JP6988057B2 - Laser processing equipment and laser processing method - Google Patents

Description

In the present invention, a laser that forms a modified region inside the wafer along the planned cutting line of the wafer by irradiating the inside of the wafer with a plurality of devices formed on the surface with a laser beam at a condensing point. It relates to a processing device and a laser processing method.

Conventionally, in order to divide a wafer in which multiple devices are formed on the surface into individual chips, a thin grindstone with a thickness of about 30 μm formed of fine diamond abrasive grains is used to cut the wafer by making a grinding groove in the wafer. Dicing equipment was used.

In a dicing apparatus, a thin grindstone (hereinafter referred to as a dicing blade) is rotated at a high speed of, for example, 30,000 to 60,000 rpm to grind a wafer, and the wafer is completely cut (full cut) or incompletely cut (half cut or semi-full). Cut).

However, in the case of grinding with this dicing blade, since the wafer is a highly brittle material, brittle mode processing occurs, chipping occurs on the front and back surfaces of the wafer, and this chipping is a factor that deteriorates the performance of the divided chips. rice field.

To solve this problem, instead of cutting with a conventional dicing blade, a modified region by multiphoton absorption is formed inside the wafer by aligning the condensing point inside the wafer and irradiating it with laser light. A technique for dividing into individual chips has been proposed (see, for example, Patent Documents 1 and 2).

Japanese Unexamined Patent Publication No. 2009-34723 Japanese Unexamined Patent Publication No. 2010-58128

However, in the above-mentioned technique, when the laser beam is irradiated by aligning the condensing point inside the wafer in order to form a reforming region inside the wafer, the laser beam incident on the inside of the wafer is emitted inside the wafer. Some laser light that is not completely absorbed and does not contribute to the formation of the modified region leaks to the device surface of the wafer (the surface opposite to the laser light incident surface), damaging the device surface of the wafer. It may end up. In particular, damage that occurs spot-like in the direction orthogonal to the planned cutting line of the wafer may affect the device formation area (area surrounded by the planned cutting line) of the wafer, so it is necessary to minimize this damage. There is.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a laser processing apparatus and a laser processing method capable of reducing damage to a device surface of a wafer due to leakage of laser light.

In order to achieve the above object, the laser processing apparatus according to the first aspect of the present invention aligns the condensing point inside the wafer and irradiates the laser light to the inside of the wafer along the planned cutting line of the wafer. A laser processing device that forms a modified region, a laser light source that outputs laser light, a condenser lens that concentrates the laser light inside the wafer, and a wafer that moves relative to the laser light. It includes a moving means and a light beam controlling means for controlling the spot shape of the laser light focused by the condenser lens into an elliptical shape.

In the first aspect of the laser processing apparatus according to the second aspect of the present invention, the luminous flux control means controls the spot shape of the laser beam so that the spot shape is an elliptical shape with the direction of the scheduled cutting line as the minor axis.

In the first aspect of the laser processing apparatus according to the third aspect of the present invention, the luminous flux control means controls the spot shape of the laser beam so as to have an elliptical shape with the direction of the planned cutting line as the long axis.

In the laser processing apparatus according to the fourth aspect of the present invention, in any one of the first to third aspects, the spatial light modulator that modulates the laser light and the depth from the laser light irradiation surface inside the wafer are mutual. The control unit that controls the spatial optical modulator so that the laser light is simultaneously condensed by the condenser lens at multiple positions that are different and separated from each other in the moving direction of the wafer, and the spatial optical modulator are configured separately. It is provided with an aberration correction means for correcting the aberration of the laser light to be equal to or less than a predetermined error at a plurality of positions where the focusing points of the laser light are aligned inside the wafer.

In the fourth aspect of the laser processing apparatus according to the fifth aspect of the present invention, the control unit collects the laser light at the first position in the depth direction of the wafer, and the first position is the wafer. The spatial light modulator is controlled so that the laser beam is focused on a second position that differs in the depth direction.

In the laser processing method according to the sixth aspect of the present invention, a laser that forms a modified region inside the wafer along a planned cutting line of the wafer by irradiating the inside of the wafer with a laser beam by aligning a condensing point. The processing method is a condensing step of concentrating the laser light inside the wafer, a moving step of moving the wafer relative to the laser light, and a spot shape of the laser light condensing inside the wafer. Includes a light beam control step that controls the shape into an elliptical shape.

