JP7545027B2 - Wafer processing method and system - Google Patents

Description

The present invention relates to a wafer processing method and system, and to a wafer processing method and system that divides a wafer starting from a laser processing area formed inside the wafer.

Conventionally, there is known a laser processing device (also called a laser dicing device) that irradiates a laser beam along a planned division line by focusing the beam inside a wafer such as silicon, and forms a laser processing area that serves as the starting point for cutting inside the wafer along the planned division line. The wafer with the laser processing area formed therein is then divided along the planned division line by a cutting process such as expanding or breaking, and separated into individual chips (see, for example, Patent Document 1).

JP 2020-057743 A

In the wafer splitting process, the load (pressure) applied to the wafer when the back side of the wafer is ground using a grinder is used to propagate a crack from the laser processed area and split the wafer.

Figure 8 is a partial cross-sectional view showing the wafer breaking process, where Figure 8(a) shows the wafer W before grinding, Figure 8(b) shows the state in which a crack has developed inside the wafer W due to the load during grinding, and Figure 8(c) shows the state in which the wafer W has been broken along the crack. Figure 9 is a partial enlarged view of region XI in Figure 8(c).

First, as shown in FIG. 8(a), a backgrind tape (not shown in FIG. 8; BG in FIG. 10) is attached to the surface Wa of the wafer W, on which devices such as electronic circuits are formed, and the wafer W is then placed with the surface Wa facing down on a suction stage (not shown) and held by suction. Inside the wafer W, laser processing regions R1 and R2 are formed by laser processing. The laser processing regions R1 and R2 are the starting points of cracks K inside the wafer W, and are formed in two stages at different positions in the depth direction (Z direction) of the wafer W. Note that multiple laser processing regions R1 and R2 are formed along the planned dividing line of the wafer W, but are simplified to two in FIG. 8.

Next, the back surface Wb of the wafer W is ground by the grinding machine 50. A grinding wheel 54 is attached to the rotating grinding machine 52 of the grinding machine 50, and the grinding wheel 54 is brought into contact with the back surface Wb of the wafer W and the rotating grinding machine 52 is rotated to grind the wafer W.

As grinding of the wafer W progresses, the load applied to the back surface Wb of the wafer W via the grindstone 54 causes the crack K to grow in the depth direction of the wafer W, as shown in FIG. 8(b). After reaching the front surface Wa and back surface Wb of the wafer W, the wafer W is broken along the planned division line, as shown in FIG. 8(c), and the space between the broken chips C1 and C2 is pushed apart, creating a tiny gap Gap.

As shown in an enlarged view in Figure 9, grinding debris (also called sludge) SL generated by grinding may penetrate into the gap GAP between chips C1 and C2 and reach the surfaces of chips C1 and C2 (see Figure 10).

Figure 10 is an image showing the surface of the backgrind tape after the chips have been picked up after cleavage. As shown in Figure 10, grinding debris SL is attached to the surface of the backgrind tape BG in a roughly lattice pattern along the planned division lines of the wafer W. Such grinding debris SL can cause contamination of the devices formed on the surfaces of the chips C1 and C2.

The present invention was made in consideration of these circumstances, and aims to provide a wafer processing method and system that can prevent grinding debris from entering the gaps that occur between chips after the wafer is cut.

In order to solve the above problem, the wafer processing method according to the first aspect of the present invention includes a laser processing step of forming a laser processing area along a planned dividing line of the wafer inside a substrate of a wafer having a device layer stacked on the front surface of the substrate, and a grinding step of grinding the back surface of the wafer to set the thickness of the wafer to a target thickness and dividing the wafer into individual chips, and the laser processing step includes a position setting step of setting the position of the laser processing area in the thickness direction of the wafer to the position of the target thickness, and a laser processing area forming step of forming the laser processing area at the position set in the position setting step by focusing laser light inside the wafer.

