An annealing method for silicon single crystals, the prepared single-crystal silicon wafers, and their applications.

Abstract

This invention relates to the field of laser processing technology, specifically disclosing a method for annealing silicon single crystals, the prepared single-crystal silicon wafers, and their applications. The annealing method for silicon single crystals provided by this invention includes the following steps: annealing the silicon single-crystal workpiece to be annealed under a near-infrared femtosecond laser with a frequency of 10MHz to 40MHz, wherein the annealing time of the target area is controlled on the microsecond scale. This invention utilizes the thermal accumulation effect of near-infrared lasers and precisely controls the annealing time on the microsecond scale, combined with optical devices, and uses computer-controlled scanning galvanometers, three-dimensional displacement platforms, and other equipment to provide an annealing method suitable for single-crystal silicon with high instantaneous temperature, short processing time, and low thermal budget. The single-crystal silicon wafers prepared by this annealing method can effectively eliminate stress and lattice defects, and also have excellent optical uniformity, making them applicable to semiconductor devices or optical components.

Description

Technical Field

[0001] This invention relates to the field of laser processing technology, specifically to an annealing method for silicon single crystals, the single crystal silicon wafers prepared therefrom, and their applications. Background Technology

[0002] The silicon single crystal pulling process is a complex and precise technology, currently mainly achieved through the Czochralski (CZ) method or the zone melting (FZ) method, with the CZ method being more widely used. However, silicon single crystals produced using the CZ method experience stress differences at different locations within the single crystal (such as the head, tail, and internal edges) due to factors such as changes in the thermal field, temperature gradient, and solid-liquid interface shape. During the constant-diameter growth stage of single crystal pulling, as the crystal height increases, its lateral heat dissipation area gradually expands. Due to the increasing crystal height and thermal shielding, the temperature around the crystal continuously decreases, leading to a continuous increase in the longitudinal temperature gradient and a gradual accumulation of internal stress. Compared to the head, the stress accumulation at the tail of the growth is more significant, thus the thermal stress also shows an increasing trend with the increase in crystal height. In the single crystal pulling process, there is a phenomenon of internal edge stress differences during single crystal growth. In the early stages of crystal growth, it progresses in a step-like manner from the center to the edge. At this time, the crystal volume is small, the heat dissipation effect is good, and the overall stress is at a low level. As the crystal continues to grow, the lateral temperature gradient continuously increases, and the thermal stress also gradually increases, eventually forming a thermal stress ring. In addition, process parameters such as air pressure, airflow rate, crystal cooling rate, and pulling speed all affect the stress distribution of single crystals.

[0003] Although conventional high-temperature annealing can improve the overall quality (especially electrical properties) of single-crystal silicon wafers by improving their surface condition and eliminating native defects, traditional high-temperature annealing processes have drawbacks such as relatively low annealing temperatures and long annealing times. This not only easily leads to unnecessary impurity redispersibility but may also cause new stress problems. Furthermore, the optical uniformity of the annealed single-crystal silicon still needs further improvement. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention utilizes the thermal accumulation effect of near-infrared lasers and precisely controls the annealing time to the microsecond level. It combines multiple levels of half-wave plates, quarter-wave plates, polarizing beam splitters, lenses, and mirrors, and employs computer-controlled scanning mirrors and a three-dimensional displacement platform to provide a near-infrared laser annealing method suitable for single-crystal silicon. This method features high instantaneous temperature, short processing time, and low thermal budget. Single-crystal silicon wafers prepared using this annealing method not only effectively eliminate stress and lattice defects but also possess excellent optical properties, playing a positive role in promoting the development of modern integrated circuit semiconductor devices.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] In a first aspect, the present invention provides an annealing method for silicon single crystal, the annealing method comprising the following steps: annealing the silicon single crystal workpiece to be annealed under a near-infrared femtosecond laser with a frequency of 10MHz to 40MHz to obtain an annealed silicon single crystal;

[0007] During the annealing process, the annealing time for the target area is controlled on the microsecond scale.

