LIFT printing of fine metal wires

Abstract

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

Description

Technical Field

[0001] The present invention generally relates to the fabrication of electronic devices, and more particularly, to methods and systems for printing conductive lines on a substrate.

Background Art

[0002] Cross-reference to related applications This application claims priority to a provisional patent application filed on December 28, 2020, and assigned U.S. Patent Application No. 63 / 130854, the entire disclosure of which is incorporated herein by reference.

[0003] In laser direct writing (LDW) technology, a laser beam is used to create a patterned surface with a spatially resolved three-dimensional structure by controlled material ablation or deposition. Laser-induced forward transfer (LIFT) is an LDW technique that can be applied to deposit micro-patterns on a surface.

[0004] In LIFT, laser photons provide a driving force to eject a small volume of material from a donor film towards an acceptor substrate. Typically, the laser beam interacts with the inside of a donor film coated on a non-absorbing carrier substrate. In other words, the incident laser beam propagates through a highly transmissive carrier substrate before the photons are absorbed by the inner surface of the film. When a specific energy threshold is exceeded, the material is ejected from the donor film towards the surface of the acceptor substrate. By appropriately selecting the donor film and laser beam pulse parameters, the laser pulse ejects a molten droplet of the donor material from the film and then lands and cures it on the acceptor substrate.

[0005] LIFT systems are particularly (but not exclusively) useful for printing conductive metal droplets and traces for the purpose of fabricating electronic circuits. Such LIFT systems are described, for example, in U.S. Patent No. 9,925,797, which is incorporated herein by reference. This patent describes a printing apparatus including a donor supply assembly configured to provide a highly transparent donor substrate having a first surface and a second surface, and a donor film formed on the second surface to position the donor film in close proximity to a target area on an acceptor substrate. The optical assembly is configured to simultaneously direct multiple output beams of laser radiation in a predetermined spatial pattern so as to pass through the first surface of the donor substrate and strike the donor film in order to induce the ejection of material from the donor film onto the acceptor substrate and write a predetermined pattern on a target area on the acceptor substrate.

[0006] LIFT printing can also be used to repair defects in printed circuit traces. Systems and methods for this purpose are described, for example, in Korean Patent Application Publication No. 20150070028, which is incorporated herein by reference.

[0007] In addition, the LIFT system can be used in the direct printing of embedded resistors onto a substrate. For example, PCT International Publication 2019 / 138404 describes a method for fabricating an electrical device, which includes identifying a trajectory on a circuit board where a resistor having a specified resistance will be formed between a first and second endpoint of the trajectory, and which is incorporated herein by reference. A highly transparent donor substrate having a donor film comprising a resistive material formed covering the second surface, with the second surface facing the circuit board, is positioned in close proximity to the identified trajectory on the circuit board. A pulse of laser radiation is directed to strike the donor film to induce the ejection of droplets of resistive material from the donor film onto the circuit board at each neighborhood position along the trajectory, with separation between neighborhood positions selected to form a circuit trace having a specified resistance between the first and second endpoints. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] U.S. Patent Application Publication No. 2018 / 281243 [Patent Document 2] U.S. Patent Application Publication No. 2010 / 035375 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The LIFT process can print conductive traces and other circuit components on circuit boards with high precision and speed. However, due to the nature of the LIFT process, the resulting traces consist of aggregates of metal particles equivalent to hardened droplets ejected onto the substrate. These particles are typically covered and separated by a thin oxide layer, and voids and air pockets may exist scattered between the particles. These phenomena tend to increase electrical resistance and impair the mechanical integrity of the circuit traces compared to solid metal traces deposited using more conventional methods. [Means for solving the problem]

[0010] Embodiments of the present invention described below in this specification provide novel methods and systems for fabricating LIFT bases of metal traces on a substrate, as well as circuits produced by such methods.

[0011] Accordingly, embodiments of the present invention provide a method for circuit fabrication, which includes defining the trajectory of a conductive trace to be formed on a circuit board. A metal droplet is ejected from a donor substrate adjacent to the circuit board onto the defined trajectory by a laser-induced forward transfer (LIFT) process, thereby causing the droplet to adhere to the circuit board and harden along the length of the defined trajectory. After the droplet has hardened, a laser beam is directed towards the defined trajectory with sufficient energy to melt the metal in the hardened droplet and coalesce into a bulk layer extending along the length of the defined trajectory.

[0012] In some embodiments, the donor substrate is highly transparent and has opposing first and second surfaces, and a metal-containing donor film is disposed on the second surface such that the donor film is close to a defined trajectory, and ejecting molten droplets involves directing pulses of laser radiation to pass through the first surface of the donor substrate and strike the donor film in order to induce ejection of molten metal droplets from the donor film onto a defined trajectory.

[0013] In one embodiment, directing pulses of laser radiation and directing the laser beam toward a defined trajectory in the LIFT process involves using a single laser with a variable pulse duration for both ejecting molten droplets and melting metal within the hardened droplets.

[0014] Alternatively or additionally, the donor film comprises a first metal, and an adhesion film comprising a second metal is disposed on the donor substrate covering the donor film, the second metal forming an outer layer covering molten droplets of the first metal, the outer layer adhering to the circuit board upon impact of the molten droplets to the circuit board. In the disclosed embodiments, the first metal comprises copper, and the second metal is selected from the group consisting of titanium, tin, bismuth, and alloys thereof.

