Edge quality improvement method for 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 processed and modified 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).

Background Art

[0002] Semiconductor wafers such as silicon wafers used in the production of semiconductor devices and the like are surface-processed by mechanical processing processes such as cutting, grinding, lapping, and polishing. Due to this surface processing, a processed and modified layer is formed on the surface and inside, and some of the processed and modified 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] And it is known that oxygen removal treatment and improvement of crystallinity of a silicon wafer can be achieved by using laser irradiation. For example, Patent Document 1 describes irradiating a pulsed laser on a single crystal surface in a method for repairing surface defects, which are processed and modified layers on the surface of a single crystal wafer.

[0004] Also, 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 processed and modified layer on the surface of a silicon wafer and flatten the roughness after grinding processes such as chamfering.

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 prior art, what is described in Patent Document 1 is for a simple plane as surface modification by laser irradiation, and is not for irradiation under conditions corresponding to complex shapes and various surface states after grinding. Therefore, with what is described in Patent Document 1, it has been difficult to effectively improve the quality of the edges (outer peripheral part, notch part, orifice part) of a silicon wafer, for example, to remove large unevenness generated by the damage in the chamfering process.

[0007] Also, with what is described in Patent Document 2, it becomes possible to repair damage such as grinding marks by grinding and perform planarization processing. However, what is described in Patent Document 2 has limitations in laser irradiation under conditions corresponding to the surface state after grinding. Not only do the irradiation conditions and apparatus configuration become complicated, but there are also limitations in improving the quality of surface modification for internal cracks, surface roughness (especially roughness caused by anisotropy), etc. Also, just by performing laser irradiation after the etching process, although roughness can be removed, it is difficult to simultaneously remove large unevenness and undulation generated by the damage in the chamfering process.

[0008] An object of the present invention is to solve the above problems of the prior art and, for the repair of surface defects of a silicon wafer by heat treatment using a laser, to achieve both removal of roughness and removal of undulation regardless of the state of the irradiated surface. Along with the high performance of the device, it improves the yield in subsequent processes, reduces the processing time of the edge polishing process performed after the etching process, and suppresses shape collapse.

Means for Solving the Problems

[0009] To achieve the above object, the present invention is a method for improving the edge quality of a silicon wafer using laser heat treatment, comprising: after the chamfering process of the silicon wafer, irradiating the ground surface with a nanosecond pulsed laser; then, subjecting the silicon wafer to an etching process; and after the etching process, irradiating the ground surface with the nanosecond pulsed laser again.

[0010] In addition, in the above, it is desirable to select and irradiate any one of wavelengths of 355, 532, and 785 nm for the nanosecond pulsed laser.

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

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

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

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

[0015] Furthermore, in the above, it is desirable to irradiate the nanosecond pulsed laser while changing at least one of the incident angle, energy density, scan pitch, and scan speed corresponding to the surface shape of the silicon wafer.

[0016] Furthermore, in the above, it is desirable to irradiate the nanosecond pulsed laser so that the incident angle becomes 10 to 15° or less corresponding to the surface shape of the silicon wafer.

Advantages of the Invention

[0017] According to the present invention, after the chamfering process of the silicon wafer, the ground surface is irradiated with a nanosecond pulsed laser, and then the silicon wafer is subjected to an etching process. After the etching process, the nanosecond pulsed laser is irradiated again. Therefore, regardless of the state of the surface to be irradiated for the repair of surface defects, which are processed altered layers on the surface of the silicon wafer, it is possible to achieve both the removal of roughness and the removal of waviness.

Brief Description of the Drawings

[0018]

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Embodiments 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. 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 the grinding process, particularly, the ground 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 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 in the same manner.

[0020] And the damage caused by the grinding process extends not only to the surface observable with a microscope but also to a region of a predetermined depth from the surface, becoming a processed altered layer in which the amorphous layer and the dislocation layer have changed, reaching the inside. That is, the surface of the ground silicon wafer 1 has a 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 notch portion 50 has a more complex shape compared to the outer peripheral portion 53 and is surface processed by grinding. Its surface and interior have a processed and altered layer formed, and some of the processed and altered layers contain microcracks (minute cracks). Usually, the planar shape of the notch portion 50 (Figure 3) has an arc-shaped bottom 51 formed as a notch, a chamfered slope 52 that slopes towards the outer peripheral portion 53 at about 45°, followed by a rounded connection to the outer peripheral portion 53, and is shaped symmetrically. In particular, the bottom 51 is greatly affected by the processing load during grinding and contains many surface defects. The outer peripheral portion 53 has a chamfered portion 56 provided as a rounded slope with respect to the end face 54 in the side surface shape (Figure 4), connecting to the upper surface 55, and is vertically symmetric.

