LASER PROCESSING METHOD

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser processing method of removing a part of an object to be processed using laser light.

2. Description of the Related Art

Up to now, when holes or grooves are to be formed in a substrate, for example, a semiconductor material substrate, a glass substrate, or a piezoelectric material substrate, which is an object to be processed, the substrate is irradiated with laser light by a generally known laser processing method for removal and processing. In such a kind of laser processing method, a laser light irradiation time necessary for laser processing of the substrate is normally determined based on a result obtained by trial processing. However, even when a correct laser light irradiation time is determined based on the result obtained by trial processing, a removal depth may be fluctuated due to fluctuations in thickness of the substrate and surface state.

As measures to solve such a problem, there may be conceived such a laser processing method described in Japanese Patent Application Laid-Open No. H02-092482. In the laser processing method, a substrate is made of different materials, for example, an insulating material and a metal material. When a change in reflectance of laser light on the metal material is detected while the insulating material is laser-processed, processing with the laser light is stopped. Therefore, holes may be formed only in the insulating material.

However, in the conventional laser processing method described above, the object to be processed is an object including a processed portion and a non-processed portion which are made of different materials (for example, printed circuit board in which steel material is buried). When the object to be processed is made of, for example, a single material (for example, Si wafer), the laser processing method described above cannot be applied. In the conventional laser processing method described above, a processing shape is determined depending on a shape of the non-processed portion, and hence the degree of freedom of processing is low.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a laser processing method, which is capable of improving precision of a processing shape and has a higher degree of freedom of the processing shape.

The laser processing method according to the present invention includes: the modified layer forming step of forming a modified layer which becomes a boundary of a laser processing region by scanning an inner portion of an object to be processed, with a condensing point of first laser light; and the removing/processing step of removing and processing the laser processing region defined by the modified layer by irradiating a surface of the object to be processed, with second laser light which is condensed.

According to the present invention, a laser processing speed of the modified layer formed in the object to be processed in a modified layer forming step is lower than a laser processing speed of a non-modified region. Therefore, when the removal processing is performed in a removing/processing step, a processing shape may be determined by the modified layer. Thus, the precision of processing in a case where the object to be processed is removed using the second laser light is improved. The modified layer may be formed into an arbitrary shape using the first laser light, and hence the degree of freedom of the processing shape is improved.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic structure of a laser processing apparatus according to a first embodiment of the present invention.

FIGS. 2A, 2B, and 2C illustrate a modified layer forming step controlled by a control device of the laser processing apparatus according to the first embodiment of the present invention, in which FIG. 2A illustrates a state in which a substrate is scanned with modification laser light, FIG. 2B illustrates a state in which a modified layer is formed in the substrate, and FIG. 2C is a plan view illustrating the substrate.

FIGS. 3A, 3B, and 3C illustrate a removing/processing step controlled by the control device of the laser processing apparatus according to the first embodiment of the present invention, in which FIG. 3A illustrates a state in which a substrate is scanned with processing laser light, FIG. 3B illustrates a state in which the substrate is scanned with the processing laser light to continue processing, and FIG. 3C illustrates a state in which the substrate is scanned with the processing laser light to complete processing.

FIGS. 4A and 4B illustrate a recess portion formed by a laser processing method, in which FIG. 4A is a cross sectional view illustrating the recess portion in the substrate and FIG. 4B is a plan view illustrating the substrate.

FIGS. 5A, 5B, and 5C illustrate a modified layer forming step controlled by a control device of a laser processing apparatus according to a second embodiment of the present invention, in which FIG. 5A illustrates a state in which a substrate is scanned with modification laser light, FIG. 5B illustrates a state in which a modified layer is formed in the substrate, and FIG. 5C is a plan view illustrating the substrate.

FIGS. 6A and 6B illustrate a removing/processing step controlled by the control device of the laser processing apparatus according to the second embodiment of the present invention, in which FIG. 6A illustrates a state in which a substrate is scanned with processing laser light and FIG. 6B illustrates a state in which the substrate is scanned with the processing laser light to continue processing.

