Alexander P. Palov

Interaction of F atoms with SiOCH ultra low-k films. Part II: etching

The etch mechanism of porous SiOCH-based low-k films by F atoms is studied. Five types of ultra-low-k (ULK) SiOCH films with k-values from 1.8 to 2.5 are exposed to F atoms in the far downstream of an SF6 inductively coupled plasma discharge. The evolution of etching with an F dose was studied using various techniques of surface and material analysis such as FTIR, XPS, EDS and SE. It is revealed that the etch mechanism is connected with surface fluorination and formation of –CHxFy species on the surface due to H abstraction by F atoms from –CH3 groups. It is shown that the etching includes two phases. The first one is observed at the low F doses and is connected with chemical modification and etching of walls in the topmost pores, which finishes when the walls are fully etched. At the same time, the additional etching in the underlying pores due to F penetration forms the etch depth profile, after that the second etching phase starts. This phase is characterized by the higher etch rate due to the propagation of the etch depth profile further into the film. The preliminary treatment of pore walls inside porous channels effectively accelerates etching many times compared to non-porous material. The acceleration depends on the modification depth, which in turn is a function of pore structure and interconnectivity as well as the F atom reaction mechanism. The combined random walk (Monte-Carlo) & kinetics model developed to describe F penetration inside SiOCH films together with reactions of F atoms leading to –CHxFy depletion and opening SiOx bonds for F access allowed relating the increased etch rates with increasing the total number of F atom collisions inside interconnected pores. The etch mechanism of SiOCH films is found in many respects to be similar to the SiO2 etch mechanism on the elementary level, but as whole it is ruled by the SiOCH structure: porosity degree, pore size, pore interconnectivity as well as structural features of SiOx bonds.

Multi-step reactions mechanism for F atoms interactions with organosilicate glass and SiOx films

An ab initio approach with the density functional theory (DFT) method was used to study F atom interactions with organosilicate glass (OSG)-based low-k dielectric films. Because of the complexity and significant modifications of the OSG surface structure during the interaction with radicals and etching, a variety of reactions between the surface groups and thermal F atoms can happen. For OSG film etching and damage, we propose a multi-step mechanism based on DFT static and dynamic simulations, which is consistent with the previously reported experimental observations. The important part of the proposed mechanism is the formation of pentavalent Si atoms on the OSG surface due to a quasi-chemisorption of the incident F atoms. The revealed mechanism of F atom incorporation into the OSG matrix explains the experimentally observed phenomena of fast fluorination without significant modification of the chemical structure. We demonstrate that the pentavalent Si states induce the weakening of adjacent Si–O bonds and their breaking under F atom flux. The calculated results allow us to propose a set of elementary chemical reactions of successive removal of CH3 and CH2 groups and fluorinated SiO x matrix etching.

Dependence of dielectric constant of SiOCH low-k films on porosity and pore size

A simple, clear, and robust numerical approach to calculate dielectric constant of porous organosilicate (SiOCH) based films with arbitrary shaped pores is proposed. The calculations are based on modified Clausius–Mossotti equation and can be applied for the films with wide range of porosity (0.01–0.96) and pore size (0.5–5 nm). The dielectric constants calculated in assumption of preferential localization of CH3 groups on pore wall are in good agreement with the experimentally measured k-values. The advantage of the proposed calculation model is ability to analyze the dependence of dielectric constant on pore size.

Charging and the secondary electron–electron emission on a trench surface: broadening and shift of ion energy spectrum at plasma trench etching

Trench surface charging at the plasma etching of dielectrics and semiconductors is a negative phenomenon because it leads to non-uniform etching of the trench bottom, undesirable etching of its wall, etch stop and breakdown of lower level device elements. To investigate the charging of a SiO2 trench surface by argon radio frequency discharge plasma we applied the 3D Monte Carlo method for modelling the electron and ion trajectories inside a trench and used the 2D analytical method to calculate electric fields and potentials produced by the deposited charges. The secondary electron–electron emission was taken into account as a really important mechanism of electrical charge redistribution on the trench surface. The ion energy spectra were calculated for the trench aspect ratios (depth d/width w) of 1–20 and trench widths of 11, 22 and 45 nm for 180 eV ion flux. The transformation of an initial ion energy spectrum from a delta function at 180 eV into bell-shaped curves with peak shifts of 10–60 eV and broadening of 5–30 eV is obtained.

Effect of porosity and pore size on dielectric constant of organosilicate based low-k films : an analytical approach

An analytical approach allowing to analyze effect of porosity, pore size, and interconnectivity on dielectric constant of organosilicate based low-k materials is developed. Within the framework of this approach, a good agreement between the calculated and experimentally measured dielectric constants for several porogen (template) based organosilicate glasses low-k films is demonstrated. It is shown that the best agreement between the calculated and measured k-values corresponds to low-k structure with CH3 groups localized on pore wall surface. The results also demonstrate a good agreement with recently published results of similar analysis based on numerical approach.

