High-resolution laser-induced forward transfer

Abstract

In a method and system for laser-induced forward transfer (LIFT), energy (E1, E2) is applied according to a non-Gaussian intensity profile (Ixy), which is spatially tailored across an interface (11xy) of a donor material (11m) to cause the donor material (11m) to be ejected from the donor substrate (11) as an elongated jet (Je) that instantaneously bridges a transfer distance (Zt) between the donor substrate (11) and the acceptor substrate (12) during a transfer period (Tt). A locally increased intensity spike (Is) at the center of the intensity profile (Ixy) causes a relatively thick jet (J1) of donor material to bifurcate into a relatively thin jet (J2) at a bifurcation location (J12) between the donor substrate (11) and the acceptor substrate (12). The thick jet (J1) allows for a relatively large transfer distance (Zt), while the thin jet (J2) deposits a relatively small droplet (Jd) of donor material (Hm).

Description

[Technical field]

[0001] The present disclosure relates to Laser-Induced Forward Transfer (LIFT). [Background technology]

[0002] Laser-induced forward transfer is an advanced printing technique capable of depositing a wide range of structural and functional materials without the use of nozzles. In this process, a pulsed laser initiates the ejection of a small volume of ink from a thin liquid donor film supported by a laser-transparent substrate onto a parallel receiver substrate held at a distance ranging from a few tenths of a micron to several millimeters, and in extreme cases, the laser-emitted jet can even bridge the donor and receiver placed several centimeters apart. Under appropriate conditions, the transfer is mediated by high-velocity microjets that achieve a high aspect ratio before detaching from the donor film.

[0003] In principle, it can be envisaged that a long laser-emitted jet bridging donor and receiver substrates could enable a wide range of applications, ranging from face-up integration of thin and relatively thick parts to conformal printing. However, longer and thinner jets are very difficult to stabilize and require very strict process conditions. More specifically, high-resolution LIFT printing requires the use of thin donor layers (<15 μm) of highly viscous inks / pastes that are usually very difficult to apply as uniform layers. Moreover, to achieve very high resolution in classical LIFT printing, the donor and receiver need to be brought very close to each other (maintaining a gap of only a few tenths of a micron), making the process less industrially relevant. Summary of the Invention [Problem to be solved by the invention]

[0004] There is a further need to improve the high resolution of LIFT over substantially larger transfer distances. [Means for solving the problem]

[0005] Aspects of the present disclosure relate to methods and systems for laser-induced forward transfer. Typically, a donor substrate is provided with a donor material to be transferred. An acceptor substrate is positioned at a transfer distance from the donor substrate to receive the donor material. For example, respective substrate handlers may be positioned to hold the substrates at a desired transfer distance. The transfer of the donor material can be initialized by impinging the donor substrate with a laser pulse that deposits energy into the donor material for initialization.

[0006] By applying energy according to a specific intensity profile, the jet can be emitted from the donor substrate as an elongated jet that bridges a relatively large transfer distance between the donor substrate and the acceptor substrate during the transfer period. In particular, by providing the intensity profile with a first intensity range that spans a relatively large first spot size on the donor material, this can encourage the elongated jet to start from the donor substrate as a corresponding relatively thick jet. Furthermore, by providing the intensity profile with a locally increased intensity spike at its center, this can encourage the thick jet to diverge into a relatively thin jet at the divergence location between the donor substrate and the acceptor substrate due to the locally increased intensity at the center. Preferably, only the thin jet reaches the acceptor substrate to deposit its donor material droplet before collapsing. By combining a relatively thick jet with a relatively thin jet, both the transfer distance and the resolution can be improved. For example, the desired intensity profile can be achieved using two or more differently sized coincident laser pulses, preferably originating from the same beam source. Additionally, various other parameters such as relative intensity, delay, angle, etc. can be used to determine various aspects of jet formation and droplet deposition.

[0007] These and other features, aspects, and advantages of the apparatus, systems, and methods of the present disclosure will become better understood from the following description, appended claims, and accompanying drawings. [Brief description of the drawings]

[0008] [Figure 1] 1 illustrates LIFT with a shaped intensity profile that produces a relatively thick jet of donor material that branches into relatively thin jets. [Figure 2A] Illustrated are images recorded of successive stages of jet formation, transfer period and jet breakup, respectively. [Figure 2B] 1 illustrates typical steps in a LIFT process related to the kinetic energy of the donor material as a function of time. [Figure 3A] 1 illustrates a comparison of jet formation resulting from a single pulse with large spot size. [Figure 3B] 1 illustrates a comparison of jet formation resulting from a single pulse with a small spot size. [Figure 3C] 1 illustrates a comparison of the jet formation resulting from the combination of two pulses with large and small spot sizes. [Figure 4A] 1 illustrates a system for laser-induced forward transfer. [Figure 4B] 1 illustrates a system for laser-induced forward transfer. [Figure 5A] 1 illustrates the individual superimposed intensity profiles of a first laser pulse with a relatively large spot size and a second laser pulse with a relatively small spot size. [Figure 5B] 1 illustrates a composite (summed) intensity profile with spikes, resulting for example from two overlapping laser pulses of different sizes. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The terminology used to describe particular embodiments is not intended to limit the invention. As used herein, the singular forms "a," "an," and "the" are intended to include the plural, unless the context clearly dictates otherwise. The term "and / or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprises" and / or "comprising" specify the presence of a stated feature, but do not preclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as following another step, it may directly follow said other step, or that one or more intermediate steps may be performed prior to performing the particular step, unless otherwise specified. Similarly, when a connection between structures or components is described, it will be understood that this connection may be established directly or through an intermediate structure or component, unless otherwise specified.

