may also drive scanning mirror 46 and control the output of laser assembly 40.

[0070] As previously mentioned, the donor foil 26 includes a transparent donor substrate 36 with a donor film 38 formed on the lower surface. Figure 1 ) positions foil 26 so that donor film 38 is proximate to target area 28 on acceptor substrate 22. Lens 44 focuses laser pulses through the outer surface of donor substrate 36 onto donor film 38. The intense, focused laser radiation causes molten droplets 52 to be ejected from film 38 toward acceptor substrate 22. Droplets 52 land and solidify into hardened droplets 54, thereby forming patterns, such as electrical traces and other structures, on the acceptor substrate.

[0071] Ideally, the volume of droplet 52 should be substantially uniform for precise patterning on receptor substrate 22. However, the droplet volume strongly depends on the flux of incident laser radiation absorbed in donor film 38. Thus, variations in the absorbed energy will result in corresponding variations in the droplet volume. Such variations can be caused, among other things, by variations in the reflection of the laser radiation across the area of ​​donor substrate 36. More specifically, a certain fraction of the laser radiation will be reflected from the upper surface of the donor substrate, and another fraction will be reflected from the interface with donor film 38 on the lower surface of the donor substrate.

[0072] When the donor substrate 36 is thin (e.g., about 100 μm to 200 μm thick) and the laser bandwidth is narrow, there may be substantial interference between these reflections, either constructive or destructive. The overall reflectivity of the donor substrate 36 will be high when there is constructive interference and low when there is destructive interference. Small local variations in the thickness and / or refractive index of the donor substrate 36 will produce an interference pattern characterized by alternating regions of high and low reflectivity. (An example of this pattern is shown in FIG. Figure 3 .) Therefore, even if the output energy of the pulses from the laser assembly 40 is uniform, the volume of the droplet 52 will vary.

[0073] To address this issue, monitoring assembly 56 measures changes in the reflection of laser radiation across regions of donor substrate 36. For example, monitoring assembly 56 can capture an image of the interference pattern of radiation reflected from the donor substrate. Controller 50 adjusts the intensity of the laser beam or beams based on the measured changes (e.g., by providing appropriate input signals to drive circuit 48). Despite the changes in reflection, the beam intensity is controlled so as to equalize the flux of laser radiation absorbed in donor film 38 across regions of donor substrate 36. To this end, the laser pulse energy is increased proportionally to the measured reflectivity, so that regions of high reflectivity receive stronger pulse energy than regions of low reflectivity. This pulse energy compensation can be achieved, for example, by varying the drive power of laser assembly 40 or by adjusting the deflection efficiency of acousto-optic deflector 42. In this way, the volume of droplets 52 can also be roughly equalized.

[0074] For the purposes described above, the controller 50 typically includes a programmable processor and interface circuitry for receiving input signals from the monitoring assembly 56 and providing output signals to the drive circuit 48 and possibly other components of the system 20. The controller 50 can be programmed in software to perform the measurement and control functions described herein. Additionally or alternatively, at least some of these functions can be implemented by hard-wired or programmable hardware logic in the controller 50.

[0075] Figure 3is a schematic side view of a monitoring assembly 56 according to an embodiment of the present invention. Before the optical assembly writes the pattern of droplets 54 onto the target area 28, the monitoring assembly 56 measures changes in the reflectance of radiation at the wavelength of the laser assembly 40 from the area of ​​the donor foil 26 where the pattern will be written. To this end, a laser 60 emits a low-intensity beam at the wavelength of the laser assembly 40 (lower than the beam that will be used to write the pattern), and a scanner 62 scans the beam over the area of ​​the donor foil. Additionally or alternatively, the scanner 62 may include optics (not shown) that spread the laser beam over all or part of the area of ​​the donor foil, allowing reflectivity measurements to be made over a larger area at once. The functions of the laser 60 may be performed by the same laser assembly 40 that will be used to write the pattern, or alternatively, the laser 60 may be separate from the laser assembly 40 for increased throughput.

[0076] An optical sensor, such as a camera 64, measures the variation in the reflection of the radiation emitted by the laser 60 from the donor foil 26. Thus, the camera 64 can generate an image 66 of the interference pattern resulting from the variation in reflection. Alternatively, a different kind of sensor can be used for this purpose, such as an optical detector, whose field of view is scanned over an area of ​​the donor foil 26 along with the beam from the laser 60. The area of ​​the donor foil 26 captured in the image 66 is then aligned over the target area 28 on the receptor substrate 22, and the controller 50 applies the measured variation in local reflectivity, as illustrated in the image 66, to adjust the intensity of the laser radiation applied when writing a pattern onto the target area.

