CN108831827B - Device for annealing amorphous silicon by heat-assisted femtosecond laser - Google Patents
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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
Technical Field
The invention belongs to the field of phase change conversion of amorphous silicon thin films, and particularly relates to a device and a method for carrying out low-temperature annealing on an amorphous silicon thin film by using heat-assisted femtosecond laser.
Background
Silicon has the existence modes of monocrystalline silicon, polycrystalline silicon, amorphous silicon (amorphous silicon) and the like, has the advantages of rich element reserves and stable chemical properties, and is widely applied to the fields of precision detection sensing, solar energy conversion, integrated circuits and the like. The intrinsic silicon has poor carrier mobility, and the number of free electrons can be increased by doping impurity elements such as boron, phosphorus, arsenic, gold and the like to increase the conductivity of the intrinsic silicon, so that the application range of the intrinsic silicon in a semiconductor device is widened. For example, a good ohmic contact is formed by doping the source and drain of the transistor with phosphorus. Polysilicon materials are known as base stones in the photovoltaic industry and the microelectronic industry, but the preparation process is complex and tedious, and the substrate is required to be expensive high-temperature-resistant materials (such as quartz and the like). Amorphous silicon can be deposited on various substrate surfaces in a large area rapidly, but the prepared film has poor stability, low light absorption rate and low electron mobility, so how to prepare the polycrystalline silicon film by amorphous silicon phase transition becomes a hotspot problem in material and device research and development.
The traditional amorphous silicon annealing process such as a high-temperature crystallization method, a solid-phase crystallization method, an excimer laser crystallization method and the like utilizes the high-temperature thermal action to realize the phase change conversion from amorphous silicon to a polycrystalline silicon film. The amorphous silicon thin film is subjected to phase change processes of temperature rise, melting, cooling and recrystallization in sequence, and expensive and heat-resistant (such as quartz and the like) substrate materials are also required to avoid damage to the substrate caused by heat diffusion in phase change. Therefore, the low-temperature phase-change annealing of the amorphous silicon can obviously reduce the preparation cost of the polycrystalline silicon and can also avoid the thermal damage generated by a high-temperature process.
The prior art generally adopts thermal annealing or laser annealing, but does not combine the two. In addition, in the laser annealing process, an excimer laser (also referred to as a thermal temperature laser annealing process) is generally used, for example, "an excimer laser annealing temperature control system and method, and an annealing apparatus" disclosed in chinese patent document CN 108231558A. Firstly, excimer laser annealing still belongs to a thermal annealing process, wherein a temperature control system (heating and cooling) is used for adjusting the distribution of a thermal field generated in the excimer laser annealing process on the surface of an amorphous silicon film, so that the film is prevented from being damaged due to overhigh local temperature (cooling), and the film does not reach a melting point due to overlow local temperature (heating), thereby realizing an even and high-quality phase-change film. The excimer laser annealing device disclosed in CN108188598A also mentions the temperature in excimer laser annealing, and the purpose of the temperature is to reduce the influence of unnecessary high temperature generated by the thermal annealing process of excimer laser on the amorphous silicon thin film.
The interaction process of the femtosecond laser and the substance is regarded as a 'cold' process, and thus low-temperature annealing of the amorphous silicon thin film is expected. In the ultrashort femtosecond pulse time, the light energy absorbed by the material has no time to diffuse to the crystal lattice, and the outer layer electrons are directly excited to a conduction band, so that the property of the material is changed. In the femtosecond laser non-thermal phase transition process of doping amorphous silicon, the integral temperature of a crystal lattice system does not reach a melting point, and the content of activated impurities for replacing silicon atoms is limited. However, femtosecond laser annealing under a heat-assisted condition has not been applied to the phase-change transformation annealing process of amorphous silicon.
Disclosure of Invention
Aiming at the defects of phase change annealing of amorphous silicon, the invention provides a device for phase change annealing of amorphous silicon by using heat-assisted femtosecond laser, which can accelerate the laser annealing process.
