CN108831827B - A device for thermally assisted femtosecond laser annealing of amorphous silicon - Google Patents

Abstract

A device for annealing amorphous silicon by heat-assisted femtosecond laser comprises a femtosecond laser amplifier, a laser energy regulator, a convergent lens, a reflecting plane mirror and a three-dimensional displacement object stage, wherein the light energy regulator, the convergent lens and the reflecting plane mirror are sequentially arranged at the output end of the femtosecond laser amplifier; the three-dimensional displacement object stage is arranged below the reflecting plane mirror, and an electric heating stage is arranged on the three-dimensional displacement object stage; placing amorphous silicon to be annealed on a three-dimensional displacement objective table, enabling the amorphous silicon to be located in front of a geometric focal plane of a convergent lens, changing laser components by adjusting a laser energy adjuster to adjust laser energy density, controlling the temperature of the amorphous silicon by an electrothermal table, and controlling the scanning speed and interval of laser on the amorphous silicon by the operation of the three-dimensional displacement objective table. The device heats the amorphous silicon in femtosecond laser scanning, thereby accelerating the laser annealing process and realizing the low-temperature phase change process.

Description

A device for thermally assisted femtosecond laser annealing of amorphous silicon

technical field

The invention belongs to the field of phase change transformation of amorphous silicon thin films, and particularly relates to a device and method for low-temperature annealing of amorphous silicon thin films by thermally assisted femtosecond lasers.

Background technique

Silicon has the existence of monocrystalline silicon, polycrystalline silicon and amorphous silicon (amorphous silicon). It has the advantages of abundant element reserves and stable chemical properties. It is widely used in precision detection and sensing, solar energy conversion and integrated circuits. The carrier mobility of intrinsic silicon is poor. By doping impurity elements such as boron, phosphorus, arsenic, and gold, the number of free electrons can be increased to increase its conductivity, thus broadening the application range of silicon in semiconductor devices. For example, the source and drain of the transistor are doped with phosphorus to form a good ohmic contact. Polycrystalline silicon material is known as the cornerstone of the photovoltaic industry and microelectronics industry, but its preparation process is complicated and cumbersome, and the substrate is required to be an expensive high temperature resistant material (such as quartz, etc.). Amorphous silicon can be deposited on the surface of various substrates quickly and in large area, but the stability of the prepared film is poor, the light absorption rate is low, and the electron mobility is small. Therefore, how to use the phase transformation of amorphous silicon to prepare polycrystalline silicon thin film, It has become a hot issue in the research and development of materials and devices.

The traditional amorphous silicon annealing process, such as high temperature crystallization method, solid phase crystallization method, excimer laser crystallization method, etc., uses high temperature thermal action to realize the phase transition conversion from amorphous silicon to polycrystalline silicon film. The amorphous silicon film undergoes a phase transition process of heating, melting, cooling, and recrystallization in sequence, which also requires expensive and heat-resistant substrate materials (such as quartz, etc.) to avoid damage to the substrate caused by thermal diffusion during the phase transition. Therefore, the low-temperature phase change annealing of amorphous silicon can significantly reduce the production cost of polycrystalline silicon, and can also avoid thermal damage caused by high-temperature processes.

The prior art generally employs thermal annealing or laser annealing, rather than a combination of the two. In addition, in the laser annealing process, excimer laser is generally used (also regarded as a thermal temperature laser annealing process), such as "A kind of excimer laser annealing temperature control system and method and annealing device" disclosed in Chinese patent document CN108231558A . First of all, excimer laser annealing still belongs to the thermal temperature annealing process. The function of the temperature control system (heating and cooling) is to adjust the distribution of the thermal field generated in the excimer laser annealing process on the surface of the amorphous silicon film, so as to avoid local excessive temperature. High (cooling) damages the film, and local temperature is too low (heating) does not reach the melting point, thereby achieving a uniform, high-quality phase change film. "Excimer Laser Annealing Equipment" disclosed by CN108188598A also mentions temperature in excimer laser annealing, the purpose of which is to reduce the influence of unnecessary high temperature on amorphous silicon thin films caused by the thermal annealing process of excimer laser.

