LIFT Printing with a Thin Donor Foil

Abstract

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 is proximate a target area on a receptor substrate. An optical assembly directs one or more beams of laser radiation through the first surface of the donor substrate and onto the donor film to induce ejection of material from the donor film onto the receptor substrate. A component is provided for reducing or compensating for variations in reflection of the laser radiation across an area of the donor substrate so as to equalize the flux of the laser radiation absorbed in the donor film across the area of the donor substrate.

Description

Technical Field

[0001] The present invention generally relates to laser-induced material transfer, and more particularly, to methods and apparatuses for printing materials from a donor substrate onto a receptor substrate. Background Art

[0002] In laser direct writing (LDW) technology, a laser beam passes through controlled material ablation or deposition to produce a patterned surface with a spatially resolved three-dimensional structure. Laser-induced forward transfer (LIFT) technology is an LDW technology that can be applied to deposit micro-patterns on a surface.

[0003] In LIFT, laser photons provide a driving force to eject a small amount of material from a donor film towards a receptor substrate. Generally, 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 inner surface of the film. Above a certain energy threshold, the material is ejected from the donor film towards the surface of the receptor substrate. With appropriate selection of the donor film and laser beam pulse parameters, the laser pulse causes a molten droplet of the donor material to be ejected from the film and then land on the receptor substrate and harden.

[0004] Some LIFT printing systems use 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 that includes 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, so as to position the donor film adjacent to a target area on a receptor substrate. An optical assembly is configured to simultaneously direct a plurality of laser radiation output beams through the first surface of the donor substrate and illuminate the donor film in a predetermined spatial pattern, so as to induce material to be ejected from the donor film onto the receptor substrate according to the present invention, thereby writing a predetermined pattern onto the target area of the receptor substrate.

[0005] In most LIFT systems, the donor film remains 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 is different from this common model. The apparatus includes a transparent donor substrate having opposing first and second surfaces such that at least a portion of the second surface is not parallel to the receptor surface and includes 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 the non-parallel portion of the second surface with respect to the receptor surface, so as to induce molten material droplets to be ejected from the donor film onto the receptor surface. Summary of the Invention

[0006] Embodiments of the present invention described below provide a LIFT printing system and method with enhanced printing quality.

[0007] Thus, according to an embodiment of the present invention, there is provided a printing apparatus including a donor supply assembly configured to position a transparent donor substrate having opposite first and second surfaces and a donor film formed on the second surface such that the donor film is close 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 irradiate the donor film so as to induce material to be ejected from the donor film onto the receptor substrate. A monitoring assembly is configured to measure a change in reflection 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 so as to equalize the flux of the laser radiation absorbed in the donor film across the area of the donor substrate.

[0008] 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 indicates the change in the reflection.

[0009] In the disclosed embodiments, the donor film includes a metal, and the donor substrate has a thickness of no more than 200 μm between the first and second surfaces.

[0010] Additionally or alternatively, the donor substrate is included in a continuous flexible foil, and the donor supply assembly includes a feed roller configured to feed the foil across the target area. In the disclosed embodiments, the donor supply assembly is configured to continuously position different donor areas of the foil close to the receptor substrate, and the monitoring assembly is configured to separately measure the change in reflection 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.

[0011] According to an embodiment of the present invention, there is also provided a printing apparatus including a donor supply assembly configured to position a transparent donor substrate having opposite first and second surfaces, a thickness of no more than 200 μm between the first and second surfaces, and a donor film formed on the second surface such that the donor film is close to a target area on a receptor 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 so as to induce material to be ejected from the donor film onto the receptor substrate.

[0012] 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.

[0013] In some embodiments, the optical assembly comprises a laser and a nonlinear optical element, the laser generating an input beam of laser radiation having an initial bandwidth of less than 0.4 nm, the nonlinear optical element being configured to receive the input beam and broaden the bandwidth of the input beam to at least 0.8 nm. The nonlinear optical element may be configured to broaden the bandwidth using the optical Kerr effect. Additionally or alternatively, the nonlinear optical element comprises a doped optical fiber.

[0014] According to an embodiment of the present invention, there is further provided a printing apparatus comprising 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 close to a target region on a receptor substrate. The optical assembly comprises a laser and a nonlinear optical element, the laser generating an input beam of laser radiation having an initial bandwidth, the nonlinear optical element being configured to receive the input beam and generate 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 irradiate the donor film so as to induce material to be ejected from the donor film onto the receptor substrate.

[0015] According to an embodiment of the present invention, there is further provided a printing apparatus comprising a donor foil comprising a transparent donor substrate having opposing first and second surfaces, a donor film formed on the second surface, and an anti-reflection coating formed on the first surface. The donor supply assembly is configured to position the donor foil such that the donor film is close to a target region on a receptor substrate. The optical assembly is configured to direct one or more beams of laser radiation through the donor foil via the anti-reflection coating on the first surface of the donor substrate and irradiate the donor film so as to induce material to be ejected from the donor film onto the receptor substrate.

[0016] In the disclosed embodiments, the anti-reflection coating comprises a plurality of thin film layers.

[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 including a birefringent material, the transparent donor substrate having opposite 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 the selected wavelength, and a donor film formed on the second surface. A donor supply assembly is configured to position the donor foil such that the donor film is adjacent 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 so as to induce material to be ejected from the donor film onto the receptor substrate.

[0018] According to an embodiment of the present invention, there is further provided a printing apparatus comprising a film carrier, the film carrier comprising a block of transparent material having opposite first and second sides, the block of transparent material being positioned such that the second side is adjacent to a target area on a receptor substrate. A donor supply assembly is configured to position a transparent donor substrate having opposite first and second surfaces and a donor film formed on the second surface such that the second surface of the donor substrate contacts the second side of the block of transparent material and the donor film is adjacent 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 onto the donor film so as to induce material to be ejected from the donor film onto the receptor substrate.

[0019] In the 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.

