Temperature collision cascade

Temperature effects in physical sputtering are still under discussion. For the case of graphitic materials, it is known that displaced atoms throughout the collision cascade can diffuse to the surface and sublimate already at temperatures below 1000 K. It has been documented that similar effects exits for metals with low melting point. A model indicates a similar process as for graphite and a thorough investigation of this effects is necessary. 

Fig. 5. Kinetic energy distributions of SiF4 etch products evolved from a silicon surface exposed to 3-keV Ar+ ions and 5 x 10 SF molecules/cm s, at two surface temperatures, 50 and 100 K. Solid curves represent collision cascade distributions with a surface binding energy (Co) of 0.05 eV. (From Osstra et al., 1986.)...

Equation (13.5) and the marker data presented in Fig. 13.5 can be used to estimate the average atomic displacement distance of a marker atom in a collision cascade formed in a matrix of amorphous Si. For example, from the temperature-independent data in Fig. 13.5a, a typical value of DiJlcj), for both Sn and Sb markers, is 4(10 cm" ), or 0.4 nm". From Fig. 13.5b, the corresponding damage energy is 1,500 eV nm Using the atomic density of crystalline Si, 50 atoms nm for the amorphous Si value of N, the ratio of Fj lN will be 30 eV nm This indicates that should be approximately 1.6 nm for a Si displacement energy of Ad = 13 eV. 

The beam current effect on the electrical conductivity is complicated by the combination of two conditions that are a function of the beam current. These two conditions are the difference between the background temperature and the glass point temperature of the polymer, and the relaxation time of the collision cascade compared with the time between ion collisions. 

For semiconductors the story is quite different. Many defects created in the collision cascade are not mobile which can result in defect configurations that do not anneal at room temperature. Heating to higher temperatures is often necessary to anneal out most damage. But it is not unlikely that the implanted atom or part of them remains part of a defect structure. Figure 6.6 illustrates defect recovery after implantation and annealing a semiconductor host. 

The term heat spike has been used to describe the dense mixture of atoms and defects in motion in the core volume (Fig. 6.7) of a collision cascade. Electron microscopy pictures suggests that a heat spike in a semiconductor can lead to the creation of small amorphous zones in semiconductors associated with individual implanted atoms. There is certainly ample evidence that the accumulation of implantation defects can lead to the creation of amorphous layers inside the semiconductor. Implantation at higher temperature can avoid the creation of such amorphous layers and is more effective than post-implantation anneahng at this temperature after an amorphous layer has been formed. 

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