JP2025036101A - Laser processing device and laser processing method - Google Patents

Abstract

To provide a laser processing device capable of processing a workpiece in a processing range such as a fine hole smaller than a diffraction limit.SOLUTION: A laser processing device 10 has: laser irradiation means 21 for irradiating a processing laser with adjusted laser intensity; distance control means (moving means 41, height control unit 62) for adjusting a processing distance between the laser 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 intensity so that a part of a Gaussian distribution of the processing laser is greater than a processing threshold of the workpiece, at a focus of the processing laser.SELECTED DRAWING: Figure 1

Description

The present invention relates to a laser processing device. The present invention also relates to a laser processing method.

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 has the advantage that it can be done without directly touching the object being processed, making it less likely to deform the material.

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 the workpiece while being focused by a focusing lens system in the head, thereby cutting the workpiece into the desired shape. The method is characterized in that the incident beam diameter 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.

As semiconductors become more highly integrated, development is underway to mount them in three dimensions. For this three-dimensional mounting, it is necessary to create through holes called TSVs (Though-Silicon Vias). To create these through holes, etching is used, and a special mask is used.

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

Patent Document 3 discloses a technology related to laser processing. Patent Document 3 discloses a laser processing method characterized by having 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.

Japanese Patent Application Publication No. 4-253584 JP 2001-135750 A JP 2004-66322 A

Because laser processing is a process using laser irradiation, the processing range that results in holes, cuts, and other processing marks caused by the laser processing is much smaller than other general processing methods. For this reason, the processing range is hardly a problem. However, it was thought that the upper limit of the processing range was due to the laser diffraction limit. On the other hand, in semiconductor processing, etc., it is required to create through holes in wafers. In order to increase the number of design options and to increase the integration density, it is required that these through holes be made in any position. In Patent Document 1 and Patent Document 2, laser processing is used, but the holes are tapered in the thickness direction, with a difference in the processing diameter, such as a constriction near the focal point, and some methods are being considered to use them as they are or to reduce their effects.

Patent Document 3 has a laser beam intensity distribution changing step in which the intensity distribution of the laser beam is changed by a laser beam intensity distribution changing means. Specifically, an embodiment in which a current supply source is controlled is described. However, when the current supply source is controlled, it takes time for the laser beam to stabilize, the energy relative to the processing threshold tends to vary, and specific conditions and test results are difficult to reproduce.

In this situation, the present invention aims to provide a laser processing device and a laser processing method that can process a workpiece with a processing range such as a microhole that is smaller than the diffraction limit.

As a result of intensive research aimed at solving the above problems, the inventors discovered that the following invention meets the above objective, and thus arrived at the present invention. That is, the present invention relates to the following inventions.

<1> A laser irradiation means for irradiating a processing laser having an adjusted laser intensity;
a distance control means for adjusting a processing distance between the laser irradiation means and the workpiece;
A means for specifying the processing distance;
A setting unit that sets the laser intensity at a focus of the processing laser so that a part of the Gaussian distribution of the processing laser is an assumed processing region where the intensity exceeds a processing threshold value of the workpiece;
An initial focus setting unit that sets the focus of the intended processing area inside the workpiece and a top of the intended processing area on the surface side of the workpiece;
A laser processing apparatus having a focus moving unit that moves the focus of the intended processing area toward the surface side of the workpiece after the processing laser is irradiated to the workpiece by the initial focus setting unit.
<2> Having an observation means for observing the periphery of the processed region of the workpiece,
The laser processing apparatus according to <1>, further comprising an optical path adjustment unit provided between an optical path of the laser irradiation means and an optical path of the observation means.
<3> The laser processing apparatus according to <1> or <2>, wherein the distance control means is a height control means for adjusting the height of the workpiece holding means, and the laser irradiation means controls the laser output using an attenuator.
<4> The identifying means uses a laser displacement meter as a measuring means,
an optical path adjustment unit provided between an optical path of the laser irradiation means and an optical path of the laser displacement meter;
The laser processing apparatus according to <3>, wherein the laser displacement meter and the laser irradiation position of the laser irradiation means are on the same part of the workpiece.
<5> The laser processing apparatus according to any one of <1> to <4>, wherein the workpiece is a semiconductor element, and the laser processing apparatus is for drilling 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 using a laser processing apparatus having a laser irradiation means for irradiating a processing laser with an adjusted laser intensity, a distance control means for adjusting a processing distance between the laser irradiation means and the workpiece, and a specifying means for specifying the processing distance,
A setting step of setting the laser intensity at a focus of the processing laser so that a part of the Gaussian distribution of the processing laser exceeds a processing threshold value of the workpiece;
a distance control step of specifying the processing distance by the distance control means and disposing the workpiece at a position exceeding the processing threshold value;
In the distance control step, a focus of the assumed processing area is set inside the workpiece, and an initial irradiation step of irradiating the processing laser at the laser intensity by setting an initial focus where an upper part of the assumed processing area is on the surface side of the workpiece;
A laser processing method comprising: a focus shifting irradiation step in which, 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 side of the workpiece, and the processing laser is further irradiated.

