JP2006114786A - Laser beam machining method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To precisely cut a machining object with higher accuracy and superior efficiency. <P>SOLUTION: A pulse laser beam L of energy intensity generating no plasma is emitted and converged above the surface 7 of the machining object 1 through an optical system (objective lens 3). By the interaction of a pulse laser beam Ld incident at a divergence angle widened to the machining object 1 with the material of the machining object 1, a V-shaped notching 5 is formed on the surface 7 of the machining object 1. Consequently, the machining object 1 can be cut by one or two times of irradiation scan. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a laser processing method, and particularly to a laser processing method suitable for cutting a workpiece.

  With the rapid development of advanced technology fields such as electronics and photonics in recent years, further miniaturization of various device elements that support these has been greatly desired. Conventionally, a mechanical cutting technique using a diamond blade or the like has been used as means for miniaturizing a semiconductor device. While these can be said to be well-established technologies, they are approaching the essential limits such as the reduction of material yield due to “cutting” and the processing resolution of miniaturization. On the other hand, instead of these mechanical processing techniques, a laser processing method is given as a fine processing technique that is rapidly progressing.

  This is a method of irradiating a processing object with high-intensity pulsed laser light and cutting the processing object starting from damage (cracks) generated by structural destruction or modification of the substance in a minute region at the focal position. is there. In this method, after damages caused by laser irradiation are arranged along a planned cutting line, mechanical stress is applied to a material to be processed, and the material is cut into a fine chip shape. This is called “dicing”.

  As a processing object, what is particularly important in practical use is a thin plate-like device made of a dielectric substrate (for example, sapphire or glass) coated with a functional semiconductor layer (for example, silicon or gallium arsenide).

  For example, in Patent Document 1, pulsed laser light having a transparent wavelength (for example, 1064 nm) (that is, not absorbing) is applied to a substrate material such as glass in an object to be processed using an objective lens. There is a method of condensing, causing damage (cracks) having a disordered shape with a size of about several tens of μm at this condensing position, and applying a stress to cut the substrate using the arranged cracks as a starting point. It is disclosed.

  In the cutting method disclosed in Patent Document 1, since the photon energy of the laser light used for processing is as small as 1/3 or less compared to the optical band gap energy of the material to be processed, damage is caused between the band gaps. This is not caused by the optical multi-photon absorption, but rather by the so-called optical breakdown (breakdown). For this reason, the size of damage becomes relatively large (several tens of μm), and there is a certain limit to the improvement of processing resolution / accuracy.

In contrast to such an internal condensing type cutting method, Patent Document 2 discloses a cutting starting point (groove in which damage is arranged) for a transparent dielectric material such as sapphire by a surface condensing type optical arrangement. A technique for processing a material surface is disclosed.
JP 2002-192370 A JP 2004-1114075 A

  However, the technique described in Patent Document 1 has the following problems.

  That is, in the technique described in Patent Document 1, laser light is condensed inside the object to be processed. As described above, the form of the material to be processed that is actually handled is a device in which a support substrate made of glass, sapphire, or the like is coated with a thin film of a semiconductor layer such as silicon or gallium arsenide. It is this support substrate that cuts the cutting starting point by laser processing, which is usually a very thin substrate having a thickness of 100 to 500 μm or less. Since the semiconductor layer is responsible for actual functions such as conduction and light emission in device operation, if these are damaged by laser, these functions will be significantly degraded. Therefore, when laser processing is performed for the purpose of cutting such a device, only the inside of this thin support substrate must be damaged very accurately without damaging the semiconductor layer.

  However, the size of laser damage is about 30 μm in the technique described in Patent Document 1. On the other hand, miniaturization of devices is an unquestionable technology flow, and it is considered that the thickness of the support substrate will be 100 μm or less in the future. This thickness value is not significantly different from the size of the damage (30 μm). Therefore, when the technique described in Patent Document 1 is used, in order to avoid damage to the semiconductor layer, it is difficult to accurately focus the beam near the center of the 100 μm thick substrate. Otherwise, the damage of the support substrate reaches the semiconductor layer itself.

  Furthermore, as described above, if the light is condensed so as to focus on the inside of a very thin support substrate (thickness of about 100 μm), a part of the laser beam that causes damage at the focus position proceeds directly inside the support substrate. However, since the focal position and the semiconductor layer are close to several tens of μm, the semiconductor layer may have a sufficient spatial energy density to damage the semiconductor layer. As a result, the direct damage of the device layer due to such laser light can be easily imagined.

