Method for surface modification of silicon wafers

Abstract

To eliminate the influence of processing stress of a notch portion having a complicated shape and modify the surface to be uniform in heat treatment using a laser on the surface of a silicon wafer.SOLUTION: In a surface modification method for a silicon wafer 1 using laser heat treatment, after the silicon wafer 1 is ground, at least an incident angle, s-polarized light and p-polarized light components, the energy density, and the number of irradiations per unit area is changed for the surface shape (particularly, a notch portion 10 having a complicated shape), and the surface is irradiated with a pulsed laser. In particular, it is desirable to irradiate with the pulse laser such that the incident angle is vertical, and to adjust the s-polarized light and p-polarized light components according to the change in the incident angle.SELECTED DRAWING: Figure 4

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 surface modification of a silicon wafer that performs surface modification and planarization of the surface of the silicon wafer using laser heat treatment.

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. However, a processed and modified layer is formed on the surface and inside thereof, and some of the processed and modified layers contain microcracks (microscopic 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] For example, Patent Document 1 describes that, in order to improve the polishing quality of the outer peripheral portion of a semiconductor wafer as a workpiece, the pressing force against the end surface at each part of the surface of the polishing tool is made uniform, and the end surface is polished with high quality.

[0004] Also, Patent Document 2 describes that by using laser irradiation, it is possible to perform oxygen elimination treatment and improve the crystallinity of a silicon wafer, and in a method for repairing surface defects, which are processed and modified layers on the surface of a single crystal wafer, a pulsed laser is irradiated onto the single crystal surface.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] In the above-mentioned prior art, the methods described in Patent Documents 1 and 2 are particularly ineffective in preventing cracks originating from notches (cutouts on the wafer). Furthermore, existing methods such as outer edge grinding, etching, and chemical mechanical polishing (CMP) cannot completely remove internal cracks. These internal cracks propagate and cause damage, reducing the yield of silicon wafer surface processing. Cracks originating from notches (cutouts on the wafer) have a particularly significant impact on yield reduction.

[0007] Furthermore, chemical mechanical polishing (CMP) after grinding and etching involves changes from the original shape (design value), making quality control difficult, and potentially reducing the flatness of the wafer surface even if it was improved in the preceding process. In addition, chemical mechanical polishing (CMP) uses consumables such as abrasive grains, polishing threads, and cleaning solutions, resulting in high costs and a significant environmental impact.

[0008] The objective of this invention is to solve the problems of the conventional technology described above and to modify the surface of a silicon wafer to be uniform by performing a suitable heat treatment using a laser on the surface after grinding or etching, thereby eliminating the effects of processing stress. This improves strength and increases yield in subsequent processes. In particular, the objective is to perform a suitable laser treatment on notch areas, which have complex shapes. [Means for solving the problem]

[0009] To achieve the above objective, the present invention provides a method for surface modification of a silicon wafer using laser heat treatment, wherein, after grinding the silicon wafer, a pulsed laser is irradiated while changing at least one of the following: the incident angle, the s-polarized and p-polarized light components, the energy density, and the number of irradiations per unit area, corresponding to the surface shape.

[0010] Furthermore, in the above, the incident angle is changed in accordance with the surface shape, and it is desirable to irradiate the pulsed laser so that the incident angle is perpendicular.

[0011] Furthermore, in the above, the incident angle is changed in accordance with the surface shape, and it is desirable to irradiate the pulsed laser so that the incident angle is 10 to 15° or less.

[0012] Furthermore, in the above, the incident angle is changed in accordance with the surface shape, and it is desirable that the irradiation direction of the pulsed laser is varied by rotating a galvanometer mirror or prism in the scanning optical system.

[0013] Furthermore, in the above, the components of s-polarization and p-polarization are changed in accordance with the surface shape, and it is desirable to keep the irradiation direction of the pulsed laser constant and adjust the components of s-polarization and p-polarization in accordance with the change in the incident angle corresponding to the surface shape.

[0014] Furthermore, in the above, the components of s-polarization and p-polarization are changed in accordance with the surface shape, and it is desirable to keep the irradiation direction of the pulsed laser constant and adjust the components of s-polarization and p-polarization in accordance with the change in the incident angle corresponding to the surface shape.

