Basic technology of plasma enhanced chemical vapor deposition (PECVD)

1. Main processes of plasma enhanced chemical vapor deposition

Plasma enhanced chemical vapor deposition (PECVD) is a new technology for the growth of thin films by chemical reaction of gaseous substances with the help of glow discharge plasma. Because PECVD technology is prepared by gas discharge, the reaction characteristics of non-equilibrium plasma are effectively utilized, and the energy supply mode of reaction system is fundamentally changed. Generally speaking, when PECVD technology is used to prepare thin films, the growth of thin films mainly includes the following three basic processes

Firstly, in the non-equilibrium plasma, electrons react with the reaction gas in the primary stage to decompose the reaction gas and form a mixture of ions and active groups;

Secondly, all kinds of active groups diffuse and transport to the surface and the wall of the film, and the secondary reactions between the reactants occur at the same time;

Finally, all kinds of primary and secondary reaction products reaching the growth surface are adsorbed and react with the surface, accompanied by the re release of gaseous molecules.

Specifically, PECVD technology based on glow discharge method can make the reaction gas ionize to form plasma under the excitation of external electromagnetic field. In glow discharge plasma, the kinetic energy of electrons accelerated by external electric field is usually about 10ev, or even higher, which is enough to destroy the chemical bonds of reactive gas molecules. Therefore, through the inelastic collision of high-energy electrons and reactive gas molecules, the gas molecules will be ionized or decomposed to produce neutral atoms and molecular products. The positive ions are accelerated by the ion layer accelerating electric field and collide with the upper electrode. There is also a small ion layer electric field near the lower electrode, so the substrate is also bombarded by ions to some extent. As a result, the neutral substance produced by decomposition diffuses to the tube wall and substrate. In the process of drift and diffusion, these particles and groups (the chemically active neutral atoms and molecules are called groups) will undergo ion molecule reaction and group molecule reaction due to the short average free path. The chemical properties of the chemical active substances (mainly groups) that reach the substrate and are adsorbed are very active, and the film is formed by the interaction between them.

2. Chemical reactions in plasma

Because the excitation of the reaction gas in the glow discharge process is mainly electron collision, the elementary reactions in the plasma are various, and the interaction between the plasma and the solid surface is also very complex, which makes it more difficult to study the mechanism of PECVD process. So far, many important reaction systems have been optimized by experiments to obtain films with ideal properties. For the deposition of silicon-based thin films based on PECVD technology, if the deposition mechanism can be deeply revealed, the deposition rate of silicon-based thin films can be greatly increased on the premise of ensuring the excellent physical properties of materials.

At present, in the research of silicon-based thin films, hydrogen diluted silane (SiH4) is widely used as the reaction gas because there is a certain amount of hydrogen in the silicon-based thin films. H plays a very important role in the silicon-based thin films. It can fill the dangling bonds in the material structure, greatly reduce the defect energy level, and easily realize the valence electron control of the materials Since spear et al. First realized the doping effect of silicon thin films and prepared the first PN junction in, the research on the preparation and application of silicon-based thin films based on PECVD technology has been developed by leaps and bounds. Therefore, the chemical reaction in silicon-based thin films deposited by PECVD technology will be described and discussed in the following.

Under the glow discharge condition, because the electrons in the silane plasma have more than several EV energy, H2 and SiH4 will decompose when they are collided by electrons, which belongs to the primary reaction. If we do not consider the intermediate excited states, we can get the following dissociation reactions of sihm (M = 0,1,2,3) with H

e+SiH4→SiH2+H2+e (2.1)

e+SiH4→SiH3+ H+e (2.2)

e+SiH4→Si+2H2+e (2.3)

e+SiH4→SiH+H2+H+e (2.4)

e+H2→2H+e (2.5)

According to the standard heat of production of ground state molecules, the energies required for the above dissociation processes (2.1) ~ (2.5) are 2.1, 4.1, 4.4, 5.9 EV and 4.5 EV respectively. High energy electrons in plasma can also undergo the following ionization reactions

e+SiH4→SiH2++H2+2e (2.6)

e+SiH4→SiH3++ H+2e (2.7)

e+SiH4→Si++2H2+2e (2.8)

e+SiH4→SiH++H2+H+2e (2.9)

The energy required for (2.6) ~ (2.9) is 11.9, 12.3, 13.6 and 15.3 EV respectively. Due to the difference of reaction energy, the probability of (2.1) ~ (2.9) reactions is very uneven. In addition, the sihm formed with the reaction process (2.1) ~ (2.5) will undergo the following secondary reactions to ionize, such as

SiH+e→SiH++2e (2.10)

SiH2+e→SiH2++2e (2.11)

SiH3+e→SiH3++2e (2.12)

If the above reaction is carried out by means of a single electron process, the energy required is about 12 eV or more. In view of the fact that the number of high-energy electrons above 10ev in the weakly ionized plasma with electron density of 1010cm-3 is relatively small under the atmospheric pressure (10-100pa) for the preparation of silicon-based films, The cumulative ionization probability is generally smaller than the excitation probability. Therefore, the proportion of the above ionized compounds in silane plasma is very small, and the neutral group of sihm is dominant. The mass spectrum analysis results also prove this conclusion [8]. Bourquard et al. Further pointed out that the concentration of sihm decreased in the order of sih3, sih2, Si and SIH, but the concentration of SiH3 was at most three times that of SIH. Robertson et al. Reported that in the neutral products of sihm, pure silane was mainly used for high-power discharge, while sih3 was mainly used for low-power discharge. The order of concentration from high to low was SiH3, SiH, Si, SiH2. Therefore, the plasma process parameters strongly affect the composition of sihm neutral products.

In addition to the above dissociation and ionization reactions, the secondary reactions between ionic molecules are also very important

SiH2＋+SiH4→SiH3++SiH3 (2.13)

Therefore, in terms of ion concentration, sih3 + is more than sih2 +. It can explain why there are more sih3 + ions than sih2 + ions in SiH4 plasma.

In addition, there will be a molecular atom collision reaction in which the hydrogen atoms in the plasma capture the hydrogen in SiH4

H+ SiH4→SiH3+H2 (2.14)

It is an exothermic reaction and a precursor for the formation of si2h6. Of course, these groups are not only in the ground state, but also excited to the excited state in the plasma. The emission spectra of silane plasma show that there are optically admissible transition excited states of Si, SIH, h, and vibrational excited states of SiH2, SiH3