The present invention makes it possible to process workpieces with a processing range that is smaller than the diffraction limit, such as microholes.

1 is a schematic diagram of a laser processing device according to an embodiment of the present invention. FIG. 1 is a schematic diagram for explaining processing by a laser processing device according to an embodiment of the present invention. 1 is a schematic diagram for explaining the principle of processing by the laser processing apparatus etc. of the present invention. FIG. 1 is a schematic diagram for explaining the principle of processing by the laser processing apparatus etc. of the present invention. FIG. FIG. 2 is a flow diagram according to an embodiment of the laser processing method of the present invention. FIG. 2 is a flow diagram according to an embodiment of the laser processing method of the present invention. 1 is an image of a material that has been laser-processed according to a test example of the present invention. 1 is an image of a material that has been laser-processed according to a test example of the present invention. 1 is an image of a material that has been laser-processed according to a test example of the present invention.

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

[Laser processing device of the present invention]
The laser processing device of the present invention includes 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 so that a part of the Gaussian distribution of the processing laser is an assumed processing region where the intensity exceeds the processing threshold value of the workpiece. The laser processing device of the present invention may also include an initial focus setting unit for positioning the focus of the assumed processing region inside the workpiece and with the top of the assumed processing region on the surface side of the workpiece, and a focus moving unit for moving the focus of the assumed processing region to the surface side of the workpiece after the processing laser is irradiated to the workpiece by the initial focus setting unit.

[Laser processing method of the present invention]
The laser processing method of the present invention is a laser processing method for processing a workpiece with 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 includes 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 locating the workpiece at a position exceeding the processing threshold, and an initial irradiation step for irradiating the processing laser with the laser intensity by locating the focus of the assumed processing area at an initial focus inside the workpiece and the top of the assumed processing area on the surface side of the workpiece in the distance control step, after irradiating the workpiece with the processing laser in the initial irradiation step, moving the focus of the assumed processing area to the surface side of the workpiece, and further irradiating the processing laser.

In addition, in this application, the laser processing method of the present invention can also be performed using the laser processing device of the present invention, and the corresponding configurations in this application can be used mutually.

Figure 1 is a schematic diagram of a laser processing device according to an embodiment of the present invention. Figure 2 is a schematic diagram for explaining processing by a laser device according to an embodiment of the present invention. Figure 3 is a schematic diagram for explaining the principle of processing by the laser processing device of the present invention.

As shown in FIG. 1, the laser processing device 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 in the setting section. Using this laser processing device 10, as shown in FIG. 2, the distance of the workpiece 9 from the laser irradiation means 21, which is set to a predetermined condition, is adjusted to perform processing within a predetermined processing range such as a hole.

Figure 3 is a diagram for explaining the principle of processing according to the present invention. A laser is irradiated from the opening of the aperture 22 of the irradiation means that performs the laser irradiation. Laser is an abbreviation for Light Amplification by Stimulated Emission of Radiation, and irradiates light with excellent directivity and convergence. However, similar to the resolution limit of a microscope, etc., when irradiating with a laser, even if the opening is made as small as possible within the range in which laser irradiation is possible, a diffraction limit occurs at the focus. Furthermore, in practice, due to the precision and error of the equipment, it is larger than the diffraction limit. This diffraction limit and its error are essentially the smallest range of processing range by laser irradiation.