[0010] The present invention will now be described more fully with reference to the accompanying drawings, in which embodiments of the invention are illustrated. In the drawings, absolute and relative sizes of systems, components, layers and regions may be exaggerated for clarity. The embodiments may be described with reference to schematic and / or cross-sectional views of possible idealized embodiments and intermediate structures of the present invention. In the description and drawings, like numbers refer to like elements throughout. Relative terms and derivatives thereof should be construed to refer to the orientation as then described or illustrated in the drawings. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless otherwise expressly stated.

[0011] FIG. 1 shows a schematic cross-sectional view of various aspects involved in laser-induced forward transfer of donor material 11m between a donor substrate 11 and an acceptor substrate 12.

[0012] Typically, a donor substrate 11 is provided with a donor material 11m to be transferred. In the illustrated embodiment, an acceptor substrate 12 is positioned at a (controlled) transfer distance "Zt" from the donor substrate 11 to receive the donor material 11m. As described herein, the donor substrate 11 is bombarded (irradiated) by one or more laser pulses "L1", "L2" that deposit respective energies "E1", "E2" into the donor material 11m to initialize the transfer.

[0013] In some embodiments, as illustrated in the dashed top view inset, the energies "E1", "E2" are applied according to an intensity profile "Ixy" that is spatially tailored across the interface 11xy of the donor material 11m. Preferably, the intensity profile "Ixy" is configured to cause the donor material 11m to be ejected from the donor substrate as an elongated jet "Je" that instantaneously bridges the donor material 11m to a transfer distance "Zt" between the donor substrate 11 and the acceptor substrate 12 during a transfer period "Tt". In one embodiment, the intensity profile "Ixy" has a first intensity range "I1" that spans a relatively large first spot size "Ds1" on the donor material 11m to cause the elongated jet "Je" to start from the donor substrate 11 as a corresponding relatively thick jet "J1".

[0014] In a preferred embodiment, the intensity profile "Ixy" has a second intensity range "I2" higher than the first intensity range "I1" over a relatively small second spot size "Ds2" (smaller than the first spot size "Ds1") that forms a locally increased intensity spike "Is" in the center of the intensity profile "Ixy" to cause the thick jet "J1" to diverge into a relatively thin jet "J2" at a divergence position "J12" between the donor substrate 11 and the acceptor substrate 12 by the locally increased intensity in the center. The locally increased intensity spike "Is" in the intensity profile "Ixy" is generally recognized as a substantial deviation above a normal, e.g. (single) Gaussian intensity profile. Accordingly, the total intensity profile of the combined intensity range and spot size usually has a non-Gaussian shape. Various ways of quantifying the spike can be envisaged, as will be explained later, e.g. with reference to FIG. 5B. Qualitatively, spikes typically have a relatively tall and narrow profile compared to the overall trend in the rest of the (surrounding) intensity profile.

[0015] In some embodiments (not shown), the intensity profile may include multiple levels, e.g., a first spike centered over a second spike centered on the remainder of the intensity profile. For example, the first spot size may be relatively large, the second spot size may be relatively small, and the third spot size may be even smaller. Advantageously, a combination of three or more different spot sizes may be adapted to generate an elongated jet that branches two or more times into thinner jets and / or bridges to a larger transfer distance "Zt".

[0016] In a preferred embodiment, for example as shown, the spike in the intensity profile is produced by using at least two laser pulses "L1", "L2" of substantially different beam widths or spot sizes "Ds1", Ds2. By using two beams, the size of each spot can be easily controlled, for example using one or more telescopes, mirrors, lenses, etc. on each beam. Also, the (relative) intensity of each spot can be easily controlled, for example using optical filters, beam splitters, polarizers, etc. on each beam. In some embodiments (not shown), more than two pulses can also be used, for example three, four or five or more pulses. For example, the pulses may generate multiple decreasing spot sizes centered on each other.

[0017] Typically, the carrier portion 11c of the donor substrate 11 may be transparent or at least translucent to the laser pulses. Preferably, the energy "E1", "E2" of one or more laser pulses is applied to the donor material 11m. For example, the donor material may directly or indirectly absorb the energy "E1", "E2" from the one or more laser pulses, causing the material to heat up. The heated donor material and / or solvent may form a rapidly expanding bubble 11b, which initializes the transfer. Alternatively or additionally, the energy "E1", "E2" may be applied to, for example, a sacrificial layer (not shown) between the donor material 11m and the donor carrier 11c. The sacrificial layer may similarly evaporate and / or decompose to initialize the transfer. Other LIFT techniques may also be used.

[0018] In one embodiment, for example as shown, during at least a portion of the transfer period "Tt", the elongated jet "Je" bridging the transfer distance "Zt" between the donor substrate 11 and the acceptor substrate 12 includes a relatively wide jet "J1" defined by a (adjacent) smallest wide jet diameter "Dj1" extending over a wide jet length "Zj1" and a relatively narrow jet "J2" defined by a maximum narrow jet diameter "Dj2" extending over an (adjacent) narrow jet length Zj2. Preferably, the maximum narrow jet diameter "Dj2" is at least 2, 3 or 4 times smaller than the minimum wide jet diameter "Dj1", such as up to 10 or even 20 times smaller.

[0019] Preferably, only the narrow jet "J2" reaches the acceptor substrate 12 to deposit a droplet "Jd" of its donor material 11m before collapsing. In principle, the smaller the maximum narrow jet diameter "Dj2" is compared to the minimum wide jet diameter "Dj1", the higher the possible gain in resolution and the smaller the droplet "Jd" can be. On the other hand, if the narrow jet becomes too narrow, it may collapse too quickly and / or in different parts to form fragments. In some embodiments, the diameter of the narrow jet is therefore preferably between 2 and 10 times smaller than that of the wide jet.