  As described above, the technique described in Patent Document 1 has a big problem of increased damage.

  Therefore, in order to avoid the above-described problem caused by the technique described in Patent Document 1, the laser beam condensing position may be placed at a position farther from the semiconductor layer in the depth direction. In the case of irradiating the laser from the support substrate side, this corresponds to matching the light collection position (focal position) near the surface of the support substrate. That is, if damage is caused on the surface of the workpiece, the damage is arrayed along a planned cutting line, a fine groove of the damage is cut, and then mechanical stress is applied to perform cutting (dicing). Good. At this time, in order to achieve precise dicing along the desired processing line and reducing “cutting margin” as much as possible, the cross-section of the damage (groove) is sharp, so-called “V-shaped”. Clearly this is desirable.

  As described above, Patent Document 2 discloses a transparent dielectric such as sapphire by using an optical arrangement of a surface focusing type of a processing object instead of an internal focusing type of the processing object as disclosed in Patent Document 1. A technique for processing a cutting start point (a groove in which damage is arranged) on a surface of a body material is disclosed.

  According to Patent Document 2, the occurrence of damage of 30 μm or less in the horizontal width is achieved on the surface of the workpiece. Here, the “horizontal direction” refers to a direction parallel to the substrate surface and perpendicular to the optical axis direction. Further, the size in the depth direction of damage varies from several μm to about 100 μm depending on the number of laser pulses irradiated. However, this horizontal size (30 μm) does not satisfy the standard for fine dicing resolution. Generally, 10 μm or less is required. Further, in order to increase the depth of damage, that is, to increase the aspect ratio of damage, a large number of laser pulse irradiations are required, and high-throughput processing is difficult. Further, Patent Document 2 does not present any micrographs that clearly show the form / structure of the damage that occurs, and does not cause damage that is suitable for dicing, that is, V-shaped damage. This can be judged from the size of the damage in the horizontal / depth direction.

In Patent Document 2, the energy density of the laser beam at the condensing position ranges from 100 J / cm ² per pulse to remarkably 100 kJ / cm ² . At such an extremely high light energy intensity, it is considered that plasma is generated at the irradiation site in any material. When plasma is generated, the size of the damage greatly exceeds the desired size due to high temperature plasma thermal diffusion, collision process of free electrons having high kinetic energy, and the like. This is considered to be the main cause of the large damage of 30 μm presented in the embodiment of Patent Document 2. Of course, such a large damage is not preferable for precise dicing.

  The present invention has been made in view of such points, for example, causes damage having a fine and sharp shape on the surface of a transparent dielectric material substrate such as sapphire, which is a typical semiconductor device substrate, In addition, there is provided a laser processing method capable of efficiently and precisely cutting a material substrate with higher accuracy while easily avoiding damage to a device layer such as a semiconductor located on the back surface of the substrate. For the purpose.

  The present invention contemplates that the pulsed laser beam having an energy intensity that does not generate plasma is focused and irradiated on the surface of the object to be processed through an optical system, and is incident on the object to be processed with a wide divergence angle. Due to the interaction between the light and the material of the workpiece, a V-shaped damage is formed on the surface of the workpiece.

  According to the present invention, for example, a semiconductor having a fine and sharp shape on the surface of a transparent dielectric material substrate such as sapphire, which is a typical semiconductor device substrate, and located on the back surface of the substrate Thus, it is possible to obtain a laser processing method capable of efficiently and precisely cutting the material substrate while avoiding damage to the device layer.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  In order to improve the processing accuracy, the present inventor needs to control the size and shape of the damage, and also the aspect ratio of the shape (ratio of the size of the damage in the horizontal and vertical directions). I found it. Also, in order to form a damaged groove that is the starting point of precise cutting, and also to achieve this by one or at most two irradiation scans, the size and shape of the damage, and also the aspect ratio in the shape, are set. I found it necessary to control. For these controls, it is essential to avoid the generation of plasma, and the mechanism that triggers multiphoton absorption is optimal for the occurrence of damage, and for this purpose, the pulse energy of the laser is optimized. I found that it was necessary.

  Furthermore, the present inventor has found that control of the focal position of the laser beam is extremely important in order to control the size and shape of the damage and the aspect ratio in the shape. That is, as in the prior art described above, the light is condensed not on the inside or on the surface of the material but on the upper outside at a distance (d) within a certain range from the surface of the material, and is incident on the material at an angle where the beam divergence spreads. It was found for the first time that a damage having a sharp V-shape with a small size and a high aspect ratio was formed. The inventor examined the d-dependence of the size and shape of the damage in detail, and considered the beam angle of the beam and the damage occurrence process. Based on these completely new damage generation processes, the present inventor has conceived the present invention described in detail below.

