Trench Isolation Example : Trench Isolation Example

Requires: SSuprem 4, Elite
Minimum Versions: Athena 5.21.2.R

This example shows the fabrication of a trench isolation structure using Elite and SSuprem 4 to model etching and oxidation processes.

The example begins with grid definition. This is tailored to place fine grid in the area that will be exposed during the trench etch. The INITIAL statement does not specify a material type, so silicon is taken by default. A background doping of boron with concentration 1.0e15 is specified. The parameter NO.IMP specifies that the entire simulation should neglect all processing steps that introduce dopant into the structure. This results in a much faster calculation for diffusion and oxidation calculations but will not produce a structure suitable for electrical analysis because dopant will be left out. This is intended to provide a fast mode of calculation that is useful for initial simulations. The parameter SPACE.MULT specifies that a coarser grid be used than that specified by the SPACING parameter on the LINE statements. This also is intended for fast initial runs of the simulator. To disable these features for a more complete calculation, simply delete the NO.IMP and SPACE.MULT parameters from the INITIALIZE statement.

Following initialization, a nitride layer is deposited and patterned by the DEPOSIT and ETCH statements. The structure is then reflected to create a symmetric structure for trench etching.

The trench is performed by defining an etch machine with the RATE.ETCH statement. For this etch, the etch rates for all materials except silicon are assumed to be zero, so only rates for silicon are defined. If other materials were to be etched, the rates for each material should be set with a RATE.ETCH statement. For this etch process, the rates are defined for the RIE model. This simple model describes the etch process as a combination of ISOTROPIC and DIRECTIONAL components. The parameter U.M specifies that the etch rates are given in units of microns per minute.

The etch is then performed for a period of 10 minutes.

Following the etch, the nitride masking layer is removed with the ETCH NITRIDE ALL statement.

The trench is then filled with a sandwich of materials. First a layer of oxide is deposited with the simple conformal model. Then a layer of polysilicon is deposited with the Elite calculation. Finally, photoresist is deposited using Elite to model the resist flow. This is included by adding the parameters SMOOTH.WIN and SMOOTH.STEP to the deposit machine definition. These two parameters specify the width over which a simple averaging algorithm will be applied and the number of times it will be applied respectively. This results in a smoothed profile that is a fast way of modeling the flowing resist during deposit.

The structure coated with resist has a surface that is nearly planar. By applying an etch process that erodes all materials at an equal rate, the planarization process can be modeled. This is performed next by specifying the etch rates for an Elite type calculation using the WET.ETCH model. Following the etch, the grid on the surface of the structure is relaxed to remove any excess grid that may have been introduced during the etch process. The parameter DX.SURF on the RELAX statement specifies the minimum surface segment size in microns for the relax operation.

Next, a nitride layer is deposited and patterned to control the area that will be oxidized.

Some model parameters are set for the oxidation using the METHOD and OXIDE statements. Then the diffusion is performed in two steps, first in dry oxygen specified using the DRYO2 parameter, and then in water, specified by the WETO2 parameter.

Finally, the structure is plotted using TonyPlot and the structure saved to a file.

To load and run this example, select the Load button in DeckBuild > Examples. This will copy the input file and any support files to your current working directory. Select the Run button in DeckBuild to execute the example.