[0020] In some embodiments, the thick jet length "Zj1" is greater than 30 percent of the transfer distance "Zt", preferably greater than 40, 50 or 60 percent. Typically, the maximum transfer distance that material can be transferred can be determined in large part by the length of the thick jet "J1". Thus, the longer this thick jet is, the higher the transfer distance that can be obtained. On the other hand, at least a part of the elongated jet "Je" should be formed by the thin jet "J2", for example, to obtain the desired resolution and to ensure that the material in the thick jet "J1" retracts back into the layer of donor material 11m after the jet breaks up. In some embodiments, the thick jet "J1" is therefore preferably less than 90 percent of the transfer distance, more preferably less than 80 percent, for example between 50 and 75 percent.

[0021] In other or further embodiments, the thin jet length Zj2 is less than 80 percent of the transfer distance "Zt", preferably less than 60 percent, for example between 25 and 50 percent of the transfer distance "Zt". It is usually observed that the elongated jet "Je" will collapse at a collapse point "Jb" somewhere along the thin jet "J2", for example near the middle of the thin jet "J2". When this occurs, the donor material 11m forming the thin subsection "J2b" of the thin jet "J2" below the collapse point "Jb" can still become the droplet "Jd" to be deposited. The shorter the thin jet length Zj2, the less the amount of this excess material in the thin subsection "J2b" and the smaller the droplet "Jd" can ultimately be. Also, the exact location of the jet collapse "Tb" can be difficult to control. Therefore, the smaller the thin jet length Zj2, the less the uncertainty.

[0022] In a preferred embodiment, the donor material 11m on the donor substrate 11 is irradiated by at least a first laser pulse "L1" and a second laser pulse "L2" to cause the transfer of the donor material 11m towards the acceptor substrate 12 by the combined effect of the laser pulses "L1", "L2". As shown, the first laser pulse "L1" has a first spot size "Ds1" impinging on the donor material 11m, and the second laser pulse "L2" has a second spot size "Ds2" impinging on the donor material 11m. In a preferred embodiment, the second spot size "Ds2" is smaller than the first spot size "Ds1", for example by at least 2, 3 or 5 times or more. The more different the sizes of the two laser spots are, the different the jet thickness is, since the spot size can be correlated with the jet thickness. On the other hand, there may be some limit on the relative thickness at which a stable elongated jet can be formed. Accordingly, in some embodiments, the relative spot sizes may preferably differ by less than a factor of 10.

[0023] In some embodiments, the second laser pulse "L2" is delayed with respect to the first laser pulse "L1" by a time difference dt, for example of at least 10 or at least 100 nanoseconds. Preferably, the delay is not too large, for example within the time window between the absorption of the first pulse and the physical manifestation of the resulting (substantial) jet formation. For example, the time difference dt can be up to 1 microsecond, 5 microseconds or 10 microseconds or more. Without being bound by theory, the inventors have found that there can be a certain time between the impact of the laser pulse on the donor material and the start of the jet formation. By delaying the second laser pulse "L2", the second laser pulse can have more influence on the elongated jet formation, since it can arrive at a time when the effect of the first laser pulse "L1" is more exerted. Alternatively, it can be sufficient that the pulses arrive at the same time, or the first laser pulse "L1" can even arrive after the second laser pulse "L2".

[0024] In one embodiment, the layer thickness Zm of the donor material 11m on the donor substrate 11 is between 1 and 500 micrometers, preferably between 10 and 200 micrometers, more preferably between 20 and 100 micrometers. The method can allow the use of a relatively thick donor layer while still providing a small resolution, for example less than 50 micrometers. For example, a relatively wide and high-energy first laser pulse "L1" can be used to initiate transfer even in thick donor layers, while a relatively narrow second laser pulse "L2" can determine a small resolution. For example, the first spot size "Ds1" is typically at least 2 or 3 times larger than the layer thickness Zm. Advantageously, a relatively thick donor layer can also cause a longer elongated jet to be formed, thus providing a larger transfer distance "Zt". Also, the donor layer can be easier to apply if it does not need to be as thin.

[0025] In one embodiment, a first pulse "L1" impinges on the layer of donor material 11m at a first angle θ1 relative to the layer normal and a second laser pulse "L2" impinges on the layer of donor material 11m at a second angle θ2 relative to the layer normal. Preferably, the angles θ1, θ2 are less than 40 degrees, 30 degrees, 20 degrees, 10 degrees, for example 0 degrees (perpendicular to the layer).

[0026] The direction of the beams with respect to the donor layer can affect the direction of the resulting jet. Therefore, the lower the angle, the straighter the material can be transferred. In some embodiments, the beams are parallel, or as parallel as possible, within an angle difference θ1+θ2 of less than 10 degrees, less than 5 degrees. Alternatively, it may be advantageous for some applications to have a relatively high angle between the beams, for example to deposit material under an angle. For example, it may be assumed that the jets form a kink at the angle between the thick and thin jets. For these and other applications, the beams may have an angle difference θ1+θ2 of, for example, more than 10 degrees, more than 20 degrees, or even more than 40 degrees.

[0027] In some embodiments, the size of one or more of the overlapping light spots is controlled during operation, e.g., as feedback based on recorded images or videos of the elongated jet "Je" and / or based on measurements, e.g., sizes, of the resulting droplets "Jd". For example, if the thick jet "J1" is observed to be too thin, the first spot size "Ds1" can be increased until the desired thickness is reached. For example, if the thin jet "J2" is observed to be too thick, the second spot size "Ds2" can be decreased until the desired thickness is reached. For example, if the resulting droplets are observed to be too large or to have a small but unpredictable size, any of the two or more spots can be adjusted.

