Laser processing device and laser processing method

Abstract

Provided is a laser processing device capable of processing a workpiece in a processing range such as a fine hole smaller than a diffraction limit.　A laser processing device (10) has: laser beam irradiation means (21) for irradiating a processing laser beam with adjusted laser beam intensity; distance control means (moving means (41), height control unit (62)) for adjusting a processing distance between the laser beam irradiation means (21) and a workpiece (9); measurement means (laser displacement meter (31)) for the processing distance; and a setting unit (laser control unit (61)) for setting the laser beam intensity so that a part of a Gaussian distribution of the processing laser beam is greater than a processing threshold value of the workpiece, at a focus of the processing laser beam.

Description

Laser processing device and laser processing method

[0001] The present invention relates to a laser processing apparatus and a laser processing method.

[0002] Laser processing is a processing method in which high-density laser light is irradiated onto various materials, causing them to melt and vaporize, drilling holes or cutting. Laser processing is widely used for molding products made from materials such as metal, wood, and resin. Laser processing is characterized by the fact that it can be done without directly touching the object being processed, making it less likely to deform the material.

[0003] Patent Document 1 discloses a laser cutting method in which a laser beam is guided from a laser oscillator into a processing head, and is guided onto a workpiece while being focused by a focusing lens system in the head, thereby cutting the workpiece into a desired shape. The laser cutting method is characterized in that the diameter of the incident beam is varied by changing the optical path length using the spread angle (divergence angle) of the laser beam guided to the focusing lens system in the head based on a variable corresponding to at least the thickness of the workpiece.

[0004] With the increasing integration of integrated circuits, development is underway for semiconductors to be able to be mounted in three dimensions. This requires the creation of through-holes called TSVs (Through-Silicon Vias). These through-holes are created using techniques such as etching, which requires the use of specialized masks.

[0005] Patent Document 2 discloses a method for manufacturing a substrate for a semiconductor package, which comprises the steps of: (1) forming blind holes in a double-sided copper-clad laminate by laser processing, leaving only the copper foil on one side; (2) forming through holes in the copper foil on one side by etching; (3) performing a desmearing process; (4) sealing the through holes by plating, thereby establishing electrical continuity between the front and back of the substrate; and (5) forming solder connection terminals at the positions of the sealed holes.

[0006] Furthermore, as a technology relating to laser processing, Patent Document 3 discloses a laser processing method characterized by comprising an oscillation step of oscillating a laser beam by a laser output means, an image transfer step of transferring an image of the laser beam onto a workpiece by a transfer optical system, and a laser beam intensity distribution changing step of changing the intensity distribution of the laser beam by a laser beam intensity distribution changing means.

[0007] Japanese Patent Laid-Open No. 4-253584 Japanese Patent Laid-Open No. 2001-135750 Japanese Patent Laid-Open No. 2004-66322

[0008] Because laser processing involves laser irradiation, the processing area resulting in holes, cuts, and other processing marks is much smaller than that of other common processing methods. Therefore, the processing area is rarely an issue. However, the laser diffraction limit has been considered the upper limit of the processing area. Meanwhile, in semiconductor processing, for example, there is a demand for through-holes in wafers. These through-holes must be able to be made at any position, allowing for higher integration and more design options. Patent Documents 1 and 2 use laser processing, but the holes have a tapered shape in the thickness direction, resulting in a narrowed area near the focal point, resulting in a difference in processing diameter. While these holes can be used as is, some methods for reducing their impact are being considered.

[0009] Patent Document 3 includes a laser beam intensity distribution changing step in which a laser beam intensity distribution changing means changes the intensity distribution of the laser beam. Specifically, it describes an embodiment in which a current supply source is controlled. However, controlling the current supply source takes time for the laser beam to stabilize, the energy relative to the processing threshold tends to vary, and it is difficult to reproduce specific conditions and test results.

[0010] Under these circumstances, an object of the present invention is to provide a laser processing apparatus and a laser processing method that can process a workpiece in a processing range such as a microhole that is smaller than the diffraction limit.

[0011] The present inventors have conducted extensive research to solve the above problems and have found that the following inventions meet the above objectives, thereby completing the present invention.

