HINTS & TIPS

Q: How can di-electric reflow be modeled? Which calibration parameters are important for tuning the reflow?

A: ATHENA contains a model for the reflow of materials as part of the Elite module. The model treats the di-electric material (eg.SiO2, BPSG) as an incompressible viscous fluid. The material is then deformed under the driving force of the surface tension of the topography. The calculation of the changing topography of the material then proceeds according to the applied time and temperature.

The reflow model for a given material is enabled by setting the REFLOW parameter on a MATERIAL statement. In addition the parameter REFLOW should be given on a DIFFUSE statements corresponding to the flow heat cycle. The following is typical syntax:

MATERIAL OXIDE VISC.0=1.862E-20 GAMMA.REFLO=1E3 REFLOW
DIFF TIME={time} TEMP={temp} REFLOW

This example syntax also includes two of the most useful tuning parameters. VISC.0 sets the viscosity of the oxide. GAMMA.REFLO sets the surface tension factor for the flow calculation.

Figure 1 shows the results of an example of reflow calculation with ATHENA. The initial structure has a set of 1 micron contacts with a 2 micron pitch after the anisotropic contact etch. The final profile shows the reflow shoulders and the proximity effects seen following a 10 minute reflow heat cycle at 950 C.

Figure 1

Q: Can dopant diffusion be modeled simultaneously with the material reflow?

A: An extremely important feature of ATHENA is that simulation of topography effects such as reflow in Elite can be combined with in-wafer simulation of dopant diffusion or oxidation in SSuprem4. A previous Hints and Tips column (April 1995) showed how ATHENA can simulate individual process steps from SSuprem4 and Elite with seamless integration. In this case the Elite and SSuprem4 simulation is done on the same process step. The reflow heat cycle will also trigger diffusion of the dopants in the silicon, including transient enhanced diffusion effects where appropriate.

A single DIFFUSE statement with the REFLOW parameter can both produce reflow and dopant diffusion. Figure 2 shows an example of a 0.5mm contact cut to an arsenic diffusion. During the reflow cycle at 875C the edges of the contact cut are flowed while the arsenic is diffusing.

Figure 2