JP7785488B2 - Method and apparatus for performing laser ablation - Google Patents

Description

The present invention relates to methods and apparatus for performing laser ablation, particularly for forming fine metal mesh.

Fine metal mesh (FMM) is used in the manufacture of organic light-emitting diode (OLED) displays. Specifically, it is used as an OLED deposition mask in display manufacturing. The FMM defines where the OLED molecules are deposited on the display, ultimately determining the resolution of the OLED display.

Current technologies for manufacturing fine metal meshes include photolithography and electroforming processes. However, the costs of such processes are high, and the resolution of OLED displays using FMMs manufactured using such techniques is typically less than 600 pixels per inch (ppi). Modern applications, such as mobile phones and virtual reality headsets, require higher resolutions, e.g., 1000 ppi or higher. FMMs manufactured by photolithography and electroforming processes struggle to achieve such high resolutions.

Other conventional techniques for fabricating FMMs involve splitting a single laser beam into multiple laser beams and scanning these laser beams across the surface of a substrate to form the FMM by ablation. Such techniques typically use femtosecond pulsed infrared lasers. However, such techniques require complex projection optics to capture the multiple laser beams. Scaling this technique to thousands of laser beams is difficult, if not impossible, limiting the rate at which FMMs can be fabricated this way. Such systems are capable of fabricating FMMs with apertures having critical dimensions of 10 μm and resolutions of hundreds of dots per inch (dpi).

Embodiments of the present disclosure are intended to at least partially address one or more of the problems discussed above and/or other problems.

According to one aspect of the present invention, there is provided a method of performing laser ablation, comprising: directing an ultraviolet laser beam through a mask to image a portion of an ablation pattern defined by the mask onto a material layer; and scanning the laser beam over the mask to sequentially image different portions of the ablation pattern onto different respective regions of the layer, thereby ablating structures in the layer corresponding to the ablation pattern, wherein the laser beam comprises an ultrafast pulsed laser beam having a pulse length of less than 20 picoseconds.

Thus, a method is provided for defining an ablation pattern formed by a mask. The ablation pattern can be defined by multiple transparent regions within the mask. Compared to the prior art described above, a single laser beam can be used to simultaneously illuminate multiple transparent regions within the mask, thereby contributing to the ablation of a correspondingly large number of features within the material layer being ablated. This is achieved without the need for complex beam-splitting and balancing optics and can be scaled to achieve simultaneous processing of a very large number of features. High laser powers can be used because the laser power can be distributed across many different features of the ablation pattern. The use of high-power lasers promotes high throughput. The use of ultraviolet illumination allows for high spatial resolution at reasonable operating costs. Defining the ablation pattern using a mask (rather than via individual beam spots directly from the laser) allows for high-precision definition of the ablation pattern while easing the requirements of the laser beam used to illuminate the mask. The laser can be easily "swept" across the mask with relatively low resolution.

In one embodiment, the structure corresponding to the ablation pattern includes a regular array of openings. The openings may all have approximately the same size and shape. Thus, the ablation pattern may be used to form an FMM.

In one embodiment, the imaging portion of the ablation pattern contributes to forming multiple openings in the layer. The multiple openings corresponding to each imaging portion can include at least 100 openings. Thus, the laser pulse energy is spread across at least 100 openings without the need for complex beam splitting and balancing. Furthermore, in one embodiment, sequential imaging of different portions of the ablation pattern can contribute to forming at least 100,000 openings in the layer. Thus, the number of openings formed can be significantly increased simply by scanning the laser beam over the mask. This approach can be scaled up to process more than 500,000 openings, more than 750,000 openings, or even more than 1 million openings using a single mask.

In one embodiment, each of the openings is tapered to have a cross-sectional area that decreases in the downstream direction of the laser beam. The directing and scanning steps can then be repeated for multiple mask patterns, each defining a cross-sectional area of the tapered opening at a different depth. This approach allows for efficient and precise control of the tapered opening profile. Optimizing the taper of the FMM openings can improve the performance of OLED fabrication processes that use FMMs by minimizing blurring of the pattern edges during deposition of the OLED molecule pattern. Typically, the tapered openings of the FMM are positioned to face the substrate onto which the OLED molecules are deposited (i.e., open outward from the substrate). Controlling the taper angle allows for an optimal balance between providing a spatially precise FMM with high resolution (which can potentially limit the maximum amount of taper allowed) and minimizing redirection of OLED molecular orbitals due to undesired interactions (collisions) with the sidewalls of the FMM opening (which can generally be improved by increasing the amount of taper).