In the sixth aspect of the laser processing method according to the seventh aspect of the present invention, the luminous flux control step controls the spot shape of the laser beam to be an elliptical shape with the direction of the planned cutting line as the minor axis.

In the sixth aspect of the laser processing method according to the eighth aspect of the present invention, the luminous flux control step controls the spot shape of the laser beam to be an elliptical shape with the direction of the planned cutting line as the long axis.

In any of the sixth to eighth aspects, the laser processing method according to the ninth aspect of the present invention includes a modulation step of modulating the laser light with a spatial light modulator and a depth from the laser light irradiation surface inside the wafer. The control process that controls the spatial optical modulator so that the laser beam is simultaneously focused at multiple positions that are different from each other and separated from each other in the moving direction of the wafer, and the spatial optical modulator are configured separately from the wafer. The present invention includes an aberration correction step of correcting the aberration of the laser light to be equal to or less than a predetermined error at a plurality of positions where the focusing points of the laser light are aligned.

In the ninth aspect of the laser processing method according to the tenth aspect of the present invention, in the control step, the laser light is focused on the first position in the depth direction of the wafer, and the first position is the wafer. The spatial light modulator is controlled so that the laser beam is focused on a second position that differs in the depth direction.

According to the present invention, by making the spot shape of the laser beam focused inside the wafer into an elliptical shape, it is possible to effectively reduce the damage to the device surface of the wafer due to the leakage light of the laser beam.

A block diagram showing an outline of a laser processing apparatus according to an embodiment of the present invention. Conceptual diagram showing how a reformed region is formed inside a wafer Conceptual diagram of how cracks extend in the wafer depth direction from the reformed region formed inside the wafer. The figure which showed the example which the spot shape of a laser beam is controlled to an elliptical shape as a 1st control example. The figure which showed the state of the damage which occurs on the device surface of a wafer by the leakage light of a laser beam when the spot shape as the 1st control example shown in FIG. 4 is adopted. The figure which showed the other example in the case where the spot shape of a laser beam is controlled to an elliptical shape as a second control example. The figure which showed the state of the damage which occurs on the device surface of a wafer by the leakage light of a laser beam when the spot shape as the 2nd control example shown in FIG. 6 is adopted. The figure which showed the example of the case where the spot shape of a laser beam is a circular shape as a comparative control example. The figure which showed the state of the damage which occurs on the device surface of a wafer by the leakage light of a laser beam when the spot shape as a comparative control example shown in FIG. 8 is adopted. Graph showing the relationship between the amount of aberration correction and the total crack length Graph showing the relationship between the amount of aberration correction and the length of the lower end of the crack A block diagram showing the outline of the laser processing apparatus according to another embodiment of the present invention. It is a conceptual diagram showing how a reformed region is formed inside a wafer, and is a diagram for explaining other forms of two positions (condensed positions) where laser light is focused.

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

FIG. 1 is a block diagram showing an outline of a laser processing apparatus according to an embodiment of the present invention. As shown in FIG. 1, the laser processing apparatus 1 of the present embodiment mainly includes a wafer moving unit 11, a laser head 20, a control unit 50, and the like.

The wafer moving unit 11 includes a suction stage 13 that sucks and holds the wafer W, and an XYZθ table 12 that is provided on the main body base 16 of the laser processing apparatus 1 and precisely moves the suction stage 13 in the XYZθ direction. The wafer W is precisely moved in the XYZθ direction in the figure by the wafer moving unit 11. The wafer moving unit 11 is an example of moving means.

The wafer W has a back grind tape (hereinafter referred to as BG tape) having an adhesive material attached to the front surface on which the device is formed, and is placed on the suction stage 13 so that the back surface faces upward. The thickness of the wafer W is not particularly limited, but is typically 700 μm or more, and more typically 700 to 800 μm.

The wafer W may have a dicing sheet having an adhesive material attached to one surface thereof, and may be placed on the suction stage 13 in a state of being integrated with the frame via the dicing sheet.

The laser head 20 mainly includes a laser light source 22, a spatial light modulator 28, a condenser lens 38, a luminous flux controller 42, and the like.

The laser light source 22 outputs a laser beam L for processing for forming a modified region inside the wafer W under the control of the control unit 50. The conditions for the laser beam L are, for example, that the light source is a semiconductor laser-excited Nd: YAG laser, the wavelength is a wavelength of 1.1 μm, the cross-sectional area of the laser beam spot is 3.14 × ^10-8 cm ² , and the oscillation mode is a Q switch pulse. The repetition frequency is 80 to 120 kHz, the pulse width is 180 to 280 ns, and the output is 10 W.