In the wafer processing method according to the second aspect of the present invention, in the laser processing area forming step of the first aspect, multiple layers of laser processing areas are formed in the thickness direction of the wafer, and in the position setting step, the laser processing area closest to the back side of the multiple layers of laser processing areas is set to the position of the target thickness.

In the wafer processing method according to the third aspect of the present invention, in the position setting step of the first or second aspect, the position of the laser processing area is set so that the position of the target thickness overlaps an area ranging from the back side to one-third of the laser processing area.

In the wafer processing method according to the fourth aspect of the present invention, in the position setting step of the first or second aspect, the position of the laser processing area is set so that the position of the target thickness overlaps an area ranging from the back side to one-third of the area of the laser processing area excluding the void at the bottom end.

The wafer processing system according to the fifth aspect of the present invention includes a laser processing device that forms a laser processing area along a planned dividing line of the wafer inside a substrate of a wafer having a device layer stacked on the front surface of the substrate, and a grinding device that grinds the back surface of the wafer to set the thickness of the wafer to a target thickness and divides the wafer into individual chips. The laser processing device includes a position setting unit that sets the position of the laser processing area in the thickness direction of the wafer to the position of the target thickness, and a laser processing unit that forms the laser processing area at the position set by the position setting unit by focusing laser light inside the wafer.

According to the present invention, by forming a laser processing area according to the target thickness of the wafer, it is possible to prevent grinding debris from entering the gaps between the chips after the wafer is cut.

FIG. 1 is an image showing a processed cross section of a chip after a wafer is divided. FIG. 2 is a block diagram showing the configuration of a laser processing device in the wafer processing system according to one embodiment of the present invention. FIG. 3 is a block diagram showing the configuration of a grinding device in the wafer processing system according to one embodiment of the present invention. FIG. 4 is a cross-sectional view showing the relationship between the laser processing area and the target thickness Ht. FIG. 5 is an image showing the processed cross section of the chip. FIG. 6 is an image showing the processed cross section of the chip after grinding. FIG. 7 is an image showing the surface of the backgrind tape after the chips have been picked up after cleavage. FIG. 8 is a partial cross-sectional view showing the wafer breaking process. FIG. 9 is a partial enlarged view of region XI in FIG. FIG. 10 is an image showing the surface of the backgrind tape after the chips have been picked up after cleavage.

Below, an embodiment of the wafer processing method and system according to the present invention will be described with reference to the attached drawings.

[Overview of the embodiment]
FIG. 1 is an image showing a processed cross section of a chip after a wafer is divided.

As shown in FIG. 1, when the laser processing regions R1 and R2 remain on the processed cross section (side wall) CW of the chip C after dividing the wafer W, it can be seen that the grinding debris (sludge) SL generated by the grinding process is collected in the laser processing region R2 on the upper side (+Z side, the back surface Wb (see FIG. 2) side of the wafer W) and does not invade the front surface Wa side of the laser processing region R2. From this, it is considered that the laser processing regions R1 and R2 have the function of collecting the grinding debris SL that invades from the surface side of the wafer W that is being ground like a net. Specifically, the surfaces of the laser processing regions R1 and R2 have more irregularities than the surfaces that are broken by the cracks that have developed from the laser processing regions R1 and R2, and it is considered that these irregularities function as a net to collect the grinding debris SL.

In this embodiment, the Z-direction position of the laser processing area R2 on the back surface Wb side of the wafer W is set to correspond to the target thickness value Ht of the chip C (hereinafter referred to as the target thickness), thereby preventing grinding debris SL from penetrating the front surface Wa side of the laser processing area R2 (see FIG. 4).

[Wafer Processing System]
The wafer processing system 10 according to this embodiment includes a laser processing device 10-1 (see FIG. 2) and a grinding device 10-2 (see FIG. 3). The laser processing device 10-1 forms a laser processing area inside the wafer W along a planned dividing line of the wafer W (laser processing step). The grinding device 10-2 grinds the back surface Wb of the wafer W to set the thickness of the wafer W to the target thickness Ht, and divides the wafer W into individual chips C by utilizing the load applied to the wafer W during grinding (grinding step).

