Silicon wafer surface modification method

Abstract

To eliminate the effect of processing stress and modify the surface to be uniform and flattened by performing suitable heat treatment using a laser on the surface of a silicon wafer after an etching process.SOLUTION: In a silicon wafer surface modification method using laser heat treatment, after alkali etching treatment, the accumulated irradiation energy corresponding to the crystal orientation of an irradiated portion is determined, and the nanosecond pulse laser is emitted. Further, an irradiation condition is determined such that the cumulative irradiation energy of the irradiated portion is equal to or less than a predetermined threshold value. Furthermore, after the alkali etching treatment, irradiation is performed by changing at least one of the energy density, scan pitch, and the number of times of irradiation according to the curvature of the irradiated portion.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to repairing surface defects, which are layers that have been altered due to processing on the surface of a silicon wafer, and in particular to a method for modifying the surface of a silicon wafer using a laser heat treatment after an alkaline etching treatment (AE). [Background technology]

[0002] Semiconductor wafers such as silicon wafers used in the manufacture of semiconductor devices are surface-processed 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 that a silicon wafer that has been subjected to a chamfering process is subjected to alkaline etching using an aqueous potassium hydroxide solution, an aqueous sodium hydroxide solution, or the like to remove distortion of the silicon wafer caused by processing in the previous process.

[0004] Furthermore, Patent Document 2 describes that after grinding such as chamfering, the damaged layer on the surface of the silicon wafer is repaired and the roughness is flattened efficiently and effectively by irradiating the surface with a pulsed laser. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2018-101698 A [Patent Document 2] JP 2020-131218 A Summary of the Invention [Problem to be solved by the invention]

[0006] In the above-mentioned conventional techniques, the alkaline etching described in Patent Document 1 strongly manifests the anisotropy of the silicon wafer, and it is difficult to prevent the cracks caused by the anisotropy. In addition, the existing peripheral edge grinding, etching, and chemical mechanical polishing (CMP) may not be able to completely remove internal cracks, etc. Furthermore, the internal cracks, etc. progress as cracks and break the wafer, which reduces the yield of the surface processing of the silicon wafer.

[0007] In addition, the technique described in Patent Document 2 makes it possible to repair damage such as grinding marks caused by grinding and perform flattening treatment. However, the technique described in Patent Document 2 has limitations in terms of laser irradiation conditions according to the surface state after grinding.

[0008] In particular, when flattening the surface after alkaline etching (AE) processing by laser irradiation, if an appropriate irradiation method is not used, internal damage such as dislocations will occur, resulting in reduced quality and a lower yield in subsequent processes. Furthermore, there is a limit to the improvement of the quality of surface modification for internal cracks, surface roughness (especially roughness caused by anisotropy), etc.

[0009] The object of the present invention is to solve the above-mentioned problems of the conventional technology, and to perform suitable heat treatment using a laser on the surface of a silicon wafer after etching, thereby eliminating the influence of processing stress and modifying the surface to a uniform and flat surface, thereby improving strength and increasing yield in subsequent processes. In particular, the object is to perform suitable laser treatment on the wafer edge, which has a shape in which the crystal orientation changes. [Means for solving the problem]

[0010] 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 alkaline etching treatment, a cumulative irradiation energy corresponding to the crystal orientation of the irradiated area is determined and a nanosecond pulse laser is irradiated.

[0011] In the above-described method for modifying a surface of a silicon wafer using laser heat treatment, it is preferable to determine irradiation conditions so that the accumulated irradiation energy at the irradiated portion is equal to or less than a predetermined threshold value.

[0012] Furthermore, in the above-mentioned method for modifying a silicon wafer surface using laser heat treatment, it is desirable to change at least one of the energy density, the scan pitch, and the number of irradiations in response to the curvature of the irradiated area after the alkaline etching treatment.

[0013] Furthermore, in the above-mentioned method for modifying the surface of a silicon wafer using laser heat treatment, it is preferable to use a CW (continuous wave) laser irradiation in addition to the nanosecond pulse laser irradiation after the alkaline etching treatment.

[0014] Furthermore, in the above-mentioned method for modifying the surface of a silicon wafer using laser heat treatment, it is desirable that the irradiation area having a curvature is divided into sections to change the angle of incidence, that the irradiation surface of each section is irradiated perpendicularly, and that the scan pitch is increased.