[0015] In some embodiments, ejecting molten droplets and directing a laser beam toward a defined trajectory includes ejecting a first layer of molten droplets onto a circuit board and directing the laser beam to melt the hardened droplets within the first layer in order to form a lower layer of a conductive trace, and ejecting at least a second layer of molten droplets onto the lower layer and directing the laser beam to melt the hardened droplets within at least the second layer in order to complete the conductive trace.

[0016] In one embodiment, directing the laser beam involves using the laser beam to apply sufficient energy to the cured droplet to melt its entire volume within the conductive trace. Alternatively, directing the laser beam involves using the laser beam to apply sufficient energy to the cured droplet to melt only the outer layer of the cured droplet along the length of a defined trajectory, without melting its entire volume. Typically, the outer layer forms a protective skin that encloses the volume of the cured droplet within the conductive trace.

[0017] In the disclosed embodiments, directing a laser beam involves directing a sequence of pulses of laser energy to strike a hardened droplet along a defined trajectory length. In some of these embodiments, each pulse has a pulse duration of less than 10 μs and may be less than or equal to 1 μs. Alternatively or additionally, directing one or more pulses involves scanning the laser beam along the trajectory such that each pulse has a predetermined overlap with a preceding pulse in the sequence.

[0018] In some embodiments, ejecting molten droplets involves depositing droplets onto a circuit board in a single row extending along a defined trajectory length, thereby forming a conductive trace by melting the single row. In one such embodiment, each droplet overlaps with a preceding droplet in the single row by no more than 50% of its diameter.

[0019] In a further embodiment, defining a trajectory includes identifying a gap between a first terminal and a second terminal on a circuit board, and ejecting a molten droplet includes depositing the molten droplet to fill the gap. In one embodiment, the first and second terminals comprise a first metal, the droplet comprises a second metal having a different composition from the first metal, and directing a laser beam includes melting the first and second metals to form a dissimilar metal joint at the first and second terminals. Alternatively, identifying a gap includes detecting a defect in a circuit trace formed on a circuit board, and the defect is repaired by depositing a molten droplet and then directing a laser beam to melt the hardened droplet.

[0020] In the disclosed embodiments, the locus has a predetermined width, and directing the laser beam includes melting only the hardened droplets deposited on the circuit board within the predetermined width of the locus. The method includes applying an etching process to remove the hardened droplets deposited on the circuit board outside the predetermined width of the locus after directing the laser beam.

[0021] Embodiments of the present invention also provide an apparatus for fabricating conductive traces on a circuit board. The apparatus includes a deposition module configured to eject molten droplets of metal from a donor substrate proximate to the circuit board onto a defined locus of a conductive trace by a process of laser-induced forward transfer (LIFT), whereby the droplets adhere and harden to the circuit board along the length of the defined locus. A laser module is configured to direct a laser beam towards the defined locus with sufficient energy to melt the metal within the hardened droplets and fuse it into a bulk layer extending along the length of the defined locus.

[0022] The present invention will be more fully understood from the following detailed description of embodiments of the invention in conjunction with the drawings.

Brief Description of the Drawings

[0023] [Figure 1] FIG. 1 is a schematic side view of a system for printing a conductive trace on a substrate according to an embodiment of the present invention. [Figure 2A] FIG. 2 is a diagram showing a micrograph of a line of metal droplets printed on a substrate according to an embodiment of the present invention. [Figure 2B] FIG. 3 is a diagram showing a micrograph of the line of FIG. 2A after laser melting according to an embodiment of the present invention. [Figure 3] FIG. 4 is a schematic cross-sectional view of a donor film exemplifying the ejection of molten droplets from the film under laser irradiation according to an embodiment of the present invention. [Figure 4A] FIG. 5 is a schematic cross-sectional view of an aggregate of metal droplets deposited on a substrate in a LIFT process defining a circuit trace according to an embodiment of the present invention. [Figure 4B] This is a schematic cross-sectional view of a circuit trace formed by the complete laser melting of the metal droplet aggregate shown in Figure 4A, according to an embodiment of the present invention. [Figure 4C] This is a schematic cross-sectional view of a circuit trace formed by partial laser melting of the metal droplet aggregate shown in Figure 4A, according to an alternative embodiment of the present invention. [Figure 5A] This is a schematic cross-sectional view of an aggregate of metal droplets deposited in the gaps within a circuit trace by a LIFT process according to an embodiment of the present invention. [Figure 5B] Figure 5A is a schematic cross-sectional view of an assembly illustrating the application of a laser melting process to an assembly according to an embodiment of the present invention. [Figure 5C] This is a schematic cross-sectional view of a circuit trace formed by the laser melting process shown in Figure 5B, according to an embodiment of the present invention. [Figure 6A] This is a schematic cross-sectional view illustrating the application of a laser melting process to an aggregate of metal droplets deposited in gaps within a circuit trace by a LIFT process, according to an embodiment of the present invention. [Figure 6B] This is a schematic cross-sectional view of a partial circuit trace formed by the laser melting process shown in Figure 6A, according to an embodiment of the present invention. [Figure 6C] This is a schematic cross-sectional view illustrating the application of a laser melting process to an aggregate of metal droplets deposited by a LIFT process to cover a partial circuit trace in Figure 6B, according to an embodiment of the present invention. [Figure 6D] This is a schematic cross-sectional view of a complete circuit trace formed by the laser melting process shown in Figure 6C, according to an embodiment of the present invention. [Figure 7] This is a schematic cross-sectional view of heterogeneous circuit traces printed by a LIFT process according to an embodiment of the present invention. [Figure 8A] This is a schematic top view of a circuit trace on an object, illustrating a series of steps in a LIFT-based process for repairing gaps in a circuit trace according to an alternative embodiment of the present invention. [Figure 8B]This is a schematic top view of a circuit trace on an object, illustrating a series of steps in a LIFT-based process for repairing gaps in a circuit trace according to an alternative embodiment of the present invention. [Figure 8C] This is a schematic top view of a circuit trace on an object, illustrating a series of steps in a LIFT-based process for repairing gaps in a circuit trace according to an alternative embodiment of the present invention. [Figure 8D] This is a schematic top view of a circuit trace on an object, illustrating a series of steps in a LIFT-based process for repairing gaps in a circuit trace according to an alternative embodiment of the present invention. [Figure 8E] This is a schematic top view of a circuit trace on an object, illustrating a series of steps in a LIFT-based process for repairing gaps in a circuit trace according to an alternative embodiment of the present invention. [Modes for carrying out the invention]

