Methods for circuit manufacturing and equipment for manufacturing conductive traces

Abstract

A method for circuit fabrication includes defining a trajectory for conductive traces to be formed on a circuit substrate. Molten metal droplets are ejected from a donor substrate near the circuit substrate onto the defined trajectory via a laser-induced forward transfer (LIFT) process, whereby the droplets adhere to and harden on the circuit substrate along the length of the defined trajectory. After the droplets have hardened, a laser beam with sufficient energy is directed toward the defined trajectory to cause the metal in the hardened droplets to melt and coalesce into a bulk layer extending along the length of the defined trajectory.

Description

[0001] Cross-reference of related applications

[0002] This application claims priority to provisional patent application No. 63 / 130854, filed on December 28, 2020, and assigned to U.S. Application No. 63 / 130854, the disclosure of which is hereby incorporated by reference. Technical Field

[0003] The present invention generally relates to the manufacture of electronic devices, and more particularly to methods and systems for printing conductive lines on a substrate. Background Technology

[0004] In laser direct writing (LDW) technology, a laser beam is used to create patterned surfaces with spatially resolved three-dimensional structures obtained through controlled material ablation or deposition. Laser-induced forward transfer (LIFT) is an LDW technique applicable to depositing micropatterns onto surfaces.

[0005] In LIFT, laser photons provide the driving force to eject a small amount of material from the donor membrane toward the acceptor substrate. The laser beam typically interacts with the inside of the donor membrane 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 membrane. Above a certain energy threshold, material is ejected from the donor membrane toward the surface of the acceptor substrate. With appropriate selection of the donor membrane and laser beam pulse parameters, the laser pulse causes molten droplets of the donor material to be ejected from the membrane, subsequently falling onto and hardening on the acceptor substrate.

[0006] LIFT systems are specifically (but not exclusively) used for printing conductive metal droplets and traces for electronic circuit fabrication. For example, such a LIFT system is described in U.S. Patent 9,925,797, the disclosure of which is incorporated herein by reference. This patent describes printing apparatus comprising an 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 to position the donor film close to a target region on a recipient substrate. An optical assembly is configured to simultaneously guide multiple output beams of laser radiation through the first surface of the donor substrate and impact the donor film according to a predetermined spatial pattern to induce material ejection from the donor film onto a corresponding recipient substrate, thereby writing a predetermined pattern onto a target region of the recipient substrate.

[0007] LIFT printing can also be used to repair defects in printed circuit traces. For example, a system and method for this purpose are described in Korean Patent Publication No. KR20150070028, the disclosure of which is incorporated herein by reference.

[0008] Additionally, the LIFT system can be used to directly print embedded resistors onto a substrate. For example, PCT International Publication WO2019 / 138404 (the disclosure of which is incorporated herein by reference) describes a method for manufacturing an electrical device, comprising identifying a trace on a circuit substrate, wherein a resistor having a specified resistance is formed on the circuit substrate between a first and a second endpoint of the trace. A transparent donor substrate having opposing first and second surfaces and a donor film comprising resistive material formed on the second surface is positioned close to the identified trace on the circuit substrate, wherein the second surface faces the circuit substrate. A pulse of guided laser radiation is struck to the donor film to induce droplets of resistive material to be ejected from the donor film to individual adjacent locations along the trace on the circuit substrate, wherein the spacing between adjacent locations is selected to form a circuit trace having a specified resistance between the first and second endpoints. Summary of the Invention

[0009] The embodiments of the present invention described below provide novel methods and systems for fabricating metal traces on a substrate based on LIFT, and circuits produced by such methods.

[0010] Therefore, according to embodiments of the present invention, a method for circuit fabrication is provided, comprising defining a trajectory for conductive traces to be formed on a circuit substrate. A droplet of metal is ejected from a donor substrate near the circuit substrate onto the defined trajectory via a laser-induced forward transfer (LIFT) process, whereby the droplet adheres to and hardens on the circuit substrate along the length of the defined trajectory. After the droplet hardens, a laser beam with sufficient energy is directed toward the defined trajectory to cause the metal in the hardened droplet to melt and coalesce into a bulk layer extending along the length of the defined trajectory.

[0011] In some embodiments, the donor substrate is transparent and has opposing first and second surfaces, and a donor film containing the metal is disposed on the second surface such that the donor film is close to the defined trajectory, and the molten droplet is ejected with a pulse containing guiding laser radiation through the first surface of the donor substrate and impacts the donor film to induce the molten droplet ejected from the donor film onto the defined trajectory of the metal.

[0012] In an embodiment, guiding the laser radiation pulses and directing the laser beam toward the defined trajectory during the LIFT process includes using a single laser with a variable pulse duration to eject the molten droplet and melt the metal in the hardened droplet.

[0013] Alternatively or additionally, the donor film comprises a first metal, and an adhesion film comprising a second metal is disposed on the donor film on the donor substrate, such that the second metal forms an outer layer on the molten droplet of the first metal, and the outer layer adheres to the circuit substrate after the molten droplet impacts the circuit substrate. In a disclosed embodiment, the first metal comprises copper, and the second metal is selected from the group consisting of titanium, tin, bismuth, and alloys thereof.

[0014] In some embodiments, ejecting the molten droplets and guiding the laser beam toward the defined trajectory includes ejecting a first layer of the molten droplets onto the circuit substrate and guiding the laser beam to melt the hardened droplets in the first layer to form a lower layer of the conductive trace, and ejecting at least a second layer of the molten droplets onto the lower layer and guiding the laser beam to melt the hardened droplets in the at least second layer to complete the conductive trace.