[0015] Furthermore, in the above-mentioned method for modifying the surface of a silicon wafer using laser heat treatment, when the irradiated portion is a curved surface, the ratio of the irradiating energy on the curved surface to the irradiating energy on a flat surface is defined as C i The scan pitch of the curved surface is the scan pitch of the flat surface × 1 / C i It is desirable to do so.

[0016] Furthermore, in the above-mentioned method for modifying the surface of a silicon wafer using laser heat treatment, it is preferable that the scan pitch is set to 0.5 to 0.7 times the spot diameter D of the nanosecond pulse laser, and that the irradiation position i where the first irradiation energy E1 is at its peak and the irradiation position I' where the second irradiation energy E2 is at its peak are set to i + the scan pitch / 2, so that the cumulative irradiation energy at the irradiated location is equal to or less than a predetermined threshold value.

[0017] Furthermore, in the above-mentioned method for modifying a silicon wafer surface using laser heat treatment, it is preferable that the scan pitch is smaller than 1 / 2 of the spot diameter D of the nanosecond pulse laser, the number of irradiations is multiple, and the cumulative irradiation energy of the irradiated area is equal to the sum of the non-overlapping area and the overlapping area. Effect of the Invention

[0018] According to the present invention, after alkaline etching, a nanosecond pulse laser is irradiated by determining a cumulative irradiation energy corresponding to the crystal orientation of the irradiated area, thereby eliminating the effects of processing stress and modifying the surface to a uniform one, thereby improving strength and improving the yield in subsequent processes. [Brief description of the drawings]

[0019] [Figure 1] A perspective view showing an example of the distribution of crystal orientations on a silicon wafer surface. [Diagram 2] Upper half cross-sectional view showing an example of crystal orientation distribution at the wafer edge [Diagram 3] Flowchart showing the laser irradiation procedure [Figure 4] A diagram showing the difference between the flat and R parts of the wafer edge when irradiated with laser light. [Diagram 5] Graph showing the relationship between spot diameter D and laser radiation intensity I(r) [Figure 6] A graph explaining how to determine the spot diameter D and the scan pitch SP according to an embodiment of the present invention. [Figure 7] Graph for explaining how to determine spot diameter D and scan pitch SP according to another embodiment DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Fig. 1 is a perspective view showing an example of the distribution of crystal orientation on the surface of a silicon wafer 1, and Fig. 2 is a cross-sectional view of the upper half showing an example of the distribution of crystal orientation at the edge of the wafer. Note that Fig. 2 shows a cross-section of the upper half of the circled portion of Fig. 1 (Si(100) plane). In general, even for the surface of the exact same material, its properties differ depending on the direction of the plane along which the crystal is cut. Crystal planes are specified by Miller indices, and for example, a cut surface of a silicon single crystal cut along a lattice plane with Miller indices (111) is called a Si(111) plane with a crystal orientation.

[0021] For example, the crystal structure of the Si(100) surface is one in which atoms are packed almost uniformly in the depth direction, whereas the Si(111) surface is layered in the depth direction. Therefore, it is believed that the propagation of kinetic energy in the depth direction due to lattice vibration, i.e., heat conduction in the depth direction, is easier on the Si(111) surface. In other words, it can be said that the Si(111) surface is easier to melt to a greater depth than the Si(100) surface due to its poorer heat conduction. The order of magnitude is Si(110) surface>Si(100) surface>Si(111) surface.

[0022] In addition, the propagation speed of the laser shock wave differs between the Si(100) surface and the Si(111) surface. This is because the atomic arrangement in the depth direction (Si(111) direction) is layered on the Si(111) surface, so lattice vibrations are more easily transmitted. It is believed that heat conduction on the scale of several microns is dominated by the propagation of lattice vibrations (waves), with ballistic heat conduction occurring due to phonons (quanta of lattice vibrations in crystals).

[0023] The Si(111) surface has a higher thermal conductivity due to the heat input from the laser than the Si(100) surface and Si(110) surface, and the heat (lattice vibration) is transferred quickly, so that the deformation (contraction) cannot keep up during the subsequent cooling process and cracks occur. In other words, distortion due to thermal stress occurs, and internal damage such as dislocations occurs.

