JP7686413B2 - Method for improving edge quality of silicon wafers - Google Patents

Description

The present invention relates to the repair of surface defects, which are layers that have been altered by processing on the surface of a silicon wafer, and in particular to a method for improving the quality of silicon wafers by using laser heat treatment to improve the quality of the edges (periphery, notch, and orientation flat).

Semiconductor wafers such as silicon wafers used in the manufacture of semiconductor devices are surface-treated by mechanical processes such as cutting, grinding, lapping, and polishing. This surface treatment causes a process-induced deterioration layer to form on the surface and inside, and some of the process-induced deterioration layers contain microcracks. These internal cracks are mainly removed by chemical and mechanical methods such as etching and chemical mechanical polishing (CMP).

It is known that the use of laser irradiation can eliminate oxygen from silicon wafers and improve their crystallinity. For example, Patent Document 1 describes a method for repairing surface defects, which are layers that have been altered by processing on the surface of a single crystal wafer, in which a pulsed laser is irradiated onto the single crystal surface.

Patent Document 2 also describes that after grinding such as chamfering, in order to repair the process-affected layer on the silicon wafer surface and smooth out the roughness, the surface condition after grinding is checked before laser irradiation, and the laser irradiation is performed under conditions appropriate to the surface condition.

JP 2008-147639 A JP 2020-131218 A

In the above-mentioned conventional technology, the one described in Patent Document 1 is a surface modification by laser irradiation for simple flat surfaces, and does not irradiate complex shapes or under conditions according to various surface conditions after grinding. Therefore, with the one described in Patent Document 1, it is difficult to effectively improve the quality of the edge (periphery, notch, and orientation flat) of a silicon wafer, for example, to remove large irregularities caused by damage during the chamfering process.

In addition, the technique described in Patent Document 2 makes it possible to repair damage such as grinding marks caused by grinding and perform flattening processing. However, the technique described in Patent Document 2 has limitations in terms of laser irradiation under conditions according to the surface state after grinding, which not only complicates the irradiation conditions and device configuration but also limits the quality improvement of surface modification for internal cracks, surface roughness (particularly roughness caused by anisotropy), etc.
Furthermore, even if the roughness can be removed by simply irradiating a laser after the etching process, it is difficult to simultaneously remove the large irregularities and undulations caused by the damage in the chamfering process.

The object of the present invention is to solve the problems of the conventional technology and to achieve both removal of roughness and waviness when repairing surface defects on silicon wafers by heat treatment using a laser, regardless of the condition of the surface being irradiated. This leads to improved device performance, improved yield in subsequent processes, reduced processing time in the edge polishing process performed after the etching process, and suppressed deformation.

To achieve the above object, the present invention provides a method for improving edge quality of a silicon wafer using laser heat treatment, in which after a chamfering process of the silicon wafer, the ground surface is irradiated with a nanosecond pulsed laser, the silicon wafer is then etched, and after the etching process, the ground surface is again irradiated with the nanosecond pulsed laser.

In addition, in the above, it is preferable that the nanosecond pulse laser be selected to irradiate one of the wavelengths 355, 532, or 785 nm.

Furthermore, in the above, it is desirable to irradiate a CW laser (continuous wave laser) before irradiating the nanosecond pulse laser.

In the above, it is also desirable to irradiate the femtosecond laser before irradiating the nanosecond pulse laser.

Furthermore, in the above, it is desirable that the wavelength of the femtosecond laser is λ=800 nm.

Furthermore, in the above, it is preferable that the femtosecond laser is a single shot laser.

Furthermore, in the above, it is desirable to irradiate the nanosecond pulsed laser by changing at least one of the incidence angle, energy density, scan pitch, and scan speed in response to the surface shape of the silicon wafer.

Furthermore, in the above, it is desirable to irradiate the nanosecond pulse laser so that the angle of incidence is 10 to 15 degrees or less in accordance with the surface shape of the silicon wafer.

According to the present invention, after the chamfering process of the silicon wafer, the ground surface is irradiated with a nanosecond pulsed laser, the silicon wafer is then etched, and after the etching process, the nanosecond pulsed laser is irradiated again. This makes it possible to simultaneously remove roughness and waviness, regardless of the state of the surface to be irradiated in order to repair surface defects, which are the processing-affected layers on the surface of the silicon wafer.