FIGS. 7A and 7B illustrate a through portion formed by a laser processing method, in which FIG. 7A is a cross sectional view illustrating the through portion in the substrate and FIG. 7B is a plan view illustrating the substrate.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention are described in detail with reference to the attached drawings.

First Embodiment

FIG. 1 illustrates a schematic structure of a laser processing apparatus according to a first embodiment of the present invention. A laser processing apparatus 100 illustrated in FIG. 1 has a structure capable of irradiating a substrate W which is an object to be processed with laser light to form, in the substrate W, a recess portion, for example, a dot (hole) or a linear or curved groove. The laser processing apparatus 100 includes a laser oscillator 1, a condensing lens 2 (12), an XY-stage 3, and a control device 4. A YAG laser, a CO₂laser, an excimer laser, a solid laser, or a dye laser is used as the laser oscillator 1. The substrate W which is the object to be processed is set on the XY-stage 3. The condensing lens 2 (12) is provided between the laser oscillator 1 and the XY-stage 3 (substrate W). The control device 4 controls a laser light emission time and laser light emission timing of the laser oscillator 1, and controls the movement of the XY-stage 3 in an X-axis direction and a Y-axis direction. Further, the control device 4 controls the movement of the condensing lens 2 (12) in a Z-axis direction. Therefore, a condensing point of the laser light which is produced by the condensing lens 2 (12) may be shifted in the X-axis direction, the Y-axis direction, and the Z-axis direction relative to the substrate W. An XYZ-stage instead of the XY-stage 3 may be used to move the substrate W in the Z-axis direction. Note that, each of the X-axis direction and the Y-axis direction is a direction parallel to a surface of the substrate W and the Z-axis direction is a direction perpendicular to the surface of the substrate W.

The laser light emitted from the laser oscillator 1 is condensed by the condensing lens 2 (12) and the substrate W is irradiated with the condensed laser light. In the first embodiment, the substrate W is moved in the X-axis direction and the Y-axis direction to scan the substrate W with the laser light. When the substrate W is to be scanned with the laser light, for example, a mirror may be used to shift the laser light in the X-axis direction and the Y-axis direction relative to the substrate W. The substrate W is made of a single material. Examples of the substrate W include a semiconductor material substrate (for example, silicon wafer), a piezoelectric material substrate made of LiTaO₃, and a glass substrate.

In the first embodiment, laser processing for the substrate W is controlled by the control device 4. The laser processing method broadly includes a modified layer forming step of forming a modified layer in an inner portion of the substrate W and a removing/processing step of performing removal processing on a laser processing region defined by the modified layer.

In the modified layer forming step, modification laser light which is first laser light is condensed by the condensing lens 2 and the substrate W is irradiated with the condensed modification laser light to form the modified layer in the inner portion of the substrate W. Laser light transparent to the substrate W is used as the modification laser light. To be specific, the modification laser light is desired to have a characteristic of “(transmittance on substrate W)>(absorption coefficient on incident surface of substrate W)”. In the removing/processing step, the substrate W is irradiated with processing laser light which is second laser light to form a recess portion in the substrate W. The laser removal processing is performed so that the processing laser light is condensed by the condensing lens 12 for irradiation to melt and vaporize (or ablate) a part of the substrate W.

In the first embodiment, the modification laser light and the processing laser light are emitted from the common laser oscillator 1. The condensing lens 2 used for the modified layer forming step is changed to the condensing lens 12 in the removing/processing step. The laser oscillator may be changed to another laser oscillator between the modified layer forming step and the removing/processing step. The modification laser light and the processing laser light may have the same property.