[0015] In the region where the angle of incidence is 10 to 15° or more, it is desirable to adjust the s-polarized and p-polarized components.

[0016] Preferably, the pulsed laser is focused onto the surface of the silicon wafer via a pulsed laser oscillator, a polarizing optical system, and a focusing lens.

[0017] The polarization optical system preferably comprises a polarizing plate, an electro-optical element that delays the phase of the slow axis in accordance with the applied voltage, and a quarter-λ wave plate, and generates a polarization direction state by voltage driving.

[0018] The energy density is changed in accordance with the surface shape, and it is desirable to keep the energy density constant in the region where the angle of incidence is 10 to 15° or less, and to increase the energy density as the angle of incidence increases in the region where the angle of incidence is 10 to 15° or more.

[0019] The pulsed laser has a wavelength of 532 nm, a pulse irradiation time within the range of 3 nanoseconds to 4 nanoseconds, an energy per pulse of 0.5 μJoule to 30 μJoules, and an energy density of 0.125 J / cm 2 to 7.5 J / cm 2 and is preferably so.

Advantages of the Invention

[0020] According to the present invention, after grinding the silicon wafer, corresponding to the surface shape of the silicon wafer, at least one of the incident angle, the components of s-polarized light and p-polarized light, the energy density, and the number of irradiations per unit area is changed to irradiate the pulsed laser. Therefore, by eliminating the influence of processing stress and modifying the surface into a uniform one, the strength can be improved, and the yield in subsequent processes can be improved.

Brief Description of the Drawings

[0021] [Figure 1] Plan view showing the shape of the notch portion [Figure 2] Side view showing the shape of the notch portion [Figure 3] Diagram showing the relationship between the irradiation direction of the pulsed laser and the incident angle [Figure 4] Configuration diagram showing the laser optical system according to the first embodiment of the present invention [Figure 5] Configuration diagram showing a part of FIG. 4 [Figure 6] Configuration diagram showing the laser optical system according to the second embodiment of the present invention [Figure 7] Block diagram showing the configuration of the polarization optical system in the second embodiment [Figure 8] Graph showing the relationship of the energy density applied to the incident angle

Embodiments for Carrying Out the Invention

[0022] Figure 1 is a plan view showing the shape of a notch portion 10 provided in a part of the silicon wafer 1, and Figure 2 is a side view. The notch portion 10 has a more complex shape compared to the outer peripheral portion 13 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.

[0023] Typically, the planar shape of the notch portion 10 (Figure 1) is formed with an arc-shaped bottom portion 11 as a notch, followed by a slope 12 that curves approximately 45° toward the outer circumference portion 13, and then continues to the outer circumference portion 13 with a rounded shape, resulting in a symmetrical shape. In particular, the bottom portion 11 is greatly affected by the processing load due to grinding and contains many surface defects. In the side shape (Figure 2) of the outer circumference portion 13, a chamfered portion 16 is provided as a rounded slope 12 toward the end face 14, and it connects to the top surface 15, resulting in an up-down symmetrical shape.

[0024] The processed and altered layer is mainly an amorphous silicon layer or a polycrystalline state. Although this processed and altered layer is extremely thin, it has a significant impact on the mechanical, electrical, and optical performance. Therefore, after grinding the silicon wafer 1, the notched portion 10 and the outer peripheral portion 13, or the upper and lower planes of the silicon wafer 1, are subjected to laser heat treatment by irradiating them with a pulsed laser (nanoseconds) to modify and flatten the surface of the processed and 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] 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.

[0028] Therefore, in the case of complex shapes such as the notch portion 10, it is desirable to change at least one of the following pulse laser conditions—incident angle, energy density, or number of irradiations per unit area (scan pitch, scanning speed)—corresponding to the surface shape after grinding the silicon wafer. In particular, for repairing the processed altered layer, energy can be effectively supplied to the material by reducing the incident angle, which is the angle between the vertical line with respect to the interface and the pulse laser irradiation direction. However, the practical difference is small when the incident angle is 10 to 15° or less.

[0029] Laser light is defined as s-polarized light, where the electric field vibrates within the plane of incidence, and p-polarized light, where the electric field vibrates perpendicular to the plane of incidence, when light is reflected at the interface between different materials, depending on the plane of incidence and the direction of vibration of the electric or magnetic field. The reflectivity of s-polarized light increases as the angle of incidence increases, while the reflectivity of p-polarized light decreases. When the angle of incidence is small, for example, between 10 and 15°, the reflectivity of s-polarized and p-polarized light is the same, and there is no distinction between s-polarized and p-polarized light.