As noted above, the layer may include a metal layer (e.g., to form an FMM). The metal layer may have a variety of compositions depending on the purpose of the FMM. The metal layer may be formed from a material with a very low coefficient of thermal expansion, such as Invar. Other materials, including non-metallic materials, may also be used. The layer may include, for example, a dielectric material and/or a polymer.

In one embodiment, the structure comprises part of a deposition mask for depositing OLED molecules during the manufacture of an OLED-based display. Thus, a method for depositing OLED molecules can be provided in which the disclosed method for performing laser ablation is used to form a deposition mask, and the resulting deposition mask is used to deposit organic light-emitting molecules in a pattern defined by the deposition mask.

According to another aspect of the present invention, there is provided an apparatus for performing laser ablation, comprising:
An apparatus is provided that includes an ultraviolet laser configured to generate an ultrafast pulsed laser beam having a pulse length of less than 20 picoseconds, a mask that defines an ablation pattern, an optical system configured to direct the laser beam through the mask to image a portion of the ablation pattern onto a material layer, and a scanning device configured to scan the laser beam over the mask to sequentially image different portions of the ablation pattern onto the layer, thereby ablating structures in the layer that correspond to the ablation pattern.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

1 is a schematic side view of an apparatus for performing laser ablation. FIG. 2 is a top view of a mask that can be used in the apparatus shown in FIG. 1; FIG. 1 is a top view of a mask showing how a laser beam can be scanned onto the mask. 2 is a top view of the layer shown in FIG. 1 with an ablation pattern ablated into the layer. 10A-10C are side cross-sectional views showing different stages of ablation of a tapered opening. 10A-10C are side cross-sectional views showing different stages of ablation of a tapered opening. 10A-10C are side cross-sectional views showing different stages of ablation of a tapered opening. 1 shows multiple mask patterns for different scans over the same region of a layer, where the mask patterns are provided on separate masks. 1 shows multiple mask patterns for different scans over the same region of a layer, where the mask patterns are provided on different regions of the same mask. 10A-10C are side cross-sectional views illustrating different aperture taper profiles formed by decreasing or increasing laser energy density (fluence). 10A-10C are side cross-sectional views illustrating different aperture taper profiles formed by decreasing or increasing laser energy density (fluence).

FIG. 1 illustrates an exemplary apparatus 2 for performing laser ablation. The apparatus 2 uses an ultraviolet laser 6 configured to provide an ultrafast pulsed laser beam 8. The ultrafast pulsed laser beam 8 has a pulse length of less than 20 picoseconds, optionally less than 15 picoseconds, optionally less than 10 picoseconds, optionally less than 8 picoseconds, optionally less than 6 picoseconds, and optionally less than 5 picoseconds. A mask 10 defining an ablation pattern 18 (shown in FIG. 2) is provided. A material layer 4 to be processed is provided on a support 12 (e.g., a substrate). The support 12 may be provided on a movable table (not shown) for stepping the support 12 to different positions under the laser beam 8. An optical system 13 is provided to direct the laser beam 8 through the mask 10 onto the layer 4. The optical system 13 images a portion of the ablation pattern 18 onto the layer 4.

The scanning device 14 scans the laser beam 8 over the mask 10 to sequentially image different portions of the ablation pattern 18 defined by the mask 10 onto different respective areas of the layer 4, thereby ablating structures corresponding to the ablation pattern into the layer 4. The laser beam 8 is typically scanned over the mask 10 without corresponding movement of the laser 6 or the mask 10 (e.g., by appropriate scanning optics).

The use of ultraviolet wavelengths allows structures to be formed in layer 4 with high resolution without the need for complex and/or expensive optics. Typically, resolutions up to 1000 dpi are formed, including features such as depressions or openings with critical dimensions of about 3 μm or less.

Apparatus 2 may further include a controller 15 for controlling the overall operation of apparatus 2. Controller 15 may control operation of laser 6 (e.g., to control when the laser is on and off and/or to change parameters of the laser such as energy per pulse or pulse repetition rate), scanning device 14, and optical system 13 (e.g., to control focal height), as well as movement of support 12 relative to laser 6 (e.g., via a movable table and associated motors).