[0028] In other or further embodiments, the (relative) intensity of each spot is controlled during operation, for example as a feedback based on recorded images of the elongated jet "Je" and / or based on measurements of the resulting droplets "Jd". For example, if it is observed that the thick jet length "Zj1" of the thick jet "J1" is too small and / or that the thick jet "J1" is insufficiently developed, the intensity I1 of the first spot and / or the energy "E1" of the first laser pulse "L1" can be increased. For example, if it is observed that the thin jet length Zj2 of the thin jet "J2" is too small and / or that the thin jet "J2" is insufficiently developed, the intensity I2 of the second spot and / or the energy E2 of the second laser pulse "L2" can be increased.

[0029] Other parameters such as the relative angle, pulse length and / or timing of one or more pulses may also be controlled, for example as a function of the desired droplet size Dx and / or the variance of this size. Other parameters such as the transfer distance "Zt" may also be varied. In one embodiment, the method or system includes a calibration step of the function in which one or more of the pulses or other system parameters are varied until a (consistent) target droplet size Dx is achieved. Alternatively, or in addition to measuring the droplet size Dx, an image of the elongated jet "Je" may be recorded during the transfer period "Tt", and various parameters such as jet thickness and length may be measured directly and used as alternative or additional feedback in the calibration.

[0030] 2A illustrates recorded images of the successive stages of jet formation "Tf", transfer period "Tt" and jet collapse "Tb", here using a combination of two pulses. In the image, the jet-forming donor material 11m is illustrated in black. The donor and acceptor substrates are located at the top and bottom of the image, respectively (not clearly visible here).

[0031] As shown in the left image, after energy is applied to the donor material, jet formation "Tf" may begin, typically with a relatively thick jet beginning to elongate from the upper material layer. As shown in the center image, an elongated jet "Je" may form and begin to flow between the donor and acceptor substrates during the transfer period "Tt". During the transfer period "Tt", the elongated jet "Je" may include a relatively thick jet "J1" that branches into a relatively thin jet "J2". After a period of transfer, jet collapse "Tb" begins, as shown in the right image. If the collapse is properly controlled, most of the jet material in the thick jet "J1" retracts back into the donor substrate.

[0032] As will be appreciated, various properties of the jet formation can be tailored using the present methods and systems. In particular, the formation can be tailored to precisely deposit relatively small size droplets "Jd" over relatively large transfer distances. For example, the intensity profile of one or more light pulses can be adapted spatially and / or temporally to shape the jet formation process as desired.

[0033] Typically, the droplet size can be determined by a combination of factors such as the diameter of the jet (especially the narrow jet "J2"), the amount of material flowing through this jet per unit time (transfer rate), the duration of the flow (transfer duration) and the location along the jet where the collapse occurs. Preferably, the jet collapse "Tb" is configured such that most of the material in the jet retracts back into the donor substrate, leaving only a small droplet of donor material "Jd" on the acceptor substrate. The exact collapse point can be difficult to control, so there may be some variation in droplet size.

[0034] Typically, it is observed that the collapse point "Jb" of the elongated jet "Je" is somewhere along the length of the thin jet "J2". Accordingly, even if the exact collapse point in the thin jet is not perfectly controlled, the absolute variation of the material droplet "Jd" can be small in view of the relatively small diameter of the thin jet. Furthermore, the collapse point often occurs along the thin jet, typically near or near the middle between the thick jet "J1" and the acceptor substrate 12. Thus, splitting the thick and thin jets also helps to have the collapse point "Jb" closer to the acceptor substrate 12 than if the elongated jet were, for example, of unitary thickness.

[0035] Generally, after reaching the acceptor substrate 12, the thin jet "J2" collapses into a first subsection "J2b" of thin subsections "J2b" that is deposited on the acceptor substrate 12 as part of the droplet "Jd" and a second subsection that remains connected to the donor substrate 11 via the elongated jet and is retracted towards the donor substrate 11 due to material properties.

[0036] Advantageously, the inventors have found that the relative variation in droplet size can be minimized if the amount of droplet material is determined primarily by the transfer period / speed at which the material flows through the elongated jet. Thus, in one preferred embodiment, the majority of the donor material in the transferred droplet "Jd" originates from the flow Jf of donor material 11m during the material transfer period "Tt" when the jet "Je" extends uninterruptedly between the donor substrate 11 and the acceptor substrate 12. Additionally or alternatively, it has been found that it is preferable for relatively little of the droplet material to originate from the portion of the jet "J2b" that falls towards the acceptor substrate 12 after the jet breakup "Tb". For example, the percentage of material in the droplet that originates from the material transfer period "Tt" is greater than 50 percent, such as 70 percent, 80 percent or 90 percent or more. The higher the percentage of material determined by the material flow, the less impact a breakup point "Jb" that was not fully controlled can have on the total droplet size. Accordingly, the droplet size can be more consistently controlled.

[0037] In some embodiments enabled by the present teachings, the transfer distance "Zt" between the donor substrate 11 and the acceptor substrate 12 is at least 4 times, preferably at least 10 times, or even more than 20 times, larger than the size Dx (e.g., cross-sectional diameter) of the droplet "Jd" to be deposited. For example, the size of the droplet "Jd" is smaller than 50 micrometers, and the transfer distance "Zt" is larger than 200 micrometers. It will be appreciated that the present method and system can tolerate a relatively large transfer distance while still providing a relatively good resolution. For example, to achieve dots of less than 50 micrometers with a single beam, a typical maximum transfer distance has been found to be less than 150 micrometers. Typically, maintaining such a small distance can be difficult. With the present method using dual spots, the inventors have found that the transfer distance for printing 40 micrometer dots can be increased to 500 micrometers or more. It can be relatively easy to keep the distance between the substrates at around half a millimeter.