The interaction process of femtosecond laser and matter is regarded as a kind of 'cold' process, so it is expected to realize low-temperature annealing of amorphous silicon thin films. In the ultra-short femtosecond pulse time, the light energy absorbed by the material does not have time to diffuse to the lattice, and directly excites the outer electrons to the conduction band, realizing the change of material properties. In the femtosecond laser athermal phase transition process of doped amorphous silicon, the overall temperature of the lattice system does not reach the melting point, and the content of activated impurities to replace silicon atoms is limited. However, femtosecond laser annealing under thermally assisted conditions has not been applied to the transformation annealing process of amorphous silicon.

SUMMARY OF THE INVENTION

Aiming at the shortcomings of phase change annealing of amorphous silicon, the present invention provides a device for phase change annealing of amorphous silicon by heat-assisted femtosecond laser capable of accelerating the laser annealing process.

The device for thermally assisted femtosecond laser annealing amorphous silicon of the present invention adopts the following technical solutions:

The device includes a femtosecond laser amplifier, a laser energy regulator, a condensing lens, a reflective plane mirror and a three-dimensional displacement stage. The optical energy regulator, the converging lens and the reflective plane mirror are sequentially arranged at the output end of the femtosecond laser amplifier; the three-dimensional displacement carrier The object stage is arranged below the reflection plane mirror, and an electric heating stage is arranged on the three-dimensional displacement stage;

Place the amorphous silicon to be annealed on the three-dimensional displacement stage, adjust the height of the amorphous silicon so that the amorphous silicon is located in front of the geometric focal plane of the converging lens, and adjust the laser energy by adjusting the laser energy regulator to change the laser component Density, the temperature of the amorphous silicon is controlled by the electric heating stage on the three-dimensional displacement stage, and the laser is controlled by the operation on the three-dimensional displacement stage (the running speed is changed by changing the controller of its stepper motor) in the amorphous silicon. The speed and interval of the upper scanning can realize the phase change annealing of amorphous silicon by thermally assisted femtosecond laser.

The center wavelength of the femtosecond laser amplifier is 800nm, the pulse width is 35fs～200fs, the repetition frequency is 10Hz～1000Hz, and the single pulse energy generated is 1.0mJ/cm ² ～3.5mJ/cm ² .

The laser energy conditioner is composed of a half-wave plate and a polarizer, the half-wave plate is located in front of the polarizer, and the half-wave plate is close to the femtosecond laser amplifier. Rotate the half-wave plate in the laser energy conditioner to change the laser component on the polarizer, thereby adjusting the laser energy density.

The condensing lens is a plano-convex lens with a focal length of 20cm-30cm, and focuses the femtosecond laser beam in the air.

The reflecting plane mirror is an aluminum mirror (suitable for broad-band light) or a high-reflection lens with a specific wavelength (800 nm), which is used to adjust the propagation direction of the femtosecond laser beam.

The three-dimensional displacement stage has a resolution of 0.002mm and a stroke of 30mm for moving in the horizontal and vertical directions in the plane.

The electric heating stage adopts a local heating stage or an integral heating stage. The heating temperature of the electric heating stage is 25°C to 400°C, and the accuracy is 1°C.

The position where the amorphous silicon is located in front of the geometric focal plane of the condensing lens means that the amorphous silicon is located 3 cm to 4 cm in front of the geometric focal plane of the condensing lens. In this way, the laser action area can be increased, and at the same time, the damage to the amorphous silicon caused by the ultra-high laser power can be avoided.

The adjustment of the laser energy density refers to adjusting the repetition frequency of the femtosecond laser amplifier to 500 Hz, and the femtosecond laser single pulse energy continuously changes from complete extinction to 3.0 mJ/cm ² .

The temperature of the amorphous silicon is controlled so that the amorphous silicon is at 25°C to 200°C.

The control of the speed and interval of the laser scanning on the amorphous silicon means that the scanning speed is 5 mm/s to 50 mm/s, and the scanning interval is 50 m to 1000 m.

Through femtosecond laser annealing, the above device is an athermal phase change process (the overall temperature of the amorphous silicon material does not reach the melting point of the material, the phase change can occur), and a low temperature (athermal) phase change process is realized. In the non-thermal phase change femtosecond laser phase change annealing process, the overall temperature of the material has not yet reached the melting point, the thermal energy only plays an auxiliary role, and the femtosecond laser annealing still occupies the dominant position. , so that the overall system is in a high-energy state, and scanning annealing by femtosecond laser is performed at the same time, which increases the probability of free electrons in the conduction band directly excited by the femtosecond laser, and increases the content of doped elements to replace silicon atoms.