[0012] <1> A laser processing apparatus comprising: a laser irradiation means for irradiating a processing laser with adjusted laser intensity; a distance control means for adjusting the processing distance between the laser irradiation means and a workpiece; a specifying means for specifying the processing distance; a setting unit for setting the laser intensity at the focus of the processing laser to an intensity having an expected processing region where part of the Gaussian distribution of the processing laser exceeds the processing threshold of the workpiece; an initial focus setting unit for arranging the focus of the expected processing region to be inside the workpiece and with an upper part of the expected processing region on the surface side of the workpiece; and a focus moving unit for moving the focus of the expected processing region to the surface side of the workpiece after the initial focus setting unit irradiates the workpiece with the processing laser. <2> The laser processing apparatus according to <1>, comprising an observation means for observing the periphery of the processing region of the workpiece, and an optical path adjustment unit provided between the optical path of the laser irradiation means and the optical path of the observation means. <3> The laser processing device according to <1> or <2>, wherein the distance control means is height control means for adjusting the height of the workpiece holding means, and the laser irradiating means controls the laser output using an attenuator. <4> The laser processing device according to <3>, wherein the specifying means uses a laser displacement meter as a measuring means, and wherein an optical path adjustment unit is provided between the optical path of the laser irradiating means and the optical path of the laser displacement meter, and wherein the laser irradiation positions of the laser displacement meter and the laser irradiating means are the same part of the workpiece. <5> The laser processing device according to any of <1> to <4>, wherein the workpiece is a semiconductor element, and wherein the laser processing device is used to drill a hole in the semiconductor element that is smaller than the diffraction limit of the processing laser.<6> A laser processing method for processing a workpiece with a laser processing device having laser irradiation means for irradiating a processing laser with adjusted laser intensity, distance control means for adjusting a processing distance between the laser irradiation means and the workpiece, and specifying means for specifying the processing distance, the laser processing method comprising: a setting step for setting the laser intensity at a focus of the processing laser to an intensity having an expected processing region where part of the Gaussian distribution of the processing laser exceeds a processing threshold of the workpiece; a distance control step for specifying the processing distance with the distance control means and locating the workpiece at a position where the processing threshold is exceeded; an initial irradiation step for locating the focus of the expected processing region at an initial focus where the focus is inside the workpiece and an upper part of the expected processing region is on the surface side of the workpiece in the distance control step, and irradiating the workpiece with the processing laser at the laser intensity; and a focus movement irradiation step for irradiating the workpiece with the processing laser in the initial irradiation step, after irradiating the workpiece with the processing laser, moving the focus of the expected processing region towards the surface side of the workpiece, and further irradiating the workpiece with the processing laser.

[0013] According to the present invention, it is possible to process a workpiece in a processing range such as a microhole that is smaller than the diffraction limit.

[0014] FIG. 1 is a schematic diagram of a laser processing apparatus according to an embodiment of the present invention. FIG. 2 is a schematic diagram for explaining processing by the laser processing apparatus according to an embodiment of the present invention. FIG. 3 is a schematic diagram for explaining the principle of processing by the laser processing apparatus etc. of the present invention. FIG. 4 is a schematic diagram for explaining the principle of processing by the laser processing apparatus etc. of the present invention. FIG. 5 is a flow diagram of an embodiment of a laser processing method of the present invention. FIG. 6 is a flow diagram of an embodiment of a laser processing method of the present invention. FIG. 7 is an image of a material that has been laser processed according to a test example of the present invention. FIG. 8 is an image of a material that has been laser processed according to a test example of the present invention.

[0015] The following describes in detail an embodiment of the present invention, but the following description of the constituent elements is one example (typical example) of an embodiment of the present invention, and the present invention is not limited to the following content unless the gist of the present invention is changed. Note that when the expression "to" is used in this specification, it is used as an expression including the numerical values ​​before and after it.

[0016] [Laser processing apparatus of the present invention] The laser processing apparatus of the present invention comprises a laser irradiation means for irradiating a processing laser with an adjusted laser intensity, a distance control means for adjusting the processing distance between the laser irradiation means and the workpiece, a specifying means for specifying the processing distance, and a setting unit for setting the laser intensity at the focus of the processing laser to be an expected processing region where a part of the Gaussian distribution of the processing laser exceeds the processing threshold of the workpiece. The laser processing apparatus of the present invention may also comprise an initial focus setting unit for positioning the focus of the expected processing region inside the workpiece with an upper part of the expected processing region on the surface side of the workpiece, and a focus moving unit for moving the focus of the expected processing region toward the surface side of the workpiece after the initial focus setting unit irradiates the workpiece with the processing laser.

[0017] [Laser processing method of the present invention] The laser processing method of the present invention is a laser processing method for processing a workpiece using a laser processing device having a laser irradiation means for irradiating a processing laser with an adjusted laser intensity, a distance control means for adjusting the processing distance between the laser irradiation means and the workpiece, and a specifying means for specifying the processing distance. The laser processing method of the present invention can include a setting step for setting the laser intensity at the focus of the processing laser so that a part of the Gaussian distribution of the processing laser exceeds the processing threshold of the workpiece, a distance control step for specifying the processing distance with the distance control means and positioning the workpiece at a position exceeding the processing threshold, an initial irradiation step for positioning the focus of the expected processing area at an initial focus inside the workpiece and with an upper part of the expected processing area facing the surface of the workpiece in the distance control step, and irradiating the processing laser at the laser intensity, and a focus movement irradiation step for irradiating the workpiece with the processing laser at the initial irradiation step, after irradiating the workpiece with the processing laser in the initial irradiation step, moving the focus of the expected processing area toward the surface of the workpiece and further irradiating the workpiece with the processing laser.

[0018] In the present application, the laser processing method of the present invention can also be performed using the laser processing apparatus of the present invention, and the corresponding configurations in the present application can be used interchangeably.

[0019] Fig. 1 is a schematic diagram of a laser processing apparatus according to an embodiment of the present invention. Fig. 2 is a schematic diagram for explaining processing by the laser apparatus according to an embodiment of the present invention. Fig. 3 is a schematic diagram for explaining the principle of processing by the laser processing apparatus etc. of the present invention.

