Silicon impurity profile

Figure 11. Experimental impurity profile of phosphorus implanted in silicon showing the as-implanted profile and the profile after annealing. (Reproduced with permission from Reference 29, copyright 1978, American Institute of Physics.)...

NAA cannot be used for some important elements, such as aluminum (in a Si or Si02 matrix) and boron. The radioactivity produced from silicon directly interferes with that ftom aluminum, while boron does not produce any radioisotope following neutron irradiation. (However, an in-beam neutron method known as neutron depth profiling C3J be used to obtain boron depth profiles in thin films. ) Another limitation of NAA is the long turn-around time necessary to complete the experiment. A typical survey measurement of all impurities in a sample may take 2-4 weeks. 

The length of the zone and the diameter of the rod are chosen in such a way that surface tension and interactions between circulating electric currents in the molten zone and the radio-frequency (r-f) field from the surrounding induction coil keep the molten zone in place. As of this writing (ca 1996), the maximum silicon rod diameter that can be purified in this manner is ca 125 mm. Initially, additional purification can be obtained by making more sweeps of the zone. Eventually, however, more sweeps do not remove any additional impurities. The limiting profile is given by equation 4 ... 

Theoretical studies of diffusion aim to predict the distribution profile of an exposed substrate given the known process parameters of concentration, temperature, crystal orientation, dopant properties, etc. On an atomic level, diffusion of a dopant in a silicon crystal is caused by the movement of the introduced element that is allowed by the available vacancies or defects in the crystal. Both host atoms and impurity atoms can enter vacancies. Movement of a host atom from one lattice site to a vacancy is called self-diffusion. The same movement by a dopant is called impurity diffusion. If an atom does not form a covalent bond with silicon, the atom can occupy in interstitial site and then subsequently displace a lattice-site atom. This latter movement is believed to be the dominant mechanism for diffusion of the common dopant atoms, P, B, As, and Sb (26). 

Depth profiles are usually presented as atomic concentrations versus sputter time, assuming we know the rate at which the sample sputters. A typical depth profile is shown in Figure 25. It is interesting to see that at the surface there is carbon, silicon dioxide and some molybdenum. As soon as the surface layer is sputtered off (300 A), the oxygen and carbon impurities drop to constant and small values. For this CVD film, the molybdenum silicide came out to be very silicon rich. We can also see that the stoichiometry of the silicide changed with position (depth) in the film. 

SIMS is used for quantitative depth profile determinations of trace elements in solids. These traces can be impurities or deliberately added elements, such as dopants in semiconductors. Accurate depth prohles require uniform bombardment of the analyzed area and the sputter rate in the material must be determined. The sputter rate is usually determined by physical measurement of the crater depth for multilayered materials, each layer may have a unique sputter rate that must be determined. Depth prohle standards are required. Government standards agencies like NIST have such standard reference materials available for a limited number of applications. For example, SRM depth profile standards of phosphorus in silicon, boron in silicon, and arsenic in silicon are available from NIST for calibration of SIMS instmments. P, As, and B are common dopants in the semiconductor industry and their accurate determination is critical to semiconductor manufacture and quality control. 

Shabani, M.B., Yoshimi, T., Okuuchi, S., Kaniava, A. (1997) A quantitative method of metal impurities depth profiling for gettering evaluation in silicon wafers. Solid State Phenomena, 57-58,81-90. 

Figure 7.7 Shows the relative concentration of vacancies in silicon at equilibrium for various Fermi level positions at 300 K. Redrawn with permission from Fair, Richard B, Concentration profiles of diffused dopants in Silicon, in Wang, F.F.Y., ed.. Impurity Doping Processes in Silicon (North Holland, Amsterdam, 1981), chapter 7. Copyright Elsevier, 1981.

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