In the non-thermal laser annealing process (femtosecond laser annealing), the present invention heats the amorphous silicon, thereby accelerating the laser annealing process, which is substantially different from the existing excimer laser annealing. Second laser annealing of amorphous silicon, combined with the advantages of thermal temperature annealing and femtosecond laser annealing, increases the content of free electrons in the conduction band directly excited by the femtosecond laser, and increases the activation content of doping elements; it achieves better optical and optical properties. Preparation of polysilicon for electrical properties.

Description of drawings

FIG. 1 is a schematic structural diagram of a device based on thermally assisted femtosecond laser low temperature annealing of amorphous silicon according to the present invention.

2 is a scanning electron microscope (SEM) micrograph of an amorphous silicon thin film before and after thermally assisted femtosecond laser annealing in Examples. (a) is the SEM image of the original deposited doped amorphous silicon film; (b) is the SEM image of the femtosecond laser annealed doped amorphous silicon film at room temperature; (c) is the femtosecond laser annealed doped amorphous silicon film at 200 degrees Celsius SEM image of an amorphous silicon film.

FIG. 3 is a Raman spectrum diagram of the doped amorphous silicon film before and after femtosecond laser annealing in the embodiment. Among them, the point-shaped curve corresponds to the Raman spectrum of the original deposited doped amorphous silicon film, the dashed curve corresponds to the Raman spectrum of the femtosecond laser annealed doped amorphous silicon film at room temperature, and the solid line curve corresponds to Raman spectra of femtosecond laser annealed doped amorphous silicon films at 200 degrees Celsius.

FIG. 4 is the reflectance spectrum of the doped amorphous silicon film before and after femtosecond laser annealing in the embodiment. The dashed curve corresponds to the reflectance spectrum of the original deposited doped amorphous silicon film, the dotted curve corresponds to the reflectance spectrum of the femtosecond laser annealed doped amorphous silicon film at room temperature, and the solid line curve Corresponds to reflectance spectra of femtosecond laser annealed doped amorphous silicon films at 200 degrees Celsius.

In the figure: 1. Femtosecond laser amplifier, 2. Laser energy conditioner, 3. Converging lens, 4. Reflecting plane mirror, 5. Amorphous silicon to be annealed, 6. Three-dimensional displacement stage.

Detailed ways

As shown in FIG. 1, the device for phase change annealing of amorphous silicon thin films by thermally assisted femtosecond laser of the present invention includes a femtosecond laser amplifier 1, a laser energy regulator 2, a condensing lens 3, a reflecting plane mirror 4 and a three-dimensional displacement Stage 6. The light energy conditioner 2 , the condensing lens 3 and the reflecting plane mirror 4 are sequentially arranged at the output end of the femtosecond laser amplifier 1 . The three-dimensional displacement stage 6 is provided below the reflection plane mirror 4 (in the light output direction). The center wavelength of the femtosecond laser amplifier 1 is 800nm, the pulse width is 35fs-200fs, the repetition frequency is 10Hz-1000Hz, and the generated single pulse pulse energy is 1.0mJ/cm ² -3.5mJ/cm ² . The laser energy conditioner 2 is composed of a polarizer and a half-wave plate, and the half-wave plate is closer to the femtosecond laser amplifier, that is, the half-wave plate is in front and the polarizer is in the back. The condensing lens 3 is a plano-convex lens with a focal length of 20cm-30cm. The three-dimensional displacement stage 6 is provided with an electric heating stage. The electric heating table adopts a local heating table (suitable for small-sized samples with a diameter of less than 30mm) or an integral heating table (for large-sized samples with a size of 30mm to 200mm). Accuracy is 1°C. The resolution of the three-dimensional displacement stage 6 moving in the horizontal and vertical directions in the plane is 0.002 mm, and the stroke is 30 mm.

In the laser energy modifier 2, the half-wave plate was rotated to change the laser component on the polarizer, thereby adjusting the laser energy density in the experiment. In the three-dimensional precision displacement stage 6 equipped with the electric heating stage, the temperature of the amorphous silicon film can be changed by setting the parameters of the electric heating stage, which can be changed by setting the speed of the stepping motor in the three-dimensional displacement stage 6 The scanning parameters of the femtosecond laser on the surface of the amorphous silicon film finally realize the phase change annealing of the amorphous silicon by the femtosecond laser under the thermal assistance.

