George Kokkoris

Controlling roughness: from etching to nanotexturing and plasma-directed organization on organic and inorganic materials

We describe how plasma–wall interactions in etching plasmas lead to either random roughening/nanotexturing of polymeric and silicon surfaces, or formation of organized nanostructures on such surfaces. We conduct carefully designed experiments of plasma–wall interactions to understand the causes of both phenomena, and present Monte Carlo simulation results confirming the experiments. We discuss emerging applications in wetting and optical property control, protein immobilisation, microfluidics and lab-on-a-chip fabrication and modification, and cost-effective silicon mould fabrication. We conclude with an outlook on the plasma reactor future designs to take advantage of the observed phenomena for new micro- and nanomanufacturing processes, and new contributions to plasma nanoassembly.

Etching of SiO2 and Si in fluorocarbon plasmas: A detailed surface model accounting for etching and deposition

A surface model is presented for the etching of silicon (Si) and silicon dioxide (SiO2) in fluorocarbon plasmas. Etching and deposition are accounted for using a generalized concept for the “polymer surface coverage,” which is found to be equivalent to a normalized fluorocarbon film thickness covering the etched surfaces. The model coefficients are obtained from fits to available beam experimental data, while the model results are successfully compared with high-density plasma etching data.

A global model for C4F8 plasmas coupling gas phase and wall surface reaction kinetics

A global or zero-dimensional model for C4F8 plasmas is formulated by coupling gas phase and wall surface reaction kinetics. A set of surface reactions implements experimental findings and quantifies the effect of the fluorocarbon film formed on the reactor walls on the densities of species in the gas phase. The model allows the calculation of the pressure change after the ignition of the discharge and the effective sticking (surface loss) coefficients of the neutral species on the wall surface. The model is validated by comparison with experimental measurements, i.e. pressure rise and densities of F atoms, CF2 and CF radicals, in an inductively coupled plasma reactor. It is predicted that C4F8 is vastly dissociated and CF4 becomes the dominant species even at low power conditions. A net production of CF3 radical and a net consumption of CF2 radical at the reactor walls are predicted. A study on the contribution of each reaction to the production and consumption of the species shows that at least one surface reaction is among the major sinks or sources of CF4, CFx radicals and F.

Simulation of SiO2 and Si feature etching for microelectronics and microelectromechanical systems fabrication: A combined simulator coupling modules of surface etching, local flux calculation, and profile evolution

A combined simulator linking gas flux data from a plasma reactor (experimental or simulated) to the feature profile evolution during etching/deposition processes is described. This combined simulator results from the coupling of surface etch, local flux calculation, and profile evolution modules. It is a modular tool, in the sense that different phenomena, surface models, and structures can be incorporated. In this work the combined simulator is applied in two processes: (a) in SiO2 feature etching to simulate reactive ion etching (RIE) lag and inverse RIE lag and (b) in etching of high aspect ratio Si trenches using the Bosch process. The profile evolution algorithm of the combined simulator, namely, the level set method, is applied to a process where etching and deposition occur, and tracking of two materials (Si and polymer formed during the Bosch deposition step) is needed.

A global model for SF6 plasmas coupling reaction kinetics in the gas phase and on the surface of the reactor walls

Gas phase and reactor wall-surface kinetics are coupled in a global model for SF6 plasmas. A complete set of gas phase and surface reactions is formulated. The rate coefficients of the electron impact reactions are based on pertinent cross section data from the literature, which are integrated over a Druyvesteyn electron energy distribution function. The rate coefficients of the surface reactions are adjustable parameters and are calculated by fitting the model to experimental data from an inductively coupled plasma reactor, i.e. F atom density and pressure change after the ignition of the discharge. The model predicts that SF6, F, F2 and SF4 are the dominant neutral species while and F? are the dominant ions. The fit sheds light on the interaction between the gas phase and the reactor walls. A loss mechanism for SFx radicals by deposition of a fluoro-sulfur film on the reactor walls is needed to predict the experimental data. It is found that there is a net production of SF5, F2 and SF6, and a net consumption of F, SF3 and SF4 on the reactor walls. Surface reactions as well as reactions between neutral species in the gas phase are found to be important sources and sinks of the neutral species.

Integrated framework for the flux calculation of neutral species inside trenches and holes during plasma etching

An integrated framework for the neutral flux calculation inside trenches and holes during plasma etching is described, and a comparison between the two types of structure in a number of applications is presented. First, a detailed and functional set of equations for the neutral and ion flux calculations inside long trenches and holes with cylindrical symmetry is explicitly formulated. This set is based on early works [T. S. Cale and G. B. Raupp, J. Vac. Sci. Technol. B 8, 1242 (1990); V. K. Singh et al., J. Vac. Sci. Technol. B 10, 1091 (1992)], and includes new equations for the case of holes with cylindrical symmetry. Second, a method for the solution of the respective numerical task, i.e., one or a set of linear or nonlinear integral equations, is described. This method includes a coupling algorithm with a surface chemistry model and resolves the singularity problem of the integral equations. Third, the fluxes inside trenches and holes are compared. The flux from reemission is the major portion of the ...