1 is a flowchart illustrating a method for improving edge quality according to an embodiment of the present invention. Schematic process diagram according to one embodiment of the present invention A plan view showing the shape of the notch A side view showing the shape of the notch FIG. 1 is a block diagram showing an apparatus configuration according to a first embodiment of the present invention. FIG. 11 is a block diagram showing the configuration of an apparatus according to a second embodiment of the present invention. FIG. 13 is a block diagram showing the configuration of an apparatus according to a third embodiment of the present invention.

Figure 1 is a flow chart showing an edge quality improvement method according to one embodiment, Figure 2 is a schematic process diagram, Figure 3 is a plan view showing the shape of a notch portion 50 provided in a part of a silicon wafer 1, and Figure 4 is a side view. First, a grinding process is performed as a chamfering process (S1). After the grinding process, a semiconductor wafer such as a silicon wafer 1 has fine grinding marks (scratches or streaks) formed on the ground surface including the chamfered slope 52 of the notch portion 50 after the chamfering process, resulting in a rough surface state. In addition, the grinding marks are formed not only on the notch portion 50 but also on the peripheral end face 54 and the peripheral chamfered slope 52 of the silicon wafer 1.

The damage caused by the grinding process does not only occur on the surface, which can be observed under a microscope, but also extends to the inside, where an area from the surface to a certain depth becomes an amorphous layer or a dislocation layer, forming a process-affected layer. In other words, the surface of the ground silicon wafer 1 becomes a rough surface with undulations and internal damage, with the surface area of the original single crystal layer becoming a process-affected layer.

In particular, the notch portion 50 has a complex shape compared to the outer peripheral portion 53, and is surface-processed by grinding. A process-degraded layer is formed on the surface and inside, and some of the process-degraded layers contain microcracks. Normally, the planar shape of the notch portion 50 (Fig. 3) has a circular arc-shaped bottom 51 formed as a notch, a chamfered slope 52 that faces the outer peripheral portion 53 at an angle of about 45°, and then a rounded connection to the outer peripheral portion 53, resulting in a shape that is symmetrical from left to right. In particular, the bottom portion 51 is greatly affected by the processing load due to the grinding process, and contains many surface defects. In the side shape (Fig. 4) of the outer peripheral portion 53, a chamfered portion 56 is provided as a rounded slope on the end face 54, which is connected to the top face 55, making it vertically symmetrical.

After the chamfering process, laser irradiation (S2) is performed on the edge of the silicon wafer 1, which is the ground surface. The surface area of the edge irradiated with the laser melts and becomes flat due to surface tension, while the area near the surface is restored by single crystallization. In other words, the ground portion of the silicon wafer 1 has crystal defects in the layer near the surface of the processed portion due to the machining process, and has become a processing-induced layer such as an amorphous layer or a dislocation layer.

Figure 5 is a block diagram showing the device configuration of the first embodiment. In Figure 5, the silicon wafer 1 is rotating clockwise. In the first embodiment, as the laser irradiation (S2, S4), a nanosecond pulsed laser is irradiated from a pulsed laser oscillator 2 to the silicon wafer surface via a prism system 20 and a focusing optical system 21. After the chamfering process of the silicon wafer 1, a laser heat treatment is performed by irradiating the notch portion 50 and the outer periphery 53, or the upper and lower flat surfaces of the silicon wafer 1 with a pulsed laser (nanosecond) in order to modify and flatten the surface of the processing-affected layer.

The laser absorption rate of the process-affected layer is significantly higher than that of the single crystal region, so the process-affected layer melts when irradiated with a laser (nanosecond pulsed laser), and the molten area expands due to thermal conduction, while the surface of the molten area is flattened by surface tension. Therefore, in the first embodiment, after the chamfering process of the silicon wafer 1, a pulsed laser (nanosecond) is irradiated to the notch portion 50 and the outer periphery 53, or the upper and lower flat surfaces of the silicon wafer 1, to modify and flatten the surface of the process-affected layer, to perform laser heat treatment.