[0024] The LIFT process can print conductive traces and other circuit components on circuit boards with high precision and speed. However, due to the nature of the LIFT process, the resulting traces consist of aggregates of metal particles equivalent to hardened droplets ejected onto the substrate. These particles are typically covered and separated by a thin oxide layer, and voids and air pockets may exist scattered between the particles. These phenomena tend to increase electrical resistance and impair the mechanical integrity of the circuit traces compared to solid metal traces deposited using more conventional methods.

[0025] Embodiments of the present invention described herein solve these problems by adding a controlled laser melting step within the deposition process. In these embodiments, after defining the trajectory of a conductive trace to be formed on a circuit board, a LIFT process is applied to eject molten droplets of metal from a donor substrate adjacent to the circuit board onto the defined trajectory. ("Trajectory" typically includes a line of a specified width extending between two endpoints, for example, between a pair of metal terminals on a substrate, but traces extending along trajectories of other shapes can be similarly defined and fabricated.) The droplets adhere to and harden on the circuit board along the length of the trace trajectory, but at this stage they still retain their distinct particle structure.

[0026] Therefore, after the droplets have hardened, the laser beam is directed towards the trace trajectory with sufficient energy to melt the metal within the hardened droplets and coalesce into a bulk layer extending along the defined trajectory length. The terms “coalition” and “bulk layer” are used in the context of this description and the claims to refer to a layer in which the boundaries between the hardened droplets are reduced to a noticeable extent in size and distribution compared to the boundaries before melting. For example, in some embodiments, at least 50% of the boundaries present between the hardened droplets after LIFT deposition and before laser melting are no longer detectable under microscopic examination after laser melting.

[0027] In the disclosed embodiments, the laser beam is pulsed, and a sequence of pulses of laser energy is applied along the length of the trajectory. The use of pulsed radiation is advantageous in that it locally concentrates the resulting heat within the metal of the trace, as well as minimizing heat loss and potential damage through heat conduction from the trace to the circuit board. Depending on the thickness and width of the trace, the pulse duration may be less than 10 μs, or even less than 1 μs for narrow traces. For example, in one embodiment, metal droplets may be deposited on a circuit board in a single row extending along the length of a defined trajectory, with predetermined overlap between the droplets. Short laser pulses are then applied to melt the hardened droplets and coalesce into a circuit trace having a width of 10 μs or less.

[0028] The controlled laser melting process provided by embodiments of the present invention can be applied to the entire volume of the cured droplet within the trace (especially when the trace is thin, as in the examples described above). Alternatively, the laser melting can be applied only to the outer layer of the cured droplet, forming 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 technique can also be applied at the edges of the trajectory, in which case the LIFT printed circuit trace comes into contact with existing terminals on the circuit board, thereby strengthening the electrical and mechanical connection between the trace and the terminals. This type of controlled laser melting can be used to form dissimilar metal joints when the terminals and droplets contain different metal compositions.

[0029] Figure 1 is a schematic side view of a system 20 for printing conductive traces 22 on a substrate 24 according to an embodiment of the present invention. The substrate 24 may include any suitable type of circuit board known in the art, such as semiconductor substrates, ceramic substrates, metal substrates, organic substrates, and other dielectric substrates. The substrate 24 may be either highly rigid or highly flexible, and the techniques described herein are particularly well suited for printing circuit traces and other conductive structures on delicate substrates that may not be resistant to the thermal and corrosive chemicals commonly used in printed circuit fabrication. During the printing process, the substrate 24 is held on a suitable mount, such as an adjustable mount, for example, a translational moving stage 50.

[0030] The system 20 comprises a laser module 26, which includes one or more lasers and suitable optical elements for directing the appropriate laser beam(s) toward the substrate 24. In the described embodiment, the laser module 26 includes both a LIFT laser 28 and a melting laser 30. For brevity, the functions and characteristics of these lasers are described here as if the lasers were separate units (one potential implementation form of the laser module 26). Alternatively, a single laser emitting short, high-energy pulses with variable pulse duration may perform the functions of both the LIFT laser 28 and the melting laser 30. Lasers 28 and 30 emit light emission in the visible, ultraviolet, and / or infrared regions at suitable wavelengths and with suitable time-pulse length and focus quality for performing the functions described herein, as will be further detailed in the following description.

