CN113523579A

CN113523579A - Method and apparatus for laser ablation

Info

Publication number: CN113523579A
Application number: CN202110407590.XA
Authority: CN
Inventors: A·N·布伦顿
Original assignee: M Solv Ltd
Current assignee: M Solv Ltd
Priority date: 2020-04-16
Filing date: 2021-04-15
Publication date: 2021-10-22
Also published as: TW202140821A; GB202005541D0

Abstract

Methods and apparatus for performing laser ablation are disclosed. In one arrangement, an ultraviolet laser beam is directed through a mask to image a portion of an ablation pattern defined by the mask onto a material layer. The laser beam is scanned over the mask to sequentially image different portions of the ablation pattern onto different corresponding areas of the layer. Thereby ablating structures corresponding to the ablation pattern into the layer. The laser beam comprises an ultrafast pulsed laser beam having a pulse length of less than 20 picoseconds.

Description

Method and apparatus for laser ablation

Technical Field

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

Background

A Fine Metal Mesh (FMM) is used to manufacture an Organic Light Emitting Diode (OLED) display. In particular, they are used as OLED evaporation masks in the manufacture of displays. The FMM defines the deposition locations of the OLED molecules on the display, ultimately determining the resolution of the OLED display.

Current techniques for producing fine metal meshes include photolithography and electroforming processes. However, the cost of such processes is high, and the resolution of OLED displays using FMMs manufactured by employing such techniques is typically less than 600 pixels per inch (ppi). Modern applications, such as cell phones and virtual reality headsets, require higher resolution, such as 1000ppi or higher. FMMs manufactured by photolithography and electroforming processes have difficulty in obtaining such high resolution.

Other prior art techniques for producing FMMs include splitting a single laser beam into multiple laser beams and scanning the 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 obtain multiple laser beams. It is difficult, if not impossible, to extend the technology to thousands of laser beams, and thus the speed at which FMMs can be produced in this manner is limited. Such a system is capable of producing FMMs with 10 μm critical size holes and resolution of several hundred dots per inch (dpi).

Disclosure of Invention

Embodiments of the present disclosure are directed to at least partially solving one or more of the problems set forth above and/or other problems.

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

Thus, a method is provided in which a mask defines an ablation pattern to be formed. The ablation pattern may be defined by a plurality of transparent regions in the mask. When compared to the prior art described above, a single laser beam may be used to simultaneously irradiate multiple transparent regions in a mask, thereby facilitating ablation of a corresponding plurality of features in a layer of material to be ablated. This is achieved without the need for complex beam splitting and balancing optics and can be scaled to achieve simultaneous processing of a large number of features. High laser power may be used because the laser power may be spread over many different features of the ablation pattern. The use of high power lasers can increase throughput. The use of ultraviolet light allows high spatial resolution to be achieved at reasonable operating costs. Defining the ablation pattern with a mask (rather than by a single beam spot directly from a laser) allows the ablation pattern to be defined with high accuracy while relaxing the requirements on the laser beam used to illuminate the mask. The laser can simply be "cleaned" on the mask with a relatively low resolution.

In one embodiment, the structure corresponding to the ablation pattern comprises a regular array of holes. The apertures may all be of substantially the same size and shape. The ablation pattern can thus be used to form an FMM.

In one embodiment, ablating the imaged portions of the pattern facilitates forming a plurality of holes in the layer. The plurality of holes corresponding to each imaging portion may include at least 100 holes. The laser pulse energy is thus spread over at least 100 holes without the need for complex beam splitting and balancing. Further, in one embodiment, sequential imaging of different portions of the ablation pattern may facilitate formation of at least 100000 holes in the layer. Therefore, the number of holes formed can be greatly increased only by scanning the laser beam over the mask. The method can be extended to use a single mask to process over 500000 holes, over 750000 holes or even over 100 ten thousand holes.