The modified layer forming step is specifically described with reference to FIGS. 2A, 2B, and 2C. As illustrated in FIG. 2A, a condensing point L_S1of modification laser light L1 emitted from the laser oscillator 1 is set in the inner portion of the substrate W by the condensing lens 2. The condensing point L_S1set in the inner portion of the substrate W and a region close to the condensing point have a higher energy density than a region except the condensing point L_S1and the close region, through which the modification laser light L1 passes. Therefore, in the condensing point L_S1set in the inner portion of the substrate W and the close region, a change in absorption coefficient and a phenomenon including multi-photon absorption are induced due to local heating, to thereby perform modification. When the scanning with the condensing point L_S1of the modification laser light L1 is performed as illustrated in FIG. 2A, a modified layer Wr which becomes a boundary of a laser processing region is formed as illustrated in FIG. 2B. In the first embodiment, the scanning with the condensing point L_S1of the modification laser light L1 is performed in the X-axis direction (or Y-axis direction) to form the modified layer Wr which becomes a bottom part of a recess portion, for example, a hole or a groove, as illustrated in FIG. 2C. In other words, in the first embodiment, the bottom part of the recess portion corresponds to a boundary of a laser processing region R1. The modified layer Wr is formed in the boundary.

The modified layer Wr is a region which is obtained by irradiating a part of a material of the substrate W with the modification laser light L1 and which is different in characteristic and structure from a non-irradiated region. To be specific, the modified layer Wr has the following three states (1), (2), and (3). Note that, of the following three modification actions, multiple modification actions may be simultaneously caused.

(1) Case where Modified Layer is Melting Processing Region

A melting processing region is a region in which, for example, a change in crystalline structure is induced in a case where the material of the substrate W1 is melted and then solidified again. The melting processing region may be a phase-changed region or a region in which a crystalline structure is changed. Alternatively, the melting processing region may be a region in which, of a single-crystalline structure, an amorphous structure, and a polycrystalline structure, a structure is changed to another structure. In this case, the condensing point L_S1of the modification laser light L1 is focused on the inner portion of the substrate W (semiconductor material substrate, for example, silicon wafer). The substrate W is irradiated with the modification laser light L1 in a condition in which an electric field strength at the condensing point L_S1is equal to or larger than 1×10⁸(W/cm²) and a pulse width is equal to or smaller than 1 μsec. Therefore, multi-photon absorption occurs in the inner portion of the substrate W and the substrate W is locally heated, and hence the melting processing region is formed in the inner portion of the substrate W. When the substrate W has a single-crystal silicon structure, the melting processing region has, for example, an amorphous silicon structure. In this case, an upper limit value of the electric field strength is, for example, 1×10⁸(W/cm²). The pulse width is preferred to be, for example, in a range of 1 nsec. to 200 nsec.

(2) Case where Modified Layer is Crack Region

A crack region is a region in which a crack is caused by stress generated in an irradiation area of the condensing point L_S1on the substrate W and a region close to the irradiation area by expansion of the irradiation area. In this case, the condensing point L_S1of the modification laser light L1 is focused on the inner portion of the substrate W (for example, glass or piezoelectric material (LiTaO₃)). The substrate W is irradiated with the modification laser light L1 in a condition in which the electric field strength at the condensing point L_S1is equal to or larger than 1×10⁸(W/cm²) and the pulse width is equal to or smaller than 1 μsec. Therefore, multi-photon absorption occurs in the inner portion of the substrate W, and hence the crack region is formed in the inner portion of the substrate W without unnecessary damage to the substrate W. An upper limit value of the electric field strength is, for example, 1×10⁸(W/cm²). The pulse width is preferred to be, for example, in a range of 1 nsec. to 200 nsec.

(3) Case where Modified Layer is Refractive Index Change Region

A refractive index change region is a region in which a change in density or refractive index is induced by local exposure with high energy. In this case, the condensing point L_S1of the modification laser light L1 is focused on the inner portion of the substrate W (for example, glass). The substrate W is irradiated with the modification laser light L1 in a condition in which the electric field strength at the condensing point L_S1is equal to or larger than 1×10⁸(W/cm²) and the pulse width is equal to or smaller than 1 nsec. When the pulse width is extremely shortened to cause multi-photon absorption in the inner portion of the substrate W, energy generated by the multi-photon absorption is not changed to thermal energy, and hence a structure change, for example, crystallization or a change in ion valence occurs in the inner portion of the substrate W. Therefore, the refractive index change region is formed. An upper limit value of the electric field strength is, for example, 1×10¹²(W/cm²). The pulse width is preferred to be, for example, equal to or smaller than 1 nsec., and more preferred to be equal to or smaller than 1 psec.