[0022] Laser irradiation (S2) is performed on the edge portion of the silicon wafer 1, which is the ground surface, after the chamfering process. The edge portion irradiated with the laser has its surface region melted and becomes a flat surface due to surface tension, and the region near its surface is recrystallized and restored again. That is, in the ground portion of the silicon wafer 1, the surface near layer of the processed portion has crystal defects caused by machining and has become a processed and altered layer such as an amorphous layer or a dislocation layer.

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

[0024] Since the laser absorption rate of the machined and modified layer portion is significantly higher than that of the single crystal region, when irradiated with a laser (nanosecond pulsed laser), the machined and modified layer melts, the molten region expands by heat conduction, and the surface of the molten region flattens due to surface tension. Therefore, in the first embodiment, after the chamfering process of the silicon wafer 1, in order to modify and flatten the surface of the machined and modified layer, the notch portion 50 and the outer peripheral portion 53, or the upper and lower planes of the silicon wafer 1 are irradiated with a pulsed laser (nanosecond) to perform laser heat treatment.

[0025] The pulsed laser (nanosecond) irradiation melts the amorphous layer of the machined and modified layer at nanosecond speed. And the melting by the pulsed laser (nanosecond) irradiation promotes recrystallization (epitaxial growth) with aligned crystal orientations, and can eliminate crystal defects generated by machining. Also, the amorphous silicon layer has strong absorption of light with a wavelength of 532 nm.

[0026] Therefore, the pulsed laser conditions are preferably such that the wavelength is 532 nm and the pulse irradiation time is in the range of 3 nanoseconds to 4 nanoseconds. And the energy per pulse width is from 0.5 μJoule to 30 μJoule, and the energy density is from 0.125 J / cm 2 to 7.5 J / cm 2 It is said to be good.

[0027] When the laser irradiation stops, liquid phase epitaxial crystals grow using the single crystal region as a seed, whereby the lattice defects of the crystals generated during grinding are eliminated, and the portion irradiated with the laser is restored to the original single crystal.

[0028] However, the incident light (radiant flux) on an object is divided into three destinations of "reflection", "absorption", and "transmission" when viewed macroscopically, and changes depending on the incident angle. And the surface melting of the silicon wafer 1 contributes more to processing as the reflection is reduced and the absorption rate is higher.

[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 pulse laser conditions, namely, the incident angle, the energy density, and the number of irradiations per unit area (scan pitch, scanning speed), in accordance with the surface shape. In particular, for a nanosecond pulse laser, it is preferable to irradiate while changing at least one of the incident angle, the energy density, the scan pitch, and the scanning speed in accordance with the surface shape of the silicon wafer.

[0030] In addition, for the repair of the processed affected layer, if the incident angle, which is the angle formed by the vertical line to the interface and the pulse laser irradiation direction, is decreased, energy can be effectively supplied to the material. However, when the incident angle is 10 to 15° or less, the substantial difference is small.

[0031] The incident angle is the angle formed by the irradiation direction and the vertical line with respect to the incident plane. Therefore, in FIGS. 3 and 4, when the pulse laser is irradiated from the arrow direction, (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 that the laser irradiation is continuously changed so that the incident angle of the pulse laser becomes perpendicular to the surface of the silicon wafer 1 and the irradiation surface corresponding to the regions (1) and (2) which are the surface shapes of the silicon wafer 1.

[0032] By the chamfering process (S1) and the laser irradiation (S2), the repair of damages such as grinding marks caused by grinding and the flattening process are performed. Then, the undulations, roughness, and internal damages are removed as much as possible. However, there is a limit to laser irradiation under conditions according to the surface state after grinding. Therefore, only by performing the laser irradiation (S2) after the chamfering process (S1), the anisotropy becomes strong, and there is a limit to the improvement of the surface modification quality for the surface roughness (particularly, the roughness caused by anisotropy) and the like.