[0020] According to an embodiment of the present invention, there is also provided a printing apparatus comprising a donor supply assembly, the donor supply assembly being configured to position a transparent donor substrate having opposite, mutually parallel first and second surfaces and a donor film formed on the second surface, wherein the donor film is adjacent 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 deviating from the normal to the first surface by at least 1°, and onto the donor film so as to induce material to be ejected from the donor film onto the receptor substrate.

[0021] 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 deviates from the normal to the first surface by at least 5°, or possibly 10°.

[0022] In the disclosed embodiments, the optical assembly includes a lens having an optical axis, and one or more beams of the laser radiation are incident on the lens off the optical axis, so that the one or more beams are off the normal of the first surface.

[0023] According to an embodiment of the present invention, a printing apparatus including a donor sheet is further provided. The donor sheet includes a transparent donor substrate having opposite first and second planar surfaces that are not parallel to each other and a donor film formed on the second planar surface. The donor supply assembly is configured to position the donor sheet such that the donor film is close to a target area on the receptor substrate. The optical assembly is configured to direct one or more beams of laser radiation through the donor sheet and irradiate the donor film to induce material to be ejected from the donor film onto the receptor substrate.

[0024] In the disclosed embodiments, the first planar surface is wedged with a wedge angle of at least 1° relative to the second planar surface.

[0025] In one embodiment, the donor supply is configured to position the donor sheet such that the second planar surface is inclined relative to the receptor substrate. Alternatively, the donor supply is configured to position the donor sheet such that the second planar surface is parallel to the receptor substrate. In one such embodiment, the optical assembly is configured to direct the one or more beams of laser radiation to irradiate the first planar surface at an angle not normal to the first planar surface.

[0026] According to an embodiment of the present invention, a printing apparatus including a donor foil is further provided. The donor foil includes a transparent donor substrate having opposite first and second surfaces and a donor film formed on the second surface, wherein at least one of them is sufficiently deviated from flat such that within the printable area of the donor foil, over any given section having a lateral dimension of at least 1 mm, the thickness variation of the donor substrate is at least 10%. The donor supply assembly is configured to position the donor foil such that the donor film is close to a target area on the receptor substrate. The 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 material to be ejected from the donor film onto the receptor substrate.

[0027] According to an embodiment of the present invention, there is further provided a printing apparatus comprising a donor foil including a transparent donor substrate having opposite 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 the radiation reflected from the second surface at an angle greater than 5° with respect to the normal of the second surface. A donor supply assembly is configured to position the donor foil such that the donor film is close 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 so as to induce material to be ejected from the donor film onto the receptor substrate.

[0028] 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 scale of the roughening of the second surface is less than half of 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 of the predetermined thickness.

[0029] Furthermore, according to an embodiment of the present invention, there is also provided a method for printing, which includes positioning a transparent donor substrate having opposite first and second surfaces and a donor film formed on the second surface such that the donor film is close 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 irradiate the donor film so as to induce material to be ejected from the donor film onto the receptor substrate. The change in the reflection 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 so as to equalize the flux of the laser radiation absorbed in the donor film across the area of the donor substrate.

[0030] According to an embodiment of the present invention, there is also provided a method for printing, which includes positioning a transparent donor substrate having opposite first and second 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 close to a target area on a receptor 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 so as to induce material to be ejected from the donor film onto the receptor substrate.

[0031] According to an embodiment of the present invention, there is further provided a method for printing, which includes positioning a transparent donor substrate having opposite first and second surfaces and a donor film formed on the second surface such that the donor film is close to a target area on a receptor substrate. An input beam of laser radiation having an initial bandwidth is guided into a nonlinear optical element, which generates an output beam having an output bandwidth that is at least twice the initial bandwidth. The output beam is guided through the first surface of the donor substrate and irradiates the donor film to induce material to be ejected from the donor film onto the receptor substrate.

[0032] According to an embodiment of the present invention, there is further provided a method for printing, which includes providing a donor foil, the donor foil including a transparent donor substrate having opposite first and second surfaces, a donor film formed on the second surface, and an anti-reflection coating formed on the first surface. The donor foil is positioned such that the donor film is close to a target area on a receptor substrate. One or more beams of laser radiation are guided through the donor foil via the anti-reflection coating on the first surface of the donor substrate and irradiate the donor film to induce material to be ejected from the donor film onto the receptor substrate.

[0033] According to an embodiment of the present invention, there is further provided a method for printing, which includes providing a donor foil, the donor foil including a transparent donor substrate including a birefringent material, the transparent donor substrate having opposite first and second 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 the selected wavelength. The donor foil is positioned such that the donor film is close to a target area on a receptor substrate. One or more beams of laser radiation at the selected wavelength are guided through the donor foil and irradiate the donor film to induce material to be ejected from the donor film onto the receptor substrate.

[0034] According to an embodiment of the present invention, there is further provided a method for printing, which includes positioning a film carrier, the film carrier including a block of transparent material having opposite first and second sides, such that the second side is close to a target area on a receptor substrate. A transparent donor substrate having opposite 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 close to the target area on the receptor substrate. One or more beams of pulsed laser radiation are guided through the first side of the block of transparent material and through the donor substrate and irradiate the donor film to induce material to be ejected from the donor film onto the receptor substrate.

[0035] According to an embodiment of the present invention, there is also provided a method for printing, which includes positioning a transparent donor substrate having opposite, mutually parallel first and second surfaces and a donor film formed on the second surface, the donor film being close to a target area on a receptor substrate and parallel to the target area. One or more beams of laser radiation are guided to pass through the first surface of the donor substrate at an angle deviating from the normal of the first surface by at least 1°, and irradiate the donor film to induce material to be ejected from the donor film onto the receptor substrate.

[0036] According to an embodiment of the present invention, there is further provided a method for printing, which includes providing a donor sheet, the donor sheet including a transparent donor substrate having opposite first and second planar surfaces that are not parallel to each other and a donor film formed on the second planar surface. The donor sheet is positioned such that the donor film is close to a target area on a receptor substrate. One or more beams of laser radiation are guided through the donor sheet and irradiate the donor film to induce material to be ejected from the donor film onto the receptor substrate.