The device for annealing the amorphous silicon by the heat-assisted femtosecond laser adopts the following technical scheme:
the device comprises a femtosecond laser amplifier, a laser energy regulator, a convergent lens, a reflecting plane mirror and a three-dimensional displacement objective table, 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;
the method comprises the steps of placing amorphous silicon to be annealed on a three-dimensional displacement objective table, adjusting the height of the amorphous silicon to enable the amorphous silicon to be located in front of a geometric focal plane of a convergent lens, adjusting laser energy density by adjusting a laser energy adjuster to change laser components, controlling the temperature of the amorphous silicon by an electrothermal table on the three-dimensional displacement objective table, and controlling the speed and the interval of laser scanning on the amorphous silicon by operation on the three-dimensional displacement objective table (changing the operation speed by changing a controller of a stepping motor of the three-dimensional displacement objective table), so that phase-change annealing of the amorphous silicon by heat-assisted femtosecond laser on silicon is realized.
The center wavelength of the femtosecond laser amplifier is 800nm, the pulse width is 35 fs-200 fs, the repetition frequency is 10 Hz-1000 Hz, and the energy of the generated single pulse is 1.0mJ/cm2～3.5mJ/cm2。
The laser energy regulator consists of a half-wave plate and a polaroid, wherein the half-wave plate is positioned in front of the polaroid, and the half-wave plate is close to the femtosecond laser amplifier. And the half-wave plate in the laser energy regulator is rotationally regulated to change the laser component on the polaroid so as to regulate the laser energy density.
The convergent lens is a plano-convex lens, has a focal length of 20-30 cm, and focuses the femtosecond laser beam in the air.
The reflecting plane mirror is an aluminum mirror (suitable for broadband light) or a high-reflection lens with specific wavelength (800nm) and is used for adjusting the transmission direction of the femtosecond laser beam.
The resolution of the three-dimensional displacement object stage moving in the horizontal and vertical directions in a plane is 0.002mm, and the stroke is 30 mm.
The electric heating platform adopts a local heating platform or an integral heating platform. The heating temperature of the electric heating platform is 25-400 ℃, and the precision is 1 ℃.
The position of the amorphous silicon in front of the geometric focal plane of the convergent lens means that the amorphous silicon is 3 cm-4 cm in front of the geometric focal plane of the convergent lens. Therefore, the laser action area can be increased, and the damage of the ultrahigh laser power to the amorphous silicon can be avoided.
The laser energy density is adjusted by adjusting the repetition frequency of the femtosecond laser amplifier to 500Hz, and the femtosecond laser single pulse energy is continuously changed from complete extinction to 3.0mJ/cm2。
The temperature of the amorphous silicon is controlled to be 25-200 ℃.
The speed and the interval of the laser scanning on the amorphous silicon are controlled, namely the scanning speed is 5-50 mm/s, and the scanning interval is 50-1000 m.
The device is a non-thermal phase change process (the phase change can occur without the integral temperature of the amorphous silicon material reaching the melting point of the material) through femtosecond laser annealing, and the low-temperature (non-thermal) phase change process is realized. In the femtosecond laser phase change annealing process of non-thermal phase change, the integral temperature of the material does not reach a melting point, heat energy only plays an auxiliary role, femtosecond laser annealing still occupies a leading position, an electrothermal platform is utilized to heat amorphous silicon, so that the integral system is in a high-energy state, and simultaneously, the femtosecond laser is scanned and annealed, the probability of conduction band free electrons directly excited by the femtosecond laser is increased, and the content of doped elements for replacing silicon atoms is improved.
In the non-thermal laser annealing process (femtosecond laser annealing), the amorphous silicon is heated, so that the laser annealing process is accelerated, the process is substantially different from the existing excimer laser annealing, the amorphous silicon is annealed by thermally-assisted femtosecond laser, the advantages of thermal temperature annealing and femtosecond laser annealing are combined, the content of conduction band free electrons directly excited by the femtosecond laser is increased, and the activation content of doping elements is improved; the preparation of the polysilicon with better optical and electrical characteristics is realized.
Drawings
Fig. 1 is a schematic structural diagram of a device for low-temperature annealing of amorphous silicon based on a thermally assisted femtosecond laser according to the present invention.