In one embodiment, the apparatus 2 further includes an optical system 16 (e.g., including a lens) downstream from the mask 10. The optical system 16 can focus the laser radiation 8 from the mask 10 onto the layer 4. In one embodiment, the optical system 16 provides demagnification between the mask 10 and the layer 4. Thus, features formed on the layer 4 by ablation are smaller than corresponding features in the mask 10. This approach allows for high-resolution patterns to be formed in the layer 4 while distributing the laser energy over a larger area in the mask 10. Therefore, the laser energy density (fluence) is lower in the mask 10 than it would otherwise be. This allows for higher laser pulse energies and higher laser powers to be used, which improves throughput without risking damage to the mask 10. Furthermore, the mask 10 can be fabricated with a lower resolution than the pattern required in the layer 4, thereby facilitating the manufacture of the mask 10.

In some embodiments, the ablation-generating structure in layer 4 corresponding to the ablation pattern of mask 10 comprises a regular array of openings in layer 4. The pitch of the array in layer 4 can be very small, e.g., 10 microns or less. All of at least a subset of the openings can have substantially the same size and shape and/or be otherwise configured in a manner suitable for forming all or part of an FMM for use in manufacturing OLED-based displays. FIG. 2 is a top view of an exemplary mask 10 for forming such an ablation pattern. In this example, mask 10 comprises a regular array of transparent regions 20. Each transparent region 20 corresponds to a respective opening formed in layer 4. The pitch of the transparent regions 20 on mask 10 is typically larger than the pitch of the corresponding openings in layer 4 due to shrinkage. For ease of illustration, mask 10 in FIG. 2 includes only a relatively small number of transparent regions 20. In practice, many more transparent regions 20 (e.g., 100,000 or more, as described below) will likely be provided per mask.

In some embodiments, each imaged portion of the ablation pattern 18 in the mask 10 contributes to forming multiple openings in the layer 4. The multiple openings preferably include at least 100 openings. For example, each opening in the layer 4 can be formed (partially or completely) by directing laser radiation through a corresponding transparent region 20 on the mask 10, and the imaged portion of the ablation pattern 18 can be formed by simultaneously irradiating multiple (e.g., 100 or more) such transparent regions 20 on the mask 10. Simultaneous illumination of multiple transparent regions 20 in this manner can simultaneously contribute to the formation of multiple openings in the layer 4. This maximizes the use of available laser pulse energy and increases throughput. When combined with a scanning step, a very large number of openings can be rapidly formed. Sequential imaging of different portions of the ablation pattern can contribute to the formation of at least 10,000 openings in the layer, for example, through 1,000 or more portions of the ablation pattern, each contributing at least 100 openings.

In the example shown schematically in FIG. 2, the ablation pattern 18 defined by the mask 10 includes an array of square transparent regions 20. In other embodiments, the transparent regions 20 can have other shapes, thereby forming differently shaped features or openings in the layer 4. The transparent regions 20 can be, for example, rectangular, circular, or elliptical.

3 shows an exemplary scanning path 22 of the laser beam spot 9 on the mask 10 during the scanning step (as viewed by the laser 6). The laser beam spot 9 is the portion of the mask 10 that is illuminated by the laser beam 8 at any given time and defines the corresponding portion of the ablation pattern (the "imaged portion") that is imaged onto the layer 4 at that time. The scanning path 22 can be described as a raster scan. Other scanning paths may also be used. The scanning path 22 can be adapted to avoid areas in the layer 4 where no structure is required. The scanning path 22 can additionally or alternatively take into account other factors, such as the nature of the laser 6 used (e.g., power and/or spot size 9 on the mask 10), the ablation pattern 18 in the mask 10, and/or the properties of the layer 4.

FIG. 4 is a top view of layer 4 after ablation into layer 4 of structures 24 corresponding to the ablation pattern 18 defined by mask 10 (e.g., a square array of openings). Structures 24 can be formed by a single scan across mask 10 or by multiple scans across the mask. For example, in a first scan across mask 10, at least a subset of the features of the structures can be ablated through layer 4 to only a portion of their intended depth, thereby providing partially formed features. By repeating the scanning process, each partially formed feature can be irradiated multiple times until it is fully formed (e.g., so that the openings extend completely through layer 4). This approach can advantageously help dissipate heat between successive ablation processes, thereby preventing undesired damage to areas outside the ablation target area. In one embodiment, laser beam spot 9 is scanned multiple times along scan path 22, as discussed above with reference to FIG. 3. The multiple scans can be performed without providing relative motion between mask 10 and layer 4 during the process.