[0038] FIG. 2B illustrates typical stages of the LIFT process in relation to the kinetic energy "Ekin" of the donor material 11m as a function of time "T."

[0039] Without being bound by theory, the following explanation is provided to facilitate understanding of the various observations. The process begins with the absorption of pulse energy into the donor material at time "T0". This can lead to the formation of a growing bubble 11b, for example as illustrated in FIG. 1. For example, the bubble can be formed by evaporated and / or decomposed donor material, solvent, or for example a sacrificial layer. Around time "T1", for example as a result of the expanding bubble, a jet of donor material 11m can be propelled forward from the remainder of the donor layer by kinetic energy "Ekin". For example, there can be several (here more than 10) microseconds between the impact of the laser pulse and the formation of the jet. It can be noted that the kinetic energy "Ekin" of the material is initially high and can begin to decrease with time as, for example, viscous or cohesive forces of the material can partially dissipate the energy. Around time "T2", a portion of the material can begin to collect kinetic energy. Around time "T3", a portion of the material can be drawn back into the donor layer again, for example by fluid cohesion, while another portion continues to leak down. Finally, at about time "T4", an elongated jet of material may form and reach the acceptor substrate. Donor material may flow through the elongated jet between donor substrate 11 and acceptor substrate 12 until the jet collapses after a period of time (not shown here).

[0040] In a preferred embodiment, the donor material 11m is a liquid, such as a paste or ink. More preferably, the donor material 11m has a relatively high viscosity. For example, the (dynamic) viscosity is preferably at least 1 Pascal-second (Pa·s), more preferably at least 2 Pascal-seconds, most preferably at least 3 Pascal-seconds (=3000 centipoise, cP) or more. For example, the viscosity can be up to 10, 20 or 100 Pascal-seconds or more. By providing the donor material with a particular viscosity, the jet formation process can be appropriately controlled to shape the jet into a desired shape. For example, the viscosity is measured at room temperature (20° C. or 293 Kelvin). For example, the viscosity can be greater than 1 s per second (1 s -1 ) or less. Most preferably, the donor material 11m is a shear-thinning liquid, i.e., the viscosity decreases with shear strain rate. Accordingly, the material can become relatively "flowable" (less viscous) upon application of high shear forces, for example through bubble expansion. This can be advantageous for processes such as funnel feeding and material feed as used herein.

[0041] In some embodiments, the donor material 11m comprises a conductive material. Also, semiconductive or nonconductive / insulating (dielectric) materials can be used. For example, the method and system can be applied to form electronic circuits. For example, relatively thin circuit lines or other electrical components / structures can be written (printed) on the acceptor substrate 12 by applying an interconnected pattern of multiple droplets "Jd". In one embodiment, the conductive material comprises solder. In some embodiments, the donor material 11m comprises other components or materials that facilitate transfer (at least when it is still applied to the donor substrate 11). In one embodiment, the donor material 11m comprises a shear thinning material, such as a solder paste with a shear thinning solvent or conductive adhesive.

[0042] In principle, the donor material 11m may consist of any material that is liquid or can be brought to a liquid state under the influence of one or more laser pulses. For example, it may be envisaged to apply the method to pure metals, optionally using a delay between the pulses. For example, a first pulse may cause ablation and / or thrust, and subsequent pulses may arrive to further propel and / or shape the molten excess jet.

[0043] 3A-3C illustrate a comparison of jet formation resulting from a single pulse with a large spot size, a single pulse with a small spot size, and a combination of two pulses with large and small spot sizes. As illustrated in FIG. 3A, a single pulse with a relatively wide spot size can generate a relatively wide jet that bridges a significant transfer distance. However, it can be difficult to write high-resolution features with such a thick jet. As illustrated in FIG. 3B, a single pulse with a relatively narrow spot size can generate a relatively thin jet bridge that can in principle generate small droplets. However, the transfer distance of the thin jet is relatively small and can be difficult to control. As illustrated in FIG. 3C, a combination of two coincident pulses and / or a spike intensity profile can generate a thick jet that branches into a thin jet. As will be appreciated, the present teachings can thus provide synergistic advantages including relatively small droplet deposition over a relatively large transfer distance.

[0044] 4A and 4B illustrate a system 100 for laser-induced forward transfer LIFT of donor material 11m from a donor substrate 11 to an acceptor substrate 12 over a transfer distance "Zt."

[0045] In one embodiment, a substrate handler (not shown), such as a platform and / or roller, is configured to position the donor substrate 11 and the acceptor substrate 12 at respective positions separated by a transfer distance "Zt". For example, one or more of the donor substrate 11 and / or the acceptor substrate 12 may be foil-based. Typically, at least one light source 50 is configured to impinge the donor substrate 11 with one or more laser pulses "L1", "L2" that deposit respective energy into the donor material 11m to initialize the transfer. Preferably, the system is configured to deposit energy according to a spatially modulated intensity profile "Ixy" as described herein.

[0046] In a preferred embodiment, the intensity spike in the intensity profile "Ixy" is generated by impinging at least two laser pulses "L1", "L2" on the donor material 11m, as illustrated in FIG. 4A for example. Most preferably, the at least two laser pulses "L1", "L2" are generated from a single laser pulse. For example, the original laser pulse is split along a first optical path and a second optical path that intersect again with overlapping laser spots on the donor material 11m. Also, more pulses can be split into three or four or more different optical paths. Alternatively, two or more separate lasers can be used to generate overlapping pulses on the donor substrate 11 (not shown).