[0031] The control circuit 52 controls the operation of the laser module 26 and other elements of the system 20, either autonomously or under the control of a human operator. For the purpose of evaluating the printing process and aligning the printing process with features on the substrate 24, an inspection module 54 equipped with one or more optical sensors may be incorporated into the system 20 to capture an image of the substrate and pass the image data to the control circuit 52 for analysis. The control circuit 52 typically comprises a general-purpose computer processor programmed in software to perform the functions described herein, along with a suitable interface for communicating with and controlling other components of the system 20. Alternatively or additionally, at least some of the functions of the control circuit 32 may be performed by a digital signal processor (DSP) or hardware logic component that may be wired or programmable.

[0032] The LIFT laser 28 emits short pulses toward the donor assembly 36 under the control of the control circuit 52, typically with a pulse duration of about 1 ns. The donor assembly 36 functions as a deposition module that ejects molten metal droplets 42 onto the defined trajectory of the conductive trace 22 by a LIFT process driven by the LIFT laser 28. The donor assembly 36 comprises a donor substrate 38, which typically includes a thin, flexible sheet of a highly permeable material, and the donor substrate 38 is coated with a donor film 40 containing a specified metal or combination of metals on the side adjacent to the circuit board 24. (The donor film may include sublayers such as an adhesive film, as described below herein with respect to Figure 3.) Alternatively, the donor substrate 38 may include a rigid or semi-rigid material. A beam deflector 32, such as a rotating mirror and / or an acousto-optical device, and a focusing optical element 34 direct pulses of radiation from the LIFT laser 28 to pass over the upper surface of the donor substrate 38 and strike the donor film 40 on the lower surface, according to a spatial pattern determined by the control circuit 52.

[0033] Each laser pulse induces the ejection of one or more molten droplets 42 of metal from the donor film 40 onto the substrate 24. The duration and energy of the laser pulses (typically having pulse durations in the nanosecond range), as well as the thickness of the donor film 40, can be selected so that each laser pulse ejects a single molten droplet 42 from the donor film towards the circuit board with precise directionality and high speed. Further details of this type of LIFT operation are described in U.S. Patent No. 9,925,797 above. The droplets 42 adhere to the substrate and harden, and in the depicted example, define a line of hardened droplets 44. Each droplet adds a specific amount of metallic material to the line. The control circuit 52 sets the number of droplets to be deposited and the spacing between consecutive droplets, depending on the desired thickness of the line 22. Thus, to create a very thin line, the droplets 42 can be deposited with only partial overlap between consecutive droplets, such that the width and height of the line 22 is approximately equal to the width and height of a single droplet 44. This technique makes it possible to create extremely fine lines with widths reduced to the micron range. Alternatively, thicker aggregates of droplets 44 can be used to create wider and deeper lines.

[0034] After the deposition of the droplet 44, the melting laser 30 irradiates the droplet line with sufficient energy to melt the metal so that the droplets fuse together into the bulk material along the line 22. A beam deflector 46, such as a scanning mirror and / or an acousto-optical device, as well as a focusing optical element 48, directs the radiation from the melting laser 30 to strike 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 (for example, to fuse the thick outer skin of the droplet without necessarily melting the entire volume).

[0035] In some embodiments, the melting laser 30 emits a sequence of pulses of laser energy rather than a CW beam to ensure that the thermal effects of the melting step are well localized and that the impact on the substrate 24 and surrounding structures is minimized. The use of short pulses is also beneficial in preventing the metal droplets from coalescing into spheres so that the trace maintains its desired shape. The optical element 48 focuses the beam so that it strikes the target line with a beam diameter small enough not to melt neighboring structures. For this purpose, the beam diameter may be less than the linewidth. However, the beam diameter is large enough to melt the entire area of ​​the trace trajectory covered by droplets 44. The beam deflector 46 scans the beam of the melting laser 30 along the trajectory of the trace 22 such that each pulse has a predetermined overlap with the preceding pulse in the sequence. The scanning speed is adjusted so that an appropriate amount of heat is applied uniformly along the entire trace.

[0036] Typically, the pulse duration of the pulses output by the melting laser 30 is less than 100 μs, and when melting fine feature areas, the pulses can be even shorter, for example, less than 10 μs. Depending on the droplet composition and the desired melting depth, the duration of each pulse may even be less than 1 μs. The pulse energy is typically in the range of 0.1 μJ to a maximum of 100 μJ, depending on the material and trace dimensions. The use of short, powerful laser pulses is beneficial in both reducing heat transfer to the substrate 24 and reducing metal oxidation during the melting process, allowing the process to be performed under ambient atmospheric conditions. The use of short laser pulses is also advantageous in reducing the tendency for the metal in the droplet to coalesce into separate spheres and lose the desired shape features of the trace 22. The time between pulses in the sequence may be long enough for heat to dissipate from the previous pulse so that heat accumulation does not become a problem.

