APPLICATION OF THE COMBINED OPTICAL METHOD FOR THE STUDY OF SILICON NITRIDE THIN FILMS ON THE SURFACE OF MONOCRYSTALLINE SILICON

Abstract

In this work, the combined optical technique is used to study the structures of a-SiN_x:H – Si. Thin films of silicon nitride, a-SiN_x:H, were obtained by chemical vapor deposition (CVD). The films were deposited on substrates of single-crystal Si at a temperature of ≈1000 ° C from a gas mixture of silane, SiH₄, and ammonia, NH₃, and the working carrier-gas N₂. After precipitation, the films were annealed to reduce the concentration of bound hydrogen. The thickness range of the studied films of silicon nitride was 100-140 nm. Ellipsometric measurements were performed on a laser ellipsometer LEF-3M-1 (λ = 632.8 nm). The reflection spectra of the investigated films were measured on a Specord M40 spectrophotometer in the spectral range of 185-900 nm. The results of studies of a-SiN_x:H – Si structures make it possible, first, to somewhat refine the combined optical method for monitoring the physical parameters of thin dielectric films on the surface of single-crystal silicon. As in its original version, this method consists of two stages. In the first stage, ellipsometric measurements of dielectric films on the surface of semiconductor substrates are performed with the obligatory use of a multi-angle measurement technique. This approach allows us to confidently answer the question of the applicability of the model "transparent optically isotropic film - absorbing optically isotropic substrate" to the obtained in the technological process structures "film-substrate", as well as unambiguously and accurately determine the film thickness. In the second stage, the reflection spectra of the dielectric films are measured in order to obtain a clear interference pattern. Secondly, on the basis of the measurements carried out in this work, it is safe to say that this technique can be effectively applied to Si₃N₄ films with a thickness of d > 80 nm. In addition, the results of ellipsometric measurements of the dynamics of change of refractive index n₁ from film thickness d, performed on the inverse surface of the silicon wafer partially covered with an inhomogeneous silicon nitride film, confirmed the conclusion that the chemical composition of the film directly depends on the gas mixture pressure in the chemical deposition process. In particular, it is shown that the decrease in the pressure of the gas mixture leads to the deposition of a-SiN_x:H films with a deviation from the stoichiometry in the direction of increasing the molar fraction of nitrogen N (x > 4/3), and possibly hydrogen H. It should be emphasized that the main emphasis in our work is on the possibility of effective use of the proposed combined optical technique for microchip technology. Perhaps some of our conclusions about the chemical composition of films and the dynamics of its change during deposition can be the subject of discussion. However, this, in our opinion, does not reduce the effectiveness of the application of this method in the technological process of manufacturing microchips. Detailed knowledge of all process parameters (such as chemical composition and gas mixture pressure, flow rate, time and temperature of film deposition) in combination with this control technique allows to effectively optimize the technological process of dielectric film deposition.

Key words: thin dielectric films, ellipsometry, interference, dispersion of the refractive index, silicon nitride