[0030] Therefore, when the irradiation direction of the pulsed laser is kept constant, the s-polarized and p-polarized light components may be adjusted via a polarizer to match the change in the incident angle corresponding to the surface shape of the silicon wafer 1. Adjusting the s-polarized and p-polarized light components allows for irradiation with good absorption regardless of the shape of the surface, even for complex shapes where the incident angle changes.

[0031] Furthermore, when adjusting the s-polarization and p-polarization components, it is more desirable to change the laser energy density and the number of irradiations per unit area (scan pitch, scanning speed) in accordance with the change in laser absorption rate due to the change in the incident angle. This allows for more efficient and uniform surface modification of the silicon wafer 1.

[0032] Figure 3 shows the relationship between the irradiation direction and the incident angle of a pulsed laser. The incident angle is the angle between the irradiation direction and the vertical line with respect to the incident surface. Therefore, in Figures 1 and 2, 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. To correspond to regions (1) and (2), which are the surface shapes of silicon wafer 1, (a) a prism, galvano-fθ lens (focusing lens 30) optical system is used to continuously change the incident angle of the pulsed laser so that it is perpendicular to the surface and irradiation surface of silicon wafer 1.

[0033] Alternatively, (b) a polarizer can be used to control the s-polarized and p-polarized components, as well as various laser parameters (energy density, number of irradiations per unit area (scan pitch, scanning speed)), in accordance with the shape of each part of the notch section 10. Of course, combining (a) and (b) is even more desirable.

[0034] A first embodiment of the laser optical system is illustrated in Figures 4 and 5. Figure 4 is a diagram showing the configuration of a laser optical system that irradiates with a pulsed laser. This configuration is a laser optical system used as a scanning optical system. A scanning optical system can perform processing by scanning the laser beam when processing at high speed and over a wide area. The laser beam emitted from the laser oscillator is focused onto the surface of the silicon wafer 1 via prism 20, prism 21, galvanometer mirror 40, and fθ lens (focusing lens 30).

[0035] The direction of pulsed laser irradiation can be varied quickly and over a wide range by rotating the galvanometer mirror 40 or the prism 21. Furthermore, if the incident angle is 10 to 15° or less, the actual difference in absorption rate is small. Therefore, it is desirable to precisely control the pulsed laser irradiation direction by rotating the galvanometer mirror 40 or the prism 21 so that the incident angle is 10 to 15° or less, even in the region (2) of Figures 1 and 2, in conjunction with the shape of the notch portion 10. In the first embodiment, the processing range is limited by the effective area of ​​the fθ lens (focusing lens 30), but the laser irradiation direction can be changed quickly and over a wide range.

[0036] Figure 5 is a configuration diagram showing a part of Figure 4. In the region where the incident angle is 10 to 15° or less, (1) in Figures 1 and 2, the prism 20 is moved in the direction of the arrow to use a part of the laser optical system of Figure 4 as shown in Figure 5. Since the incident angle is 10 to 15° or less, the laser light, which travels in a straight line in one direction with a certain spread, is focused onto the surface of the silicon wafer 1 via the prism 20 and the focusing lens 30. Since the laser light is focused onto the silicon wafer 1 by the focusing lens 30 for processing, a very simple lens configuration can be used and the optical path length can be relatively short.

[0037] Figure 6 shows a laser optical system of a second embodiment in which the laser irradiation direction is kept constant and the s-polarized and p-polarized components are adjusted. In Figure 6, the laser light emitted from the pulsed laser oscillator 3 is focused onto the surface of the silicon wafer 1 via the polarization optical system 50 and the focusing lens 32. Unlike the first embodiment, the laser irradiation direction is not variable. Therefore, the region (2) in Figures 1 and 2 has an effective incident angle, for example, 10 to 15° or more.

