Surface modification method for silicon wafer

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 repairing surface defects, which are process-altered layers on the surface of a silicon wafer, and more particularly to a method for modifying the surface of a silicon wafer by using laser heat treatment to modify and flatten the surface. [Background technology]

[0002] 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. However, the surface and interior of the wafer are damaged by the process, and some of the damaged layers contain microcracks. These internal cracks are mainly removed by chemical and mechanical methods such as etching and chemical mechanical polishing (CMP).

[0003] For example, Patent Document 1 describes a method for improving the polishing quality of the outer periphery of a semiconductor wafer as a workpiece by applying a uniform pressing force to the edge face at each point on the surface of a polishing tool, thereby polishing the edge face to a high quality.

[0004] Furthermore, Patent Document 2 describes that the use of laser irradiation makes it possible to remove oxygen from silicon wafers and improve their crystallinity, and describes that a method for repairing surface defects, which are layers that have been altered by processing on the surface of a single crystal wafer, involves irradiating the single crystal surface with a pulsed laser. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2009-297842 A [Patent Document 2] JP 2008-147639 A Summary of the Invention [Problem to be solved by the invention]

[0006] In the above-mentioned conventional techniques, those described in Patent Documents 1 and 2, in particular, have difficulty in preventing cracks from occurring from notches (notches provided on the wafer). In addition, existing peripheral edge grinding, etching, and chemical mechanical polishing (CMP) cannot completely remove internal cracks, etc. Furthermore, internal cracks, etc., progress as cracks and cause breakage, resulting in a decrease in the yield of surface processing of silicon wafers. In particular, cracks that occur from notches (notches provided on the wafer) have a significant impact on the decrease in yield.

[0007] Furthermore, chemical mechanical polishing (CMP) after grinding and etching involves changes from the original shape (design value), making quality control difficult, and there is a possibility that the flatness of the wafer surface, which was improved in the previous process, will be reduced.Furthermore, chemical mechanical polishing (CMP) uses consumable materials such as abrasive grains, polishing thread, and cleaning liquid, which is costly and has a large environmental impact.

[0008] The object of the present invention is to solve the problems of the conventional technology and to perform suitable heat treatment using a laser on the surface of a silicon wafer after grinding or etching, thereby eliminating the influence of processing stress and modifying the surface to a uniform surface, thereby improving strength and increasing yield in subsequent processes. In particular, the object is to perform suitable laser treatment on a notch portion having a complex shape. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, the present invention provides a method for modifying the surface of a silicon wafer using laser heat treatment, in which after grinding the silicon wafer, a pulsed laser is irradiated by changing 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 in accordance with the surface shape.

[0010] In the above, it is preferable that the angle of incidence is changed in accordance with the surface shape, and that the pulse laser is irradiated so that the angle of incidence is perpendicular.

[0011] Furthermore, in the above, it is preferable that the incidence angle is changed in response to the surface shape, and that the pulse laser is irradiated so that the incidence angle is 10 to 15° or less.

[0012] Furthermore, in the above, it is desirable that the angle of incidence is changed in response to the surface shape, and that a scanning optical system is used in which the irradiation direction of the pulse laser is varied by rotating a galvanometer mirror or a prism.

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

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

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

[0016] It is desirable that the pulsed laser be focused on the surface of the silicon wafer via a pulsed laser oscillator, a polarization optical system, and a focusing lens.

[0017] It is desirable that the polarization optical system has a polarizing plate, an electro-optical element that delays the phase of the slow axis in response to an applied voltage, and a 1 / 4 λ wavelength plate, and creates a polarization direction state by voltage driving.

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

[0019] The pulse laser has a wavelength of 532 nm, a pulse irradiation time within a range of 3 nanoseconds to 4 nanoseconds, a pulse width of 0.5 μJ to 30 μJ per pulse, and an energy density of 0.125 J / cm 2 to 7.5 J / cm 2 It is desirable that: Effect of the Invention

[0020] According to the present invention, after grinding a silicon wafer, a pulsed laser is irradiated by changing at least one of the incident angle, the s-polarized and p-polarized light components, the energy density, and the number of irradiations per unit area in accordance with the surface shape of the silicon wafer. This eliminates the effects of processing stress and modifies the surface to a uniform shape, thereby improving strength and improving the yield in subsequent processes. [Brief description of the drawings]

[0021] [Figure 1] A plan view showing the shape of the notch [Diagram 2] A side view showing the shape of the notch [Diagram 3] Diagram showing the relationship between the pulsed laser irradiation direction and the incidence angle [Figure 4] FIG. 1 is a diagram showing the configuration of a laser optical system according to a first embodiment of the present invention. [Diagram 5] A block diagram showing part of Figure 4. [Figure 6] FIG. 1 is a diagram showing the configuration of a laser optical system according to a second embodiment of the present invention. [Figure 7] FIG. 11 is a block diagram showing the configuration of a polarization optical system according to a second embodiment. [Figure 8] A graph showing the relationship between the incident angle and the energy density DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Fig. 1 is a plan view showing the shape of a notch portion 10 provided in a part of a silicon wafer 1, and Fig. 2 is a side view. The notch portion 10 has a complex shape compared to the outer periphery 13, and its surface is processed by grinding. A process-affected layer is formed on the surface and inside, and some of the process-affected layers contain microcracks.