Pulsed laser (nanosecond) irradiation melts the amorphous layer that has been altered by machining at nanosecond speeds. Furthermore, melting caused by pulsed laser (nanosecond) irradiation promotes recrystallization (epitaxial growth) with aligned crystal orientation, eliminating crystal defects caused by mechanical processing. In addition, amorphous silicon layers have strong absorption of light with a wavelength of 532 nm.

Therefore, it is considered that the pulse laser conditions are preferably such that the wavelength is 532 nm, the pulse irradiation time is within the range of 3 nanoseconds to 4 nanoseconds, the pulse width is preferably such that the energy per pulse is 0.5 μJ to 30 μJ, and the energy density is preferably such that the energy density is 0.125 J/ ^cm2 to 7.5 J/ ^cm2 .

When the laser irradiation is stopped, liquid phase epitaxial crystals grow using the single crystal region as a seed, eliminating the crystal lattice defects that occurred during the grinding process and restoring the laser-irradiated area to its original single crystal state.

However, the light (radiant flux) incident on an object is divided into three destinations macroscopically: "reflection," "absorption," and "transmission," and these destinations change depending on the angle of incidence. The less reflection there is and the higher the absorption rate, the more the surface melting of the silicon wafer 1 contributes to processing.

Therefore, in the case of a complex shape such as the notch portion 50, it is desirable to change at least one of the pulse laser conditions, the angle of incidence, the energy density, and the number of irradiations per unit area (scan pitch, scan speed), in accordance with the surface shape. In particular, it is desirable to irradiate the nanosecond pulse laser by changing at least one of the angle of incidence, energy density, scan pitch, and scan speed in accordance with the surface shape of the silicon wafer.

In addition, to repair a processed and altered layer, energy can be effectively supplied to the material by reducing the angle of incidence, which is the angle between the vertical line and the direction of the pulsed laser irradiation relative to the boundary surface. However, there is little practical difference when the angle of incidence is 10 to 15 degrees or less.

The angle of incidence is the angle between the vertical line and the irradiation direction relative to the incident surface. Therefore, in Figures 3 and 4, when a pulsed laser is irradiated from the direction of the arrow, (1) is the region where the angle of incidence is 10 to 15° or less, and (2) is the region where the angle of incidence is 10 to 15° or more. Therefore, it is preferable to continuously change the laser irradiation so that the angle of incidence of the pulsed laser is perpendicular to the surface and irradiation surface of silicon wafer 1 in accordance with regions (1) and (2), which are the surface shape of silicon wafer 1.

The chamfering process (S1) and laser irradiation (S2) repair damage such as grinding marks caused by the grinding process and perform flattening processing. Waviness, roughness, and internal damage are then removed as much as possible. However, there are limitations to the laser irradiation conditions that correspond to the surface state after grinding. Therefore, simply irradiating the laser (S2) after the chamfering process (S1) will result in strong anisotropy, and there are limitations to the quality improvement of the surface modification for surface roughness (especially roughness caused by anisotropy), etc.

Therefore, following the chamfering step (S1) and laser irradiation (S2), an etching process (S3) is performed. Usually, the etching process of the silicon wafer 1 removes the fractured layer closest to the surface among the surface-degraded layers. The surface that has been subjected to the etching process is composed of SiO ₂ (a silicon oxide film) near the surface, and although this process-degraded layer is extremely thin, it has a significant effect on the mechanical, electrical, and optical performance. For example, the miniaturization and high integration of MOS devices progresses as the gate oxide film becomes thinner, and the film thickness has reached 2 nm or less.

After the etching process (S3), laser irradiation (S4) is performed again. The laser irradiation (S4) is a laser heat treatment in which a nanosecond pulse laser with wavelengths of 355, 532, or 785 nm is selected and irradiated to melt and epitaxially grow the material into a single crystal with a uniform crystal orientation. Then, following the chamfering process (S1) and laser irradiation (S2), an etching process (S3) is performed, and then laser irradiation (S4) is performed again, thereby making it possible to simultaneously remove both roughness and waviness.

Fig. 6 is a block diagram showing the device configuration of the second embodiment. The difference from the first embodiment is that a CW laser (continuous laser) of 1080 nm, which has a high absorption rate for _SiO2 , is irradiated before the nanosecond pulse laser is irradiated. In Fig. 6, the silicon wafer 1 rotates clockwise. The CW laser oscillator 5 has a wavelength λ = 1080 nm, and heats the surface point a of the silicon wafer 1 via the prism system 10 and the focusing optical system 11. The pulse laser oscillator 2 has a wavelength λ = 355, 532, or 785 nm, for example, a wavelength λ = 532 nm.