[0037] To allow the pulse duration to be adjusted for different trace dimensions and melting depths, the melting laser 30 may include, for example, a suitable fiber laser or a high-power diode laser. If the laser has a sufficiently wide range of adjustment for pulse duration, reduced to the nanosecond range, it may also function as a LIFT laser 28.

[0038] The accompanying drawings and accompanying description present several techniques that may be applied to LIFT printing of metal traces in conjunction with controlled laser melting. For clarity and specificity, these techniques are described below herein with respect to the elements of System 20. However, these techniques are not limited to the specific system configuration shown in Figure 1, and the principles of the present invention may be applied alternatively in other systems having the necessary capabilities, as will be obvious to those skilled in the art after reading this description. All such alternative implementations are considered to be within the scope of the present invention.

[0039] Printing of metal wires of different widths and thicknesses Figure 2A is a micrograph of a line of metal droplets 44 printed on a circuit board 24 by a LIFT laser 28 according to an embodiment of the present invention. The droplets 44 are approximately 1 μm in diameter and are printed in a single row extending along the length of the trace 22, with an overlap of approximately 50% of droplet diameter between consecutive droplets in the sequence. Alternatively, the overlap between consecutive droplets can be less than 50% when a very narrow trace is desired.

[0040] Figure 2B is a micrograph of a trace 22 after laser melting by a melting laser 30 according to an embodiment of the present invention. A sequence of overlapping laser pulses is scanned across droplets 44, and these are merged into a single trace 22 shown in this figure with a line width of approximately 1 μm. The optimal laser pulse parameters for achieving this type of uniform metal trace depend on the materials and geometric dimensions involved and can be optimized each time through calculation and empirical trial and error.

[0041] Figure 3 is a schematic cross-sectional view of a donor assembly 36 illustrating the ejection of molten droplets 42 from a donor film 40 under laser irradiation according to an embodiment of the present invention. This embodiment aims to solve the problem of poor adhesion between droplets 44 and substrate 24, which can occur particularly when printing very fine traces and when printing on smooth substrates such as glass.

[0042] To solve this problem, the donor film 40 includes an adhesion film 62 that overlaps the primary metal donor film 60 on the donor substrate 38. For example, assuming that the film 60 contains copper, which is a good conductor but may not adhere well to a dielectric substrate, the adhesion film 62 may contain another metal that oxidizes faster than copper, such as titanium, tin, bismuth, or alloys of these metals. Optionally, an intermediate layer 64 is also deposited between the donor substrate 38 and the primary metal donor film 60 to enhance the adhesion of the donor film to the donor substrate and reduce the reflection of laser energy at the substrate / film interface. In this embodiment, the primary metal donor film is typically 50-700 nm thick, while the adhesion film is thinner, for example, 50-200 nm thick.

[0043] As shown in Figure 3, when the laser pulse strikes the donor film 40, the metal in the adhesion film 62 forms an outer layer that covers the droplet 42, surrounding the primary metal from the film 60. This outer layer adheres to the circuit board 24 upon impact of the molten droplet onto the circuit board. Due to the speed of the jet process, the outer layer is not substantially mixed into the metal nucleus of the droplet 42 while the droplet is in the air. However, as an alternative to this method, the donor film 40 may contain an alloy with enhanced adhesion properties. Additionally or alternatively, the surface of the substrate may be roughened or prepared separately before the LIFT jet to improve adhesion.

[0044] Figure 4A is a schematic cross-sectional view of an aggregate of metal droplets 44 deposited on a substrate 24 in a LIFT process defining a circuit trace according to an embodiment of the present invention. In this embodiment, the trace is wider and deeper than in the example shown in Figures 2A / 2B.

[0045] Figure 4B is a schematic cross-sectional view of a circuit trace 70 formed by the complete laser melting of an aggregate of metal droplets 44 shown in Figure 4A, according to an embodiment of the present invention. In this case, the melting laser 30 applies sufficient energy to the hardened droplets 44 to melt the entire volume of the hardened droplets within the trace 70. This technique is beneficial in maximizing mechanical integrity and thermal conductivity, as well as minimizing the electrical resistance of the trace, but must be applied carefully to avoid damage to the circuit board 24 and surrounding structures. In one embodiment (not shown), a beam deflector 46 directs the beam from the melting laser 30 to strike the volume of the droplets 44 over a range of incident angles in order to achieve more uniform melting.

[0046] Figure 4C is a schematic cross-sectional view of a circuit trace formed by partial laser melting of an aggregate of metal droplets 44 shown in Figure 4A, according to an alternative embodiment of the present invention. In this case, the melting laser 30 applies sufficient energy to the hardened droplets along the length of the trace to melt only the outer layer of the hardened droplets without melting the entire volume of the hardened droplets. This outer layer forms a protective skin 72 that encloses the volume of hardened droplets 44 within the conductive trace. The skin 72 enhances the mechanical and electrical integrity of the trace, as well as its resistance to etching and corrosion. This technique requires a much smaller investment of laser energy and therefore can increase process throughput while reducing the risk of damage to the substrate 24 and deformation of the trace compared to complete melting of the trace volume. In one embodiment, the laser beam used in the melting process is focused to a spot size smaller than the width of the trace and scanned across the surface of the trace until the entire area is covered.