[0024] As shown in Figure 1, the distribution of crystal orientations on the surface of a silicon wafer 1 has the same orientation distributed at every 90 degrees of three-dimensional rotation angle. Also, as shown in Figure 2, the distribution of crystal orientations on the wafer edge changes at the R portion 3, even if the end surface 2 is a Si(100) surface, and for example, the Si(111) surface appears at 36 degrees from the horizontal, and becomes a Si(100) surface on the top surface 4. The surface state of each crystal orientation after alkaline etching is as follows: Si(110) surface: Etch pits are widely distributed and the surface is rough overall. Si(100), Si(111) surface: some etch pits scattered Etch pits are corrosion holes that appear on the surface during etching, and correspond to the points where dislocations, which are linear crystal lattice defects, intersect with the crystal surface. Etch pits occur because the area near the lattice defect is more chemically reactive or corroded than other areas.

[0025] In one embodiment, in a surface modification method for silicon wafer 1 using laser heat treatment, irradiation conditions are determined according to the crystal orientation and shape so that the cumulative value of the laser irradiation energy (cumulative irradiation energy) at the irradiation point is equal to or less than a predetermined threshold value in order to reduce the effect of thermal stress. In another embodiment, a CW (continuous wave) laser is used in combination to prevent a sudden temperature change, reduce the load caused by repeated thermal stress, and suppress the occurrence of dislocations.

[0026] 3 is a flow chart showing the procedure of laser irradiation. In step S1, the surface state (roughness, waviness) of the silicon wafer 1 after grinding is measured and mapped to obtain distribution data corresponding to the shape of the silicon wafer 1. It is preferable to measure the surface state and / or shape of the silicon wafer 1 by a non-destructive method. The non-destructive method may be any one selected from an image measuring means, a capacitance change measuring means, a droplet contact angle measuring means, a Raman spectroscopic measuring means, a surface roughness meter, an acoustic measuring means, an eddy current characteristic measuring means, a reflectance measuring means, an X-ray Lang's method measuring means, an electron beam diffraction measuring means, an SEM measuring means, and the like, or a combination of two or more of them.

[0027] Next, in step S2, the internal state of the irradiated area is inspected by Raman spectroscopy. That is, in step S2, the change in the crystalline state inside the silicon wafer 1 is detected by measuring the distortion from the Raman shift. Here, those with large internal damage are excluded.

[0028] In step S3, irradiation conditions are determined and irradiation is performed. In step S3-1, irradiation conditions are determined according to the crystal orientation and shape. In other words, conditions must be determined that flatten the surface of the irradiated area, which has a different surface condition depending on the orientation and shape, while not causing internal damage. As already mentioned, the surface condition after alkaline etching (roughness due to the distribution of etch pits) and thermal conductivity differ depending on the Si(110), Si(100), and Si(111) surfaces, and the irradiation energy required for the desired flattening differs.

[0029] The required irradiation energy has a magnitude relationship of Si(110)>Si(100)>Si(111), and the irradiation conditions are determined according to the crystal orientation. For example, the energy required for a Si(110) surface to be flattened from several hundred nanometers to a roughness of about several nanometers by laser irradiation without generating internal dislocations is approximately 1.3 times that required for a Si(100) surface. Note that the irradiation energy required for a Si(111) surface is approximately the same as that for a Si(100) surface, but only 0.7 times that.

[0030] Regarding the laser conditions according to the shape, it is desirable that the laser be approximately perpendicular to the irradiated surface and the laser incidence angle be 10 to 15° or less at irradiated areas that can be considered as flat areas such as the inclined surface 5, the end surface 2, and the top surface 4. It is difficult to irradiate the entire curved surface with the laser perpendicularly to the R-section 3, and the effective laser radiation intensity of the laser is lower than that of a flat area.

[0031] The laser irradiation is a nanosecond pulsed laser, which melts the amorphous layer of the processing-affected layer at nanosecond speeds. The melting caused by the nanosecond pulsed laser irradiation promotes recrystallization (epitaxial growth) with aligned crystal orientation, eliminating crystal defects caused by mechanical processing. In addition, the amorphous silicon layer has strong absorption of light with a wavelength of 532 nm.

[0032] Therefore, the nanosecond pulse laser has a wavelength λ of 355, 532, or 785 nm, for example, λ=532 nm, and the pulse irradiation time is preferably within a range of 3 nanoseconds to 4 nanoseconds. The pulse width and energy per pulse are preferably 0.5 μJ to 30 μJ, and the energy density is preferably 0.125 J / cm. 2 to 7.5 J / cm 2 It is considered good to be.