[0037] According to an embodiment of the present invention, there is further provided a method for printing, which includes providing a donor foil, the donor foil including a transparent donor substrate having opposite first and second surfaces and a donor film formed on the second surface, wherein at least one of them is sufficiently deviated from flatness such that within the printable area of the donor foil, the thickness of the donor substrate varies by at least 10% in any given section having a lateral dimension of at least 1 mm. The donor foil is positioned such that the donor film is close to a target area on a receptor substrate. One or more beams of laser radiation are guided through the printable area of the donor foil and irradiate the donor film to induce material to be ejected from the donor film onto the receptor substrate.

[0038] According to an embodiment of the present invention, there is also provided a method for printing, which includes providing a donor foil, the donor foil including a transparent donor substrate having opposite 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 the radiation reflected from the second surface into an angle greater than 5° with respect to the normal of the second surface. The donor foil is positioned such that the donor film is close to a target area on a receptor substrate. One or more beams of laser radiation are guided through the printable area of the donor foil and irradiate the donor film to induce material to be ejected from the donor film onto the receptor substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present invention will be more fully understood from the following detailed description of embodiments of the present invention in conjunction with the drawings, in which:

[0040] Figure 1 Schematic illustration of a LIFT printing system according to an embodiment of the present invention;

[0041] Figure 2 Schematic illustration of an optical assembly used in a LIFT printing system according to an embodiment of the present invention;

[0042] Figure 3 Schematic side view of a monitoring assembly used in a LIFT printing system according to an embodiment of the present invention;

[0043] Figure 4 Schematic side view of a laser assembly with an increased spectral bandwidth used in a LIFT printing system according to an embodiment of the present invention;

[0044] Figure 5 Schematic cross-sectional view of a LIFT donor foil with an anti-reflection coating according to an embodiment of the present invention;

[0045] Figure 6 Schematic detailed view of a foil carrier in a LIFT printing system according to an embodiment of the present invention;

[0046] Figure 7A 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;

[0047] Figure 7B and 7C is a plot schematically showing the angle of incidence of a laser beam on a donor substrate, which will be set according to the laser spot size to avoid interference effects;

[0048] Figure 8A and 8B is a schematic cross-sectional view of a donor foil with a non-uniform surface according to a further embodiment of the present invention; and

[0049] Figure 9A 、 9B and 9C are schematic cross-sectional views of a wedge-shaped donor substrate on which a laser beam is incident at different angles according to a further embodiment of the present invention. Detailed Description

[0050] Overview

[0051] The LIFT-based system described in the above-mentioned U.S. Patent No. 9,925,797 is capable of printing patterns on a variety of different substrates with a wide range of donor materials at high throughput. In the disclosed embodiments, the donor substrate includes a continuous flexible donor foil that is fed onto a feed roller across the target area and includes a film such as a metal or a rheological material that is coated on one side of a 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 towards the target area according to a predefined spatial pattern.

[0052] 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 pulses incident on the donor film. Generally, it is desirable for the volume of the droplet volume to be as uniform as possible to ensure that the desired pattern is written onto the receptor substrate with well-controlled line widths and thicknesses. However, in using such systems, the inventors have found substantial variations in droplet volume, which they were able to trace to variations in the absorption of laser energy in the donor film due to interference effects within the donor foil. More specifically, variations in the interference between the reflections of the laser beam from different surfaces within the donor substrate - due to varying substrate thicknesses and internal inhomogeneities - result in large variations in the fraction of laser energy absorbed at different positions in the film, along with variations in droplet temperature and volume. Such variations in droplet temperature can have critical effects, especially in applications that are highly sensitive to the electrical behavior of the printed pattern.

[0053] Embodiments of the invention described herein use a variety of different measures to address this problem. These measures are applicable to a variety of donor sheets, including both flexible donor foils and donor sheets based on more rigid substrates. The disclosed measures 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 substantially any combination. They share the property of reducing interference such that the amount of laser energy absorbed at all positions in the donor film is substantially uniform, and the temperature and volume of the ejected droplets are also uniform.

[0054] System description

[0055] Figure 1Schematic 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 includes a thin flexible sheet of a transparent donor substrate that is coated with a donor film on the side facing the receptor substrate. Alternatively, the donor substrate may include a rigid or semi-rigid material. Receptor substrate 22 may include any suitable material, such as glass, ceramic, or plastic, as well as other dielectric, semi-conductive, or even conductive materials.

[0056] Donor supply assembly 30 positions the printable area of donor foil 26 proximate to a target area 28 on receptor substrate 22. A donor film (as shown in the figure below) is formed on the lower surface of at least the printable area of the donor foil. In the depicted example, feed rollers 32 advance and hold 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.

[0057] Optical assembly 24 directs one or more output beams of laser radiation through the upper surface of donor foil 26 in a predefined spatial pattern and thus irradiates 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 in order to induce material to eject from the donor film onto receptor substrate 22. The selection of the laser beam parameters depends in particular 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 able to write substantially any suitable pattern onto the target area of the receptor substrate.

[0058] System 20 also includes a positioning assembly, which may include an X-Y stage 34, for example, on which receptor substrate 22 is mounted. Stage 34 displaces receptor substrate 22 relative to optical assembly 24 and donor supply assembly 30 so that system 20 is able to write spatial patterns onto different target areas 28 on the surface of the receptor substrate. Additionally or alternatively, the positioning assembly may include a motion assembly (not shown) that displaces optical assembly 24 and donor supply assembly 30 (where appropriate) on the surface of the receptor substrate. In the depicted embodiment, feed rollers 32 advance foil 26 in order to provide a fresh area of the donor film at target area 28 at each location where a pattern is to be written. Optical assembly 24 is typically programmed and controlled to write different spatial patterns on different target areas.

[0059] System 20 can be used to print a wide variety of different donor materials from a foil 26 onto a substrate 22. For example, the donor materials can include suitable metals 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 materials can include rheological materials such as metal pastes (silver, copper or nickel, and other metals) or ceramic pastes. Further still, additionally or alternatively, the donor materials on the film 26 can include metal or dielectric inks, which include modified metals and dielectric inks with additives to provide a controlled viscosity, as well as viscous adhesives, conductive adhesives or other pastes (such as solder pastes). The metal pastes or inks can include pure metals or metal alloys. Non-conductive solids such as polymer, oligomer or monomer solutions can also be incorporated into the donor materials.