In one embodiment, each hole is tapered to have a cross-sectional area that decreases in a downstream direction of the laser beam. The directing and scanning steps may then be repeated for a plurality of mask patterns, each mask pattern defining the cross-sectional area of the tapered aperture at a different depth. This method allows the profile of the tapered hole to be controlled efficiently and with high accuracy. Optimizing the taper of the holes in the FMM can improve the performance of OLED manufacturing processes using FMMs by minimizing the blurring of the pattern edges in depositing the OLED molecular pattern. Typically, the tapered holes of the FMM are arranged to face (i.e., face outward) the substrate on which the OLED molecules are to be deposited. Controlling the angle of taper may achieve an optimal balance between providing a high resolution and spatially accurate FMM (which may limit the amount of maximum taper allowed) and minimizing the reorientation of OLED molecular trajectories (which can generally be improved by increasing taper) caused by unwanted interactions (collisions) with the sidewalls of the holes in the FMM.

As described above, this layer may include a metal layer (e.g., to form a FMM). The metal layer may have various compositions according to the purpose of the FMM. The metal layer may, for example, be formed of a material having a very low coefficient of thermal expansion, such as invar. Other materials, including non-metallic materials, may also be used. The layer may, for example, comprise a dielectric material and/or a polymer.

In one embodiment, the structure includes a portion of an evaporation mask for depositing OLED molecules in fabricating an OLED-based display. Thus, a method of depositing OLED molecules may be provided wherein an evaporation mask is formed using the method of performing laser ablation of the present disclosure, and the resulting evaporation mask is used to deposit organic light emitting molecules in a pattern defined by the evaporation mask.

According to another aspect of the present invention, there is provided an apparatus for performing laser ablation, the apparatus comprising: an ultraviolet laser configured to generate an ultrafast pulsed laser beam having a pulse length of less than 20 picoseconds; a mask defining an ablation pattern; an optical system configured to direct a laser beam through the mask to image a portion of the ablation pattern onto the 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 corresponding to the ablation pattern into the layer.

Drawings

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

FIG. 1 is a schematic side view of an apparatus for performing laser ablation;

FIG. 2 is a top view of a mask that may be used in the apparatus of FIG. 1;

FIG. 3 is a top view of a mask showing how a laser beam is scanned over the mask;

FIG. 4 is a top view of the layer shown in FIG. 1 with an ablation pattern ablated into the layer;

FIGS. 5-7 are side cross-sectional views showing different stages of ablation of a tapered hole;

FIG. 8 depicts multiple mask patterns for different scans over the same area of a layer, wherein the mask patterns are disposed on separate masks;

FIG. 9 depicts multiple mask patterns for different scans over the same area of a layer providing mask patterns over different areas of the same mask; and

fig. 10 and 11 are side cross-sectional views illustrating different aperture taper profiles formed by decreasing or increasing laser fluence (fluence).

Detailed Description

Fig. 1 shows an example apparatus 2 for performing laser ablation. The apparatus 2 uses an ultraviolet laser 6, the ultraviolet laser 6 being configured to provide an ultrafast pulsed laser beam 8. The pulse length of the ultrafast pulsed laser beam 8 is less than 20 picoseconds, optionally less than 15 picoseconds, optionally less than 10 picoseconds, optionally less than 8 picoseconds, optionally less than 6 picoseconds, optionally less than 5 picoseconds. A mask 10 is provided that defines an ablation pattern 18 (shown in fig. 2). A layer 4 of material to be processed is disposed on a support 12 (e.g., a substrate). The support 12 may be arranged on a movable table (not shown) for stepping the support 12 to different positions below the laser beam 8. An optical system 13 is provided, which optical system 13 will direct the laser beam 8 through the mask 10 and onto the layer 4. Optical system 13 images a portion of ablation pattern 18 onto layer 4.

Scanning device 14 scans laser beam 8 over mask 10 to sequentially image different portions of ablation pattern 18 defined by mask 10 onto different corresponding areas of layer 4. Thereby ablating structures corresponding to the ablation pattern 18 to the layer 4. The laser beam 8 will typically be scanned over the mask 10 without any corresponding movement of the laser 6 or mask 10 (e.g., by suitable scanning optics).

The use of ultraviolet wavelengths allows structures to be formed in layer 4 at high resolution without the need for complex and/or expensive optics. Typically, resolutions of up to 1000dpi can be formed, including features such as dimples or holes having a critical dimension of about 3 μm or less.