[0047] In some embodiments, a delay stage (not shown) in at least one of the optical paths can be used to delay one pulse relative to another. This can be particularly advantageous when the pulses originate from one light source, such as a laser 50. Alternatively, or in addition, the pulses may originate from different sources (not shown) where the pulsing of one pulse is delayed relative to the other. This can more easily tolerate relatively large pulse delays.

[0048] In some embodiments, the system comprises one or more beam splitters "BS" for splitting a light pulse from one light source 50 into at least a first laser pulse "L1" traveling along a first optical path and a second laser pulse "L2" traveling along a second optical path. In another or further embodiment, the system comprises an optical component T2 for reducing a beam diameter of the second laser pulse "L2" relative to a beam diameter of the first laser pulse "L1" and / or increasing a beam diameter of the first laser pulse "L1" relative to the second laser pulse "L2".

[0049] Preferably, the system is configured to overlap the beam paths onto the donor substrate 11 as matching spots of different sizes. For example, a beam combiner "BC" can be used to overlap both the beam positions and the beam directions. For example, a combination of a polarizing beam splitter and a beam combiner can be used. Alternatively or additionally, a semi-transparent mirror can be used. Alternatively, or in addition to a beam combiner, the beams can be projected onto the donor substrate simply with overlapping spots, for example at different angles or near-parallel beams. Alternatively, or in addition to splitting the beam paths, in some embodiments, the system includes a beam shaper "SH" to help achieve the desired intensity profile "Ixy". For example, as illustrated in FIG. 4B, the beam shaper can allow for a relatively simple beam path. On the other hand, the use of two or more separate pulses allows more versatility in shaping the individual sides of the overlapping laser spots, and may also allow further optimization of the relative timing and angles, etc.

[0050] In some embodiments, one, preferably two or more, laser pulses are nanosecond pulses, which can be relatively easy to generate and provide the desired transfer characteristics. Alternatively, other pulse lengths can also be used. In one embodiment, a single picosecond pulse is used. For example, the nonlinear absorption of the picosecond pulse can at least partially contribute to the spike intensity in the center of the intensity profile "Ixy" without the need for a second pulse. However, the process can be more difficult to control.

[0051] Other or additional processes may also play a role. For example, when using a single pulse of high enough energy, the pulse may cause damage to the donor carrier 11c, e.g. a glass plate. In one embodiment, a single pulse is used with energy and / or intensity adjusted so that it not only heats the donor material 11m, but also causes some damage at the interface of the donor carrier 11c between the donor carrier 11c and the donor material 11m, e.g. generating a plasma or ablation of the donor carrier 11c at the interface, preferably concentrated as a spike in the center of the beam spot. The inventors have found that such a process may provide an additional "kick" to initiate the formation of a secondary (thin) jet, similar to other mechanisms described herein. For example, this may be used in combination with a disposable carrier. Preferably, the energy and / or intensity of the pulse is however low enough that the damage does not extend into the bulk carrier material, e.g. the damage does not extend into the carrier more than 10% of the carrier thickness. Otherwise the damage may for example impede the transmission of light.

[0052] In some embodiments, for example as shown, the laser beam is moved across a fixed surface of the donor substrate 11 to generate a pattern of droplets on the acceptor substrate 12. This may allow the laser to perform a series of rapid droplet depositions, e.g., jet formation is minimally affected by the substrate motion. Alternatively or additionally, the donor substrate 11 and / or acceptor substrate 12 may be moved relative to a stationary laser spot, e.g., between depositions.

[0053] Although the figures depict a single beam impinging on the substrate, deposition can also be performed with multiple beams. For example, two or more beams can write a pattern in parallel at different locations. In one embodiment, a mask is placed between the (broad beam) light sources to write the entire pattern of droplets simultaneously. For example, the mask includes a pixelated pattern, with each pixel acting as a beam shaper to generate an intensity profile "Ixy" as described herein. The pattern can also be generated using multiple masks. For example, a first mask has a pattern of relatively large holes that generate a first spot size, and a second mask has the same pattern but relatively small holes that generate a second spot size. The two patterns can be overlaid on the donor substrate. It is also possible to envision masks being placed in series, for example where the masking material of at least the small hole mask is semi-transparent so that some of the blocked light is transmitted to the other mask or to the donor substrate. In some embodiments, one or more beam shapers can act as masks, each generating multiple parallel intensity profiles as described herein. Alternatively, or in addition to lasers, other light sources such as flash lamps can also be used. This is particularly useful for irradiating large mask patterns.

[0054] FIG. 5A illustrates the separate intensity profiles of a first laser pulse "L1" with a relatively large spot size "Ds1" and a second laser pulse "L2" with a relatively small spot size "Ds2."

[0055] Preferably, as described herein, the second spot size "Ds2" is relatively small. For example, the second spot size "Ds2" is smaller than 100 micrometers, smaller than 50 micrometers, or smaller than 30 micrometers, for example between 5 and 20 micrometers or less. It will be appreciated that a relatively small second spot size "Ds2" may correlate with a relatively small maximum thin jet diameter "Dj2" and therefore with good resolution. Preferably, the first spot size "Ds1" is relatively large. For example, the first laser pulse "L1" has a spot size between 30 and 300 micrometers or more. As described herein, the spot size may be determined, for example, as the full width at half maximum (FWHM) of the spot intensity on the donor material 11m.

[0056] Preferably, the laser spots of the first laser pulse "L1" and the second laser pulse "L2" coincide on the donor material, e.g., the centers of the spots coincide within a distance of 50, 30, 20, or 10 percent of the maximum spot size. Most preferably, the intensity profile "Ixy" is rotationally symmetric. Accordingly, the intensity profile may have a similar shape in the X and Y directions along the donor substrate interface. Also, the laser spots may have substantially circular coincident profiles.

