(Invited) Oxygen Precipitation and Defect Generation in Cz Silicon during Second and Millisecond Annealing

Oxygen precipitation in Czochralski silicon is still an issue in various device technologies. On the one hand it is exploited for internal gettering of metallic impurities by oxygen precipitates and on the other hand it is undesired because oxygen precipitates are known to degrade the lifetime of minority carriers. At the end, it depends on the application of the silicon material how precipitation of interstitial oxygen is controlled.

The reduced thermal budgets due to continuous scaling of features sizes in high end device technologies lead to the development of faster and faster anneal processes. The earlier device technologies were dominated by conventional annealing processes. Today rapid thermal processes working on the second scale are common steps in microelectronic processing. Recent developments utilize already thermal processes on the millisecond scale as e. g. flash and laser anneals. All these different time scales influence the generation of intrinsic point defects and thus also the kinetics of oxygen precipitation in a different way.

Rapid thermal annealing (RTA) is a widely used thermal pretreatment to create an internally gettering bulk defect zone and a defect denuded zone below the surface. This is possible because the precipitation of interstitial oxygen strongly depends on the supersaturation of intrinsic point defects. During RTA vacancies supersaturate facilitating the formation of VmOn complexes which are the initial stages of oxygen precipitation. However, the strength of the effect depends on the temperature and cooling rate of the RTA pre-treatment.

Flash lamp annealing is used first of all for the creation of shallow dopant profiles in high end electronic device technologies. The surface of a silicon wafer can be heated with this technique to temperatures up to the melting point within a few milliseconds. With the help of a thermal model we can simulate the temperature profiles. We combined this model with a second model describing the generation and annihilation of intrinsic point defects. The results of modeling help to understand the experimental results of oxygen precipitation. After 3 ms of annealing a defect denuded zone below the front surface is generated while after 20 ms annealing oxygen precipitation is suppressed in the wafer.

Flash lamp annealing leads to steep temperature profiles in the wafers causing stress. Silicon wafers with etched, ground, and sawed surfaces were subjected to flash lamp annealing for 20 ms. Such wafers contain different densities of pre-existing dislocations which propagate into the wafer during flash lamp annealing up to a certain depth which is different on the front and back sides. Plastic deformation was not observed if no pre-existing dislocations were present in the wafer before flash lamp annealing. The elastic and plastic deformation was simulated by a mechanical model.

In summary, the difference between thermal processing on the second and on the millisecond scales is based on the temperature profiles generated by the different types of processing. These profiles influence the shrinking of grown-in oxide precipitate nuclei and the generation, diffusion, and annihilation of intrinsic point defects and thus the oxygen precipitation during further processing.