Investigation of the side walls roughness effect of the Si3N4 lightguide layer of different thickness on optical losses in an integral waveguide formed on a quartz substrate
This article presents results of the influence of the side walls roughness of the 100 nm and 200 nm silicon nitride. Calculation of the main parameters of the side walls roughness of the lightguide layer, which have the greatest effect on the optical loss in the waveguide, carried out by finite difference time domain method (FDTD), is presented. Based on this calculation, the optimal thickness of the lightguide layer of nitride was established, allowing the light flow to be retained. Calculation of the model was based on the data obtained during the study of SEM images of manufactured waveguide structures. The results of these calculations are consistent with the data obtained using optical frequency domain reflectometry (OFDR) in the optical backscatter reflectometer (OBR) of manufactured waveguides with a thickness of silicon nitride 200 nm and width of 3 µm and 8 µm.
Recently, the rapidly growing pace of development of modern information processing systems has given rise to a trend towards improving the methods of their implementation. The ever-growing amount of data transmitted over fiber-optic communication lines has been the driving force behind extensive research and development in photonic integrated circuit (PIC) technology . Compared to fiber optic systems, photonic integrated circuits can provide improved performance and stability with a smaller footprint and lower cost . The use of PIC allows you to solve important problems in the field of data transmission, using the scale of integration to reduce the cost, occupied space and power, and increase productivity . Although optical communication is one of the main applications and the main driver of photon integration, active use of PIC can also be found in such applications as sensing , spectroscopy , Raman spectroscopy , optical coherence tomography , biochemical sensors [8–10] and the defense complex . One of the significant advantages of PIC is its compatibility with standard silicon technologies. This makes it possible to manufacture silicon photonics products using standard technological processes within existing production sites, predominantly loaded with the production of electronic integrated circuits. This approach allows you to minimize possible financial costs associated with the launch of specialized production .
Integral optical waveguide structures (IWFs), where Si, SiO2, and Si3N4 are used as the fiber layer, are optimal from the point of view of compatibility with silicon technologies. While the main materials of the integrated technology – silicon and silicon dioxide – usually suffer from high losses or delocalized optical modes, Si3N4 provides the advantages of both high luminous flux retention and high Q factor [14–18]. In addition, silicon nitride has a number of nonlinear properties, such as parametric amplification , broadband supercontinuum generation , and transparency in the near infrared and visible spectral regions . These properties provide a wide range of new possibilities for using silicon nitride in FIS.
Thus, the substantiation of the choice of the 200 nm silicon nitride fiber layer thickness for the fabrication of symmetric waveguide structures with a silicon oxide cladding is one of the urgent problems at the initial stage of creating a PIC. Therefore, on the basis of the previously described method for calculating the influence of various roughness parameters , we present new results of studying the magnitude of optical losses for waveguides of different thicknesses (100 nm and 200 nm).
RESEARCH METHODS AND RESULTS
As a result of a series of experiments, in the course of which films of various thicknesses of deposited silicon nitride LPCVD were formed on silicon and quartz substrates, it was concluded that these films have high optical quality. Low stress silicon nitride films can be grown using Plasma Enhanced Chemical Vapor Deposition (PECVD) and Modified Low Pressure Chemical Vapor Deposition (LPCVD) processes, but these films result in higher absorption levels caused by dangling bonds of H and O with Si and N in IOBS . In this regard, to determine the optimal thickness of the silicon nitride film, which provides the maximum localization of the light flux, it is necessary to comprehensively study a number of design and technological parameters.
At the first stage of the study, in order to determine the current state of the technological process of the silicon nitride fiber layer formation test samples of symmetric integrated waveguides with a silicon nitride fiber layer on a quartz substrate were made. At all fabrication stages of these waveguides, in order to control the quality of the operations performed, simultaneously with the working structures on quartz substrates, all technological operations were also performed on the control silicon wafers. This made it possible to obtain visual images of the dielectric structure profile using a scanning electron microscope, which are shown in Fig.1 . The roughness data obtained during the analysis of these SEM images formed the basis for the calculation carried out by the finite time difference method.