The spatial light modulator 28 is a phase modulation type, and is a predetermined hologram that inputs the laser light L output from the laser light source 22 and modulates the phase of the laser light L in each of a plurality of pixels arranged in two dimensions. A pattern is presented and the laser beam L after the phase modulation is output. This hologram pattern is a superposition of a plurality of Fresnel lens patterns having different light collection positions. As a result, as will be described in detail later, the depths from the laser beam irradiation surface (back surface of the wafer W) inside the wafer W are different from each other, and the wafer moving direction (machining feed direction) parallel to the X direction in FIG. The laser beam L is simultaneously condensed by the condenser lens 38 at two positions separated from each other by M.

The spatial light modulator 28 includes, for example, a liquid crystal on silicon (LCOS).
) Spatial Light Modulator (SLM) is used. The operation of the spatial light modulator 28 and the hologram pattern presented by the spatial light modulator 28 are controlled by the control unit 50. Since the specific configuration of the spatial light modulator 28 and the hologram pattern presented by the spatial light modulator 28 are already known, detailed description thereof will be omitted here.

The condenser lens 38 is an objective lens (infrared objective lens) that concentrates the laser beam L inside the wafer W. The numerical aperture (NA) of the condenser lens 38 is 0.6 to 0.8 (for example, 0.65).

The condenser lens 38 includes a correction ring 40 for correcting the aberration of the laser beam L generated inside the wafer W. The correction ring 40 is manually rotatably configured, and when the correction ring 40 is rotated in a predetermined direction, the distance between the lens groups constituting the condenser lens 38 is changed, and the laser light irradiation surface of the wafer W is changed. The aberration can be corrected so that the aberration of the laser beam L is equal to or less than the predetermined aberration at a position at a predetermined depth from the (back surface). The correction ring 40 is an example of aberration correction means.

The correction ring 40 may be configured to be electrically rotated by a correction ring drive unit (not shown). In this case, the control unit 50 controls the operation of the correction ring drive unit and rotates the correction ring 40 to correct the aberration of the laser beam L so as to be in a desired state.

The luminous flux controller 42 controls the spot shape (cross-sectional shape of the speed of light at the condensing point) of the laser beam L condensed by the condensing lens 38. The luminous flux controller 42 is preferably arranged on the optical path of the laser beam L of the laser beam L, and is preferably configured by a variable slit in which the shape and size of the opening through which the laser beam L passes are variable. The luminous flux controller 42 in the present embodiment controls so that the spot shape of the laser beam L becomes an elliptical shape having the direction of the planned cutting line as the short axis or the long axis. The luminous flux controller 42 is an example of the luminous flux control means.

In addition to the above configuration, the laser head 20 includes a beam expander 24, a λ / 2 wave plate 26, a reduction optical system 36, and the like.

The beam expander 24 expands the laser beam L output from the laser light source 22 to an appropriate beam diameter for the spatial light modulator 28. The λ / 2 wave plate 26 adjusts the plane of polarization of laser light incident on the spatial light modulator 28. The reduction optical system 36 is an afocal optical system (bilateral telecentric optical system) composed of a first lens 36a and a second lens 36b, and collects laser light L modulated by a spatial light modulator 28 into a condenser lens 38. Reduced projection to.

Although not shown, the laser head 20 has an alignment optical system for aligning with the wafer W, and an auto for keeping the distance (working distance) between the wafer W and the condenser lens 38 constant. It is equipped with a focus unit and the like.

The control unit 50 includes a CPU, a memory, an input / output circuit unit, and the like, and controls the operation of each unit of the laser processing apparatus 1. Specifically, the thickness of the wafer W and the feed rate of the wafer W are controlled, the operation of each part (wafer moving part 11, laser head 20, etc.) is controlled under optimum conditions, and a reforming region is formed.

Further, the control unit 50 controls the operation of the spatial light modulator 28, and causes the spatial light modulator 28 to present a predetermined hologram pattern. Specifically, two positions inside the wafer W that are different from each other (that is, two positions that are different in depth from the laser beam irradiation surface and are separated from each other in the wafer moving direction M parallel to the X direction in FIG. 1). ), The spatial light modulator 28 presents a hologram pattern for modulating the laser light L so that the laser light L is simultaneously focused by the condenser lens 38. The hologram pattern is derived in advance based on the formation position of the modified region, the wavelength of the laser light L to be irradiated, the refractive index of the condenser lens 38 and the wafer W, and the like, and is stored in the control unit 50.

