Paper PS2-TuA3
Simulation of Bosch Process Deep Silicon Etching - A Multi-scale Approach

Tuesday, October 29, 2013, 2:40 pm, Room 104 C

Deep etching of silicon is a very used process in the semi-conductor industry. Such high aspect ratio etchings can be obtained using Bosch process. This plasma process consists in alternating many etching and deposition steps at high etch rate (few seconds each). Two gases are mainly used: SF₆ for the etching step and C₄F₈ for the passivation.

To predict the silicon etch profile through the mask, we have developed an etching Bosch simulator. It permits to investigate plasma/surface interactions for both the deposition and the etching step. Based on a multi-scale approach the silicon etching simulator is composed of three modules: plasma, sheath and surface models. This allows to predict the etch profile as a function of the operating conditions (pressure, power, gas flow rates, time steps for deposition and etching cycles).

The plasma module is based on a global kinetic model which allows the calculation of the neutral and ion densities and fluxes as well as the electronic temperature and density using the machine parameter of the ICP reactor as input parameters.

The sheath module is based on the Monte-Carlo technique to calculate the Ion Angular and Energetic Distribution Functions (IAEDFs). Positive ion fluxes and electronic temperature and density calculated from de plasma module are used as input parameters in the evaluation of average sheath thickness and proportion of each considered positive ion.

The fluxes of neutral and ion species calculated from the plasma module and the IAEDF calculated from the sheath model are introduced as input parameters in the surface model. This model is based on the cellular Monte-Carlo method to describe the 2D etch profile through the mask and evaluate the etch rate evolution with time. Both the silicon substrate and the mask are discretized in uniform cells named super-sites. Each super-site contains a number of real atomic silicon sites which depends on the cell size. In our etching simulations, 1nm by 1nm cells are considered. This allows a good compromise between computing time and spatial resolution. The particles used are atomic fluorine for the reactive etching, atomic carbon, CF and CF₂ for the passivation polymer growth, and positive ions for the sputtering.

The simulation results show the influence of the input Bosch process parameters (RF powers, pressure, gas flow rates, time steps for deposition and etching cycles and bias voltage) on the etching processes like the etch rate and the deep etch profile evolution with etch time. Such an etching simulation tool can contribute to improve the deep silicon etching processes in terms of anisotropy and scalloping reduction.