[0008] Conventional rapid annealing (RTP) in existing technologies uses a halogen lamp to focus light onto the wafer, rapidly heating it to the required process temperature (200℃~1300℃), and then introducing nitrogen gas for rapid cooling. The heating / cooling rate is typically around 20℃ / s~250℃ / s, and this annealing process can take several seconds to tens of seconds to complete. This annealing method not only fails to provide differentiated treatment based on stress differences within the silicon wafer (such as the head, tail, or inner edge), but also cannot achieve precise control over the annealing time.

[0009] Femtosecond laser pulse intervals are in the microsecond or even nanosecond range, meaning that the heat generated by the preceding pulse on the material has not completely dissipated before the subsequent pulse acts on the material, leading to heat accumulation from the laser pulse. Heat release is achieved by transferring energy to the crystal lattice through collisions between free electrons and silicon single crystal materials, with the timescale of this process being on the microsecond scale. The silicon single crystal annealing method provided by this invention optimizes a series of femtosecond laser parameters, including frequency, and utilizes the thermal accumulation effect of near-infrared lasers to achieve instantaneous high temperatures. Based on the specific characteristics of the target area, the method precisely and flexibly controls the duration of localized instantaneous heating of the target area's surface, followed by rapid cooling to room temperature. This localized heating allows for highly precise control of position and intensity, enabling selective elimination of areas with high stress and thorough removal of lattice defects, thereby improving the overall quality of single-crystal silicon wafers, including optical properties.

[0010] Preferably, the present invention controls the annealing time of the target area to 5μs~50μs by adjusting the scanning speed of the scanning galvanometer.

[0011] In this invention, the far-field focusing Gaussian spot of the scanning galvanometer irradiates single-crystal silicon. When the polarization state and incident pulse energy are fixed, the annealing time can be precisely controlled by adjusting the number of pulses deposited in each step within the near-field scanning step, i.e., the exposure time per unit interval.

[0012] Preferably, the near-infrared femtosecond laser has an output pulse width of 185fs~195fs and a wavelength of 1540nm~1560nm. This invention uses an output pulse width of 190fs and a wavelength of 1550nm as an example for illustration; however, comparable technical effects can also be achieved with an output pulse width of 185fs or 195fs and a wavelength of 1540nm or 1560nm.

[0013] Preferably, the near-infrared femtosecond laser emitted by the femtosecond laser passes through a first half-wave plate and a polarizing beam splitter, then enters a first convex lens and a second convex lens for beam expansion. After being reflected by a first reflecting mirror, it enters a second half-wave plate and a quarter-wave plate, and is incident on a scanning galvanometer. The three-dimensional displacement platform is adjusted by a computer so that the laser emitted from the scanning galvanometer is focused onto the annealed silicon single crystal element after passing through a third convex lens, a fourth convex lens, a second reflecting mirror, and an objective lens.

[0014] Rotating the optical axis of the first half-wave plate changes the angle of linearly polarized light; the polarization beam splitter is a depolarizer. When natural light is incident, linearly polarized light with a high extinction ratio can be obtained at the transmission end of the PBS, allowing only linearly polarized light in one direction to pass through. Therefore, by controlling the half-wave plate and the polarization beam splitter, the processing power of the laser can be controlled, achieving high-temperature annealing.

[0015] Preferably, during the annealing process, the optical axis of the second half-wave plate is rotated, and the linear polarization of the laser is adjusted so that it forms a 45° angle with the optical axis of the quarter-wave plate, thereby changing the laser polarization state to circular polarization.

[0016] If linearly polarized light is used to irradiate the surface of a single-crystal silicon wafer, the anisotropic non-uniformity of the light field may cause circular damage to the wafer. When the polarization is perpendicular to the scanning direction, near-field enhancement occurs along the scanning direction; when the polarization is parallel to the scanning direction, near-field destructive occurs. If circularly polarized light is used to irradiate the surface of a single-crystal silicon wafer, the uniformity of the light field is better after annealing due to its isotropic uniformity.

[0017] Preferably, during the annealing process, the exposure time of the near-field scanning unit interval in the scanning galvanometer is adjusted to be 5μs~20μs.

[0018] More preferably, the three-dimensional displacement stage includes a Z-axis piezoelectric positioning platform, a three-axis stepper motor, a dual-axis adjustment platform, and a sample stage;

[0019] The light source emitted by the illumination source located above the sample stage can illuminate the surface of the silicon single crystal workpiece to be annealed through the third reflector and the second reflector.