[0033] Therefore, following the chamfering process (S1) and the laser irradiation (S2), an etching process (S3) is performed. Usually, the etching process of the silicon wafer 1 removes the crushed layer closest to the surface among the surface-modified layers. The surface after the etching process is composed of SiO2 (silicon oxide film) near the surface. Although this processed modified layer is extremely thin, it has a great influence on mechanical, electrical, and optical properties. 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 2 nm or less.

[0034] Furthermore, after the etching process (S3), laser irradiation (S4) is performed again. The laser irradiation (S4) selects and irradiates a nanosecond pulse laser with wavelengths of 355, 532, and 785 nm as a laser heat treatment, melts it, and epitaxially grows it into a single crystal with aligned crystal orientations. By performing the etching process (S3) following the chamfering process (S1) and the laser irradiation (S2), and then performing the laser irradiation (S4) again, it is possible to achieve both the removal of roughness and the removal of undulation.

[0035] 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) with a wavelength of 1080 nm, which has a high absorption rate for SiO2, is irradiated before irradiating the nanosecond pulse laser. In FIG. 6, the silicon wafer 1 is rotating in the clockwise direction. The CW laser oscillator 5 has a wavelength λ = 1080 nm and heats point a on the surface of the silicon wafer 1 through the prism system 10 and the condensing optical system 11. The pulse laser oscillator 2 has one of wavelengths λ = 355, 532, 785 nm, for example, the wavelength λ = 532 nm.

[0036] The pulse laser oscillator 2 irradiates a nanosecond pulse laser at point b through the prism system 20 and the condensing 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 c-Si (silicon carbide) melts and solidifies at point b. The second embodiment heats the irradiated portion and transfers heat to the underlying c-Si (silicon carbide) portion.

[0037] After that, for c-Si, any one of nanosecond pulsed lasers with wavelengths of 355, 532, and 785 nm, which are wavelengths with high absorption rates, is selected and irradiated according to the surface shape (roughness) after etching. The irradiation of the nanosecond pulsed laser heats the SiO2 on the surface, and c-Si is also heated by heat conduction. Then, c-Si is melted and solidified, and flattening is promoted.

[0038] The pulsed laser conditions are preferably such that the wavelength is 532 nm and the pulse irradiation time is in the range of 3 nanoseconds to 4 nanoseconds. And the energy per pulse width is from 0.5 μJoule to 30 μJoule, and the energy density is from 0.125 J / cm 2 to 7.5 J / cm 2 is good.

[0039] Figure 7 is a block diagram showing the device configuration of the third embodiment. The difference from the second embodiment is that instead of irradiating with a CW laser (continuous laser), it is irradiated with a femtosecond 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 through the prism system 30 and the condensing optical system 31. The irradiated femtosecond laser is a single shot and has a low intensity below the processing threshold of the c-Si (silicon carbide) layer.

[0040] Also, for the prism system 30, a polarizer such as a λ / 2 wavelength plate is also used so that the deflection is parallel in the scanning direction. Special absorption different from the normal absorption characteristics occurs at point a on the surface of the silicon wafer 1, and it is a-Si (amorphous silicon) -ized.

[0041] The pulsed laser oscillator 4 irradiates point b with a nanosecond pulsed laser through the prism system 40 and the condensing optical system 41. The nanosecond pulsed laser has any one of wavelengths λ = 355, 532, 785 nm, for example, the wavelength λ = 532 nm. If the rotation direction of the silicon wafer 1 is clockwise, the outer peripheral surface of the silicon wafer 1 is a-Si (amorphous silicon) -ized at point a, melted at point b, epitaxially grown, and solidified.

[0042] A femtosecond laser is an optical laser that deals with time units in "femtoseconds" (one billionth) and emits light only between several femtoseconds and several hundred femtoseconds. Then, a pulsed laser with wavelengths of 355, 532, and 785 nm, which have a high absorption rate with respect to a-Si (amorphous silicon), is selected and irradiated onto the a-Si (amorphous silicon) conversion part, melted, and epitaxially grown to form a single crystal with aligned crystal orientations.

[0043] The femtosecond laser to be irradiated is a single shot with a wavelength of 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 made lower than the fluence at which ablation occurs on the surface of the silicon wafer 1.