  In the present invention, pulsed laser light having energy intensity that does not generate plasma is irradiated so as to be focused above the surface of the material to be processed, thereby causing sharp and fine damage to the surface of the material.

  First, the principle of the present invention will be described.

  There are two possible laser damage mechanisms in solids: 1) avalanche and 2) multiphoton absorption. “Electronic avalanche” here is almost synonymous with “electrical breakdown”. The modification / destruction inside the workpiece due to the dielectric breakdown is accompanied by difficulty in controlling the region. In other words, internal cracks caused by dielectric breakdown are large in diameter, and irregular irregularities are generated in the peripheral region thereof, so that it is considered unsuitable for precise and fine processing / modification.

  When the wavelength of the laser used is longer than 1060 nm, it is understood that the destruction theory by “electron avalanche” of 1) above is applied. On the other hand, the laser frequency increases (that is, the wavelength decreases), or the band gap Eg of the material decreases, and the relationship between the photon energy hν and the band gap Eg is hν> Eg / 3. Then (that is, when the energy of three photons exceeds the band gap), the destruction mechanism is considered to be a mechanism based on “multiphoton absorption” rather than “electron avalanche”. In other words, destruction in a pure multiphoton process is hardly important beyond the three-photon process, and it can be said that the four-photon absorption and the five-photon absorption are practically negligible.

  For example, when a laser beam having a wavelength of 1064 nm with photon energy hν = 1.165 eV is used and the material to be processed is silicon (band gap Eg≈1.12 eV) or Pyrex (registered trademark) glass (Eg≈4 eV or more). Does not cause multiphoton absorption. The reason is that, in silicon, since the one-photon energy is already almost equal to the band gap, it is considered that damage is induced by simple one-photon absorption rather than multiphoton absorption, and Pyrex (registered trademark) This is because, in glass, the above relationship hν> Eg / 3 does not hold in the first place. Even when sapphire (Eg≈8 eV) having a photon energy of hν = 1.165 eV is irradiated to the processing target sapphire (Eg≈8 eV), the relationship of hν> Eg / 3 does not hold. In this case, 7hν≈Eg, and 7-photon absorption is required to induce multi-photon absorption, but such multi-photon absorption is practically negligible. In other words, the laser damage mechanism in these cases is considered to be due to the “electron avalanche” destruction mechanism rather than “multi-photon absorption”.

  In the present embodiment, laser light having a wavelength of 355 nm with photon energy hν = 3.5 eV is used, and this is irradiated onto sapphire that is a workpiece. The band gap Eg of sapphire is about 8 eV. In this case, a relationship of hν> Eg / 3 is established. The relationship of hν> Eg / 3 is the same when the Pyrex (registered trademark) glass having a band gap Eg of about 4 eV or more is irradiated with laser light having a wavelength of 355 nm.

  As shown in Examples described later, when sapphire was irradiated with laser light having a wavelength of 355 nm, a small crack of 1/10 or less was formed compared to the case where laser light having a wavelength of 1064 nm was used. This is because the occurrence of cracks is due to laser damage induced by “multiphoton absorption”.

  The laser beam used in this embodiment has a Gaussian beam characteristic. Here, the Gaussian beam will be described. Usually, the intensity distribution of laser light in the direction perpendicular to the optical axis follows a Gaussian function, and such laser light is called a Gaussian beam. In the Gaussian beam, the light intensity is highest at the center of the beam, and the light intensity decreases toward the periphery. When a Gaussian beam travels through a transparent material such as glass or sapphire, if the light intensity is low, the laser beam only travels straight, and even if it is collected by a lens, it is collected at an angle according to normal geometric optics. Light is emitted.

  FIG. 1A shows a state in which a pulse laser beam L having the characteristics of a Gaussian beam is irradiated on the surface of the workpiece 1 after the focal point P is once focused by the objective lens 3. At this time, the distance between the condensing position (focal position) P of the pulse laser beam L and the surface of the workpiece 1 is defined as “d”. The optimum value of d depends on the precondition. According to experiments, the value of d suitable for occurrence of V-shaped damage is in the range of 1 to 100 μm. For example, when sapphire is used as the material to be processed, it is preferably in the range of 4 to 20 μm. Therefore, in the present embodiment, d is 20 μm or less. Since the pulse laser beam L is already focused before reaching the workpiece 1, when the pulse laser beam L reaches the workpiece 1, the beam becomes a beam Ld having a spread divergence, and the light intensity is optimized. A sharp and fine “V-shaped” damage 5 is formed on the surface of the workpiece 1. The workpiece 1 is, for example, a sapphire substrate (for example, a two-layer structure including a sapphire layer 1a and a GaN layer 1b).

