Annealing method of silicon single crystal, silicon single crystal wafer prepared by annealing method and application of silicon single crystal wafer

Abstract

The invention relates to the technical field of laser processing, and particularly discloses an annealing method of a silicon single crystal, a monocrystalline silicon wafer prepared by the annealing method and application of the monocrystalline silicon wafer. The annealing method of the silicon single crystal provided by the invention comprises the following steps: carrying out annealing treatment on a silicon single crystal workpiece to be annealed under near-infrared femtosecond laser with the frequency of 10MHz-40MHz, wherein the annealing treatment controls the annealing time of a target area at a microsecond scale. The invention provides an annealing treatment method which is suitable for monocrystalline silicon and has the advantages of high instantaneous temperature, short action time and low thermal budget by utilizing the thermal accumulation effect of near-infrared laser, accurately controlling the annealing time at microsecond magnitude, combining with an optical device and jointly controlling equipment such as a scanning galvanometer and a three-dimensional displacement platform through a computer. The monocrystalline silicon wafer prepared by the annealing method can effectively eliminate stress and lattice defects, has excellent optical uniformity, and can be applied to the fields of semiconductor devices or optical elements and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of laser processing, and in particular to a method for annealing a silicon single crystal, a single crystal silicon wafer prepared therefrom, and applications thereof. Background Art

[0002] The process of pulling silicon single crystals is a complex and precise process. At present, it is mainly achieved by the CZ method or the FZ method, among which the CZ method is more widely used. However, due to the changes in thermal field, temperature gradient and solid-liquid interface shape, silicon single crystals produced by the CZ method will cause stress differences in different positions inside the single crystal (such as the head, tail and internal edge). In the isodiameter growth stage of pulling single crystals, as the height of the crystal increases, its side heat dissipation area gradually expands. As the crystal height increases and the heat shield is insulated, the temperature around the crystal continues to decrease, which makes the longitudinal temperature gradient continue to increase, and the internal stress of the crystal gradually accumulates. Compared with the head, the stress accumulation at the growth tail is more significant, so the thermal stress also shows an upward trend with the increase of crystal height. In the process of pulling single crystals, there is a phenomenon of internal edge stress difference when the single crystal grows. In the early stage of crystal growth, it advances from the center to the edge in a step-by-step manner. At this time, the crystal volume is small and the heat dissipation effect is good, so the overall stress is at a low level. As the crystal continues to grow, the lateral temperature gradient continues to increase, and the thermal stress gradually increases, thereby forming a thermal stress ring. In addition, process parameters such as air pressure conditions, air flow rate, crystal cooling rate and pulling speed will all affect the stress distribution of the single crystal.

[0003] Although the traditional high-temperature annealing treatment of silicon single crystals can improve the surface state of single crystal silicon wafers and eliminate native surface defects, thus improving the overall quality of single crystal silicon (especially electrical performance) to a certain extent, the traditional high-temperature annealing process has disadvantages such as relatively low annealing temperature and long annealing time, which not only easily causes unnecessary impurity re-diffusion, but also may cause new stress problems, and the optical uniformity of annealed silicon single crystals still needs to be further improved. Summary of the invention

[0004] In view of the above technical problems, the present invention utilizes the thermal accumulation effect of near-infrared laser and accurately controls the annealing time in microseconds, combines multi-stage half-wave plates, quarter-wave plates, polarization beam splitters, lenses, reflectors and other optical devices, and jointly controls scanning galvanometers, three-dimensional displacement platforms and other equipment through computers, to provide a near-infrared laser annealing treatment method suitable for single crystal silicon with high instantaneous temperature, short action time and low thermal budget. The single crystal silicon wafer prepared based on the annealing method not only effectively eliminates stress and lattice defects, but also has excellent optical properties, which plays a positive role in promoting the development of modern integrated circuit semiconductor devices.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions: In a first aspect, the present invention provides a method for annealing a silicon single crystal, the method comprising the following steps: annealing a silicon single crystal workpiece to be annealed under a near-infrared femtosecond laser with a frequency of 10 MHz to 40 MHz to obtain an annealed silicon single crystal; Wherein, during the annealing process, the annealing time of the target area is controlled in microsecond scale.