[0047] The following table lists examples of process parameters that can be used for controlled laser melting of LIFT-deposited metal traces of various dimensions. In these examples, the droplets 44 contain copper, and the aggregate of droplets on the substrate 24 has the general form shown in Figure 4A. The number of melting laser pulses applied to each location along the trace can be selected according to the desired melting depth, which can range from about 1 μm (shown in Figure 4C) to the total thickness of the trace (shown in Figure 4B). [Table 1]

[0048] The above example illustrates the broad applicability and range of controllable parameters provided by this technique, particularly in the formation of stable, narrow traces that are difficult or impossible to fabricate by other techniques. The pulse width can be selected according to the melting depth, and may involve performing multiple LIFT / melt cycles when complete melting of thick traces is desired. The pulse pitch and repetition rate can also affect the overall thermal profile and therefore the melting depth. The laser spot size is generally selected to roughly match the trace width. The laser wavelength can also be selected so that the laser energy is well absorbed by the trace rather than the substrate, thereby minimizing damage to the substrate when the laser spot extends over an area wider than the trace.

[0049] Fabrication and repair of circuit elements Referring back to Figure 1, in some embodiments, the trajectory on which the trace 22 is printed includes a gap between a pair of terminals on the circuit board. For example, the control circuit 52 may identify such a gap by analyzing an image captured by the inspection module 54 of the circuit board 24. The control circuit 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 a defect detected on the circuit board 24, such as a misformed trace or an open circuit trace. In this case, before filling the gap, the defective trace and the underlying substrate may be cleaned and prepared using, for example, laser ablation as described in Korean Patent Application Publication No. 20150070028. This preparation may include shaping the ends of the circuit trace adjacent to the gap to form well-defined terminals on which the droplets 42 will adhere. Next, the defect is repaired by depositing molten droplets 42 into the gaps between the terminals, and then directing the beam of the melting laser 30 to melt the hardened droplets 44.

[0050] In other embodiments, circuit traces including gaps are intentionally formed on the circuit board 24, for example, by photolithography. These gaps can then be filled with a different material from the circuit trace, such as a resistive material like NiCr, by LIFT printing. This process, illustrated in Figure 7, can be used to produce circuit components such as resistors and strain gauges.

[0051] Figures 5A to 5C are schematic cross-sectional views illustrating the steps in a process for filling gaps 82 in a circuit trace according to embodiments of the present invention. Figure 5A shows an aggregate of hardened metal droplets 44 deposited in gaps 82 within a circuit trace 80 by a LIFT process. In this case, the gaps 82 are thought to have been created due to a defect in the initial fabrication of the trace 80. The edges of the gaps 82 are squared, including a stepped shape at the edges of the gaps, to create well-defined terminals. This type of pretreatment allows for uniform deposition of droplets 44 in the gaps and good electrical contact between the droplets and terminals. In this example, the droplets 44 are deposited in a single LIFT deposition step to fill the entire depth of the gaps 82.

[0052] Figure 5B illustrates the application of the laser melting process to an aggregate of droplets 44. A pulsed beam 84 from the melting laser 30 is focused on the outer surface of the aggregated droplets and scans the gaps 82, as indicated by the arrow 86.

[0053] Figure 5C shows the circuit trace 80 formed by the laser melting process in Figure 5B. The upper layer 88 of the droplet 44 is melted and bonded to the metal of the trace 80, forming a skin that covers the hardened droplet 44 below. The depth of the layer 88 is determined by the intensity and scanning pattern of the laser beam 84.

[0054] Figures 6A to 6D are schematic cross-sectional views illustrating the steps in a process for filling gaps within a circuit trace 80 according to another embodiment of the present invention. Figure 6A illustrates the application of a laser melting process to an aggregate of metal droplets 90 deposited in the gaps within the circuit trace 80 by a LIFT process. In this case, a layering technique is applied such that the droplets 90 do not fill the entire depth of the gap, but form a first layer on the circuit board. A laser beam 84 is scanned across the gap to melt the hardened droplets within this first layer.

[0055] Figure 6B shows a partial circuit trace formed by the laser melting process in Figure 6A. In this example, the entire depth of the droplet 90 is melted by the laser beam 84 so that it forms an underlying layer 92 of the conductive trace within the gaps in the circuit trace 80.

[0056] Figure 6C illustrates the application of the laser melting process to a further aggregate of metal droplets 94 by scanning the beam 84. The droplets 94 are deposited by the LIFT process, covering the underlying layer 92, and the laser beam 84 is then scanned over this additional aggregate of droplets 94 to melt the hardened droplets.

[0057] Figure 6D shows the complete circuit trace formed by the laser melting process of Figure 6C. The controlled laser melting process in Figure 6C formed an upper layer 96 of the conductive trace by covering the lower layer 92, filling the gaps within the trace 80 and completing the trace. This multilayering technique is useful in ensuring that the trace melts completely and fuses into the bulk material throughout its entire depth, while reducing heat dissipation into the trace 80 and substrate 24 (and mitigating potential thermal damage). Although Figures 6A–6D show only a two-layer process for the sake of simplicity, the principles of this technique can be similarly applied to creating three or more layers, depending on the required trace thickness.

