
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.
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.