In addition to this, the laser processing apparatus 1 includes a wafer transfer means (not shown), an operation plate, a television monitor, an indicator lamp, and the like.

Switches and display devices for operating the operation of each part of the laser processing device 1 are attached to the operation plate. The TV monitor displays a wafer image captured by a CCD camera (not shown), or displays program contents, various messages, and the like. The indicator lamp displays the operating status of the laser processing apparatus 1 during processing, processing completion, emergency stop, and the like.

The operation of the laser processing apparatus 1 of the present embodiment configured as described above will be described. Here, a case where a silicon substrate having a thickness of 775 μm is processed as the wafer W will be described as an example.

First, by manually (or electrically) rotating the correction ring 40 provided in the condenser lens 38, the laser beam is emitted at a position (processing depth of the modified region) in which the laser beam L is condensed inside the wafer W. The aberration correction amount is adjusted so that the aberration of L is equal to or less than a predetermined aberration. In the present specification, the "aberration correction amount" is a value converted into the depth of the wafer W from the laser beam irradiation surface. That is, for example, when the aberration correction amount is 500 μm, it means that the laser aberration is minimized when the depth of the wafer W from the laser beam irradiation surface is close to 500 μm. In this example, as will be described in detail later, when the thickness of the wafer W is 775 μm, it is preferable that the aberration correction amount by the correction ring 40 is set to 500 μm.

Next, after the wafer W to be processed is placed on the suction stage 13, the wafer W is aligned using an alignment optical system (not shown).

Next, while processing and feeding the XYZθ table 12 in the wafer moving direction M (that is, moving the wafer W relative to the laser beam L in the wafer moving direction M), the laser head 20 to the wafer W. Irradiate the laser beam L.

At this time, the laser beam L output from the laser light source 22 is expanded in beam diameter by the beam expander 24, reflected by the first mirror 30, and the polarization direction is changed by the λ / 2 wave plate 26 for spatial light modulation. It is incident on the vessel 28.

The laser light L incident on the spatial light modulator 28 is modulated according to a predetermined hologram pattern presented on the spatial light modulator 28. At that time, the control unit 50 spatially light-modulates the hologram pattern for modulating the laser light L so that the laser light L is simultaneously focused by the condenser lens 38 at two different positions inside the wafer W. The control to be presented to the vessel 28 is performed.

The laser light L emitted from the spatial light modulator 28 is sequentially reflected by the second mirror 31 and the third mirror 32, then passes through the first lens 36a, and is further passed by the fourth mirror 33 and the fifth mirror 34. It is reflected, passes through the second lens 36b, and is incident on the condenser lens 38 via the light beam controller 42. As a result, the laser beam L emitted from the spatial light modulator 28 is reduced and projected onto the condenser lens 38 by the reduction optical system 36 including the first lens 36a and the second lens 36b. Then, the laser light L incident on the condenser lens 38 is focused by the condenser lens 38 at two different positions inside the wafer W.

FIG. 2 is a conceptual diagram showing how a reformed region is formed inside the wafer W. As shown in FIG. 2, the laser light L modulated by the spatial light modulator 28 has different depths from the laser light irradiation surface (back surface of the wafer W) inside the wafer W by the condenser lens 38, and The light is collected at two positions separated from each other in the wafer moving direction M at the same time. As a result, modified regions P1 and P2 due to multiphoton absorption are formed in the vicinity of the respective light collection positions. Further, when the modified regions P1 and P2 are formed, cracks K1 and K2 extending from the modified regions P1 and P2 in the wafer depth direction (wafer thickness direction) are formed.

Here, the two positions where the laser light L is focused (condensing position) will be described in detail. In the present embodiment, the first position Q1 in the wafer depth direction inside the wafer W (left side in FIG. 2). The laser light L is focused on the condensing point), and the laser light is focused on the second position Q2 (condensing point on the right side in FIG. 2) which is different from the first position Q1 in the wafer depth direction. The light.

