During VLSI technology processes, mainly for shallow junction formation,
dopant concentrations higher than solid solubility and off equilibrium
conditions are generally employed. At high dopant concentrations not all
dopant atoms are ionized due to clustering and precipitation. The total
concentration of an impurity 
will be made up by a mobile (active) part 
and an immobile (clustered) part 
,
(
). This cluster formation causes a drastic
reduction of the diffusion coefficient at high concentrations because the
gradient of the active atoms almost vanishes and the clustered part of the
dopants does not diffuse (see Figure 3.2-1).
For boron and antimony the inactive dopants form relatively large
precipitates, whereas the inactive arsenic and phosphorus atoms form very
small coherent precipitates or clusters [Sol90]. An impurity cluster
is composed of 
impurity atoms and 
electrons. The cluster
formation process (3.2-10) does not occur instantaneously,
several hours at 
and a few minutes at 
are necessary to
attain equilibrium conditions [Sol90].
Commonly used process simulations are inadequate to correctly predict the dopant redistribution in supersaturated conditions. SUPREM III [Ho83b] and PREDICT [Fai88], even though assuming a maximum electrically active solubility, consider all dopants mobile during thermal treatment. On the contrary, ICECREM [Pic90] hypothesizes the instantaneous aggregation of dopant atoms to form immobile clusters.
In PROMIS' model library we supply two models for clustering effects, a kinetic (dynamic) clustering model DIFDCL and an equilibrium (static) clustering model DIFSCL.