[0020] 1, the laser processing apparatus 10 has a laser irradiation means 21, a stage 40 serving as a holding means, a laser displacement meter 31 serving as a measuring means, a moving means 41 serving as a distance control means, and a control unit 6 of a setting section. Using this laser processing apparatus 10, as shown in FIG. 2, the distance of the workpiece 9 from the laser irradiation means 21, which is set to predetermined conditions, is adjusted to perform processing within a predetermined processing range such as a hole.

[0021] FIG. 3 is a diagram illustrating the principles of processing according to the present invention. A laser is emitted from the aperture of the aperture section 22 of the irradiation means. Laser, which stands for Light Amplification by Stimulated Emission of Radiation, emits light with excellent directionality and convergence. However, similar to the resolution limit of a microscope, even when the aperture is made as small as possible within the range of laser irradiation, a diffraction limit occurs at the focus. Furthermore, in practice, the diffraction limit is larger than the diffraction limit due to the precision and error of the equipment. This diffraction limit and its error essentially constitute the smallest range of processing by laser irradiation.

[0022] As shown in (x') of Figure 3, in conventional laser processing, in order to ensure a sufficient depth of focus, it is common to irradiate the workpiece with a sufficiently strong laser intensity so that the entire range of the diffraction limit near the focus exceeds the threshold for processing the workpiece. This method, taking into account the diffraction limit and its error, results in a substantially smaller processing range than conventional processing, with a laser of a practical wavelength, with a lower limit of approximately φ1.2 μm. Furthermore, from the perspective of efficient laser processing and the difficulty of controlling the focus of laser processing, it is common to set processing conditions with a sufficiently strong laser intensity that provides a wide, tolerable depth of focus from the diffraction-limited focus. Therefore, in conventional laser processing, the range beyond the laser optical path 2a near the diffraction-limited depth of focus is the processing range.

[0023] However, the inventors focused on the Gaussian distribution of laser light to perform finer processing. The intensity distribution of laser light also has a Gaussian distribution. If only a portion of the Gaussian distribution exceeds the threshold for processing the workpiece at the focal point, only that area becomes the processing area. That is, as shown in Figure 3, in the laser optical path 2a, only the processing threshold range 2b becomes the processing area for the workpiece. Therefore, areas away from the focal point where the diffraction limit is reached do not exceed the threshold, as shown in Figure 3(a), and processing is not possible. Furthermore, as shown in Figure 3(c), only a portion of the focal point exceeds the threshold, becoming the processing area. That is, within the diffraction limit diameter L0, only the processing diameter L1 is processed. With this setting, even at a small distance from the focal point, it becomes difficult to exceed the threshold, and only a narrower area exceeds the threshold for processing, as shown in Figure 3(b). This is thought to be due to the presence of a diamond-shaped processing area 2b within the laser optical path 2a. The assumed processing area in this invention is the wide central portion of this diamond-shaped processing area. The expected processing area is an area where the laser intensity is expected to exceed the processing threshold of the workpiece. In the present invention, the expected processing area is the area where part of the Gaussian distribution at the focus exceeds the processing threshold and is expected to be diamond-shaped when viewed from the side, for example, as shown in 2b in Figure 3.

[0024] The present invention enables processing at a range smaller than the diffraction limit by processing with such a laser beam irradiation intensity. This is a technology that makes it possible to drill extremely small holes, such as those less than 1 μm in size. Because the processing conditions are within a very narrow range, the depth of focus is limited, and it is important to accurately grasp the distance between the laser irradiation means and the workpiece. For this reason, the actual distance between the workpiece and the laser irradiation means is measured and combined with a means for controlling the height. The present invention is based on this finding.

[0025] [Laser Irradiation Means] The laser irradiation means 21 is a means for irradiating a processing laser with adjusted laser intensity. Any irradiation means can be used for laser irradiation. For example, a CO2 laser, YAG laser, fiber laser, etc. can be used. The laser wavelength can be any, and processing can be performed within a range smaller than the diffraction limit of the laser of the selected wavelength. In particular, when a small processing range is desired, it is preferable to use a laser with an ultraviolet wavelength. The laser irradiated by the laser irradiation means 21 is a laser with a diffraction limit based on the laser wavelength and the opening of the diaphragm unit 22. Furthermore, the laser intensity of the laser irradiation means 21 can be set according to the heat amount, irradiation time, number of times, etc., depending on the workpiece 9.

[0026] [Attenuator] The laser irradiation means 21 is preferably equipped with an attenuator. Controlling the laser output with an attenuator improves the stability of the laser irradiation energy. The attenuator is also called an attenuator, and an absorption-type attenuator, a variable beam splitter-type attenuator, a non-polarizing beam splitter, etc. can be used. For laser processing according to the present invention, it is important to operate the energy irradiation intensity while highly stabilizing it near the processing threshold. Since it often takes time for the laser intensity to stabilize when adjusting the current, etc., it is effective to control the output with an attenuator.

[0027] [Determining Means] The determining means is a means for determining the processing distance (FIG. 2(b) processing distance) between the laser irradiation means 21 and the workpiece 9. When the set value of the height direction stage (Z stage) is highly reliable and the position from the lens can be determined based on that set value, the processing distance can be determined by that set value. Alternatively, the processing distance may be determined by measuring the actual processing distance. Furthermore, the processing distance can also be set and determined as appropriate while referring to whether the periphery of the processing area of ​​the image observed by the observation means is in focus.