Further, in the present embodiment, the second position Q2 is arranged on the upstream side (right side in FIG. 2) of the wafer moving direction M from the first position Q1, and is the first position from the laser beam irradiation surface of the wafer W. It is arranged at a position deeper than the position Q1. As an example of the distance (interval) between the first position Q1 and the second position Q2, the distance in the wafer depth direction is 50 μm. Further, the distance in the wafer moving direction M is 30 μm.

As the wafer W moves relative to the laser beam L in this state, as shown in FIG. 3, the wafer W moves along the movement loci of the two focusing points (condensing positions Q1 and Q2) of the laser beam L. The reforming regions P1 and P2 are formed inside the wafer W. As a result, along the planned cutting line of the wafer W, the reforming region P1 and the reforming region P2 are formed one line at a time in a state where the reforming region P1 and the reforming region P2 are overlapped at predetermined intervals inside the wafer W.

When the reforming region P1 and the reforming region P2 are formed in a state of being overlapped at a predetermined interval in this way, of the two reforming regions P1 and P2 overlapping in the wafer depth direction, the modification region P1 and P2 are delayed in time. As shown in FIG. 3, the crack K1 in the modified region P1 and the crack K2 in the modified region P2 are connected by the impact during the formation of the modified region P1 to be formed, and the crack K2 extending from the modified region P2 is formed in the wafer. It extends toward the surface (surface of the wafer W) opposite to the laser beam irradiation surface of W.

That is, by modulating the laser light L with the spatial light modulator 28, the laser light L is simultaneously focused at different positions inside the wafer W, so that the throughput is not reduced and the laser light L is collected from the surface of the wafer W. The crack can be extended to a desired position between the target surface indicating the position of the final thickness T2 of the wafer W and the surface of the wafer W. In FIGS. 2 and 3, T1 indicates the initial thickness of the wafer W (775 μm in this example).

When the reformed regions P1 and P2 are formed one line at a time along the planned cutting line, the XYZθ table 12 is indexed and fed one pitch in the Y direction, and the reformed regions P1 and P2 are similarly formed on the next line. ..

When the modified regions P1 and P2 are formed along all the planned cutting lines parallel to the X direction, the XYZθ table 12 is rotated by 90 °, and all the lines orthogonal to the previous line are also modified regions P1. , P2 is formed.

As a result, the modified regions P1 and P2 are formed along all the planned cutting lines. The reformed regions P1 and P2 have the same position on the plane (position seen from the back surface or the front surface of the wafer W) and are both formed along the planned cutting line, but in the thickness direction of the wafer W (wafer). Only the position in the depth direction) is different.

After the reformed regions P1 and P2 are formed along the planned cutting line as described above, the back surface of the wafer W is ground using a grinding device (not shown) to obtain the thickness (initial thickness) T1 of the wafer W. Is processed to a predetermined thickness (final thickness) T2 (for example, 30 to 50 μm) by performing a back surface grinding step.

After the back surface grinding process, an expand tape (dicing tape) is attached to the back surface of the wafer W, the BG tape attached to the front surface of the wafer W is peeled off, and then tension is applied to the expand tape attached to the back surface of the wafer W. The expanding process of adding and stretching is performed.

As a result, the wafer W is cut starting from a crack extending to the surface side of the wafer W. That is, the wafer W is cut along the scheduled cutting line and divided into a plurality of chips.

Here, in the present embodiment, when the modified regions P1 and P2 are formed inside the wafer W along the planned cutting line as described above, the laser beam L condensed by the condenser lens 38 The spot shape (cross-sectional shape of the speed of light at each condensing position Q1 and Q2) is controlled to an elliptical shape by the luminous flux controller 42. Hereinafter, a control example of the spot shape of the laser beam L (first control example, second control example) and a comparative control example will be described.

(First control example)
FIG. 4 is a diagram showing an example in which the spot shape of the laser beam L is controlled to an elliptical shape as a first control example, and is a planar position between the spot shape of the laser beam L and the direction of the planned cutting line. It is a figure which showed the relationship. Further, FIG. 5 is a diagram showing a state of damage caused to the device surface of the wafer W by the leakage light of the laser beam L when the spot shape is used as the first control example shown in FIG. The lower row schematically shows the spot shape of the laser beam L adopted at that time.

As shown in FIG. 4, the spot shape of the laser beam L as the first control example is an elliptical shape having the short axis in the direction of the planned cutting line (direction parallel to the wafer moving direction). When such a spot-shaped laser beam L is irradiated from the laser beam irradiation surface of the wafer W, as shown in FIG. 5, the device surface of the wafer W is hardly damaged by the leakage light of the laser beam L.