(Laser processing)
FIG. 2 is a block diagram showing the configuration of a laser processing device in the wafer processing system according to one embodiment of the present invention.

As shown in FIG. 2, the laser processing device 10-1 includes a control unit 12, a wafer moving unit 14, and a laser processing unit 16.

The control unit 12 has a CPU (Central Processing Unit), memory (e.g., ROM (Read Only Memory), RAM (Random Access Memory), etc.), storage (e.g., HDD (Hard Disk Drive), SSD (Solid State Drive), etc.), input/output circuitry, etc., and controls the operation of each part of the laser processing device 10-1. The control unit 12 is realized, for example, by a personal computer or a workstation.

The wafer moving unit 14 includes a suction stage T1 that holds the wafer W by suction, and an XYZθ table that is provided on the main body base (not shown) of the laser processing device 10-1 and moves the suction stage T1 in the XYZθ directions.

The wafer W is, for example, a disk-shaped semiconductor wafer made of silicon. The surface Wa of the substrate of the wafer W is partitioned into lattice-like regions by a number of planned division lines extending in the XY direction, for example, and devices (device layers) such as electronic circuits are formed (stacked) in each of these lattice-like regions.

When dividing the wafer W into individual chips C, first, a backgrind tape (protective tape) BG is applied to the surface Wa of the wafer W, and the wafer is placed with the surface Wa facing down on the holding surface of the table T of the laser processing device 10-1. The wafer W is then suction-held by the suction stage T1. Note that the wafer W may also be suction-held without the backgrind tape BG being applied.

The laser processing unit 16 includes a laser light source, optical elements such as a condensing lens, and a driving means for slightly moving the laser light L in the Z direction relative to the wafer W. As the laser light source, for example, a semiconductor laser excited Nd:YAG (Yttrium Aluminum Garnet) laser is used. The laser light L emitted from the laser light source is focused inside the wafer W by the condensing lens. As a result, laser processing regions R1 and R2 are formed inside the wafer W.

The laser processed regions R1 and R2 refer to regions in which the physical properties of the inside of the wafer W, such as density, refractive index, and mechanical strength, become different from those of the surrounding area due to irradiation with laser light, resulting in a lower strength than the surrounding area. The laser processed regions R1 and R2 include, for example, crack regions.

When forming the laser processing regions R1 and R2 on the wafer W, laser light L is emitted from the laser processing unit 16 and irradiated onto the wafer W via an optical system such as a condenser lens. The Z-direction position of the focal point FP of the irradiated laser light L is accurately set to a predetermined position inside the wafer W by adjusting the Z-direction position of the wafer W using the XYZθ table and controlling the position of the condenser lens.

In this state, the XYZθ table is fed for processing in the X direction, which is the dicing direction. As a result, one line of laser processing regions R1 and R2 is formed along the planned dividing line of the wafer W. Then, once one line of laser processing regions R1 and R2 has been formed along the planned dividing line, the XYZθ table is indexed and fed one pitch in the Y direction, and laser processing regions R1 and R2 are also formed on the next planned dividing line. Next, once laser processing regions R1 and R2 have been formed along all of the planned dividing lines in the X direction, the XYZθ table is rotated 90° around the Z axis, and laser processing regions R1 and R2 are similarly formed on the planned dividing line in the X direction after the rotation.

The procedure for forming the laser processing regions R1 and R2 is not particularly limited. For example, as shown in FIG. 2, the first layer of laser processing region R1 may be formed on the entire surface of the wafer W, and then the second layer of laser processing region R2 (on the back surface Wb side of the wafer W) may be formed. In this case, the laser processing conditions for forming the laser processing regions R1 and R2 may be different from each other or may be the same. Alternatively, a branching section (e.g., a beam splitter, etc.) that branches the laser light L may be provided, and the laser light L may be focused on two focusing points at different positions (depths) in the Z direction inside the wafer W, thereby forming two layers of laser processing regions R1 and R2 in one scan.