[0077] For accurate control of the laser radiation intensity, it is desirable to have monitoring assembly 56 close to target area 28 and therefore capture image 66 immediately before optics assembly 24 writes the desired pattern from donor film 38 onto receptor substrate 22. It is also desirable to assess and compensate for changes that will occur in interference due to the laser beam heating foil 26 as it writes the pattern.

[0078] Interference mitigation through spectral broadening

[0079] The interference between the laser radiation reflected from the upper and lower surfaces of the donor substrate 36 varies rapidly with wavelength, with the frequency increasing with the thickness of the substrate. Therefore, one solution to the interference problem and the resulting variation in the absorption of the laser radiation in the donor film 38 is to use a thicker donor substrate. In this case, even with a narrow-band laser source having a bandwidth of 0.2 nm, for example, the interference variation across the laser emission band will average out, making the net effect on absorption negligible.

[0080] However, for system 20 ( Figure 1) for efficient operation, it is desirable that the donor foil 26 be thin, on the order of 100 μm to 200 μm thick. In this case, using a laser beam having a wavelength of 532 nm and a typical bandwidth of 0.2 nm or less, the inventors have found that the total transmission of the laser radiation through the donor substrate 36 can vary by as much as 40% to 50% over a given target area.

[0081] In some embodiments of the present invention, this effect is mitigated by increasing the bandwidth of the laser beam. Specifically, for laser radiation in the visible range, for example at 532 nm, and a foil thickness of 100 μm to 200 μm, increasing the bandwidth of the laser radiation to at least 0.8 nm is sufficient to average out local variations in interference even from such a thin foil. This bandwidth is wider than the bandwidth of typical lasers that can be used for this purpose, such as frequency-doubled Nd:YAG lasers, which typically have bandwidths less than 0.4 nm and typically in the range of 0.1 nm to 0.2 nm. However, it is desirable not to increase the laser bandwidth beyond what is necessary to achieve the purpose of interference mitigation, as other components of the optical assembly 24, such as the acousto-optic deflector 42 and nonlinear optical elements, such as crystals used for frequency conversion, may be sensitive to bandwidth.

[0082] Thus, in a typical embodiment, using visible laser radiation and a foil thickness of 100 μm to 200 μm, the laser bandwidth should be roughly in the range of 0.8 nm to 1 nm. These bandwidth requirements will vary depending on the foil thickness and the laser wavelength. For ultraviolet laser radiation, a smaller bandwidth is required, while in the infrared, the bandwidth should be larger. For example, at 355 nm, a bandwidth of 0.4 nm to 0.5 nm should be sufficient to offset interference effects, but at 1064 nm, a bandwidth of 3 nm to 4 nm is required. The desired wide bandwidth can be achieved using lasers with very short pulse durations and / or poor mode quality; however, for precise ejection of droplets 52, it is desirable to have laser pulses with a duration of at least 0.5 ns and a well-controlled mode structure. Thus, in some embodiments of the present invention, a nonlinear optical element receives an input beam generated by a laser and outputs a beam having an output bandwidth that is at least twice the initial bandwidth of the input beam.

[0083] Figure 4 FIG2 is a schematic side view of a laser assembly 40 having a spectral bandwidth increased in this manner, according to an embodiment of the present invention. In this example, a laser 70 generates an input beam with an initial bandwidth of 0.2 nm. This beam is focused into a nonlinear optical element—in this case, an appropriately doped optical fiber 74, such as a fused silica fiber doped with germanium, fluorine, or erbium. Self-phase modulation due to the optical Kerr effect within the fiber widens the output bandwidth by an amount determined by the type and amount of doping and the length of the fiber. In this example, these parameters are selected so that the output bandwidth is approximately 0.8 nm.

[0084] Alternatively, if a fiber laser is used, self-phase modulation within the laser itself can be applied in a similar manner to achieve the required bandwidth.

[0085] Alternatively, other types of nonlinear optical elements can be used to broaden the spectral bandwidth of the laser beam. For example, the bandwidth can be increased by Brillouin scattering or frequency down-conversion, or by configuring the laser to operate in multiple longitudinal modes.