  FIG. 1B is a detailed view of the main part of FIG. 1A, and is a diagram illustrating in more detail how sharp and fine V-shaped damage occurs.

  When the pulsed laser light L, which is a Gaussian beam, is condensed by the objective lens 3, the beam shape in the vicinity of the focal position P is reduced to an approximate parallel light Lw called a beam waist, and then has a certain spread divergence. The beam Ld is applied to the workpiece 1. Again, the laser beam Ld does not lose its characteristics as a Gaussian beam, and its energy density increases as it approaches the center line C of the beam. Since the laser beam Ld has a spread angle, the beam diameter diverges and the spatial energy density decreases as the penetration depth increases after entering the workpiece 1. Therefore, on the surface 7 of the workpiece 1, since the spatial energy density of the laser beam Ld is large, most of the vicinity of the beam spot is damaged. However, since the spatial energy density decreases as the laser beam Ld enters, the damage occurs only in the vicinity of the beam center line C having a high light energy intensity. Further, if the laser beam Ld penetrates deeper into the workpiece 1, the spatial energy density of the laser beam Ld becomes even weaker, and damage is caused by the spatial energy density of the laser beam Ld becoming the threshold value of the energy density of damage. It will only occur at the higher beam center point 9. That is, as the laser beam Ld penetrates into the workpiece 1, the horizontal size of the damage gradually decreases. By this action, a fine and sharp “V-shaped” damage 5 is formed on the surface 7 of the workpiece 1.

  Such a V-shaped damage 5 is not formed by scanning a plurality of times of beam irradiation along a predetermined scheduled cutting line, but is performed once for one scheduled scanning line or at most twice. It is also a feature of the present invention that it is sufficiently caused by the beam scanning. That is, the V-shaped damage can be formed deeper by the second irradiation. This is because a mechanism similar to the first V-shaped damage formation described above occurs. Since the second irradiation has a certain depth due to the first irradiation, the distance d is made smaller than that at the first irradiation, for example, damage is caused at a position slightly deeper by several μm. Like that.

  In the above damage formation process, the concern is the generation of air plasma (air breakdown) at the focal point in the beam waist where the spatial energy density of the pulse laser beam is the highest. The plasma generated directly above the workpiece 1 may damage the surface 7 of the workpiece 1. In this case, the attack of free electrons having high thermal energy and high kinetic energy of the plasma will increase the size of the damage and greatly disturb its shape. Therefore, in order to form precise and shape-controlled damage, such plasma generation must be avoided even if it is not.

  In order to avoid plasma generation, the pulse energy E at the stage of passing through the irradiation optical system / objective lens must be sufficiently reduced. Specifically, when the laser is a nanosecond pulse oscillation YAG laser with a wavelength of 355 nm and the object to be processed is sapphire, E is preferably 10 μJ or less (measured after passing through the objective lens). Furthermore, in order to reduce the spatial energy density of the pulsed laser beam at the focal position P, it is necessary to increase the spatial volume of the beam waist portion Lw. For this purpose, the numerical aperture (NA: Numerical Aperture) of the focusing objective lens 3 is required. ) Must be reduced, and the specific NA value is preferably 0.4 or less. That is, the length (L) of the beam waist in the depth direction depends on the numerical aperture (NA) of the objective lens, and the L value decreases as the NA value increases. For example, when NA = 0.25, L = 4 μm, and when NA = 0.4, L = 2.8 μm. In the present embodiment, the range of the NA value of the objective lens that can be used is 0.1 to 0.8, and for more preferable V-shaped processing, NA = 0.25 to 0.4. preferable.

  The formation of the damage can be controlled according to the processing mode. That is, in the “V-shaped” damage, the V-shaped width (horizontal size) and the V-shaped height (depth direction size) are controlled to a desired size. For that purpose, the wavelength, focal position, and pulse energy of the pulse laser beam, the numerical aperture of the objective lens, and the number of times of beam scanning (one or two times) may be controlled so that the damage has a desired optimum shape. .

  FIG. 2 is a block diagram showing a configuration of a laser processing apparatus that realizes a laser processing method according to an embodiment of the present invention.