[0057] In one embodiment, the first laser pulse "L1" has a pulse energy higher than the second laser pulse "L2", for example at least 2, 3 or 4, for example between 2 and 10 times, or more. The relatively high energy of the first laser pulse "L1" can help initiate material transfer over a large donor area. Because the energy of the first laser pulse "L1" is spread over a relatively large irradiation area, the maximum intensity of the first laser pulse "L1" can still be similar to that of the second laser pulse "L2", or even lower. Also, a higher intensity second laser pulse can be used if desired.

[0058] FIG. 5B illustrates a composite (summed) intensity profile "Ixy". For example, this may correspond to the two overlapping laser pulses of FIG. 5A. Alternatively to overlapping two simultaneous pulses, at least a portion of the desired effect may be achieved with a single pulse that is shaped to effectively provide the same intensity profile on the donor layer as the two pulses described herein. For example, the single pulse may be shaped to have an intensity profile with locally high intensity in a subsection of the total spot size.

[0059] In some embodiments, the locally high intensity spikes can be quantified by comparing the full widths of the intensity profile "Ixy" at different heights. For example, full width at three quarters maximum ("FWTQM") is compared to full width at one quarter maximum ("FWOQM"). As can be noticed by comparing FIG. 5A and FIG. 5B, when using two pulses of similar maximum intensity but different spot sizes or beam widths, the FWOQM can roughly correspond to the FWHM of the wide first pulse "L1", i.e., Ds1, while the FWTQM can roughly correspond to the FWHM of the narrow second pulse "L2", i.e., Ds2. Therefore, taking into account the relative spot sizes when using two pulses can translate to similar relative values ​​of the FWTQM and FWOQM. In particular, it is preferred that the FWTQM is much smaller than the FWOQM, e.g., at least 4, 5, 6, 7, 10 or 20 times or more.

[0060] Without being bound by theory, this is I(x)=I max exp(-x 2 / 2σ 2 ) where I max is the maximum intensity, x is the distance from the maximum, and σ is the variance or standard deviation. So a Gaussian curve has a variance of about 1.51 times σ.

[0061]

number

[0062] , and is about 3.33 times the variance σ

[0063]

number

[0064] Therefore, for a single Gaussian curve, the FWTQM is only 2.19 times smaller than the FWOQM (compared to over 4 times as mentioned above).

[0065] Of course, other ways of quantifying spikes are conceivable. For example, the total energy in the intensity profile "Ixy" may be compared to the percentage of that energy in the central subsection. For example, in a typical Gaussian profile, about 68 percent of the energy is contained within one standard deviation (1 sigma) from the center, while the addition of a central spike may raise this percentage to more than 70 percent, more than 75 percent, more than 80 percent, or even more than 90 percent. The more prominent the central spike, the higher the percentage of the total energy that will be located there.

[0066] When interpreting the appended claims, the following should be understood: unless specifically stated otherwise, the word "comprising" does not exclude the presence of other elements or acts than those recited in a given claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Any reference signs in the claims do not limit their scope. Several "means" may be represented by the same or different details or structures or functions implemented. Any of the disclosed devices or parts thereof may be combined or separated into further parts. When one claim refers to another claim, this may indicate synergistic advantages achieved by a combination of their respective features. However, the mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to the same advantage. The present embodiment may therefore include all functional combinations of claims, where each claim may in principle refer to any preceding claim, unless clearly excluded by the context. [Explanation of symbols]

[0067] 11 Donor Substrate 11b Bubbles 11c Donor Carrier 11m Donor material 11xy interface 50 light source 100 Systems 12 Acceptor substrate BC Beam Combiner BS Beam Splitter Dj1 Smallest jet diameter Dj2 Maximum fine jet diameter Ds1 First spot size Ds2 Second spot size dt Time difference Dx droplet size E1 Energy E2 Energy I1 First intensity range I2 Second Intensity Range Is Intensity Spike Ixy Intensity Profile J1 Relatively thick jet J12 Branch position J2 Relatively thin jet J2b subdivision Jb collapse point Jd droplet Je Extension Jet Jf flow L1 First laser pulse L2 Second laser pulse SH Beam Shaper T2 Optics Tb jet decay Tf jet formation Tt Transcription period Zj1 Thick jet length Zj2 Thin Jet Length Zm layer thickness Zt Transfer distance θ1 First angle θ2 Second angle

Claims

1. 1. A method for laser induced forward transfer (LIFT), comprising: - providing a donor substrate (11) with a donor material (11m) to be transferred; - providing an acceptor substrate (12) at a transfer distance (Zt) from said donor substrate (11) for receiving said donor material (11m); - impinging the donor substrate (11) with one or more laser pulses that deposit respective energy into the donor material (11m) to initiate the transfer, - said energy is applied according to an intensity profile (Ixy) that is spatially adjusted across the interface (11xy) of said donor material (11m) so as to cause said donor material (11m) to be ejected from said donor substrate as an elongated jet (Je) that instantaneously bridges said transfer distance (Zt) between said donor substrate (11) and said acceptor substrate (12) during a transfer period (Tt); said intensity profile (Ixy) having a first intensity range (I1) spanning a first relatively large spot size (Ds1) on said donor material (11m) causing said elongated jet (Je) to start from said donor substrate (11) as a corresponding relatively thick jet (J1); - said intensity profile (Ixy) has a second intensity range (I2) higher than said first intensity range (I1) over a relatively small second spot size (Ds2) smaller than said first spot size (Ds1), said intensity profile (Ixy) of said intensity ranges (I1, I2) and spot sizes (Ds1, Ds2) combined has a non-Gaussian shape with a locally increased intensity spike (Is) in the center of said intensity profile (Ixy), causing said thick jet (J1) to diverge into a relatively thin jet (J2) at a branching position (J12) between said donor substrate (11) and acceptor substrate (12) due to said locally increased intensity in the center, and only said thin jet (J2) reaches said acceptor substrate (12) for depositing a droplet (Jd) of said donor material (11m) before collapsing.

