Method for preparing polycrystalline silicon thin film

1 Introduction

Polycrystalline silicon thin film The material has the advantages of high mobility of single crystal silicon material and large-scale and low-cost preparation of amorphous silicon material. Therefore, the research on polycrystalline silicon thin film materials has attracted more and more attention. The preparation process of polycrystalline silicon thin film can be divided into two categories: one is high-temperature process, the temperature during the preparation process is higher than 600℃, and the substrate uses expensive quartz, but the preparation process is relatively simple. The other is low-temperature process, the temperature of the entire processing process is lower than 600℃, and cheap glass can be used as the substrate, so it can be produced on a large area, but the preparation process is relatively complicated. At present, there are mainly the following methods for preparing polycrystalline silicon thin films:

2 Low Pressure Chemical Vapor Deposition (LPCVD)

This is a method for directly generating polysilicon. LPCVD is a standard method commonly used in the preparation of polysilicon films used in integrated circuits. It has the characteristics of fast growth rate, dense and uniform film formation, and large film loading capacity. Polysilicon film can be directly deposited on the substrate by LPCVD using silane gas. The typical deposition parameters are: silane pressure of 13.3~26.6Pa, deposition temperature Td=580~630℃, and growth rate of 5~10nm/min. Due to the high deposition temperature, such as the softening temperature of ordinary glass at 500~600℃, cheap ordinary glass cannot be used and expensive quartz must be used as the substrate. The polysilicon film grown by LPCVD has a <110> preferred orientation of the grains, a "V" shape, a high density of micro-twist defects, a small grain size, and insufficient carrier mobility, which limits its device application. Although reducing the silane pressure helps to increase the grain size, it is often accompanied by an increase in surface roughness, which has an adverse effect on the carrier mobility and the electrical stability of the device.

3 Solid Phase Crystallization (SPC)

The so-called solid phase crystallization refers to the temperature at which amorphous solids crystallize at a temperature lower than the temperature at which they crystallize after melting. This is an indirect method for generating polycrystalline silicon. First, silane gas is used as the raw material, and a-Si:H film is deposited at about 550°C by LPCVD method. Then the film is melted at a high temperature above 600°C, and then the crystal nucleus appears at a slightly lower temperature. As the temperature decreases, the molten silicon continues to crystallize on the crystal nucleus, causing the grain to increase and transform into a polycrystalline silicon film. Using this method, the grain size of the polycrystalline silicon film depends on the thickness of the film and the crystallization temperature. Annealing temperature is an important factor affecting the crystallization effect. In the annealing temperature range below 700°C, the lower the temperature, the lower the nucleation rate, and the larger the grain size that can be obtained when the annealing time is equal; above 700°C, due to the movement of the grain boundaries at this time, the grains are swallowed up by each other, so that in this temperature range, the grain size increases with the increase of temperature. A large number of studies have shown that the grain size of polycrystalline silicon obtained by this method is also closely related to the disorder degree of the initial film sample. T. Aoyama et al. studied the effect of the deposition conditions of the initial material on solid phase crystallization and found that the more disordered the initial material is, the lower the nucleation rate is during the solid phase crystallization process and the larger the grain size is. Since the formation of crystal nuclei is spontaneous during the crystallization process, the crystal plane orientation of the SPC polycrystalline silicon film grains is random. The different crystal plane orientations of adjacent grains will form a higher potential barrier, and hydrogenation treatment is required to improve the performance of SPC polycrystalline silicon. The advantage of this technology is that it can prepare large-area films with grain sizes larger than directly deposited polycrystalline silicon. In-situ doping can be performed, with low cost, simple process, and easy production line formation. Since SPC is crystallized at the melting temperature of amorphous silicon, it is a high-temperature crystallization process with a temperature higher than 600°C, usually requiring about 1100°C, and an annealing time of more than 10 hours. It is not suitable for glass substrates. The substrate material uses quartz or single crystal silicon for the production of small-size devices such as liquid crystal light valves and camera viewfinders.