The wafer W on which the laser processing regions R1 and R2 are formed is transferred from the laser processing device 10-1 to the grinding device 10-2, where the back surface of the wafer W is ground to remove a portion of the laser processing region R2 on the back surface, and the wafer W is divided into individual chips (see Figure 3).

(Grinding)
3 is a block diagram showing the configuration of a grinding device in a wafer processing system according to an embodiment of the present invention, which illustrates a state in which the back surface Wb of the wafer W has been ground down to Wb1.

As shown in FIG. 3, the grinding device 10-2 according to this embodiment includes a grinding control unit 18, a thickness measurement unit 20, a wafer moving unit 22, and a grinding machine (grinder) 50. The grinding machine 50 includes a rotary grinding disk 52 and a grinding wheel 54 attached to the rotary grinding disk 52.

The grinding control unit 18 includes a motor for rotating the rotary grinding machine 52 around a shaft. In response to a command from the control unit 12, the grinding control unit 18 adjusts the Z-direction position of the rotary grinding machine 52 while supplying slurry to the back surface Wb of the wafer W from a slurry supply port (not shown) to rotate the grinding wheel 54 in contact with the back surface Wb of the wafer W.

The wafer moving unit 22 includes a chuck table T2 that holds the wafer W by suction, and an XY table (not shown) that moves the chuck table T2 in the X and Y directions within the grinding device 10-2.

After the laser processing regions R1 and R2 are formed in the laser processing device 10-1, the wafer W is transported to the grinding device 10-2. The wafer W is then placed with its front surface Wa facing down on the holding surface of the chuck table T2 and is adsorbed and held by the chuck table T2. Next, while the wafer moving unit 22 moves the chuck table T2 in the XY directions, the grinding control unit 18 abuts the grindstone 54 against the back surface Wb of the wafer W and rotates it, thereby grinding the entire back surface Wb of the wafer W.

The thickness measuring unit 20 is a means for measuring the thickness of the wafer W. The thickness measuring unit 20 is capable of measuring the thickness of the wafer W in-situ while the back surface Wb of the wafer W is being ground. The thickness measuring unit 20 may be a contact type means for measuring by contacting a contact height gauge with the back surface Wb of the wafer W. The thickness measuring unit 20 may also be a non-contact type means (e.g., a ToF (Time-of-Flight) method) for measuring the distance to the back surface Wb of the wafer W by irradiating the back surface Wb of the wafer W with laser light from a laser light source (e.g., a semiconductor laser).

The grinding control unit 18 grinds the back surface Wb of the wafer W while calculating the thickness of the wafer W based on the output from the thickness measurement unit 20. This allows the thickness of the wafer W to be set to the target thickness Ht.

In the example shown in FIG. 3, the grinding device 10-2 is controlled by the same control unit 12 as the laser processing device 10-1, but it may be controlled by a control unit (e.g., a personal computer or a workstation) separate from the laser processing device 10-1. In other words, the laser processing device 10-1 and the grinding device 10-2 may be separate and independent devices. In this case, the control unit of the grinding device 10-2 may share information regarding the positions and sizes of the laser processing regions R1 and R2, and information regarding the target thickness Ht of the wafer W, from the control unit 12 of the laser processing device 10-1 via a communication line or a storage device, etc.

[Laser processing area]
Next, the relationship between the laser processing region R2 and the target thickness Ht of the chip C will be described. Fig. 4 is a cross-sectional view showing the relationship between the laser processing region and the target thickness Ht, and Fig. 5 is an image showing the processed cross section of the chip. For simplicity, Fig. 4 shows only the laser processing region R2 on the back surface Wb side of the wafer W.

As shown in FIG. 4, the laser processed region R2 includes a void R20 located at its lower end and an upper void region R22 formed above the void R20 (on the back surface side).

