Method for improving edge quality of silicon wafer

Abstract

To achieve both roughness removal and waviness removal for the repair of surface defect of a silicon wafer by heat treatment using a laser.SOLUTION: An edge quality improvement method for a silicon wafer 1 using laser heat treatment includes a chamfering step (S1) of a silicon wafer 1, a step (S2) of irradiating the ground surface with a nanosecond pulse laser after the chamfering step (S1), a step (S3) of etching the silicon wafer 1 after the step (S2), and a step (S4) of irradiating the ground surface with the nanosecond pulse laser again after the etching step (S3). It is desirable to select and irradiate any one of the wavelengths of 355, 532, and 785 nm as the nanosecond pulse laser.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to the repair of surface defects, which are processing-affected layers on the surface of a silicon wafer, and particularly to a method for improving the quality of a silicon wafer by using laser heat treatment to improve the quality of the edge (outer peripheral part, notch part, orifice part). [[ID=II]]

Background Art

[0002] Semiconductor wafers such as silicon wafers used in the fabrication of semiconductor devices and the like are surface-processed by mechanical processing processes such as cutting, grinding, lapping, and polishing. Through this surface processing, a processing-affected layer is formed on the surface and inside thereof, and some of the processing-affected layers contain microcracks (minute cracks). The removal of these internal cracks and the like is mainly performed by chemical and mechanical methods such as etching and chemical mechanical polishing (CMP).

[0003] Moreover, it is known that oxygen removal treatment and improvement of crystallinity of a silicon wafer are possible by using laser irradiation. For example, Patent Document 1 describes irradiating a single crystal surface with a pulsed laser in a method for repairing surface defects, which are processing-affected layers on the surface of a single crystal wafer.

[0004] In addition, Patent Document 2 describes examining the surface state after grinding before laser irradiation and performing laser irradiation under corresponding conditions in order to repair the processing-affected layer on the surface of a silicon wafer and flatten the roughness after performing grinding such as chamfering.[[ID=HI]]

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the above-mentioned prior art, the method described in Patent Document 1 is for surface modification using laser irradiation on simple planes and does not involve irradiation under conditions corresponding to complex shapes or various surface conditions after grinding. Therefore, with the method described in Patent Document 1, it was difficult to effectively improve the quality of silicon wafer edges (outer periphery, notch, orientation flat), for example, by removing large irregularities caused by damage during the chamfering process.

[0007] Furthermore, the method described in Patent Document 2 makes it possible to repair damage such as grinding marks caused by grinding and to perform flattening treatment. However, the method described in Patent Document 2 has limitations in that it is possible to irradiate the surface with a laser under conditions that correspond to the surface condition after grinding. This not only complicates the irradiation conditions and equipment configuration, but also limits the quality improvement of surface modification for internal cracks, surface roughness (especially roughness caused by anisotropy), etc. Furthermore, simply irradiating with a laser after the etching process may remove roughness, but it is difficult to remove both the large irregularities and undulations caused by the chamfering process.

[0008] The objective of the present invention is to solve the problems of the conventional technology described above and to achieve both the removal of roughness and waviness when repairing surface defects of silicon wafers by laser heat treatment, regardless of the condition of the irradiated surface. Furthermore, it will improve the performance of the device, improve yield in subsequent processes, reduce the processing time for the edge polishing process performed after the etching process, and suppress shape deformation. [Means for solving the problem]

[0009] To achieve the above objective, the present invention provides a method for improving the edge quality of a silicon wafer using laser heat treatment, wherein after a chamfering process of the silicon wafer, a nanosecond pulsed laser is irradiated onto the ground surface, the silicon wafer is then etched, and after the etching process, the nanosecond pulsed laser is irradiated onto the ground surface again.

[0010] Furthermore, in the above, it is desirable to select one of the following wavelengths for the nanosecond pulsed laser: 355, 532, or 785 nm.

[0011] Furthermore, in the above, it is desirable to irradiate with a CW laser (continuous wave laser) before irradiating with the nanosecond pulsed laser.

[0012] Furthermore, in the above, it is desirable to irradiate with a femtosecond laser before irradiating with the nanosecond pulsed laser.