[0086] The spectral broadening of the light beam in the optical fiber 74 due to the nonlinear refractive index n2 can be expressed as follows:

[0087] ω(t)=ω0+α·t

[0088] where α is:

[0089]

[0090] In this expression, ω is the laser optical frequency and t represents time. L is the length of the fiber, I0 is the pulse intensity, λ0 is the central wavelength, and τ is the pulse duration. Assuming the laser pulse has a Gaussian temporal profile, the spectral broadening Δλ is given by:

[0091]

[0092] Where c is the speed of light. Depending on the input beam parameters, the required length of optical fiber 74 is typically in the range of 5 m to 500 m to achieve the desired degree of spectral broadening.

[0093] As mentioned previously, the increased bandwidth of the laser assembly 40 will result in increased dispersion in the AOD 42, which can cause a loss of scanning accuracy and distortion of the laser spot across the scan field. To counteract these effects, the optical assembly 24 may include an additional AOM or diffractive optical element (not shown) whose dispersion is opposite to that of the deflector 42.

[0094] Anti-reflection treatment of donor foil

[0095] Figure 5 FIG. 8 is a schematic cross-sectional view of a LIFT donor foil 80 having an antireflective coating 82 according to an embodiment of the present invention. The donor foil 80 may be used in the system 20 ( Figure 1 ), for example, instead of foil 26.

[0096] The donor foil 80 includes a transparent donor substrate 36, as described above, with a donor film 38 formed on one of its surfaces ( Figure 580 .

[0097] The antireflective coating 82 can be tuned for the specific wavelength output by the laser assembly 40. Thus, for a relatively small number of layers, or even a single quarter-wave layer, the coating 82 should be able to substantially reduce the amount of optical energy reflected from the upper surface of the donor substrate 36, and thus reduce variations in reflected intensity due to interference.

[0098] In an alternative embodiment (not shown), a birefringent donor foil can be used to suppress interference: the thickness of the birefringent foil is chosen so that it behaves as a quarter-wave plate at the wavelength of the incident beam. Thus, when the incident laser beam is linearly polarized, the polarization of the beam reflected back from the inner surface of the foil will be rotated 90° relative to the incident beam, and interference will therefore be minimal.

[0099] Increase the effective thickness of the donor foil

[0100] As mentioned previously, increasing the thickness of the donor foil 26 can substantially reduce interference effects for a given laser radiation bandwidth. However, for practical reasons, such as cost and foil flexibility, it is desirable to keep the foil thin, in the range of 100 μm to 200 μm. The need for greater thickness can be met by increasing the effective optical thickness of the donor foil using a transparent film carrier.

[0101] Figure 6 is a schematic detailed view of a foil carrier 90 used in a LIFT printing system, such as system 20, according to an embodiment of the present invention. Foil carrier 90 comprises a block of transparent material, such as glass, the underside of which is positioned proximate to a target area on receptor substrate 22. Donor supply assembly 30 ( Figure 1 ) The donor foil 26 is positioned and advanced along this underside so that the upper surface of the donor substrate contacts the underside of the film carrier 90 and the donor film is proximate to the target area on the acceptor substrate. The optical assembly 24 directs one or more beams of laser radiation through the upper side of the foil carrier 90 so that the one or more beams pass through the foil carrier and the donor substrate and thereby illuminate the donor film.

[0102] In order to achieve the desired effective thickening of the donor foil, it is important that the refractive indices of film carrier 90 and donor substrate 36 are closely matched at the wavelength of the laser radiation. To this end, the transparent materials used in the film carrier and donor substrate are desirably selected so that their respective refractive indices differ by no more than 10% at the wavelength of the radiation, or even less. Additionally or alternatively, an index-matched antireflection layer may be formed on the underside of film carrier 90 or the upper surface of foil 26, or both.

[0103] It is also desirable that the underside of the foil carrier 90 has a convex shape, such as a cylindrical curve, as shown in FIG. Figure 6 . This shape serves both to maintain close contact between carrier 90 and foil 26 and to prevent distortion of the optical phase of the laser beam. In this regard, it is desirable that the gap between carrier 90 and foil 26 be substantially smaller than the laser wavelength, e.g., no more than a few tens of nanometers. For refractive index matching purposes, a liquid or gel may be introduced between the carrier and foil.

[0104] Use oblique incidence to reduce interference

[0105] As previously explained, the pattern of different reflectivities that produces local differences in absorbed laser beam energy in the donor film 38 is due to interference between reflections of the laser beam from the upper and lower surfaces of the donor substrate 36. This problem can be mitigated if the two reflections (from the upper and lower surfaces) are laterally displaced relative to each other: if there is little or no overlap between the reflected beams, there will be little or no interference.

