Prediction of plasma-induced damage distribution during silicon nitride etching using advanced three-dimensional voxel model

Abstract

The authors modeled SiN film etching with hydrofluorocarbon (CHxFy/Ar/O2) plasma considering physical (ion bombardment) and chemical reactions in detail, including the reactivity of radicals (C, F, O, N, and H), the area ratio of Si dangling bonds, the outflux of N and H, the dependence of the H/N ratio on the polymer layer, and generation of by-products (HCN, C2N2, NH, HF, OH, and CH, in addition to CO, CF2, SiF2, and SiF4) as ion assistance process parameters for the first time. The model was consistent with the measured C-F polymer layer thickness, etch rate, and selectivity dependence on process variation for SiN, SiO2, and Si film etching. To analyze the three-dimensional (3D) damage distribution affected by the etched profile, the authors developed an advanced 3D voxel model that can predict the time-evolution of the etched profile and damage distribution. The model includes some new concepts for gas transportation in the pattern using a fluid model and the property of voxels called “smart voxels,” which contain details of the history of the etching situation. Using this 3D model, the authors demonstrated metal–oxide–semiconductor field-effect transistor SiN side-wall etching that consisted of the main-etch step with CF4/Ar/O2 plasma and an over-etch step with CH3F/Ar/O2 plasma under the assumption of a realistic process and pattern size. A large amount of Si damage induced by irradiated hydrogen occurred in the source/drain region, a Si recess depth of 5 nm was generated, and the dislocated Si was distributed in a 10 nm deeper region than the Si recess, which was consistent with experimental data for a capacitively coupled plasma. An especially large amount of Si damage was also found at the bottom edge region of the metal–oxide–semiconductor field-effect transistors. Furthermore, our simulation results for bulk fin-type field-effect transistor side-wall etching showed that the Si fin (source/drain region) was directly damaged by high energy hydrogen and had local variations in the damage distribution, which may lead to a shift in the threshold voltage and the off-state leakage current. Therefore, side-wall etching and ion implantation processes must be carefully designed by considering the Si damage distribution to achieve low damage and high transistor performance for complementary metal–oxide–semiconductor devices.

Advanced simulation technology for etching process design for CMOS device applications

“…It is also etched by H and N, which are generated from the reactive layer as the outfluxes of À ER H and À ER N , respectively, whereas molecules such as CH, HCN, HF, CF 2 , C 2 N 2 , OH, NH, and CO are generated as by-products during etching. The existence of these molecules is indicated by the measured OES data, 115) previously reported experimental results, [118][119][120][121][122] and classical MD calculations. 29,33,35,123,124) The effect of the H=N ratio in the polymer layer is a key factor in the accurate reproduction of the experimental data, and the three surface models proposed for the polymer layer are dependent on the H=N ratio.…”

“…16. By using a voxel model 113) connected to the slab model, 114) which is then called the voxel-slab model, a simulation technology for the SiN SW etching process with new concepts for gas transportation, detailed surface reactions that are dependent on the H=N ratio, and etch front evolution has recently been reported; 115) this technology can calculate the etched profile and damage distribution simultaneously in a 3D space with a short calculation run time. Figure 17 shows a schematic image of the calculation flow of the 3D voxelslab model described above.…”

“…(17) or (18). In contrast, the 3D ion flux can be derived from the extended 2D model based on the works of Tuda et al and Kuboi et al,44,115) depending on the incident ion energy.…”

“…i and j are used for voxel identification and slab depth, respectively. The physical damage to the SiN film, D a , is expressed simply using the θ value and neglecting the specific types of defects (interstitial Si, substitutional Si, vacancy, and complex), which are given by 115) D a ði; j; tÞ ¼ L Ã ði; jÞ Â ði; j; tÞ; ð22Þ L Ã ði; jÞ ¼ ERði; j; tÞ ERði; tÞ Â ðD P À T CF Þ Â :…”

Optical and electrical characterization methods of plasma-induced damage in silicon nitride films

“…Recently, the formation of the plasma-damaged region in an SiN film has been focused on as a cause of property degradation -plasma-induced damage (PID). Kuboi et al reported 4) that not only the topological feature change but also the formation of damaged regions is induced in SiN films by plasma exposure. In general, PID is classified on the basis of mechanisms such as "charging damage", "radiation damage", and "physical damage".…”