The amorphous silicon 5 to be annealed is placed on the three-dimensional displacement stage 6 so that it is located in front of the geometric focal plane of the converging lens 3 . Adjust the laser energy regulator 2 to control the laser energy density used. By adjusting the electric heating stage and the three-dimensional displacement stage 6, and controlling the temperature of the amorphous silicon 5 and the speed and interval of the laser scanning, the thermally assisted femtosecond laser can realize the phase change annealing of the amorphous silicon thin film. The specific process is as follows:

(1) The amorphous silicon 5 is placed on the three-dimensional displacement stage 6, and the height of the stage is adjusted so that the amorphous silicon 5 is located 3 cm to 4 cm in front of the geometric focal plane of the converging lens 3, and the laser action area is increased. Avoid high laser power damage to the amorphous silicon film;

(2) Adjust the repetition frequency of the femtosecond laser amplifier to 500Hz, and rotate the half-wave plate in the laser energy conditioner 2 to realize continuous change of the femtosecond laser single-pulse energy, ranging from complete extinction to 3.0mJ/cm ² ;

(3) Control the stepping motor controller to control the scanning speed and scanning interval of the femtosecond laser. Set the scanning speed of the precision displacement platform to 5mm/s～50mm/s, and the scanning interval to be 50m～1000m.

(4) Through the setting of the electric heating stage, the amorphous silicon is in the range of 25° C. (room temperature) to 200° C., so as to realize the phase change annealing of the amorphous silicon film by the thermally assisted femtosecond laser.

Examples of setting specific parameters are given below.

Amorphous silicon 5 is selected from doped amorphous silicon film, and in the process of depositing amorphous silicon by PEVCD method, phosphine diluted with hydrogen is added for growth, and the specific ratio is SiH ₄ : H ₂ : PH ₃ =10: 4.37: 0.23, wherein the growth temperature is 260 degrees Celsius; the thickness of the final doped amorphous silicon is 400nm, and it is cut into 20mm×20mm.

1. Adjust the repetition frequency of the femtosecond laser amplifier 1 to 500Hz, use a power meter (Spectral Physics) to test the laser power, and adjust the half-wave plate in the laser energy conditioner 2 so that the single-pulse laser power used is 0.3mJ/cm ² , using a converging lens 3 with a focal length of 30cm.

3. The amorphous silicon film 5 is placed on the three-dimensional displacement stage 6, and the height of the stage 6 is adjusted so that the amorphous silicon film 5 is positioned 4 cm in front of the geometric focal plane of the converging lens 3;

4. Set the running speed of the displacement platform on the three-dimensional displacement stage 6 to 50 mm/s, and the scanning interval to 250 m. The operating temperatures of the microscopic hot stage are respectively 25°C (room temperature) and 200°C; the scanning annealing of amorphous silicon by femtosecond laser under thermal assistance can be realized.

The surface morphology of the doped amorphous silicon films before and after annealing was characterized by scanning electron microscopy. Figure 2 presents the scanning electron microscope (SEM) micrographs of the amorphous silicon films before and after thermally assisted femtosecond laser annealing. (a) is the SEM image of the original deposited doped amorphous silicon film; (b) is the SEM image of the femtosecond laser annealed doped amorphous silicon film at room temperature; (c) is the doped amorphous silicon film at 200 degrees Celsius SEM image.

Raman spectrometer was used to test the change of crystal orientation on the surface of doped amorphous silicon films before and after annealing. Figure 3 shows the Raman spectra of the doped amorphous silicon films before and after femtosecond laser annealing. Among them, the point-shaped curve corresponds to the Raman spectrum of the original deposited doped amorphous silicon film, the dashed curve corresponds to the Raman spectrum of the femtosecond laser annealed doped amorphous silicon film at room temperature, and the solid line curve corresponds to Raman spectra of femtosecond laser annealed doped amorphous silicon films at 200 degrees Celsius.

The reflectance spectrum of the doped amorphous silicon film before and after femtosecond laser annealing is shown in Figure 4. The dashed curve corresponds to the reflectance spectrum of the original deposited doped amorphous silicon film, the dotted curve corresponds to the reflectance spectrum of the femtosecond laser annealed doped amorphous silicon film at room temperature, and the solid line curve Corresponds to reflectance spectra of femtosecond laser annealed doped amorphous silicon films at 200 degrees Celsius.