[0006] Conventional rapid annealing (RTP) in the prior art uses a halogen lamp to focus light onto the wafer, quickly heats it to the temperature required for the process (200°C~1300°C), and introduces nitrogen to achieve rapid cooling. The temperature rise / fall rate is generally about 20°C / s~250°C / s. This annealing process may take several seconds to tens of seconds to complete. This annealing method not only cannot achieve differentiated processing based on different locations with stress differences in the silicon wafer (such as the head, tail or internal edge), but also cannot achieve precise control of the annealing time.

[0007] The pulse interval of a femtosecond laser is microseconds or even nanoseconds, that is, the heat generated by the material acted on by the preceding pulse has not completely dissipated, and the subsequent pulse has already acted on the material, resulting in heat accumulation of the laser pulse. The energy can be transferred to the lattice to achieve heat release through the collision between free electrons and silicon single crystal materials, and the time scale of this process is in the order of microseconds. The annealing method of silicon single crystal provided by the present invention can achieve the characteristics of instantaneous high temperature by optimizing a series of parameters of a femtosecond laser including frequency, and utilizing the thermal accumulation effect of a near-infrared laser. According to the specific characteristics of the target area, the time for local instantaneous heating of the surface of the target area is precisely and flexibly controlled, and then the target area is rapidly cooled to room temperature. Localized heating, with very precise control of position and intensity, can achieve the selective elimination of areas with greater stress and the full elimination of lattice defects, thereby improving the overall quality of single crystal silicon wafers, including optical properties.

[0008] Preferably, the present invention controls the annealing time of the target area to 5 μs to 50 μs by adjusting the scanning speed of the scanning galvanometer.

[0009] In the present invention, the far-field focused Gaussian spot of the scanning galvanometer irradiates the single crystal silicon. When the polarization state and the incident pulse energy are fixed, the annealing time is precisely controlled by adjusting the number of pulses deposited in each step within the near-field scanning step, that is, the exposure time per unit interval.

[0010] Preferably, the near-infrared femtosecond laser has an emission pulse width of 185fs to 195fs and a wavelength of 1540nm to 1560nm. In the present invention, an emission pulse width of 190fs and a wavelength of 1550nm are used as an example for explanation. When the emission pulse width is 185fs or 195fs and the wavelength is 1540nm or 1560nm, a comparable technical effect can also be achieved.

[0011] Preferably, after being emitted by a femtosecond laser, the near-infrared femtosecond laser passes through a first half-wave plate and a polarization beam splitter, enters a first convex lens and a second convex lens for beam expansion, and then is reflected by a first reflector into 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 by the scanning galvanometer passes through a third convex lens, a fourth convex lens, a second reflector and an objective lens, and is focused on the annealed silicon single crystal element.

[0012] Rotating the optical axis of the first half-wave plate can change the angle of linear polarized light, and the polarization beam splitter is a depolarizer. When natural light is incident, linear polarized light with a high extinction ratio can be obtained at the transmission end of the PBS, and only linear polarized light in one direction is allowed to pass. Therefore, by controlling the half-wave plate and the polarization beam splitter, the processing power of the laser can be controlled to achieve high-temperature annealing.

[0013] Preferably, during the annealing treatment, the optical axis of the second half-wave plate is rotated, and the linear polarization of the laser is adjusted to form an angle of 45° with the optical axis of the quarter-wave plate, so that the polarization state of the laser is changed to circular polarization.

[0014] If the surface of single crystal silicon is irradiated with linear polarized light, the anisotropy of the light field is not uniform, which may cause the single crystal silicon to be circularly destroyed. When the polarization is perpendicular to the scanning direction, the near field will be enhanced along the scanning direction; when the polarization is parallel to the scanning direction, the near field will be destructed along the scanning direction. If the surface of single crystal silicon is irradiated with circular polarized light, the uniformity of the single crystal silicon wafer will be better after annealing due to the uniform isotropy of the light field.

