JP7087401B2 - Manufacturing method of semiconductor device - Google Patents

Description

The techniques disclosed herein relate to methods of manufacturing semiconductor devices.

Patent Document 1 discloses a method for manufacturing a semiconductor device. This manufacturing method includes a step of irradiating the surface of the semiconductor wafer with a laser. In this step, the laser spot is moved on the surface of the semiconductor wafer to irradiate the entire surface of the semiconductor wafer with the laser. By irradiating the laser, the surface electrodes of the semiconductor wafer make ohmic contact with the semiconductor layer.

Japanese Unexamined Patent Publication No. 2016-127157

With the thinning of semiconductor wafers, the problem of the tensile strength of semiconductor wafers has become apparent. If the bending strength of the semiconductor wafer is low, the semiconductor wafer cracks in the manufacturing process of the semiconductor device. The present specification provides a technique for suppressing a decrease in the contact strength of a semiconductor wafer in a laser irradiation step.

The method for manufacturing a semiconductor device disclosed in the present specification includes a first step and a second step of irradiating the surface of a semiconductor wafer with a laser. In the first step, the laser is irradiated so that the laser spot moves along the first direction at the energy density at which the semiconductor wafer melts. In the second step, the laser is irradiated so that the laser spot moves along the region irradiated with the laser in the outer peripheral portion of the laser spot in the first step at an energy density at which the semiconductor wafer is not melted.

The above-mentioned "outer peripheral portion of the laser spot in the first step" means a portion of the laser spot in the first step having an energy density lower than the energy density at which the semiconductor wafer melts. That is, the portion of the laser spot in the first step having the energy density for melting the semiconductor wafer is the main portion, and the peripheral portion thereof is the outer peripheral portion.

In the first step, the laser is applied to the semiconductor wafer so that the laser spot moves along the first direction on the surface of the semiconductor wafer. Here, the temperature distribution in the laser spot becomes high in the main part of the laser spot and low in the outer peripheral part of the laser spot. Further, in the outer peripheral portion of the laser spot, the temperature is distributed so that the temperature becomes lower toward the outer peripheral side, and the temperature difference depending on the position becomes large. Therefore, a high thermal stress is generated in the region irradiated with the laser in the outer peripheral portion of the laser spot, and many crystal defects are formed inside the semiconductor wafer. Therefore, by carrying out the first step, the tensile strength of the semiconductor wafer is lowered. In the second step, the laser is irradiated at an energy density at which the semiconductor wafer is not melted along the region irradiated with the laser (the region where many crystal defects are formed) in the outer peripheral portion of the laser spot in the first step. Therefore, the region with many crystal defects is heated to a relatively low temperature, and most of the crystal defects disappear. As a result, the tensile strength of the semiconductor wafer is restored. Therefore, according to this manufacturing method, the laser irradiation step can be carried out while suppressing the decrease in the contact strength of the semiconductor wafer.

The plan view which shows the irradiation path of a laser. Sectional drawing of semiconductor wafer. The figure which shows the energy distribution in the laser spot of the 1st process. The figure which shows the locus of the laser spot of the 1st process. The figure which shows the energy distribution in the laser spot of the 2nd process. The figure which shows the locus of the laser spot of the 2nd process.

The semiconductor wafer 12 shown in FIGS. 1 and 2 has a semiconductor substrate 14, an upper electrode 16 that covers the upper surface of the semiconductor substrate 14, and a lower electrode 18 that covers the lower surface of the semiconductor substrate 14. The semiconductor substrate 14 is made of SiC. The lower electrode 18 is made of nickel. A switching element or the like is formed inside the semiconductor wafer 12. The semiconductor wafer 12 is later divided into a plurality of semiconductor chips to become a semiconductor device. Hereinafter, a laser irradiation step of making the lower electrode 18 ohmic contact with the semiconductor substrate 14 by irradiating the lower surface of the semiconductor wafer 12 (that is, the surface of the lower electrode 18) with a laser will be described.

The laser irradiation step has a first step and a second step. The arrows 100 in FIGS. 1 and 2 show the loci of the laser spot 20 moving on the lower surface of the semiconductor wafer 12 in the first step. As shown in FIG. 1, the laser spot 20 moves in the x direction while reciprocating in the y direction on the lower surface of the semiconductor wafer 12. As a result, the entire lower surface of the semiconductor wafer 12 is irradiated with the laser.