[0058] Figure 7 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, the trace 80 includes a first metal, e.g., copper, etched or ablated to define terminals 100. The donor film 40 includes a different metal, e.g., NiCr, with a different composition than the first metal. The LIFT laser 28 is operated to deposit droplets of NiCr into the gaps between terminals 100. The melting laser 30 then operates to melt at least the upper layer of terminals 100 to form heterogeneous metal junctions at the terminals, as well as to melt the NiCr droplets and coalesce them into the trace 102. These junctions are useful for creating metal-to-metal contacts with low resistance and high mechanical strength. As previously mentioned, the trace 102 can function, for example, as an embedded resistor or strain gauge.

[0059] Similarly, even when the trace and terminals contain the same metal, the operation of the fusion laser 30 is useful in forming homogeneous metal junctions between the trace and terminals. As with dissimilar junctions, these metal junctions enhance mechanical strength and resistance to etching and corrosion and reduce electrical resistance.

[0060] The following table lists examples of process parameters that can be used for controlled laser melting of LIFT-deposited NiCr traces that intersect with copper circuit traces, as shown in Figure 7. [Table 2]

[0061] Figures 8A to 8E are schematic top views of a circuit trace 110 on a substrate 24, showing sequential steps in a LIFT-based process for repairing gaps 112 in a circuit trace according to an alternative embodiment of the present invention. Figure 8A shows the gap 112 before the LIFT process is initiated. In this case, droplets 114 are deposited by the LIFT process over an area larger than the gap 112, as shown in Figure 8B. This type of deposition pattern is created, for example, when the LIFT laser 28 emits shorter pulses, for example in the picosecond range, with higher peak power, resulting in each pulse ejecting many submicron droplets toward the substrate 24. Operation in this form may be advantageous in that the cured droplets are smaller and can adhere better to the substrate, but the directionality of droplet ejection is not very precise.

[0062] To reduce the width of the area covered by droplet 114, the melting laser 30 is applied to melt only the hardened droplets deposited on the substrate 24 within a predetermined width of the trace trajectory. Thus, as shown in Figure 8C, the trace region 116 melts and coalesces to form a solid trace that joins to the circuit trace 110. To increase the thickness of the LIFT deposited trace, the LIFT step may be repeated to deposit one or more additional layers of droplet 118 over the region of the gap 112, as shown in Figure 8D. After each such step, the controlled laser melting step in Figure 8C is repeated to melt and coalesce additional droplets within (but not outside of) the trace region 116.

[0063] Once the metal within the trace region 116 reaches the desired depth, an etching process is applied to the circuit board 24 to remove hardened droplets deposited on the circuit board outside the trace region 116. Because the separate droplets outside region 116 have a larger surface area relative to their volume and are therefore more susceptible to the etching process, this step can be performed using, for example, chemical etching or galvanic etching methods known in the art. Alternatively, the hardened droplets can be removed by laser ablation. A clean trace after the etching step is shown in Figure 8E.

[0064] It should be understood that the embodiments described above are for illustrative purposes only, and that the present invention is not limited to those specifically shown and described above. Rather, the scope of the present invention includes both combinations and partial combinations of the various features described above, as well as variations and modifications thereof that can be recalled by those skilled in the art by reading the foregoing description and that are not disclosed in the prior art.

Claims

1. A method for constructing circuits, To define the trajectory of conductive traces formed on a circuit board, The process involves ejecting molten metal droplets from a donor substrate adjacent to the circuit board along a defined trajectory using a laser-induced forward transfer (LIFT) process, thereby causing the molten droplets to adhere to and harden on the circuit board along the length of the defined trajectory. After the molten droplets harden, the metal within the hardened molten droplets is melted, and the hardened molten droplets are combined to form a bulk layer, thereby directing the laser beam toward the defined trajectory with sufficient energy to form a conductive trace on the circuit board that extends along the length of the defined trajectory. Includes, The donor substrate has a first surface and a second surface facing each other, and the donor film containing the metal is disposed on the second surface such that the donor film is close to the defined trajectory. Discharging the molten droplets includes directing pulses of laser radiation to pass through the first surface of the donor substrate and strike the donor film in order to induce the discharge of the molten droplets of the metal from the donor film along the defined trajectory, The donor film contains a first metal, An adhesive film containing a second metal is disposed on the donor substrate, covering the donor film, the second metal forming an outer layer that covers the molten droplets of the first metal, and the outer layer adhering to the circuit board when the molten droplets collide with the circuit board. A method characterized by the following:

2. A method according to claim 1, characterized in that in the process of the LIFT, directing the pulses of the laser radiation and directing the laser beam toward the defined trajectory includes using a single laser having a variable pulse duration for ejecting the molten droplets and melting the metal within the hardened molten droplets.

3. A method according to claim 1, characterized in that the first metal includes copper, and the second metal is selected from the group consisting of titanium, tin, bismuth, and alloys thereof.

4. The method according to claim 1, Discharging the molten droplets and directing the laser beam toward the defined trajectory are, To eject the first layer of molten droplets onto the circuit board and form the lower layer of the conductive trace, the laser beam is directed to melt the hardened molten droplets within the first layer. To discharge at least a second layer of the molten droplet onto the lower layer and complete the conductive trace, the laser beam is directed to melt the hardened molten droplet within at least the second layer. including A method characterized by the following:

5. A method according to claim 1, characterized in that directing the laser beam includes using the laser beam to apply to the hardened molten droplet sufficient energy to melt the entire volume of the hardened molten droplet within the conductive trace.