[0015] Preferably, during the annealing treatment, the exposure time of the near-field scanning unit interval in the scanning galvanometer is adjusted to be 5 μs to 20 μs.

[0016] Further preferably, the three-dimensional displacement stage includes a Z-axis piezoelectric positioning platform, a three-axis stepping motor, a two-axis adjustment platform and a sample stage; The light source emitted by the illumination light source disposed above the sample stage can be irradiated onto the surface of the silicon single crystal workpiece to be annealed through the third reflector and the second reflector; The reflected light irradiated to the surface of the silicon single crystal workpiece to be annealed is reflected by the second reflector, the third reflector, and the fourth reflector to the fifth convex lens and then focused to the camera; The femtosecond laser, three-dimensional displacement platform, scanning galvanometer and camera are electrically connected to the computer respectively.

[0017] The computer in the present invention is used for: (1) controlling a femtosecond laser to emit a femtosecond pulse laser with specific parameters; (2) controlling a scanning galvanometer to precisely adjust the annealing time of different target areas on the surface of a silicon single crystal; (3) controlling the movement of a three-dimensional processing platform; and (4) observing the number and distribution of light beams through CCD images.

[0018] The annealing method provided by the present invention regulates the laser power by selecting appropriate laser repetition rate and laser wavelength, thereby regulating the laser annealing depth; controls the laser exposure time and pulse redundancy by regulating the scanning speed of the scanning galvanometer, thereby realizing precise control of the laser annealing time; and utilizes circularly polarized femtosecond laser to improve the uniformity of the thermal annealing temperature during the annealing process, thereby realizing improved uniformity of the light field. A three-dimensional displacement platform and a scanning galvanometer are combined to realize large-scale laser thermal annealing. The annealing method provided by the present invention can selectively perform localized heating to eliminate areas with greater stress, thereby fully eliminating lattice defects. Circularly polarized femtosecond laser is used to improve the uniformity of the light field, further improving the uniformity of the thermal annealing temperature. Large-area thermal annealing of the single-crystal silicon workpiece to be annealed is performed by combining a three-dimensional displacement platform and a field mirror, which can effectively reduce the cost of thermal annealing and is more suitable for industrial production.

[0019] In a second aspect, the present invention provides a single crystal silicon wafer produced by the annealing method of a silicon single crystal provided in the first aspect.

[0020] Compared with the single crystal silicon wafer prepared by the traditional annealing method, the single crystal silicon wafer provided by the present invention not only has significantly reduced stress, but also has better optical properties.

[0021] Preferably, the optical uniformity of the single crystal silicon wafer provided by the present invention at a wavelength of 3 μm is 0.23-0.34, the optical uniformity at a wavelength of 4 μm is 0.19-0.29, and the optical uniformity at a wavelength of 5 μm is 0.19-0.28.

[0022] On the third aspect, in view of the excellent optical properties and smaller stress of the single crystal silicon wafer provided by the second aspect of the present invention, it can be applied to fields such as semiconductor devices or optical elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0024] Figure 1 A structural diagram of a near-infrared femtosecond laser annealing device in the silicon single crystal annealing method provided by the present invention; In the figure: HWP1-first half-wave plate, PBS-polarization 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 light source, R3-third reflector, L5-fifth convex lens, CCD-camera, R4-fourth reflector, PC-computer; Figure 2 Schematic diagram of the principle of the annealing method of silicon single crystal in Example 1 of the present invention, wherein: Figure 2 a) is a schematic diagram of the beam waist radius of the light spot. Figure 2 b) is a simulation diagram of the thermal field distribution of a single-crystal silicon wafer at the focus of laser processing. Figure 2 c) is 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; Figure 3 It is the stress distribution diagram of the single crystal silicon wafer before annealing and silicon wafers I to II in the effect example 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 DESCRIPTION

[0025] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0026] The annealing method of silicon single crystal provided by the present invention can be performed using a near-infrared femtosecond laser annealing device, wherein the structure of the near-infrared femtosecond laser annealing device is shown in FIG. Figure 1 shown. Figure 1 Middle: HWP1-first half-wave plate, PBS-polarization 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 light source, R3-third reflector, L5-fifth convex lens, CCD-camera, R4-fourth reflector, PC-computer.

