Method for modifying the surface of silicon wafers

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 affected by processing on the surface of a silicon wafer, and more particularly to a method for modifying the surface of a silicon wafer using laser heat treatment after alkaline etching (AE). [Background technology]

[0002] Semiconductor wafers, such as silicon wafers used in the fabrication of semiconductor devices, are surface-processed using mechanical processes such as cutting, grinding, lapping, and polishing. However, the surface and interior of these wafers are subject to the formation of process-affected layers, some of which contain microcracks. These internal cracks are typically removed using 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 that has occurred during processing up to 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 it with a pulsed laser. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-101698 [Patent Document 2] Japanese Patent Publication No. 2020-131218 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 exhibits anisotropy in the silicon wafer, making it difficult to prevent cracks caused by this. Furthermore, existing peripheral edge grinding, etching, and chemical mechanical polishing (CMP) may not be able to completely remove internal cracks, etc. Furthermore, internal cracks, etc., propagate as cracks and cause damage, reducing the yield of surface processing of silicon wafers.

[0007] Furthermore, 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 planarizing a surface after alkaline etching (AE) processing by laser irradiation, if the appropriate irradiation method is not used, internal damage such as dislocations will occur, resulting in reduced quality and a decrease in yield in subsequent processes. Furthermore, there are limits 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 problems of the conventional technology and to perform a 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 present invention aims to perform a suitable laser treatment on the wafer edge, which has a shape where the crystal orientation changes. [Means for solving the problem]

[0010] In order to achieve the above 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 the surface of a silicon wafer using laser heat treatment, it is desirable to determine irradiation conditions so that the cumulative irradiation energy at the irradiated location is equal to or less than a predetermined threshold value.

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

[0013] Furthermore, in the above-mentioned method for modifying the surface of a silicon wafer using laser heat treatment, it is desirable to use 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, the angle of incidence is changed, the irradiation surface of each section is irradiated perpendicularly, and 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 irradiation energy on the curved surface to the irradiation 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 desirable 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 reaches its peak is set to i+the scan pitch / 2 where the second irradiation energy E2 reaches its peak, so that the cumulative irradiation energy at the irradiated location is equal to or less than a predetermined threshold.

[0017] Furthermore, in the above-mentioned method for modifying the surface of a silicon wafer using laser heat treatment, it is desirable 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. [Effects of the Invention]

[0018] According to the present invention, after alkaline etching, the cumulative irradiation energy corresponding to the crystal orientation of the irradiated area is determined and then the nanosecond pulse laser is irradiated. This eliminates the influence of processing stress and modifies the surface to a uniform one, thereby improving strength and increasing yield in subsequent processes. [Brief explanation of the drawings]

[0019] [Figure 1] A perspective view showing an example of the distribution of crystal orientation on the surface of a silicon wafer. [Figure 2] Upper half cross section showing an example of crystal orientation distribution at the wafer edge [Figure 3] Flowchart showing the laser irradiation procedure [Figure 4] Diagram showing the difference between the flat and rounded areas of the wafer edge when irradiating with laser light [Figure 5] Graph showing the relationship between spot diameter D and laser radiation intensity I(r) [Figure 6] Graph explaining how to determine the spot diameter D and scan pitch SP according to one embodiment of the present invention. [Figure 7] Graphs illustrating how to determine the spot diameter D and the scan pitch SP according to another embodiment DETAILED DESCRIPTION OF THE INVENTION

[0020] Figure 1 is a perspective view showing an example of the distribution of crystal orientation on the surface of a silicon wafer 1, and Figure 2 is a cross-sectional view of the upper half showing an example of the distribution of crystal orientation at the wafer edge. Note that Figure 2 shows a cross-section of the upper half of the circled area in Figure 1 (Si(100) plane). In general, even the surface of the exact same material will have different properties depending on the direction of the plane along which the crystal is cut. Crystal planes are specified by Miller indices; for example, a cut surface of a silicon single crystal cut along a lattice plane with Miller indices (111) will be 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 thought 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, the Si(111) surface is more likely to melt to a greater depth than the Si(100) surface due to its better heat conduction. The order of magnitude is Si(110) surface > Si(100) surface > Si(111) surface.