(Second control example)
FIG. 6 is a diagram showing another example in which the spot shape of the laser beam L as the second control example is controlled to have an elliptical shape, and is a plan view of the spot shape of the laser beam L and the direction of the planned cutting line. It is a figure which showed the positional relationship. Further, FIG. 7 is a diagram showing a state of damage caused to the device surface of the wafer W by the leakage light of the laser beam L when the spot shape is used as the second control example shown in FIG. The lower row schematically shows the spot shape of the laser beam L adopted at that time.

As shown in FIG. 6, the spot shape of the laser beam L as the second control example is an elliptical shape having the planned cutting line direction as the long axis. When such a spot-shaped laser beam L is irradiated from the laser beam irradiation surface of the wafer W, as shown in FIG. 7, the laser beam L leaks to the device surface of the wafer W as in the first control example described above. There is almost no damage from light.

(Comparison control example)
FIG. 8 is a diagram showing an example of a case where the spot shape of the laser beam L as a comparative control example is a circular shape. Further, FIG. 9 is a diagram showing the state of damage caused to the device surface of the wafer W by the leakage light of the laser beam L when the spot shape is used as the comparative control example shown in FIG. 8, and is shown in the lower part of FIG. Schematically shows the spot shape of the laser beam L adopted at that time.

When the spot shape of the laser beam L is circular as in the comparative control example shown in FIG. 8, as shown in FIG. 9, when the laser beam L is irradiated from the laser beam irradiation surface of the wafer W, the laser beam L becomes The leakage light causes spot damage on the device surface of the wafer W in the direction orthogonal to the planned cutting line direction. If this damage extends to the device forming area of the wafer, it becomes a factor that deteriorates the quality of the chip. In particular, as in the present embodiment, the laser beam L is simultaneously focused on two positions inside the wafer W that are different from each other in the depth direction of the wafer, and the modified regions P1 and P2 are simultaneously formed at the respective focusing positions. When the two-step processing is performed, the damage caused by the leakage light of the laser beam L becomes remarkable.

As can be seen from the above comparison, in the present embodiment, the luminous flux controller 42 controls the spot shape of the laser beam L so as to have an elliptical shape, so that the device surface of the wafer W due to the leakage light of the laser beam L is reached. Damage can be effectively reduced. Although the detailed principle of this is not known, the power of the laser beam L concentrated on the condensing point is more appropriate when the spot shape of the laser beam L is circular (see FIG. 8) than when the spot shape is circular. It is presumed that the laser beam L leaking to the device surface of the wafer W can be reduced and the damage generated on the device surface of the wafer W can be reduced.

As described above, in the present embodiment, the spot shape of the laser beam L is controlled by the light beam controller 42 to be an elliptical shape (preferably an elliptical shape having the planned cutting line direction as the minor axis or the major axis). Compared with the case where the spot shape of L is a circular shape, it is possible to effectively reduce the damage to the device surface of the wafer W due to the leakage light of the laser beam L.

In this embodiment, the spot shape of the laser beam L controlled by the light beam controller 42 is an elliptical shape having a short axis or a long axis in the direction of the planned cutting line (see FIGS. 4 and 6). However, the present invention is not limited to this, and an elliptical shape may be formed in which the direction diagonal to the planned cutting line direction is the minor axis or the major axis.

Further, in the present embodiment, the spatial light modulator 28 is used to simultaneously condense the laser light L at a plurality of positions by the condensing lens 38, and the correction ring 40 of the condensing lens 38 is used to collect the laser light L. Since the aberration at the position where the light spots are aligned is corrected so as to be equal to or less than the predetermined aberration, the laser beam L is efficiently focused at a deep position in the depth direction of the wafer even when the thickness of the wafer W is thick. Is possible. Further, the cracks generated when the reformed regions P1 and P2 overlapping in the wafer depth direction are formed can be sufficiently extended to the surface (surface of the wafer W) opposite to the laser beam irradiation surface of the wafer W. can. Therefore, the wafer W can be efficiently divided into chips, and chips of stable quality can be efficiently obtained.

Here, an experiment conducted by the present inventors in order to verify the above effect will be described.