As shown in FIG. 4, the structure 24 corresponding to the ablation pattern 18 provided by one mask 10 may cover only a small portion of the layer 4 being processed. Therefore, this method can be repeated to process other required portions of the layer 4. In one embodiment, a step-and-scan process is used, whereby the structure 24 is formed in the layer 4 provided at a first position relative to the mask 10. Relative motion is then provided between the layer 4 and the mask 10 to move the layer 4 to a second position relative to the mask 10 (typically by moving the layer 4 while keeping the mask 10 and optics 16 in place), and the scanning process is repeated to form another instance of the structure 24 adjacent to the previously formed instance. This process can then be repeated to process the entire layer 4. Thus, the above orienting and scanning steps are repeated for multiple different positions of the layer 4 relative to the mask 10 to ablate structures corresponding to the ablation pattern at multiple different locations on the layer 4, thereby allowing much larger structures to be built in the layer 4 than would be possible without stepping the layer 4.

In one embodiment, as shown in FIGS. 5-7 , each of the openings 25 in the ablated structure 24 is tapered to have a cross-sectional area that decreases downstream of the laser beam 8. In one embodiment, the taper is controlled by repeatedly directing the laser beam 8 onto the layer 4 through a mask 10 and scanning the laser beam 8 over the mask 10 for a plurality of different mask patterns, each mask pattern defining a cross-sectional area of the tapered openings 25 at a different depth. For example, a first mask pattern may include a first plurality of transparent regions 20 corresponding to each of the openings 25 formed in the layer 4, a second mask pattern may include a second plurality of transparent regions 20 corresponding to the same respective openings 25, and a third mask pattern may include a third plurality of transparent regions 20 corresponding to the same respective openings 25, with the transparent regions 20 of the first mask pattern being larger than the transparent regions 20 of the second mask pattern, which in turn are larger than the transparent regions 20 of the third mask pattern. An exemplary result of processing using a first mask pattern is shown schematically in FIG. 5, where a shallow depression having a diameter 26 is formed. An exemplary result of processing using a second mask pattern is shown schematically in FIG. 6, where the depression is deepened and has a narrower diameter 28. An exemplary result of processing using a third mask pattern is shown schematically in FIG. 7, where ablation has penetrated layer 4, forming a tapered opening 25 with a diameter 30 at the deepest point of depression 25. Varying the size of the transparent regions in the first, second, and third mask patterns affects the diameters 26, 28, and 30 at different points along the taper, allowing for precise control of the taper profile.

The approach of repeating the directing and scanning steps for multiple mask patterns defining different respective ablation patterns is not limited to cases where the structure being formed is a regular array of openings, with different ablation patterns corresponding to different opening depths. This approach can be applied to different or more complex structures. This approach is useful whenever it is advantageous to control the shape of the depressions or openings in layer 4 as a function of the depression or opening depth. To achieve control of shape as a function of depth, it will usually be desirable for the repeated directing and scanning steps to result in the application of different laser ablation patterns to the same or overlapping regions of layer 4. Typically, this will involve repeating the directing and scanning without changing the relative position between mask 10 and layer 4, e.g., so that it is the same portion of layer 4 being processed each time. The multiple mask patterns can be provided on separate masks 101, 102, and 103, as shown schematically in FIG. 8, or as different regions 10A, 10B, and 10C on the same mask 10, as shown schematically in FIG. 9.

Other approaches to taper control can be used in combination with or as an alternative to the above. For example, in one class of embodiments, the taper is controlled at least in part by controlling the fluence (pulse energy density—the energy of the laser pulse divided by the area illuminated by the laser pulse) of the laser beam 8 onto the mask 10. For example, the laser beam 8 can be scanned multiple times over the mask 10, with at least two of the scans being performed at different fluences. The fluence of the laser beam 8 as it impinges on the layer 4 affects the taper angle of the walls of the ablated pockets within the layer. A higher fluence results in a smaller taper angle (more vertical). A lower fluence results in a larger taper angle (less vertical). Providing the ability to vary the fluence as a function of depth within the layer 4 provides a useful additional degree of freedom for tailoring the internal shape (e.g., taper profile) of the structures formed within the layer 4.

Based on the above, in one class of embodiments, for each of the one or more openings, the fluence of the laser beam 8 at the mask 10 is varied during the formation of the opening. Varying the fluence at the mask 10 results in a corresponding change in fluence at the layer 4. The variation is such that the fluence at the mask (and therefore at the layer 4) is different during the formation of portions of the opening at different depths within the layer 4, thereby controlling the variation in the taper angle of the opening as a function of depth within the layer 4.