[0023] Typically, the planar shape of notch portion 10 (Fig. 1) has an arc-shaped bottom 11 formed as a notch, a slope 12 that slopes toward outer periphery 13 at an angle of about 45°, and then a rounded shape that connects to outer periphery 13, making it a bilaterally symmetrical shape. Bottom portion 11 in particular is heavily affected by the processing load caused by the grinding process, and contains many surface defects. In the side shape (Fig. 2), outer periphery 13 has a chamfered portion 16 formed as a rounded slope 12 at end face 14, which connects to top face 15, making it vertically symmetrical.

[0024] The process-affected layer is mainly an amorphous silicon layer or a polycrystalline state. Although this process-affected layer is extremely thin, it has a significant effect on the mechanical, electrical, and optical performance. Therefore, after grinding the silicon wafer 1, in order to modify and flatten the surface of the process-affected layer, a laser heat treatment is performed by irradiating the notch portion 10 and the outer periphery 13, or the upper and lower flat surfaces of the silicon wafer 1 with a pulsed laser (nanoseconds).

[0025] Pulsed laser (nanosecond) irradiation melts the amorphous layer of the processing-induced deformed layer at nanosecond speed. The 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.

[0026] Therefore, the pulse laser conditions are preferably a wavelength of 532 nm, a pulse irradiation time within a range of 3 nanoseconds to 4 nanoseconds, a pulse width of 0.5 μJ to 30 μJ per pulse, and an energy density of 0.125 J / cm. 2 to 7.5 J / cm 2 It is considered good to be.

[0027] However, the light (radiant flux) incident on an object is divided into three destinations macroscopically: "reflection," "absorption," and "transmission," and changes 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.

[0028] Therefore, in the case of a complex shape such as the notch portion 10, it is desirable to change at least one of the pulse laser conditions, the incidence angle, the energy density, and the number of irradiations per unit area (scan pitch, scanning speed), according to the surface shape after grinding the silicon wafer. In particular, for repairing a processing-affected layer, energy can be effectively supplied to the material by reducing the incidence angle, which is the angle between the vertical line to the boundary surface and the pulse laser irradiation direction. However, when the incidence angle is 10 to 15 degrees or less, the substantial difference is small.

[0029] When laser light is reflected at the boundary between different materials, it is defined as s-polarized light, where the electric field oscillates within the plane of incidence, and p-polarized light, where the electric field oscillates perpendicular to the plane of incidence, depending on the "plane of incidence" and the "direction of vibration of the electric or magnetic field." The reflectance of s-polarized light increases as the angle of incidence increases, while the reflectance of p-polarized light decreases. The reflectances of s-polarized light and p-polarized light are the same when the angle of incidence is small, for example, when the angle of incidence is 10 to 15 degrees, and there is no distinction between s-polarized light and p-polarized light.

[0030] Therefore, when the irradiation direction of the pulsed laser is kept constant, the damaged layer may be repaired by adjusting the s-polarized and p-polarized components via a polarizer in accordance with the change in the incident angle corresponding to the surface shape of the silicon wafer 1. Adjusting the s-polarized and p-polarized components allows irradiation with good absorptivity regardless of the shape on a surface with a complex shape where the incident angle changes.

[0031] Furthermore, when adjusting the s-polarized and p-polarized components, it is more desirable to change the laser energy density and the number of irradiations per unit area (scan pitch, scanning speed) in response to the change in laser absorptance caused by the change in incident angle. This allows the surface of the silicon wafer 1 to be modified more efficiently and uniformly.

[0032] FIG. 3 is a diagram showing the relationship between the irradiation direction of a pulsed laser and the angle of incidence. The angle of incidence is the angle between the irradiation direction and a vertical line with respect to the incident surface. Therefore, in FIGS. 1 and 2, when a pulsed laser is irradiated from the direction of the arrow, (1) is a region where the angle of incidence is 10 to 15° or less, and (2) is a region where the angle of incidence is 10 to 15° or more. Therefore, in order to correspond to the region (1) and region (2) which are the surface shape of the silicon wafer 1, an optical system (a) including a prism and a galvano-fθ lens (condenser lens 30) is used to continuously change the angle of incidence of the pulsed laser so that it is perpendicular to the surface of the silicon wafer 1 and the irradiation surface.

[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, scan speed)) in accordance with the shape of each part of the notch portion 10. Of course, a combination of (a) and (b) is more preferable.

