Non-planar Optical Lithography

anex01.in : Non-planar Optical Lithography

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

This example uses Optolith to show non-planar lithography over a complex topography created using SSuprem 4 and Elite .

The example begins with a silicon substrate that is 24 microns wide and 1 micron deep. Two parameters on the INITIALIZE statement specify that the no impurity (geometric) mode and the coarse grid mode be employed. The parameter NO.IMP specifies that the introduction of impurities will be bypassed. This results in a much faster calculation for many steps and still produces similar topographies to the calculation including impurities. For simulations that are concerned primarily with topography, the no impurity mode can typically be applied without loss of significant information.

The parameter SPACE.MULT specifies that all grid spacings specified by the parameter SPACING on the LINE statement should be multiplied by the specified value. This results in a global grid coarsening by the specified factor. This speeds the simulation by reducing the discretization.

The process sequence begins with deposition of a thin oxide to model the effect of gate oxidation. Next, nitride is deposited and patterned simply by specifying the positions of a vertical plane that defines the boundary of the etch region. Following the nitride patterning, field oxidation is performed and the nitride mask is stripped.

Next, gate polysilicon is deposited and patterned using the geometric etch capability and a structure file is saved.

Following the structure file save, the structure is implanted. Since the no impurity mode is being used, the implant step will be ignored. Next, the sidewall spacers are formed by depositing oxide and etching it using the vertical etch model. The source/drain implant is performed next, but due to the use of the no impurity mode, it is bypassed. The next step is the source/drain implant anneal. This step runs much more quickly in no impurity mode. This is because impurity diffusion, which typically limits the time step size during diffusion and causes more equations to be solved, does not need to be calculated.

The next step is to deposit spin-on-glass. The glass deposition and reflow process are approximated here by performing a deposit that includes a geometric smoothing performed during the deposit. This is accomplished by defining an Elite machine using the RATE.DEPO statement and including the parameters SMOOTH.WIN and SMOOTH.STEP as part of the statement. The Elite machine is invoked by specifying the DEPOSIT statement with the MACHINE parameter used to specify the previously defined machine.

The next sequence of process steps deposits photoresist, patterns the resist with the geometric etch capability, defines a wet etch machine and employs that machine on the structure, defines a directional etch machine and employs that machine on the structure, saves the structure, and strips the photoresist.

Following this, a short wet etch is performed to remove fillets from the structure, and aluminum is deposited using the Elite hemispherical model.

Next, photoresist is deposited using the same geometric smoothing approach that is applied to model spin-on-glass reflow. This results in a nearly planar top surface following the resist deposit. The parameter NAME.RESIST specifies the type of resist that will be used from the library of resists included in the models file. In this case, the photoresist named OiR32 is applied.

After the resist is deposited, the simulation begins the Optolith portion of the process. This performs a detailed analysis of the photolithographic process of this final structure. Optolith simulation begins by performing imaging. The ILLUMINATION , ILLUM.FILTER , PROJECTION , and PUPIL.FILTER statements describe the illumination system. The LAYOUT statements define two mask features. The parameter LAY.CLEAR removes any previously defined layout information so that a new layout can be initialized. Once the illumination system and the layout have been defined, the imaging module is invoked. The IMAGE statement invokes the imaging module and specifies the window in which imaging will be performed. The parameter DX specifies the discretization along the x-dimension and the parameter ONE.D specifies that the imaging should consider only one dimension. The parameter CLEAR specifies that the mask should be considered as a clear field with dark features defined by the LAYOUT statements.

The OPTICAL statement defines the index of refraction for BPSG material at a wavelength of 0.365 microns. The EXPOSE statement performs the resist exposure from the results of the imaging module.

Following exposure, the BAKE statement performs post-exposure bake and calculates the diffusion of the photoactive component. Finally, resist development is performed using the DEVELOP statement. The MACK model is specified along with the development TIME , number of STEPS at which regridding will be performed, and the number of non-regridded SUBSTEPS that will be taken per step.

Finally the results of the simulation are plotted using TonyPlot . The non-ideal features in the photoresist due to reflections from the underlying topography can be seen in the lines of resist that remain.

To load and run this example, select the Load example button. This action will copy all associated files to your current working directory. Select the DeckBuild run button to execute the example.