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

In one embodiment, apparatus 2 further includes optics 16 (e.g., including a lens) located downstream of mask 10. The optics 16 may focus the laser radiation 8 from the mask 10 onto the layer 4. In one embodiment, optics 16 provide demagnification between mask 10 and layer 4, so that features formed on layer 4 by ablation are smaller than corresponding features in mask 10. This method allows high resolution patterning of layer 4 while distributing laser energy over a large area on mask 10. Therefore, the laser energy density (fluence) on the mask 10 is lower than it would otherwise be. This allows the use of higher laser pulse energies and higher laser powers, which improves the productivity without risking damage to the mask 10. Furthermore, the mask 10 may be manufactured with a lower resolution than the pattern required in the layer 4, which facilitates the production of the mask 10.

In some embodiments, the ablation-producing structures in layer 4 corresponding to the ablation pattern in mask 10 comprise a regular array of holes in layer 4. The pitch of the array in layer 4 can be made very small, for example 10 microns or less. All of at least a subset of the apertures may have substantially the same size and shape and/or be configured in a manner suitable for forming all or a portion of a FMM used to fabricate an OLED-based display. Fig. 2 is a top view of an exemplary mask 10 for forming such an ablation pattern. In this example, the mask 10 includes transparent regions 20 arranged regularly. Each transparent area 20 corresponds to a respective hole to be formed in the layer 4. Due to the shrinking, the pitch of the transparent areas 20 on the mask 10 is typically larger than the pitch of the corresponding holes in the layer 4. For ease of illustration, the mask 10 of FIG. 2 contains only a relatively small number of transparent regions 20. In practice, more transparent regions 20 (e.g., 100000 or more as described below) are likely to be provided per mask.

In some embodiments, each imaged portion of ablation pattern 18 in mask 10 facilitates the formation of a plurality of holes in layer 4. The plurality of holes may preferably comprise at least 100 holes. For example, each aperture in layer 4 may be formed (partially or completely) by directing laser radiation through a respective transparent region 20 on mask 10, and the imaged portion of ablation pattern 18 may be formed by simultaneously illuminating a plurality of such transparent regions 20 (e.g., 100 or more) on mask 10. By simultaneously irradiating a number of transparent areas 20 in this manner, it is possible to facilitate the simultaneous formation of a number of holes in the layer 4. This allows full use of the available laser pulse energy and improves productivity. When combined with the scanning step, a large number of holes can be formed quickly. Sequential imaging of different portions of the ablation pattern can facilitate formation of at least 100000 holes in the layer, for example, by creating at least 100 holes through each of 1000 or more portions of the ablation pattern.

In the example schematically illustrated 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 region 20 may have other shapes, forming different shaped features or holes in the layer 4. The transparent area 20 may be, for example, rectangular, circular or oval.

Fig. 3 depicts an exemplary scan path 22 of the laser beam spot 9 over the mask 10 during a scanning step (as viewed from 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 that is now imaged onto the layer 4 (the "imaged portion"). Scan path 22 may be described as a raster scan. Other scan paths may be used. The scan path 22 may be adapted to avoid areas in the layer 4 where no structures are required, the scan path 22 may 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 at the mask 10), the ablation pattern 18 in the mask 10 and/or properties of the layer 4.

Fig. 4 is a top view of layer 4 after ablation into layer 4 of structure 24, which corresponds to ablation pattern 18 defined by mask 10 (e.g., a square array of holes). Structure 24 may be formed by a single scan over mask 10 or multiple scans over the mask. For example, in a first scan over mask 10, at least a subset of the features of the structure may be ablated through layer 4 to a portion of its intended depth, thereby providing partially formed features. The repeated scanning process allows multiple exposures to be made to each partially formed feature until each partially formed feature is fully formed (e.g., with holes extending all the way through layer 4). The method may advantageously allow heat to be dissipated between successive ablation processes, thereby helping to prevent unnecessary damage to areas outside the ablation target area. In one embodiment, the laser beam spot 9 is scanned multiple times along a scan path 22 such as discussed above with reference to FIG. 3. During this process, multiple scans may be performed without providing any relative movement between the mask 10 and the layer 4.

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 to be treated. Thus, the method may be repeated to process other desired portions of layer 4. In one embodiment, a step-and-scan process is used to form structure 24, wherein layer 4 is disposed at a first position relative to mask 10. A relative motion is then provided between layer 4 and mask 10 (typically by moving layer 4 and holding mask 10 and optics 16 fixed in position) to bring layer 4 to a second position relative to mask 10, and the scanning process is repeated to form another instance of structure 24 adjacent to the previously formed instance. The process may then be repeated to treat the entire layer 4. The above-described directing and scanning steps may therefore be repeated for a plurality of different positions of layer 4 relative to mask 10 in order to ablate structures corresponding to the ablation pattern at a plurality of different positions on layer 4, thereby creating a structure in layer 4 that is much larger than it would otherwise be without entering layer 4.