The pulsed laser oscillator 2 irradiates point b with a nanosecond pulsed laser via a prism system 20 and a focusing optical system 21. If the rotation direction of the silicon wafer 1 is clockwise, the outer peripheral surface of the silicon wafer 1 is heated at point a, and then the c-Si (silicon carbide) melts and solidifies at point b. In the second embodiment, the irradiated portion is heated and the heat is transferred to the underlying c-Si (silicon carbide) portion.

Then, the c-Si is irradiated with a nanosecond pulsed laser of 355, 532, or 785 nm, which is a wavelength with a high absorption rate, depending on the surface shape (roughness) after etching. The irradiation with the nanosecond pulsed laser heats the _SiO2 on the surface, and the c-Si is also heated by heat transfer. The c-Si is then melted and solidified, promoting planarization.

The pulse laser conditions are preferably such that the wavelength is 532 nm, the pulse irradiation time is within the range of 3 nanoseconds to 4 nanoseconds, the pulse width is preferably such that the energy per pulse is 0.5 μJ to 30 μJ, and the energy density is preferably such that the energy density is 0.125 J/cm ² to 7.5 J/cm ² .

Figure 7 is a block diagram showing the device configuration of the third embodiment. The difference from the second embodiment is that a femtosecond laser is used instead of a CW laser (continuous wave laser). In Figure 7, the femtosecond laser oscillator 3 has a wavelength λ = 800 nm, and is irradiated to point a on the surface of the silicon wafer 1 via a prism system 30 and a focusing optical system 31. The irradiated femtosecond laser is a single shot with a low intensity below the processing threshold of the c-Si (silicon carbide) layer.

The prism system 30 also uses a polarizer such as a λ/2 wave plate so that the polarization is parallel to the scanning direction. Point a on the surface of the silicon wafer 1 experiences special absorption that differs from normal absorption characteristics, and is converted into a-Si (amorphous silicon).

The pulsed laser oscillator 4 irradiates point b with a nanosecond pulsed laser via a prism system 40 and a focusing optical system 41. The nanosecond pulsed laser has a wavelength λ of 355, 532, or 785 nm, for example, λ = 532 nm. If the rotation direction of the silicon wafer 1 is clockwise, the outer peripheral surface of the silicon wafer 1 is turned into a-Si (amorphous silicon) at point a, and melts and epitaxially grows at point b, then solidifies.

A femtosecond laser is an optical laser that measures time in "femto" (one quadrillionth) units and emits light for only a few to a few hundred femtoseconds. The a-Si (amorphous silicon) part is then irradiated with a pulsed laser of wavelengths of 355, 532, or 785 nm, which have a high absorption rate for a-Si (amorphous silicon), to melt it and cause epitaxial growth to produce a single crystal with a uniform crystal orientation.

The femtosecond laser used for irradiation is a single shot with a wavelength λ = 800 nm, and has a low intensity below the processing threshold of the c-Si (silicon carbide) layer. The fluence (energy per unit area) of the femtosecond laser is set lower than the fluence at which ablation occurs on the surface of the silicon wafer 1.

The c-Si (silicon carbide) layer has a unique processing threshold, and if the pulse width is sufficiently short, such as femtoseconds, special absorption that differs from normal absorption characteristics occurs, and the outer peripheral surface of the silicon wafer 1 is converted to a-Si (amorphous silicon).

In addition, when femtosecond laser irradiation is performed with a laser fluence sufficient to cause ablation, nano-periodic structures are formed in a self-organized manner in a direction perpendicular to the polarization of the laser light. The period of the nano-periodic structure varies depending on the fluence, wavelength, and number of incident pulses of the incident laser. On the other hand, it is known that when femtosecond laser irradiation is performed with a fluence lower than the ablation threshold, striped nanostructures are formed by a mechanism different from that described above.