[0060] These donor materials can be coated onto a variety of donor foils for use in system 20. For example, the foil 26 can include polymer materials such as PET, PEN, polyimide or PEEK. Alternatively, the donor foil 26 can include thin flexible glass. The foil can be smooth or structured (e.g., having indentations in the foil), and can be intentionally non-flat, as further explained below with reference to Figure 8A / B. The foil can be coated with a single layer or multiple layers, such as a thin transparent dielectric layer, or a thin metal layer, or a combination coating of such layers.

[0061] System 20 can similarly be used to print on a wide variety of different receptor types. Generally, as shown in the figures, system 20 is used to print on a flat receptor substrate such as glass, polymer foil (such as PET, PEN, polyimide or PEEK), thermosetting material or printed circuit substrate (which can be epoxy-based, epoxy composite or glass / epoxy resin, such as FR4). Additionally or alternatively, system 20 can be used to print on paper using various surface treatments for applications such as encapsulation, where the paper includes different types of paper. The paper surface treatment can include a thin coating with an organic layer. Further additionally or alternatively, system 20 can be used to print on other kinds of substrates such as ceramic substrates, metal foils or composites. As a further alternative, the system can be modified to print on non-flat substrates, which include curved substrates made of some of the materials mentioned above (molded plastics, polymeric foils, molded ceramics, etc.). Further applications of such systems are described in U.S. Patent No. 9,925,797 mentioned above.

[0062] The beam parameters of the optical assembly 24 are indicated by the type of material to be printed (rheological or solid, metallic or dielectric) and the thickness of the donor film, and are selected to give the desired droplet size and good printing quality. For example, when printing a metallic donor film with a thickness ranging from 0.5 μm to 1 μm on the donor foil 26, the optical assembly 24 can typically be adjusted to give laser pulses having a pulse duration in the nanosecond range (usually 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 figures below.

[0063] Interference detection and compensation

[0064] Figure 2 is a schematic illustration showing details of the optical assembly 24 according to an embodiment of the present invention. The laser assembly 40 emits a pulsed beam of optical radiation, which may include visible, ultraviolet, or infrared radiation. For example, the laser assembly 40 may include a Nd:YAG laser having 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, the laser assembly 40 may include other components, such as non-linear optical elements (e.g., as described below with reference to Figure 4 ).

[0065] In Figure 2 the embodiment shown, the acousto-optic deflector 42 splits the input beam into a plurality of output beams. For this purpose, the multi-frequency drive circuit 48 applies a drive signal to the piezoelectric crystal 49, which drives the deflector 42 to generate acoustic waves in the deflector that split the input beam. Alternatively, the acousto-optic deflector 42 may be driven to output and scan a single beam. Further alternatively, the acousto-optic deflector 42 may 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 having suitable amplification and interface circuitry for generating appropriate drive signals for the various components of the optical assembly 24.

[0066] At least one scanning mirror 46 scans the beam on the foil 26 via the scanning lens 44. Although Figure 2 only a single mirror 46 is shown in

[0067] alternative embodiments (not shown in the figures) may employ two-axis mirrors and / or any other suitable type of beam scanner known in the art that can be scanned together or independently. The drive circuit 48 can drive the acousto-optic deflector 42 in various different 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. 8,395,083Figure 2 The description of A / B describes several suitable drive techniques and assist focusing and scanning optics that may be adapted for use in the optical assembly 24, the disclosure of which patent is incorporated herein by reference. According to one of these techniques, the drive circuit 48 generates a multi-frequency drive signal that causes the acousto-optic deflector to diffract an input beam at different corresponding angles into a plurality of output beams. The drive circuit 48 may also drive the scanning mirror 46 and control the output of the laser assembly 40.

[0068] As previously mentioned, the donor foil 26 includes a transparent donor substrate 36, on the lower surface of which the donor film 38 is formed. The donor supply assembly 30( Figure 1 ) positions the foil 26 such that the donor film 38 is close to the target area 28 on the receptor substrate 22. The lens 44 focuses the laser pulse through the outer surface of the donor substrate 36 onto the donor film 38. The intense, focused laser radiation causes a molten droplet 52 to be ejected from the film 38 toward the receptor substrate 22. The droplet 52 lands and solidifies into a hardened droplet 54, thereby forming a pattern, such as electrical traces and other structures, on the receptor substrate.

[0069] Ideally, for precise patterning on the receptor substrate 22, the volume of the droplet 52 should be substantially uniform. However, the droplet volume strongly depends on the flux of the incident laser radiation absorbed in the donor film 38. Thus, variations in the absorbed energy will result in corresponding variations in the droplet volume. Such variations can be caused, in particular, by variations in the reflection of the laser radiation across the area of the 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 the donor film 38 on the lower surface of the donor substrate.

[0070] 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 constructive or destructive interference between these reflections. The total 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 Figure 3 .) Thus, even if the output energy of the pulses from the laser assembly 40 is uniform, the volume of the droplet 52 will vary.

[0071] To solve this problem, the monitoring assembly 56 measures changes in the reflection of laser radiation across the region of the donor substrate 36. For example, the monitoring assembly 56 can capture an image of an interference pattern of the radiation reflected from the donor substrate. The controller 50 adjusts the intensity of the laser beam or beams based on the measured changes (e.g., by providing an appropriate input signal to the drive circuit 48). Although the reflection changes, the beam intensity is controlled so as to equalize the flux of laser radiation absorbed in the donor film 38 across the region of the donor substrate 36. For this purpose, the laser pulse energy is increased in proportion to the measured reflectivity such that regions of high reflectivity will receive stronger pulse energy than regions of low reflectivity. This pulse energy compensation can be achieved, for example, by varying the drive power of the laser assembly 40 or by adjusting the deflection efficiency of the acousto-optic deflector 42. In this way, the volume of the droplets 52 can also be substantially equalized.