[0022] Furthermore, the propagation speed of laser shock waves differs between the Si(100) and Si(111) surfaces. This is because the atomic arrangement on the Si(111) surface is layered in the depth direction (Si(111) direction), making it easier for lattice vibrations to propagate. It is believed that heat conduction on a 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] Compared to the Si(100) and Si(110) surfaces, the Si(111) surface has a higher thermal conductivity due to the heat input from the laser, and the heat (lattice vibration) is transferred quickly, so that the deformation (contraction) cannot keep up during the subsequent cooling process, resulting in cracks. 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 one surface of a silicon wafer is such that the same orientation is distributed at every 90 degrees of rotation angle in three dimensions. 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; for example, the Si(111) surface appears at 36 degrees from the horizontal, and the top surface 4 becomes a Si(100) surface. After alkaline etching, the surface state of each crystal orientation is as follows: Si(110) surface: Etch pits are widely distributed and the surface is rough overall. Si(100), Si(111): 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 around these lattice defects is more chemically reactive or susceptible to corrosion 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 laser irradiation energy (cumulative irradiation energy) at the irradiated point is equal to or less than a predetermined threshold value in order to reduce the effects of thermal stress. In another embodiment, a CW (continuous wave) laser is also used to prevent sudden temperature changes, reduce the load caused by repeated thermal stress, and suppress the occurrence of dislocations.

[0026] 3 is a flowchart showing the laser irradiation procedure. In step S1, the surface condition (roughness, waviness), etc. of the silicon wafer 1 after grinding is measured and mapped to obtain distribution data corresponding to the shape, etc., of the silicon wafer 1. It is desirable to measure the surface condition and / or shape of the silicon wafer 1 using a non-destructive method. The non-destructive method may be any one selected from image measurement means, capacitance change measurement means, droplet contact angle measurement means, Raman spectroscopy measurement means, surface roughness meter, acoustic measurement means, eddy current characteristic measurement means, reflectance measurement means, X-ray Lang method measurement means, electron beam diffraction measurement means, SEM measurement means, etc., or a combination of two or more of these may be used.

[0027] Next, in step S2, the internal state of the irradiated area is inspected by Raman spectroscopy. That is, in step S2, the strain from the Raman shift is measured to detect changes in the crystalline state inside the silicon wafer 1. Here, wafers with significant 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 different surface conditions depending on the orientation and shape, while not causing internal damage. As mentioned above, 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 to achieve the desired flatness differs.

[0029] The required irradiation energy has the magnitude relationship Si(110)>Si(100)>Si(111), and irradiation conditions are determined according to the crystal orientation. For example, to flatten a Si(110) surface from several hundred nanometers to a roughness of a few nanometers without generating internal dislocations by laser irradiation, the energy required is approximately 1.3 times that of 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 laser conditions according to the shape, it is desirable that the laser be nearly perpendicular to the irradiation surface, with an incident angle of 10 to 15° or less, at irradiation points that can be considered flat, such as the inclined surface 5, the end surface 2, and the top surface 4. For the curved portion 3, it is difficult to irradiate the entire surface with a curved beam perpendicularly, and the effective laser radiation intensity of the laser is lower than that of a flat surface.

[0031] The laser irradiation is a nanosecond pulse laser, which melts the amorphous layer affected by the processing at nanosecond speeds. The melting caused by the nanosecond pulse laser irradiation promotes recrystallization (epitaxial growth) with aligned crystal orientation, eliminating crystal defects caused by the machining process. In addition, the amorphous silicon layer has a 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 the range of 3 to 4 nanoseconds. The pulse width and energy per pulse are preferably 0.5 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] Furthermore, after alkaline etching, areas with large etch pits or undulations are irradiated with a CW (continuous wave) laser in addition to the nanosecond pulse laser. For example, before irradiating with the nanosecond pulse laser, a CW (continuous wave) laser with a wavelength of 1080 nm, which has a high absorption rate for SiO2, is irradiated.