[0020] The reflected light that is irradiated onto the surface of the silicon single crystal workpiece to be annealed is reflected by the second, third and fourth mirrors and then focused into the camera by the fifth convex lens.

[0021] The femtosecond laser, three-dimensional displacement platform, scanning galvanometer, and camera are all electrically connected to the computer.

[0022] The computer in this invention is used to: (1) control a femtosecond laser to emit femtosecond pulsed lasers with specific parameters; (2) control a scanning galvanometer to precisely adjust the annealing time of different target areas on the silicon single crystal surface; (3) control the movement of a three-dimensional processing platform; and (4) observe the number and distribution of beams through CCD images.

[0023] The annealing method provided by this invention controls the laser annealing depth by selecting appropriate laser repetition rate and wavelength to regulate laser power; it achieves precise control of laser annealing time by adjusting the scanning speed of the scanning galvanometer to control laser exposure time and pulse redundancy; and it utilizes circularly polarized femtosecond lasers during annealing to improve the uniformity of the thermal annealing temperature, thereby enhancing the uniformity of the optical field. A large-area laser thermal annealing is achieved by combining a three-dimensional displacement platform and a scanning galvanometer. The annealing method provided by this invention can selectively perform localized heating, eliminating areas with high stress and thus effectively eliminating lattice defects. The use of circularly polarized femtosecond lasers to improve the uniformity of the optical field further enhances the uniformity of the thermal annealing temperature. The combination of a three-dimensional displacement platform and a field mirror for large-area thermal annealing of the single-crystal silicon workpiece effectively reduces the cost of thermal annealing and is more suitable for industrial production.

[0024] In a second aspect, the present invention provides a single-crystal silicon wafer prepared by the annealing method for single-crystal silicon provided in the first aspect.

[0025] Compared with single-crystal silicon wafers prepared by traditional annealing methods, the single-crystal silicon wafers provided by this invention not only have significantly reduced stress, but also have superior optical properties.

[0026] Preferably, the single-crystal silicon wafer provided by the present invention has an optical uniformity of 0.23~0.34 at a wavelength of 3μm, an optical uniformity of 0.19~0.29 at a wavelength of 4μm, and an optical uniformity of 0.19~0.28 at a wavelength of 5μm.

[0027] Thirdly, given the excellent optical properties and lower stress of the single-crystal silicon wafer provided in the second aspect of the present invention, it can be applied to fields such as semiconductor devices or optical components. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 A structural diagram of the near-infrared femtosecond laser annealing device in the silicon single crystal annealing method provided by the present invention;

[0030] In the diagram: HWP1 - First half-wave plate, PBS - Polarizing beam splitter, L1 - First convex lens, L2 - Second convex lens, R1 - First reflector, HWP2 - Second half-wave plate, QWP - Quarter-wave plate, GS - Scanning galvanometer, L3 - Third convex lens, L4 - Fourth convex lens, R2 - Second reflector, OL - High-power objective lens, LED - Illumination source, R3 - Third reflector, L5 - Fifth convex lens, CCD - Camera, R4 - Fourth reflector, PC - Computer;

[0031] Figure 2 This is a schematic diagram illustrating the principle of the silicon single crystal annealing method in Embodiment 1 of the present invention, wherein: Figure 2 a) is a schematic diagram of the beam waist radius. Figure 2 b) is a simulation diagram of the thermal field distribution of the single-crystal silicon wafer at the laser processing focal point. Figure 2 c) A two-dimensional simulation diagram of the energy distribution of the laser annealing spot; Figure 2 d) is a three-dimensional simulation diagram of the energy distribution of the laser annealing spot;

[0032] Figure 3 This is a stress distribution diagram of the single-crystal silicon wafer and silicon wafers I-II before annealing in the example of the effect of the present invention; wherein, Figure 3 a) Represents a single-crystal silicon wafer before annealing. Figure 3 b) represents silicon wafer I. Figure 3 c) represents silicon wafer II. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0034] The silicon single crystal annealing method provided by this invention can be performed using a near-infrared femtosecond laser annealing device, the structure of which is shown in the figure below. Figure 1 As shown. Figure 1 In the middle: HWP1 - First half-wave plate, PBS - Polarizing beam splitter, L1 - First convex lens, L2 - Second convex lens, R1 - First reflector, HWP2 - Second half-wave plate, QWP - Quarter-wave plate, GS - Scanning galvanometer, L3 - Third convex lens, L4 - Fourth convex lens, R2 - Second reflector, OL - High-magnification objective lens, LED - Illumination source, R3 - Third reflector, L5 - Fifth convex lens, CCD - Camera, R4 - Fourth reflector, PC - Computer.

