Laser machining method and laser machining device

Abstract

To provide a laser machining method capable of forming a through-hole having no fusion residual on a low dielectric loss tangent ceramic substrate through only a laser machining process using a carbon dioxide gas laser.SOLUTION: This laser machining method is for forming a through-hole by using a laser machining device provided with an oscillator that oscillates a carbon dioxide gas laser pulse, a modulator that further modulates the laser pulse oscillated from the oscillator into a machining laser pulse of prescribed pulse energy, a condenser lens that condenses the machining laser pulse on a workpiece, a table on which the workpiece is placed, a beam diameter adjustment optical system for adjusting a beam diameter at a machining point of the machining laser pulse, and a general control unit that controls an operation of the device as a whole. The workpiece is a low-temperature sintered glass ceramic substrate, and the pulse energy of the machining laser pulse is 6.7 mJ or greater.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a laser processing method and a laser processing apparatus for forming a through hole in a ceramic substrate by using a carbon dioxide gas laser. [Background technology]

[0002] Conventionally, the materials used for substrates (interposers) for semiconductor packages and the core layers of printed wiring boards have mainly been organic materials, and the formation of through holes for forming through electrodes in these has mainly been carried out using laser processing equipment from the standpoint of production efficiency.

[0003] In recent years, in order to meet the next generation communication speeds of 5G and 6G, the use of materials with low dielectric loss tangents as substrates and other materials to improve frequency characteristics has been considered, and fired low dielectric loss tangent ceramics are one such candidate. However, it was known that when through holes are formed in fired ceramics using a laser processing device, molten residue adheres to the inner walls. If molten residue adheres to the inner walls of the through holes, it becomes difficult to form plating to make the through holes conductive. The generation of molten residue when a through hole is formed using laser processing is a phenomenon that occurs in all fired ceramic substrates.

[0004] Therefore, as described in Patent Document 1, for example, a method has been proposed in which an alumina ceramic substrate, which is generally known as a ceramic with a low dielectric tangent, is laser-processed in a chamber with an inert atmosphere containing hydrochloric acid gas, and then the molten residue on the inner wall of the through hole is removed using a remover such as a sodium hydroxide solution. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2013-180938 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the method described in Patent Document 1, a separate process is required to remove the molten residue on the inner wall after the laser drilling process, which reduces production efficiency, so a method of forming through holes without molten residue by only the laser processing process was desired. It is becoming clear that if a laser with a UV wavelength range is used, through holes without molten residue on the inner wall can be formed in a ceramic substrate by only the laser processing process, but the through hole formation speed is slow with a UV laser, resulting in poor productivity. In contrast, if a carbon dioxide gas laser can be used, the through hole formation speed can be fast, and high productivity can be expected.

[0007] In order to solve the above problems, the present invention aims to find processing conditions that allow for the formation of a through hole without molten residue on its inner wall by only a laser processing process using a carbon dioxide laser, which was previously thought to be impossible, and a type of ceramic with a low dielectric tangent that can be processed. [Means for solving the problem]

[0008] In order to solve the above problems, the inventors conducted various studies and experiments, and as a result, they discovered that a low-temperature co-fired ceramic substrate, which is a type of low dielectric tangent ceramic, can be processed without producing molten residue on the inner wall of a through hole by processing it with a predetermined pulse energy, thereby completing the present invention.

[0009] That is, in order to solve the above-mentioned problems, the present invention provides a laser processing method for forming a through hole using a laser processing apparatus including an oscillator that oscillates a carbon dioxide laser pulse, a modulator that further modulates the laser pulse oscillated from the oscillator into a processing laser pulse having a predetermined pulse energy, a focusing lens that focuses the processing laser pulse on a workpiece, a table on which the workpiece is placed, a beam diameter adjusting optical system for adjusting the beam diameter of the processing laser pulse at the processing point, and an overall control unit that controls the operation of the entire apparatus, characterized in that the workpiece is a low-temperature fired glass ceramic substrate, and the pulse energy of the processing laser pulse is 6.7 mJ or more. Effect of the Invention

[0010] According to the present invention, through holes without any molten residue on the inner walls can be formed in a low-temperature co-fired glass ceramic substrate, which is a type of ceramic substrate having a low dielectric tangent, by only a laser processing step using a carbon dioxide gas laser. [Brief description of the drawings]