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

[0035] In step S3-2, the laser is irradiated by overlapping scanning with a predetermined spot diameter D (spot diameter of the Gaussian laser) 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 area to be irradiated is scanned overlappingly multiple times to uniformly flatten the entire edge without any gaps. In the case of cumulative irradiation with the predetermined spot diameter D, there are areas that are irradiated more than once, i.e., areas where irradiation overlaps.

[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 similar to step S2 in which it is confirmed whether the surface of the silicon wafer 1 has been repaired. 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 repair is confirmed.

[0038] FIG. 4 shows the difference between the flat portion and the rounded portion 3 of the wafer edge in laser irradiation. FIG. 4 shows an example of step S3-1. The inclined surface 5, which is the flat portion, is made approximately perpendicular to the irradiation surface, but the rounded portion 3 is an irradiation area with curvature, so it is difficult to irradiate all irradiation areas with the laser perpendicularly. Therefore, as shown in FIG. 4, the irradiation area with curvature is divided (discretized) into appropriate sections, the angle of incidence is changed, and each section is irradiated perpendicularly to the irradiation surface. Furthermore, for irradiation areas with curvature, it is desirable to increase the scan pitch SP (or reduce the cumulative number of irradiations) to shorten 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 and forms a rounded portion 3. The vertical axis is laser radiation intensity I(r), and the horizontal axis is radial distance r (= D / 2) from the beam center. i is the radius of curvature of the irradiated area of ​​the R portion 3. The angle α at the spot diameter D O is D / R i Therefore, the angle α(r) at any r is α O / 2×r / (D / 2)=r / R i This 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).

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[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 following.

[0042] 6 and 7 are graphs 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 × 0.5 to 0.7, and Fig. 7 shows the case where the scan pitch SP is set to the spot diameter D × ~ 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 each case. 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 below a threshold value.

[0043] In Figure 6, E1 is the peak of the first irradiation energy, and the cumulative irradiation energy (point b in the arrow) at the overlapping portion between irradiation position i where this peak occurs and the next irradiation position (i+1) is slightly smaller than E1 even when they are overlapped. The second irradiation is then performed with an irradiation energy of E2. (However, if the scan pitch is SP so that the cumulative irradiation energy at the overlapping portion is approximately the same as E1, then irradiation with E1 alone is sufficient.) In this case, the irradiation position I' where E2 peaks is shifted in phase so that it is i+SP / 2.

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

[0045] In Figure 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 half the spot diameter D. The cumulative irradiation energy at point b by E1 is greater than E1 because of overlap, and similarly, the cumulative irradiation energy at point b by E2 is greater than E2.

[0046] The cumulative irradiation energy due to irradiation of E1 and E2 is then averaged so that it is equal to the sum of E1+E2 at point a, which is the non-overlapping portion, and point b, which is the overlapping portion. As in Figure 6, the cumulative irradiation energy at the irradiated point 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: A method for modifying the surface of a silicon wafer by irradiating the surface with a nanosecond pulse laser after alkaline etching so that the cumulative irradiation energy is below 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 location includes irradiating the irradiation location in accordance with a curvature of the irradiation location.

3. 3. The surface modification method of 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 energy density, scan pitch, and number of irradiations.

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

5. The surface modification method of a silicon wafer according to claim 3, characterized in that the irradiation area having a curvature is divided into sections, the angle of incidence is changed, the irradiation surface of each section is irradiated perpendicularly, and the scan pitch is increased.

6. When the irradiation location is a curved surface, the ratio of the irradiation energy on the curved surface to the irradiation 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 6. The method for modifying the surface of a silicon wafer according to claim 3 or 5, wherein:

7. The surface modification method of a silicon wafer according to claim 6, wherein 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 second irradiation energy E2 is at its peak, so that the cumulative irradiation energy at the irradiated point is equal to or less than a predetermined threshold.

8. 7. The surface modification method of claim 6, wherein 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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