FIG. 2 is a Scanning Electron Microscope (SEM) micrograph of an amorphous silicon thin film before and after femtosecond laser annealing with heat assistance in example. Wherein (a) is an SEM image of an as-deposited doped amorphous silicon thin film; (b) is SEM picture of doped amorphous silicon film by femtosecond laser annealing at room temperature; (c) is SEM image of the doped amorphous silicon film by femtosecond laser annealing at 200 ℃.
FIG. 3 is a Raman spectrum of the doped amorphous silicon thin film before and after the femtosecond laser annealing in the example. Wherein, the dotted line-shaped curve is corresponding to the Raman spectrum of the original deposition doped amorphous silicon film, the dotted line-shaped curve is corresponding to the Raman spectrum of the femtosecond laser annealing doped amorphous silicon film at room temperature, and the solid line-shaped curve is corresponding to the Raman spectrum of the femtosecond laser annealing doped amorphous silicon film at 200 ℃.
FIG. 4 is a reflectance spectrum of the doped amorphous silicon thin film before and after the femtosecond laser annealing in the example. The dotted line-shaped curve corresponds to the reflectivity spectrum of the originally deposited doped amorphous silicon film, the point-shaped line-shaped curve corresponds to the reflectivity spectrum of the femtosecond laser annealing doped amorphous silicon film at room temperature, and the solid line-shaped curve corresponds to the reflectivity spectrum of the femtosecond laser annealing doped amorphous silicon film at 200 ℃.
In the figure: 1. the laser annealing device comprises a femtosecond laser amplifier, 2 a laser energy regulator, 3 a convergent lens, 4 a reflecting plane mirror, 5 amorphous silicon to be annealed and 6 a three-dimensional displacement object stage.
Detailed Description
As shown in FIG. 1, the device for phase-change annealing of amorphous silicon thin film by heat-assisted femtosecond laser comprises a femtosecond laser amplifier 1, a laser energy regulator 2, a converging lens 3 and a reflecting plateA mirror 4 and a three-dimensional displacement stage 6. The light energy regulator 2, the convergent 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 disposed below the reflecting plane mirror 4 (in the light output direction). The center wavelength of the femtosecond laser amplifier 1 is 800nm, the pulse width is 35 fs-200 fs, the repetition frequency is 10 Hz-1000 Hz, and the energy of the generated single pulse is 1.0mJ/cm2～3.5mJ/cm2. The laser energy adjuster 2 consists of a polarizer and a half-wave plate, the half-wave plate is closer to the femtosecond laser amplifier, namely the half-wave plate is in front and the polarizer is in back. The converging lens 3 is a plano-convex lens, and the focal length is 20 cm-30 cm. An electric heating table is arranged on the three-dimensional displacement object stage 6. The electric heating platform adopts a local heating platform (suitable for small-size samples with the diameter less than 30 mm) or an integral heating platform (suitable for large-size samples with the size of 30 mm-200 mm), the heating temperature of the electric heating platform is 25-400 ℃, and the precision is 1 ℃. The resolution of the movement of the three-dimensional displacement stage 6 in both the horizontal and vertical directions in the plane was 0.002mm, and the stroke was 30 mm.
In the laser power adjuster 2, the half-wave plate was rotated to change the laser component on the polarizing plate, thereby adjusting the laser power density in the experiment. In the three-dimensional precise displacement object stage 6 provided with the electric heating stage, the temperature of the amorphous silicon film can be changed by setting the parameters of the electric heating stage, the scanning parameters of the femtosecond laser on the surface of the amorphous silicon film can be changed by setting the rotating speed of the stepping motor in the three-dimensional displacement object stage 6, and finally the phase change annealing of the femtosecond laser under the assistance of heat to the amorphous silicon is realized.