[0011] [Figure 1] 1 is a schematic diagram of an overall configuration of a laser processing apparatus suitable for implementing the present invention; [Diagram 2] 1 is a photograph showing a cross section of a through hole machined in an embodiment; [Diagram 3] 1 is a photograph showing a cross section of a through hole machined in an embodiment; [Figure 4] 1 is a photograph showing a cross section of a through hole machined in an embodiment; [Diagram 5] 1 is a photograph showing a cross section of a through hole machined in a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the laser processing method according to the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of the overall configuration of a laser processing apparatus suitable for carrying out the present invention. In the drawing, each component and connection line is mainly shown as being necessary for explaining this embodiment, and does not show all components necessary for the laser processing apparatus. The laser processing apparatus includes a laser oscillator 1 that oscillates a carbon dioxide gas laser pulse with a wavelength of 9.4 μm, an acousto-optic modulator (AOM) 2 that modulates the laser pulse oscillated from the laser oscillator 1 in accordance with processing conditions and branches a laser with a predetermined pulse width and a predetermined pulse energy as a processing laser pulse to an optical axis for processing, a galvano deflection unit 3 that deflects the processing laser pulse branched by the AOM 2 in the direction of a processing position on a low-temperature fired ceramic substrate 5 (hereinafter, also simply referred to as "substrate 5"), which is a workpiece, a focusing lens (fθ lens) 4 that focuses the processing laser pulse deflected by the galvano deflection unit 3 on the substrate 5, and a damper 7 that absorbs a non-processing laser that is not branched by the AOM 2. Furthermore, a moving table 6 is provided on which the substrate 5 is fixed and which is movable in the X and Y directions.

[0013] Although not shown, a dust collection mechanism is provided between the substrate 5 and the moving table 6. The dust collection mechanism is composed of a top plate having a hole (discharge hole) with a diameter larger than the processing diameter at the same position as the target processing position of the substrate 5, and a dust collection base that holds the top plate and has a dust collection space inside. When the recess formed by the laser irradiation penetrates the substrate 5, the molten material scatters upward and is discharged from the through hole by scattering and dropping into the dust collection space through the discharge hole in the top plate.

[0014] Although not shown, a duct is provided between the condenser lens 4 and the substrate 5 to suck in molten material generated by irradiating the substrate 5 with the laser. Also, although not shown, a beam diameter adjustment optical system such as an aperture and a lens for adjusting the beam diameter (beam waist) at the processing point is provided between the laser oscillator 1 and the AOM 2.

[0015] The low-temperature co-fired ceramic substrate 5, which is the workpiece, is also called LTCC (Low Temperature Co-fired Ceramics), and is made by firing a green sheet in which a low-melting-point glass material is added to a ceramic material such as alumina. The sintering temperature is about 1000°C or less, and it is fired at a lower temperature than alumina ceramic, which is generally known as a type of low dielectric tangent ceramic (the sintering temperature of alumina ceramic is about 1500°C). In addition, since the low-temperature co-fired ceramic substrate 5 is added with a low-melting-point glass material, it has a lower melting point than generally known alumina ceramic.

[0016] Returning to Fig. 1, the laser processing apparatus further includes a laser oscillation control unit 11 that controls the oscillation of laser pulses by the laser oscillator 1, an AOM control unit 12 that controls the operation of the AOM 2, a galvanometer control unit 13 that controls the operation of the galvanometer deflection unit 3, and an overall control unit 14 that controls the operation of these control units and the entire apparatus. The overall control unit 14 is realized by, for example, a program-controlled processing device, and has a storage unit that stores various information therein. The overall control unit 14 has control functions other than those described here, and is also connected to blocks not shown.

