The term topography simulation covers all processes dealing with structural changes of the simulated structure by partially or completely removing material layers (etching processes) or by adding new material layers (deposition processes). Within a modern semiconductor fabrication process a lot of etching and deposition steps have to be performed in the front end of line as well as in the backend of line processes and therefore accurate and predictable modelling of these processes are a must. Simulation enables to predict resulting three dimensional structures and eventually voids which must be avoided in voidless filling of deep trenches.
The simulation tools TOPO3D, ELSA2D and ELSA3D can be used for the simulation of topography processes. These simulation tools make use of the cellular technique, empirical techniques and the levelset technique to handle the topological modifications of the simulated structures. TOPO3D is an integrated (coupled with the other process simulation tools) topography process simulation tool which currrently uses the cellular technique and empirical methods, while ELSA2D and ELSA3D are levelset based standalone tools for the simulation of trench filling and etching with advanced kinetic models.
By the levelset technique the material interface is described by the iso-surface with the value zero of the so called levelset function. The levelset function mainly determines the distance of a point in space from the the interface. Providing a distributed speed function which describes the mobility of each point in space the motion of the levelset function and thereby the motion of the material interface can be described by a partial differential equation. The model for the physical process which is simulated has to determine the speed function which is provided to the levelset equation.
For the cellular technique the surface of the simulated structure is discretized by cells, whereby each cell either contains material or non-material. The motion of the surface is performed by cellular morphological operations. This means that so called structuring elements are applied to each surface cell. All cells contained in these structuring elements change their material property (material -> non-material or non-material -> material). The modeled physical process determines the shape and the size of the structuring elements. The interface between material cells and non-material cells defines the surface of the structure after each time step.
Low Pressure PECVD Process Simulation
In a PECVD (plasma enhanced chemical vapour deposition) process as it is used for instance for the deposition of silicon nitride, gases are introduced into a deposition chamber. Within the reactor the gases are ionized and accelerated towards the wafer surface where the deposition of the desired film is achieved by a chemical reaction.
For the simulation of depostion with low pressure CVD (chemical vapour deposition) processes the radiosity model is applied within the simulation tools ELSA2D/3D. Thereby the local deposition rate at each surface position is determined by the particle flux coming from the reactor, the reflected particle flux from other parts of the simulated structure and from the sticking coefficient of the deposited atoms at the surface. Shading effects are implicitly considered within this model.
Directional or anisotropic etching can be achieved with dry etching processes. Dry etching techniques are sputter etching, ion beam etching, and plasma etching. These techniques are mainly used to transfer structures created by lithography into the wafer of for self-aligned structures like the spacer of an MOS transistor.
For the simulation of dry etching processes highly anisotropic structuring elements are used within TOPO3D in order to obtain differential vertical and horizontal etch rates.
By immersing the wafers into baths of liquid chemicals, the exposed material layers are etched. Due to the high selectivity of the chemical reactions just single materials can be attacked. These kinds of processes are used to strip layers that have been used as masks for other processes or to remove layers that have been selectively generated during a preceeding process (e.g. silicidation).
For the simulation of wet etching processes TOPO3D uses isotropic structuring elements with individual etch rates for each material.