The sheet resistance of the surface was tested using a four-probe testing system (4Probes Tech RTS-5). Among them, the sheet resistance of the original deposited doped amorphous silicon film is very high (beyond the test range), and the sheet resistance of the femtosecond laser annealed doped amorphous silicon film at room temperature is ~80Ω/□, while the femtosecond laser annealed at 200 degrees Celsius has a sheet resistance of ~80Ω/□. The sheet resistance of the doped amorphous silicon film is further reduced to about 15Ω/□.

The present invention combines the advantages of thermal temperature annealing and femtosecond laser annealing, by controlling the working temperature of the electric heating stage and the operating parameters of the three-dimensional displacement stage 6, and under the condition of thermal and temperature assistance, the femtosecond laser annealing of amorphous materials is simultaneously performed. Phase-change annealing of silicon thin films, thereby realizing low-temperature preparation of polysilicon thin films with lower reflectivity and lower resistivity, plays an important role in the fields of microelectronic devices and photovoltaic devices.

The invention can realize the phase change annealing of doped amorphous silicon by femtosecond laser under thermal assistance, and combines the advantages of thermal temperature annealing and femtosecond laser annealing, and compares the situation of unannealed and femtosecond laser annealing at room temperature. The doped amorphous silicon thin films subjected to femtosecond laser annealing show a higher degree of crystallization (lower reflectivity, lower surface resistance), which is beneficial to its further application in photovoltaic and microelectronic devices.

Claims (9)

1. a device for thermally assisted femtosecond laser annealing amorphous silicon, it is characterized in that: comprise femtosecond laser amplifier, laser energy regulator, convergent lens, reflection plane mirror and three-dimensional displacement stage, light energy regulator, convergent lens and the reflection plane mirror are sequentially arranged at the output end of the femtosecond laser amplifier; the three-dimensional displacement stage is arranged under the reflection plane mirror, and an electrothermal stage is arranged on the three-dimensional displacement stage; Place the amorphous silicon to be annealed on the three-dimensional displacement stage, adjust the height of the amorphous silicon so that the amorphous silicon is located in front of the geometric focal plane of the converging lens, and adjust the laser energy by adjusting the laser energy regulator to change the laser component Density, the temperature of the amorphous silicon is controlled by the electric heating stage on the three-dimensional displacement stage, and the speed and interval of the laser scanning on the amorphous silicon are controlled by the operation of the three-dimensional displacement stage, so as to realize the thermally assisted femtosecond laser pairing. Phase change annealing of amorphous silicon; The position where the amorphous silicon is located in front of the geometric focal plane of the condensing lens means that the amorphous silicon is located 3 cm to 4 cm in front of the geometric focal plane of the condensing lens. 2 . The device for thermally assisted femtosecond laser annealing amorphous silicon according to claim 1 , wherein the center wavelength of the femtosecond laser amplifier is 800 nm, the pulse width is 35 fs～200 fs, and the repetition frequency is 10 Hz～1000 Hz. 3 . , the generated single pulse pulse energy is 1.0mJ/cm ² -3.5mJ/cm ² . 3 . The device for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1 , wherein the laser energy regulator is composed of a half-wave plate and a polarizer, and the half-wave plate is located in front of the polarizer. 4 . 4 . The device for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1 , wherein the condensing lens is a plano-convex lens, and the focal length is 20cm-30cm. 5 . 5 . The device for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1 , wherein the reflecting plane mirror is an aluminum mirror or a high-reflection lens with a specific wavelength. 6 . 6 . The device for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1 , wherein the three-dimensional displacement stage has a resolution of moving in two horizontal and vertical directions in a plane: 6 . 0.002mm with 30mm stroke. 7 . The device for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1 , wherein the heating temperature of the electric heating stage is 25°C to 400°C, and the accuracy is 1°C. 8 . 8. The device for thermally assisted femtosecond laser annealing amorphous silicon according to claim 1, wherein the adjusting laser energy density refers to adjusting the repetition frequency of the femtosecond laser amplifier to 500 Hz, and the femtosecond laser single pulse The energy varies continuously from complete extinction to 3.0 mJ/cm ² . 9 . The device for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1 , wherein: the temperature of the amorphous silicon is controlled so that the temperature of the amorphous silicon is 25° C. to 200° C.; The speed and interval of scanning on amorphous silicon means that the scanning speed is 5 mm/s to 50 mm/s, and the scanning interval is 50 μm to 1000 μm.

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