 # Hints, Tips and Solutions

How can I develop my own etch model in Victory Process?

Background: Victory Process is packaged with a wide range of etch models. Victory Process also supports an “Open Modelling Interface” (OMI). The OMI enables users to develop their own models inside Victory Process. Both etch and deposit models can be defined by the user in the OMI.

Example: In this example we are going to demonstrate how to define your own etch model.

Shown in Figure 1 is the starting structure. Figure 1. Starting structure for customised etch modelling.

The demonstration structure is a simple piece of silicon with uniform geometry in the y direction and simple stepped surface.

The etch model is a CMP style model which will control the etch rate as a function of distance from the surface. As such, the surface high point (left hand side of the structure) will etch at a higher rate than the lower point of the surface (right hand side of the structure).

The C interpreter file / code to create the etch model is shown below

/***********************************
* cmp_physical
* Comment: rate dependent on point’s
* distance to the
* top surface point
**************************************/
int
cmp_physical(double max_z,

double x, double y,
double z,
double r0,
double ratio0,
double* rate)

{

if ( z >= max_z )
{
*rate = r0
}
else
{
*rate = r0 -( max_z - z) *
ratio0;
}
return 0;

}

The first section of the file defines the inputs and output of the file.

Here the inputs are:

• the z-coordinate of the highest point of the surface (max_z)
• the x, y, z location of the surface point
• the maximum etch rate, r0. This is set in the deck (defined in the material characteristics)
• the decrease of the etch rate with the distance from the highest surface point, ratio0, again set in the deck (defined in the material characteristics)
• the value returned to the simulator is *rate. This is the etch rate at point (X/Y/Z)

Victory Process will call this C-function for every surface point (once every time step) of the simulated structure.

The last part of the file manipulates theses inputs and returns the output. In this example we assume that the etch rate decreases linearly with the distance from the highest point of the surface: *rate = r0 -( max_z - z) * ratio0

The deck used in this example is shown below:

GO victoryprocess

INIT material=”silicon” from=”0, 0” to=”2, 0.2” depth=1.5 gasheight=0.5 resolution=0.08flow.dim=3d meshDepth=2

REACTION name=my_name c_function=cmp_physical depend1=highestSurfaceCoordinate depend2=position depend3=rate depend4=ratio

TOPOGRAPHYMODEL name=”my_etch_model” reactionmodel=my_name fluxmodel= “constant”

ETCHDEPOPROPERTIES chemicalname=”etcherrate” material=”silicon” rate=1 ratio=0.3

ETCH model=”my_etch_model” chemicalname=”etcherrate” time=0.30

SAVE name=”after_cmp”
EXPORT structure=”after_cmp_ex.str”

QUIT

The first part of the deck is quite conventional, setting up the starting material. The REACTION statement first gives a name to the reaction model that is to be created it then instructs Victory Process to use the C Interpreter function shown earlier. Finally this statement lists the dependencies (DEPEND<n>) which are the set of parameters sent to the C-function.

The topography model is then flagged, named and set to call the reaction model as well as defining the flux model.

Finally the etch can be performed, the etch is initiated by the ETCH statement. This calls in the etch machine that has been defined in the previous statements.

Shown in Figure 2 is the product of this executed deck. Figure 2. The structure shown in Figure 1 once it has been through the depth dependent rate etch process.

Shown in Figure 3 is a 2D overlay of slices taken through the structures shown in Figure 1 and 2. The etch model used here is dependent on the depth from the surface. As distance increases into the substrate, the etch rate decreases. As such the left hand side of the structure (the highest point) has etched significantly faster than the lower area on the right hand side. Figure 3. Overlay of 2D cutplanes through the starting structure (red) and the final (etched) structure. The higher portion has etched faster.

The etch feature is not limited to a single material or a pseudo2D block (uniform geometry in the y direction). For example, shown in Figure 4 is a structure with an elliptical trench in the silicon into which concentric rings of nitride and polysilicon have been added. Victory Process can call in external mask files (GDS2), however in this instance the masks used to create these shapes were specified inside the Victory Process deck. Figure 4. Distance rate etch starting structure with multiple materials all with different etch characteristics. Figure 5. The result of subjecting the structure shown in Figure 4 to distance rate etch model.

The silicon, nitride and polysilicon are then subjected to the distance etch rate model introduced previously. The different materials are assigned different rate characteristics so the polysilicon is the least etched material, followed by the nitride and finally the material to be etched the fastest is the silicon. As the distance rate etch model is used on all three materials, the higher points are etched fastest and the lower points more slowly.

Conclusion

In this example we have demonstrated one of the advanced features of Victory Process, namely the Open Modelling Interface and its application for the development of user defined etch models via a CMP style model.