[0015] In one embodiment, guiding the laser beam involves using the laser beam to apply sufficient energy to the hardening droplet to melt the entire volume of the hardening droplet within the conductive track. Alternatively, guiding the laser beam involves using the laser beam to apply sufficient energy to the hardening droplet to melt only the outer layer of the hardening droplet, rather than melting the entire volume of the hardening droplet along the defined track. The outer layer typically forms a protective surface layer that encloses the volume of the hardening droplet within the conductive track.

[0016] In the disclosed embodiments, guiding the laser beam comprises a sequence of pulses guiding laser energy to strike the hardened droplet along the length of the defined trajectory. In some of these embodiments, each of the pulses has a pulse duration of less than 10 μs and may not exceed 1 μs. Alternatively or additionally, guiding the one or more pulses comprises scanning the laser beam along the trajectory such that each of the pulses has a predetermined overlap with a previous pulse in the sequence.

[0017] In some embodiments, the molten droplets are ejected in a single row extending along the defined trajectory on the circuit substrate, thereby forming the conductive trace by melting the single row. In this embodiment, each droplet overlaps with a previous droplet in the single row by no more than 50% of the droplet's diameter.

[0018] In a further embodiment, defining the trajectory includes identifying a gap between a first and a second terminal on the circuit substrate, and ejecting the molten droplet includes depositing the molten droplet to fill the gap. In an embodiment, the first and second terminals comprise a first metal, and the droplet comprises a second metal having a composition different from the first metal, and guiding the laser beam includes melting the first and second metals to form heterogeneous metallic bonds at the first and second terminals. Alternatively, identifying the gap includes detecting defects in the circuit traces already formed on the circuit substrate, and repairing the defects by depositing the molten droplet and then guiding the laser beam to melt the hardened droplet.

[0019] In a disclosed embodiment, the trajectory has a predetermined width, and guiding the laser beam comprises melting only the hardened droplets within the predetermined width of the trajectory deposited on the circuit substrate, wherein the method comprises applying an etching process after guiding the laser beam to remove the hardened droplets outside the predetermined width of the trajectory deposited on the circuit substrate.

[0020] According to embodiments of the present invention, an apparatus for fabricating conductive traces on a circuit substrate is also provided. The apparatus includes a deposition module configured to eject molten metal droplets from a donor substrate adjacent to the circuit substrate onto a defining trajectory of the conductive trace via a laser-induced forward transfer (LIFT) process, whereby the droplets adhere to and harden on the circuit substrate along the length of the defining trajectory. A laser module is configured to use sufficient energy to guide a laser beam toward the defining trajectory to cause the metal in the hardened droplets to melt and coalesce into a bulk layer extending along the length of the defining trajectory.

[0021] The invention will be more fully understood from the following detailed description of embodiments of the invention, taken in conjunction with the accompanying drawings: Attached Figure Description

[0022] Figure 1 This is a schematic side view of a system for printing conductive traces on a substrate according to an embodiment of the present invention;

[0023] Figure 2A These are microscope images of lines of metal droplets printed on a substrate according to an embodiment of the present invention;

[0024] Figure 2B It is after laser melting according to an embodiment of the present invention. Figure 2A Microscopic photograph of the line;

[0025] Figure 3 This is a schematic cross-sectional view of the donor membrane according to an embodiment of the present invention, illustrating the ejection of molten droplets from the membrane under laser irradiation;

[0026] Figure 4A This is a schematic cross-sectional view of the polymerization of metal droplets deposited on a substrate during the LIFT process for defining circuit traces according to an embodiment of the present invention.

[0027] Figure 4B This is according to an embodiment of the present invention. Figure 4A A schematic cross-sectional view of the circuit traces formed by the complete laser melting of the polymerization of molten metal droplets;

[0028] Figure 4C This is an alternative embodiment of the invention. Figure 4A A schematic cross-sectional view of the circuit traces formed by the partial laser melting of the polymerization of molten metal droplets;

[0029] Figure 5A This is a schematic cross-sectional view of the polymerization of metal droplets deposited in the gaps of circuit traces by a LIFT process according to an embodiment of the present invention;

[0030] Figure 5B This illustrates the application of a laser melting process to polymerization according to an embodiment of the present invention. Figure 5A A schematic cross-sectional view of the polymerization;

[0031] Figure 5C It is based on an embodiment of the present invention. Figure 5B A schematic cross-sectional view of the circuit traces formed during the laser melting process;

[0032] Figure 6A This is a schematic cross-sectional view illustrating the polymerization of molten metal droplets deposited in the gaps of circuit traces by a LIFT process according to an embodiment of the present invention;

[0033] Figure 6B It is based on an embodiment of the present invention. Figure 6A A schematic cross-sectional view of some circuit traces formed during the laser melting process;

[0034] Figure 6C This illustrates the application of a laser melting process to a substrate deposited by a LIFT process according to an embodiment of the present invention. Figure 6B A schematic cross-sectional view of the polymerization of metal droplets along part of the circuit traces;

[0035] Figure 6D It is based on an embodiment of the present invention. Figure 6C A schematic cross-sectional view of the complete circuit trace formed during the laser melting process;

[0036] Figure 7 This is a schematic cross-sectional view of heterogeneous circuit traces printed by the LIFT process according to an embodiment of the present invention; and

[0037] Figure 8A , 8B 8C, 8D, and 8E are schematic top views of circuit traces on a substrate according to an alternative embodiment of the present invention, illustrating the sequential steps of a LIFT-based process for repairing gaps in the circuit traces. Detailed Implementation

[0038] Overview

[0039] The LIFT process can print conductive traces and other circuit components onto a circuit substrate with high precision and speed. However, due to the nature of the LIFT process, the resulting traces consist of aggregates of metal particles corresponding to hardened droplets ejected onto the substrate. These particles are typically covered and separated by a thin oxide layer, and voids and cavitation may exist between the particles. Compared to solid metal traces deposited using more conventional methods, these phenomena tend to increase resistance and compromise the mechanical integrity of the circuit traces.

