Efficiency of resist patterning

Abstract

A direct imaging system includes an ultraviolet radiation source configured to irradiate a first region having a first width of a layer of photoreactive material on a substrate with patterned ultraviolet radiation. A thermal energy source is configured to heat a second region of the photoreactive material layer when the ultraviolet radiation source irradiates the first region. The second region contains a portion of the first region and has a second width less than the first width.

Description

Technical Field

[0001] The present invention generally relates to the manufacture of electronic devices, and more particularly to methods and systems for optical patterning of photoactive layers. Background Art

[0002] Photoactive resists are widely used in the manufacture of printed circuit boards. For example, to create a solder mask pattern (also known as a solder mask) on a printed circuit board, the board is coated with an unpolymerized solder mask layer. A pattern of ultraviolet radiation is applied to polymerize the solder mask, after which the unpolymerized portions of the resist are washed away to expose underlying conductive pads on the substrate. Photoresists are patterned in a similar manner for creating circuit traces. The term "resist" generally refers in this specification and in the claims to photoactive materials used for patterning circuit boards, including solder masks and photoresists.

[0003] Most commonly, the pattern is applied by irradiating the photoactive resist with ultraviolet radiation through a photolithographic mask. On the other hand, in a direct imaging system, the patterned ultraviolet radiation is dynamically generated, for example using a scanned laser beam, an addressable array of radiation sources, or an addressable spatial light modulator, such as a liquid crystal device or a digital micromirror device (DMD).

[0004] U.S. Patent No. 11,464,116 describes a lithographic exposure system and method for exposing and structuring a substrate coated with a solder mask. The lithographic exposure system has at least one light beam, which in embodiments is formed by two or more laser beams of different UV wavelengths, and the light beam is deflected relative to the substrate by a variable deflection device in order to create a structure on the substrate. Specifically, the light beams are spatially and temporally superimposed in the image plane by a spatially restricted high energy (in embodiments, an externally mounted heat source). In embodiments, an infrared laser diode with linear optics is used. Summary of the Invention

[0005] Embodiments of the present invention described below provide improved methods and systems for patterning resists.

[0006] Thus, according to an embodiment of the present invention, there is provided a direct imaging system including an ultraviolet radiation source configured to irradiate a first region having a first width of a layer of photoactive material on a substrate with patterned ultraviolet radiation. A heat energy source is configured to heat a second region of the photoactive material layer while the ultraviolet radiation source irradiates the first region, the second region containing a portion of the first region and having a second width smaller than the first width. A motion assembly is configured to scan at least the second region across the photoactive material layer.

[0007] In some embodiments, the first region is rectangular, having a given height perpendicular to the first width, and the second region extends across the height of the first region, thereby defining an elongated region heated by a heat source. In the disclosed embodiments, the motion assembly is configured to scan at least the second region across the photoreactive material layer in a direction perpendicular to the height of the first region. In one embodiment, the motion assembly is configured to translate the substrate in a direction perpendicular to the height, and the ultraviolet radiation source is configured to modify the pattern of ultraviolet radiation as the substrate is translated. Additionally or alternatively, the heat source is configured to direct a radiation beam to impinge on the second region, wherein the beam is shaped to conform to the elongated region.

[0008] In one embodiment, the ultraviolet radiation source includes an array of light-emitting diodes. Alternatively or additionally, the ultraviolet radiation source includes a spatial light modulator that applies a predefined pattern to the ultraviolet radiation.

[0009] In some embodiments, the heat source includes an infrared radiation source and projection optics configured to project infrared radiation above the second region. In one embodiment, the infrared radiation source includes one or more infrared lasers.

[0010] Alternatively, the heat source includes a convection heater or a conduction heating element configured to direct a jet of heated gas to impinge on the second region, the conduction heating element configured to contact the photoreactive material across the second region and transfer heat to the photoreactive material.

[0011] In the disclosed embodiments, the photoreactive material includes a solder mask.

[0012] In some embodiments, the ultraviolet radiation is configured to irradiate a plurality of first regions spaced apart on the substrate and staggered with respect to the scanning direction of the motion assembly, and the heat source is configured to heat a plurality of second regions that contain portions of the corresponding ones of the first regions.

[0013] According to an embodiment of the present invention, there is also provided a method for direct imaging, the method including irradiating a first region having a first width of a layer of photoreactive material on a substrate with patterned ultraviolet radiation. Applying heat to a second region of the photoreactive material layer while irradiating the first region with the patterned ultraviolet radiation, the second region containing a portion of the first region and having a second width less than the first width. Scanning at least the second region across the photoreactive material layer.