[0072] For the foregoing purposes, the controller 50 generally includes a programmable processor and an interface circuit 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 hardwired or programmable hardware logic in the controller 50.

[0073] Figure 3 is a schematic side view of the monitoring assembly 56 according to an embodiment of the present invention. Before the optical assembly writes the pattern of the droplets 54 onto the target area 28, the monitoring assembly 56 measures changes in the reflection of radiation at the wavelength of the laser assembly 40 from the region of the donor foil 26 that will be used to write the pattern. For this purpose, the laser 60 emits a low-intensity beam (lower compared to the beam that will be used to write the pattern) at the wavelength of the laser assembly 40, and the scanner 62 scans the beam across the region of the donor foil. Additionally or alternatively, the scanner 62 can include optics (not shown) that spread the laser beam over all or part of the region of the donor foil such that reflectivity measurements can be made over a larger area at one time. The function of the laser 60 can be implemented by the same laser assembly 40 that will be used to write the pattern, or alternatively, for increased throughput, the laser 60 can be separate from the laser assembly 40.

[0074] An optical sensor, such as camera 64, measures variations in the reflection of the radiation emitted by laser 60 from donor foil 26. Thus, camera 64 can produce an image 66 of the interference pattern resulting from the variations in reflection. Alternatively, different kinds of sensors, such as optical detectors, whose field of view scans over the area of donor foil 26 together with the beam from laser 60, can be used for this purpose. Then, the area of donor foil 26 captured in image 66 is aligned over target area 28 on receptor substrate 22, and controller 50 applies the measured local reflectivity variations, as illustrated in image 66, to adjust the intensity of the laser radiation applied when writing the pattern onto the target area.

[0075] For accurate control of the laser radiation intensity, it is desirable to monitor assembly 56 close to target area 28 and thus capture image 66 immediately before optical assembly 24 writes the desired pattern from donor film 38 onto receptor substrate 22. It is also desirable to evaluate and compensate for the changes that will occur in the interference due to heating of foil 26 by the laser beam when writing the pattern.

[0076] Mitigation by spectrally broadened interference

[0077] The interference between the laser radiation reflected from the upper and lower surfaces of donor substrate 36 varies rapidly with wavelength, where the frequency increases with the thickness of the substrate. Thus, one solution to the interference problem and the resulting variations in the absorption of laser radiation in donor film 38 is to use a thicker donor substrate. In this case, for example, even with a narrowband laser source with a bandwidth of 0.2 nm, the interference variations across the laser emission band will be averaged such that the net effect on absorption can be neglected.

[0078] However, for the efficient operation of system 20 ( Figure 1 ), it is desirable for donor foil 26 to be thinner, approximately 100 μm to 200 μm thick. In this case, using a laser beam with 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 donor substrate 36 can vary by up to 40% to 50% over a given target area.

[0079] 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, e.g., at 532 nm, and for foil thicknesses from 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 the interference even from such thin foils. This bandwidth is wider than that of typical lasers available for this purpose, such as frequency-doubled Nd:YAG lasers, which typically have a bandwidth of less than 0.4 nm and usually in the range of 0.1 nm to 0.2 nm. However, it is desirable not to increase the laser bandwidth beyond what is required to achieve the purpose of interference mitigation, as other components of the optical assembly 24, such as the acousto-optic deflector 42 and non-linear optical elements, such as crystals for frequency conversion, may be sensitive to the bandwidth.

[0080] Accordingly, in a typical implementation, using visible laser radiation and a foil thickness from 100 μm to 200 μm, the laser bandwidth should be approximately 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 counteract the interference effect, but at 1064 nm, a bandwidth of 3 nm to 4 nm is required. The desired broadbandwidth can be achieved using lasers with a very short pulse duration and / or poor mode quality; but for the precise ejection of the droplets 52, it is desirable for the laser pulse to have a duration of at least 0.5 ns and a well-controlled mode structure. Accordingly, in some embodiments of the present invention, the non-linear optical element receives the input beam generated by the laser and outputs a beam having an output bandwidth that is at least twice the initial bandwidth of the input beam.

[0081] Figure 4 FIG. 7 is a schematic side view of a laser assembly 40 having an increased spectral bandwidth in this manner according to an embodiment of the present invention. In this example, the laser 70 generates an input beam having an initial bandwidth of 0.2 nm. This beam is focused into the non-linear optical element - in this case a suitably 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 broadens 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 such that the output bandwidth is approximately 0.8 nm.

[0082] 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.

[0083] Alternatively, other types of non-linear 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.

[0084] The spectral broadening of the beam in the optical fiber 74 due to the non-linear refractive index n2 can be expressed as follows:

[0085] ω(t) = ω0 + α·t

[0086] where α is:

[0087]

[0088] In this expression, ω is the optical frequency of the laser, t represents time. L is the length of the optical fiber, I0 is the pulse intensity, λ0 is the central wavelength, and τ is the pulse duration. Assuming that the laser pulse has a Gaussian time distribution, then the spectral broadening Δλ will be given by the following formula:

[0089]

[0090] where c is the speed of light. Depending on the input beam parameters, the required length of the optical fiber 74 is typically in the range of 5m to 500m to achieve the desired degree of spectral broadening.

[0091] As previously mentioned, the increased bandwidth of the laser assembly 40 will result in an increase in the dispersion in the acousto-optic deflector 42, which may cause a loss of scanning accuracy and distortion of the laser spot across the scanning field. To counteract these effects, the optical assembly 24 can include additional acousto-optic modulators or diffractive optical elements (not shown in the figure), the dispersion of which is opposite to that of the deflector 42.

[0092] Anti-reflection treatment of the donor foil

[0093] Figure 5 is a schematic cross-sectional view of a LIFT donor foil 80 with an anti-reflection coating 82 according to an embodiment of the present invention. The donor foil 80 can be used in the system 20 ( Figure 1 ) for example, instead of the foil 26.