[0028] [Measurement Means] The measurement means is a means for determining the processing distance (height h1 in FIG. 2(b)) between the laser irradiation means 21 and the workpiece 9 by measuring it. The measurement means may be any means capable of determining the distance between the laser irradiation means and the workpiece with high accuracy. For example, in an apparatus configured to irradiate laser light in the vertical direction, the height may be measured based on a control value in the height direction (Z direction) of the stage on which the workpiece is placed. The laser processing apparatus 10 uses a laser displacement meter 31 as the measurement means. The laser displacement meter 31 is suitable for the present invention because it can measure distance with high accuracy and can also measure distances corresponding to the depth of focus, which is an extremely narrow range.

[0029] The laser processing apparatus 10 has an optical path adjustment unit 51 provided between the optical path of the laser irradiation means 21 and the optical path of the laser displacement meter 31. When irradiating from the laser irradiation means 21 through this optical path adjustment unit 51, the laser is irradiated directly onto the workpiece 9. On the other hand, when measuring the distance (height h1) to the workpiece 9, the optical path can be switched via the optical path adjustment unit 51, and the distance can be measured by the laser displacement meter 31. Therefore, the laser irradiation positions of the laser displacement meter 31 and the laser irradiation means 21 are substantially the same part of the workpiece 9. Note that in order to prevent reflected light from the processing laser or the like from entering the laser displacement meter 31, a shutter 52 or the like can be provided between the laser displacement meter 31 as appropriate, so that the optical path to be used can be adjusted according to the operation.

[0030] Furthermore, the laser processing device 10 may be configured to be able to observe the processing position of the workpiece 9 by using the optical path adjustment unit 51 or any built-in microscope configuration, and further adding a camera and lighting.

[0031] [Observation Means] The observation means is a means for observing the periphery of the processing area of ​​the workpiece 9. The observation means 32 can be installed in place of the laser displacement meter 31 and can utilize the optical path adjustment unit 51. The laser processing apparatus 10 may be equipped with either the laser displacement meter 31 or the observation means 32, or may be equipped with an optical path adjustment unit or a switching unit to enable the use of both. The observation means can, for example, capture an image appropriately magnified by an objective lens using a CMOS (Complementary Metal Oxide Semiconductor) sensor and display it on a monitor or the like. By processing while observing a specific portion of the processing target, it is possible to process while understanding the processing status, such as the processing position and hole diameter.

[0032] [Holding Means] The holding means is a means for holding the workpiece 9. The holding means may be any means that does not obstruct the optical path of the laser irradiated from the laser irradiation means 21, and may be a means for placing the workpiece 9 on or a means for gripping the periphery of the workpiece 9. The laser processing apparatus 10 places the workpiece 9 on the stage 40 as the holding means.

[0033] [Distance Control Means] The distance control means adjusts the processing distance between the laser irradiation means 21 and the workpiece 9. As long as the distance control means can adjust the relative distance, the optical system side having the laser irradiation means 21 may approach the workpiece 9, or the means for holding the workpiece 9 may move toward the optical system.

[0034] The laser processing device 10 irradiates a laser beam from above the workpiece 9 placed on the stage 40 using the laser irradiation means 21. Therefore, the distance control means is height control means that controls the height of the stage 40, which serves as a holding means.

[0035] The moving means 41 also functions as a height control means and can also control the processing position. A combination of an XYZ stage and a rotary stage can be used. In particular, it is preferable to use a stage that can control the movement distance in each of the X, Y, and Z directions, i.e., width (X), depth (Y), and height (Z), by approximately 0.1 to 10 μm, and preferably approximately 0.1 to 1 μm, so that the height can be controlled in accordance with the depth of focus. The movement distance in the X, Y, and Z directions can be controlled independently, as needed, or multiple stages can be adjusted in conjunction with each other as needed.

[0036] [Setting Unit] The setting unit sets the laser intensity at the focus of the processing laser so that a portion of the Gaussian distribution of the processing laser exceeds the processing threshold of the workpiece. As described above with reference to FIG. 3 and other figures, even if the slit or the like is adjusted, laser light has a diffraction limit due to the wavelength of the laser light, the slit shape, and the lens resolution. Because lasers can only process a narrow range even at a size close to the diffraction limit, conventionally, lasers have been irradiated at such a high laser intensity that the Gaussian distribution is ignored at the focus, in order to deepen the allowable range of the focal depth. On the other hand, in the present invention, the Gaussian distribution of the laser light is taken into account and the laser is irradiated at an intensity such that only a portion of the Gaussian distribution at the focus exceeds the processing threshold.

[0037] [Processing Threshold] The processing threshold varies depending on the type and settings of the laser, depending on the material. To determine the specific processing threshold, first determine the focal point at the diffraction limit from the design of the optical system, etc. Then, at the focal point, a high laser intensity is applied to confirm whether the processing range (through-hole diameter, etc.) is processed to the same extent as or exceeding the diffraction limit, thereby determining whether the laser is excessively intense. Next, by determining the intensity at which no through-holes are formed even with repeated irradiation, the insufficient intensity is determined.