Hereinafter, a specific example in a case where the substrate W is a silicon wafer is described. The substrate W is a silicon wafer and has a thickness of 625 μm and an outer size of 6 inches. The condensing lens 2 has magnification of 50 and a NA of 0.55. A transmittance of the modification laser light L1 is 60%. The modified layer Wr is one of the melting processing region, the crack region, and the refractive index region described above. The laser oscillator 1 is a YAG laser. With respect to the modification laser light L1 emitted in the modified layer forming step, a wavelength is 1,064 nm, an oscillation mode is a Q-switch pulse, a pulse width is 30 nm, an output power is 20 μJ/pulse, a laser spot cross sectional area is 3. 1×10⁸cm², and a repetition frequency is 80 kHz. When the modification laser light L1 is condensed to the inner portion of the substrate W by the condensing lens 2, an energy density of the modification laser light L1 on a surface Wa of the substrate W is smaller than 1×10⁸W/cm²and an energy density of the modification laser light L1 at the condensing point L_S1is equal to or larger than 1×10⁸W/cm². Scanning with the condensing point L_S1illustrated in FIG. 2A is performed at a speed of 100 mm/sec. Therefore, as illustrated in FIGS. 2B and 2C, the modified layer Wr is formed in a region scanned with the condensing point L_S1.

The condensing lens 2 and the substrate W are moved relative to each other to successively form the modified layer Wr. A thickness of the modified layer Wr may be adjusted by scanning with the condensing point L_S1at changed depth positions to stack multiple modified layer regions. When a relative movement locus between the condensing lens 2 and the substrate W is designed, it is necessary to prevent a region which has already been modified from being located between a region to be modified and the condensing lens 2. This reason is to prevent the modification laser light L1 from entering the region which has already been modified and being scattered. Therefore, modification is started from a region far away from the surface Wa.

With respect to the modified layer Wr formed as described above, a laser processing speed in the removing/processing step is lower than a laser processing speed of a non-modified region. Therefore, the modified layer Wr is formed in a region to stop removal processing performed in the removing/processing step later, that is, at least a boundary of the laser processing region R1 to be removed. In other words, in the first embodiment, before the removal processing with laser light, the bottom part of the recess portion which is removed is defined by the modified layer Wr. The modified layer Wr may have not a linear shape in each side as illustrated in FIG. 2C but a curved shape. The modified layer Wr may be located in any position of the substrate W and may reach the surface Wa of the substrate W. The modified layer Wr may be formed in a region except the region to stop the removal processing. That is, the modified layer Wr may be formed in a region larger in area than the region to stop the removal processing.

Next, the removing/processing step in the first embodiment is described in detail with reference to FIGS. 3A, 3B, and 3C. The control device 4 performs control so as to irradiate the surface Wa of the substrate W with condensed processing laser light L2 to remove the laser processing region R1 defined by the modified layer Wr.

Note that, the condensing lens 12 and the processing laser light L2 are not particularly limited as long as a characteristic to remove a part of the substrate W is obtained. For example, any one of a solid laser, an excimer laser, and a dye laser may be used as a laser source for the processing laser light L2. In the first embodiment, as described above, the processing laser light L2 is emitted from the common laser oscillator 1. The condensing lens 12 is preferred to be prevented from being broken by the processing laser light L2. A transmittance of the condensing lens 12 with respect to the processing laser light L2 is preferred to be equal to or larger than 20%. In this condition, the processing laser light L2 is preferred to be condensed to a condensing point L_S2. The condensing lens 12 has a magnification of 50 and a NA of 0.55. A transmittance of the processing laser light L2 is 60%. In the removing/processing step, with respect to the processing laser light L2 emitted from the laser oscillator 1, a wavelength is 532 nm, an oscillation mode is a Q-switch pulse, a pulse width is 30 nm, an output power is 20 μJ/pulse, a laser spot cross sectional area is 3. 1×10⁸cm², and a repetition frequency is 80 kHz. In this case, an electric field strength at the condensing point L_S2is preferred to be equal to or larger than 1×10⁸(W/cm²) and a pulse width is preferred to be equal to or smaller than 1 μsec.