[0094] The donor foil 80 includes a transparent donor substrate 36, as described above, where the donor film 38 is formed on one of its surfaces ( Figure 5on the lower surface). The donor film 38 may include, for example, a metal having a high reflectivity and thus increase the fraction of the incident laser radiation that is reflected back towards the upper surface when irradiating the foil 80 in the system 20. In the depicted example, an anti-reflection coating 82 is formed on the opposite (upper) surface of the foil 80. Generally, this anti-reflection coating includes a thin film layer or multiple thin film layers deposited on the donor substrate 36, such as dielectric layers having different refractive indices. Alternatively, the anti-reflection coating may be formed on the lower surface of the foil 80 between the substrate 36 and the donor film 38.

[0095] The anti-reflection coating 82 can be adjusted for a 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 the variation in the reflection intensity due to interference.

[0096] In an alternative embodiment (not shown in the figures), a birefringent donor foil can be used to suppress interference: the thickness of the birefringent foil is selected such 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 thus the interference will be minimized.

[0097] Increasing the effective thickness of the donor foil

[0098] As previously mentioned, increasing the thickness of the donor foil 26 can substantially reduce the interference effects for a given laser radiation bandwidth. However, for practical reasons, such as cost and the flexibility of the foil, it is desirable for the foil to remain thin, in the range of 100 μm to 200 μm. By using a transparent film carrier to increase the effective optical thickness of the donor foil, the need for a greater thickness can be met.

[0099] 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. The foil carrier 90 includes a block of transparent material, such as glass, whose lower side is positioned close to the target area on the receptor substrate 22. The donor supply assembly 30 ( Figure 1 ) is positioned and advanced along this lower side to feed the donor foil 26 such that the upper surface of the donor substrate contacts the lower side of the film carrier 90 and the donor film is close to the target area on the receptor substrate. The optical assembly 24 directs one or more beams of laser radiation through the upper side of the foil carrier 90 such that the one or more beams pass through the foil carrier and the donor substrate and thus irradiate the donor film.

[0100] For the effective thickening of the desired donor foil to be achieved, it is important that the refractive indices of the membrane carrier 90 and the donor substrate 36 closely match at the wavelength of the laser radiation. For this purpose, the transparent materials used in the membrane carrier and the donor substrate are desirably selected such that the difference in their respective refractive indices at the radiation wavelength does not exceed 10%, or even less. Additionally or alternatively, an anti-reflection layer with a refractive index match can be formed on the lower side of the membrane carrier 90 or the upper surface of the foil 26, or both.

[0101] It is also desirable for the lower side of the foil carrier 90 to have a convex shape, such as a cylindrical curve, as Figure 6 shown. This shape can be used both to maintain close contact between the carrier 90 and the foil 26 and to prevent distortion of the optical phase front of the laser beam. In this regard, it is desirable for the gap between the carrier 90 and the foil 26 to be substantially less than the laser wavelength, for example, not exceeding a few tens of nanometers. For the purpose of refractive index matching, a liquid or gel can be introduced between the carrier and the foil.

[0102] Reducing interference using oblique incidence

[0103] As previously explained, the different reflectivity patterns that create local differences in the energy of the laser beam absorbed in the donor film 38 are attributed to the interference between the reflections of the laser beam from the upper and lower surfaces of the donor substrate 36. If the two reflections (from the upper and lower surfaces) are laterally displaced relative to each other, then this problem can be alleviated: if there is little or no overlap between the reflected beams, then there will be little or no interference.

[0104] If one or more beams of laser energy are incident on at least one surface of the donor substrate 36 at an oblique angle (i.e., an angle deviating from the normal to the plane of the donor film), then such a lateral displacement can be achieved. The desired oblique incidence can be achieved by directing the laser beam at an oblique angle or by making one or both of the surfaces of the donor film uneven.

[0105] Figure 7A is a schematic detailed view of the LIFT donor foil 26 on which a laser beam is incident at an oblique angle θ1 according to an embodiment of the present invention. In this embodiment (as in the previous embodiment), it is assumed that the upper and lower surfaces of the donor substrate 36 are parallel to each other, and the thickness between the surfaces is d. The donor film 38 is coated on the lower surface of the donor substrate 36 and is parallel to the target area on the receptor substrate 22.

[0106] In this embodiment, as in the previous embodiment, the optical assembly 24 ( Figure 1)The laser beam guiding the laser radiation passes through the donor substrate 36 and irradiates the donor film 38, except that in this case the laser beam is incident at an oblique angle θ1. As will be shown below, for a typical thickness of the donor substrate 36 and a typical size of the incident laser spot 100, setting θ1 to at least 5° is sufficient to avoid the overlap between the reflected beam spots 102 and 104 and thus prevent interference.

[0107] Reference Figure 7A , let d be the thickness of the substrate 36 with a refractive index of n. The relationship between the input angle θ1 required to neglect the overlap and the spot size w is as follows:

[0108] θ2 = sin -1 (sinθ1 / n)

[0109] AC = w / cosθ1 (2 b )AC = d · tan(θ2)

[0110] w = 2 · d · tan(θ2) · cosθ1

[0111] Figure 7B is a plot schematically showing the incident angle of the laser beam on the donor foil 26 according to an embodiment of the present invention, and the incident angle will be set according to the size (diameter) of the incident laser spot 100 to avoid the interference effect between the reflected beams. The plot shows the minimum value of the incident angle θ1. For different substrate thickness d ranges from 100 μm to 200 μm, the incident angle θ1 will result in no overlap between the reflected beam spots 102 and 104. Although the calculation assumes that the spots 102 and 104 have sharp edges, since the amount of energy other than the diameter is very small, the result can be approximately applied to a Gaussian beam with a comparable 1 / e diameter.

[0112] As can be seen from the plot, for a small laser spot size (20 μm) and a relatively thick foil, setting θ1 = 5° will substantially eliminate any interference between the reflected beams. For a thinner film, the required angle increases to 10°, where d = 100 μm, and if the laser spot size is larger, then the angle may be even larger.