In this experiment, the laser processing apparatus 1 of the above-described embodiment was used as an example. Further, as a comparative example, using a device having the same configuration as that of Patent Document 2, that is, a spatial light modulator, the laser light is focused on two different positions inside the wafer, and the respective positions are focused. A device that corrects aberrations in the above (first comparative example), and a device that corrects aberrations using a correction ring without using a spatial light modulator (that is, condensing laser light at one condensing position). (Second comparative example) was used.

As an experimental condition, for a wafer (silicon substrate) with a thickness of 775 μm, the total crack length (crack length) generated in the wafer depth direction when a modified region is formed while changing the amount of error correction in each device. The overall length) and the crack lower end length (the length of the crack extending from the lower end of the reformed region toward the wafer surface side) were measured (see FIG. 3).

FIG. 10 is a graph showing the relationship between the aberration correction amount and the total crack length. FIG. 11 is a graph showing the relationship between the aberration correction amount and the crack lower end length. In FIGS. 10 and 11, the horizontal axis indicates the amount of aberration correction, and the vertical axis indicates the total crack length generated by each device and the crack lower end length, respectively.

As can be seen from FIGS. 10 and 11, in the embodiment, the total crack length is longer over all the aberration correction amounts, and the crack lower end length is also longer than in the first comparative example and the second comparative example. Results were obtained.

In particular, when the aberration correction amount is 500 μm, in the examples, the total crack length is about 250 μm and the crack lower end length is about 80 μm, which is much better than the first comparative example and the second comparative example. Has been obtained. From this result, in the laser processing apparatus 1 of the present embodiment, when processing a wafer W having a thickness of 775 μm, the aberration correction amount by the correction ring 40 is 500 μm (that is, the laser beam irradiation surface (wafer) of the wafer W). It is preferable to set (so that the aberration is minimized at a position where the depth from the back surface of W) is 500 μm). Then, by modulating the laser light with the spatial light modulator 28 so that the laser light L is simultaneously focused by the condenser lens 38 at two positions having different depths in the vicinity of this depth (500 μm). , The crack extending in the depth direction of the wafer from the reforming region formed inside the wafer W can be sufficiently extended. As a result, the wafer can be efficiently divided into chips, and chips of stable quality can be efficiently obtained.

In the present embodiment, the aberration correction means is configured by the correction ring 40 provided in the condenser lens 38, but the present invention is not limited to this, and the condenser lens is not limited to this, for example, as shown in FIG. It may be configured by the correction optical system 44 arranged on the optical path of the laser beam L between the 38 and the spatial light modulator 28. In this case, the aberration of the laser beam L generated inside the wafer W can be corrected by changing the spacing between the plurality of lens groups constituting the correction optical system 44 by a driving means (not shown). In the example shown in FIG. 12, it is arranged between the luminous flux controller 42 and the second lens 36b.

Further, instead of using an objective lens with a correction ring, a correction function is incorporated in advance as a condenser lens 38 so that the aberration of the laser beam L is minimized at a predetermined position in the wafer depth direction inside the wafer W. An objective lens (infrared objective lens) may be used. In this case, the aberration correction amount is fixed depending on the thickness of the wafer W and the focusing position of the laser beam (processing depth of the modified region). For example, the aberration is generated at the position where the aberration correction amount is 500 μm. By using the minimum objective lens, cracks can be sufficiently extended from the modified region in the wafer depth direction.

Further, in the present embodiment, the laser beam L is simultaneously condensed at two positions different from each other in the wafer depth direction inside the wafer W, and the modified regions P1 and P2 are simultaneously formed at the respective condensing positions. A method of dividing the wafer W into individual chips by performing a back surface grinding process for grinding the back surface of the wafer W after performing two-step processing is adopted, but the method is not limited to this, and for example, the wafer W is required. Laser processing may be performed a plurality of times while changing the position (processing depth of the modified region) for condensing the laser beam L inside the. At that time, according to the processing depth of the modified region, the hologram pattern presented to the spatial light modulator 28 has a correction pattern for correcting the aberration inside the wafer W (in this case, the direction in which the correction by the correction ring 40 is canceled). By superimposing the pattern) on the wafer W, it is possible to appropriately correct aberrations even in a portion relatively deep from the laser beam irradiation surface of the wafer W.