Etching of SiO2 features in fluorocarbon plasmas: Explanation and prediction of gas-phase-composition effects on aspect ratio dependent phenomena in trenches

A model to calculate etching rates in SiO2 features in fluorocarbon plasmas is presented. The model can predict several aspect ratio dependent phenomena such as reactive ion etching (RIE) lag, etch stop, inverse RIE lag, and aspect ratio independent etching (ARIE) at least for a limited range of aspect ratio values. The model includes three components: (a) a surface model for open area etching of SiO2 (and Si) [Gogolides et al., J. Appl. Phys. 88, 5570 (2000)]; (b) a flux calculator, which calculates local fluxes on each elementary surface of the feature being etched; and (c) a coupling of the two models (a) and (b), the focal point of coupling being the simultaneous calculation of the neutral species fluxes and the corresponding effective sticking coefficients. The model is applied for trench etching and the gas phase conditions considered correspond to a generic fluorocarbon gas. A different approach is presented by which the gas phase composition is divided (i.e., mapped) into regions leading to (a) de...

Characterization and global modelling of low-pressure hydrogen-based RF plasmas suitable for surface cleaning processes

In this paper we present results of measurements and global modelling of low-pressure inductively coupled H2 plasma which is suitable for surface cleaning applications. The plasma is ignited at 1 Pa in a helicon-type reactor and is characterized using optical emission measurements (optical actinometry) and electrical measurements, namely Langmuir and catalytic probe. By comparing catalytic probe data obtained at the centre of the chamber with optical actinometry results, an approximate calibration of the actinometry method as a semi-quantititative measure of H density was achieved. Coefficients for conversion of actinometric ratios to H densities are tabulated and provided. The approximate validity region of the simple actinometry formula for low-pressure H2 plasma is discussed in the online supplementary data (stacks.iop.org/JPhysD/46/475206/mmedia). Best agreement with catalytic probe results was obtained for (Hβ, Ar750) and (Hβ, Ar811) actinometric line pairs. Additionally, concentrations of electrons and ions as well as plasma potential, electron temperature and ion fluxes were measured in the chamber centre at different plasma powers using a Langmuir probe. Moreover, a global model of an inductively coupled plasma was formulated using a compiled reaction set for H2/Ar gas mixture. The model results compared reasonably well with the results on H atom and charge particle densities and a sensitivity analysis of important input parameters was conducted. The influence of the surface recombination, ionization, and dissociation coefficients, and the ion–neutral collision cross-section on model results was demonstrated.

Minimum energy paths of wetting transitions on grooved surfaces.

A method that computes minimum energy paths (MEPs) of wetting transitions is developed. The method couples the Cahn-Hilliard formulation of a modified phase-field method with the simplified string method. Its main computational kernel is the fast Fourier transform that is efficiently performed on graphics processing units. The effectiveness of the proposed method is demonstrated on two types of transitions of droplets on grooved surfaces. The first is the transition from the Cassie-Baxter wetting state to the Wenzel state, where it is shown that it progresses in a sequential manner with the droplet wetting each groove successively. The second transition type is a lateral displacement of the droplet against the grooves, where the droplet successively detaches/attaches from/to the rear/front protrusion of the surface (a transition in the reverse order is also possible). The energy barriers of both the transitions are extracted from the MEP; they are useful for the evaluation of the robustness of superhydrophobic surfaces (resistance to the Cassie-Baxter to Wenzel transition) and the droplet mobility on those surfaces (high mobility/small resistance to lateral displacements). The relation of the MEP with the potential transition paths coming from the solution space mapping is discussed.

Line-edge-roughness transfer during plasma etching: modeling approaches and comparison with experimental results

Both modeling and experimental results for the effects of plasma etching on photoresist line edge roughness and linewidth roughness (LER/LWR) and their transfer to underlayer films are presented and compared. In particular, we investigate the roughness formation on both photoresist and underlayer sidewalls during (1) isotropic trimming of photoresist, and (2) anisotropic plasma etching and LER transfer to substrate. The trimming process is modeled with an isotropic movement of the resist sidewall. In the anisotropic plasma etching process, the resist sidewall is used as a mask to anisotropically transfer the pattern to the underlying film. Experiments include trimming of a photoresist patterned with 193-nm photolithography in O2 plasma with no bias voltage and anisotropic etching of BARC and Si underlayers in CF4 and HBr/Cl2/O2 with bias. Both model and experimental results show that resist trimming causes reduction of resist LER and increase of the correlation length and roughness exponent with trimming time. This means that surface nano-protrusions versus trimming time become shorter and wider. In the case of anisotropic etching, the model predicts noticeable reduction of LWR, whereas correlation length and roughness exponent remain almost unaffected. The first experimental results seem to confirm these predictions.