[0040] The embodiments of the invention described herein address these problems by adding a controlled laser melting stage to the deposition process. In these embodiments, after defining the trajectory of the conductive trace to be formed on the circuit substrate, a LIFT process is applied to eject molten metal droplets from a donor substrate close to the circuit substrate onto the defined trajectory. (A “trajectory” typically comprises a line of a specified width extending between two endpoints (e.g., a pair of metal terminals on the substrate); however, traces extending along other shapes can be similarly defined and manufactured.) The droplets adhere to and harden on the circuit substrate along the length of the trace's trajectory, but retain their separated granular structure during this stage.

[0041] Therefore, after droplet hardening, a laser beam with sufficient energy is directed toward the trajectory of the trace to cause the metal in the hardened droplets to melt and coalesce into a bulk layer extending along the length of the defined trajectory. The terms "coalescing" and "bulk layer" are used in the context of this specification and the claims to refer to a layer whose boundaries between hardened droplets are significantly reduced in size and distribution compared to the boundaries before melting. For example, in some embodiments, at least 50% of the boundaries present between hardened droplets after LIFT deposition but before laser melting are no longer visible under a microscope after laser melting.

[0042] In the disclosed embodiments, a pulsed laser beam is applied, consisting of a sequence of pulses providing laser energy along the length of the trajectory. Pulsed radiation advantageously concentrates the resulting heat locally within the metal of the trace and minimizes heat loss and potential damage by conducting heat from the trace to the circuit substrate. Depending on the thickness and width of the trace, the pulse duration can be less than 10 μs or even less than 1 μs for narrow traces. For example, in one embodiment, metal droplets can be deposited on the circuit substrate in a single row extending along the length of the defined trajectory, with a predetermined overlap between the droplets. Laser pulses with a width of 10 μs or even less are then applied to cause the hardened droplets to melt and coalesce into the circuit trace.

[0043] The controlled laser melting process provided by embodiments of the present invention can be applied to the entire volume of a hardened droplet in a trace (particularly when the trace is thin, as in the examples above). Alternatively, laser melting can be applied only to the outer layer of the hardened droplet to thus form a protective "skin" that encloses the remaining volume of the trace. In either case, the controlled laser melting process improves both the mechanical and electrical integrity of the resulting trace. In some cases, the melting process also improves the adhesion of the trace to the substrate and its ability to withstand subsequent etching steps. The invention can also be applied to the ends of traces where LIFT printed circuit traces contact existing terminals on the circuit substrate to thus strengthen the electrical and mechanical connection between the trace and the terminals. When the terminals and the droplet comprise different metal compositions, such controlled laser melting can be used to form heterogeneous metallic bonds.

[0044] System Description

[0045] Figure 1 This is a schematic side view of a system 20 for printing conductive traces 22 onto a substrate 24 according to an embodiment of the present invention. The substrate 24 may include any suitable type of circuit substrate known in the art, such as semiconductor, ceramic, metal, organic, and other dielectric substrates known in the art. The substrate 24 may be rigid or flexible; and among other things, the techniques described herein are particularly suitable for printing circuit traces and other conductive structures onto fragile substrates that cannot tolerate the heat and corrosive chemicals typically used in printed circuit fabrication. During the printing process, the substrate 24 is held on a suitable mounting base (e.g., an adjustable mounting base (e.g., a translation stage 50)).

[0046] System 20 includes a laser module 26 comprising one or more lasers and suitable optics for guiding one or more suitable laser beams toward substrate 24. In the depicted embodiment, laser module 26 includes both a LIFT laser 28 and a molten laser 30. For simplicity, the functions and properties of these lasers are described herein as if they were separate units (which are possible embodiments of laser module 26). Alternatively, a single laser emitting short, high-energy pulses with variable pulse durations can perform the functions of both LIFT laser 28 and molten laser 30. Lasers 28 and 30 emit optical radiation in the visible, ultraviolet, and / or infrared range at suitable wavelengths and with suitable time pulse lengths and focal quality to perform the functions described herein, as will be further described in detail below.

[0047] The control circuitry 52 controls the operation of the laser module 26 and other components of the system 20 autonomously or under the control of a human operator. To evaluate the printing process and align it with features on the substrate 24, an inspection module 54, including one or more optical sensors, may be incorporated into the system 20 to capture images of the substrate and transmit the image data to the control circuitry 52 for analysis. The control circuitry 52 typically includes a general-purpose computer processor programmed in software to implement the functions described herein and suitable interfaces for communicating with and controlling other components of the system 20. Alternatively or additionally, at least some functions of the control circuitry 52 may be implemented by a hardwired or programmable digital signal processor (DSP) or hardware logic components.