[0113] Figure 7C is a plot schematically showing the incident angle of the laser beam on a thicker donor substrate according to another embodiment of the present invention, and the incident angle will be set according to the size (diameter) of the incident laser spot 100 to avoid the interference effect between the reflected beams. In this case, the plot shows the minimum value of the incident angle θ1. For different substrate thickness d ranges from 1 to 2 mm, the incident angle θ1 will result in no overlap between the reflected beam spots 102 and 104. The plot shows that for substrates within this thickness range, an incident angle of 2 to 3° is sufficient to avoid the interference effect, even if the laser beam diameter is relatively large.

[0114] For example, through proper design and configuration of the scanning lens 44 ( Figure 2 ), various optical configurations can be used to generate the tilted beam used in the Figure 7A embodiments described. To scan a large field of view at the large incident angles (e.g., θ1 = 5°) required for a thin donor film, the scanning lens 44 can include an afocal f-theta lens that is aligned with the lens such that the laser beam enters the lens away from the optical axis of the lens. For smaller fields of view and smaller incident angles, other types of scanning lenses can be used, where the laser beam is only slightly off-axis.

[0115] When a tilted beam is used in LIFT printing, any variation 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 thus into a lateral shift in the position of the droplets ejected towards the receptor substrate 22. To maintain good printing accuracy under these conditions, the wheel 32 ( Figure 1 ) should be precisely designed to minimize any static spatial differences in the foil height. Additionally or alternatively, a vacuum or compressed air can be applied to keep the foil 26 flat at a desired distance from the target area 28. Further additionally or alternatively, the monitoring assembly 56 ( Figure 2 ) can be configured to monitor changes in the height of the foil 26, and the controller 50 can then control the drive circuit 48 such that the deflector 42 and / or the scanning mirror 46 compensate for small height variations.

[0116] Reducing interference using non-uniform donor surfaces

[0117] Figure 8A and 8B are schematic cross-sectional views of donor foils 110 and 120, respectively, having non-uniform surfaces according to another embodiment of the present invention. The foils 110 and 120 can be used in the system 20, for example, instead of the foil 26. In the foil 110, the outer surface 112 of the donor substrate 36 is non-uniform, while in the foil 120, the inner surface 122 of the donor substrate is non-uniform (and thus the donor film 38 is also non-uniform). The surfaces are "non-uniform" in that they deviate substantially from flat. The scale of the non-uniform surfaces in these figures is arbitrary and is shown only for illustrative purposes.

[0118] Due to the non-uniformity of surfaces 112 and 122, one or more beams of laser radiation in system 20 will impinge on these surfaces at locally inclined angles. The profiles of surfaces 112 and 122 are selected to have sufficient inclination to prevent any substantial overlap between the beams reflected from the upper and lower surfaces of donor substrate 36. Any suitable profile that meets this criterion can be used, such as a sawtooth profile or a pseudo-random profile. In one embodiment, the surface profile is selected such that the thickness of donor substrate 36 varies by at least 10% over any given section with a lateral dimension of at least 1 mm.

[0119] In some embodiments, the non-uniform surface 112 or 122 is formed by roughening the surface in question. Such roughening reduces the intensity of the interfering portion of the reflected laser radiation, to an extent depending on the level of scattering from the rough surface. Roughening the inner surface 122 is beneficial because in this case, the optical distortion of the incident laser beam will be minimal. The roughening characteristics (such as the size and depth of the roughening pattern) should be selected such that at least 50% of the radiation reflected from the rough surface is scattered at an angle greater than 5° with respect to the normal of the surface and thus does not interfere with the radiation specularly reflected from the other surface of the substrate.

[0120] The type and degree of roughening of the inner surface 122 are selected to cause sufficient scattering to reduce the interference effect while not impairing the ejection of material from donor film 38. To avoid non-uniform deposition and ejection of the donor material, it is desirable that the lateral roughness scale of surface 122 be much smaller than the laser spot size, for example, less than half of the laser spot diameter, which is typically 20 μm to 50 μm in the above embodiments. 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, in the case of σ = 100 nm, Λ = 2 μm, and λ = 532 nm, approximately 64% of the incident light reflected from surface 122 is scattered at an angle greater than 5° with respect to the normal, which means that the interference effect 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 donor film 38 (e.g., less than half of the donor film thickness), the roughening of surface 122 will have no significant effect on the ejection uniformity of the donor material.

[0121] Figure 9A 、 9B Figures 9A, 9B, and 9C are schematic cross-sectional views of donor foils 130, 140 into which donor substrate 36 according to an embodiment of the present invention is wedged. In these three embodiments, laser beams 132, 142, and 150 impinge on substrate 36 at different angles.

[0122] A wedge-shaped donor substrate with a wedge angle α can be used to deflect an incident laser beam and avoid interference due to multiple reflections, provided that the lateral shift Δ of the beam after a round trip is greater than the laser spot size d. In Figure 9A In the example shown in

[0123]

[0124] For a spot 134 with a diameter of d, the criterion for avoiding interference is Δ > d. For small angles α, the above formula can be approximated as Δ ∼ 4αh, and the condition for avoiding interference is:

[0125]

[0126] where h is the thickness of the substrate 36 at its thinner edge.

[0127] Assume that the donor foil 130 has a width W (the dimension of the foil in the Figure 1 Y direction in

[0128]

[0129] For example, for a spot size d = 30 μm and W = 25 mm, a wedge angle α of about 1° will be sufficient to meet the above conditions, which means that the average thickness of the foil 130 is about 0.5 mm.

[0130] For the wedge-shaped structure of the foil 140, where the beam 142 produces a spot 144, as shown in Figure 9B the following conditions define the minimum wedge angle required to avoid interference:

[0131]

[0132] where n is the refractive index of the substrate.