[0038] The processing threshold used in the present invention exists between the excessive intensity and the insufficient intensity, and the processing threshold can be found by gradually checking the processing range, such as the through-hole diameter, by laser irradiation between these levels. It should be noted that, since processing through-holes near the processing threshold tends to be confirmed by increasing the irradiation time or pulse width or by repeating irradiation more frequently, it is better to find the processing threshold by increasing the irradiation time or number of irradiations under conditions lower than the excessive intensity. In the present invention, laser processing is performed with the range exceeding the processing threshold as the expected processing area.

[0039] When a part of the Gaussian distribution of the processing laser exceeds the processing threshold of the workpiece, this part of the Gaussian distribution corresponds to the ratio of the diffraction limit diameter L0 to the processing diameter L1 in Figure 3 when processing at a point to obtain a through hole or the like. This diameter is L0 > L1. L1 / L0 is preferably 0.95 or less, and more preferably 0.90 or less.

[0040] Conventional laser irradiation often exceeds the theoretical diffraction limit. This is thought to be because the theoretical diffraction limit is a value that ignores the thickness direction, assuming an ideal lens and ideal laser with extremely little aberration. Therefore, in reality, even a slight deviation around the depth of the diffraction limit will result in an expanded processing range. Furthermore, when viewed in the direction of laser light propagation, the focal point is the narrowest range, and the processing range before and after that will expand in width. For this reason, when viewed in the thickness direction of the processed workpiece, it will have a tapered shape that narrows toward the focal point.

[0041] According to the present invention, processing can be performed at the focal point within a processing range narrower than the diffraction limit, and by moving the focal point in stages when viewed in the thickness direction, the focal point becomes the widest part. Therefore, by moving this widest part up and down, tapering does not occur in the thickness direction, and hole diameters and line widths in the thickness direction can be processed to a certain extent.

[0042] [Condition Input] After finding the processing threshold suitable for the material or raw material of the workpiece, conditions such as "material / material" and "thickness" are selected and input to control the laser irradiation conditions of the laser irradiation means with a laser intensity suitable for those conditions. The laser irradiation conditions include the amount of heat, time, number of times, etc. to be controlled. The condition input unit 72 is a section for inputting conditions for determining such control conditions. In addition, specific control conditions can be read and used from information stored in the memory unit 71 as a condition table, etc.

[0043] [Control Unit] The control unit 6 performs various controls for controlling the laser processing apparatus 10. The control unit 6 includes a laser control unit 61, a height control unit 62, a position control unit 63, etc. The laser control unit 61 sets and controls the laser irradiation conditions of the laser irradiation means 21 according to the workpiece 9 input from the condition input unit 72, etc. The height control unit 62 controls the measurement of the distance (height) from the diaphragm unit 22 to the workpiece 9, and controls the operation of the laser displacement meter 31, the setting of the optical path of the optical path adjustment unit 51, and height conversion based on information input from the laser displacement meter 31. The position control unit 63 controls the position of the stage 40 via the moving means 41 based on the height measurement results and processing position information so that the workpiece 9 is positioned within the processing threshold range 2b.

[0044] The control unit 6 may further include an initial focus setting unit 64 and a focus moving unit 65. The initial focus setting unit 64 is a unit that performs control to position the focus of the expected processing area inside the workpiece 9 and the top of the expected processing area on the surface side of the workpiece 9 in order to irradiate the processing laser. The focus moving unit 65 is a unit that performs control to move the focus of the expected processing area toward the surface side of the workpiece 9 in order to further irradiate the processing laser after the initial focus setting unit 64 irradiates the workpiece 9 with the processing laser.

[0045] [Display Unit] The display unit 73 is a part that displays information for confirming the conditions input for these processing, and for visually confirming the actual processing position and the processing status by appropriately combining a microscope or the like with the optical system. In addition, these controls, processing conditions, and processing results are stored in the memory unit 71 and can be read out and used as appropriate.

[0046] [Workpiece] Any object to be laser processed can be used as the workpiece 9. The workpiece 9 is preferably a semiconductor element. Furthermore, the laser processing apparatus 10 is preferably used to drill a through hole in the semiconductor element.

[0047] Under test condition (1), when a laser with a wavelength of 355 nm is used, the measured value of the source slit is 0.7 μm, the objective lens magnification is 100x, and the objective lens NA value is 0.5, the theoretical diffraction limit is 0.87 μm. Conventional laser irradiation with excessive intensity results in through-holes exceeding 1.2 μm in size. However, according to the present invention, through-holes of approximately 0.8 μm can also be obtained.

[0048] In accordance with an embodiment of the present invention, when a semiconductor wafer was processed under the above-described test condition (1), 100 shots were performed at an output intensity of 2 mJ, which corresponds to an excessive intensity in accordance with conventional laser irradiation, and the through-hole diameter was 1.2 μm. On the other hand, when 100 shots were performed at an output intensity of 1 mJ, which is the processing threshold, through-holes with a diameter of 0.8 μm, which is smaller than the diffraction limit, could be processed.

[0049] Diversity in semiconductor device microfabrication technology is being sought in order to broaden processing options. Currently, through-holes of less than 1 μm are being sought for through electrodes and the like. Conventional laser irradiation has not been able to achieve this. However, the present invention makes it possible to form through-holes of less than 1 μm, which is below the diffraction limit.