  The laser processing apparatus 100 is an apparatus that cuts an object to be processed using a specific laser beam based on the above principle. For example, the laser processing apparatus 100 includes a laser light source 101, a telescope optical system 103, a polarizing plate 105, a dichroic mirror. 107, objective lens 109, protective window plate 111, stage 113, measurement light source 115, beam shaper 117, half mirror 119, photodetector 121, controller 123, illumination light source 125, CCD camera 127, computer 129, and A monitor 131 is provided. An object 1 to be processed by the laser processing apparatus 100 is a two-layer structure made of, for example, a sapphire / GaN layer.

  The laser light source 101 generates laser light for processing. As the laser light source, a nanosecond pulse oscillation pulse laser capable of efficiently causing multiphoton absorption with respect to sapphire is used. For example, the laser light source 101 is an Nd: YAG laser that generates pulsed laser light having a wavelength of 355 nm, a pulse width of 10 nanoseconds (variable from 1 to 50 nanoseconds), and an oscillation repetition frequency of 50 to 300 kHz. As described above, sapphire is transparent to 355 nm laser light (ie, has no absorption).

In addition to the Nd: YAG laser, a laser that can be used for the laser light source 101 includes an Nd: YVO ₄ laser, an Nd: YLF laser, a titanium sapphire laser, and the like. Moreover, as a wavelength to be used, in addition to 355 nm that induces three-photon absorption with respect to sapphire, 266 nm that induces two-photon absorption with respect to sapphire may be used. Or near infrared light (for example, 1064 nm).

  The telescope optical system 103 optimizes the beam diameter of the processing laser light output from the laser light source 101 in order to obtain a preferable processing shape.

  In order to obtain a preferable processing shape, the polarizing plate 105 adjusts the laser beam that has passed through the telescope optical system 103 to linearly polarized light that is parallel / perpendicular to the processing line or circularly polarized light.

  The dichroic mirror 107 is a mirror that reflects almost 100% of the processing laser light that has passed through the polarizing plate 105 and transmits almost 100% of the measurement laser light from the measurement light source 115.

  The objective lens 109 is an objective lens for a microscope and condenses the processing laser light reflected by the dichroic mirror 107. Condensing characteristics vary depending on the numerical aperture (NA) of the objective lens 109. In the present embodiment, as described above, the objective lens 109 with NA <0.4 is used. In addition, a condensing position is located in front of a workpiece.

  The protective window plate 111 is provided in order to protect the objective lens 109 from minute debris scattered from the surface by processing when the surface of the workpiece 1 is processed.

  The stage 113 has a mounting table (not shown), and the workpiece 1 to be irradiated with the laser beam condensed by the objective lens 109 is mounted on the mounting table. The stage 113 has a drive mechanism (not shown) that can move the mounting table in the XYZ axis directions and rotate around the XYZ axes. With this drive mechanism, the stage 113 causes the workpiece 1 on the stage 113 to be damaged along the planned machining line (XY axis direction) and the planned machining position (Z axis direction). Driven (translated and rotated).

  Specifically, the Z-axis direction is a direction orthogonal to the surface of the workpiece 1, that is, a direction parallel to the laser beam incident on the workpiece 1 (depth direction of the workpiece 1). By moving the stage 113 in the Z-axis direction, the condensing position of the laser beam with respect to the workpiece 1 can be adjusted to a predetermined position in the Z-axis direction. Further, the scanning of the irradiation position of the laser beam on the workpiece 1 is performed by moving the stage 113 in the XY axis direction (that is, the horizontal direction). The tilt control of the stage 113 is performed by rotating the stage 113 around the XYZ axes. By such a stage 113, the position and orientation of the workpiece 1 are three-dimensionally controlled. FIG. 3 schematically shows that the front surface of the workpiece 1 is processed.

  The measurement light source 115 generates laser light for measuring the position of the surface of the workpiece 1 on the stage 113.

  The beam shaper 117 adjusts the beam shape of the laser light output from the measurement light source 115 in order to optimize the measurement laser light.

  The half mirror 119 is a mirror that reflects / transmits the measurement laser beam translucently. The measurement laser light that has passed through the beam shaper 117 passes through the half mirror 119, the dichroic mirror 107, and the objective lens 109, reaches the surface of the workpiece 1, and is reflected. This reflected light passes through the objective lens 109 and the dichroic mirror 107 again, and a part of the reflected light is reflected by the half mirror 119 and reaches the photodetector 121.