[0014] The present invention will be more fully understood from the following detailed description of embodiments of the invention in conjunction with the drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1is a schematic side view of a direct imaging system for patterning a printed circuit board (PCB) according to an embodiment of the present invention;

[0016] Figure 2 is a schematic detail view of an optical patterning assembly according to an embodiment of the present invention; and

[0017] Figure 3 is a schematic front view of a printed circuit substrate according to an embodiment of the present invention showing a radiation pattern projected onto a substrate during a direct imaging process. DETAILED DESCRIPTION

[0018] Proper polymerization of a resist layer on a printed circuit board typically requires at least a certain minimum flux of ultraviolet energy on the surface of the layer. "Flux" is the energy received per unit area of the layer and thus depends on the incident intensity of the ultraviolet radiation and the duration of the exposure. In a direct imaging system, the available intensity is limited by the capabilities of the ultraviolet radiation source and its associated patterning and imaging optics. Therefore, in order to achieve the desired flux, it becomes necessary to increase the exposure time, thereby limiting the throughput of the system.

[0019] Embodiments of the present invention described herein solve this problem by applying thermal energy to heat the areas of the resist that are undergoing ultraviolet irradiation. Increasing the temperature of the resist increases the polymerization rate, thereby reducing the required ultraviolet flux, reducing the required exposure time, and increasing the throughput of the direct imaging system. The thermal energy is applied instantaneously and locally, i.e., only to a small portion of the area that is undergoing patterned ultraviolet irradiation at any given time. The inventors have found that such instantaneous local heating is beneficial in reducing the thermal expansion of the printed circuit substrate, which otherwise could cause distortion of the printed pattern and loss of accuracy.

[0020] Accordingly, the disclosed embodiments provide a direct imaging system that includes an ultraviolet radiation source and a thermal energy source. The ultraviolet radiation source applies patterned ultraviolet radiation to irradiate a patterned area of a resist layer on a substrate having a specific height and width. (In this disclosure and the claims, for convenience, the terms "height" and "width" are arbitrarily used to denote the dimensions of an area on the surface of the substrate, without implying any particular physical orientation of these dimensions.) While the ultraviolet radiation source irradiates the entire patterned area, the thermal energy source simultaneously heats the resist layer only within a portion of the patterned area such that, for example, at least 80% of the thermal energy is applied to a heated area that does not exceed 20% of the patterned area. For example, the thermal energy source can be radiative, convective, or conductive.

[0021] In the embodiments described below, the width of the heated region is less than the width of the patterned region. In these embodiments, the heated region is an elongated region that extends across the height of the rectangular patterned region but has a much narrower width. The motion assembly scans at least the heated region across the resist layer and may also scan the patterned region, typically in a scan direction perpendicular to the height of the heated region. For example, the motion assembly may translate the substrate in a direction perpendicular to the height. Alternatively or additionally, the motion assembly may offset the heated region and possibly offset the patterned region across the substrate.

[0022] Figure 1 FIG. 4 is a schematic side view of a direct imaging system 20 for patterning a printed circuit board (PCB) substrate 22 according to an embodiment of the present invention. The system 20 includes one or more ultraviolet radiation sources 24 that generate patterned ultraviolet radiation corresponding to a pattern of a conductive material, such as a solder mask, to be formed on the substrate 22. Prior to irradiation, the substrate 22 is coated with a layer of a suitable resist 30. The optics 26 focus the patterned radiation generated by each radiation source 24 to irradiate a corresponding patterned region 28 of the resist layer.

[0023] While the ultraviolet irradiation is being performed by the source 24, one or more radiation sources 32 generate thermal energy. In this example, it is assumed that the radiation sources 32 emit near-infrared radiation; alternatively, however, the radiation sources 32 may emit electromagnetic radiation in any suitable wavelength band in the range from visible light to far-infrared (at wavelengths between 500 nm and 12 μm). The projection optics 34 shape and project the infrared radiation to produce an elongated infrared beam 36 that heats a corresponding elongated region of the resist 30 within the patterned region 28. The radiation sources 32 may be coupled to the projection optics 34 by suitable high-power optical fibers. As Figure 3 illustrated, the region heated by each beam 36 contains a portion of the corresponding patterned region 28 that has a width less than the width of the patterned region.

[0024] The motion assembly 37 translates the substrate 22 in the process direction indicated by the arrow 38. (In the illustrated example, the process direction is considered the X direction, and the widths of the patterned region and the heated region are considered the respective dimensions of the regions in the X direction, while the height is the dimension in the Y direction.) The translation of the substrate 22 causes the patterned region 28 and the region heated by the beam 36 to scan across the resist layer 30 in the X direction perpendicular to the height of the elongated region heated by the infrared beam 36. As the substrate 22 is translated, the ultraviolet radiation source 24 modifies the pattern of the ultraviolet radiation output therefrom in order to produce the desired exposure pattern on the resist 30.