Further, in the present embodiment, as shown in FIG. 2, the second position Q2 is the first position as the arrangement relationship between the two positions (condensing position) Q1 and Q2 where the laser beam L is focused. A configuration was adopted in which the wafer is placed on the upstream side (right side in FIG. 2) of the wafer moving direction M from the position Q1 and is placed at a position deeper than the first position Q1 from the laser beam irradiation surface of the wafer W. However, as shown in FIG. 13, a configuration opposite to that shown in FIG. 2 may be used. That is, the second position Q2 is arranged on the upstream side (right side in FIG. 13) of the wafer moving direction M from the first position Q1, and is shallower than the first position Q1 from the laser beam irradiation surface of the wafer W. It may be configured to be arranged at a position.

Further, in the present embodiment, the spatial light modulator 28 modulates the laser light L so that the laser light L is simultaneously focused by the condenser lens 38 at two different positions inside the wafer W. The position for condensing the laser beam L is not limited to two, and may be three or more.

Further, in the present embodiment, a reflective spatial light modulator (LCOS-SLM) is used as the spatial light modulator 28, but the present invention is not limited to this, and a MEMS-SLM, DMD (deformable mirror device), or the like can be used. There may be. Further, the spatial light modulator 28 is not limited to the reflection type, and is not limited to the reflection type.
It may be a transparent type. Further, examples of the spatial light modulator 28 include a liquid crystal cell type and an LCD type.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above examples, and it goes without saying that various improvements and modifications may be made without departing from the gist of the present invention. ..

10 ... Laser processing equipment, 11 ... Wafer moving part, 12 ... XYZθ table, 13 ... Adsorption stage, 20 ... Laser head, 22 ... Laser light source, 24 ... Beam expander, 26 ... λ / 2 wave plate, 28 ... Spatial light Modulator, 36 ... reduction optics, 38 ... condenser lens, 40
... correction ring, 42 ... correction optical system, 44 ... luminous flux controller, 50 ... control unit

Claims (6)

A laser processing device that irradiates a laser beam from the back surface opposite to the device surface of the wafer along a planned cutting line to condense the laser beam inside the wafer.
In order to reduce the damage to the device surface due to the leakage of the laser light, the spot shape of the laser light focused inside the wafer is controlled to be elliptical, and the power of the laser light at the focusing point is controlled. Equipped with a laser beam shape control means to disperse
The laser light shape control means controls the spot shape of the laser light to be an elliptical shape having a minor axis or a major axis in a direction oblique to the direction along the planned cutting line.
Les Za processing equipment. The spatial light modulator that modulates the laser light, and
The spatial light modulator is controlled so that the laser light is simultaneously focused at a plurality of positions inside the wafer, which have different depths from the back surface and are separated from each other in the direction along the planned cutting line. Spatial light modulator control means and
An aberration correction means that is configured separately from the spatial light modulator and corrects the aberration of the laser light to be equal to or less than a predetermined aberration at the plurality of positions where the focusing points of the laser light are aligned inside the wafer. ,
The laser processing apparatus according to claim 1. In the spatial light modulator control means, the laser light is focused on a first position in the depth direction of the wafer, and a second position different from the first position in the depth direction of the wafer. Control the spatial light modulator so that the laser light is focused on .
The laser processing apparatus according to claim 2. It is a laser processing method that irradiates a laser beam from the back surface opposite to the device surface of the wafer along a planned cutting line to condense the laser beam inside the wafer.
In order to reduce the damage to the device surface due to the leakage of the laser light, the spot shape of the laser light focused inside the wafer is controlled to be elliptical, and the power of the laser light at the focusing point is controlled. The laser light shape control step includes a laser light shape control step of dispersing, and the laser light shape control step includes an elliptical shape having a direction in which the spot shape of the laser light is oblique to a direction along the planned cutting line as a minor axis or a major axis. Control to be,
Les Za processing method. The modulation step of modulating the laser light with a spatial light modulator,
The spatial light modulator is controlled so that the laser light is simultaneously focused at a plurality of positions inside the wafer, which have different depths from the back surface and are separated from each other in the direction along the planned cutting line. Spatial light modulator control process and
Aberration correction means configured separately from the spatial light modulator is used so that the aberration of the laser light is equal to or less than a predetermined aberration at the plurality of positions where the focusing points of the laser light are aligned inside the wafer. Aberration correction process to correct to
The laser processing method according to claim 4. In the spatial light modulator control step, the laser beam is focused on a first position in the depth direction of the wafer, and a second position different from the first position in the depth direction of the wafer. Control the spatial light modulator so that the laser light is focused on.
The laser processing method according to claim 5.

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