For example, when the laser light L is a pulsed laser, the laser light L focused inside the wafer W is locally absorbed at the focal point FP and the laser light absorption region nearby. At this time, the temperature of the focal point FP and the laser light absorption region nearby rises instantaneously (about 10,000 K in one example), and the silicon in the laser light absorption region vaporizes all at once, forming a void (vacancy) R20.

Next, the high temperature portion expands rapidly as a thermal shock wave above the void R20, i.e., toward the side irradiated with the laser light L. At this time, the temperature difference between the high temperature portion and the surrounding low temperature portion becomes very large, and the thermal expansion of the high temperature portion is suppressed by the surrounding low temperature portion, so that the high temperature portion is subjected to very strong compression. As a result, the high temperature portion does not melt, and a high dislocation density layer is formed.

Then, when the next pulse wave is irradiated, dislocations in the high dislocation density layer in the high temperature area act as nuclei, and cracks K propagate from the region R22 above the void to the low temperature area of the single crystal.

The region R22 above the void refers to the region above the void R20 where laser processing marks (such as a high dislocation density layer or microcracks) can be confirmed.

As described above, the laser processing regions R1 and R2 are considered to have the function of collecting, like a net, grinding debris SL that has entered from the surface of the wafer W that is being ground. For this reason, it is possible to grind away part of the laser processing region R2 to expose the laser processing region R2 on the back side and sidewall CW of the chip C.

In this regard, the inventors of the present invention have discovered that when grinding a portion of the laser processing region R2, if grinding is performed up to the vicinity of the void R20 in the laser processing region R2, debris and meandering are likely to occur on the back side (upper side in the figure) of the chip C. For this reason, in this embodiment, a portion of the region R22 above the void in the laser processing region R2 is ground (without grinding the vicinity of the void R20) to expose the region R22 above the void on the back side and sidewall CW of the chip C. This makes it possible to improve the function of collecting grinding debris SL by the laser processing region R20 without generating debris and meandering on the back side of the chip C.

Therefore, in this embodiment, as shown in FIG. 4, laser processing is performed so that a region R22 above the void in the laser processing region R2 is formed at a position of the target thickness Ht from the surface Wa of the chip C.

The control unit 12 adjusts the laser processing conditions of the laser processing unit 16 to set the position and size of the laser processing region R2 (void R20 and the region R22 above the void) (position setting step). Specifically, first, for each material of the wafer W and for each laser processing condition such as the position of the focal point FP during laser processing and the intensity, frequency, numerical aperture, and irradiation time of the laser light L, laser processing data for the position and size of the laser processing region R2 (void R20 and the region R22 above the void) is obtained in advance experimentally or by simulation. Then, based on the material and target thickness Ht of the wafer W and the laser processing data, the control unit 12 adjusts the position of the focal point FP during laser processing and the laser processing conditions such as the intensity, frequency, numerical aperture, and irradiation time of the laser light L so that the region R22 above the void of the laser processing region R2 overlaps with the position of the target thickness Ht. Here, the control unit 12 functions as a position setting unit.

The laser processing unit 16 forms a laser processing area R2 inside the wafer W along the planned dividing line of the wafer W in accordance with instructions from the control unit 12 (laser processing area forming step).

Here, it is preferable to set the formation position of the laser processing region R2 so that when the wafer W is ground to the target thickness Ht, a part of the upper end of the laser processing region R2 (for example, a region ranging from the top (back side) of the laser processing region R2 to about one-third of the area) is ground. More specifically, when the dimension of the laser processing region R2 in the Z direction is H1, it is preferable to adjust the laser processing conditions so that the distance Δ from the upper end of the laser processing region R2 in the figure to the position of the target thickness Ht is 0 < Δ ≦ H1/3.