[0035] In this invention, the quarter-wave plate is an electrically controlled quarter-wave plate; the half-wave plate is an electrically controlled half-wave plate, namely the first electrically controlled half-wave plate and the second electrically controlled half-wave plate, both of which are driven by stepper motors; the three-dimensional displacement platform consists of four parts: a Z-axis piezoelectric positioning platform, a three-axis stepper motor, a dual-axis adjustment platform, and a sample stage.

[0036] This invention uses a femtosecond laser with a wavelength of 1550 nm and a pulse width of 190 fs as an example, with a beam diameter of approximately 5 mm. The first convex lens L1 has a focal length of 10 cm, the second convex lens L2 has a focal length of 20 cm, the third convex lens L3 has a focal length of 25 cm, the fourth convex lens L4 has a focal length of 25 cm, and the fifth convex lens L4 has a focal length of 10 cm. The first convex lens L1 and the second convex lens L2 form a beam amplification system, increasing the original femtosecond laser beam diameter by a factor of 2 to 1 cm. Matching the entrance pupil diameter of the scanning galvanometer GS, the third convex lens L3 is placed 25cm from the exit of the scanning galvanometer GS, and the fourth convex lens L4 is placed 25cm from the surface of the displacement platform. The third convex lens L3 and the fourth convex lens L4 form a 4f system for imaging and filtering. The three-dimensional displacement stage consists of four parts: a Z-axis piezoelectric positioning platform, a three-axis stepper motor, a dual-axis adjustment platform, and a sample stage. While the illumination source LED is transmitting light, the sample position can be electrically controlled by a computer PC, and the Z-axis position of the sample can be precisely adjusted during the processing.

[0037] Example 1

[0038] This embodiment provides an annealing method for silicon single crystals. The method used in this embodiment is as follows: Figure 1 The near-infrared femtosecond laser annealing apparatus shown is used for annealing. A schematic diagram of the principle of this annealing method is as follows: Figure 2 As shown, where: Figure 2 a) is a schematic diagram of the beam waist radius. Figure 2 b) is a simulation diagram of the thermal field distribution of the single-crystal silicon wafer at the laser processing focal point. Figure 2 c) A two-dimensional simulation diagram of the energy distribution of the laser annealing spot; Figure 2 d) A three-dimensional simulation of the energy distribution of the laser annealing spot. The laser scans along a predetermined path, and its energy distribution exhibits a typical Gaussian distribution pattern at the focal point. At the focal point, the energy is highly concentrated by precisely controlling the size of the laser spot. The annealing method includes the following steps:

[0039] S1. Leveling of the sample stage

[0040] The femtosecond laser is turned on, and the emitted femtosecond laser beam passes through the first half-wave plate HWP1 and the polarizing beam splitter PBS, then enters the first convex lens L1 and the second convex lens L2 for beam expansion, doubling the beam size. It is then reflected sequentially by the first reflecting mirror R1 and enters the second half-wave plate HWP2 and the quarter-wave plate QWP. After entering the entrance pupil of the scanning galvanometer GS, it is filtered by a 4f system consisting of the third convex lens L3 and the fourth convex lens L4, and then enters the objective lens OL (NA = 0.65, magnification × 40) through the second reflecting mirror R2. After rotating the optical axis of the first electrically controlled half-wave plate HWP1 at the femtosecond laser exit, the beam passes through the second reflecting mirror R2 and is focused by the objective lens OL, illuminating the surface of the silicon single crystal workpiece to be annealed. In this embodiment, the silicon single crystal workpiece used is a single crystal silicon wafer with a diameter of 300 mm and a thickness of 5 mm, prepared by the Czochralski method.