[0050] FIG. 2 is a diagram for explaining processing by the laser processing apparatus 10 in more detail. As shown in FIG. 2(a), first, the workpiece 9 is placed on the stage 40. If the distance h0 between the aperture 22 and the workpiece 9 is too large, the distance will not fall within the processing threshold range, and repeated laser irradiation will not result in processing. Next, as shown in FIG. 2(b), while measuring the distance to the workpiece 9 with the laser displacement meter 31, the stage 40 is moved to a distance h1 near the focus at the diffraction limit. Then, as shown in FIG. 2(c), the processing laser emitted by the laser irradiation means 21 is controlled and irradiated so as to process within the processing threshold range 2b. This results in the workpiece 9 having a hole 91, as shown in FIG. 2(d).

[0051] FIG. 4 is a schematic diagram illustrating the processing principle of the laser processing apparatus of the present invention. As will be explained in the test examples below, the inventors, after examining various test conditions, unexpectedly discovered that by performing laser processing with the focal point of the intended processing area set inside the workpiece and then shifting the focal point toward the surface of the workpiece, a hole with a small diameter and a high aspect ratio can be obtained. Note that the side of the workpiece facing the laser irradiation means is referred to as the surface of the workpiece. Furthermore, with regard to the intended processing area, the widest point of the diamond shape is referred to as the focal point of the intended processing area, and the side facing the laser irradiation means is referred to as the top of the intended processing area.

[0052] Figure 4(a) shows the state in which the position of the workpiece relative to the processing laser is set by the initial focus setting unit. The processing range 2b is the expected processing area, and the focus of this expected processing area is positioned near the center in the thickness direction of the workpiece 9, with the upper part near the upper end of the expected processing area positioned near the surface of the workpiece 9. This position can be adjusted by adjusting the laser of the laser irradiation means or by controlling the relative distance between the laser irradiation means and the workpiece, and in Figure 4, it is adjusted by controlling the height of the workpiece 9. In Figure 4, the height of this initial focus is set to height h0.

[0053] When the thickness of the workpiece 9 is d1, the initial focus can be set at a position from the surface of the workpiece 9 that is approximately 0.3×d1 to 0.7×d1 or 0.4×d1 to 0.6×d1. For example, when the thickness is 50 μm, the focus is set at a position that is approximately 15 μm to 35 μm or 20 μm to 30 μm from the surface. It is preferable to set the focus at approximately the center in the thickness direction.

[0054] In the initial irradiation process using the initial focus setting unit, the processing laser is irradiated in this position. In this state, sufficient processing energy does not reach the interior of the workpiece 9 because the light is actually blocked by the top of the workpiece 9. On the other hand, near the top of the expected processing area, the width is narrower than the focus and the energy intensity only slightly exceeds the threshold, resulting in a small hole. On the other hand, if irradiation is continued in this state, the laser light will easily enter the hole, resulting in a hole in the thickness direction. However, since the light is blocked on the front side of the workpiece 9, it is thought that the light is not sufficiently focused and only a processing area smaller than the size of the expected processing area will be processed. For this reason, even if processing is performed in the state shown in Figure 4(a), the hole may not extend to the back side.

[0055] Fig. 4(b) shows a state in which, after irradiation with the processing laser according to Fig. 4(a), the focus of the expected processing area is moved to a position on the surface side of the workpiece in order to further irradiate the processing laser. Fig. 4(b) shows a state in which the focus of the expected processing area is moved to the surface side of the workpiece 9 by moving the workpiece 9, which was positioned at height h0 in Fig. 4(a), to height hn, which is further away from the laser irradiation means.

[0056] By moving the laser beam in this manner, the range of the expected processing area on the surface side of the workpiece 9 is expanded. Furthermore, since a hole is formed near the surface during initial irradiation at the initial focus position, a certain amount of light penetrates into this hole. This operation makes it easier to further deepen the hole after it has been formed near the surface without changing the laser intensity of the laser processing means. This allows the hole depth to be efficiently increased in the workpiece 9, as shown in FIG. 4( c), and a hole 92 with a high aspect ratio can be efficiently processed.

[0057] The initial focus in Fig. 4(a) can be set to the center of the thickness of the workpiece, and the focus shift in Fig. 4(b) can be one or more times, depending on the thickness, or can be shifted multiple times from the initial focus near the center.

[0058] Furthermore, when the light is blocked near the surface of the workpiece 9, the slit size may be temporarily adjusted to increase the amount of light in order to temporarily increase the processing energy above the focus of the expected processing area, thereby improving processing efficiency. Changing the intensity by current results in large fluctuations and a long period of instability, but adjusting the slit size allows for stable adjustment of the light amount in a short period of time.

[0059] The fact that holes with a high aspect ratio can be obtained by initially irradiating with this type of initial focus setting unit, and then shifting the focus to the surface side with the focus shifting unit and irradiating is based in part on experimental findings as mentioned above, and the principle is not limited to these, but is thought to be as follows: Processing by laser irradiation is thought to occur when the energy exceeds the processing threshold.