In one embodiment, as shown in fig. 5-7, each hole 25 of the structure 24 formed by ablation is tapered to have a cross-sectional area that decreases in the downstream direction of the laser beam 8. In one embodiment, taper is controlled by repeatedly directing the laser beam 8 through the mask 10 and onto the layer 4 and scanning the laser beam 8 over the mask 10 for a plurality of different mask patterns, each mask pattern defining the cross-sectional area of one tapered aperture 25 at different depths. For example, a first mask pattern may be provided with a first plurality of transparent regions 20 corresponding to respective plurality of apertures 25 to be formed in the layer 4, a second mask pattern may be provided with a second plurality of transparent regions 20 corresponding to the same respective plurality of apertures 25, and a third mask pattern may be provided with a third plurality of transparent regions 20 corresponding to the same respective plurality of apertures 25, wherein the transparent regions 20 of the first mask pattern are larger than the transparent regions 20 of the second mask pattern and the transparent regions of the second mask pattern are larger than the transparent regions 20 of the third mask pattern. An exemplary result of the process using the first mask pattern is schematically shown in fig. 5, in which shallow dents having a diameter of 26 are formed. An exemplary result of the process using the second mask pattern is schematically shown in fig. 6, where the dimples have deepened and have a narrower diameter 28. The schematic result of the process using the third mask pattern is schematically shown in fig. 7, where the ablation has passed through the layer 4 and a tapered hole 25 of diameter 30 is formed at the deepest point of the tapered hole 25. The dimensional variations of the transparent regions in the first, second and third mask patterns affect the

diameters

26, 28 and 30 at different points along the taper, thus allowing the taper profile to be controlled with high accuracy.

The method of repeating the directing and scanning steps for a plurality of mask patterns defining different respective ablation patterns is not limited to the following: the structure formed is a regular array of holes, and the different ablation patterns correspond to different depths of the holes. The method may be applied to different or more complex structures. This method is useful as long as the shape of the indentations or holes in the control layer 4 is advantageous as a function of the depth of the indentations or holes. To achieve shape control as a function of depth, it is often necessary to repeat the directing and scanning steps so that different laser ablation patterns are applied to the same or overlapping regions of layer 4. Typically, this will involve repeated steering and scanning without any change in the relative position between the mask 10 and the layer 4, for example so that it is the same part of the layer 4 that is to be processed each time. Multiple mask patterns may be provided on

separate masks

101, 102 and 103 as schematically depicted in fig. 8, or may be provided in

different areas

10A, 10B, 10C on the same mask 10 as schematically depicted in fig. 9.

Other methods of controlling taper may be used in combination with or in lieu of the above. For example, in one class of embodiments, taper is controlled, at least in part, by controlling the fluence of the laser beam 8 onto the mask 10 (pulse fluence — the energy of the laser pulse divided by the area illuminated by the laser pulse). For example, the laser beam 8 may be scanned over the mask 10a plurality of times, wherein at least two scans are performed at different fluxes. The fluence of the laser beam 8 incident on the layer 4 influences the cone angle of the walls of the ablated indentation in this layer. The higher the flux, the smaller the cone angle (more vertical). Lower flux results in a larger (smaller perpendicular) cone angle. Providing the ability to vary the energy density as a function of depth in the layer 4 provides a useful additional degree of freedom for adjusting the internal shape (e.g., tapered profile) of the structures formed in the layer 4.

Based on the foregoing, in one class of embodiments, for each of one or more apertures, the fluence of the laser beam 8 at the mask 10 is varied in forming the aperture. Changing the flux at the mask 10 will result in a corresponding change in the flux at the layer 4. This variation causes the flux at the mask (and hence at layer 4) to be different during the formation of the holes with portions of different depth in layer 4, thereby controlling the cone angle of the holes as a function of depth in layer 4.