[0017] The processing method according to the present invention is performed, for example, by using the above-mentioned laser processing device as follows. A processing program instructing a processing position is input to the overall control unit 14 in advance, and processing conditions such as the pulse width and pulse energy of the processing laser pulse are further input. The beam diameter adjustment optical system is adjusted so that the beam diameter at the processing point has a desired value, and the low-temperature fired glass ceramic substrate 5 is placed on the moving table 6. The processing pulse energy is set to a value of 6.7 mJ or more. Then, when the overall control unit 14 is instructed to start processing, the overall control unit 14 controls each unit to operate the processing device as follows. First, the galvano deflection unit 3 is operated so that the laser optical axis is positioned at the processing position instructed by the processing program. Then, a laser pulse is oscillated from the laser oscillator 1, and the laser pulse is incident on the AOM2. The AOM2 modulates the incident laser pulse so that the pulse width and pulse energy are input as processing conditions, and branches it toward the galvano deflection unit 3 as a processing laser pulse. In this manner, the processing laser pulse incident on the galvano deflection unit 3 is deflected and enters the processing position of the substrate 5 via the condenser lens 4 .

[0018] When the laser is incident on the processing position of the substrate 5, the surface of the corresponding position is instantly melted, scattered, and evaporated by the heat of the laser irradiation, forming a recess. Each time the processing laser pulse is repeatedly irradiated, the recess becomes deeper, and when the recess reaches the back surface of the substrate 5, a through hole is formed. EXAMPLES

[0019] A specific embodiment of the present invention will be described below. The inventor performed processing to form a through hole in a low-temperature co-fired ceramic substrate using the above-mentioned laser processing device under conditions in which the pulse energy of the processing laser pulse was changed in various ways. Specifically, the pulse energy of the processing laser pulse was changed in increments of 10% (0.95 mJ) within a range of ±40% (5.7 mJ to 13.3 mJ) based on 9.5 mJ. The low-temperature co-fired ceramic substrate to be processed had a thickness of 0.4 mm. The pulse width of the processing laser pulse branched by the AOM2 was set to 20 μs, and the beam diameter (beam waist) at the processing point was set to 120 μm. Furthermore, in each embodiment, the laser was irradiated at each of a plurality of predetermined processing positions with one pulse each, and then the laser was irradiated at each of all processing positions with one pulse each, repeating the process until the substrate was penetrated, which is called cycle processing. The irradiation interval of the pulse laser at the same processing position in each embodiment is about 3 seconds. The presence or absence of molten residue on the inner wall of the through hole machined in each Example was examined, and the results are summarized in Table 1. Furthermore, each energy density is also shown in Table 1. Figures 2 to 4 are photographs substituted for drawings showing the cross sections of the through holes machined in each Example.

[0020] [Table 1]

[0021] As can be seen from Table 1, when the pulse energy was 5.7 mJ (Example 1), molten residue 51 was observed on the inner wall of the through hole, as shown in Fig. 2(A). On the other hand, when the pulse energy was 6.7 mJ or more (Examples 2 to 9), no molten residue was observed on the inner wall of any of the through holes, as shown in Figs. 2(B) to 4(C).

[0022] This is thought to be because, when the pulse energy was 6.7 mJ or more, the surface of the laser irradiated area instantly became hot and melted, and the viscosity was low and the material scattered vigorously, so no molten material remained on the inner wall. On the other hand, when the pulse energy was 5.7 mJ, the temperature of the irradiated area instantly rose but did not become very hot, and although it melted, the viscosity was high, so it was unable to scatter vigorously and remained on the inner wall.

[0023] In addition, when the surface of the recess during the formation of the through hole (a state in which a recess is formed at the processing point but does not penetrate to the back surface) was observed, a molten residue was observed on the surface of the recess when the pulse energy was 5.7 mJ (Example 1), but no molten residue was observed when the pulse energy was 6.7 mJ or more (Examples 2 to 9). Therefore, it can be said that when the pulse energy is 6.7 mJ or more, a recess without molten residue can be formed by laser pulse irradiation. Therefore, although a substrate with a thickness of 0.4 mm was processed in each example, when the pulse energy is 6.7 mJ or more, a through hole without molten residue on the inner wall can be formed even in a substrate thicker than this. In other words, it can be said that when the pulse energy is 6.7 mJ or more, a through hole without molten residue on the inner wall can be formed not only in a substrate thinner than 0.4 mm, but also in a substrate thicker than this. Therefore, for example, even for a plate thickness of about 50 μm (0.05 mm) to 1500 μm (1.5 mm), which is generally expected for a substrate (interposer) for a semiconductor package, by setting the pulse energy to 6.7 mJ or more, it is possible to process a through hole without any adhesion of molten residue.