[0025] In Figure 1In the illustrated example, system 20 includes two ultraviolet radiation sources 24 and two thermal radiation (e.g., IR) sources 32, which have corresponding optics, all of which operate in parallel. This arrangement is beneficial for increasing the throughput of system 20 because it enables the system to irradiate multiple regions of the substrate simultaneously. Alternatively, system 20 may include a large number of ultraviolet and infrared radiation sources, or only a single source of each type. In one embodiment, the plurality of patterned regions 28 are staggered to cover a large area or even the entire area of substrate 22 in a single pass, such as as Figure 3 illustrated.

[0026] In some embodiments, radiation source 32 includes a high-power semiconductor device, such as a semiconductor laser, such as a laser diode, vertical cavity surface emitting laser (VCSEL), or light emitting diode (LED). These infrared sources 32 may operate at any suitable wavelength in the near-infrared range. The wavelength may be optimized for the type of resist 30 that has been applied to substrate 22, for example, using a longer wavelength for a white solder mask, which tends to scatter light at wavelengths close to the visible range. As another example, each radiation source 32 may include a linear filament with a suitable reflector (in this case, additional projection optics 34 may not be required).

[0027] Alternatively, in addition to or instead of infrared source 32, system 20 may include other types of heat sources. For example, the heat source may include a convection heater that directs a long, narrow jet of heated air or other heated gas against an elongated region to be heated. As another example, the heat source may include a conduction heating element, such as a heating rod, that contacts resist 30 or substrate 22 across the region to be heated.

[0028] Figure 2 is a schematic detail view of an optical patterning assembly including an ultraviolet radiation source 24 and optics 26 according to an embodiment of the present invention. In the illustrated embodiment, an ultraviolet emitter 40 (such as a discharge lamp, laser, or solid-state device) emits a beam of ultraviolet radiation. Collimating optics 42 collimate the ultraviolet beam and direct it towards a spatial light modulator 44. In this example, spatial light modulator 44 includes a digital micromirror device (DMD) that includes an addressable array of micromirrors 46 that are controlled to apply a predefined pattern to the ultraviolet radiation for projection onto patterned region 28. Alternatively, other types of spatial light modulators, such as a liquid crystal pixel array, may be used.

[0029] In an alternative embodiment, ultraviolet radiation source 24 includes an addressable array of LEDs that emit ultraviolet light, which are turned on and off to produce the desired pattern. In this case, spatial light modulator 44 may not be required.

[0030] Figure 3 is a schematic front view of a PCB substrate 22 showing the pattern of radiation projected onto the substrate during a direct imaging process in system 20 according to an embodiment of the present invention. The optical patterning assembly projects a plurality of patterns of ultraviolet radiation onto corresponding patterned regions 28, each pattern including an array of pixels 52. A thermal energy source heats an elongate region 54 that extends across the height (Y dimension) of each patterned region 28 but is significantly narrower in width (X dimension) than the patterned region. Although Figure 3 two examples of regions 28 and 54 are shown, three or more regions of each type may be irradiated in parallel to increase the throughput of system 20. Alternatively, system 20 may operate on a single patterned region with a single heated region.

[0031] Translation of the substrate 22 in the process direction indicated by arrow 38 causes regions 28 and 54 to scan across the resist 30. During the scan, the radiation source 24 modifies the values of the pixels 52 to create the desired pattern over the entire surface of the PCB substrate 22. The array of pixels 52 is tilted with respect to the process direction (indicated by arrow 38) to enable patterning of features with a resolution finer than the pitch of the pixels 52. The regions 28 are staggered to cover a wide area in the Y direction (and possibly the entire substrate in a single scan) in a single scan of the substrate 22. As Figure 3 shown, staggering the regions 28 along the process direction helps to alleviate the spatial constraints on the optics 26 and helps to spread out the heated regions 54 to avoid local overheating. Although the heated region 54 is Figure 3 shown as centered in the X direction within the patterned region 28, the heated region 54 may alternatively be shifted from the center. For example, the heated region 54 may be shifted towards the leading edge of the patterned region 28.

[0032] A controller may be used to control the motion assembly 37, radiation source 32, ultraviolet radiation source 24, and / or other components in system 20. The controller generally includes: a programmable processor programmed in software and / or firmware to perform the functions described herein, and suitable digital and / or analog interfaces for connecting to the other elements of system 20. Alternatively or additionally, the controller includes hardwired and / or programmable hardware logic circuitry that performs at least some of the functions of the controller. The controller may include a single control unit or multiple interconnected control units, having suitable interfaces for receiving and outputting the signals illustrated in the figures and described in the text. The program code or instructions for the controller to implement the various methods and functions disclosed herein may be stored in a readable storage medium (such as a memory in the controller or other memory).