The condensing point L_S2is a point in which an energy density of the processing laser light L2 is maximum in a case where the processing laser light L2 is condensed by the condensing lens 12. As illustrated in FIG. 3A, the surface Wa of the substrate W is scanned with the condensing point L_S2to perform the removal processing in scanning positions of the condensing point L_S2. A scanning speed of the condensing point L_S2is 100 mm/sec.

FIG. 3B illustrates a state during the removal processing. A bottom part Vb of a region V removed by the processing laser light L2 is illustrated in FIG. 3B. In the region V which is removed by the processing laser light L2, the amount of removal is fluctuated depending on processing dust discharged by the removal processing or a fluctuation in intensity of the processing laser light L2, and hence a shape of the bottom part Vb fluctuates. Even when a portion of the bottom part Vb of the removed region V early reaches the modified layer Wr because of the processing fluctuation, the laser processing speed of the modified layer Wr is lower than the laser processing speed of the non-modified region, and hence the modified layer Wr is not easily removed by the processing laser light L2. Therefore, as illustrated in FIG. 3C, the fluctuation in shape of the bottom part Vb of the removed region V finally obtained by removal processing, that is, a fluctuation in removal depth is reduced. In other words, the processing shape may be defined by the modified layer Wr.

FIGS. 4A and 4B illustrate a recess portion Wc formed by the laser processing method described above. As illustrated in FIGS. 4A and 4B, a bottom part Wb of the recess portion Wc corresponds to the modified layer Wr. When the modified layer Wr is formed, the precision of processing in the case where the substrate W is subjected to the removal processing with the processing laser light L2 is improved.

According to the first embodiment, the laser processing speed of the modified layer Wr during the removing/processing step is lower than that in the non-modified region, and hence the non-modified region is easily processed. Therefore, even when a spatial intensity distribution or pulse energy of the processing laser light L2 is temporally varied, a fluctuation in processing amount is reduced by the modified layer Wr, and hence the flatness of the bottom part Wb of the recess portion We may be improved. Thus, the precision of processing of the substrate W is improved. The modified layer Wr may be formed into an arbitrary shape by the modification laser light L1 during the modified layer forming step, and hence the degree of freedom of the processing shape is improved.

Second Embodiment

In the first embodiment, the case where the recess portion is formed in the substrate is described. In a second embodiment, a case where a through portion is formed in the substrate is described. The same laser processing apparatus as in the first embodiment is used, and hence the description is made with reference to the laser processing apparatus illustrated in FIG. 1.

Even in the second embodiment, the control device executes a modified layer forming step and a removing/processing step. The modified layer forming step is specifically described with reference to FIGS. 5A, 5B, and 5C. In the modified layer forming step, the inner portion of the substrate W which is the object to be processed is scanned with the condensing point L_S1of the modification laser light L1 which is the first laser light to form a modified layer Wra which becomes a boundary of a laser processing region R2. A condition of the modification laser light L1 is the same as that in the first embodiment.

Hereinafter, a detailed description is made. As illustrated in FIG. 5A, the condensing point L_S1of modification laser light L1 emitted from the laser oscillator 1 is set in the inner portion of the substrate W by the condensing lens 2. The condensing point L_S1set in the inner portion of the substrate W and a region close to the condensing point have a higher energy density than a region except the condensing point L_S1and the close region, through which the modification laser light L1 passes. Therefore, in the condensing point L_S1set in the inner portion of the substrate W and the close region, a change in absorption coefficient and a phenomenon including multi-photon absorption are induced due to local heating, to thereby perform modification. When the scanning with the condensing point L_S1of the modification laser light L1 is performed as illustrated in FIG. 5A, the modified layer Wra which becomes the boundary of the laser processing region R2 is formed as illustrated in FIG. 5B. In the second embodiment, the condensing point L_S1of modification laser light L1 is shifted for scanning in the X-axis direction, the Y-axis direction, and the Z-axis direction to form the modified layer Wra which becomes a side wall of the through portion as illustrated in FIG. 5C. In other words, in the second embodiment, a portion which becomes the side wall of the through portion correspond to the boundary of the laser processing region R2, and hence the modified layer Wra is formed in the boundary. The formed modified layer Wra is one of the melting processing region, the crack region, and the refractive index region. The modified layer Wra is formed by scanning with the condensing point L_S1at changed depth positions in the Z-axis direction to stack multiple modified layer regions. When a relative movement locus between the condensing lens 2 and the substrate W is designed, it is necessary to prevent a region which has already been modified from being located between a region to be modified and the condensing lens 2. This reason is to prevent the modification laser light L1 from entering the region which has already been modified and being scattered. Therefore, modification is successively started from a region far away from the surface Wa.