[0027] The quarter-wave plate in the present invention is an electrically controlled quarter-wave plate; the half-wave plate is an electrically controlled half-wave plate, namely a first electrically controlled half-wave plate and a second electrically controlled half-wave plate, both of which are driven by a stepper motor; the three-dimensional displacement platform is composed of four parts: a Z-axis piezoelectric positioning platform, a three-axis stepper motor, a two-axis adjustment platform and a sample stage.

[0028] In the present invention, a femtosecond laser with a wavelength of 1550nm and a pulse width of 190fs is used as an example for explanation, and its beam diameter is about 5mm; the focal length of the first convex lens L1 is 10cm, the focal lengths of the second convex lens and the second convex lens L2 are 20cm, the focal length of the third convex lens L3 is 25cm, the focal length of the fourth convex lens L4 is 25cm, and the focal length of the fifth convex lens L4 is 10cm; the first convex lens L1 and the second convex lens L2 form a beam expansion system, which expands the original femtosecond laser beam diameter by 2 times to 1cm, Matching the entrance pupil diameter of the scanning galvanometer GS, the third convex lens L3 is placed 25 cm away from the exit of the scanning galvanometer GS, and the fourth convex lens L4 is placed 25 cm away 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 stepping motor, a two-axis adjustment platform and a sample stage. While the illumination light source LED is transmitting light, the sample position can be electronically controlled by a computer PC and the Z-axis position of the sample can be precisely adjusted during the processing.

[0029] Example 1 This embodiment provides a method for annealing a silicon single crystal. Figure 1 The annealing is performed using the near-infrared femtosecond laser annealing device shown in FIG. 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 a single-crystal silicon wafer at the focus of laser processing. Figure 2 c) is 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. The laser scans along a predetermined path, and its energy distribution at the focus shows a typical Gaussian distribution pattern. At the focus, the size of the laser spot is precisely controlled to ensure a high concentration of energy. The annealing method includes the following steps: S1. Leveling of the sample stage The femtosecond laser is turned on, and the emitted femtosecond laser passes through the first half-wave plate HWP1 and the polarization beam splitter PBS, enters the first convex lens L1 and the second convex lens L2 for beam expansion, and the light spot is enlarged by 2 times, and is reflected by the first reflector R1 in turn to enter the second half-wave plate HWP2 and the quarter-wave plate QWP, and is incident on the entrance pupil of the scanning galvanometer GS, and is filtered by the 4f system composed of the third convex lens L3 and the fourth convex lens L4, and then is incident on the objective lens OL (NA=0.65, magnification × 40) through the second reflector R2, and then the light spot is focused by the objective lens OL and shines on the surface of the silicon single crystal workpiece to be annealed after the optical axis of the first electrically controlled half-wave plate HWP1 at the exit of the femtosecond laser is rotated, and then passes through the second reflector R2, and then is focused by the objective lens OL. The silicon single crystal workpiece used in this embodiment is a single crystal silicon wafer with a diameter of 300 mm and a thickness of 5 mm prepared by the Czochralski process.

[0030] The light source emitted by the illumination light source LED disposed above the sample stage passes through the third reflector R3 and the second reflector R2 in sequence and then irradiates the surface of the single crystal silicon wafer; the reflected light irradiated to 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), so that the sample stage leveling process can be monitored in real time in the computer.

[0031] Take the two mutually perpendicular reference sides of the sample as the X-axis and Y-axis, adjust the three-axis stepper motor to move between (0, 0) and (1cm, 0) through the computer PC, and then adjust the X-direction leveling knob until the size and position of the laser spot displayed in the camera (CCD) do not change during the movement of the laser between the two points, and the X-direction leveling is completed; similarly, adjust the three-axis stepper motor to scan and move between (0, 0) and (0, 1cm) through the computer PC, and adjust the Y-direction leveling knob until the laser does not change during movement, and the Y-direction leveling is completed. After the X-direction and Y-direction leveling, the sample stage is perpendicular to the focused laser beam, and the position of the sample stage is fixed.