[0033] In addition, after the alkaline etching process, for areas with large etch pits or waviness, the nanosecond pulsed laser is irradiated together with the CW (continuous wave) laser. For example, before irradiating with the nanosecond pulsed laser, a 1080 nm CW (continuous wave) laser, which has a wavelength with high absorption rate for SiO2, is irradiated.

[0034] Furthermore, the nanosecond pulse laser is irradiated by changing at least one of the energy density, the scan pitch SP, and the number of irradiations in response to the curvature of the irradiated area. This adjusts the irradiation conditions according to the crystal orientation and shape, and the surface of the silicon wafer 1 is modified more uniformly.

[0035] In step S3-2, the laser is irradiated by overlapping scanning with a predetermined spot diameter D (Gaussian laser spot diameter) so that the cumulative irradiation energy is averaged regardless of the irradiation position and is below the threshold. In other words, in step S3-2, the irradiation location is scanned overlappingly multiple times to uniformly flatten the entire edge without any breaks. Accumulative irradiation with a predetermined spot diameter D means that there are locations that are irradiated more than twice, i.e., overlapping irradiation.

[0036] In the overlapping areas, the cumulative irradiation energy may exceed the threshold at which dislocations do not occur. Therefore, important conditions for reducing waviness in the laser-irradiated areas are the spot diameter D, the scan pitch SP, and multiple irradiations, for example, the phase between the first and second irradiations in two irradiations, and the cumulative irradiation energy is set below the threshold at which dislocations do not occur and averaged.

[0037] Step S4 is a step of confirming whether or not the surface of the silicon wafer 1 has been repaired, similar to step S2. If the surface has been repaired, the laser irradiation is terminated, and if the surface has not been repaired, the laser irradiation is resumed or continued until the surface is confirmed to be repaired.

[0038] FIG. 4 is a diagram showing the difference between the flat portion and the R portion 3 of the wafer edge portion in laser irradiation. FIG. 4 shows an example of step S3-1. The inclined surface 5, which is the flat portion, is approximately perpendicular to the irradiation surface, but the R portion 3 is an irradiation portion with a curve, so it is difficult to irradiate all irradiation portions with the laser perpendicularly. Therefore, as shown in FIG. 4, the irradiation portion with a curve is divided (discretized) into appropriate sections, the angle of incidence is changed, and the irradiation surface of each section is irradiated perpendicularly. In addition, it is desirable to increase the scan pitch SP for the irradiation portion with a curve (or reduce the cumulative number of irradiations) to shorten the throughput and improve efficiency.

[0039] FIG. 5 is a graph showing the relationship between spot diameter D and laser radiation intensity I(r), and shows the relationship between spot diameter D (radial distance r from the beam center = D / 2) and laser radiation intensity I(r) when the irradiated area is a curved surface, forming R portion 3. The vertical axis is laser radiation intensity I(r), the horizontal axis is radial distance r (= D / 2) from the beam center, and R i is the radius of curvature of the irradiated spot of the R portion 3. The angle α O D / R i Therefore, the angle α(r) at any r is α O / 2×r / (D / 2)=r / R i It becomes.

[0040] The relationship between the irradiation energy when the irradiation surface is flat and when it is curved is as follows. Since the laser radiation intensity I(r) is a Gaussian laser, it can be expressed as (Equation 1).

number

number

number

number

[0041] From the above, if the curvature of R part 3 is constant, Scan pitch of R section 3 = Scan pitch of flat section × 1 / C i If the curvature changes, Scan pitch of R section 3 = Scan pitch of flat section × 1 / C i,i+1 It is preferable to set the above.

[0042] 6 and 7 are graphs for explaining how to determine the spot diameter D and scan pitch SP in step S3-2. Fig. 6 shows the case where the scan pitch SP is set to the spot diameter D x 0.5 to 0.7, and Fig. 7 shows the case where the scan pitch SP is set to the spot diameter D x ~ 0.5 (smaller than 1 / 2 of the spot diameter D), with the vertical axis representing the irradiation energy and the horizontal axis representing the irradiation position. In order to uniformly flatten the entire surface, the irradiation energy needs to be averaged regardless of the irradiation position, and the cumulative irradiation energy needs to be equal to or less than a threshold value.