[0106] This lateral displacement can be achieved if one or more beams of laser energy are incident at an oblique angle (i.e., an angle that is offset from the normal to the plane of the donor film) on at least one surface of the donor substrate 36. The desired oblique incidence can be achieved by directing the laser beam at an oblique angle or by fabricating the donor film so that one or both of its surfaces are non-planar.

[0107] Figure 7A FIG2 is a schematic detailed view of a LIFT donor foil 26 with a laser beam incident thereon at an oblique angle θ1, according to an embodiment of the present invention. In this embodiment (as in the previous embodiments), it is assumed that the upper and lower surfaces of donor substrate 36 are parallel to each other, with a thickness d between the surfaces. Donor film 38 is applied to the lower surface of donor substrate 36, parallel to the target area on acceptor substrate 22.

[0108] In this embodiment, as in the previous embodiment, the optical assembly 24 ( Figure 1) directs a beam of laser radiation through donor substrate 36 and irradiates donor film 38, except that in this case the laser beam is incident at an oblique angle θ1. As will be shown below, for typical thicknesses of donor substrate 36 and typical sizes of incident laser spot 100, setting θ1 to at least 5° is sufficient to avoid overlap between reflected beam spots 102 and 104 and, therefore, to prevent interference.

[0109] refer to Figure 7A , let d be the thickness of substrate 36 and its refractive index be n. The relationship between the input angle θ1 required to neglect overlap and the spot size w is as follows:

[0110] θ2=sin -1 (sinθ1 / n)

[0111] AC=w / cosθ1(2 b )AC=d·tan(θ2)

[0112] w=2·d·tan(θ2)·cosθ1

[0113] Figure 7B is a plot schematically illustrating the incident angle of the laser beam on donor foil 26, which is set according to the size (diameter) of the incident laser spot 100 to avoid interference effects between the reflected beams, according to an embodiment of the present invention. The plot shows a minimum value for the incident angle θ1, which results in no overlap between the reflected beam spots 102 and 104 for a range of substrate thicknesses d from 100 μm to 200 μm. Although the calculations assume that spots 102 and 104 have sharp edges, the results can be approximated to a Gaussian beam of comparable 1 / e diameter because the amount of energy outside the diameter is small.

[0114] As can be seen in the plot, for a small laser spot size (20 μm) and a relatively thick foil, setting θ1 = 5° will virtually eliminate any interference between the reflected beams. For thinner films, the required angle increases to 10° with d = 100 μm, and can be even larger if the laser spot size is larger.

[0115] Figure 7C FIG2 is a plot schematically illustrating the incident angle of a laser beam on a thicker donor substrate according to another embodiment of the present invention, which is set according to the size (diameter) of the incident laser spot 100 to avoid interference effects between the reflected beams. In this case, the plot shows a minimum value for the incident angle θ1, which results in no overlap between the reflected beam spots 102 and 104 for substrate thicknesses d ranging from 1 to 2 mm. The plot shows that for substrates within this thickness range, an incident angle of 2 to 3° is sufficient to avoid interference effects, even with relatively large laser beam diameters.

[0116] For example, by scanning lens 44 ( Figure 2 ) with appropriate design and configuration, various optical configurations can be used to produce Figure 7A The tilted beam used in the embodiment of FIGC. To scan a large field of view at the large angle of incidence required for thin donor films (e.g., θ1 = 5°), the scan lens 44 may comprise a non-telecentric f-theta lens aligned with the lens so that the laser beam enters the lens away from the lens' optical axis. For smaller fields of view and smaller angles of incidence, other types of scan lenses may be used where the laser beam is only slightly off-axis.

[0117] When using an inclined beam in LIFT printing, any change in the height of the donor foil 26 will translate into a lateral shift in the position of the laser spot on the donor film 38 and, therefore, into a lateral shift in the position of the droplets ejected towards the receptor substrate 22. In order to maintain good printing accuracy under these conditions, the wheel 32 ( Figure 1 ) should be precisely designed to minimize any static spatial differences in foil height. Additionally or alternatively, vacuum or compressed air may be applied to keep foil 26 flat at a desired distance from target area 28. Further additionally or alternatively, monitoring assembly 56 ( Figure 2 ) may be configured to monitor height variations of the foil 26, and the controller 50 may then control the drive circuit 48 such that the deflector 42 and / or the scanning mirror 46 compensate for small height variations.