Charging of submicron structures during silicon dioxide etching in one- and two-frequency gas discharges

A model that combines the Monte Carlo method for calculating electron and ion trajectories in three-dimensional geometry and an analytic approach developed for calculating an electric field in two-dimensional geometry is used to simulate the charging of the surface of periodic submicron SiO2 structures by electron and ion fluxes in the plasma of a one- and a two-frequency capacitive RF discharge. The energy distribution function of the electrons and ions that come to the bottom of a submicron structure in an argon and an argon-containing plasma is calculated for structures with a width of 11–45 nm and an aspect ratio of d/w = 1–10 (where d and w are the depth and width of the structure). It is shown that secondary electronelectron emission plays an important role in the redistribution of the electric charge and, accordingly, of the electric potential in a submicron structure. It is demonstrated that, when the secondary electron-electron emission mechanism is taken into account, the ion energy spectrum at the bottom of a submicron structure is shifted toward lower energies and becomes broader in comparison with the spectrum of an ion flux from an RF discharge plasma. Moreover, the shift and broadening depend only on the secondary electron-electron emission coefficient, the energy of the charged particles, and the aspect ratio.

Simulation of optical properties of silicon solar cells textured with penetrating V-shaped grooves

The coefficients of reflection (R), transmission (T), and absorption (A) of light for two wavelengths λ = 1000 and 1100 nm for silicon wafers that have thicknesses t = 50, 100, and 200 μm and are textured with penetrating V-shaped grooves with various geometries have been calculated; the half-width of groove’s base w (10, 20, and 30 μm) and the depth of the groove d (0 ≤ d ≤ t) have been varied. In the case of an increase in the aspect ratio d/w (in the case of λ = 1100 nm), the absorption curve A(d/w) monotonically ascends from 6.6 to 67.6%, whereas, for λ = 1000 nm, a nontrivial dependence A(d/w) is observed: the absorption coefficient first increases to 54%, attains then a maximum of 97% at d/w = 3, and then decreases at d > t/2 for all values of w. This effect of a decrease in absorption with an increase in d/w distinguishes texturing with penetrating grooves from conventional surface texturing. Distributions of angles of deviations of photons in the plane of bottoms of grooves are obtained; these distributions are represented by a set of δ-type functions.

Mixing of post-discharge O2/He flow with NO2/He flow: 3D modeling of experimental data

Flow mixing and chemical kinetics in the afterglow of an electro-discharge source of singlet oxygen O2(a1&Dgr;g) are studied theoretically using both a simple analytical approach and an advanced three-dimensional (3D) simulation with the FLUENT code. Calculated results are compared with titration measurements of atomic oxygen flow in the discharge oxygen-iodine laser (DOIL) system in which the first positive gain and continuous-wave laser oscillation were demonstrated. It was shown that atomic oxygen atoms in rf post-discharge O2/He mixtures effectively deplete O2(a1&Dgr;g) molecules through quenching of the excited iodine atoms. Thus, the precise technique for O atom flow measurements and methods of removing O atoms are of importance for DOIL operation. One technique is through the ad-mixing of NO2 to the post-discharge flow which allows control of the O atom flow. Theoretical predictions of NO2* production (and equivalently the emission), using both an analytical solution of a simplified system of equations and thorough 3D modeling of the reactive mixing flows, are compared with experimental measurements of NO2* emission. Analysis of 3D calculated distributions of species concentrations and flow velocities shows how titration measurements should be calibrated correctly and validates the use of a simple analytical approach for the interpretation of titration measurements. It is recommended that NO2 titrations for the flow conditions near to those examined use the value obtained from fully extinguishing the NO2* emission.

Etching low-k films by F atoms: Inside view

The multistep reactions mechanism of F atoms interaction with SiOCH low-κ dielectric films, developed on the base of the measured evolution of various surface groups (e.g., Si-CH3) and systematic density functional theory quantum mechanical calculations, was incorporated into the three-dimensional Monte Carlo model of the damage and etching processes. The model is realized on model maps of porous films and allows us to obtain dynamic 3D images of etching porous films and a layer by layer distribution of components that are formed during the etching. Comparison of calculated etching rates of SiOx matrix by fluorine atoms with the experimental data is used to determine the effective etching probabilities (reciprocal values of F atoms collisions with SiOx matrix groups that are required to remove one of them). The detailed space-resolved dynamics of damage and etching processes of low-κ films with different parameters (porosity, pore, and interpore channels sizes, dielectric permittivity) was obtained and di...

Sputtering of Si by Ar: A binary collision approach based on quantum-mechanical cross sections

A new binary collision approach for the calculation of the sputtering yield of Si under nonreactive ionic bombardment by Ar+ is presented for the energy range from threshold to 200 eV. Unlike conventional Monte Carlo approaches that use a classical calculation of the scattering angle from a known potential, their approach employs quantum-mechanical methods to compute the scattering angle. Comparison of the energy and angular dependence of sputtering yields computed using their new quantum-based method with experimental data and with transport of ions in matter (TRIM) and molecular dynamics (MD) calculations supports the accuracy and usefulness of their approach. It is shown that their new approach leads to results of an accuracy intermediate between that of the TRIM and MD methods. The authors expect the new approach to be useful in plasma processing applications.A new binary collision approach for the calculation of the sputtering yield of Si under nonreactive ionic bombardment by Ar+ is presented for the energy range from threshold to 200 eV. Unlike conventional Monte Carlo approaches that use a classical calculation of the scattering angle from a known potential, their approach employs quantum-mechanical methods to compute the scattering angle. Comparison of the energy and angular dependence of sputtering yields computed using their new quantum-based method with experimental data and with transport of ions in matter (TRIM) and molecular dynamics (MD) calculations supports the accuracy and usefulness of their approach. It is shown that their new approach leads to results of an accuracy intermediate between that of the TRIM and MD methods. The authors expect the new approach to be useful in plasma processing applications.