[0041] The light source LED located above the sample stage emits light that passes through the third reflector R3 and the second reflector R2 in sequence before illuminating the surface of the single-crystal silicon wafer. The reflected light illuminating the surface of the single-crystal silicon wafer is reflected by the second reflector R2, the third reflector R3, and the fourth reflector R4 to the fifth convex lens L5 and then focused into the camera (CCD), thereby enabling real-time monitoring of the sample stage leveling process in the computer.

[0042] Using the two mutually perpendicular reference edges of the sample as the X and Y axes, the three-axis stepper motor is moved between points (0, 0) and (1cm, 0) using a computer PC. The X-axis leveling knob is then adjusted until the size and position of the laser spot displayed in the camera (CCD) remain unchanged during the laser's movement between these two points; this completes the X-axis leveling. Similarly, the three-axis stepper motor is moved between points (0, 0) and (0, 1cm) using a computer PC, and the Y-axis leveling knob is adjusted until the laser's position remains unchanged during movement; this completes the Y-axis leveling. After X and Y-axis leveling, the sample stage is perpendicular to the focused laser beam; the position of the sample stage is then fixed.

[0043] S2. Annealing treatment

[0044] The femtosecond laser emitted by the femtosecond laser was set to a wavelength of 1550 nm, a pulse width of 190 fs, a frequency of 40 MHz, and a power of 10 W. The optical axis of the second half-wave plate HWP2 was rotated to change the laser polarization to circular polarization. The sample stage was moved, and the Gaussian spot was focused in the far field by the scanning galvanometer GS onto the preset position of the single-crystal silicon wafer to be annealed. Under irradiation conditions where the laser pulse did not exceed the single-crystal silicon damage threshold, the annealing time was controlled by adjusting the number of pulses deposited per step (i.e., the exposure time per unit interval) within the near-field scanning step of the scanning galvanometer GS. The computer PC controlled the scanning galvanometer GS and the three-dimensional displacement platform, and combined with the analysis of the camera (CCD) imaging, controlled the movement of the focused spot to the center and edge of the head and tail of the single-crystal silicon, performing a large-scale high-temperature rapid thermal annealing of the single-crystal silicon. After annealing, a single-crystal silicon wafer was obtained, denoted as silicon wafer I.

[0045] The focused spot radius is 1.45 μm. The near-field scanning coordinate point spacing is set to 100 nm, the exposure time is 5 μs, and the actual exposure count for each coordinate point is 100.

[0046] Example 2

[0047] This embodiment provides an annealing method for silicon single crystals, which is basically the same as that in Embodiment 1. The only difference is that the parameters of the femtosecond laser emitted by the femtosecond laser are different and the near-field scanning parameter settings are different. All other steps and parameters are the same.

[0048] In this embodiment, the specific parameters of the femtosecond laser are as follows: wavelength 1550nm, pulse width 190fs, frequency 10MHz, and power 2.5W. The near-field scanning settings are as follows: coordinate point spacing 100nm, exposure time 20μs, and 100 actual exposures per coordinate point.

[0049] The single-crystal silicon wafer obtained after annealing in this embodiment is denoted as silicon wafer II.

[0050] Example 3

[0051] This embodiment provides an annealing method for silicon single crystals, which is basically the same as that in Embodiment 1. The only difference is that the parameters of the femtosecond laser emitted by the femtosecond laser are different and the near-field scanning parameter settings are different. All other steps and parameters are the same.

[0052] In this embodiment, the specific parameters of the femtosecond laser are as follows: wavelength 1550nm, pulse width 190fs, frequency 30MHz, and power 7.5W. The near-field scanning settings are as follows: coordinate point spacing 100nm, exposure time 8μs, and 100 actual exposures per coordinate point.

[0053] The single-crystal silicon wafer obtained after annealing in this embodiment is denoted as silicon wafer III.

[0054] Comparative Example 1

[0055] This comparative example provides an annealing method for silicon single crystals, which is basically the same as that in Example 1, except that the frequency of the femtosecond laser emitted by the femtosecond laser is replaced by 45MHz instead of 40MHz, and the exposure time for near-field scanning is replaced by 3μs instead of 5μs. All other steps and parameters are the same. The single-crystal silicon wafer obtained after annealing in this comparative example is denoted as silicon wafer pair I.