Nanosecond pulsed laser irradiation melts and solidifies the c-Si that has been converted to a-Si (amorphous silicon), promoting flattening. The pulsed laser conditions are the same as those in the first and second embodiments, with a wavelength of 532 nm and a pulse irradiation time in the range of 3 to 4 nanoseconds.

The first, second, and third embodiments have in common that they irradiate a nanosecond pulsed laser. In particular, the nanosecond pulsed laser is characterized by being irradiated with a wavelength of 355, 532, or 785 nm.

In any of the first, second and third embodiments, when the surface shape is complex, such as the notch portion 50, it is desirable to irradiate the nanosecond pulse laser by changing at least one of the incidence angle, energy density, scan pitch and scan speed in accordance with the surface shape. This ensures that the performance of the final product is not impaired even if the miniaturization and high integration of devices progresses.

Nanosecond pulsed lasers can be easily configured as scanning optical systems that can change the direction of irradiation. Therefore, it is preferable to configure a nanosecond pulsed laser as a scanning optical system and irradiate the material with an incident angle that is approximately perpendicular to the surface shape, as this allows energy to be supplied to the material effectively.

However, when the angle of incidence is 10 to 15 degrees or less, the substantial difference is small, and it is sufficient to ensure that the angle of incidence is 10 to 15 degrees or less. Therefore, from the perspective of ease of configuration, it is preferable to use a nanosecond pulse laser as a scanning optical system and irradiate the surface approximately perpendicularly in accordance with the surface shape, specifically at an angle of incidence of 10 to 15 degrees or less.

In addition, when irradiating with a femtosecond laser, a polarizer is used for the purpose of amorphization, so the irradiation direction can be kept constant and the polarizer can be used to adjust the s-polarized and p-polarized components in accordance with changes in the angle of incidence corresponding to the surface shape.

Although the case where the rotation direction of the silicon wafer 1 is clockwise has been described, the rotation direction of the silicon wafer 1 may be counterclockwise. In this case, in FIG. 5 and FIG. 6, which are block diagrams showing the device configurations in the second and third embodiments, point b, which is the irradiation position of the pulse laser oscillators 2 and 4, is arranged in the opposite direction with respect to point a, which is the irradiation position of the CW laser oscillator 5 and the femtosecond laser oscillator 3.

1...silicon wafer 50...notch portion 51...bottom portion 52...beveled slope 53...periphery 54...end surface 55...upper surface 56...beveled portion 2, 4...pulse laser oscillator (nanosecond pulse laser)
5...CW laser oscillator 3...Femtosecond laser oscillator (femtosecond laser)
10, 20, 30, 40... prism system 11, 21, 31, 41... focusing optical system

Claims (8)

1. A method for improving edge quality of a silicon wafer using laser heat treatment, comprising:
After a chamfering process of grinding the silicon wafer, a nanosecond pulse laser is irradiated onto the ground surface formed by the grinding process ;
Thereafter, the silicon wafer is etched,
A method for improving edge quality of a silicon wafer, comprising irradiating the ground surface with the nanosecond pulse laser again after the etching process. The method for improving edge quality of silicon wafers according to claim 1, characterized in that the nanosecond pulse laser is irradiated with a wavelength selected from one of 355, 532, and 785 nm. 3. The method for improving edge quality of a silicon wafer according to claim 1 or 2, characterized in that after the chamfering process and/or after the etching process, a CW laser (continuous wave laser) is irradiated before the nanosecond pulse laser is irradiated. 3. The method for improving edge quality of a silicon wafer according to claim 1 or 2, further comprising irradiating a femtosecond laser before irradiating the nanosecond pulsed laser after the chamfering process and/or after the etching process . The method for improving edge quality of a silicon wafer according to claim 4, characterized in that the femtosecond laser has a wavelength of λ = 800 nm. The method for improving edge quality of a silicon wafer according to claim 4 or 5, characterized in that the femtosecond laser is a single shot. The method for improving edge quality of a silicon wafer according to any one of claims 1 to 6, characterized in that the nanosecond pulse laser is irradiated by changing at least one of the incidence angle, energy density, scan pitch, and scan speed in response to the surface shape of the silicon wafer. The method for improving edge quality of a silicon wafer according to claim 7, wherein the nanosecond pulse laser is irradiated so that the incident angle is 10 to 15 degrees or less in accordance with the surface shape of the silicon wafer.