  The photodetector 121 detects the reflected light from the surface of the workpiece 1 to detect the surface position of the workpiece 1. The detection result is output to the controller 123.

  The controller 123 has a feedback circuit, and based on the information on the surface position of the workpiece 1 obtained by the photodetector 121, the condensing position of the processing laser beam is in the planned processing line XY axis direction) and the planned processing. The stage 113 is feedback controlled so as to match the position (Z-axis direction).

  The illumination light source 125 is disposed below the stage 113 and generates illumination light for observing the processing portion of the processing target 1 on the stage 113.

  The CCD camera 127 takes in the illumination light emitted from the illumination light source 125 and transmitted through the processing object 1, images the processing site of the processing object 1, and outputs the imaging data to the computer 129.

  The computer 129 is connected to the laser light source 101, the measurement light source 115, the controller 123, and the CCD camera 127, and comprehensively controls these units. For example, the computer 129 drives the stage 113 through feedback control by the controller 123 in accordance with a predetermined program, thereby scanning the condensing position of the laser light along any scheduled processing line and planned processing position.

  The monitor 131 displays an image captured by the CCD camera 127. That is, the processing part of the processing object 1 (V-shaped damage formed on the surface) is observed by the CCD camera 127 and the monitor 131.

  In addition to the sapphire, the processing object 1 may be glass such as silica glass (Eg≈9 eV) or Pyrex (registered trademark), diamond (Eg≈5.5 eV), or the like.

  Next, a processing process using the laser processing apparatus 100 having the above configuration will be described with reference to a flowchart shown in FIG. Note that the explanatory diagram shown in FIG. 4 is also referred to as appropriate. Here, the workpiece 1 is a two-layer structure including a sapphire layer 1a and a GaN layer 1b.

  First, in step S1000, the optimum laser intensity of the laser light source 101 for the workpiece 1 is determined. As described above, 355 nm laser light can efficiently induce multiphoton absorption. As described above, sapphire is suitable for cutting the workpiece 1 without generating plasma by using laser light having an energy of 10 μJ or less per pulse (measured after passing through the objective lens). Damage can occur. By using such an optical arrangement and laser light with low pulse energy, optical damage to the semiconductor layer (GaN layer 1b) can be largely avoided.

  In step S1100, the condensing position of the processing laser beam is determined. In order to form sharp V-shaped damage, the laser beam is focused on the workpiece 1 (see FIG. 4A). The distance d between the focal position and the substrate surface is preferably 20 μm or less, more preferably 10 μm or less.

  In step S1200, the computer 129 is programmed for the line to be cut.

  In step S1300, the workpiece 1 is placed on the stage 113 and placed. At this time, the measurement light source 115 and the illumination light source 125 are turned on.

  In step S1400, the laser light source 101 is turned on, and the cutting target line of the workpiece 1 is irradiated with 355 nm laser light (see FIG. 4A). Then, the stage 113 is scanned in the XY axis direction (horizontal direction) along the planned cutting line, and the damage 11 is formed on the surface of the workpiece 1 along the planned cutting line (see FIG. 4B). ).

  In step S1500, in order to ensure the cutting, the second laser irradiation is performed along the same scheduled cutting line as in step S1400. At this time, the light collection position is slightly changed as compared with the case of step S1400. As a result, the aspect ratio of the V-shaped damage can be improved. According to the experiment, by such a process, a fine damage 11 having a width of 10 μm and a depth of 50 μm and having a high aspect ratio and a sharp V-shaped cross section can be carved on the surface.

  Note that the operation in step S1500 is optional and need not always be performed. However, depending on the thickness of the workpiece 1, it is possible to reliably cut by adding this process.

  That is, through step S1400 and step S1500, fine damage 11 having a sharp V-shaped cross section is formed on the front surface of the workpiece 1 (see FIG. 4A). By performing such a laser beam irradiation process while scanning the workpiece 1 along the planned cutting line in the horizontal direction, damage 11 along the planned cutting line is formed (FIGS. 4B and 4). (See (C)).

  In step S1600, cutting is performed using the damage formed on the surface of the workpiece 1 along the planned cutting line through steps S1400 and S1500. That is, if a mechanical stress is applied to the workpiece 1, desired precision cutting is achieved. As a result, the workpiece 1 (here, a sapphire substrate / device plate made of a GaN layer) is cut (divided) into minute chips.

  As described above, according to the present embodiment, the laser beam capable of inducing multiphoton absorption reliably and efficiently is used, and the focal position is set outside (above) the workpiece. Even for crystals, fine damage with a sharp V-shaped cross section can be achieved by one or at most two irradiation scans.