A laser processing speed of the modified layer Wra formed as described above during the removing/processing step is lower than that in a non-modified region. Therefore, in the second embodiment, the modified layer Wra is formed to surround the laser processing region R2. In other words, in the second embodiment, before the removal processing with laser light, the side wall of the through portion which is removed is defined by the modified layer Wra. The modified layer Wra may have not a linear shape in each side as illustrated in FIG. 5B but a curved shape. The modified layer Wra may be located in any position of the substrate W and may reach the surface Wa of the substrate W. The modified layer Wra may be formed in a region except the region to define the removal processing. That is, the modified layer Wra may be formed in a region larger in area than the region to define the removal processing.

Next, the removing/processing step in the second embodiment is described in detail with reference to FIGS. 6A and 6B. The control device 4 performs control so as to irradiate the surface Wa of the substrate W with condensed processing laser light L2 to remove the laser processing region R2 defined by the modified layer Wra. Note that, the conditions of the processing laser light L2 are the same as those in the first embodiment.

The condensing point L_S2is a point in which an energy density of the processing laser light L2 is maximum in a case where the processing laser light L2 is condensed by the condensing lens 12. As illustrated in FIG. 6A, the surface Wa of the substrate W is scanned with the condensing point L_S2to perform the removal processing in scanning positions of the condensing point L_S2. A scanning speed of the condensing point L_S2is 100 mm/sec.

FIG. 6B illustrates a state during the removal processing. A bottom part Vb of a region V removed by the processing laser light L2 is illustrated in FIG. 6B. In the region V which is removed by the processing laser light L2, the amount of removal is fluctuated depending on processing dust discharged by the removal processing or a fluctuation in intensity of the processing laser light L2, and hence a shape of the bottom part Vb fluctuates. In contrast to this, the laser processing speed of the modified layer Wra is lower than that in the non-modified region, and hence the modified layer Wra is not easily removed by the processing laser light L2 and the non-modified region is removed. Therefore, the processing shape may be defined by the modified layer Wra.

FIGS. 7A and 7B illustrate a through portion Wt formed by the laser processing method described above. As illustrated in FIGS. 7A and 7B, a side wall Ws of the through portion Wt corresponds to the modified layer Wra. When the modified layer Wra is formed, the precision of processing in the case where the substrate W is subjected to the removal processing with the processing laser light L2 is improved.

According to the second embodiment, the laser processing speed of the modified layer Wra during the removing/processing step is lower than that in the non-modified region, and hence the non-modified region is easily processed. Therefore, even when a spatial intensity distribution or pulse energy of the processing laser light L2 is temporally varied, a fluctuation in processing amount is reduced by the modified layer Wra, and hence the flatness of the side wall Ws of the through portion Wt may be improved. Thus, the precision of processing of the substrate W is improved. The modified layer Wra may be formed into an arbitrary shape by the modification laser light L1 during the modified layer forming step, and hence the degree of freedom of the processing shape is improved.

In the first embodiment, the modified layer is formed in only the bottom part of the recess portion. However, as in the second embodiment, the modified layer may be formed in the side wall of the recess portion. The modified layer may be freely formed according to the processing shape.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2009-255897, filed Nov. 9, 2009, which is hereby incorporated by reference herein in its entirety.

LASER PROCESSING METHOD

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