4 Excimer Laser Crystallization (ELA)

Laser crystallization is more ideal than solid phase crystallization for preparing polycrystalline silicon. It uses the high energy generated by instantaneous laser pulses to irradiate the surface of amorphous silicon film, and only produces thermal energy effects at a depth of 100nm thick on the surface of the film, so that the a-Si film reaches about 1000℃ in an instant, thereby realizing the transformation of a-Si to p-Si. In this process, the instantaneous energy (15~50ns) of the laser pulse is absorbed by the a-Si film and converted into phase change energy. Therefore, there will not be too much heat energy transmitted to the film substrate. By reasonably selecting the wavelength and power of the laser, the a-Si film can be heated by laser to reach the melting temperature and ensure that the temperature of the substrate is lower than 450℃. Glass substrates can be used as substrates, which not only realizes the preparation of p-Si films, but also meets the requirements of LCD and OEL for transparent substrates. Its main advantages are short pulse width (15~50ns) and low substrate heating. Mixed crystallization, that is, a mixture of polycrystalline silicon and amorphous silicon, can also be obtained by selection. Mechanism of excimer laser annealing crystallization: laser irradiation to the surface of a-Si causes the surface to reach the crystallization threshold energy density Ec when the temperature reaches the melting point. a-Si absorbs energy under laser radiation, excites unbalanced electron-hole pairs, increases the conductive energy of free electrons, and the hot electron-hole pairs transfer their energy to the lattice by non-radiative recombination during the thermalization time, resulting in extremely rapid temperature rise near the surface. Since amorphous silicon materials have a large number of gap states and deep energy levels, non-radiative transitions are the main recombination process, and thus have a high light-to-heat conversion efficiency. If the energy density of the laser reaches the threshold energy density Ec, that is, the semiconductor is heated to the melting point, the surface of the film will melt, and the melting front will be about 10m /s speed to penetrate into the material. 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, the film crystallizes into polycrystalline. As the laser energy density increases, the size of the grain increases. When the amorphous film is completely melted, the film crystallizes into microcrystals or polycrystalline. If the laser energy density is less than the threshold energy density Ec, that is, the absorbed energy is not enough to raise the surface temperature to the melting point, the film will not crystallize. In general, as the energy density increases, the grain size increases, and the mobility of the film increases accordingly. When the Si film is almost completely melted, the grain size is the largest. However, the energy is limited by the laser and cannot be increased indefinitely. Too much energy density will reduce the mobility. The laser wavelength also has a great influence on the crystallization effect. The longer the wavelength, the deeper the laser energy is injected into the Si film, and the better the crystallization effect. The polysilicon film prepared by ELA method has large grains, good spatial selectivity, high doping efficiency, few intracrystalline defects, good electrical properties, and a high mobility of 400cm2/vs. It is currently the low-temperature polysilicon film with the best comprehensive performance. The process is highly mature and there are large-scale production line equipment, but it also has its own shortcomings. The grain size is sensitive to laser power and the uniformity of large areas is poor. Poor repeatability, high equipment cost, and complex maintenance

5 Rapid Thermal Annealing (RTA)

Generally speaking, the rapid annealing process includes three stages: heating stage, stabilization stage and cooling stage. When the power of the annealing furnace is turned on, the temperature rises with time, and this stage is called the heating stage. The temperature change per unit time is very easy to control. After the heating process is over, the temperature is in a stable stage. Finally, when the power of the annealing furnace is turned off, the temperature decreases with time, and this stage is called the cooling stage. Use hydrogenated amorphous silicon as the initial material for annealing. When the equilibrium temperature is controlled above 600°C, nano-silicon grains can be formed in the amorphous silicon film, and the size of the formed nano-silicon grains varies with the speed of heating during the annealing process. During the heating process, if the temperature change per unit time is large (such as 100°C/s), the formed nano-silicon grains are small (1.6~15nm); if the temperature change per unit time is small (such as 1°C/s), the nano-silicon grains are large (23~46nm). Further experiments show that extending the annealing time and increasing the annealing temperature cannot change the size of the formed nano-silicon grains; and during annealing, the speed of temperature rise directly affects the size of the formed nano-silicon grains. In order to clarify the influence of the speed of temperature rise on the size of the formed nano-silicon grains, the nucleation theory in crystal growth is adopted. There are two steps in crystal growth: the first step is nucleation and the second step is growth. In other words, a sufficient amount of growing seed crystals is required in the first step. The results show that the speed of temperature rise affects the density of the seed crystals formed. If the temperature change per unit time is large, the seed crystal density is large; conversely, if the temperature change per unit time is small, the seed crystal density is small. Increasing the annealing temperature or extending the annealing time during RTA annealing cannot eliminate the amorphous part in the film. Xue Qing et al. proposed a growth mechanism for fractal growth of nano-silicon from amorphous silicon: fractal growth. From bottom to top, as long as the temperature is not too high so that the adjacent nano-silicon islands do not melt, then even if the annealing temperature is increased or the annealing time is extended, the amorphous part cannot be completely eliminated. The polysilicon prepared by the RTA annealing method has small grain size, large grain boundary density inside the crystal, high material defect density, and is a high-temperature annealing method, which is not suitable for preparing polysilicon using glass as a substrate.