[0048] Under the control of the control circuitry system 52, the LIFT laser 28 emits short pulses with a pulse duration typically of about 1 ns toward the donor assembly 36. The donor assembly 36 acts as a deposition module that ejects molten metal droplets 42 onto the defined path of the conductive trace 22 via a LIFT process driven by the LIFT laser 28. The donor assembly 36 includes a donor substrate 38, which typically comprises a thin flexible sheet of transparent material coated on the side adjacent to the circuit substrate 24 and a donor film 40 comprising a specified metal or metal combination. (The donor film may include, for example, sublayers of an adhesive film, as will be referenced below.) Figure 3 (To describe.) Alternatively, the donor substrate 38 may comprise a rigid or semi-rigid material. The beam deflector 32 (e.g., a rotating mirror and / or acousto-optic device) and focusing optics 34 guide the radiation pulses from the LIFT laser 28 through the upper surface of the donor substrate 38 and thus impact the donor film 40 on the lower surface according to a spatial pattern determined by the control circuit system 52.

[0049] Each laser pulse induces one or more molten droplets 42 of metal to be ejected from the donor film 40 onto the substrate 24. The duration and energy of the laser pulse (typically having a pulse duration in the nanometer range) and the thickness of the donor film 40 can be selected such that each laser pulse causes a single molten droplet 42 to be ejected from the donor film with precise directionality and high speed toward the circuit substrate. Further details of such LIFT operations are described in U.S. Patent 9,925,797, mentioned above. In the depicted example, droplets 42 adhere to and harden on the substrate, thus defining a line of hardened droplets 44. Each droplet adds a certain amount of metal material to the line. The control circuitry 52 sets the number of droplets to be deposited and the spacing between consecutive droplets according to the desired thickness of the line 22. Thus, to produce a very thin line, droplets 42 can be deposited with only partial overlap between consecutive droplets, such that the width and height of the line 22 will be approximately equal to the width and height of a single droplet 44. This method can be used to produce very thin lines with widths down to the micrometer range. Alternatively, thicker polymerization of droplet 44 can be used to produce wider and deeper lines.

[0050] After the droplet 44 is deposited, the melting laser 30 irradiates the line of the droplet with sufficient energy to induce metal melting, causing the droplet to melt along the line 22 into the bulk material. A beam deflector 46 (e.g., a scanning mirror and / or acousto-optic device) and focusing optics 48 guide the radiation from the melting laser 30 to impact the target line. The beam energy and other parameters of the melting laser 30 are selected to melt the metal in the droplet 44 while minimizing thermal damage to the substrate 24 and surrounding structures. The beam may have sufficient energy to melt the entire volume of the line or only a portion of the volume (e.g., melting the thick outer layer of the droplet and not necessarily the entire volume).

[0051] In some embodiments, the melting laser 30 emits a pulse sequence of laser energy instead of a CW beam to ensure that the thermal effects of the melting step are well localized, with minimal impact on the substrate 24 and surrounding structures. Using short pulses also helps prevent metal droplets from coalescing into spheres, allowing the trace to maintain its desired shape. Optics 48 focuses the beam to strike the target line with a beam diameter too small to melt adjacent structures. For this purpose, the beam diameter may be smaller than the line width. However, the beam diameter is large enough to melt the entire area of ​​the trace already covered by droplets 44. Beam deflector 46 scans the beam of the melting laser 30 along the trajectory of trace 22 such that each pulse has a predetermined overlap with previous pulses in the sequence. The scan rate is adjusted to apply an appropriate thermal dose uniformly along the entire trace.

[0052] The pulse duration of the pulses output by the melting laser 30 is typically less than 100 μs; and for fine melting features, the pulses are even shorter, for example, less than 10 μs. Depending on the droplet composition and the desired melting depth, the duration of each pulse can even be less than 1 μs. The pulse energy is typically in the range of 0.1 μJ to 100 μJ, depending on the material and trace size. Using short, dense laser pulses is beneficial for reducing heat transfer to the substrate 24 and reducing metal oxidation during the melting process, allowing the process to be carried out under ambient atmospheric conditions. Using short laser pulses also advantageously reduces the tendency of metal in the droplet to coalesce into individual spheres and lose the desired shape characteristics of trace 22. The time between pulses in the sequence can be long enough for heat from the previous pulse to dissipate, so that heat accumulation does not become a problem.

[0053] To enable adjustment of pulse duration for different trace sizes and melting depths, the melting laser 30 may include, for example, a fiber laser or a high-power diode laser. If the laser has a sufficiently wide adjustment range of pulse duration down to the nanometer range, it may also function as a LIFT laser 28.

[0054] The following figures and accompanying descriptions illustrate several techniques that can be combined with controlled laser melting for LIFT printing of metal traces. For clarity and specificity, these techniques will be described below with reference to the components of system 20. However, these techniques are by no means limited to... Figure 1 The invention is illustrated with respect to the specific system configurations shown; and those skilled in the art will understand, upon reading this invention, that the principles of the invention can be alternatively applied to other systems with the necessary capabilities. All such alternative embodiments are considered to be within the scope of this invention.

[0055] Printing metal wires with different widths and thicknesses

[0056] Figure 2A This is a microscopic photograph of a line of metal droplets 44 printed on a circuit substrate 24 by a LIFT laser 28 according to an embodiment of the present invention. The droplets 44 have a diameter of about 1 μm and are printed as a single row extending along the length of the trace 22, wherein the overlap between consecutive droplets in the sequence is about 50% of the droplet diameter. Alternatively, when very narrow traces are desired, the overlap between consecutive droplets may be even less than 50%.