6. A method according to claim 1, characterized in that directing the laser beam includes using the laser beam to apply to the hardened molten droplet sufficient energy to melt only the outer layer of the hardened molten droplet without melting the entire volume of the hardened molten droplet along the length of the defined trajectory.

7. A method according to claim 1, characterized in that directing the laser beam includes directing a pulse of laser energy to strike the molten droplet that has hardened along the length of the defined trajectory.

8. A method according to claim 7, characterized in that each of the pulses has a pulse duration of less than 10 μs.

9. A method according to claim 7, characterized in that directing the pulses includes scanning the laser beam along the trajectory such that each of the pulses has a predetermined overlap with a preceding pulse.

10. The method according to claim 1, Discharging the molten droplets involves depositing the molten droplets onto the circuit board in a single row extending along the length of the defined trajectory, thereby forming the conductive trace by melting the single row. Each of the molten droplets overlaps with a preceding molten droplet in the single row by no more than 50% of the diameter of the molten droplet. A method characterized by the following:

11. The method according to claim 1, Defining the aforementioned trajectory includes identifying the gap between the first terminal and the second terminal on the circuit board. Discharging the molten droplets includes depositing the molten droplets to fill the gaps. A method characterized by the following:

12. The method according to claim 11, The first terminal and the second terminal each contain a first metal, and the molten droplet each contain a second metal having a different composition from the first metal. Directing the laser beam includes melting the first metal and the second metal so as to form a dissimilar metal joint at the first terminal and the second terminal. A method characterized by the following:

13. The method according to claim 11, Identifying the gap includes detecting defects in conductive traces formed on the circuit board, The aforementioned defect is repaired by depositing the molten droplets and then directing the laser beam to melt the hardened molten droplets. A method characterized by the following:

14. An apparatus for fabricating conductive traces on a circuit board, A deposition module configured to eject molten metal droplets from a donor substrate adjacent to the circuit board onto a defined trajectory of the conductive trace by a laser-induced forward transfer (LIFT) process, wherein the molten droplets adhere to the circuit board along the length of the defined trajectory and harden. A laser module configured to direct a laser beam toward the defined trajectory with sufficient energy to form a conductive trace on the circuit board that extends along the length of the defined trajectory, by melting the metal in the hardened molten droplets and combining the molten droplets to form a bulk layer. Equipped with, The donor substrate has a first surface and a second surface facing each other, and the donor film containing the metal is disposed on the second surface such that the donor film is close to the defined trajectory. Discharging the molten droplets includes directing pulses of laser radiation to pass through the first surface of the donor substrate and strike the donor film in order to induce the discharge of the molten droplets of the metal from the donor film along the defined trajectory, The donor film contains a first metal, An adhesive film containing a second metal is disposed on the donor substrate, covering the donor film, the second metal forming an outer layer that covers the molten droplets of the first metal, and the outer layer adhering to the circuit board when the molten droplets collide with the circuit board. A device characterized by the following features.

15. The apparatus according to claim 14, wherein the laser module comprises a single laser having a variable pulse duration for directing pulses of the laser radiation in the LIFT process and for directing the laser beam to melt the metal in the hardened molten droplet.

16. The apparatus according to claim 14, wherein the deposition module and the laser module are configured to: direct the laser beam to melt the hardened molten droplets in the first layer in order to eject a first layer of molten droplets onto the circuit board to form a lower layer of the conductive trace; and direct the laser beam to melt the hardened molten droplets in at least the second layer in order to eject at least a second layer of molten droplets onto the lower layer to complete the conductive trace.

17. The apparatus according to claim 14, wherein the laser module is configured to use the laser beam to apply sufficient energy to the hardened molten droplet to melt the entire volume of the hardened molten droplet within the conductive trace.

18. The apparatus according to claim 14, wherein the laser module is configured to use the laser beam to apply to the hardened molten droplet sufficient energy to melt only the outer layer of the hardened molten droplet without melting the entire volume of the hardened molten droplet along the length of the defined trajectory.

19. The apparatus according to claim 18, wherein the laser module is configured to direct pulses of laser energy to strike the molten droplets that have hardened along the length of the defined trajectory.

20. The apparatus according to claim 19, characterized in that each of the pulses has a pulse duration of less than 10 μs.

21. The apparatus according to claim 19, The laser module is configured to scan the laser beam along the trajectory such that each of the pulses has a predetermined overlap with the preceding pulse. Each of the molten droplets overlaps with a preceding molten droplet in a single row by no more than 50% of its diameter. A device characterized by the following features.

22. The apparatus according to claim 14, wherein the deposition module is configured to deposit the molten droplets on the circuit board in a single row extending along the length of the defined trajectory, thereby forming the conductive trace from the single row.

23. The apparatus according to claim 14, comprising a control circuit configured to identify the gap between a first terminal and a second terminal on the circuit board, and to control the deposition module to deposit the molten droplets so as to fill the gap.

24. The apparatus according to claim 23, The first terminal and the second terminal each contain a first metal, and the molten droplet each contain a second metal having a different composition from the first metal. The laser module is configured to direct the laser beam so as to melt the first metal and the second metal, thereby forming a dissimilar metal joint at the first terminal and the second terminal. A device characterized by the following features.

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