3.7.4 RTA-Experiments



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3.7.4 RTA-Experiments

In this section we verify our model RTA (Section 3.2.4) by simulating boron diffusion for temperatures ranging from to for diffusion times and [Sol92]. With the large number of parameters it is possible to perfectly fit virtually any single profile, but the parameter set is not unambiguous. We tried to find a consistent set of parameters for various profiles which still achieves good agreement between simulation and experimental results.

For the first three experiments boron was implanted at an energy of with a dose of through a Å screening oxide. The boron concentrations are below the solubility limit, such that cluster effects are negligible (cluster effects are disregarded by the model).

As initial conditions for the diffusion simulation we used the measured implantation profile for boron (, , ) and calculated damage profiles for the interstitials. We calculated the initial damage profiles with PROMIS' analytic implant model DAMPAR (see Section 2.4) and corrected the local interstitial concentration by a factor of 1/8 [Hob89], [Pic91] (cf. page gif).

 

To resemble the temperature profile observed during rapid thermal annealing, we split the RTA cycle into three phases (see Figure 3.7-10). In the first phase the temperature rises with a rate of , which is suitable for RTA equipment, until it reaches the process temperature. After the second phase of constant temperature, the temperature decays exponentially with a time constant of to room temperature in the third phase.

 

Figures 3.7-11 to 3.7-13 show the simulated and measured boron profiles. The implanted boron profile (, , ) was annealed in inert ambient for at , and , respectively. There is good agreement between simulation and experimental results for all three temperatures. This conformity verifies the model's temperature dependence of the transient diffusion enhancement, i.e. the activation energy of the reaction velocity . However, for the verification of our consistent parameter set we need to simulate profiles at one temperature for different diffusion times.

  

In the fourth experiment boron was implanted with an energy of with a dose of through a Å screening oxide. Again, the boron peak concentration is well below the solubility limit, such that cluster effects are negligible. An RTA cycle was performed for a process time of at . For these conditions, normal diffusion models report a motion of boron of only about - in the experiment we observe a displacement of . This anomalous transient displacement is reproduced quite acceptably by our simulations (Figure 3.7-14). The transient enhanced diffusion is predicted quantitatively correct by the model also for short time anneal. Nevertheless, only with a more complete set of experiments the parameters can be proven.

 

During the extraction of the parameters with PROFILE [Ouw89] we observed the following influences of the different parameters. The most critical model parameter is the reaction velocity . It determines the time dependence of the transient enhancement effect. Discrepancies in the equilibrium ratio may be partially compensated by the reaction coefficients and . The model is fairly insensitive with respect to the precise values of the Frenkel-pair constant and the surface self-interstitial annihilation rate .

Physically, the transient enhanced diffusion is attributed to two facts, a high interstitial component of implanted boron and an enhanced point defect concentration related to the implantation damage. We noticed that the initial substitutional component of boron, i.e. in the initial condition (3.2-70), has only minor influence on the final distribution. No changes in the final profiles can be recognized, as long as the initial substitutional component does not exceed () of the total boron concentration . Figure 3.7-15 shows the peak concentrations of substitutional boron for different .

 

The given pair diffusion model has been successfully verified against experimental data. The model is able to predict the transient enhanced diffusion effect during low thermal budget processes quantitatively correct.

The model does not include any high concentration effects (clustering). It neglects reactions according the Frank-Turnbull mechanism ( and ), which might not be valid in the early stage of the diffusion. For the calculation of the electric field the model disregards the charges caused by point defects and dopant point-defect pairs, which is only justified as long as dopants are primarily at substitutional sites. Anyhow, the electric field is not that important for the transient enhanced diffusion effect (particularly at the relatively low concentrations considered in our examples).



next up previous contents
Next: 4 Oxidation Up: 3.7 Applications Previous: 3.7.3 Field Oxide Isolation



Martin Stiftinger
Wed Oct 19 13:03:34 MET 1994