[0057] Figure 2B This is a microscopic photograph of the trace 22 after laser melting by a melting laser 30 according to an embodiment of the present invention. An overlapping laser pulse sequence was scanned on the droplet 44 to cause it to coalesce into the integral trace 22 shown in this figure with a linewidth of approximately 1 μm. The optimal laser pulse parameters for achieving such uniform metallic traces depend on the material and geometry involved and can be optimized in each case through computation and empirical trial and error.

[0058] Figure 3 This is a schematic cross-sectional view of the donor assembly 36 according to an embodiment of the present invention, illustrating the ejection of molten droplets 42 from the donor film 40 under laser irradiation. This embodiment aims to address the problem of poor adhesion between droplets 44 and the substrate 24, which can occur particularly when printing very fine traces and on smooth substrates such as glass.

[0059] To address this issue, the donor film 40 includes an adhesion film 62 overlaid on the host metal donor film 60 on the donor substrate 38. For example, assuming film 60 comprises copper (which is a good conductor but does not adhere well to a dielectric substrate), then the adhesion film 62 may comprise another metal that oxidizes faster than copper, such as titanium, tin, bismuth, or an alloy of these metals. An intermediate layer 64 is also optionally deposited between the donor substrate 38 and the host metal donor film 60 to enhance the adhesion of the donor film to the donor substrate and reduce laser energy reflection at the substrate / film interface. In this embodiment, the thickness of the host metal donor film is typically between 50 nm and 700 nm, while the adhesion film is thinner, for example, between 50 nm and 200 nm.

[0060] like Figure 3 As shown, when the laser pulse irradiates the donor film 40, the metal in the adhesion film 62 forms an outer layer on the droplet 42 to surround the main metal from the film 60. This outer layer adheres to the circuit substrate 24 immediately after the molten droplet impacts the circuit substrate. Due to the speed of the printing process, the outer layer does not substantially mix into the metal core of the droplet 42 during droplet flight. However, as an alternative to this method, the donor film 40 may comprise an alloy with enhanced adhesion properties. Additionally or alternatively, the surface of the substrate may be roughened or otherwise prepared prior to LIFT printing to improve adhesion.

[0061] Figure 4A This is a schematic cross-sectional view of the polymerization of metal droplets 44 deposited on substrate 24 during the LIFT process, defining circuit traces according to an embodiment of the present invention. In this embodiment, the traces are more... Figure 2A The examples shown in / B are wider and deeper.

[0062] Figure 4B According to an embodiment of the present invention, complete laser melting is used. Figure 4AThe diagram shows a schematic cross-sectional view of the circuit trace 70 formed by the polymerization of the metal droplet 44. In this case, the melting laser 30 applies sufficient energy to the hardening droplet 44 to melt the entire volume of the hardened droplet in the trace 70. This method is beneficial for maximizing mechanical integrity and thermal conductivity while minimizing the resistance of the trace, but it should be applied carefully to avoid damaging the circuit substrate 24 and surrounding structures. In an embodiment (not shown in the figure), the beam deflector 46 guides the beam from the melting laser 30 within the incident angle range to impact the volume of the droplet 44 to achieve more uniform melting.

[0063] Figure 4C This is an alternative embodiment of the invention using partial laser melting. Figure 4A The diagram shows a schematic cross-sectional view of the circuit trace formed by the polymerization of metal droplets 44. In this case, the melting laser 30 applies sufficient energy to the hardened droplet to melt only the outer layer of the hardened droplet, rather than the entire volume of the hardened droplet along the length of the trace. This outer layer forms a protective surface layer 72, which encloses the volume of the hardened droplet 44 within the conductive trace. Surface layer 72 enhances the mechanical and electrical integrity of the trace, as well as its resistance to etching and corrosion. This method requires a much smaller laser energy input, thus increasing process throughput, while reducing the risk of damage to the substrate 24 and trace deformation relative to the complete melting of the trace volume. In this embodiment, the laser beam used in the melting process is focused to a point size smaller than the width of the trace and scans the surface of the trace until it covers the entire area.

[0064] The table below lists examples of process parameters that can be used in controlled laser melting for LIFT-deposited metal traces of various sizes. In these examples, droplet 44 comprises copper, and the polymerization of the droplets on substrate 24 has… Figure 4A The general form shown in the figure. It can be adjusted according to the desired melting depth (which can range from approximately 1 μm). Figure 4C ) to the full thickness of the trace (e.g. Figure 4B Within the range of ), the number of melting laser pulses applied to each position along the trace in each case can be selected.

[0065] Table I – Examples of Controlled Laser Melting

[0066] trace width laser wavelength Pulse repetition rate Laser dot size Pulse width Pulse pitch Pulse energy 20-40 µm Near-infrared 20 kHz 20 µm 250 ns 0.5-2 µm 15-30 µJ 20-40 µm Near-infrared 40 kHz 20 µm 1-2 µs 0.5-2 µm 20-40 µJ 7-12 µm Near-infrared 20 kHz 20 µm 0.5-2 µs 0.5 µm 12-25 µJ 20 µm Visible light (532 nm) 20 kHz 20 µm 600 ns 1 µm 30-45 µJ 5-10 µm Near-infrared 20 kHz 7 µm 0.5-2 µs 0.5 µm 5-10 µJ

[0067] The above examples illustrate the broad applicability and scope of the controllable parameters provided by the technology of this invention, especially in forming stable narrow traces that are difficult or impossible to manufacture by other technologies. The pulse width can be selected according to the melt depth, and multiple LIFT / melt cycles can be performed when complete melting of thick traces is required. The pulse pitch and repetition rate can also affect the overall heat distribution and thus the melt depth. The laser spot size is generally selected to approximately match the width of the trace. The laser wavelength can also be selected so that the laser energy is well absorbed by the trace but not by the substrate, thereby minimizing damage to the substrate when the laser spot extends over a region wider than the trace.

