The latest version of the etching tool uses a hybrid approach  to
simulate the etching process. The discretization of the simulation domain is
restricted to a thin layer around the attacked surface. The surface is
discretized in two levels of (recursive) cell resolution, where a cell can
either contain the material information or a regular grid of sub cells with a
smaller resolution. Additionally to the cell information the topology of the
simulation domain as it is stored in the WAFER-STATE-SERVER is used.
Upon start of the simulation the data are transferred from the
persistent Wafer description into memory using the I/O layer of the
WAFER-STATE-SERVER. Since the etching tool performs a lot point location operations the
finite oct-tree data structures (as opposed to jump-and-walk, c. f. Section 3.4.3) are used to
hold the data. Once the data are available in the WAFER-STATE-SERVER data structures the
cellular discretization process takes place in the following manner.
- The surface to attack is extracted from the WAFER-STATE-SERVER.
- Around each point of the extracted surface a refined cell is
created. The material type of a cell is thereby determined via the point location
operation of the WAFER-STATE-SERVER.
- The so-called structuring elements are determined depending on the
material type. Structuring elements are used to model different etching rates
(for different materials) .
Fig. 4.14 illustrates the discretization process.
cellular discretization process. The cells at the attacked surface (refined
cells) contain sub cells of smaller size. The other cells directly contain
the material information determined via the WAFER-STATE-SERVER.
Once the discretization is finished the topography simulation is
initiated. Thereby, the structural elements are applied to the simulation
domain. Cells that are covered by the smallest possible rectangular prism that
surrounds a structural element are defined. Defining means that the cell is
allocated and assigned the material determined via the
WAFER-STATE-SERVER. The actual cellular simulation algorithm is quite similar to
traditional algorithms with the following exceptions.
- Memory allocation for cells is deferred until a cell is actually
- Cells that are close to the attacked surface are refined.
- As the etch front moves, refined cells move away from the front. If all
refined cells of their corresponding unrefined counterpart contain only one
material type they are converted back to the unrefined cell and the memory
associated with the refined cells is freed.