In one example procedure, a first scan on the mask 10 is performed using a laser beam 8 that provides a first fluence at the mask 10. The scan can, for example, follow the scan path 22 as described above with reference to FIG. 3. The fluence of the laser beam 8 can be such that after this first scan on the mask 10, the structure formed in layer 4 extends only partway through layer 4 (similar to the situation in FIG. 5). A second scan on the mask 10 is then performed using a laser beam 8 that provides a second fluence that is lower than the first fluence. This scan results in the structure formed in the first scan being deepened. However, due to the lower fluence of the laser beam 8 during the second scan, the taper angle also increases. A third scan on the mask 10 is then performed using a laser beam 8 that provides a third fluence that is lower than the second fluence. This scan results in the structure formed in the second scan being deepened until ablation breaks through to the other side of layer 4. The lower fluence of the laser beam 8 during the third scan means that the taper angle increases further at the newly reached depth. Figure 10 shows an aperture profile created in this manner. This approach therefore provides an alternative or additional method for controlling the shape of the aperture taper. While this embodiment has been illustrated by reference to three scans, any number of scans can be used. Furthermore, the fluence does not necessarily have to be adjusted in the manner described above; instead, it can be varied in any suitable manner. A gradual increase in fluence with successive scans creates an aperture profile of the type shown in Figure 11. Furthermore, the fluence of the laser beam 8 does not have to be different for every scan, as long as there is a difference between at least two scans. The fluence can be increased or decreased between different scans.

Claims (16)

1. A method of performing laser ablation, comprising:
directing an ultraviolet laser beam through a mask to image a portion of an ablation pattern defined by the mask onto a material layer;
scanning the laser beam over the mask using scanning optics, without corresponding movement of either the laser or the mask, to sequentially image different portions of the ablation pattern onto different respective regions of the layer, thereby ablating structures in the layer corresponding to the ablation pattern;
the laser beam comprises an ultrafast pulsed laser beam having a pulse length of less than 20 picoseconds;
the structure corresponding to the ablation pattern comprises a regular array of openings, each of the openings tapering to have a decreasing cross-sectional area in a downstream direction of the laser beam . The method of claim 1 , wherein all of at least a subset of the openings have approximately the same size and shape. 3. The method of claim 1 or 2, wherein for each of one or more of the openings, the fluence of the laser beam at the mask is varied during formation of the opening, the variation being such that the fluence is different during formation of portions of the opening at different depths in the material layer, thereby controlling the variation of a taper angle of the opening as a function of depth in the material layer. 4. The method of claim 1, wherein the directing and scanning steps are repeated for a plurality of mask patterns, each mask pattern defining a cross-sectional area of the tapered opening at a different depth. The method of claim 1 , wherein each imaging portion of the ablation pattern contributes to forming a plurality of openings in the layer. The method of claim 5 , wherein the plurality of openings corresponding to each imaging portion includes at least 100 openings. The method of claim 6 , wherein the successive imaging of different portions of the ablation pattern contributes to forming at least 100,000 openings in the layer. The method of any one of claims 1 to 7 , wherein the pitch of the array is less than 10 microns. The method of any one of claims 1 to 8 , wherein the directing and scanning steps are repeated for a plurality of mask patterns defining different respective ablation patterns. 10. The method of claim 9 , wherein the repetitions of the directing and scanning steps apply different laser ablation patterns to the same or overlapping regions of the layer. The method of claim 9 or 10 , wherein the multiple mask patterns are provided on separate masks. The method according to claim 9 or 10 , wherein the multiple mask patterns are provided in different areas of the same mask. The method of any one of claims 1 to 12 , wherein the layer comprises a metal layer. 14. The method of claim 1, wherein the directing and scanning steps are repeated for a plurality of different locations of the layer relative to the mask, thereby ablating the structure corresponding to the ablation pattern at a plurality of different locations on the layer. The method of any one of claims 1 to 14 , wherein the structure comprises part of an evaporation mask for depositing organic light-emitting molecules during the manufacture of an organic light-emitting molecule-based display. 1. A method for depositing organic light-emitting molecules, comprising:
forming a deposition mask by carrying out the method of claim 15 ;
using the deposition mask to deposit organic light-emitting molecules in a pattern defined by the deposition mask.