[0068] Manufacturing and repair of circuit components

[0069] Return to reference Figure 1 In some embodiments, the trace 22 to be printed includes a gap between a pair of terminals on a circuit substrate. For example, the control circuitry system 52 can identify this gap by analyzing an image of the circuit substrate 24 captured by the inspection module 54. The control circuitry system 52 then directs the laser module 26 to eject molten droplets 42 from the donor film 40 onto the substrate 24 to fill the gap. In some embodiments, the gap thus identified may be due to defects, such as deformation or open-circuit traces detected on the circuit substrate 24. In this case, before filling the gap, the defective traces and underlying substrate may be cleaned and prepared, for example, using laser ablation as described in Korean Patent Publication Application KR20150070028 mentioned above. This preparation may include shaping the ends of the circuit traces adjacent to the gap to form well-defined terminals for which the droplets 42 will adhere. The defect is then repaired by depositing molten droplets 42 into the gap between the terminals and then guiding the beam of the molten laser 30 to melt and harden the droplets 44.

[0070] In other embodiments, the circuit traces containing gaps are intentionally formed on the circuit substrate 24, for example, by photolithography. These gaps can then be filled by LIFT printing with a material different from the circuit traces (e.g., a resistive material, such as NiCr). Figure 7 The process described herein can be used to produce circuit components such as resistors and strain gauges.

[0071] Figures 5A to 5C This is a schematic cross-sectional view illustrating a stage in the process of filling gap 82 in the circuit trace according to an embodiment of the present invention. Figure 5AThis illustrates the polymerization of hardened metal droplets 44 deposited in gaps 82 within a circuit trace 80 by a LIFT process. In this case, it is assumed that gaps 82 are due to defects in the initial fabrication of trace 80. The edges of gaps 82 have been squared to create well-defined terminals, including a stepped shape at the edges of the gaps. This pretreatment enables the uniform deposition of droplets 44 within the gaps and achieves good electrical contact between the droplets and the terminals. In this example, droplets 44 have been deposited in a single LIFT deposition step to fill the entire depth of gap 82.

[0072] Figure 5B This illustrates the application of a laser melting process to the polymerization of droplet 44. A pulsed beam 84 from the melting laser 30 is focused onto the outer surface of the polymerized droplet and scans across the gap 82, as indicated by arrow 86.

[0073] Figure 5C The display is by Figure 5B The circuit trace 80 is formed by the laser melting process. The upper layer 88 of the droplet 44 has melted and bonded to the metal of the trace 80 to form a surface layer covering the underlying hardened droplet 44. The depth of the layer 88 is determined by the intensity of the laser beam 84 and the scanning pattern.

[0074] Figures 6A to 6D This is a schematic cross-sectional view illustrating a stage of the process of filling gaps in circuit trace 80 according to another embodiment of the present invention. Figure 6A This describes the application of a laser melting process to the polymerization of molten metal droplets 90 deposited in the gaps of circuit traces 80 by a LIFT process. In this case, a layering method is applied so that the droplets 90 do not fill the entire depth of the gap but instead form a first layer on the circuit substrate. A laser beam 84 scans across the gap to melt the hardened droplets in this first layer.

[0075] Figure 6B The display is by Figure 6A The laser melting process forms a portion of the circuit traces. In this example, the entire depth of the droplet 90 has been melted by the laser beam 84 to form a lower layer 92 of conductive traces within the gaps of the circuit traces 80.

[0076] Figure 6C This describes the application of a laser melting process to the further polymerization of the molten metal droplet 94 via a scanning beam 84. The droplet 94 is deposited onto the underlying layer 92 by a LIFT process, and then the laser beam 84 is used to scan the droplet 94 to further polymerize it, thus melting and hardening the droplet.

[0077] Figure 6D The display is by Figure 6C The full circuit trace formed by the laser melting process. Figure 6CThe controlled laser melting process has formed an upper layer 96 on the lower layer 92 to fill the gaps in the trace 80 and thus complete the trace. This layering method is used to ensure that the trace is completely melted and coalesces into the bulk material throughout its entire depth, while reducing heat dissipation to the trace 80 and the substrate 24 (and thus mitigating potential thermal damage). Although Figure 6A For simplicity, only a two-layer process is shown up to D, but the principles of the invention can also be applied to generate three or more layers, depending on the desired trace thickness.

[0078] Figure 7 This is a schematic cross-sectional view of a heterogeneous circuit trace printed by a LIFT process according to an embodiment of the present invention. In this embodiment, trace 80 includes a first metal, such as copper, which is etched or stripped to define terminals 100. A donor film 40 includes a different metal, such as NiCr, having a composition different from the first metal. A LIFT laser 28 is operated to deposit droplets of NiCr into the gaps between the terminals 100. Subsequently, a melting laser 30 is operated to not only cause the NiCr droplets to melt and coalesce into trace 102, but also to melt at least the upper layer of the terminals 100 to form heterogeneous metal bonds 104 at the terminals. These bonds 104 are used to create intermetallic contacts with low resistance and high mechanical strength. As previously mentioned, trace 102 can serve as, for example, an embedded resistor or strain gauge.