[0032] S2. Annealing The wavelength of the femtosecond laser emitted by the femtosecond laser is set to 1550nm, the pulse width is 190fs, the frequency is 40MHz, and the power is 10W. The optical axis of the second half-wave plate HWP2 is rotated to change the laser polarization into circular polarization. The sample stage is moved, and the Gaussian spot is focused in the far field at the preset position of the single crystal silicon wafer to be annealed after the scanning galvanometer GS. When the laser pulse is not higher than the damage threshold of the single crystal silicon, the annealing time is controlled by adjusting the number of pulses deposited in each step within the near-field scanning step of the scanning galvanometer GS (i.e., the exposure time per unit interval). The computer PC controls the scanning galvanometer GS and the three-dimensional displacement platform. Combined with the analysis of the camera (CCD) imaging, the movement of the focused spot is controlled to the center and edge of the head and tail of the single crystal silicon, and the line scanning is performed to implement a large-scale high-temperature rapid thermal annealing of the single crystal silicon to be annealed. After annealing, a single crystal silicon wafer is obtained, which is recorded as silicon wafer I.

[0033] The spot radius after focusing is 1.45 μm. The near-field scanning coordinate point spacing is set to 100 nm, the execution exposure time is 5 μs, and the actual exposure number of each coordinate point is 100.

[0034] Example 2 This embodiment provides a method for annealing a silicon single crystal. The annealing method is basically the same as that of Embodiment 1, except that the parameters of the femtosecond laser emitted by the femtosecond laser are different and the near-field scanning parameter settings are different. The remaining steps and parameters are the same.

[0035] The specific parameters of the femtosecond laser in this embodiment are as follows: wavelength 1550nm, pulse width 190fs, frequency 10MHz, power 2.5W. The near-field scanning setting parameters are as follows: the coordinate point spacing is set to 100nm, the execution exposure time is 20μs, and the actual exposure number of each coordinate point is 100.

[0036] The single crystal silicon wafer obtained after annealing in this embodiment is denoted as silicon wafer II.

[0037] Example 3 This embodiment provides a method for annealing a silicon single crystal. The annealing method is basically the same as that of Embodiment 1, except that the parameters of the femtosecond laser emitted by the femtosecond laser are different and the near-field scanning parameter settings are different. The remaining steps and parameters are the same.

[0038] The specific parameters of the femtosecond laser in this embodiment are as follows: wavelength 1550nm, pulse width 190fs, frequency 30MHz, power 7.5W. The near-field scanning setting parameters are as follows: the coordinate point spacing is set to 100nm, the execution exposure time is 8μs, and the actual exposure number of each coordinate point is 100.

[0039] The single crystal silicon wafer obtained after annealing in this embodiment is denoted as silicon wafer III.

[0040] Comparative Example 1 This comparative example provides a method for annealing a silicon single crystal, which is basically the same as Example 1, except that the frequency of the femtosecond laser emitted by the femtosecond laser is replaced from 40 MHz to 45 MHz, and the exposure time of the near-field scanning is replaced from 5 μs to 3 μs, and the remaining steps and parameters are the same. The single crystal silicon wafer obtained after annealing in this comparative example is recorded as silicon wafer pair I.

[0041] Effect Example 1 The present invention investigates the optical uniformity of single crystal silicon wafers before annealing, silicon wafers I to III provided in the embodiments of the present invention, silicon wafer pair I, and silicon wafers obtained by conventional high temperature annealing, and the specific contents are as follows: Optical uniformity measurement steps: Measure the stress birefringence of single crystal silicon, and then use Fourier infrared spectroscopy to measure the transmittance of single crystal silicon at near-infrared wavelengths. Calculate the refractive index of single crystal silicon based on the transmittance of single crystal silicon and the Fresnel reflection formula. The ratio of stress birefringence to refractive index represents optical uniformity.