In one example process, a first scan on mask 10 is performed with laser beam 8 providing a first flux at mask 10. The scan may, for example, follow a scan path 22 such as described above with reference to fig. 3. The fluence of the laser beam 8 may be such that, after a first scan of the mask 10, the structures formed in the layer 4 extend only partially through the layer 4 (similar to the situation in fig. 5). A second scan is then made over the mask 10 with the laser beam 8 providing a second fluence which is lower than the first fluence. The result of this scan is that the structures formed in the first scan are deepened. However, since the flux of the laser beam 8 is low in the second scan, the cone angle also increases. A third scan is then performed over the mask 10 with the laser beam 8 providing a third fluence which is lower than the second fluence. The result of this scan is that the structure formed in the second scan is deepened until ablation penetrates to the other side of the layer 4. The lower fluence in the third scan of the laser beam 8 means that the cone angle is further increased at the newly reached depth. Figure 10 shows the profile of the hole produced by this method. Thus, the method provides an alternative or additional way to control the shape of the taper in the hole. Although this embodiment has been illustrated with reference to three scans, any number of scans may be used. Furthermore, the flux does not necessarily need to be adjusted in the above-described manner, but may be changed in any suitable manner. If the flux is gradually increased in successive scans, an aperture profile of the type shown in figure 11 will be created. Further, the flux of the laser beam 8 does not have to be different in each scan as long as there is a difference in at least two scans. The flux may be increased or decreased between different scans.

Claims

1. A method of performing laser ablation, the method 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 areas of the layer to ablate a structure corresponding to the ablation pattern into the layer,

wherein the laser beam comprises an ultrafast pulsed laser beam having a pulse length of less than 20 picoseconds.

2. The method of claim 1, wherein the structure corresponding to the ablation pattern comprises a regular array of holes.

3. The method of claim 2, wherein at least a subset of all of the holes have substantially the same size and shape.

4. A method according to claim 2 or 3, wherein each of the holes is tapered so as to have a cross-sectional area that decreases in a downstream direction of the laser beam.

5. A method according to claim 4 wherein, for each of the one or more holes, the fluence of the laser beam at the mask is varied during formation of the hole, the variation being such that the fluence is different in the formation of parts of the hole at different depths in the layer of material, whereby the variation in the cone angle of the hole is controlled in dependence on the depth in the layer of material.

6. The method of claim 4 or 5, wherein the directing and scanning steps are repeated for a plurality of mask patterns, each mask pattern defining the cross-sectional area of a tapered aperture at a different depth.

7. The method of any of claims 2-6, wherein each imaged portion of the ablation pattern facilitates forming a plurality of holes in the layer.

8. The method of claim 7, wherein the plurality of apertures corresponding to each imaging portion comprises at least 100 apertures.

9. The method of claim 8, wherein sequential imaging of different portions of the ablation pattern facilitates formation of at least 100000 holes in the layer.

10. The method of any of claims 2-9, wherein the pitch of the array is less than 10 microns.

11. A method according to any preceding claim, wherein the directing and scanning steps are repeated for a plurality of mask patterns defining different respective ablation patterns.

12. The method of claim 11, wherein repeating the directing and scanning steps applies the different laser ablation patterns to the same or overlapping regions of the layer.

13. The method of claim 11 or 12, wherein the plurality of mask patterns are provided on a single mask.

14. The method of claim 11 or 12, wherein the plurality of mask patterns are provided on different areas of the same mask.

15. The method of any preceding claim, wherein the layer comprises a metal layer.

16. A method according to any preceding claim, wherein the directing and scanning steps are repeated for a plurality of different positions of the layer relative to the mask, thereby ablating structure corresponding to the ablation pattern at a plurality of different positions on the layer.

17. A method according to any preceding claim, wherein the structure comprises a portion of an evaporation mask for depositing organic light emitting molecules in the manufacture of an organic light emitting molecule based display.

18. A method of depositing an organic light-emitting molecule, comprising:

forming an evaporation mask by performing the method of claim 16; and

the evaporation mask is used to deposit organic light-emitting molecules in a pattern defined by the evaporation mask.

19. An apparatus for performing laser ablation, the apparatus comprising:

an ultraviolet laser configured to generate an ultrafast pulsed laser beam having a pulse length of less than 20 picoseconds;

a mask defining 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 layer of material; 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 to ablate structures corresponding to the ablation pattern into the layer.