The amorphous silicon 5 to be annealed is placed on a three-dimensional displacement stage 6 so that it is in front of the geometric focal plane of the converging lens 3. The laser energy regulator 2 is adjusted to control the energy density of the laser used. And adjusting the electric heating table and the three-dimensional displacement object stage 6, and controlling the temperature of the amorphous silicon 5 and the speed and interval of laser scanning, so that the phase change annealing of the amorphous silicon film by the heat-assisted femtosecond laser can be realized. The specific process is as follows:
(1) placing the amorphous silicon 5 on a three-dimensional displacement object stage 6, and adjusting the height of the object stage to ensure that the amorphous silicon 5 is 3-4 cm in front of the geometric focal plane of the convergent lens 3, so as to increase the laser action area and simultaneously avoid the damage of ultrahigh laser power to the amorphous silicon film;
(2) the repetition frequency of the femtosecond laser amplifier is adjusted to 500Hz, and the half-wave plate in the laser energy adjuster 2 is rotated to realize the continuous change of the femtosecond laser single pulse energy in the range of complete extinction to 3.0mJ/cm2；
(3) And controlling a stepping motor controller to realize the control of the scanning speed and the scanning interval of the femtosecond laser, and setting the scanning speed of the precision displacement platform to be 5-50 mm/s and the scanning interval to be 50-1000 m.
(4) By setting the electric heating platform, the amorphous silicon is in the range of 25 ℃ (room temperature) to 200 ℃, so that the phase-change annealing of the amorphous silicon film by the heat-assisted femtosecond laser is realized.
Examples of setting specific parameters are given below.
Selecting a doped amorphous silicon thin film from the amorphous silicon 5, adding phosphine diluted by hydrogen to grow in the process of depositing the amorphous silicon by a PEVCD method, wherein the specific proportion is SiH4：H2：PH310: 4.37: 0.23, wherein the growth temperature is 260 degrees celsius, and the final doped amorphous silicon thickness is 400nm and cut to 20mm × 20 mm.
1. Adjusting the repetition frequency of the femtosecond laser amplifier 1 to 500Hz, testing the laser power by using a power meter (Spectral Physics), and adjusting the half-wave plate in the laser energy adjuster 2 to make the used single-pulse laser power be 0.3mJ/cm2The focal length of the condensing lens 3 used is 30 cm.
3. Placing the amorphous silicon thin film 5 on a three-dimensional displacement object stage 6, and adjusting the height of the object stage 6 to enable the amorphous silicon thin film 5 to be positioned 4cm in front of a geometric focal plane of the convergent lens 3;
4. the running speed of the displacement platform on the three-dimensional displacement object stage 6 is set to be 50mm/s, and the scanning interval is set to be 250 m. Setting working temperatures of the microscopic heating table to be 25 ℃ (room temperature) and 200 ℃; thus realizing the scanning annealing of the amorphous silicon by the femtosecond laser under the assistance of heat.
And characterizing the surface appearance of the doped amorphous silicon thin film before and after annealing by using a scanning electron microscope. FIG. 2 shows Scanning Electron Microscope (SEM) micrographs of amorphous silicon thin films before and after femtosecond laser annealing with thermal assistance. Wherein (a) is an SEM image of an as-deposited doped amorphous silicon thin film; (b) is SEM picture of doped amorphous silicon film by femtosecond laser annealing at room temperature; (c) is an SEM image of a doped amorphous silicon thin film at 200 ℃.
And testing the change condition of the crystal orientation of the surface of the doped amorphous silicon thin film before and after annealing by using a Raman spectrometer. FIG. 3 shows Raman spectra of the doped amorphous silicon thin film before and after femtosecond laser annealing. Wherein, the dotted line-shaped curve is corresponding to the Raman spectrum of the original deposition doped amorphous silicon film, the dotted line-shaped curve is corresponding to the Raman spectrum of the femtosecond laser annealing doped amorphous silicon film at room temperature, and the solid line-shaped curve is corresponding to the Raman spectrum of the femtosecond laser annealing doped amorphous silicon film at 200 ℃.
The reflection spectrum of the doped amorphous silicon thin film is characterized by utilizing an ultraviolet-visible-near infrared spectrophotometer (L ISR-UV3600), and a reflectivity spectrogram of the doped amorphous silicon thin film before and after femtosecond laser annealing is given in figure 4, wherein a dotted line-shaped curve corresponds to the reflectivity spectrum of the originally deposited doped amorphous silicon thin film, a point line-shaped curve corresponds to the reflectivity spectrum of the femtosecond laser annealing doped amorphous silicon thin film at room temperature, and a solid line-shaped curve corresponds to the reflectivity spectrum of the femtosecond laser annealing doped amorphous silicon thin film at 200 ℃.