[0056] Example 1

[0057] This invention investigated the optical uniformity of single-crystal silicon wafers before annealing, silicon wafers I to III provided in the embodiments of this invention, silicon wafer pair I, and silicon wafers obtained by conventional high-temperature annealing. The specific details are as follows:

[0058] Optical uniformity determination procedure: The stress birefringence of monocrystalline silicon is measured, followed by the transmittance of monocrystalline silicon at near-infrared wavelengths using Fourier transform infrared spectroscopy. The refractive index of monocrystalline silicon is calculated based on its transmittance and Fresnel reflection formula. Optical uniformity is represented by the ratio of stress birefringence to refractive index.

[0059] First, based on the stress birefringence measurement method, the light source is passed through a filter and a polarizer to obtain horizontally linearly polarized light. This horizontally linearly polarized light is then passed through a quarter-wave plate at a 45° angle to the horizontal to obtain circularly polarized light. Due to the birefringent structure of the sample, the circularly polarized light becomes elliptically polarized light after passing through the sample. The elliptically polarized light passes through two liquid crystal phase retardation plates (LCB and LCA) to modulate the phase of the incident light, and finally passes through an image analyzer. The image analyzer measures the light intensity after passing through all the devices. Using the four rotations of the liquid crystal phase retardation plates and the light field intensity obtained from the detector, the phase retardation and slow axis azimuth angle are calculated for the incident light. This method is then used to measure the stress birefringence of single-crystal silicon after laser annealing.

[0060] Then, the single-sided transmittance of monocrystalline silicon was measured in the range of 3μm to 5μm using Fourier transform infrared spectroscopy. The transmittance of the entire monocrystalline silicon mass was calculated from the known single-sided transmittance; and the refractive index of monocrystalline silicon in different wavelength bands was calculated according to the Fresnel reflection formula.

[0061] Ultimately, the ratio of the calculated stress birefringence to the refractive index represents the optical homogeneity of the corresponding material; the lower the optical homogeneity, the better the optical homogeneity after annealing.

[0062] for <100> For silicon with a uniform crystal orientation, it is isotropic and has no birefringence. However, once a defect or dislocation forms at a certain location, this symmetry is broken, causing a displacement of the electron cloud between atoms, which macroscopically manifests as stress birefringence. Stress birefringence is usually obtained by measuring the optical path difference between the o-ray and e-ray per unit length of the aperture. By comparing the magnitude of stress birefringence, the intensity of defect and dislocation formation can be quantified. In the mid-infrared band (3μm~5μm), the electronic energy levels of silicon single crystals are 0.24eV~0.4eV. The oxygen donors and oxygen precipitates formed during annealing are located at these energy levels. The difference in refractive index of single-crystal silicon at different wavelengths can be used as a standard to measure the quality within the crystal. In summary, the ratio of stress birefringence to refractive index can be used as a concept to measure optical homogeneity.

[0063] The conventional high-temperature annealing method selected in this invention is as follows: the single-crystal silicon wafer to be annealed selected in Example 1 is placed in an annealing furnace, heated to 800°C for 4 hours, kept at this temperature for 4 hours, then cooled to 200°C at a cooling rate of 1°C / min, the power supply to the annealing furnace is turned off, the annealed single-crystal silicon wafer is taken out and allowed to cool naturally, which is the silicon wafer obtained by high-temperature annealing.

[0064] The results of the optical uniformity measurement of single-crystal silicon wafers before annealing, silicon wafers I~III, silicon wafer pair I, and silicon wafers obtained by conventional high-temperature annealing are shown in Table 1 below.

[0065] Table 1

[0066]

[0067] This invention compares the ability of different annealing methods to improve the optical uniformity of silicon wafers based on optical uniformity values; a smaller optical uniformity value indicates better optical uniformity. Table 1 shows that the silicon wafers annealed using the methods provided in Examples 1-3 exhibit superior optical uniformity. Comparative Example 1, employing a higher femtosecond laser frequency, even with a lower exposure time, could not achieve uniform annealing of the silicon wafer, resulting in poor optical uniformity. Conventional high-temperature annealing methods, due to the relatively large size of the silicon wafer, cannot differentiate the annealing time during the heating and cooling process because of the varying rates of temperature change at different internal locations, leading to significantly inferior optical uniformity of the annealed silicon wafers compared to Examples 1-3.