[0042] First, based on the stress birefringence measurement method, the light source obtains horizontal linear polarized light through a filter and a polarizer. The horizontal linear polarized light is passed through a quarter-wave plate at a 45° angle to the horizontal direction to obtain circularly polarized light. Since the sample has a birefringent structure, the circularly polarized light becomes elliptically polarized light after passing through the sample. The elliptically polarized light passes through two liquid crystal phase retarders, LCB and LCA, to phase modulate the incident light, and finally passes through the image analyzer. The image analyzer is used to measure the light intensity after passing through all devices. Using the four rotations of the liquid crystal phase retarder, the phase delay amount and the slow axis azimuth of the incident light are calculated according to the light field intensity obtained by the detector. The stress birefringence of single crystal silicon after laser annealing is measured using this method.

[0043] Then, the single-side transmittance of single-crystal silicon is measured in the range of 3μm~5μm using Fourier transform infrared spectroscopy. The transmittance of the entire single-crystal silicon block is deduced from the known single-side transmittance; the refractive index of single-crystal silicon in different bands is calculated based on the Fresnel reflection formula.

[0044] Finally, the calculated ratio of stress birefringence to refractive index represents the optical homogeneity of the corresponding material, and the lower the optical homogeneity, the better the optical homogeneity after annealing.

[0045] for <100> For silicon in the crystal direction, it is isotropic and has no birefringence. However, once defects and dislocations are formed at a certain position, this symmetry will be broken, causing the electron cloud between atoms to shift, which is manifested as stress birefringence on a macro scale. Usually, stress birefringence can be obtained by measuring the optical path difference between o-light and e-light per unit length of the aperture. By comparing the size of stress birefringence, the generation intensity of defects and dislocations can be quantified. The electron energy level of silicon single crystal in the mid-infrared band of 3μm~5μm corresponds to 0.24eV~0.4eV. The oxygen donors and oxygen precipitates generated during annealing are just at this energy level. The difference in the refractive index of single crystal silicon in different bands can be used as a standard for measuring the quality of the crystal. In summary, the ratio of stress birefringence to refractive index can be used as a concept to measure optical uniformity.

[0046] Among them, the conventional high-temperature annealing method selected in the present invention is: the single crystal silicon wafer to be annealed selected in Example 1 is placed in an annealing furnace, the temperature is raised to 800°C in 4 hours, and after maintaining the temperature at this temperature for 4 hours, the temperature is reduced to 200°C at a rate of 1°C / min, the power of the annealing furnace is turned off, the annealed single crystal silicon wafer is taken out, and it is naturally cooled to obtain a silicon wafer obtained by high-temperature annealing.

[0047] After calculation, the relevant results of the optical uniformity measurement process of the single crystal silicon wafer before annealing, silicon wafers I to III, silicon wafer pair I and the silicon wafer obtained by conventional high temperature annealing are shown in Table 1 below.

[0048] Table 1

[0049] The present invention compares the ability of different annealing methods to improve the optical uniformity of silicon wafers based on the optical uniformity value; wherein the smaller the optical uniformity value, the better the optical uniformity effect. As can be seen from Table 1, the optical uniformity performance of the silicon wafer obtained after annealing by the annealing method provided in Examples 1 to 3 is better. Comparative Example 1 uses a higher femtosecond laser frequency, and even if a lower exposure time is set, it cannot achieve the purpose of uniformly annealing the silicon wafer, resulting in poor optical uniformity. Due to the relatively large size of silicon wafers, conventional high-temperature annealing methods cannot differentiate the annealing time during the heating and cooling process due to the differences in the temperature change rates at different internal positions, resulting in the optical uniformity of the silicon wafer obtained after annealing being significantly inferior to that of Examples 1-3.

[0050] Effect Example 2 The present invention investigates the stress distribution of the single crystal silicon wafer before annealing and the silicon wafers I to II, and the specific contents are as follows: At room temperature, the stress birefringence of silicon wafers before and after annealing is tested, and the stress distribution is 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.