[0118] Using non-uniform donor surfaces to reduce interference

[0119] Figure 8A and 8B 1 are schematic cross-sectional views of donor foils 110 and 120, respectively, having non-uniform surfaces, according to another embodiment of the present invention. Foils 110 and 120 can be used in system 20, for example, in place of foil 26. In foil 110, outer surface 112 of donor substrate 36 is non-uniform, while in foil 120, inner surface 122 of the donor substrate is non-uniform (and thus, donor film 38 is also non-uniform). Surfaces are "non-uniform" in the sense that they deviate substantially from being flat. The scale of the non-uniform surfaces in these figures is arbitrary and is shown for illustrative purposes only.

[0120] Because surfaces 112 and 122 are non-uniform, one or more beams of laser radiation in system 20 will be incident on these surfaces at locally oblique angles. The profiles of surfaces 112 and 122 are selected to have sufficient inclination to prevent any substantial overlap between beams reflected from the upper and lower surfaces of donor substrate 36. Any suitable profile that meets this criterion may be used, such as a sawtooth profile or a pseudo-random profile. In one embodiment, the surface profile is selected so that the thickness of donor substrate 36 varies by at least 10% over any given segment having a lateral dimension of at least 1 mm.

[0121] In some embodiments, the non-uniform surface 112 or 122 is formed by roughening the surface in question. This roughening reduces the intensity of the interfering portion of the reflected laser radiation, the extent of which depends on the level of scattering from the roughened surface. Roughening the inner surface 122 is beneficial because, in this case, optical distortion of the incident laser beam will be minimal. The roughening characteristics (e.g., the size and depth of the roughening pattern) should be selected so that at least 50% of the radiation reflected from the roughened surface is scattered at an angle greater than 5° relative to the normal to the surface and thus does not interfere with radiation specularly reflected from the other surface of the substrate.

[0122] The type and degree of roughening of the inner surface 122 are selected so as to induce sufficient scattering to reduce interference effects while not impairing the ejection of material from the donor film 38. To avoid uneven deposition and ejection of the donor material, it is desirable that the lateral roughness scale of the surface 122 be much smaller than the laser spot size, for example, less than half the laser spot diameter, which in the above-described embodiment is typically 20 μm to 50 μm. The scattering intensity can be estimated based on the wavelength λ, the RMS (root mean square) roughness σ of the scattering surface, and the autocorrelation length Λ (representing the lateral variation of the roughness). For example, with σ = 100 nm, Λ = 2 μm, and λ = 532 nm, approximately 64% of the incident light reflected from the surface 122 is scattered at angles greater than 5° from the normal, meaning that the effects of interference will be reduced by at least 64%. At the same time, since the autocorrelation length Λ is much smaller than the laser spot size and the roughness is much smaller than the thickness of the donor film 38 (e.g., less than half the thickness of the donor film), the roughening of the surface 122 will not significantly affect the ejection uniformity of the donor material.

[0123] Figure 9A 、 9B 9C are schematic cross-sectional views of donor foils 130, 140 into which donor substrate 36 is wedged, according to embodiments of the present invention. In these three embodiments, laser beams 132, 142, and 150 are incident on substrate 36 at different angles.

[0124] A wedge-shaped donor substrate with a wedge angle α can be used to deflect the incident laser beam and avoid interference due to multiple reflections, provided that the lateral displacement Δ of the beam after the round trip is larger than the laser spot size d. Figure 9A In the example shown in , the displacement due to the wedge angle is given by:

[0125]

[0126] For a spot 134 with a diameter of d, the criterion for avoiding interference is Δ>d. For a small angle α, the above formula can be approximated as Δ~4αh, and the condition for avoiding interference is:

[0127]

[0128] Where h is the thickness of substrate 36 at its thinner edge.

[0129] Assume that the body foil 130 has a width W (the foil is Figure 1 The minimum wedge angle that meets the above criteria will be:

[0130]

[0131] For example, for a spot size d=30 μm and W=25 mm, a wedge angle α of approximately 1° will be sufficient to meet the above conditions, which means that the average thickness of the foil 130 is approximately 0.5 mm.

[0132] For a wedge-shaped structure of foil 140, where light beam 142 generates light spot 144, as shown Figure 9B As shown in , the following conditions define the minimum wedge angle required to avoid interference:

[0133]

[0134] where n is the refractive index of the substrate.