[0038] Furthermore, the reflectivity of the silicon wafer 1 surface increases with increasing angle of incidence, with the reflectivity of s-polarized light increasing and the reflectivity of p-polarized light decreasing. Also, the reflectivity of s-polarized and p-polarized light coincides at an angle of incidence of 5°, with a difference of 1.5% at an angle of incidence of 12°, and there is virtually no difference at angles of incidence of 10-15°. Therefore, in the region where the angle of incidence is 10-15° or greater, the components of s-polarized and p-polarized light are adjusted in the polarization optical system 50 so that the absorption rate remains constant.

[0039] Furthermore, in the region (2) shown in Figures 1 and 2 where the incident angle is 10 to 15° or more, the s-polarization and p-polarization components can be adjusted in the polarization optical system 50, and the laser energy density and the number of irradiations per unit area (scan pitch, scanning speed) can also be changed. In this case, the various laser parameters (energy density, number of irradiations per unit area (scan pitch, scanning speed)) are controlled in accordance with the shape of each part of the notch section 10. It is desirable to use an external pulse generator to ensure that the absorption rate remains constant when irradiating with a pulsed laser (nanosecond).

[0040] Furthermore, in the case of p-polarization, the absorption rate increases sharply in the region of large incident angles, and reaches its maximum value at the Brewster angle (polarization angle). Therefore, by repeatedly performing multiple reflections at large incident angles, effective irradiation energy can be supplied to the silicon wafer 1.

[0041] Figure 7 is a block diagram showing the configuration of the polarization optical system 50. The polarization optical system 50 is an optical system capable of creating any polarization direction state by voltage drive. In Figure 7, the pulsed laser emitted from the pulsed laser oscillator 3 is linearly polarized by the polarizer plate 51 and passes through the electro-optic element 52.

[0042] The electro-optic element 52 is, for example, a Kasel or Pockels polarizer, which is an element that delays the phase of the slow axis in accordance with the applied voltage. The slow axis of the electro-optic element 52 and the direction of linear polarization are set to 45 degrees.

[0043] As a result, the light that passes through the electro-optic element 52 becomes elliptically polarized in any direction. Linearly polarized light in any direction can be created by passing it through a quarter-λ wave plate 53, whose original linear polarization direction and the direction of the slow axis are aligned.

[0044] In other words, the polarization optical system 50 can create any polarization state with controllable phase delay by using an electro-optic element 52 with controllable phase delay. Furthermore, the polarization optical system 50 can convert between linearly polarized, elliptically polarized, and circularly polarized light by changing the amount of phase delay.

[0045] Figure 8 is a graph showing the relationship between the incident angle and the energy density. In the region where the incident angle is 10 to 15° or less, (1) in Figures 1 and 2, the energy density is kept almost constant. In the region (2) in Figures 1 and 2 where the incident angle is 10 to 15° or more, the energy density increases as the incident angle increases. As a result, the surfaces of the notch portion 10 and the outer peripheral portion 13 are modified to a uniform surface, regardless of their shape, eliminating the influence of processing stress. Consequently, the silicon wafer 1 has improved strength and yield in subsequent processes. [Explanation of Symbols]

[0046] 1…Silicon wafer 2… Laser light 3. Pulse laser oscillator 10... Notch section 11...Bottom 12…Slope 13…Outer perimeter 14...End face 15…Top surface 16… Chamfered section 20, 21... Prism 30, 32… Focusing lenses 40... Galvano Mirror 50…Polarization Optics 51…Polarizing plate 52… Electro-optic elements 53…1 / 4λ wavelength plate

Claims

1. A method for surface modification of a notched portion of a silicon wafer using laser heat treatment, A method for surface modification of a silicon wafer notch, comprising grinding the silicon wafer and then irradiating the arc-shaped bottom of the notch with a pulsed laser with a constant energy density, and increasing the energy density on the slope of the notch as the angle of incidence increases.

2. In a plan view, the notched portion has the arc-shaped bottom portion and, A method for surface modification of a notch portion of a silicon wafer according to claim 1, comprising the inclined surfaces extending from both ends of the arc-shaped bottom portion toward the outer circumference and connected to the outer circumference in a rounded manner.

3. The method for surface modification of a notch portion of a silicon wafer according to claim 1 or 2, wherein the pulsed laser is irradiated from the side of the silicon wafer with a constant irradiation direction.

4. Furthermore, the method for surface modification of a notched portion of a silicon wafer according to claim 1, wherein the pulsed laser is irradiated by changing the s-polarized and p-polarized components and / or the number of irradiations per unit area.

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