[0013] Furthermore, in the above, it is desirable that the femtosecond laser has a wavelength of λ = 800 nm.

[0014] Furthermore, in the above, it is preferable that the femtosecond laser be a single-shot laser.

[0015] Furthermore, in the above, it is desirable that the nanosecond pulsed laser irradiates the silicon wafer by changing at least one of the incident angle, energy density, scan pitch, or scanning speed in accordance with the surface shape of the silicon wafer.

[0016] Furthermore, in the above, it is desirable to irradiate the silicon wafer with the nanosecond pulse laser such that the incident angle is 10 to 15° or less, corresponding to the surface shape of the silicon wafer. [Effects of the Invention]

[0017] According to the present invention, after the chamfering process of a silicon wafer, the ground surface is irradiated with a nanosecond pulsed laser, then the silicon wafer is etched, and after the etching process, the nanosecond pulsed laser is irradiated again. Therefore, regardless of the condition of the irradiated surface, it is possible to remove both roughness and waviness in order to repair surface defects, which are the processed altered layer on the surface of the silicon wafer. [Brief explanation of the drawing]

[0018] [Figure 1]Flowchart showing a method for improving edge quality according to an embodiment of the present invention [Figure 2] Schematic process diagram according to an embodiment of the present invention [Figure 3] Plan view showing the shape of the notch portion [Figure 4] Side view showing the shape of the notch portion [Figure 5] Block diagram showing the device configuration according to the first embodiment of the present invention [Figure 6] Block diagram showing the device configuration in the second embodiment of the present invention [Figure 7] Block diagram showing the device configuration in the third embodiment of the present invention

Mode for Carrying Out the Invention

[0019] FIG. 1 is a flowchart showing a method for improving edge quality according to an embodiment. FIG. 2 is a schematic process diagram, FIG. 3 is a plan view showing the shape of the notch portion 50 provided in a part of the silicon wafer 1, and FIG. 4 is a side view. First, grinding is performed as a chamfering process (S1). The semiconductor wafer such as the silicon wafer 1, in a state after grinding, particularly, the grinding surface including the chamfered slope 52 of the notch portion 50 after the chamfering process has fine grinding marks (scratches or streaks) formed thereon, resulting in a rough surface state. Further, the grinding marks are similarly formed not only on the notch portion 50 but also on the end surface 54 of the outer periphery of the silicon wafer 1 and the chamfered slope 52 of the outer periphery.

[0020] And the damage caused by the grinding extends not only to the surface observable with a microscope but also to a region with a predetermined depth from the surface, becoming a processed altered layer in which the amorphous layer and the dislocation layer are changed, reaching inside. That is, the surface of the ground silicon wafer 1 has the surface region of the original single crystal layer become a processed altered layer, resulting in a rough surface with undulations and internal damage.

[0021] In particular, the notched portion 50 has a more complex shape compared to the outer circumference 53 and its surface is processed by grinding. A processed altered layer is formed on its surface and interior, and some of the processed altered layer contains microcracks. Typically, the planar shape of the notched portion 50 (Figure 3) has an arc-shaped bottom 51 formed as a notch, a chamfered slope 52 that extends towards the outer circumference 53 at approximately 45°, and then connects to the outer circumference 53 with a rounded shape, resulting in a symmetrical shape. In particular, the bottom 51 is greatly affected by the processing load from grinding and contains many surface defects. In the side shape (Figure 4) of the outer circumference 53, a chamfered portion 56 is provided as a rounded slope relative to the end face 54, connecting to the top surface 55, resulting in an up-down symmetrical shape.

[0022] Laser irradiation (S2) is performed on the edge of the silicon wafer 1, which is the ground surface, after the chamfering process. When the edge is irradiated with a laser, the surface region melts and becomes flat due to surface tension, while the region near the surface is restored by single crystallization. In other words, in the ground portion of the silicon wafer 1, the near-surface layer of the processed portion develops crystal defects due to machining, becoming a processed altered layer such as an amorphous layer or a dislocation layer.