As shown in (x') of FIG. 3, in conventional laser processing, in order to ensure the focal depth, it is common to irradiate the workpiece with a sufficiently strong laser intensity so that the entire range of the diffraction limit near the focal point exceeds the threshold for processing the workpiece. This is a method in which, when using a laser with a practical wavelength, the processing range is substantially limited to approximately φ1.2 μm, which is considerably smaller than general processing, including the diffraction limit and its error. In addition, from the viewpoint 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 has a wide focal depth that is an allowable range from the diffraction limit focus. Therefore, in the case of general laser processing, the range that exceeds the laser optical path 2a near the focal depth where the diffraction limit is reached is the processing range as it is.

However, the inventors focused on the Gaussian distribution of laser light in order to perform finer processing. The intensity distribution of laser light is also a Gaussian distribution. When only a part of the Gaussian distribution exceeds the threshold for processing the workpiece at the focus, only that range becomes the processing range. That is, as shown in FIG. 3, in the laser light path 2a, only the processing threshold range 2b becomes the range for processing the workpiece. Therefore, in the part away from the focus where the diffraction limit is reached, the threshold is not exceeded as shown in FIG. 3(a) and processing is not possible. Also, as shown in FIG. 3(c), only a part at the focus exceeds the threshold and becomes the processing range. That is, within the range of the diffraction limit diameter L0, only the range of the processing diameter L1 is processed. With such a setting, even at a slight distance from the focus, it becomes difficult to exceed the threshold, and as shown in FIG. 3(b), only the threshold for processing a narrower range is exceeded. This is considered to be due to the presence of a diamond-shaped processing range 2b in the laser light path 2a. The assumed processing area of the present invention is the focus of the wide central part of this diamond-shaped processing range.

The present invention makes it possible to process a range smaller than the diffraction limit by processing with such a laser light irradiation intensity. This is a technology that makes it possible to drill extremely small holes, such as those less than 1 μm in size. For this reason, since the processing conditions are in a very narrow range, it is important to accurately grasp the distance between the laser irradiation means and the workpiece, as the focal depth is limited. For this reason, the distance between the actual workpiece and the laser irradiation means is measured and combined with a means for controlling the height. The present invention is based on such findings.

[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, a YAG laser, a fiber laser, etc. can be used. The wavelength of the laser can be any, and processing can be performed in a range smaller than the diffraction limit of the laser of the selected wavelength. In particular, when it is desired to reduce the processing range, 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 wavelength of the laser and the opening of the diaphragm unit 22. In addition, the intensity of the laser of the laser irradiation means 21 can be set according to the heat amount, irradiation time, number of times, etc., according to the workpiece 9.

[Attenuator]
The laser irradiation means 21 is preferably equipped with an attenuator. The stability of the laser irradiation energy is improved by controlling the laser output with the attenuator. The attenuator is also called an attenuator, and an absorption type attenuator, a variable beam split type attenuator, a non-polarized beam splitter, etc. can be used. For the laser processing according to the present invention, it is important to operate the energy irradiation intensity with high stability near the processing threshold. Since it is likely to take time for the laser intensity to stabilize when current adjustment is performed, it is effective to control the output with an attenuator.

[Specification Means]
The specifying means is a means for specifying the processing distance (FIG. 2(b) processing distance) between the laser irradiation means 21 and the workpiece 9. When the set value of the stage in the height direction (Z stage) is highly reliable and the position from the lens can be determined based on the set value, the processing distance can be specified by the set value. Alternatively, the actual processing distance may be measured to specify the processing distance. Furthermore, the processing distance can be set and specified by appropriately referring to whether the periphery of the processing area of the image observed by the observation means is in focus.