[0133] The choice between the foils 130 and 140 depends on system considerations, such as the desired distance and orientation between the donor and the acceptor in a LIFT system. For example, in Figure 9A the configuration of Figure 9A the lower surface of the donor substrate 36 can be inclined relative to the acceptor substrate, while in Figure 9B the configuration of

[0134] Figure 9C Figure 9B the configuration of

[0134] Figure 9C the configuration of

[0134] Figure 9C

[0134] Figure 9CEmbodiments avoid this displacement by directing the light beam 150 at an angle nα with respect to the normal to the upper surface of the foil 140. As a result of refraction, the light beam 150 propagates in a direction perpendicular to the lower surface within the substrate, and thus creates a light spot 152 directly below the incident point on the upper surface on the donor film 38. In this case, the minimum wedge angle is given by:

[0135]

[0136] Although the figures and description provided above present several different solutions to the problems of interference and different reflectivities that can each be applied separately to the donor sheet, two or more of these solutions can be applied alternately in combination to achieve further mitigation of the problems. Accordingly, it should be understood that the embodiments described above are cited as examples, and the present invention is not limited to what has been particularly shown and described above. Rather, the scope of the present invention includes combinations and sub - combinations of the various features described above, as well as variations and modifications that would occur to those skilled in the art upon reading the foregoing description and that are not disclosed in the prior art.

Claims

1. A printing device, comprising: 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 region on a receptor substrate; An optical assembly configured to direct one or more beams of laser radiation through the first surface of the donor substrate and onto the donor film to induce material to eject from the donor film onto the receptor substrate; A monitoring assembly configured to measure changes in the reflection of the laser radiation across a region of the donor substrate; and A controller configured to adjust the intensity of the laser radiation in response to the measured changes to equalize the flux of the laser radiation absorbed in the donor film across the region of the donor substrate.

2. The device according to claim 1, wherein the monitoring assembly is configured to capture an image of an interference pattern formed by the laser radiation, wherein the interference pattern indicates the change in the reflection.

3. The device according to claim 1, wherein the donor film comprises a metal, and the donor substrate has a thickness of no more than 200 μm between the first surface and the second surface.

4. The device according to claim 1, wherein the donor substrate is included in a continuous flexible foil, and wherein the donor supply assembly comprises a feed roller configured to feed the foil across the target area.

5. The device according to claim 4, wherein the donor supply assembly is configured to continuously position different donor areas of the foil adjacent to the receptor substrate, and wherein the monitoring assembly is configured to respectively measure the change in the reflection 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 respectively measured changes.

6. A printing device, comprising: A donor supply assembly configured to position a transparent donor substrate having opposing first and second surfaces and having a thickness of no greater 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 region on a receptor substrate; and An optical assembly 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 onto the donor film to induce material to eject from the donor film onto the receptor substrate.

7. The device according to claim 6, wherein the pulsed laser radiation comprises pulses having a pulse duration of at least 0.5 ns.

8. The device according to claim 6, wherein the bandwidth of the visible laser radiation guided through the first surface of the donor substrate is no more than 1.0 nm.

9. The device according to claim 6, wherein the optical assembly comprises: A laser that produces an input beam of the laser radiation having an initial bandwidth of less than 0.4 nm; and A nonlinear optical element configured to receive the input beam and broaden the bandwidth of the input beam to at least 0.8 nm.

10. The device according to claim 9, wherein the nonlinear optical element is configured to broaden the bandwidth using the optical Kerr effect.

11. The device according to claim 6, wherein the donor film comprises a metal.

12. The device according to claim 6, wherein the donor substrate is included in a continuous flexible foil, and wherein the donor supply assembly includes a feed roller configured to feed the foil across the target area.

13. A printing device, comprising: 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 region on a receptor substrate; An optical assembly comprising: A laser that produces an input beam of laser radiation having an initial bandwidth; A nonlinear optical element configured to receive the input beam and produce an output beam having an output bandwidth that is at least twice the initial bandwidth; An optical device configured to direct the output beam through the first surface of the donor substrate and onto the donor film to induce material to eject from the donor film onto the receptor substrate; A monitoring assembly configured to measure changes in the reflection of the laser radiation across a region of the donor substrate; and A controller configured to adjust the intensity of the laser radiation in response to the measured changes to equalize the flux of the laser radiation absorbed in the donor film across the region of the donor substrate.

14. The device according to claim 13, wherein the nonlinear optical element is configured to broaden the bandwidth using the optical Kerr effect.

15. The device according to claim 13, wherein the donor film includes metal.

16. The device according to claim 13, wherein the donor substrate is included in a continuous flexible foil, and wherein the donor supply assembly includes a feed roller configured to feed the foil across the target area.

17. A printing device, comprising: A donor supply assembly configured to position a transparent donor substrate having opposing, parallel first and second surfaces and having a thickness of no greater than 200 μm between the first and second surfaces and having a donor film formed on the second surface, wherein the donor film is proximate to and parallel to a target region on a receptor substrate; and An optical assembly 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 at an angle deviating from the normal of the first surface by at least 1° and irradiate the donor film to induce material to be ejected from the donor film onto the receptor substrate.

18. The device according to claim 17, wherein the donor film includes metal.

19. The device according to claim 17, wherein the optical assembly is configured such that one or more beams of the laser radiation pass through the first surface of the donor substrate at an angle deviating from the normal of the first surface by at least 5°.

20. The device according to claim 19, wherein the optical assembly is configurable such that one or more beams of the laser radiation pass through the first surface of the donor substrate at an angle deviating from the normal of the first surface by at least 10°.

21. The device according to claim 19, wherein the donor substrate is included in a continuous flexible foil, and wherein the donor supply assembly includes a feed roller configured to feed the foil across the target area.

22. The device according to claim 17, wherein the optical assembly includes a lens having an optical axis, and one or more beams of the laser radiation are incident on the lens deviating from the optical axis, thereby deviating the one or more beams from the normal of the first surface.

23. A printing device, comprising: A donor foil, comprising: A transparent donor substrate having opposing first and second surfaces, wherein the second surface is sufficiently roughened to scatter at least 50% of the radiation reflected from the second surface into an angle greater than 5° relative to the normal of the second surface, and A donor film formed on the second surface; A donor supply assembly configured to position the donor foil such that the donor film is close to a target area on the receptor substrate; and An optical assembly 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 material to be ejected from the donor film onto the receptor substrate.

24. The device according to claim 23, wherein the optical assembly is configured to focus the one or more beams of the laser radiation to irradiate the donor film with a selected spot size, and a lateral scale of the roughening of the second surface is less than half of the spot size.

25. The device according to claim 23, 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.

Images