[0079] Even when the traces and terminals consist of the same metal, the operation of the molten laser 30 is used in a similar manner to form homogeneous metallic bonds between the traces and terminals. In the case of heterogeneous bonds, these metallic bonds enhance mechanical strength, etch resistance, and corrosion resistance while reducing electrical resistance.

[0080] The following table lists the options available for use with Figure 7 Examples of controlled laser melting process parameters for LIFT-deposited NiCr traces at the interface of copper circuit traces are shown in the figure.

[0081] Table II – Examples of Controlled Laser Melting of NICR Traces

[0082] trace width laser wavelength Pulse repetition rate Laser dot size Pulse width Pulse pitch Pulse energy 10 µm Visible light (532 nm) 1000 Hz 20 µm 600 ns 0.5-1 µm 20-40 µJ 10 µm Visible light (532 nm) 1000 Hz 20 µm 600 ns 10 µm 50-75 µJ

[0083] Figures 8A to 8E This is a schematic top view of a circuit trace 110 on a substrate 24, illustrating successive steps in a LIFT-based process for repairing gaps 112 in the circuit trace according to an alternative embodiment of the present invention. Figure 8A Show the gap 112 before the LIFT process is triggered. In this case, as by Figure 8BAs shown, droplets 114 are deposited by a LIFT process on a region wider than the gap 112. This type of deposition pattern will be generated, for example, when the LIFT laser 28 emits short pulses (e.g., in the picosecond range) with high peak power, causing each pulse to induce the ejection of numerous submicron droplets toward the substrate 24. The advantage of this approach is that the hardened droplets are smaller and adhere better to the substrate, but the directionality of the droplet ejection is less accurate.

[0084] To reduce the width of the area covered by droplet 114, the melting laser 30 is applied to harden the droplet only within a predetermined width of the trajectory of the deposited trace on the substrate 24. Therefore, as... Figure 8C As shown, trace region 116 melts and coalesces to form a solid trace, which bonds to circuit trace 110. To increase the thickness of the LIFT-deposited trace, the LIFT step can be repeated to deposit one or more additional layers of droplets 118 onto the region of gap 112, such as... Figure 8D As shown in the diagram. After each step, repeat. Figure 8C The controlled laser melting step allows additional droplets to melt and coalesce within (but not outside) the trace region 116.

[0085] After the metal in trace region 116 reaches the desired depth, an etching process is applied to circuit substrate 24 to remove hardened droplets deposited on the circuit substrate outside trace region 116. This step can be performed, for example, using chemical etching or electrochemical etching methods known in the art, because individual droplets outside region 116 have a large surface area relative to their volume and are therefore more susceptible to the etching process. Alternatively, hardened droplets can be removed by laser ablation. Figure 8E The image shows the clean traces after the etching process.

[0086] It should be understood that the above embodiments are illustrative by way of example, and the present invention is not limited to the content specifically shown and described above. Specifically, the scope of the present invention includes combinations and sub-combinations of the various features described above, as well as variations and modifications thereof that will be understood by those skilled in the art after reading the above description and that are not disclosed in the prior art.

Claims

1. A method for circuit fabrication comprising: defining a trajectory of a conductive trace to be formed on a circuit substrate; ejecting molten droplets of a metal from a donor substrate proximate to the circuit substrate onto the defined trajectory by a process of laser induced forward transfer (LIFT), whereby the droplets adhere to and harden on the circuit substrate along a length of the defined trajectory; and after the droplets have hardened, directing a laser beam toward the defined trajectory with sufficient energy to cause the metal in the hardened droplets to melt and coalesce into a bulk layer extending along the length of the defined trajectory, wherein the donor substrate is transparent and has opposing first and second surfaces, and includes a donor film of the metal disposed on the second surface such that the donor film is proximate to the defined trajectory, wherein the donor film includes a first metal, and wherein an adhesion film including a second metal is disposed on the donor film on the donor substrate, and wherein the second metal oxidizes more quickly than the first metal, such that when the laser beam illuminates the donor film, the second metal forms an outer layer on the molten droplets of the first metal, and the outer layer adheres to the circuit substrate as soon as the molten droplets impact the circuit substrate.

2. The method of claim 1, wherein ejecting the molten droplets includes directing pulses of laser radiation through the first surface of the donor substrate and impinging the donor film to induce ejection from the donor film onto the defined trajectory of the molten droplets of the metal.

3. The method of claim 2, wherein directing the pulses of laser radiation in the process of laser induced forward transfer and directing the laser beam toward the defined trajectory includes using a single laser with variable pulse duration to eject the molten droplets and to melt the metal in the hardened droplets.

4. The method of claim 1, wherein the adhesion film is thinner than the donor film.

5. The method of claim 1, wherein the first metal includes copper, and wherein the second metal is selected from the group consisting of titanium, tin, bismuth, and alloys thereof.

6. The method of claim 1, wherein ejecting the molten droplets and directing the laser beam toward the defined trajectory includes: ejecting a first layer of the molten droplets onto the circuit substrate and directing the laser beam to melt the hardened droplets in the first layer to form a lower layer of the conductive trace; and ejecting at least a second layer of the molten droplets onto the lower layer and directing the laser beam to melt the hardened droplets in the at least second layer to complete the conductive trace.