[0034] A first embodiment of the laser optical system will be described with reference to FIGS. Fig. 4 is a configuration diagram showing a laser optical system that irradiates a pulsed laser. This configuration is a laser optical system that functions as a scanning optical system. The scanning optical system can perform processing by scanning a laser beam when performing processing over a wide area at high speed. The laser beam emitted from a laser oscillator is focused on the surface of a silicon wafer 1 via a prism 20, a prism 21, a galvanometer mirror 40, and an fθ lens (focusing lens 30).

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

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

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

[0038] As the angle of incidence increases, the reflectance of s-polarized light on the surface of silicon wafer 1 increases, while the reflectance of p-polarized light decreases. The reflectances of s-polarized light and p-polarized light match at an angle of incidence of 5°, have a difference of 1.5% at an angle of incidence of 12°, and are virtually insignificant at angles of incidence between 10 and 15°. Therefore, in the region where the angle of incidence is 10 to 15° or more, the s-polarized and p-polarized light components are adjusted by polarization optical system 50 so that the absorptance is constant.

[0039] Furthermore, in the region (2) in Figs. 1 and 2 where the angle of incidence is 10 to 15° or more, the s-polarized and p-polarized components may be adjusted by the polarization optical system 50, and the energy density of the laser and the number of irradiations per unit area (scan pitch, scanning speed) may be changed. In this case, various parameters of the laser (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 portion 10. It is desirable to irradiate the pulsed laser (nanoseconds) using an external pulse generator so that the absorption rate is constant.

[0040] In the case of p-polarized light, the absorptance increases rapidly in the region where the angle of incidence is large, and reaches a maximum value at the Brewster angle (polarization angle). Therefore, by repeating multiple reflections at a large angle of incidence, effective irradiation energy can be supplied to the silicon wafer 1.

[0041] FIG. 7 is a block diagram showing the configuration of the polarization optical system 50. As shown in FIG. The polarization optical system 50 is an optical system capable of producing an arbitrary polarization direction state by voltage driving. In FIG. 7, the pulse laser emitted from the pulse laser oscillator 3 is linearly polarized by the polarizing plate 51 and passes through the electro-optical element 52.

[0042] The electro-optic element 52 is, for example, a Kerr-cell or Pockels polarizer, and is an element that delays the phase of the slow axis in response to an applied voltage. The slow axis of the electro-optic element 52 and the direction of the linearly polarized light are set at 45 degrees.

[0043] As a result, the light that has passed through the electro-optic element 52 becomes an arbitrary elliptically polarized light. Linearly polarized light in an arbitrary direction can be created by finally passing the light through a ¼λ wave plate 53 whose slow axis direction is aligned with the original linear polarization direction.

[0044] In other words, the polarization optical system 50 can create any polarization state in which the phase delay can be controlled by using the electro-optical element 52 that can control the phase delay. In addition, the polarization optical system 50 can convert light into linearly polarized light, elliptically polarized light, or circularly polarized light by changing the amount of phase delay.

[0045] FIG. 8 is a graph showing the relationship of energy density to the incident angle. The energy density is almost constant in the region where the incident angle is 10 to 15° or less, (1) in FIGS. 1 and 2. In the region (2) in FIGS. 1 and 2 where the incident angle is 10 to 15° or more, the energy density is increased as the incident angle increases. As a result, the surfaces of the notch portion 10 and the outer periphery 13 are modified to a uniform surface without being affected by processing stress, regardless of the shape. The strength of the silicon wafer 1 is improved, and the yield in the subsequent process is improved. [Explanation of symbols]

[0046] 1. Silicon wafer 2...Laser light 3...Pulse laser oscillator 10...Notch 11...Bottom 12...Slope 13...Outer periphery 14...End face 15…Top surface 16…Beveled part 20, 21…Prism 30, 32...Condenser lens 40…Galvano mirror 50...Polarizing optical system 51...Polarizing plate 52...Electro-optical element 53…1 / 4λ wavelength plate

Claims

1. A method for modifying the surface of a notch portion of a silicon wafer using laser heat treatment, comprising the steps of: a pulsed laser is irradiated onto a circular arc bottom of the notch portion with a constant energy density and onto a slope of the notch portion with an energy density that increases as an incidence angle increases, after grinding the silicon wafer.

2. The notch portion has, in a plan view, the arc-shaped bottom portion and 2. The method for modifying a surface of a notch portion of a silicon wafer according to claim 1, further comprising the inclined surfaces extending from both ends of the arc-shaped bottom portion toward the outer periphery and connecting to the outer periphery in a rounded manner.

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

4. 2. The method for modifying the surface of a notch portion of a silicon wafer according to claim 1, further comprising the step of irradiating the pulsed laser by changing s-polarized and p-polarized components and / or the number of irradiations per unit area.

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