[0060] On the other hand, laser irradiation is thought to involve not only the irradiation of light but also melting and ablation (burning) that accompany heating due to energy accumulation. If the focus of the intended processing area is placed on the surface of the workpiece or above that surface and processing is performed in a so-called digging manner, it is thought that it is possible to dig up to the width of the focus, but in reality, the hole may be tapered, with a wider width on the surface and a narrower width on the back side, or it may be difficult to reach the back side. This is thought to be because the processed area is more susceptible to light reaching, resulting in effects such as diffuse reflection of light, and energy is more likely to accumulate only from the surface side.

[0061] Next, a method of processing the substrate in the state shown in FIG. 4(a) is also conceivable. However, this would likely leave the surface side in a light-shielded state, resulting in a slow processing speed on the surface side and difficulty in processing in the depth direction. When processing is performed as shown in FIGS. 4(a) and 4(b) according to the preferred embodiment of the present application, it is believed that a certain amount of energy reaches the focal point, and regions where the threshold value is sufficiently exceeded and light reaches the substrate are generated starting from the surface, thereby enabling efficient processing with a high aspect ratio in the depth direction. The high aspect ratio in the present invention depends on the material and processing purpose, but can be, for example, an aspect ratio of 5 or more, 7 or more, 10 or more, 15 or more, etc. Although no upper limit is particularly required, it may be set to approximately 50 or less, or 40 or less.

[0062] 5 is a flow diagram of an embodiment of the laser processing method of the present invention. Step S11 is a step of inputting and setting the conditions of the workpiece. Step S21 is a step of measuring the distance (height) between the workpiece and the laser irradiation means and moving it to an appropriate height to set it. Step S31 is a step of irradiating the workpiece with a laser. This allows laser processing to be performed on the workpiece.

[0063] FIG. 6 is a flow diagram of an embodiment of the laser processing method of the present invention. This flow corresponds in particular to the processing in FIG. 4. First, in step S12, processing conditions for the workpiece are set. Based on these conditions, step S22 positions the workpiece at the initial focus. This positions the focus of the intended processing area at the initial focus, which is inside the workpiece and the top of the intended processing area is on the surface side of the workpiece, and an initial irradiation step is performed in which the processing laser is irradiated. Then, step S32 moves the focus position of the workpiece. After irradiating the workpiece with the processing laser in the initial irradiation step, the focus of the intended processing area is moved toward the surface of the workpiece, and a focus movement irradiation step is performed in which the processing laser is further irradiated. This allows for the creation of a hole with a high aspect ratio.

[0064] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as the gist of the present invention is not changed.

[0065] Experimental Example 1: A laser processing test based on the present invention was conducted using a polysilicon film of approximately 400 μm thickness used for silicon wafers. The following specifications were used for the laser source and objective lens. The diffraction limit was approximately 0.87 μm. Wavelength: 355 nm, source slit setting: 0.7 μm, objective lens magnification: 100x, objective lens NA value: 0.5

[0066] As a preliminary test, a shot laser with a heat amount and pulse width of 2 mJ / 5 nsec, 1 mJ / 5 nsec, 0.7 mJ / 5 nsec, and 0.6 mJ / 5 nsec was used at approximately 6 Hz, and the focal point was irradiated from the periphery of the surface of the polysilicon film in the depth direction at 50 shots every 10 μm to verify the processing threshold.

[0067] As a result of this verification, we decided to adopt a heat quantity of 0.7 mJ as the condition under which only a part of the Gaussian distribution exceeds the processing threshold. Note that a heat quantity of 1 mJ or more results in large holes, while a heat quantity of 0.6 mJ makes it difficult to confirm holes and makes processing difficult.

[0068] Based on these condition verifications, a test was conducted to form through-holes in a polysilicon film, and then an image of the portion where the through-holes had been formed was observed with a scanning electron microscope (SEM), as shown in Figure 7. It was confirmed that by performing the processing in this manner, it was possible to form through-holes of 0.79 μm, which is smaller than the theoretical diffraction limit of 0.87 μm under these conditions.

[0069] Experimental Example 2: Under the same conditions as Experimental Example 1, the following specifications were used for the laser source and objective lens. The diffraction limit at this time was approximately 1.3 μm: wavelength: 532 nm, radiation source slit setting: 0.7 μm, objective lens magnification: 100x, objective lens NA value: 0.5

[0070] A test was conducted to form through-holes in a polysilicon film with reference to the condition verification in Experimental Example 1. After that, an image of the portion where the through-holes had been formed was observed with a scanning electron microscope (SEM), as shown in Fig. 8. It was confirmed that by performing the processing in this manner, it was possible to form through-holes of 0.77 µm in diameter, which is smaller than the theoretical diffraction limit of 1.3 µm.

[0071] [Experimental Example 3] Laser processing was performed on a 50 μm thick silicon wafer film. A YAG laser processing machine was used, with a wavelength of 355 nm. The irradiation intensity was adjusted to create an expected processing area on the silicon wafer that was wide near the focal point and shaped like a diamond when viewed from the side of the laser's direction of travel. This expected processing area had a diameter of 2 μm at the focal point and a height of approximately 50 μm. A single shot of 1 mJ of laser irradiation was also performed. The following preliminary tests, reference tests, and tests were performed using this laser irradiation. Images of the processing results and test positions from these preliminary tests, reference tests, and tests are shown in Figure 9.