[Measurement means]
The measuring means is a means for determining the processing distance (height h1 in FIG. 2B) between the laser irradiation means 21 and the workpiece 9 by measuring it. The measuring 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 configuration in which the laser is irradiated 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 measuring 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 equivalent to the focal depth, which is an extremely narrow range.

The laser processing device 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 directly irradiated 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. In addition, in order to prevent reflected light of 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, and the optical path used can be adjusted according to the operation.

The laser processing device 10 may also 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.

[Observation Method]
The observation means is a means for observing the periphery of the processing area of the workpiece 9. The observation means 32 can be attached instead of the laser displacement meter 31, and can be one that uses the optical path adjustment unit 51. The laser processing device 10 may be attached with either the laser displacement meter 31 or the observation means 32, or may be one that uses both by providing an optical path adjustment unit or a switching unit. The observation means can be, for example, one that captures an image appropriately enlarged by an objective lens using a CMOS (Complementary Metal Oxide Semiconductor) sensor and displays it on a monitor or the like. By processing while observing a specific part of the processing target, it is also possible to process while grasping the processing situation such as the processing position and hole diameter.

[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 thereon or a means for gripping the periphery thereof. The laser processing apparatus 10 places the workpiece 9 on the stage 40 as the holding means.

[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 holding means for the workpiece 9 may move toward the optical system.

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

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 moving distance in each of the XYZ directions, i.e., width (X), depth (Y), and height (Z), by about 0.1 to 10 μm, and preferably about 0.1 to 1 μm, so that the height can be controlled according to the focal depth. The control of the moving distance in the XYZ directions can be either independent and only the necessary adjustments can be made, or multiple adjustments can be made while linking them together as necessary.

[Settings section]
The setting unit sets 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. As described above with reference to FIG. 3 and the like, even if the slit or the like is adjusted, the laser light has a diffraction limit due to the wavelength of the laser light, the slit shape, and the resolution of the lens. Since the laser can only process a narrow range even if it is about the size of the diffraction limit, it has been irradiated with a very strong laser intensity at the focus such that the Gaussian distribution is ignored from the viewpoint of deepening the allowable range of the focal depth. On the other hand, in the present invention, the laser light is irradiated with an intensity that takes into account the Gaussian distribution of the laser light and that only a part of the Gaussian distribution exceeds the processing threshold even at the focus.

[Processing threshold]
The processing threshold has conditions that depend on the type and settings of the laser depending on the material. When determining the specific processing threshold, first determine the focal point at which the diffraction limit is reached from the design of the optical system, etc. Then, at the focal position, a strong laser is irradiated to confirm whether processing is achieved at a processing range (through hole diameter, etc.) that is equal to or exceeds the diffraction limit, thereby determining whether the laser is excessively strong. Next, by determining the intensity at which no through holes are created even with repeated irradiation, the insufficient intensity is determined.

The processing threshold used in the present invention exists between this excessive intensity and insufficient intensity, so the processing threshold can be found by checking the processing range, such as the through-hole diameter, by stepwise checking the processing range by laser irradiation between these two levels. Note that, since there is a tendency for through-holes to be able to be confirmed by increasing the irradiation time or pulse width or by repeating irradiation many times when processing through-holes near the processing threshold, 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, the range exceeding this processing threshold is used as the expected processing area for laser processing.

When a portion of the Gaussian distribution of the processing laser exceeds the processing threshold of the workpiece, this portion of the Gaussian distribution corresponds to the ratio of the diffraction limit diameter L0 in FIG. 3 to the processing diameter L1 when processing at points 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.

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. For this reason, in reality, the processing range will expand even if there is a slight deviation around the depth of the diffraction limit. Also, when viewed in the direction of laser light propagation, the focal point is the narrowest range, and the processing range before and after that expands in width. For this reason, when viewed in the thickness direction after processing, the workpiece has a tapered shape that tapers toward the focal point.

According to the present invention, processing can be performed at the focal point with 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, so by moving this widest part up and down, no tapering occurs in the thickness direction, and the hole diameter and line width in the thickness direction can be processed to a certain degree.