The process of forming a symmetric integral waveguide as well as a method for calculating the critical parameters of the side wall roughness of a silicon nitride fiber layer were described in detail in . The most significant from the viewpoint of minimizing the effect of light flux scattering, due to the imperfection of the interfaces, is the method of forming a fiber layer of silicon nitride. Therefore, to assess the current state of the technological process of its formation, we analyzed the lateral surface of the silicon nitride walls of similar control structures obtained on silicon wafers in the same process with the samples under study using a scanning electron microscope (SEM). This method of assessment was chosen due to the fact that when studying dielectric structures, which are essentially the waveguides we are describing, in a scanning electron microscope, a charge accumulates in the dielectric layers which causes the frame to be "exposed" and interferes with the analysis. As a result, it was revealed that the roughness of the side wall of the silicon nitride etching profile has a wavy shape with a period of about 30 nm and amplitude of 10–20 nm .
The finite time difference method (FTDM) was used to calculate the effect of the sidewall roughness of the silicon nitride fiber layer on optical loss in a multimode optical waveguide. The data obtained in the study of the fiber layer roughness of the previously fabricated waveguides were taken as a basis. The parameters σ (root-mean-square roughness deviation) and δ (longitudinal roughness size) used for the calculation are clearly shown in Fig. 2 .
To assess reliability of the proposed method, the fabricated integrated waveguides were studied using frequency domain reflectometry (OFDR) of a backscatter reflectometer (OBR). The backscattering method is based on introduction of pulsed optical radiation into the waveguide structure and the subsequent analysis of that small part of the light flux that returns to the receiver as a result of backscattering and reflections of the light wave propagating in the fiber . This method is optimal for solving problems that require, as in our case, a combination of high speed, sensitivity and resolution in the analysis of short and intermediate lengths of transmission lines . The calculated data and the data obtained as a result of evaluating the reflectograms are in good agreement, which allows us to speak about the reliability of the proposed method . In this regard, in order to determine the effect of similar parameters of the roughness of the side walls in a waveguide with a silicon nitride fiber layer thickness of 100 nm, a calculation was performed using the indicated method.
Also, to determine the degree of the light flux localization in the studied waveguides of different widths and of different thicknesses of the fiber layer, the electric field strength distribution was calculated by the finite elemental analysis method. This calculation is necessary to determine the optimal design of the integral waveguide from the viewpoint of keeping the light flux inside the light guide layer, in other words, ensuring the maximum photon lifetime inside the waveguide. This is the most important criterion, in particular, for the implementation of high-Q resonant structures based on such waveguides.
The results of the calculation by the finite time difference method are presented in the summary Table 1, including the values obtained for a waveguide with a light guiding layer of 100 nm and 200 nm thick silicon nitride.
Based on the data obtained, it can be concluded that with a decrease in thickness of the silicon nitride fiber layer, the magnitude of optical losses increases significantly. In fact, we can say that the luminous flux completely passes into the shell. This conclusion is also confirmed by the results of the calculation by the finite element methods, which are clearly shown in Fig.3.
As a result of the calculations, it was found that a decrease in the value of the root-mean-square deviation of the roughness, even if the value of its longitudinal size is preserved, leads to a decrease in the value of optical attenuation in a waveguide with a silicon nitride fiber layer with a thickness of both 200 nm and 100 nm. This confirms the conclusion about the effect of roughness of the side walls of the fiber layer on the value of optical attenuation in the waveguide too. However, in the case of using a waveguide with a 100 nm light guide layer, the luminous flux will not be sufficiently localized regardless of the characteristics of the side walls.
CONCLUSIONS AND ACKNOWLEDGEMENTS
The performed calculations have confirmed the conclusion made earlier in  about a positive effect of reducing the root-mean-square deviation of the sidewall roughness of the Si3N4 fiber layer even if the value of its longitudinal size is preserved on the value of optical losses in the integrated waveguide. At the same time, it was found that a decrease in the thickness of the Si3N4 film, from which the light guide layer is subsequently formed, to 100 nm leads to delocalization of the light flux. This suggests that in order to form integrated waveguides with a minimum optical loss, it is necessary to improve the methods of forming the fiber layer, as well as to pay special attention to its thickness.
This article was prepared with the financial support of the Ministry of Education and Science of the Russian Federation within the framework of the state assignment for 2019 (project No. 0N59-2019-0020) "Theoretical and experimental studies of constructive and technological methods for creating integrated optical elements compatible with silicon technology." When performing the work, the equipment of the collective use center "Functional control and diagnostics of micro- and nanosystem equipment" on the basis of the SMC "Technological Center" was used. ■