  For example, according to the present embodiment, precise cutting can be performed on sapphire with an accuracy of 10 μm or less. At this time, the width of the formed V-shaped crack is about 10 μm, the cutting margin can be reduced to 5 μm or less, and the yield can be greatly improved.

  The inventor conducted an experiment to demonstrate the effect of the present invention (the advantage over the inventions described in Patent Document 1 and Patent Document 2). In addition, this invention is limited to a following example and is not interpreted.

In this example, an experiment was performed in which a V-shaped groove was formed on the surface of a workpiece to be cut using the laser processing method of the present invention. The conditions are as follows.
・ Processing object: Sapphire / GaN (Laser irradiation from sapphire side)
・ Laser: Nd: YAG laser with a wavelength of 355 nm, pulse width: 10 nanoseconds ・ Laser output: 6.3 μJ (measured at the sample position after passing through the objective lens per pulse)
・ Laser pulse oscillation repetition frequency: 130 kHz
Objective lens: numerical aperture (NA) = 0.28
・ Number of beam scans: 1 time ・ Sample stage drive speed: 10 mm / s
・ Irradiation focusing position: 6μm above the sapphire substrate

  5A is a photomicrograph showing a cross section of damage formed under the above conditions, FIG. 5B is a photomicrograph showing the upper surface (sapphire side) of the damaged groove, and FIG. 5C is an object to be processed. It is a microscope picture which shows the back surface (GaN side) of a thing. As can be seen from FIG. 5A, in this example, damage having a sharp V-shaped cross section was observed, the width was 10 μm, and the depth was 33 μm. Further, FIG. 5B shows that a damaged groove having such a cross section is carved in the sapphire substrate. Further, from FIG. 5C, it can be seen that the back surface on the irradiation side, that is, the GaN device layer is not damaged at all.

  FIG. 6A is a micrograph showing a cut surface when the workpiece is actually cut by mechanical stress starting from the damage, and FIG. 6B is an enlarged photograph thereof.

  According to the present embodiment, no crack growth or destruction is observed at any place other than the damage carved by the laser processing method of the present invention, and the depth of damage is maintained at a constant value (33 μm). It can be seen that the technology according to the present invention has high machining accuracy and machining shape controllability.

In this example, an experiment was performed in which a V-shaped groove was formed on the surface of a workpiece to be cut using the laser processing method of the present invention under the following conditions.
・ Processing object: Sapphire / GaN (Laser irradiation from sapphire side)
・ Laser: Nd: YAG laser with a wavelength of 355 nm, pulse width: 10 nanoseconds ・ Laser output: 4 μJ (measured at the sample position after passing through the objective lens per pulse)
・ Laser pulse oscillation repetition frequency: 130 kHz
Objective lens: numerical aperture (NA) = 0.28
・ Number of beam scans: 2 times ・ Sample stage drive speed: 9 mm / s
・ Irradiation condensing position: First scan at a focal point 20 μm above the sapphire substrate, then second scan at a focal point 8 μm above

  7A is a photomicrograph showing a cross section of damage formed under the above conditions, FIG. 7B is a photomicrograph showing the upper surface (sapphire side) of the damaged groove, and FIG. 7C is an object to be processed. It is a microscope picture which shows the back surface (GaN side) of a thing. As can be seen from FIG. 7A, also in this example, damage having a sharp V-shaped cross section was observed, the width was 13 μm, and the depth was 33 μm. Although the pulse energy of Example 2 is smaller than that of Example 1, it can be seen that similar damage could be caused by two irradiation scans. Further, FIG. 7B shows that a damaged groove having such a cross section is carved in the sapphire substrate. Further, from FIG. 7C, it can be seen that the back surface on the irradiation side, that is, the GaN device layer is not damaged at all. Therefore, it can be seen from this example that the technique according to the present invention has high machining accuracy and machining shape controllability.

