Modeling of Ion Implantation Induced Damage in Silicon

Ion implantation is the primary technique in silicon technology for introducing dopant atoms into the substrate. As an unwanted side-effect, the crystal lattice is damaged which makes an annealing step necessary. Unfortunately, the generated defects cause transient enhanced diffusion of the dopants during anneal. This leads to the broadening of dopant profiles and, as a consequence, impedes device shrinkage. The amount of diffusion as well as the time and temperature necessary to anneal the defects depends on their number and type. Detailed understanding of the damage formation process and accurate models for simulation are therefore of great value to the design of silicon devices. The importance of this topic is illustrated by the International Technology Roadmap for Semiconductors having identified "implant damage, amorphization, and subsequent re-crystallization during anneal" as a difficult challenge in the field of modeling and simulation. The goal of this project is a more detailed understanding of damage formation in silicon and quantitative models of its dependence on implant parameters such as ion mass, dose, dose rate, and temperature. As simulation methods atomistic approaches are used (binary collision, kinetic Monte Carlo, and molecular dynamics simulations) which have become more popular in recent years because of the increase in available computer resources. Implantation experiments at cryogenic temperatures are performed to study collisional effects, at temperatures up to room temperature to study dynamic annealing, and under amorphizing conditions to study processes at existing amorphous/crystalline interfaces. As analytical techniques, Rutherford backscattering and channeling (RBS/C) is used to measure the total number of displaced atoms, and transmission electron microscopy (TEM) to study the damage morphology and to determine the position of the amorphous/crystalline interface. Special care is taken in RBS/C quantification to use accurate defect atom positions determined by ab-initio simulations. Dopant profiles generated by implantations into channeling direction and analyzed with secondary ion mass spectrometry (SIMS) are used to cross-check the RBS/C results.

Ion implantation is the primary technique in microelectronics for introducing dopant atoms into the substrate. As an unwanted side-effect, the crystal lattice is damaged. Upon annealing the defects cause various side-effects such as transient enhanced diffusion and clustering of dopants. Detailed understanding of the damage formation process and accurate simulation models are therefore of great value to the design of semiconductor devices. In the course of the project several scientific advances were accomplished. First, a better understanding of amorphous pocket formation in silicon by heavy ions was obtained. Previous theories explained the generation of amorphous regions around heavy-ion trajectories by the generation and flow of heat from the region of energy deposition. We showed that quantitative modelling of this process requires consideration of the heat of melting, and that it is necessary to assume the collapse of the silicon lattice once a critical density of defects in a local region is exceeded. The latter criterion is commonly applied to amorphization due to damage accumulation by light ions. It is interesting to see that the same mechanism plays an important role in the amorphization process induced by single heavy ions. Moreover, we showed that for the formation of continuous amorphous layers an effect known from high-energy (MeV) implantations, ion beam induced interfacial amorphization, is also relevant in the more common keV energy range. We implemented and coupled several techniques for the simulation of atomic-scale phenomena. E.g., molecular dynamics and transmission electron microscopy (TEM) image calculations were combined for the first time to better understand experimental TEM images. Binary collision simulations of Rutherford backscattering spectroscopy showed the importance of considering the atomic positions of the defects and the superiority of ab- initio to empirical potential calculations to obtain these positions. Coupling of binary collision and kinetic Monte Carlo simulations allowed the investigation of the damage evolution after light-ion implantation, considering the diffusion and reactions of point defects at room temperature. In particular, it was possible to explain the different efficiency of hydrogen and deuterium implantation to induce the exfoliation of silicon surface layers. These kinds of simulations may be important in the future to better understand and control the defects generated during silicon- on-insulator (SOI) wafer fabrication.