Diffusion of dopants

Oxidation of Silicon. Silicon dioxide [7631-86-9] Si02, is a basic component of IC fabrication. Si02 layers are commonly used as selective masks against the implantation or diffusion of dopants into silicon. Si02 is also used to isolate one device from another. It is a component of MOS devices, and provides electrical isolation of multilevel metalliza tion stmctures (12). A comparison of Si and Si02 properties is shown in Table 1. 

In the oxidation process, a layer of dopant is apphed to the surface of sihcon and patterned sihcon dioxide for subsequent thermal diffusion into the sihcon. The masking property of the Si02 is based on differences in rates of diffusion. Diffusion of dopant into the oxide is much slower than the diffusion into the sihcon. Thus, the dopants reach only the sihcon substrate. Oxide masks are usually 0.5—0.7 p.m thick. 

The diffusion of dopants into, and out of, conducting polymers is important for possible applications in batteries, and as conductors or semiconductors. For conductors or semiconductors, the chief requirements are that the material can be doped in a reasonable time, but that it will then not lose dopant over periods of years. This is particularly important in determining junction stability in devices. In the case of batteries, on the other hand, rapid and reversible uptake and loss of dopants is needed, since the diffusion rate controls charging and discharging rates. In addition, the accessibility of the structure to oxygen, and other degradants, will be a factor in the stability of the polymer. 

The diffusion behaviour of Shirakawa polyacetylene is complicated by its fibrillar morphology and high surface area, so that weight changes depend on pore transport and surface adsorption, as well as on diffusion into the fibrils. Chien 6) has reviewed earlier studies of the diffusion of dopant counter-ions in Shirakawa polymer and has emphasised the wide range of values of diffusion coefficient which are reported and which depend a great deal upon the morphological model chosen to interpret experimental data. 

One concern with measurements of this type, is that undoping of a film may result from the outward diffusion of dopant ions or the inward diffusion of counterions which would then form salt within the film. This has been avoided in our polyacetylene study by measuring further doping pulses in samples which have only been doped in one direction, either reduction or oxidation. 

Figure 13. Experiments that illustrate oxidation-enhanced or oxidation-retarded diffusion of dopants in silicon. The supersaturation of self-interstitials associated with the oxidation process drives both effects. (Reproduced with permission from reference 118. Copyright 1984 Noyes Publications.)...

The effects of damage by ion implantation on the low-temperature diffusion of dopant can also be studied by implanting Si+ or Ge+ ions into predeposited layers in Si. Recently, Servidori et al. (58) studied the influence of lattice defects induced by Si+ implantation. Using triple crystal X-ray diffraction and TEM, they confirmed (1) that below the original amorphous surface-crystal interface, interstitial dislocation loops and interstitial clusters exist and (2) that epitaxial regrowth leaves a vacancy-rich region in the surface. 

The thermal oxidation process is an essential feature of planar-device fabrication and plays an important role in the diffusion of dopants in Si. In the thermal oxidation process, Si reacts with either oxygen or water vapor at temperatures between 600 and 1250 °C to form Si02. The oxidation reaction may be represented by the following two reactions ... 

In the processes described so far, the resist is removed after etching, baring the patterned oxide that serves as a mask during subsequent high-temperature diffusion of dopants into the exposed silicon substrate. There are examples, however, where the resist material is left behind to become... 

When the polymer flhn is oxidized, its electronic conductivity can exceed the ionic conductivity due to mobile counterions. Then, the film behaves as a porous metal with pores of limited diameter and depth. This can be represented by an equivalent circuit via modified Randles circuits such as those shown in Figure 8.4. One Warburg element, representative of linear finite restricted diffusion of dopants across the film, is also included. The model circuit includes a charge transfer resistance, associated with the electrode/fllm interface, and a constant phase element representing the charge accumulation that forms the interfacial double... 

Indispensable and continue to be Important In the development of models for range statistics In Ion Implantation. SIMS depth profiles are also used to monitor and develop an understanding of the diffusion of dopants during laser and thermal annealing processes. Metallization and thin films have also been Investigated by SIMS. In addition SIMS depth profiles are useful for failure analysis and problem solving. 

The out diffusion of dopants from highly doped wafer areas Into the epitaxial layer during epitaxial deposition or prebake Is called autodoping. SIMS depth profiles during autodoping showed a pile up of As at the SI substrate surface. Spreading resistance measurements showed that the source of the autodoping to be electrically Inactive As (41). 

The boundary layer between the graded doping was investigated in the manner of electrical resistivity measurement. It was suggested that diffusion of dopant occurred during sintering and resulted in a broad boundary layer as wide as 3-4mm. 

The diffusion of dopants in semiconductors has been briefly discussed in Sect. 2.1.3. At an atomic scale, the diffusion of a FA in a crystal lattice can take place by different mechanisms, the most common being the vacancy and interstitial mechanisms in silicon and germanium (see for instance [25]). The interstitial/substitutional or kick-out mechanism, which is an interstitial mechanism combined with the ejection of a lattice atom (self-interstitial) and its replacement by the dopant atom is also encountered for some atoms like Pt in silicon. 

Such uniform dopant distribution in the porous layer was observed in all SiC samples in which the thicknesses of porous layers varied from 2 to 10 pm in our experiments on diffusion. This can be explained by the fact that diffusion of dopant atoms in a porous layer occurs not only via diffusion through the SiC walls of porous layer but also through gas-phase transport within pore openings and surface diffusion. 

The diffusion of impurities into Si wafers typically is done in two steps. In the first step, dopants are implanted into the substrate to a relatively shallow depth of a few thousand angstroms. After the impurities have been introduced into the Si substrate, they are diffused deeper into the substrate to provide a suitable impurity distribution in the substrate. The solid solubility and diffusion of dopant atoms in Si are given in the top and bottom, respectively, of Fig. 9.10. 

Ion implantation and diffusion of dopants Boron into silicon... 

The second property of importance involves the chemical compatibility with the semiconductor substrate, often termed passivation . Passivation includes protecting the semiconductor from external contamination (diffusion of dopants and/or external chemical attack), as well as providing electrical stability. 

Dielectric materials are used for isolating conducting layers, to facilitate the diffusion of dopants from doped oxides, as diffusion and ion implantation masks, capping doped films to prevent loss of dopant, for gettering impurities, for protection against mois-... 

Diffusion is a process that also occurs in solids. The manufacture of solid-state transistors involves the diffusion of dopants, such as boron or phosphorus, into silicon in order to create... 

Easy speciation and isotope detection have been identified as playing important roles in optimizing the production of superconducting NbsSn wires by heating a composite of bronze and niobium filaments. Application of TOF-LMMS has provided evidence of the out-diffusion and accumulation of phosphorous, inhibiting Nb3Sn formation when tin diffuses inward. The preferential incorporation of the lower tin isotopes in the NbsSn layer has confirmed the process to be driven by diffusion. The semiconductor industry has used TOF-LMMS to verify the lateral diffusion of dopants, e.g., boron in tantalum and cobalt silicide ruimers only 3 pm wide. Another application is the identification of microscopic residues left on integrated circuits after removal of the photoresist. 

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