[0023] Figure 5 is a block diagram showing the apparatus configuration of the first embodiment. In Figure 5, the silicon wafer 1 is rotating clockwise. In the first embodiment, as laser irradiation (S2, S4), a nanosecond pulsed laser is irradiated onto the silicon wafer surface from the pulsed laser oscillator 2 via the prism system 20 and the focusing optical system 21. After the chamfering process of the silicon wafer 1, the notch portion 50 and the outer peripheral portion 53, or the upper and lower planes of the silicon wafer 1, are subjected to laser heat treatment by irradiation with a pulsed laser (nanosecond) in order to modify and flatten the surface of the processed altered layer.

[0024] Since the laser absorption rate of the processed altered layer is significantly higher than that of the single-crystal region, the processed altered layer melts upon irradiation with a laser (nanosecond pulsed laser), the molten region expands due to heat conduction, and the surface of the molten region becomes flattened by surface tension. Therefore, in the first embodiment, after the chamfering process of the silicon wafer 1, laser heat treatment is performed by irradiating the notch portion 50 and the outer peripheral portion 53, or the upper and lower planes of the silicon wafer 1, with a pulsed laser (nanosecond) to modify and flatten the surface of the processed altered layer.

[0025] Pulsed laser (nanosecond) irradiation melts the amorphous layer of the processed and altered layer at nanosecond speed. This melting by pulsed laser (nanosecond) irradiation promotes recrystallization (epitaxial growth) with aligned crystal orientations, eliminating crystal defects caused by machining. Furthermore, the amorphous silicon layer exhibits strong absorption of light with a wavelength of 532 nm.

[0026] Therefore, the ideal pulsed laser conditions are a wavelength of 532 nm and a pulse duration of 3 to 4 nanoseconds. Furthermore, the energy per pulse should be between 0.5 μjoules and 30 μjoules, with an energy density of 0.125 J / cm². 2 From 7.5J / cm 2 It is considered good to be that way.

[0027] When laser irradiation is stopped, a liquid-phase epitaxial crystal grows using the single-crystal region as a seed. This eliminates the lattice defects in the crystal that occurred during grinding, and the laser-irradiated portion is restored to its original single-crystal state.

[0028] However, incident light (radiant flux) on an object is divided into three destinations macroscopically: "reflection," "absorption," and "transmission," and these destinations change depending on the angle of incidence. Furthermore, surface melting of the silicon wafer 1 contributes more to processing as reflection is reduced and absorption is increased.

[0029] Therefore, in the case of a complex shape such as the notch portion 50, it is desirable to change at least one of the following pulse laser conditions, such as the incident angle, energy density, or number of irradiations per unit area (scan pitch, scanning speed), in accordance with the surface shape. In particular, it is preferable to irradiate the silicon wafer with a nanosecond pulse laser while changing at least one of the following, such as the incident angle, energy density, scan pitch, or scanning speed, in accordance with the surface shape of the silicon wafer.

[0030] Furthermore, to repair the 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 relative to the interface and the pulsed laser irradiation direction. However, the practical difference is small when the angle of incidence is 10-15° or less.

[0031] The incident angle is the angle between the irradiation direction and the vertical line 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 incident angle is 10 to 15° or less, and (2) is the region where the incident angle is 10 to 15° or more. Thus, it is preferable to continuously change the incident angle of the pulsed laser so that it is perpendicular to the surface and irradiation surface of the silicon wafer 1, corresponding to regions (1) and (2), which are the surface shapes of the silicon wafer 1.

[0032] The chamfering process (S1) and laser irradiation (S2) repair damage such as grinding marks caused by the grinding process and perform flattening treatment. This removes undulations, roughness, and internal damage as much as possible. However, there are limitations to applying laser irradiation under conditions that correspond to the surface condition after grinding. Therefore, simply performing laser irradiation (S2) after the chamfering process (S1) results in strong anisotropy, and there are limitations to improving the quality of surface modification in terms of surface roughness (especially roughness caused by anisotropy).

[0033] Therefore, following the chamfering process (S1) and laser irradiation (S2), an etching process (S3) is performed. Typically, the etching process of silicon wafer 1 removes the fragmented layer closest to the surface of the surface-altered layer. The surface after etching is composed of SiO2 (silicon oxide film) near the surface, and although this processed altered layer is extremely thin, it has a significant impact on mechanical, electrical, and optical performance. For example, the miniaturization and high integration of MOS devices have progressed as the thinning of the gate oxide film, and its film thickness has reached less than 2 nm.

