Study on the Preparation of Low Temperature p-Si Thin Films

introduction

In recent years, active liquid crystal display technology has dominated the liquid crystal display industry and research fields. In active liquid crystal display technology, polysilicon thin film has a unique advantage due to its high mobility. Its device size is small, and it can obtain a higher aperture ratio and resolution. Due to the increase in mobility, the peripheral drive can be integrated into the display panel, which can effectively reduce material costs. At the same time, the overall weight and thickness of the LCD will be greatly reduced.

At present, the research on polysilicon thin film technology is mainly focused on reducing the crystallization temperature and reducing the crystallization time. Metal induced crystallization is a new technology with great potential. The crystallization temperature can be reduced to 500℃ and the crystallization time can be shortened. For example, Al, Cu, Au, or Ni are deposited on a-Si:H or ion-implanted into the interior of a-Si:H film. Due to the induction of metal, the crystallization temperature of a-Si is lower than the SPC temperature of a-Si. Excimer laser annealing is another promising solution for preparing p-Si thin films with higher mobility. It has a series of outstanding advantages such as high degree of crystallization, short preparation cycle, and low substrate temperature, showing a good development prospect.

Preparation of polycrystalline silicon thin films by metal induction method

1.Mechanism of metal-induced crystallization

Hayzelden believes that Ni-induced crystallization occurs due to the migration of NiSi2. Since the crystal structure of NiSi2 is similar to that of Si, the lattice constant differs by 0.4%. NiSi2 is formed below 350°C and used as a seed crystal. The crystallization temperature can be reduced to below 500°C. a-Si crystallizes into polysilicon with NiSi2 as the medium. The crystallization of silicon is nucleated on one or more faces of the NiSi2 octahedron. The resistivity of NiSi2 is very low, 35uΩ·cm, and the lattice constant matches that of polysilicon. Metal-induced crystallization is believed to be due to the covalent bond reaction between the free electrons of the metal and Si at the interface. The tiny lattice mismatch (0.4%) between NiSi2 and Si makes it easy for Si to grow on the NiSi2 (111) surface.

2. Experimental results and discussion

Figure 1 (a) ~ (c) are XRD spectra of Ni-MIC p-Si film annealed at 440℃, 480℃, and 520℃ for 4h, and Figure 2 (a) ~ (c) are XRD spectra of Ni-MIC p-Si film annealed at 440℃ for 2h, 4h, and 10h. The characteristic diffraction peaks of polycrystalline silicon appear in the XRD diffraction spectrum, d = 3.13A (111). d = 1.91A (220). And d = 1.63A (311), a-Si crystallization has no preferred crystal orientation. The intensity of the XRD diffraction peak increases with the extension of annealing time and the increase of annealing temperature. It shows that the crystallization strength of the film increases with the increase of temperature and annealing time.

Figure 3 shows the Raman spectra of a-Si and a-Si/Ni annealed at 440℃ for 4h. There is a broad TO phonon peak at 480cm-1 on the a-Si spectrum. When Ni is present, there is a sharp TO phonon peak at 520cm-1 on the Raman spectrum, and its full width at half maximum is 5.0cm-1, which is greater than the FWHM 4.5cm-1 of single crystal silicon, indicating that the film is a polycrystalline structure. Due to the presence of Ni, after a-Si is annealed at 440℃, no characteristic peak of a-Si is observed at 480cm-1, indicating that the film is completely crystallized into a polycrystalline structure.