[0044] The c-Si (silicon carbide) layer has a specific processing threshold. If it is a femtosecond with a sufficiently short pulse width, special absorption different from normal absorption characteristics occurs, and the outer peripheral surface of the silicon wafer 1 is converted into a-Si (amorphous silicon).

[0045] Normally, in femtosecond laser irradiation with a laser fluence at which ablation occurs, a nano-periodic structure is self-organized in a direction perpendicular to the polarization of the laser light. Also, 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 femtosecond laser irradiation with a fluence lower than the ablation threshold forms a striped nano-structure by a mechanism different from the above.

[0046] Nanosecond pulsed laser irradiation melts and solidifies the a-Si (amorphous silicon)-converted c-Si and promotes planarization. The pulsed laser conditions are the same as those in the first embodiment and the second embodiment, with a wavelength of 532 nm and a pulse irradiation time in the range of 3 nanoseconds to 4 nanoseconds being preferable.

[0047] Note that the first embodiment, the second embodiment, and the third embodiment are common in that they irradiate with a nanosecond pulsed laser. In particular, the nanosecond pulsed laser is characterized in that any one of wavelengths of 355, 532, and 785 nm is selected for irradiation.

[0048] In any of the first embodiment, the second embodiment, and the third embodiment, when the surface shape is complex like the notch portion 50, it is desirable that the nanosecond pulsed laser is irradiated by changing at least one of the incident angle, energy density, scan pitch, and scanning speed corresponding to the surface shape. Thereby, even if the miniaturization and high integration of the device progress, the performance of the final product will not be impaired.

[0049] The nanosecond pulsed laser can be easily configured as a scanning optical system whose irradiation direction is variable. Therefore, it is preferable that the nanosecond pulsed laser is used as a scanning optical system and irradiated so that the incident angle is substantially perpendicular corresponding to the surface shape, in terms of being able to effectively supply energy to the material.

[0050] However, when the incident angle is 10 to 15° or less, the substantial difference is small, and it is sufficient to make the incident angle 10 to 15° or less. Therefore, from the viewpoint of ease of configuration, it is preferable that the nanosecond pulsed laser is used as a scanning optical system and irradiated so as to be substantially perpendicular corresponding to the surface shape, specifically, the incident angle is 10 to 15° or less.

[0051] Also, in the case of irradiation with a femtosecond laser, since a polarizer is used for the purpose of amorphization, the irradiation direction can be fixed, and the components of s-polarized light and p-polarized light can be adjusted by the polarizer in accordance with the change in the incident angle corresponding to the surface shape.

[0052] 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 FIGS. 5 and 6 which are block diagrams showing the device configurations in the second and third embodiments, the point b which is the irradiation position of the pulse laser oscillators 2 and 4 is arranged in the reverse order with the point a which is the irradiation position of the CW laser oscillator 5 and the femtosecond laser oscillator 3 interposed therebetween.

Explanation of Signs

[0053] 1…Silicon wafer 50…Notch portion 51…Bottom 52…Chamfered slope 53…Outer peripheral portion 54…End face 55…Upper surface 56…Chamfered 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…Condensing optical system

Claims

1. Laser irradiating at least one edge selected from the group consisting of an outer peripheral end face, an outer peripheral slope, a notch portion, and an orifice portion of a silicon wafer having the edge ground, to perform surface modification; etching the silicon wafer after the surface modification; and laser irradiating the edge of the etched silicon wafer to perform surface modification. A method for processing an edge of a silicon wafer, comprising the above steps.

2. The step of laser irradiating the ground edge to perform surface modification includes repairing damage to the edge caused by the grinding and / or flattening the edge. The method for processing an edge of a silicon wafer according to Claim 1.

3. The step of laser irradiating the edge of the etched silicon wafer to perform surface modification includes melting the surface of the edge to promote epitaxial growth of crystals. The method for processing an edge of a silicon wafer according to Claim 1 or 2.

4. The step of laser irradiating the edge of the etched silicon wafer to perform surface modification includes selecting a wavelength of 355, 532, or 785 nm according to the surface shape of the edge of the etched silicon wafer, and laser irradiating the edge to perform surface modification. The method for processing an edge of a silicon wafer according to any one of Claims 1 to 3.

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