Alternatively, the formation position of the laser processing region R2 may be set so that when the wafer W is ground to the target thickness Ht, a portion of the upper end of the laser processing region R2 (for example, a region ranging from the top (back side) of the void upper region R22 of the laser processing region R2 to about one-third of the area) is ground. In other words, when the Z-direction dimension of the void upper region R22 of the laser processing region R2 is H2, the laser processing conditions may be adjusted so that the distance Δ from the upper end of the void upper region R22 of the laser processing region R2 in the figure to the position of the target thickness Ht is 0 < Δ ≦ H2/3.

In this embodiment, the laser processing area is two layers, R1 and R2, but the present invention is not limited to this. The laser processing area may be formed in only one layer or in multiple layers of three or more layers. When forming three or more layers of laser processing areas, the laser processing area closest to the back surface side (closest to the back surface Wb of the wafer W) may be formed so that the position of the target thickness Ht overlaps.

Figure 6 is an image showing the cross section of the chip after grinding, and Figure 7 is an image showing the surface of the backgrind tape after the chip has been picked up after being broken.

As shown in Figure 6, when grinding was completed about one-third of the way from the top (back side) of the laser processing area R2 and the wafer W was divided, as shown in Figure 7, almost no grinding debris SL was detected on the surface of the back grind tape BG.

According to this embodiment, by forming the laser processing region R2 so that it overlaps the position of the target thickness Ht, it is possible to prevent the grinding debris SL from entering the gap GAP between the chips C after the wafer W is cleaved.

[others]
In addition, when performing grinding, if fine grits such as 6000 or more or polish are used, surface burning is likely to occur. On the other hand, if coarse grits are used, chipping or small debris is likely to occur. For this reason, it is preferable to use coarse grits such as 3600.

10...wafer processing system, 10-1...laser processing device, 10-2...grinding device, 12...control unit, 14...wafer moving unit, 16...laser processing unit, 18...grinding control unit, 20...thickness measuring unit, 22...wafer moving unit, 50...grinding machine, 52...rotary grinding machine, 54...grinding wheel, T1...suction stage, T2...chuck table

Claims (6)

A laser processing step of forming a laser processed region including a void and a region above the void along a planned dividing line of the wafer inside the wafer having a device layer laminated on a surface of the wafer;
a grinding step of grinding the back surface of the wafer to a target thickness and dividing the wafer into individual chips;
The laser processing step includes:
a position setting step of setting a position of the laser processing area in a thickness direction of the wafer to a position of the target thickness;
a laser processing area forming step of forming the laser processing area having irregularities capable of collecting grinding debris generated in the grinding step by focusing a laser beam inside the wafer so that the region above the void is located at the position set in the position setting step;
A wafer processing method comprising: The wafer processing method of claim 1 , wherein the grinding step includes grinding away a portion of the region above the void to expose the region above the void on a backside and a sidewall of the wafer. In the laser processing region forming step, a plurality of layers of laser processing regions are formed in a thickness direction of the wafer,
3. The wafer processing method according to claim 2 , wherein in the position setting step, the laser processing region closest to the back surface side among the laser processing regions of the plurality of layers is set to a position of the target thickness. In the position setting step, the position of the laser processing area is set so that the position of the target thickness overlaps an area ranging from a back side to one third of the laser processing area.
The wafer processing method according to claim 2 or 3 . 4. The wafer processing method according to claim 2 or 3, wherein in the position setting step, the position of the laser processing area is set so that the position of the target thickness overlaps an area ranging from the back surface side to one - third of the area of the laser processing area excluding a void at a lower end. a laser processing apparatus for forming a laser processed region including a void and a region above the void along a planned dividing line of a wafer inside the wafer, the wafer having a device layer laminated on a surface of the wafer;
a grinding device that grinds the back surface of the wafer to a target thickness and divides the wafer into individual chips;
The laser processing apparatus includes:
a position setting unit that sets the position of the laser processing area in the thickness direction of the wafer to the position of the target thickness;
a laser processing unit that forms the laser processing area having irregularities capable of collecting grinding debris generated by the grinding process by focusing a laser beam inside the wafer so that the region above the void is located at a position set by the position setting unit;
A wafer processing system comprising:

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