6 Plasma Enhanced Chemical Vapor Deposition (PECVD)

Plasma enhanced chemical vapor deposition (PECVD) uses glow discharge electrons to activate chemical vapor deposition reactions. Initially, the gas is inevitably slightly ionized due to the radiation of high-energy cosmic rays such as ultraviolet rays, and there are a small number of electrons. Introducing an excitation source (for example, DC high voltage, radio frequency, pulse power supply, etc.) into a reaction vessel filled with rarefied gas, the electrons gain energy under the acceleration of the electric field. When it collides inelastically with neutral particles in the gas, it is possible to produce secondary electrons. Repeated collisions and ionizations will result in a large number of ions and electrons. Since the number of positive and negative particles is equal, it is called plasma, and the excess energy is released in the form of light, forming a "glow". In plasma, due to the large difference in mass between electrons and ions, the process of exchanging energy through collisions between the two is relatively slow, so the various charged particles inside the plasma each reach their thermodynamic equilibrium state, so there will be no uniform temperature in such a plasma, only the so-called electron temperature and ion temperature. At this time, the temperature of the electrons can reach 104°C, while the temperature of molecules, atoms, and ions is only 25-300°C. Therefore, from a macroscopic point of view, the temperature of this plasma is not high, but the electrons inside it are in a high-energy state and have high chemical activity. If the energy of the excitation exceeds the thermal energy required for the chemical reaction to activate, the energy of the excited electrons (1-10eV) is sufficient to open the molecular bonds, resulting in the production of chemically active substances. Therefore, chemical reactions that originally required high temperatures can occur at lower temperatures or even at room temperature through the action of discharge plasma.

The process of PECVD thin film deposition can be summarized into three stages:

1. SiH4 decomposes to produce active particles Si, H, SiH2 and SiH3, etc.;

2. Adsorption and diffusion of active particles on the substrate surface;

3. The active molecules adsorbed on the substrate react on the surface to form a Poly-Si layer and release H2;

Research shows that in the process of plasma-assisted deposition, the bombardment of ions and charged groups on the deposition surface is one of the important factors affecting the crystallization quality. This effect is overcome by suppressing or enhancing it through an external bias. There are currently two main views on the crystallization process of polycrystalline silicon thin films prepared by PECVD technology. One view is that the active particles are first adsorbed on the substrate surface, and then various surface processes such as migration, reaction, and dissociation occur to form a crystalline phase structure. Therefore, the surface state of the substrate plays a very important role in the crystallization of the film. The other view is that the space gas phase plays a more important role in the low-temperature crystallization of the film, that is, the particles with a crystalline phase structure are first formed in the space plasma region, and then diffuse to the substrate surface to grow into a polycrystalline film. For the SiH4:H2 gas system, studies have shown that under high hydrogen doping conditions, when depositing polycrystalline silicon thin films by RF PECVD, the substrate must be heated to above 600°C to promote the formation of crystal nuclei in the initial growth stage. When the substrate temperature is less than 300°C, only hydrogenated amorphous silicon (a-Si:H) thin films can be formed. The temperature of polysilicon deposition using SiH4:H2 as the gas source is relatively high, generally above 600°C, and is a high-temperature process that is not suitable for glass substrates.

7 Metal Lateral Induction Coating (MILC)

In the early 1990s, it was found that adding some metals such as Al, Cu, Au, Ag, Ni to a-Si and depositing them on a-Si:H or ion implanting them into the interior of a-Si:H film could reduce the phase change energy of a-Si to p-Si. Then, Ni/a-Si:H was annealed to crystallize the a-Si film, and the crystallization temperature could be lower than 500°C. However, it was not applied in TFT due to metal contamination. It was later found that Ni lateral induced crystallization could avoid the formation of twins. The lattice constant of nickel-silicon compounds was similar to that of single-crystal silicon, with low mutual solubility and appropriate phase change energy. The method of using nickel metal to induce a-Si film was used to obtain a laterally crystallized polycrystalline silicon film. The surface of the laterally crystallized polycrystalline silicon film is smooth, with the characteristics of long grains and continuous grain boundaries. The grain boundary barrier height is lower than that of SPC polycrystalline silicon. Therefore, MILC TFT has excellent performance and does not require hydrogenation. Metals such as nickel are used to form an induction layer on the surface of the amorphous silicon film. The metal Ni and a-Si form NiSi2 silicide at the interface. Using the latent heat released by the silicide and the lattice position provided by the lattice error at the interface, the a-Si atoms recrystallize at the interface to form polycrystalline silicon grains. The NiSi2 layer is destroyed, and the Ni atoms gradually migrate to the bottom layer of the a-Si layer to form NiSi2 silicide. This process is repeated until the a-Si layer is basically completely crystallized. The induction temperature is generally 500°C and the duration is about 10 hours. The annealing time is related to the film thickness.

The metal-induced amorphous silicon crystallization method for preparing polycrystalline silicon thin films has high uniformity, low cost, and amorphous silicon outside the connected metal masking area can also be crystallized, and the growth temperature is 500°C. However, the crystallization rate of MILC is still not high at present, and the rate will decrease with the increase of heat treatment time. We use a method combining MILC and light pulse radiation to achieve rapid lateral crystallization of a-Si thin films in a low temperature environment. Polycrystalline silicon strips with high mobility and low metal contamination are obtained.

8 Conclusion

In addition to the above-mentioned main methods for preparing polysilicon thin films, there are also ultra-high vacuum chemical vapor deposition (UHV/CVD), electron beam evaporation, etc. When UHV/CVD is used to grow polysilicon, when the growth temperature is lower than 550°C, high-quality fine-grained polysilicon thin films can be generated without recrystallization treatment, which is impossible with traditional CVD. Therefore, this method is very suitable for the preparation of low-temperature polysilicon thin-film transistors.