[0034] Furthermore, after the etching process (S3), laser irradiation (S4) is performed again. Laser irradiation (S4) is a laser heat treatment in which nanosecond pulsed lasers with wavelengths of 355, 532, and 785 nm are selected and irradiated to melt and epitaxially grow single crystals with aligned crystal orientations. Then, following the chamfering process (S1) and laser irradiation (S2), etching (S3) is performed, and after that, laser irradiation (S4) is performed again, which makes it possible to remove both roughness and waviness.

[0035] Figure 6 is a block diagram showing the apparatus configuration of the second embodiment. The difference from the first embodiment is that a CW laser (continuous wave laser) with a wavelength of 1080 nm, which has a high absorption rate for SiO2, is irradiated before the nanosecond pulse laser. In Figure 6, the silicon wafer 1 is rotating clockwise. The CW laser oscillator 5 has a wavelength λ=1080 nm and heats point a on the surface of the silicon wafer 1 via a prism system 10 and a 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.

[0036] The pulsed laser oscillator 2 irradiates point b with a nanosecond pulsed laser via the prism system 20 and the focusing optical system 21. If the rotation direction of the silicon wafer 1 is clockwise, the outer surface of the silicon wafer 1 is heated at point a, and then 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.

[0037] Subsequently, one of the nanosecond pulsed lasers with wavelengths of 355, 532, or 785 nm, which have high absorption rates, is selected and irradiated onto the c-Si according to the desired surface shape (roughness) after etching. Irradiation with the nanosecond pulsed laser heats the SiO2 on the surface, and the c-Si is also heated by heat transfer. As a result, the c-Si is melted and solidified, promoting planarization.

[0038] The ideal pulsed laser conditions are a wavelength of 532 nm and a pulse duration of 3 to 4 nanoseconds. Furthermore, the energy per pulse should be between 0.5 μjoules and 30 μjoules, with an energy density of 0.125 J / cm². 2 From 7.5J / cm 2 It is good to be that way.

[0039] Figure 7 is a block diagram showing the apparatus 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) for irradiation. In Figure 7, the femtosecond laser oscillator 3 has a wavelength λ = 800 nm and irradiates 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.

[0040] Furthermore, the prism system 30 also uses polarizers such as λ / 2 wave plates so that the deflection is parallel to the scanning direction. At point a on the surface of silicon wafer 1, a special absorption occurs that differs from the normal absorption characteristics, and the material is converted to a-Si (amorphous silicon).

[0041] 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 of λ = 355, 532, or 785 nm, for example, λ = 532 nm. If the rotation direction of the silicon wafer 1 is clockwise, the outer surface of the silicon wafer 1 is converted to a-Si (amorphous silicon) at point a, melts at point b, undergoes epitaxial growth, and solidifies.

[0042] A femtosecond laser is a laser that operates on a time unit of "femto" (one quadrillionth) and emits light for only a few femtoseconds to several hundred femtoseconds. Then, pulsed lasers with wavelengths of 355, 532, and 785 nm, which have a high absorption rate for amorphous silicon, are selected and irradiated onto the a-Si (amorphous silicon) portion to melt it and grow epitaxially to form a single crystal with aligned crystal orientation.

[0043] The femtosecond laser used for irradiation is a single shot with a wavelength of λ=800nm ​​and is set to 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 silicon wafer 1.

[0044] The c-Si (silicon carbide) layer has an inherent processing threshold, and if the pulse width is sufficiently short (femtoseconds), a special absorption occurs that differs from normal absorption characteristics, and the outer surface of silicon wafer 1 is converted to a-Si (amorphous silicon).

[0045] Normally, when ablation occurs with femtosecond laser irradiation at a laser fluence, nanoperiodic structures are self-organized and formed perpendicular to the polarization of the laser light. Furthermore, the period of these nanoperiodic structures varies depending on the incident laser fluence, wavelength, and number of incident pulses. On the other hand, it is known that when a femtosecond laser irradiation with a fluence lower than the ablation threshold, striped nanostructures are formed by a different mechanism than described above.

[0046] Nanosecond pulsed laser irradiation melts and solidifies a-Si (amorphous silicon) c-Si, promoting planarization. The pulsed laser conditions are the same as 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.

