Silicon nitride thin film - the most important dielectric material in the manufacture of integrated circuits

Silicon nitride (Si3N4) thin films are widely used as dielectric materials.  As amorphous insulating material, dielectric properties of silicon nitride film is better than that of silica membranes, have to the movable ions blocking ability strong, compact structure, small pinhole density, good chemical stability, high dielectric constant, etc. In the field of integrated circuit manufacturing is widely used for surface passivation coating, insulation, impurity diffusion etching mask mask, as well as the surface of the semiconductor encapsulation and so on.  

In addition, the silicon nitride film has good photoelectric properties, passivation properties and water vapor permeability resistance.  In silicon based solar cells, silicon nitride film can be used as anti-reflection film, which plays a role of surface passivation and internal passivation at the same time, so as to improve the conversion efficiency of solar cells.  

Therefore, more and more attention has been paid to the preparation technology, composition, structure and properties of silicon nitride thin films.  At present, there are several methods to prepare silicon nitride thin films, such as direct nitride, physical vapor deposition and chemical vapor deposition.  

Methods to prepare silicon nitride thin films

01 Direct nitriding  

Direct nitriding method is the simplest method to prepare silicon nitride film, that is, put silicon in nitriding atmosphere and heat it to a certain temperature, so that the silicon and nitriding atmosphere react and generate a layer of silicon nitride film on the silicon surface.  Commonly used nitriding gases are NH3, NO, N2, etc.  

The biggest characteristics of the preparation of silicon nitride by direct nitriding method are as follows: once the silicon nitride film is formed on the surface, the nitrogen and silicon are isolated, which reduces the reaction speed. Therefore, the silicon nitride film is prepared by direct nitriding, and the film thickness is generally less than 10nm.  

The advantages of this method are that the silicon nitride film is compact and stable compared with other methods, with good stoichiometry and less hydrogen content. However, high temperature will cause impurity redistribution in the substrate, resulting in stacking errors, thus reducing the performance of the equipment.  

02  Physical Vapor deposition (PVD)  

1. Vacuum evaporation coating  

The substrate or workpiece to be coated is placed in a high vacuum chamber, and the evaporating material is vaporized (or sublimated) by heating and deposited on the surface of the substrate or workpiece at a certain temperature, thus forming a layer of thin film. This process is called vacuum evaporation coating.  Film formation in high vacuum environment can prevent pollution and oxidation of the film, easy to get clean, compact, in line with the predetermined requirements of the film.  The limitation of this method is that the refractory metal is difficult to make at low vapor pressure.  

2. Magnetron reactive sputtering method  

Sputtering, a physical phenomenon discovered by Grove as early as 1852, is the bombardment of a solid surface by charged particles, causing solid atoms (or their molecules) to eject from the surface.  But it wasn't until the 1920s that Sputtering was developed as a thin-film deposition technique by Langmuir.  The usual sputtering method sputtering efficiency is not high, the working pressure required by sputtering is higher, the possibility of gas molecules contaminating the film is higher.  

In order to improve these two shortcomings, a magnetic field parallel to the surface of the cathode can be added to restrict the movement of the initial electron to the area adjacent to the cathode, so as to increase the ionization efficiency of gas atoms and improve the sputtering rate. This sputtering method is magnetron sputtering.  

Magnetron reactive sputtering is characterized by: the application of magnetron sputtering technology, can sputter all metal and semiconductor materials with a certain heat resistance;  Easy to use, easy to control the operation, coating process through accurate control of air pressure, power and time sputtering conditions, can obtain a relatively stable deposition rate, can deposit the required thickness of the film;  Easy to organize mass production.  

03 Chemical vapor deposition (CVD)  

The chemical vapor deposition method supplies the substrate with the gas containing the elements of the thin film, and uses the energy such as heating, plasma and ultraviolet light to produce the chemical reaction to deposit the thin film.  The commonly used CVD methods are as follows:  

1. Atmospheric Chemical Vapor deposition (APVCD)  

Atmospheric chemical vapor deposition is in atmospheric pressure environment, the reaction gas is heated by N2 or Ar inert gas transported to the heated high temperature substrate, through combination reaction or thermal decomposition to generate solid film.  

As the reaction is carried out under atmospheric pressure, various by-products will be produced at the same time as the film material is formed.  Under atmospheric pressure, the diffusion rate of molecules is low, and the by-products cannot be discharged in time, which not only limits the deposition rate, but also increases the possibility of film contamination, leading to the degradation of film quality.  It has been gradually replaced by the later low pressure chemical vapor deposition and plasma enhanced chemical vapor deposition.  

2. Low pressure chemical vapor deposition (LPCVD)  

LPCVD was developed on the basis of APCVD.  LPCVD overcomes the shortcomings of APCVD, such as low deposition rate and serious film pollution, so the silicon nitride film prepared by LPCVD has good uniformity, fewer defects and high quality.  And can be deposited on a large number of substrates at the same time, easy to achieve automation, high efficiency, has become the semiconductor industry to prepare silicon nitride film main method.  

LPCVD activates the reaction gas with heat. In order to ensure the complete reaction, the reaction temperature is high, generally above 700℃.  At this temperature, the film has good stoichiometry and dense film layer, so the film performance is also better.  But on the other hand, high temperature has a high demand on the substrate, the substrate is easy to deform, and the defects will grow and spread, thus affecting the interface performance.

3. Plasma Enhanced Chemical Vapor Deposition (PECVD)  

Plasma enhanced chemical vapor deposition (CVD) is a kind of technology in which a certain gaseous substance is ionized by radio frequency to form plasma and promote chemical reaction to deposit thin film materials.  Because the technology produces coupling resonance through high-frequency electromagnetic induction and covalent bond of gas molecules, ionizing them, significantly reducing the reaction temperature, increasing the reaction rate and improving the quality of film formation.  

The method has the advantages of simple equipment, good adhesion between substrate and film, good uniformity and repeatability of film formation.  At the same time, lower deposition temperature is beneficial to achieve smaller distortion, better conformation and faster deposition rate.  The silicon nitride films prepared by PECVD are characterized by high strength, high hardness, high dielectric constant, adjustable refractive index, high transmittance, small optical attenuation coefficient and good chemical stability.