[0043] In Figure 6, E1 is the peak of the first irradiation energy, and the cumulative irradiation energy (point b indicated by arrow) of the overlapping portion between the irradiation position i where the peak occurs and the next irradiation position (i+1) is slightly smaller than E1 even when they are overlapped. The second irradiation is then set to the irradiation energy of E2. (However, if the scan pitch is SP where the cumulative irradiation energy of the overlapping portion is almost the same as E1, then it is sufficient to irradiate only E1.) At this time, the irradiation position I' where E2 peaks is shifted in phase to be i+SP / 2.

[0044] As a result, the cumulative irradiation energy at points a and b indicated by the arrows is equalized and averaged. The cumulative irradiation energy at the irradiated area is set to a predetermined threshold value or less that does not cause dislocations even if the number of irradiations is multiple, e.g., E1+E2 for two times, or E1+E2+... for many times. Furthermore, in order to reduce throughput (processing time), it is desirable to perform the cumulative irradiation with about two times or less.

[0045] In Fig. 7, E1 is the peak of the first irradiation energy, and the second irradiation is performed with an irradiation energy of E2. The scan pitch SP is smaller than 1 / 2 of the spot diameter D. The cumulative irradiation energy of E1 at point b is greater than E1 because of overlap, and similarly, the cumulative irradiation energy of E2 at point b is greater than E2.

[0046] The cumulative irradiation energy due to irradiation with E1 and E2 is averaged so that it is equal to the sum of E1+E2 at point a, which is the non-overlapping part, and point b, which is the overlapping part. As in Fig. 6, the cumulative irradiation energy at the irradiated part is set to a predetermined threshold or less at which dislocations do not occur even if it is E1+E2 or E1+E2+... [Explanation of symbols]

[0047] 1. Silicon wafer 2...End face 3...R section 4…Top surface 5. Slope SP…Scan pitch

Claims

1. A method for modifying a surface of a silicon wafer using laser heat treatment, comprising the steps of: A method for modifying the surface of a silicon wafer, which comprises irradiating the surface with a nanosecond pulse laser after alkaline etching so that the cumulative irradiation energy is equal to or less than a predetermined threshold value depending on the shape of the irradiated area.

2. 2. The method for modifying a surface of a silicon wafer according to claim 1, wherein irradiating the nanosecond pulse laser in accordance with the shape of the irradiation spot includes irradiating the irradiation spot in accordance with a curvature of the irradiation spot.

3. 3. The method for modifying a silicon wafer according to claim 1, wherein irradiating the nanosecond pulse laser so that the cumulative irradiation energy of the irradiated area is equal to or less than a predetermined threshold includes irradiating the area while changing at least one of an energy density, a scan pitch, and a number of irradiations.

4. 4. The method for modifying a surface of a silicon wafer according to claim 1, further comprising the step of irradiating the surface with a CW (continuous wave) laser in addition to the irradiation with the nanosecond pulse laser after the alkaline etching treatment.

5. 4. The method for modifying a surface of a silicon wafer according to claim 3, wherein the irradiation area having a curvature is divided into sections to change the angle of incidence, the irradiation surface of each section is irradiated perpendicularly, and the scan pitch is increased.

6. When the irradiated portion is a curved surface, the ratio of the irradiating energy on the curved surface to the irradiating energy on a flat surface is C ｉ The scan pitch of the curved surface is the scan pitch of the flat surface × 1 / C ｉ 6. The method for modifying a surface of a silicon wafer according to claim 3 or 5, characterized in that

7. The surface modification method of a silicon wafer according to claim 6, characterized in that the scan pitch is set to 0.5 to 0.7 times the spot diameter D of the nanosecond pulse laser, and the irradiation position i where the first irradiation energy E1 is at its peak is set to i + the scan pitch / 2 where the irradiation position I' where the second irradiation energy E2 is at its peak, so that the cumulative irradiation energy at the irradiated location is equal to or less than a predetermined threshold value.

8. The surface modification method of a silicon wafer according to claim 6, characterized in that the scan pitch is smaller than 1 / 2 of the spot diameter D of the nanosecond pulse laser, the number of irradiations is multiple, and the cumulative irradiation energy of the irradiated area is equal to the sum of the non-overlapping area and the overlapping area.

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