The sheet resistance of the surface was tested using a four-probe test system (4Probes Tech RTS-5). The sheet resistance of the originally deposited doped amorphous silicon thin film is very high (exceeds the test range), the sheet resistance of the doped amorphous silicon thin film subjected to femtosecond laser annealing at room temperature is 80 omega/□, and the sheet resistance of the doped amorphous silicon thin film subjected to femtosecond laser annealing at 200 ℃ is further reduced to about 15 omega/□.
The invention combines the advantages of thermal annealing and femtosecond laser annealing, controls the working temperature of the electric heating platform and the operating parameters of the three-dimensional displacement object stage 6, and simultaneously carries out phase change annealing on the amorphous silicon film by femtosecond laser under the condition of thermal temperature assistance, thereby realizing the low-temperature preparation of the polycrystalline silicon film with lower reflectivity and lower resistivity, and having important function in the fields of microelectronic devices and photovoltaic devices.
The invention can realize the phase change annealing of the doped amorphous silicon by the femtosecond laser under the thermal assistance, combines the advantages of thermal annealing and femtosecond laser annealing, and compared with the conditions of non-annealing and femtosecond laser annealing at room temperature, the doped amorphous silicon film under the thermal assistance femtosecond laser annealing effect shows higher crystallization degree (lower reflectivity and lower surface resistance) and is beneficial to further application in photovoltaic and microelectronic devices.
Claims (9)
1. A device for annealing amorphous silicon by heat-assisted femtosecond laser is characterized in that: the device comprises a femtosecond laser amplifier, a laser energy regulator, a convergent lens, a reflecting plane mirror and a three-dimensional displacement objective table, 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, adjusting the height of the amorphous silicon to enable the amorphous silicon to be located in front of a geometric focal plane of a convergent lens, adjusting the laser energy density by adjusting a laser energy adjuster to change laser components, controlling the temperature of the amorphous silicon by an electrothermal table on the three-dimensional displacement objective table, and controlling the scanning speed and interval of laser on the amorphous silicon by the operation of the three-dimensional displacement objective table to realize phase change annealing of the amorphous silicon by heat-assisted femtosecond laser;
the position of the amorphous silicon in front of the geometric focal plane of the convergent lens means that the amorphous silicon is 3 cm-4 cm in front of the geometric focal plane of the convergent lens.
2. The apparatus for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1, wherein: the center wavelength of the femtosecond laser amplifier is 800nm, the pulse width is 35 fs-200 fs, the repetition frequency is 10 Hz-1000 Hz, and the energy of the generated single pulse is 1.0mJ/cm2～3.5mJ/cm2。
3. The apparatus for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1, wherein: the laser energy regulator consists of a half-wave plate and a polaroid, and the half-wave plate is positioned in front of the polaroid.
4. The apparatus for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1, wherein: the convergent lens is a plano-convex lens, and the focal length is 20 cm-30 cm.
5. The apparatus 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 specific wavelength.
6. The apparatus for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1, wherein: the resolution of the three-dimensional displacement object stage moving in the horizontal and vertical directions in a plane is 0.002mm, and the stroke is 30 mm.
7. The apparatus for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1, wherein: the heating temperature of the electric heating platform is 25-400 ℃, and the precision is 1 ℃.
8. The apparatus for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1, wherein: the laser energy density is adjusted by adjusting the repetition frequency of the femtosecond laser amplifier to 500Hz, and the femtosecond laser single pulse energy is continuously changed from complete extinction to 3.0mJ/cm2。
9. The apparatus for thermally assisted femtosecond laser annealing of amorphous silicon according to claim 1, wherein: the temperature of the amorphous silicon is controlled to be 25-200 ℃; the speed and the interval of the laser scanning on the amorphous silicon are controlled, namely the scanning speed is 5 mm/s-50 mm/s, and the scanning interval is 50 mu m-1000 mu m.
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| CN101369526A (en) * | 2007-08-15 | 2009-02-18 | 应用材料股份有限公司 | Pulsed laser anneal system architecture |
| CN103268852A (en) * | 2013-05-02 | 2013-08-28 | 中国科学院半导体研究所 | Method for preparing supersaturated-doping semiconductor thin film |
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