Preparation of polycrystalline silicon thin films by laser annealing

1. Principle of preparing polycrystalline silicon thin film by laser annealing

a-Si absorbs energy under laser radiation, excites unbalanced electron-hole pairs, and increases the energy of free electrons in the conduction band. The "hot" electron-hole pairs transfer their energy to the lattice through the thermal pathway of non-radiative recombination within the thermalization time (about 10-11~10-9s), resulting in extremely rapid (about 1010K/s) heating of the near-surface layer. Since a-Si material has a large number of gap states and deep energy levels, non-radiative transition is the main recombination process, so it has a high photothermal conversion efficiency. If the energy of the laser reaches the threshold energy density EC, even if the semiconductor is heated to the melting point, the surface of the film will melt, and the melting front will penetrate into the material at a speed of about 10m/s. After laser irradiation, the film forms a melt layer of a certain depth. After stopping irradiation, the melt layer begins to cool at a speed of 108-1010K/s, and the interface between the solid phase and the liquid phase will return to the surface at a speed of 1~2m/s. After cooling, as the overall temperature of the film drops, non-uniform nucleation will occur preferentially at the solid-liquid interface with a lower temperature, and the nuclei will grow in the lateral and longitudinal directions of the film. When the grains collide with each other, the lateral growth of the grains stops. Only some "lucky" grains are larger in size. If the laser energy is less than the threshold energy EC, that is, the absorbed laser energy is not enough to raise the surface temperature to the melting point, the film will not crystallize.

2. Experimental results and discussion

Figure 4 shows the XRD diffraction results of thin film materials obtained by scanning at different laser energy densities. The substrate temperature used in the experiment was 400℃, the background vacuum was 3.2x10-4Pa, and the laser pulse frequency was 3HZ. The results show that within the laser energy density range used, a-Si:H thin film with a thickness of 100nm has been transformed into polycrystalline silicon after laser crystallization sintering, and its critical energy density is about 160mJ/cm2. When the energy density is 240mJ/cm2, the characteristic diffraction peaks (111) and (200) of polycrystalline silicon have appeared in the XRD spectrum. Later, with the increase of laser energy density, the intensity of the (111) and (200) diffraction peaks increases. At an energy density of 340mJ/cm2, the (311) diffraction peak also begins to appear. As can be seen from the figure, within the range of scanning energy density used, the a-Si:H film has already undergone a transition from an amorphous phase to a crystalline phase, and the order of the optimal orientation of the crystal plane is (111)>(220)>(311). As the scanning energy increases, the intensity of the (111) peak gradually increases, while the (220) peak grows slowly, especially when the laser energy density increases to 456mJ/cm2, the (220) peak weakens, indicating that at higher scanning energy densities, the (111) has a significant optimal orientation trend.

Figure 5 shows the XRD diffraction results of the thin film material obtained after scanning with laser beams of different pulse frequencies. The laser energy density used at this time is constant at 340mJ/cm2, and the substrate temperature and background vacuum are still 400℃ and 3.2x10-4Pa respectively. It can be seen from the figure that at a lower scanning laser pulse frequency, the a-Si:H film also undergoes a transition from an amorphous phase to a crystalline phase. At the same time, with the increase of the scanning laser pulse frequency, (111) and (220) are gradually enhanced, indicating that the grain size of the material is also gradually increasing. This is because as the number of times the film is exposed to light per unit time increases, the film temperature drops slower, the solidification rate decreases, the grain growth time is prolonged, and the size increases. It was found in the experiment that, due to similar reasons, in laser scanning sintering, increasing the substrate temperature can also play a role in reducing the melt solidification rate and prolonging the grain growth time. Therefore, under the premise that the substrate can withstand the temperature, using a higher substrate temperature is conducive to obtaining polycrystalline silicon thin film materials with larger grain sizes.

Figure 6 is a Raman spectrum of the thin film material obtained by annealing with different laser energy densities. Since the Raman absorption of a-Si:H film appears near 480cm-1, while that of Poly-Si appears near 520cm-1. Therefore, the crystallinity of polysilicon thin film material annealed by excimer laser can also obtain important information from the Raman spectrum. As can be seen from the figure, with the increase of laser energy density, the crystallinity gradually increases at the beginning, and then with the increase of energy density, the crystallinity begins to decrease. This is because, compared with the case of relatively low energy density, the increase in energy density leads to a longer melting time of the film and an increase in nucleation density, so that microcrystallization or amorphization occurs in local areas, which reduces the overall crystallinity of the film.

summary

We used laser annealing and metal induction to prepare p-Si thin film materials based on a-Si∶H thin film, and studied the process conditions for preparing polycrystalline silicon thin film materials on glass substrates and the structural characteristics of the obtained thin film materials. On this basis, low-temperature p-SiTFT liquid crystal display devices can be made, which can further provide some useful explorations for the technology of my country's liquid crystal display industry.