FIG. 3 shows the laser spot 20 that irradiates the lower surface of the semiconductor wafer 12 in the first step and the energy density distribution of the laser. In FIG. 3, the energy density E1 is the energy density required to melt the semiconductor layer of the semiconductor substrate 14, and the energy density E2 is half the maximum energy density within the laser irradiation range. In FIG. 3, the portion having an energy density lower than the energy density E2 has an extremely low ability to heat the semiconductor wafer 12. Therefore, in the present embodiment, the portion having an energy density higher than the energy density E2 is referred to as a laser spot 20. Further, a portion of the laser spot 20 having an energy density higher than that of the energy density E1 is referred to as a main portion 20a. Further, the portion of the laser spot 20 having an energy density lower than the energy density E1 is referred to as an outer peripheral portion 20b. The main portion 20a is located in the center of the laser spot 20, and the outer peripheral portion 20b is located around the main portion 20a. As shown in FIG. 3, in the main portion 20a, the energy density is distributed substantially uniformly at a high value. In the outer peripheral portion 20b, the energy density decreases toward the outer peripheral side of the laser spot 20. Within the outer peripheral portion 20b, there is a large difference in energy density depending on the position in the radial direction.

FIG. 4 shows how the laser spot 20 moves in the y direction in the first step. In the first step, the laser is pulsed while moving the laser irradiation position in the y direction. That is, the laser is irradiated intermittently in an extremely short cycle. Therefore, the laser spot 20 draws a trajectory as shown in FIG. In FIG. 4, the gray-hatched region 22 is a region through which the outer peripheral portion 20b of the laser spot 20 passes. That is, the region 22 is a region heated by the outer peripheral portion 20b without being heated by the main portion 20a. Further, in FIG. 4, the range sandwiched between the two regions 22 is a region through which the main portion 20a of the laser spot 20 passes.

In the region through which the main portion 20a passes, the semiconductor layer is instantaneously melted in the vicinity of the lower surface of the semiconductor substrate 14, and then solidified immediately. As a result, the semiconductor layer (that is, SiC) and the lower electrode 18 (nickel) are alloyed, and the lower electrode 18 changes to nickel silicide. As a result, the lower electrode 18 makes ohmic contact with the semiconductor substrate 14.

On the other hand, in the region 22 through which the outer peripheral portion 20b passes, the energy density is low, so that the semiconductor layer of the semiconductor substrate 14 does not melt. Further, as shown in FIG. 3, in the outer peripheral portion 20b, the energy density is distributed so as to decrease toward the outer peripheral side. Therefore, at the time of laser irradiation, the temperature is distributed in the region 22 so that the temperature becomes lower toward the outer peripheral side of the laser spot 20. That is, a large temperature difference occurs inside the region 22. Therefore, a high thermal stress is generated in the semiconductor layer in the region 22. As a result, as shown in FIG. 2, a large number of crystal defects 90 are generated in the semiconductor layer in the region 22. In particular, since the region 22 is located at the boundary between the region where the semiconductor layer melts (the region through which the main portion 20a passes) and the region where the semiconductor layer does not melt, many crystal defects are generated in the region 22. As described above, many crystal defects are generated in the region 22, so that the tensile strength of the semiconductor wafer 12 is lowered.

After the first step is carried out, the second step is carried out. In the second step, the lower surface of the semiconductor wafer 12 (that is, the surface of the lower electrode 18) is irradiated with the laser at a lower energy density than in the first step. In the second step, the laser is irradiated with an energy density at which the semiconductor layer of the semiconductor substrate 14 does not melt. FIG. 5 shows the energy density distribution of the laser irradiated on the lower surface of the semiconductor wafer 12 in the second step. In FIG. 5, the energy density E3 is an energy density of 90% of the maximum value of the energy density in the laser irradiation range, and the energy density E4 is an energy density of half the maximum value of the energy density in the laser irradiation range. In the second step, the portion having an energy density higher than the energy density E4 is referred to as a laser spot 30. Further, the portion of the laser spot 30 having an energy density higher than that of the energy density E3 is referred to as a main portion 30a. Further, the portion of the laser spot 30 having an energy density lower than the energy density E3 is referred to as an outer peripheral portion 30b. Further, in FIG. 5, the energy density E1 shows the same value as the energy density E1 in FIG. 3 (energy density required for melting the semiconductor substrate 14). As shown in FIG. 5, the energy density of the entire laser spot 30 is lower than the energy density E1. The main portion 30a is located in the central portion of the laser spot 30, and the outer peripheral portion 30b is located around the main portion 30a.