[Condition input]
After finding the processing threshold suitable for the material or raw material of the workpiece, the laser irradiation conditions of the laser irradiation means are controlled with a laser intensity suitable for the conditions by selecting and inputting conditions such as "raw material/material" and "thickness". The laser irradiation conditions include heat quantity, 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, etc. can be read and used from information stored in the memory unit 71, etc., as a condition table.

[Control unit]
The control unit 6 is a part that performs various controls for controlling the laser processing device 10. The control unit 6 has 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 is a part that performs control for measuring 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 the conversion of height based on the 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 so that the workpiece 9 is placed in the processing threshold range 2b based on the height measurement result and the information on the processing position.

The control unit 6 may further include an initial focus setting unit 64 and a focus shifting unit 65. The initial focus setting unit 64 is a unit that performs control to position the focus of the intended processing area inside the workpiece 9 and the top of the intended processing area on the surface side of the workpiece 9 in order to irradiate the processing laser. The focus shifting unit 65 is a unit that performs control to shift the focus of the intended processing area to 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.

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

[Workpiece]
Any object to be laser-processed can be used as the workpiece 9. The workpiece 9 is preferably a semiconductor element. The laser processing device 10 is preferably used to open a through hole in the semiconductor element.

When a laser with a wavelength of 355 nm is used as test condition (1), the theoretical diffraction limit is 0.87 μm when the source slit measurement value is 0.7 μm, the objective lens magnification is 100 times, and the objective lens NA value is 0.5. As in the past, when the laser is irradiated with excessive intensity, the through-holes will be larger than 1.2 μm. However, according to the present invention, it is possible to obtain through-holes of about 0.8 μm.

In accordance with an embodiment of the present invention, when a semiconductor wafer was processed under the above-mentioned test condition (1), 100 shots were performed with an output intensity of 2 mJ, which corresponds to the 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 with an output intensity of 1 mJ, which is the processing threshold, a through-hole with a diameter of 0.8 μm, which is smaller than the diffraction limit, could be processed.

Diversity is being sought in the microfabrication technology of semiconductor elements in order to expand the processing options. Currently, there is a demand for through holes of less than 1 μm in through electrodes and the like. Conventional laser irradiation has not been able to achieve a size of less than 1 μm. However, the present invention can also create through holes of less than 1 μm, which is below the diffraction limit.

2 is a diagram for explaining the processing by the laser processing device 10 in more detail. As shown in FIG. 2(a), first, the workpiece 9 is placed on the stage 40. At this time, if the distance h0 between the diaphragm 22 and the workpiece 9 is far, the processing threshold range is not reached and processing cannot be performed even if the laser is repeatedly irradiated. 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 irradiated by the laser irradiation means 21 is controlled and irradiated so that processing can be performed within the processing threshold range 2b. As a result, the workpiece 9 with a hole 91 is obtained as shown in FIG. 2(d).

Figure 4 is a schematic diagram for explaining the principle of processing using the laser processing device of the invention. As will be explained in the test examples described later, the inventors have unexpectedly found, after examining various test conditions, that by performing laser processing with the focal point of the intended processing area set inside the workpiece, and then moving the focal point to the surface side 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 described as the surface of the workpiece. In addition, the widest part of the intended processing area is called the focal point of the intended processing area, and the side facing the laser irradiation means is called the top of the intended processing area.

Figure 4 (a) shows the state in which the position of the workpiece relative to the processing laser has been 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 so that it is near the center in the thickness direction of the workpiece 9, and the top near the upper end of the expected processing area is 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 this Figure 4, the height of this initial focus is set to height h0.

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 x d1 to 0.7 x d1, or 0.4 x d1 to 0.6 x d1. For example, when the thickness is 50 μm, the focus is set at a portion 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.

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

Figure 4(b) shows the state in which, after irradiation with the processing laser according to Figure 4(a), the focus of the expected processing area has been moved to a position on the surface side of the workpiece in order to further irradiate the processing laser. In Figure 4(b), the workpiece 9, which was positioned at height h0 in Figure 4(a), has been moved to height hn, away from the laser irradiation means, and the focus of the expected processing area has been moved to the surface side of the workpiece 9.