[0051] Depend on Figure 3 It can be seen that the stress distribution in the single crystal silicon wafer before annealing is ring-shaped, which may be caused by the longitudinal temperature difference and the lateral temperature difference in the equal diameter growth process and the shoulder release process of the Czochralski single crystal silicon. The stress rings of the silicon wafers I and II obtained after annealing by the annealing method provided by the present invention are weakened, and the stress birefringence is also reduced at the same time.

[0052] In summary, the optical uniformity of silicon wafers I to III obtained by the annealing method provided in Examples 1 to 3 at a wavelength of 3 μm is 0.23 to 0.34, the optical uniformity at a wavelength of 4 μm is 0.19 to 0.29, and the optical uniformity at a wavelength of 5 μm is 0.19 to 0.28, and its optical uniformity is significantly better than that of the single crystal silicon wafer before annealing, silicon wafer pair I, and the silicon wafer obtained by conventional high temperature annealing, and the stress is significantly reduced. In view of the excellent optical properties and smaller stress of the silicon wafer provided in the embodiment of the present invention, it can be used in the fields of preparing semiconductor devices or optical elements.

[0053] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent substitution or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for annealing a silicon single crystal, characterized in that: The annealing method comprises the following steps: annealing a silicon single crystal workpiece to be annealed under a near-infrared femtosecond laser with a frequency of 10 MHz to 40 MHz to obtain an annealed silicon single crystal; Wherein, during the annealing process, the annealing time of the target area is controlled in microsecond scale.

2. The method for annealing a silicon single crystal according to claim 1, wherein: The annealing time of the target area is controlled within 5μs~50μs by adjusting the scanning speed of the scanning galvanometer (GS).

3. The method for annealing a silicon single crystal according to claim 1, wherein: The near-infrared femtosecond laser has an emission pulse width of 185 fs to 195 fs and a wavelength of 1540 nm to 1560 nm.

4. The method for annealing a silicon single crystal according to claim 1, wherein: After being emitted by the femtosecond laser, the near-infrared femtosecond laser enters the first convex lens and the second convex lens for beam expansion through the first half-wave plate and the polarization beam splitter, and then is reflected by the first reflector into the second half-wave plate and the quarter-wave plate, and is incident on the scanning galvanometer; The three-dimensional displacement platform is adjusted by a computer so that the laser emitted by the scanning galvanometer passes through the third convex lens, the fourth convex lens, the second reflecting mirror and the objective lens and then focuses on the annealed silicon single crystal element.

5. The method for annealing a silicon single crystal according to claim 4, characterized in that: During the annealing process, the optical axis of the second half-wave plate is rotated to adjust the linear polarization of the laser to form an angle of 45° with the optical axis of the quarter-wave plate, so that the laser polarization is converted into circular polarization.

6. The near-infrared laser annealing method according to claim 4, characterized in that: During the annealing process, the exposure time of the near-field scanning unit interval in the scanning galvanometer is adjusted to be 5μs~20μs.

7. The near-infrared laser annealing method according to claim 4, characterized in that: The three-dimensional displacement stage includes a Z-axis piezoelectric positioning platform, a three-axis stepping motor, a two-axis adjustment platform and a sample stage; The light source emitted by the illumination light source disposed above the sample stage can be irradiated onto the surface of the silicon single crystal workpiece to be annealed through the third reflector and the second reflector; The reflected light irradiated to the surface of the silicon single crystal workpiece to be annealed is reflected by the second reflector, the third reflector, and the fourth reflector to the fifth convex lens and then focused to the camera; The femtosecond laser, three-dimensional displacement platform, scanning galvanometer and camera are electrically connected to the computer respectively.

8. A single crystal silicon wafer, characterized in that: The silicon single crystal annealing method is described in any one of claims 1 to 7.

9. The single crystal silicon wafer according to claim 8, characterized in that: Its optical uniformity at 3μm wavelength is 0.23~0.

34.

10. Use of the single crystal silicon wafer according to claim 8 or 9 in semiconductor devices or optical elements.

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