[0024] In addition, although the pulse width is set to 20 μs in all the examples, the length of the pulse width is a factor that affects the depth of the recess formed by one irradiation of the pulse laser, and does not affect the adhesion of the molten residue to the inner wall, so the pulse width is not limited to 20 μs. However, if the pulse width is shortened, a large number of shots are required to penetrate the substrate, so the processing time is extended and the production efficiency is reduced. On the other hand, if the pulse width is lengthened, the number of shots to penetrate is reduced, but the amount of molten debris generated by one shot increases, making it necessary to frequently clean the dust collection mechanism, etc., resulting in a decrease in production efficiency. Therefore, for example, if the pulse width is set in the range of 5 μs to 30 μs, processing can be performed with good production efficiency.

[0025] From the above examples, it is evident that by processing a low-temperature co-fired glass ceramic substrate with a pulse energy of 6.7 mJ or more, a through hole without molten residue on the inner wall can be formed using a carbon dioxide pulse laser.

[0026] [Comparative Example] As comparative examples, an alumina ceramic substrate, which is widely known as a type of ceramic substrate with a low dielectric tangent, was processed by changing the pulse energy to 9.5 mJ (Comparative Example 1) and 13.3 mJ (Comparative Example 2). In both comparative examples, the pulse width was 20 μs, and the beam diameter at the processing point was 120 μm. The pulse energy density in Comparative Example 1 was 840.4 mJ / mm 2 In the case of Comparative Example 2, the pulse energy was 1176.6 mJ / mm 2 Fig. 5 is a photograph showing the cross section of each of the through holes machined in Comparative Examples 1 and 2. As shown in the figure, in both Comparative Examples, molten residue was attached to the inner wall.

[0027] As described above, according to the present invention, it is possible to form a through hole, which does not have any molten residue adhering to the inner wall, in a low-temperature co-fired ceramic substrate, which is a type of low dielectric tangent ceramic, by only a laser processing process using a carbon dioxide laser.

[0028] Although the above description has been given of machining a through hole for forming a through electrode in a substrate, the present invention is not limited to this, and can also be used for forming a through hole for cutting a substrate. [Explanation of symbols]

[0029] 1. Laser oscillator (carbon dioxide laser pulse oscillator), 2. Acousto-optical modulator (AOM), 3. Galvano deflection section, 4. Focusing lens (fθ lens), 5. Workpiece (low-temperature fired glass ceramic substrate), 51. Molten residue, 6. Moving table, 7. Damper, 11. Laser oscillation control section, 12. AOM control section, 13. Galvano control section, 14. Overall control section, 151. Molten residue

Claims

1. A laser processing method for forming a through hole using a laser processing device including an oscillator that oscillates a carbon dioxide laser pulse, a modulator that further modulates the laser pulse oscillated from the oscillator into a processing laser pulse having a predetermined pulse energy, a focusing lens that focuses the processing laser pulse on a workpiece, a table on which the workpiece is placed, a beam diameter adjusting optical system that adjusts the beam diameter of the processing laser pulse at a processing point, and an overall control unit that controls the operation of the entire device, the workpiece is a low-temperature co-fired glass ceramic substrate; The pulse energy of the processing laser pulse is 6.7 mJ or more. A laser processing method comprising:

2. The energy density of the processing laser pulse at the processing point is 592.7 mJ / mm ２ More than 2. The laser processing method according to claim 1 .

3. an oscillator for emitting a carbon dioxide gas laser pulse; a modulator that further modulates the laser pulse oscillated from the oscillator into a processing laser pulse having a predetermined pulse energy; A focusing lens that focuses the processing laser pulse on a workpiece; A table on which the workpiece is placed; a beam diameter adjusting optical system for adjusting a beam diameter at a processing point of the processing laser pulse; an overall control unit for controlling the operation of the entire device; Equipped with The modulator modulates the pulse energy of the processing laser pulse to 6.7 mJ or more.

1. A laser processing apparatus for forming a through hole in a low-temperature co-fired glass ceramic substrate.

4. (Beam diameter limitations apply) The beam diameter of the processing laser pulse at the processing point is adjusted by the beam diameter adjusting optical system, and the energy density of the processing laser pulse at the processing point is adjusted to 592.7 mJ / mm ２ More than 4. The laser processing apparatus according to claim 3.

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