By moving in this manner, the range of the anticipated processing area on the surface side of the workpiece 9 is expanded. Also, because a hole is created near the surface during initial irradiation with the initial focus position, a certain amount of light enters this hole. This type of operation makes it easier to process the hole to be deeper after it is created near the surface without changing the laser intensity of the laser processing means. This makes it possible to efficiently process the depth of the hole in the workpiece 9, as shown in Figure 4 (c), and efficiently process a hole 92 with a high aspect ratio.

The initial focus in FIG. 4(a) can be approximately near the center of the thickness of the workpiece. Also, the focus movement in FIG. 4(b) can be one or more times, depending on the thickness, or can be multiple times from the initial focus near the center.

In addition, 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 intended processing area, thereby improving processing efficiency. Changing the intensity by current fluctuates greatly and is unstable for a long period of time, but adjusting the slit size allows for a stable adjustment of the light amount in a short period of time.

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, holes with a high aspect ratio can be obtained. This 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.

On the other hand, laser irradiation is thought to involve not only the irradiation of light, but also melting and ablation (burning) that accompanies 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, a tapered hole that is wider on the surface side and narrower on the back side may be formed, or it may be difficult to reach the back side. This is thought to be because light is more likely to reach the processed area, causing effects such as diffuse reflection of light, and because energy is more likely to accumulate only from the surface side.

Next, a method of processing the state shown in FIG. 4(a) can be considered, but the surface side is considered to remain in a light-shielded state, the processing speed on the surface side is slow, and processing in the depth direction is difficult to progress. When processing is performed as shown in FIG. 4(a) and FIG. 4(b) as in the preferred embodiment of the present application, it is considered that a certain amount of energy reaches the focal point, and a portion where the light also reaches and sufficiently exceeds the threshold value is generated from the surface vicinity, and it is considered that processing with a high aspect ratio in the depth direction can be efficiently performed. The high aspect ratio in the present invention depends on the material and processing purpose, and can be, for example, an aspect ratio of 5 or more, 7 or more, 10 or more, 15 or more, etc. Although there is no particular upper limit, it may be set to 50 or less or about 40 or less.

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

Figure 6 is a flow diagram of an embodiment of the laser processing method of the present invention. In particular, this flow corresponds to the processing of Figure 4. First, step S12 sets the conditions of the workpiece for processing conditions. Based on these conditions, step S22 places the workpiece at the initial focus position. As a result, the focus of the expected processing area is placed at the initial focus inside the workpiece, and the top of the expected processing area is placed at 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. This is to perform a focus movement irradiation step in which the focus of the expected processing area is moved to the surface side of the workpiece after the processing laser is irradiated to the workpiece in the initial irradiation step, and the processing laser is further irradiated. This makes it possible to obtain a hole with a high aspect ratio.

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 unless the gist of the invention is changed.

[Experimental Example 1]
A laser processing test based on the present invention was carried out using a polysilicon film having a thickness of about 400 μm, which is used for silicon wafers.
The following specifications were used for the laser source and objective lens, and the diffraction limit in this case was approximately 0.87 μm.
Wavelength: 355 nm, radiation source slit setting: 0.7 μm, objective lens magnification: 100 times, objective lens NA value: 0.5

As a preliminary test, the heat amount and pulse width were 2 mJ/5 nsec, 1 mJ/5 nsec, 0.7 mJ/5 nsec, and 0.6 mJ/5 nsec, and a shot laser at about 6 Hz was used to irradiate the polysilicon film with a focal point at 10 μm intervals in the depth direction from the surface periphery to verify the processing threshold.

As a result of this verification, we decided to adopt a heat amount of 0.7 mJ as the condition under which only a portion of the Gaussian distribution exceeds the processing threshold. Note that a heat amount of 1 mJ or more would result in a large hole, while a heat amount of 0.6 mJ would make it difficult to see the hole and make processing difficult.

Based on these condition verifications, a test was conducted to create a through hole in the polysilicon film, and then an image of the part with the through hole machined 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 is possible to create a through hole of 0.79 μm, which is smaller than the theoretical diffraction limit of 0.87 μm for these conditions.