[0072] In this experimental example, an optical path adjustment unit was installed in the optical path of the laser irradiation means, and the image of the objective lens could be captured while being observed with a CMOS sensor, allowing processing to be performed while observing the laser processing position and processing results.In addition, the silicon wafer was fixed to a vibration-isolated table and performed on a stage attached to a precisely controlled XYZ+ rotation stage, and the height position was adjusted using the setting value of the Z stage while referring to the focus of the image obtained from the observation means.

[0073] In preliminary tests (3-1) and (3-2), the laser irradiation means was focused near the surface of the silicon wafer, and 210 shots were performed for each test. As a result, holes with a diameter slightly wider than 2 μm were obtained near the surface, but the holes did not reach the back surface.

[0074] As a reference test (3-1), under the same setting conditions of the laser irradiation means, 210 shots were performed with the focal point set at a position 25 μm from the surface of the silicon wafer in the thickness direction. As a result, holes smaller than those in the preliminary test (3-1) were obtained near the surface, but the holes did not reach the back side.

[0075] In Tests (3-1) and (3-2), under the same laser irradiation means settings, the initial focus was set at a position 25 μm from the surface of the silicon wafer in the thickness direction, and 30 shots were performed. Then, by moving the silicon wafer 2 μm at a time, the focus of the intended processing area was moved relatively toward the surface of the silicon wafer, and 30 shots were performed. Similarly, the operation of moving 2 μm and performing 30 shots (30 shots / 2 μm) was repeated. Starting with the initial focus as the first shot, the laser irradiation was terminated after six movements of 12 μm, resulting in a total of 210 shots. This test was performed twice. As a result of Tests (3-1) and (3-2), holes with a diameter of approximately 2 μm were obtained on the surface, and these holes also reached the back surface. This indicates that an aspect ratio of approximately 25 was achieved, as holes with a diameter of 2 μm were obtained in a workpiece with a thickness of 50 μm.

[0076] The present invention can be used for processing workpieces and is industrially useful.

[0077] REFERENCE SIGNS LIST 10 Laser processing device 21 Laser irradiation means 22 Diaphragm section 2a Laser optical path 2b Processing threshold range 31 Displacement meter 32 Observation means 40 Stage 41 Moving means 51 Optical path adjustment section 52 Shutter 6 Control section 61 Laser control section 62 Height control section 63 Position control section 64 Initial focus setting section 65 Focus movement section 71 Memory section 72 Condition input section 73 Display section 9 Workpiece 91 Hole

Claims

1. A laser processing device having: a laser irradiation means for irradiating a processing laser with adjusted laser intensity; a distance control means for adjusting the processing distance between the laser irradiation means and a workpiece; a determination means for determining the processing distance; a setting unit for setting the laser intensity at the focus of the processing laser to an intensity having an expected processing area where part of the Gaussian distribution of the processing laser exceeds the processing threshold of the workpiece; an initial focus setting unit for arranging the focus of the expected processing area to be inside the workpiece and the top of the expected processing area to be on the surface side of the workpiece; and a focus moving unit for moving the focus of the expected processing area to the surface side of the workpiece after the initial focus setting unit irradiates the processing laser to the workpiece.

2. A laser processing device as described in claim 1, further comprising an observation means for observing the periphery of the processed area of ​​the workpiece, and an optical path adjustment unit provided between the optical path of the laser irradiation means and the optical path of the observation means.

3. A laser processing device as set forth in claim 1, wherein said distance control means is a height control means for adjusting the height of said workpiece holding means, and said laser irradiation means controls the laser output by an attenuator.

4. A laser processing device as described in claim 3, wherein the identifying means uses a laser displacement meter as a measuring means, and an optical path adjustment unit is provided between the optical path of the laser irradiation means and the optical path of the laser displacement meter, and the laser irradiation positions of the laser displacement meter and the laser irradiation means are the same part of the workpiece.

5. A laser processing device according to any one of claims 1 to 4, wherein the workpiece is a semiconductor element, and the laser processing device is used to drill a hole in the semiconductor element that is smaller than the diffraction limit of the processing laser.

6. A laser processing method for processing a workpiece with a laser processing device having a laser irradiation means for irradiating a processing laser with adjusted laser intensity, a distance control means for adjusting the processing distance between the laser irradiation means and the workpiece, and a specifying means for specifying the processing distance, the method comprising: a setting step for setting the laser intensity at the focus of the processing laser to an intensity having an estimated processing area where part of the Gaussian distribution of the processing laser exceeds the processing threshold of the workpiece; a distance control step for specifying the processing distance with the distance control means and positioning the workpiece at a position exceeding the processing threshold; an initial irradiation step for irradiating the processing laser at the laser intensity by positioning the focus of the estimated processing area at an initial focus that is inside the workpiece and where the top of the estimated processing area is on the surface side of the workpiece in the distance control step; and a focus moving irradiation step for irradiating the workpiece with the processing laser at the initial irradiation step, after irradiating the workpiece with the processing laser in the initial irradiation step, moving the focus of the estimated processing area to the surface side of the workpiece, and further irradiating the processing laser.

Images