[0047] The first, second, and third embodiments all share the common feature of irradiating with a nanosecond pulsed laser. In particular, the nanosecond pulsed laser is characterized by the selection of one of the wavelengths of 355, 532, or 785 nm for irradiation.

[0048] In all three embodiments, including the first, second, and third embodiments, when the surface shape is complex, such as the notch portion 50, it is desirable that the nanosecond pulsed laser be irradiated by changing at least one of the incident angle, energy density, scan pitch, or scanning speed in accordance with the surface shape. This ensures that the performance of the final product is not impaired even as the miniaturization and integration of the device progresses.

[0049] A nanosecond pulsed laser can be easily configured as a scanning optical system with a variable irradiation direction. Therefore, it is preferable to use a nanosecond pulsed laser as a scanning optical system and irradiate the surface so that the incident angle is approximately perpendicular to the surface shape, in order to effectively supply energy to the material.

[0050] However, for incident angles of 10-15° or less, the practical difference is small, and it is sufficient to ensure that the incident angle is 10-15° or less. Therefore, for ease of configuration, it is preferable to use a nanosecond pulsed laser as the scanning optical system and irradiate the surface approximately perpendicular to the surface shape, specifically so that the incident angle is 10-15° or less.

[0051] Furthermore, in the case of femtosecond laser irradiation, a polarizer is used for the purpose of amorphousization, so the irradiation direction can be kept constant, and the s-polarized and p-polarized components can be adjusted by the polarizer in accordance with the change in the incident angle corresponding to the surface shape.

[0052] Although the description above describes the case where the silicon wafer 1 rotates clockwise, the silicon wafer 1 may also rotate counterclockwise. In this case, in the block diagrams of the apparatus configuration in the second and third embodiments, points b, which are the irradiation positions of the pulse laser oscillators 2 and 4, are arranged in the opposite direction to points a, which are the irradiation positions of the CW laser oscillator 5 and the femtosecond laser oscillator 3. [Explanation of symbols]

[0053] 1…Silicon wafer 50... Notch section 51...Bottom 52… Beveled slope 53...Outer perimeter 54…End face 55…Top surface 56... Chamfered section 2, 4... Pulsed laser oscillator (nanosecond pulsed laser) 5…CW laser oscillator 3… Femtosecond laser oscillator (femtosecond laser) 10, 20, 30, 40... Prism type 11, 21, 31, 41… Focusing optical system

Claims

1. 1. A method for improving edge quality of a silicon wafer using laser heat treatment, comprising: After the chamfering step of the silicon wafer, irradiating the ground surface with a nanosecond pulse laser, Thereafter, the silicon wafer is subjected to an etching process, A method for improving edge quality of a silicon wafer, characterized in that after the etching process, the ground surface is again irradiated with the nanosecond pulse laser.

2. 2. The method for improving edge quality of a silicon wafer according to claim 1, wherein the nanosecond pulse laser is irradiated with a wavelength selected from the group consisting of 355 nm, 532 nm, and 785 nm.

3. 3. The method for improving edge quality of a silicon wafer according to claim 1, further comprising irradiating the silicon wafer with a CW laser (continuous wave laser) before irradiating the silicon wafer with the nanosecond pulse laser.

4. 3. The method for improving edge quality of a silicon wafer according to claim 1, further comprising irradiating the silicon wafer with a femtosecond laser before irradiating the silicon wafer with the nanosecond pulse laser.

5. 5. The method for improving edge quality of a silicon wafer according to claim 4, wherein the femtosecond laser has a wavelength λ=800 nm.

6. 6. The method for improving edge quality of a silicon wafer according to claim 4, wherein the femtosecond laser is a single shot.

7. 7. The method for improving edge quality of a silicon wafer according to claim 1, wherein the nanosecond pulse laser is irradiated by changing at least one of the incident angle, energy density, scan pitch, and scan speed in accordance with the surface shape of the silicon wafer.

8. 8. The method for improving edge quality of a silicon wafer according to claim 7, wherein the nanosecond pulse laser is irradiated so that the angle of incidence is 10 to 15 degrees or less in accordance with the surface shape of the silicon wafer.

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