[0068] Example 2

[0069] This invention investigates the stress distribution of single-crystal silicon wafers and silicon wafers of types I and II before annealing. The specific details are as follows:

[0070] Stress birefringence of silicon wafers before and after annealing was tested at room temperature, and the stress distribution was as follows: Figure 3 As shown. Among them, Figure 3 a) Represents a single-crystal silicon wafer before annealing. Figure 3 b) represents silicon wafer I. Figure 3 c) represents silicon wafer II.

[0071] Depend on Figure 3 It is known that the stress distribution in the single-crystal silicon wafer before annealing exhibits a ring-like pattern, which may be due to the longitudinal and lateral temperature differences existing during the constant-diameter growth and shoulder formation processes of the Czochralski method. After annealing using the method provided in this invention, the stress rings in silicon wafers I and II are weakened, and the stress birefringence is also reduced simultaneously.

[0072] In summary, the silicon wafers I-III obtained by the annealing methods provided in Examples 1-3 exhibit optical uniformity of 0.23-0.34 at 3 μm wavelength, 0.19-0.29 at 4 μm wavelength, and 0.19-0.28 at 5 μm wavelength. Their optical uniformity is significantly superior to that of the unannealed single-crystal silicon wafer, silicon wafer pair I, and silicon wafers obtained by conventional high-temperature annealing, and the stress is significantly reduced. Given the excellent optical properties and lower stress of the silicon wafers provided in these embodiments, they can be applied to the fabrication of semiconductor devices or optical components.

[0073] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An annealing method for silicon single crystals, characterized in that, The annealing method includes the following steps: annealing the silicon single crystal workpiece to be annealed under a near-infrared femtosecond laser with a frequency of 10MHz~40MHz to obtain the annealed silicon single crystal. During the annealing process, the annealing time for the target area is controlled on the microsecond scale. The annealing time of the target area is controlled between 5 μs and 50 μs by adjusting the scanning speed of the scanning galvanometer (GS). The near-infrared femtosecond laser has an output pulse width of 185fs~195fs and a wavelength of 1540nm~1560nm; The near-infrared femtosecond laser emitted by the femtosecond laser passes through the first half-wave plate and polarization beam splitter, then enters the first and second convex lenses for beam expansion. After being reflected by the first mirror, it enters the second half-wave plate and quarter-wave plate, and is incident on the scanning galvanometer. The three-dimensional displacement platform is adjusted by the computer so that the laser emitted from the scanning galvanometer is focused onto the annealed silicon single crystal element after passing through the third and fourth convex lenses, the second mirror, and the objective lens. During the annealing process, the optical axis of the second half-wave plate is rotated, and the linear polarization of the laser is adjusted to form a 45° angle with the optical axis of the quarter-wave plate, so that the laser polarization becomes circular polarization.

2. The annealing method for silicon single crystal as described in claim 1, characterized in that, During the annealing process, the exposure time of the near-field scanning unit interval in the scanning galvanometer is adjusted to 5μs~20μs.

3. The annealing method for silicon single crystal as described in claim 1, characterized in that, The three-dimensional displacement platform includes a Z-axis piezoelectric positioning platform, a three-axis stepper motor, a dual-axis adjustment platform, and a sample stage; The light source emitted by the illumination source located above the sample stage can illuminate the surface of the silicon single crystal workpiece to be annealed through the third reflector and the second reflector. The reflected light that is irradiated onto the surface of the silicon single crystal workpiece to be annealed is reflected by the second, third and fourth mirrors and then focused into the camera by the fifth convex lens. The femtosecond laser, three-dimensional displacement platform, scanning galvanometer, and camera are all electrically connected to the computer.

4. A single-crystal silicon wafer, characterized in that, It is prepared by the annealing method of silicon single crystal according to any one of claims 1 to 3.

5. The single-crystal silicon wafer as described in claim 4, characterized in that, Its optical uniformity at a wavelength of 3μm is 0.23~0.

34.

6. The application of the single-crystal silicon wafer as described in claim 4 or 5 in semiconductor devices or optical components.

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