8.27 Sspd_chapter 6_part 8_caliberating athena for typical mosfet  (Page 3/4)

THETA.0 can be rapidly tuned by taking a one dimensional (1D) vertical cutline through the center of the gate and doing a 1D process simulation. You can either tune THETA.0 manually or by using the Optimize function in DECKBUILD. Theta.0 is tuned until the measured and simulated data of the long channel threshold voltage correspond. The fine tuning of THETA.0 is performed by using a full 2D simulation.

Figure 7.41 shows a typical dependence of extracted threshold voltage on the Theta.0 tuning parameter. Realistic values of THETA.0 correspond to the rising part of the curve. The glitch in the curve is due to rounding errors in the EXTRACT statement used to calculate the threshold voltage due to the automatic and independent mesh generated in the EXTRACT statement. The mesh can be changed from its default value shown here to eliminate this effect. But close examination reveals that the error is only a few millivolts off, which is accurate enough for most process parameter extractions.

Figure 7.41: A Typical Dependence of Extracted Threshold Voltage on Theta.0

X-axis = Theta.0, Y-axis = Threshold Voltage.

7.8.3: Tuning Implantation Parameters

You can now tune two implantation parameters by using the threshold voltage versus gate length data. The peak value of threshold voltage for a given process flow (the reverse short channel effect) will be a function of the initial implant damage caused by the LDD and source-drain implants. Since these implants have a high total dose and damage, the tuning parameter here is the clustering factor. In ATHENA, this parameter is called CLUST.FACT and is defined in the CLUSTER statement. The higher the clustering factor, the greater the damage, and the greater the diffusion, the greater the reverse short channel effect.

Figure 7.42 shows the effect on the threshold voltage of changing the CLUST.FACT parameter for a typical process flow.

Figure 7.42: How Changing the clust.fact parameter affects the threshold voltage

The second implantation parameter that can now be tuned is the lateral spread of the implant near the surface. In ATHENA, this parameter is called LAT.RATIO1 and is defined in the IMPLANT statement. The lateral spread of the source-drain and LDD dopant is responsible for the classical short channel effect, where the threshold voltage reduces for very short channel lengths. Simply tune the LAT.RATIO1 parameter until the onset of classical short channel effects of simulated and measured data correspond. If the LAT.RATIO1 is increased, the onset of the classical short channel effect will occur for longer gate lengths.

7.8.4: Tuning Diffusion Parameters

The final part of the threshold voltage versus gate length curve can now be used to tune the surface recombination rate of interstitials. In ATHENA, this parameter is called KSURF.0 and is specified in the INTERSTITIAL statement. The surface recombination of interstitials will dictate the roll-off rate of threshold voltage from its peak value (reverse short channel effect) to the long gate length value. Once again, simply tune KSURF.0 until the long channel threshold voltage roll off rate matches that of the measured data.