[0135] The choice between foils 130 and 140 depends on system considerations, such as the desired distance and orientation between the donor and acceptor in the LIFT system. Figure 9A In the configuration of , the lower surface of the donor substrate 36 may be tilted relative to the acceptor substrate, and Figure 9A In the configuration, the lower surface of the donor substrate is parallel to the acceptor substrate. Figure 9B In a configuration of , the light beam 142 is deflected due to refraction, resulting in a displacement in the Y direction that varies over the width of the foil 140. This displacement should be taken into account in the printing plan.

[0136] Figure 9CThe embodiment of avoids this displacement by directing the light beam 150 at an angle nα relative to the normal to the upper surface of the foil 140. As a result of refraction, the light beam 150 propagates within the substrate in a direction perpendicular to the lower surface, thus creating a spot 152 on the donor film 38 directly below the point of incidence on the upper surface. In this case, the minimum wedge angle is given by:

[0137]

[0138] While the figures and description provided above suggest several different solutions that can each be applied individually to the problem of interference and varying reflectivity of the donor sheet, two or more of these solutions can be applied in combination to further mitigate the problem. It should be understood, therefore, that the embodiments described above are cited as examples, and the present invention is not limited to what has been specifically shown and described above. Rather, the scope of the present invention encompasses combinations and subcombinations of the various features described above, as well as variations and modifications that would occur to one skilled in the art upon reading the foregoing description and that are not disclosed in the prior art.

Claims

1. A donor supply assembly configured to position a donor foil proximate a target area on an acceptor substrate, wherein the donor foil comprises a transparent donor substrate having opposing first and second surfaces and the transparent donor substrate has a thickness of no greater than 200 μm between the first and second surfaces, the donor foil further comprising a donor film formed on the second surface, such that the donor supply assembly is configured to position the donor film proximate the target area on the acceptor substrate.

2. The donor body supply assembly of claim 1, wherein the donor body film comprises a metal.

3. The donor supply assembly of claim 1, wherein the donor substrate is comprised in a continuous flexible foil.

4. The donor supply assembly of claim 1, further comprising a feed roller configured to feed the donor foil across the target area.

5. The donor supply assembly of claim 4, wherein the donor supply assembly is configured to successively position different donor regions of the donor foil proximate to the acceptor substrate.

6. The donor supply assembly of claim 1, wherein the first and second surfaces of the transparent donor substrate are parallel to each other, and the donor supply assembly is configured to position the donor film proximate to and parallel to a target area on a receptor substrate.

7. The donor supply assembly of claim 1, wherein the second surface of the transparent donor substrate is sufficiently roughened to scatter at least 50% of radiation reflected from the second surface into an angle greater than 5° relative to a normal to the second surface.

8. The donor supply assembly of claim 1, wherein the donor film has a predetermined thickness, and a root mean square (RMS) roughness of the second surface is less than half of the predetermined thickness.

9. The donor supply assembly of claim 1, wherein the donor foil further comprises an antireflective coating formed on the first surface.

10. The donor supply assembly of claim 9, wherein the antireflective coating comprises a plurality of thin film layers.

11. A donor supply assembly configured to position a donor foil proximate a target area on an acceptor substrate, wherein the donor foil comprises a transparent donor substrate having opposing first and second surfaces, wherein the second surface of the transparent donor substrate is sufficiently roughened to scatter at least 50% of radiation reflected from the second surface into an angle greater than 5° relative to a normal to the second surface, the donor foil further comprising a donor film formed on the second surface, such that the donor supply assembly is configured to position the donor film proximate the target area on the acceptor substrate.

12. The donor body supply assembly of claim 11, wherein the donor body film comprises a metal.

13. The donor supply assembly of claim 11, wherein the donor substrate is comprised in a continuous flexible foil.

14. The donor supply assembly of claim 11, further comprising a feed roller configured to feed the donor foil across the target area.

15. The donor supply assembly of claim 14, wherein the donor supply assembly is configured to successively position different donor regions of the donor foil proximate to the acceptor substrate.

16. The donor supply assembly of claim 11, wherein the first and second surfaces of the transparent donor substrate are parallel to each other, and the donor supply assembly is configured to position the donor film proximate to and parallel to a target area on a receptor substrate.

17. The donor supply assembly of claim 11, wherein the donor film has a predetermined thickness, and a root mean square (RMS) roughness of the second surface is less than half the predetermined thickness.

18. The donor supply assembly of claim 11, wherein the donor foil further comprises an antireflective coating formed on the first surface.

19. The donor supply assembly of claim 18, wherein the antireflective coating comprises a plurality of thin film layers.

Images