In the second step, as shown in FIG. 6, the laser spot 30 is moved so that the main portion 30a of the laser spot 30 passes over the region 22 (the region where the crystal defect is generated in the first step). In FIG. 6, the main portion 30a and the outer peripheral portion 30b are not shown for the sake of easy viewing. Since the energy density of the main portion 30a of the laser spot 30 is lower than the energy density E1, the semiconductor layer is heated to a temperature at which the semiconductor layer does not melt. Then, most of the crystal defects in the semiconductor layer in the region 22 disappear, and the number of crystal defects decreases. Further, when the laser spot 30 is moved as shown in FIG. 6, the temperature is distributed in the outer peripheral portion 30b (see FIG. 5) of the laser spot 30 so that the temperature becomes lower toward the outer peripheral side. However, as shown in FIG. 5, in the outer peripheral portion 30b, the gradient in which the energy density changes is smaller than that in the outer peripheral portion 20b (see FIG. 3) of the laser spot 20 in the first step. Therefore, the temperature gradient generated in the semiconductor layer in the outer peripheral portion 30b is small. Therefore, not so high thermal stress is generated in the semiconductor layer in the outer peripheral portion 30b, and crystal defects are less likely to be generated in the semiconductor layer in the outer peripheral portion 30b. In particular, in the second step, since the semiconductor layer does not melt in the entire laser spot 30, the outer peripheral portion 30b does not form a boundary between the melted region and the unmelted region. As a result, crystal defects are less likely to occur in the outer peripheral portion 30b. Therefore, in the second step, almost no crystal defects occur in the passing region of the outer peripheral portion 30b.

After the second step is performed, the semiconductor wafer 12 is divided into a plurality of pieces to manufacture a semiconductor device.

As described above, in the second step, the crystal defects generated in the first step can be reduced with almost no new crystal defects. As a result, the tensile strength of the semiconductor wafer 12 is restored in the second step. As described above, according to this manufacturing method, the laser irradiation step can be carried out while suppressing the decrease in the contact strength of the semiconductor wafer 12.

Although the embodiments have been described in detail above, these are merely examples and do not limit the scope of claims. The techniques described in the claims include various modifications and modifications of the specific examples exemplified above. The technical elements described in the present specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the techniques exemplified in the present specification or the drawings achieve a plurality of objectives at the same time, and achieving one of the objectives itself has technical usefulness.

12: Semiconductor wafer 14: Semiconductor substrate 16: Upper electrode 18: Lower electrode 20: Laser spot 20a: Main part 20b: Outer peripheral part 22: Region 30: Laser spot 30a: Main part 30b: Outer peripheral part

Claims (6)

It is a manufacturing method of semiconductor devices.
It has a first step and a second step of irradiating the surface of the semiconductor wafer with a laser.
In the first step, the laser is irradiated so that the laser spot moves along the first direction at the energy density at which the semiconductor wafer melts.
The second step is carried out after the region melted in the first step has solidified.
In the second step, the laser is irradiated so that the laser spot moves along the outer peripheral irradiation region irradiated with the laser in the outer peripheral portion of the laser spot in the first step at an energy density that does not melt the semiconductor wafer. ,
Production method. The main part of the laser spot in the first step is a portion of the laser spot in the first step having an energy density for melting the semiconductor wafer.
The outer peripheral portion of the laser spot of the first step is an outer portion of the main portion of the laser spot of the first step.
The outer peripheral portion irradiation region is a region where the laser is irradiated at the outer peripheral portion of the laser spot of the first step and the laser is not irradiated at the main portion of the laser spot of the first step. The manufacturing method described in. The portion where the laser spot of the first step has an energy density higher than half of the maximum value of the energy density within the irradiation range of the laser of the first step.
The laser spot in the second step is a portion having an energy density higher than half of the maximum value of the energy density in the irradiation range of the laser in the second step.
The manufacturing method according to claim 2. The main part of the laser spot of the second step is a portion having an energy density higher than 90% of the maximum value of the energy density within the irradiation range of the laser of the second step.
The manufacturing method according to claim 3, wherein in the second step, the laser is irradiated so that the main portion of the laser spot of the second step passes over the outer peripheral portion irradiation region. The manufacturing method according to claim 3, wherein in the second step, the laser is irradiated so that the center of the laser spot of the second step passes over the outer peripheral irradiation region. In the first step, when the laser spot of the first step is moved along the first direction, two outer peripheral irradiation regions extending along the first direction are formed.
In the second step, when the laser spot of the second step passes over one of the two outer peripheral irradiation regions, the laser spot of the second step is placed on the other of the two outer peripheral irradiation regions. Do not overlap,
The manufacturing method according to any one of claims 2 to 5.

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