7. The method of claim 1, wherein directing the laser beam includes using the laser beam to apply sufficient energy to the hardened droplets to melt an entire volume of the hardened droplets in the conductive trace.

8. The method of claim 1, wherein directing the laser beam comprises using the laser beam to apply sufficient energy to the hardened droplet to only melt an outer layer of the hardened droplet, rather than melt an entire volume of the hardened droplet along the length of the defined track.

9. The method of claim 1, wherein directing the laser beam comprises directing a sequence of pulses of laser energy to impinge the hardened droplet along the length of the defined track.

10. The method of claim 9, wherein each of the pulses has a pulse duration of less than 10 μβ.

11. The method of claim 9, wherein directing the pulses comprises scanning the laser beam along the track such that each of the pulses has a predetermined overlap with a previous pulse in the sequence.

12. The method of claim 1, wherein ejecting the molten droplet comprises depositing the droplet on the circuit substrate as a single row extending along the length of the defined track, whereby the conductive trace is formed by melting the single row, wherein each of the droplets overlaps a previous droplet in the single row by no more than 50% of a diameter of the droplet.

13. The method of claim 1, wherein defining the track comprises identifying a gap between a first terminal and a second terminal on the circuit substrate, and wherein ejecting the molten droplet comprises depositing the molten droplet to fill the gap.

14. The method of claim 13, wherein the first terminal and the second terminal comprise a first metal and the second metal has a composition different than the first metal, and wherein directing the laser beam comprises melting the first metal and the second metal to form a heterogeneous metal bond at the first terminal and the second terminal.

15. The method of claim 13, wherein identifying the gap comprises detecting a defect that has been formed in a circuit trace on the circuit substrate, and wherein the defect is repaired by depositing the molten droplet and then directing the laser beam to melt the hardened droplet.

16. An apparatus for fabricating a conductive trace on a circuit substrate, the apparatus comprising: a deposition module configured to eject molten droplets of a metal from a donor substrate in proximity to the circuit substrate onto a defined track of the conductive trace by a process of laser-induced forward transfer (LIFT), whereby the droplets adhere to the circuit substrate and harden thereon along a length of the defined track; and a laser module configured to direct a laser beam toward the defined track using sufficient energy to cause the metal in the hardened droplets to melt and coalesce into a bulk layer extending along the length of the defined track, wherein the donor substrate is transparent and has opposing first and second surfaces, and includes a donor film of the metal disposed on the second surface such that the donor film is in proximity to the defined track, wherein the donor film comprises a first metal, and wherein an adhesion film comprising a second metal is disposed on the donor film on the donor substrate, and wherein the adhesion film comprises a second metal, and wherein the donor film comprises a first metal, and wherein the adhesion film is disposed on the donor film on the donor substrate, and wherein the second metal oxidizes faster than the first metal, such that when the laser beam irradiates the donor film, the second metal forms an outer layer on the molten droplet of the first metal, and the outer layer adheres to the circuit substrate after the molten droplet impacts the circuit substrate.

17. The apparatus of claim 16, wherein the laser module is configured to direct pulses of laser radiation through the first surface of the donor substrate and impinge the donor film to induce the defined trajectory of the molten droplets of the metal ejected from the donor film.

18. The apparatus of claim 17, wherein the laser module comprises a single laser with variable pulse duration for directing the pulses of laser radiation in the process of the laser-induced forward transfer and directing the laser beam to melt the metal in the hardened droplet.

19. The apparatus of claim 16, wherein the deposition module and the laser module are configured to cause a first layer of the molten droplets to be ejected onto the circuit substrate and direct the laser beam to melt the hardened droplets in the first layer to form a lower layer of the conductive trace, and cause at least a second layer of the molten droplets to be ejected onto the lower layer and direct the laser beam to melt the hardened droplets in the at least second layer to complete the conductive trace.

20. The apparatus of claim 16, wherein the laser module is configured to use the laser beam to apply sufficient energy to the hardened droplet to melt an entire volume of the hardened droplet in the conductive trace.

21. The apparatus of claim 16, wherein the laser module is configured to use the laser beam to apply sufficient energy to the hardened droplet to melt only an outer layer of the hardened droplet, but not an entire volume of the hardened droplet along the length of the defined trajectory.

22. The apparatus of claim 21, wherein the laser module is configured to direct a sequence of pulses of laser energy to impinge the hardened droplet along the length of the defined trajectory.

23. The apparatus of claim 22, wherein each of the pulses has a pulse duration of less than 10 μβ.

24. The apparatus of claim 22, wherein the deposition module is configured to deposit the droplets on the circuit substrate in a single row extending along the length of the defined trajectory, whereby the conductive trace is formed from the single row.

25. The apparatus of claim 24, wherein the laser module is configured to scan the laser beam along the trace such that each of the pulses has a predetermined overlap with a previous pulse in the sequence, wherein each of the droplets overlaps a previous droplet in the single row by no more than 50% of a diameter of the droplet.

26. The apparatus of claim 16, comprising control circuitry configured to identify a gap between a first terminal and a second terminal on the circuit substrate and control the deposition module to deposit the molten droplets to fill the gap.

27. The apparatus of claim 26, wherein the first terminal and the second terminal comprise a first metal, and the second metal has a composition different than the first metal, and wherein the laser module is configured to direct the laser beam to melt the first metal and the second metal to form a heterogeneous metal bond at the first terminal and the second terminal.

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