2. 2. The method of claim 1, wherein the donor substrate (11) is bombarded with at least two laser pulses, a first laser pulse having an optical path that is at least partially different from a second laser pulse, the optical paths being recombined at the donor substrate (11) to cause the transfer of the donor material (11m) towards the acceptor substrate (12) by a combined effect of the laser pulses.

3. 3. The method of claim 2, wherein the first laser pulse has a first spot size (Ds1) impinging on the donor material (11m) and the second laser pulse has a second spot size (Ds2) impinging on the donor material (11m), the second spot size (Ds2) being at least two times smaller than the first spot size (Ds1).

4. 4. The method of claim 2 or 3, wherein the second laser pulse is delayed with respect to the first laser pulse by a time difference (dt) between 10 nanoseconds and 10 microseconds.

5. 5. The method of claim 2, wherein the first laser pulse has a pulse energy at least twice as high as the second laser pulse.

6. 6. The method according to claim 1, wherein the size and / or intensity of one or more of the overlapping light spots is controlled during operation based on recorded images of the elongated jet (Je) and / or based on measurements of the resulting droplet size (Dx).

7. 7. The method according to claim 1, wherein the transfer distance (Zt) between the donor substrate (11) and the acceptor substrate (12) is at least 10 times larger than the size (Dx) of the deposited droplet (Jd).

8. 8. The method according to claim 1, wherein the layer thickness (Zm) of the donor material (11m) on the donor substrate (11) is between 20 and 100 micrometers.

9. the extensional jet (Je) bridging the transfer distance (Zt) between the donor substrate (11) and the acceptor substrate (12) during at least a portion of the transfer period (Tt), - said relatively wide jet (J1) defined by a minimum wide jet diameter (Dj1) extending over a wide jet length (Zj1); said relatively thin jet (J2) defined by a maximum thin jet diameter (Dj2) extending over a thin jet length (Zj2); - said maximum narrow jet diameter (Dj2) is at least twice as small as said minimum wide jet diameter (Dj1); - said fat jet length (Zj1) is greater than 50 percent of said transfer distance (Zt); - the thin jet length (Zj2) is between 25 and 50 percent of the transfer distance (Zt); 9. The method according to any one of claims 1 to 8.

10. 10. The method according to claim 1, wherein after reaching the acceptor substrate (12), the thin jet (J2) collapses into a first subsection (J2b) of thin subsections (J2b) that is deposited as part of a droplet (Jd) on the acceptor substrate (12) and a second subsection that remains connected to the donor substrate (11) via the elongated jet and is retracted towards the donor substrate (11) due to material properties.

11. 11. The method according to claim 1, wherein a majority of the donor material in the droplet (Jd) after transfer results from a flow (Jf) of the donor material (11m) during a material transfer period (Tt) when the jet (Je) extends uninterrupted between the donor substrate (11) and the acceptor substrate (12).

12. 12. The method according to any one of claims 1 to 11, wherein the donor material (11m) consists of a shear-thinning liquid.

13. 13. The method according to any one of claims 1 to 12, wherein the donor material (11m) is made of a conductive material and the method comprises the step of printing an electronic circuit on the acceptor substrate (12) by deposition of a plurality of droplets (Jd).

14. A system (100) for laser-induced forward transfer (LIFT) of a donor material (11m) from a donor substrate (11) to an acceptor substrate (12) over a transfer distance (Zt), comprising: a substrate handler configured to position said donor substrate (11) and said acceptor substrate (12) at respective positions separated by said transfer distance (Zt); at least one light source (50) configured to impinge on the donor substrate (11) one or more laser pulses that deposit respective energy into the donor material (11m) to initiate the transfer, the system being configured to deposit the energy according to an intensity profile (Ixy) that is spatially adjusted across the interface of the donor material (11m) to cause the donor material (11m) to be ejected from the donor substrate as an elongated jet (Je) that instantaneously bridges the transfer distance (Zt) between the donor substrate (11) and the acceptor substrate (12), the intensity profile (Ixy) having a first intensity range (I1) that spans a relatively large first spot size (Ds1) on the donor material (11m) to eject the elongated jet (Je) from the donor substrate as a corresponding relatively thick jet (J1). a first spot size (Ds1) having a second intensity range (I2) higher than the first intensity range (I1) over a second spot size (Ds2) that is smaller than the first spot size (Ds1), the intensity profile (Ixy) of the combined intensity ranges (I1, I2) and spot sizes (Ds1, Ds2) having a non-Gaussian shape with a locally increased intensity spike (Is) in the center of the intensity profile (Ixy), causing the thick jet (J1) to diverge into a relatively thin jet (J2) at a divergence position (J12) between the donor substrate (11) and the acceptor substrate (12) due to the locally increased intensity at the center, and only the thin jet (J2) reaching the acceptor substrate (12) to deposit a droplet (Jd) of its donor material (11m) before collapsing.

15. 15. The system of claim 14, comprising a beam splitter (BS) for splitting a light pulse from one light source (50) into a first laser pulse traveling along a first optical path and a second laser pulse traveling along a second optical path, the system comprising an optical component (T2) for decreasing a beam diameter of the second laser pulse relative to a beam diameter of the first laser pulse and / or increasing the beam diameter of the first laser pulse relative to the second laser pulse, the system being configured to overlap beam paths as coincident spots of different sizes on the donor substrate (11).

Images