In this example, an experiment was performed in which a V-shaped groove was formed on the surface of a workpiece to be cut using the laser processing method of the present invention under the following conditions.
・ Processing object: Sapphire / GaN (Laser irradiation from sapphire side)
・ Laser: Nd: YAG laser with a wavelength of 355 nm, pulse width of 10 nanoseconds ・ Laser output: 6.3 μJ (measured at the sample position after passing through the objective lens per pulse)
・ Laser pulse oscillation repetition frequency: 130 kHz
Objective lens: numerical aperture (NA) = 0.28
・ Number of beam scanning: 2 times ・ Sample stage driving speed: 10 mm / s
・ Irradiation condensing position: First scan at a focal point 10 μm above the sapphire substrate, then second scan at a focal point 4 μm above

  8A is a photomicrograph showing a cross section of damage formed under the above conditions, FIG. 8B is a photomicrograph showing the upper surface (sapphire side) of the damaged groove, and FIG. 8C is a processing target. It is a microscope picture which shows the back surface (GaN side) of a thing. As can be seen from FIG. 8A, in this example, damage having a sharp V-shaped cross section was observed, the width was 10 μm, and the depth was 50 μm. As described above, when the number of beam scans is set to two with the same pulse energy as in the first embodiment, it is possible to carve V-shaped damage having a very sharp and high aspect ratio. Further, FIG. 8B shows that a damaged groove having such a cross section is carved in the sapphire substrate. Further, FIG. 8C shows that there is no damage at all on the back side of the irradiation side, that is, the GaN device layer.

  FIG. 9A is a photomicrograph showing a cut surface when the workpiece is actually cut by mechanical stress starting from the damage, and FIG. 9B is an enlarged photo thereof.

  According to the present embodiment, no crack growth or destruction is observed at any place other than the damage carved by the laser processing method of the present invention, and the damage depth is maintained at a constant value (50 μm). It can be seen that the technology according to the present invention has high machining accuracy and machining shape controllability.

  The laser processing method according to the present invention causes, for example, damage having a fine and sharp shape on the surface of a transparent dielectric material substrate such as sapphire which is a typical semiconductor device substrate, and on the back surface of the substrate. This is useful as a laser processing method capable of efficiently and precisely cutting a material substrate while avoiding damage to a device layer such as a semiconductor that is positioned easily.

(A) The figure which shows the state which condensed and irradiated the pulse laser beam above the surface of the workpiece, (B) The figure which shows the detail of the principal part The block diagram which shows the structure of the laser processing apparatus which implement | achieves the laser processing method concerning one embodiment of this invention The flowchart which shows the process in one embodiment of this invention Schematic by process showing processing steps in one embodiment of the present invention (A) Micrograph showing the cross section of damage in Example 1 of the present invention, (B) Micrograph showing the upper surface (sapphire side) of the damaged groove, (C) Microscope showing the back surface (GaN side) of the workpiece. Photo (A) Micrograph showing the cut surface in Example 1 of the present invention, (B) Enlarged photo (A) Micrograph showing the cross section of damage in Example 2 of the present invention, (B) Micrograph showing the upper surface (sapphire side) of the damaged groove, (C) Microscope showing the back surface (GaN side) of the workpiece. Photo (A) Micrograph showing a cross section of damage in Example 3 of the present invention, (B) Micrograph showing the upper surface (sapphire side) of the damaged groove, (C) Microscope showing the back surface (GaN side) of the workpiece. Photo (A) Micrograph showing the cut surface in Example 3 of the present invention, (B) Enlarged photo

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Processing object 3,109 Objective lens 5,11 Damage 7 Processing object surface 100 Laser processing apparatus 101 Laser light source 103 Telescope optical system 105 Polarizing plate 107 Dichroic mirror 109 Objective lens 111 Protection window plate 113 Stage 115 Measurement light source 117 Beam shaper 119 Half mirror 121 Photo detector 123 Controller 125 Illumination light source 127 CCD camera 129 Computer 131 Monitor L Pulse laser beam P Condensing position (focal position)

Claims (5)

  Through the optical system, the pulse laser beam with energy intensity that does not generate plasma is condensed and irradiated above the surface of the object to be processed, and the pulse laser light that is incident on the object to be processed with a wide divergence angle and the process A laser processing method characterized by forming a V-shaped damage on the surface of an object to be processed by interaction with the material of the object.   The laser processing method according to claim 1, wherein the irradiation scan of the pulse laser beam is performed once or twice.   When the irradiation scan of the pulse laser beam is performed twice, the focused position of the pulse laser beam in the second irradiation scan is closer to the processing object than the focused position of the pulse laser beam in the first irradiation scan. The laser processing method according to claim 2, wherein the laser processing method is changed so as to approach the surface.   When the material of the workpiece is sapphire, the distance d between the surface of the workpiece and the focused position of the pulse laser beam is in the range of 4 μm ≦ d ≦ 20 μm. The laser processing method as described.   2. The laser processing method according to claim 1, wherein the numerical aperture NA of the lens in the optical system is in a range of 0.1 ≦ NA ≦ 0.8.