[0033] Although the foregoing describes specific embodiments, for purposes of concreteness and clarity, in the context of the particular content of system 20, as Figures 1 to 3 illustrated in, the principles of the present invention can similarly be applied, with necessary modifications, to the patterning of resists in other kinds of direct imaging systems. Accordingly, the embodiments described above are cited by way of example, and the present invention is not limited to what has been specifically shown and described above. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described above, as well as variations and modifications of the present invention that will occur to those of ordinary skill in the art upon reading the foregoing description and that are not disclosed in the prior art.

Claims

1. A direct imaging system, comprising: a source of ultraviolet radiation configured to irradiate a first region having a first width of a layer of a photo-responsive material on a substrate with a pattern of ultraviolet radiation; a heat source configured to heat a second region of the photo-responsive material layer while the ultraviolet radiation source irradiates the first region, the second region containing a portion of the first region and having a second width less than the first width; and a motion assembly configured to scan at least the second region across the photo-responsive material layer.

2. The system according to claim 1, wherein the first region is rectangular, having a height perpendicular to the first width, and wherein the second region extends across the height of the first region, thereby defining an elongated region heated by the heat source.

3. The system according to claim 2, wherein the motion assembly is configured to scan at least the second region across the photo-responsive material layer in a direction perpendicular to the height of the first region.

4. The system according to claim 3, wherein the motion assembly is configured to translate the substrate in a direction perpendicular to the height, and wherein the ultraviolet radiation source is configured to modify the pattern of ultraviolet radiation while the substrate is being translated.

5. The system according to claim 2, wherein the heat source is configured to direct a radiation beam to impinge on the second region, the beam being shaped to conform to the elongated region.

6. The system according to claim 1, wherein the ultraviolet radiation source comprises an array of light-emitting diodes.

7. The system according to claim 1, wherein the ultraviolet radiation source comprises a spatial light modulator that applies a predefined pattern of ultraviolet radiation.

8. The system according to claim 1, wherein the heat source comprises an infrared radiation source and projection optics configured to project infrared radiation onto the second region.

9. The system according to claim 8, wherein the infrared radiation source comprises one or more infrared lasers.

10. The system according to claim 1, wherein the heat source comprises a convection heater configured to direct a jet of heated gas to impinge on the second region.

11. The system according to claim 1, wherein the heat source comprises a conduction heating element configured to contact the photo-responsive material across the second region and transfer heat to the photo-responsive material.

12. The system according to claim 1, wherein the photo-responsive material comprises a solder mask.

13. The system according to claim 1, wherein the ultraviolet radiation is configured to irradiate a plurality of first regions spaced apart and staggered relative to the scanning direction of the motion assembly on the substrate, and wherein the heat source is configured to heat a plurality of second regions, the plurality of second regions containing portions of the respective ones of the plurality of first regions.

14. A method for direct imaging, comprising: Irradiate a first region having a first width of a layer of a light-responsive material on a substrate with a pattern of ultraviolet radiation; Apply heat to a second region of the light-responsive material layer while irradiating the first region with the pattern of ultraviolet radiation, the second region containing a portion of the first region and having a second width less than the first width; And Scan at least the second region across the light-responsive material layer.

15. The method according to claim 14, wherein the first region is rectangular, having a height perpendicular to the first width, and wherein the second region extends across the height of the first region, thereby defining an elongated region heated by a heat source.

16. The method according to claim 15, wherein scanning at least the second region includes offsetting the first region and the second region across the light-responsive material layer in a direction perpendicular to the height of the first region.

17. The method according to claim 16, wherein offsetting the first region and the second region includes translating the substrate in the direction perpendicular to the height, and wherein irradiating the first region includes modifying the pattern of ultraviolet radiation while the substrate is being translated.

18. The method according to claim 15, wherein applying the heat includes directing a radiation beam to impinge on the second region, the beam being shaped to conform to the elongated region.

19. The method according to claim 14, wherein irradiating the first region includes operating an array of ultraviolet light-emitting diodes to irradiate the first region, or operating a spatial light modulator to apply a predefined pattern of ultraviolet radiation.

20. The method according to claim 14, wherein applying the heat includes projecting infrared radiation onto the second region using one or more infrared lasers, directing a jet of heated gas to impinge on the second region, or bringing a conduction heating element into contact with the second region.

21. The method according to claim 14, wherein the light-responsive material includes a solder resist.

22. The method according to claim 14, wherein irradiating the first region includes irradiating a plurality of first regions with the pattern of ultraviolet radiation, the plurality of first regions being spaced apart and staggered relative to the scanning direction on the substrate, and wherein applying the heat includes heating a plurality of second regions, the plurality of second regions containing portions of the respective ones of the plurality of first regions.

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