[Experimental Example 2]
The following specifications for the laser source and objective lens were used under conditions similar to those in Experimental Example 1. The diffraction limit in this case was approximately 1.3 μm.
Wavelength: 532 nm, radiation source slit setting: 0.7 μm, objective lens magnification: 100 times, objective lens NA value: 0.5

Referring to the condition verification of Experimental Example 1, a test was conducted to create a through hole in the polysilicon film. Figure 8 shows an image of the part where the through hole was created, observed with a scanning electron microscope (SEM). It was confirmed that by performing processing in this manner, it is possible to create a through hole of 0.77 μm, which is smaller than the theoretical diffraction limit of 1.3 μm.

[Experimental Example 3]
Laser processing was performed on a film for a silicon wafer with a thickness of 50 μm. A YAG laser processing machine was used with a wavelength of 355 nm, and the irradiation intensity was adjusted to adjust the expected processing area on the silicon wafer to be a diamond shape with a wide width near the focus when viewed from the side of the laser's traveling direction. This expected processing area was about 2 μm in diameter and about 50 μm in height at the focus. In addition, one shot of laser irradiation with 1 mJ was performed. The following preliminary test, reference test, and test were performed using this laser irradiation. Images of the processing results of these preliminary test, reference test, and test, and the test positions are shown in FIG. 9.

In this experimental example, an optical path adjustment unit was provided 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, making it possible to perform processing while observing the laser processing position and processing results. In addition, the silicon wafer was fixed to a vibration-proof table and placed 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.

In preliminary test (3-1) and preliminary test (3-2), the focus of the laser irradiation means was set near the surface of the silicon wafer, and 210 shots were performed for each preliminary 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 side.

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

In tests (3-1) and (3-2), 30 shots were performed under the same laser irradiation means setting conditions, with the initial focus set at a position 25 μm from the surface of the silicon wafer in the thickness direction. After that, the silicon wafer was moved away by 2 μm at a time, and the focus of the expected processing area was moved relatively toward the surface side 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. When the initial focus was moved 12 μm for six movements, a total of 210 shots were obtained, and the laser irradiation was terminated. This test was performed twice. As a result of tests (3-1) and (3-2), a hole with a diameter of about 2 μm was obtained on the surface, and this hole also reached the back surface. This is because a hole with a diameter of 2 μm was obtained for a workpiece with a thickness of 50 μm, and an aspect ratio of about 25 was achieved.

The present invention can be used to process workpieces and is industrially useful.

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 (5)

A laser irradiation means for irradiating a processing laser with adjusted laser intensity;
a distance control means for adjusting a processing distance between the laser irradiation means and the workpiece;
A measuring means for measuring the processing distance;
a setting unit that sets the laser intensity at a focus of the processing laser so that a part of the Gaussian distribution of the processing laser exceeds a processing threshold value of the workpiece. The laser processing device according to claim 1, wherein the distance control means is a height control means for adjusting the height of the workpiece holding means. the measuring means is a laser displacement meter,
an optical path adjustment unit provided between an optical path of the laser irradiation means and an optical path of the laser displacement meter;
3. The laser processing apparatus according to claim 2, wherein the laser displacement meter and the laser irradiation position of the laser irradiation means are located on the same part of the workpiece. The laser processing device according to claim 3, wherein the workpiece is a semiconductor element, and the laser processing device is for drilling a hole in the semiconductor element that is smaller than the diffraction limit of the processing laser. A laser processing method for processing a workpiece using a laser processing apparatus having a laser irradiation means for irradiating a processing laser with an adjusted laser intensity, a distance control means for adjusting a processing distance between the laser irradiation means and the workpiece, and a measurement means for measuring the processing distance,
A setting step of setting the laser intensity at a focus of the processing laser so that a part of the Gaussian distribution of the processing laser exceeds a processing threshold value of the workpiece;
a distance control step of measuring the processing distance by the distance control means and positioning the workpiece at a position exceeding the processing threshold value;
a laser irradiation step of irradiating the workpiece with the processing laser at the laser intensity.

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