A. H. van Ommen

Transition metal silicides in silicon technology

Studies of the properties and characteristics of transition metal silicides have been stimulated by their (potential) use in integrated circuit technology. This review describes some of the most recent studies in this field of research. Formation mechanisms of silicides are discussed in some detail. A division is made between near-noble and refractory metal silicidation which aids in the understanding of differences in formation mechanisms of the various silicides. The evolution of the components of thin film stress during metal silicidation is also elucidated. In the review of the practical uses of these materials, emphasis is placed on specific processes involving laterally confined (self-aligned) silicide film formation as more advanced applications require film formation only in certain localized regions on a Si wafer.

Microstructure of heteroepitaxial Si/CoSi2/Si formed by Co implantation into (100) and (111) Si

Heteroepitaxial Si/CoSi2/Si structures have been synthesized by high‐dose implantation of Co into (100) and (111) Si at an energy of 170 keV and subsequent annealing. In the as‐implanted state the implanted Co is found to be present as CoSi2. For a dose of 2×1017 Co/cm2, the Co is present in the form of epitaxial precipitates, which exhibit both the aligned (A‐type) CoSi2 and twinned (B‐type) orientation. For a higher dose of 3×1017 Co/cm2, a monocrystalline epitaxial CoSi2 layer near the top of the implanted Co distribution is formed during the implantation. The heteroepitaxial structures that are formed in this way are fully aligned. In contrast, when these structures are formed by sequential surface deposition techniques, twinning occurs at every Si/CoSi2 interface. The formation of the aligned orientation of the buried CoSi2 layer can be attributed to the larger stability of aligned precipitates as compared to twin‐oriented precipitates.

Synthesis of heteroepitaxial Si/CoSi2/Si structures by Co implantation into Si

Heteroepitaxial Si/CoSi2/Si structures have been formed in both (100) and (111)Si by high‐dose implantation of Co. During the implantation, Co is incorporated in the Si lattice as CoSi2 with the same orientation as the Si substrate. Upon annealing the implanted Co distribution is transformed into an epitaxial Si/CoSi2/Si layered structure. Sputtering of Si during the implantation of Co was studied using an implanted Si3N4 marker layer. The Si/CoSi2/Si/Si3N4/Si multilayer structure that is formed in this way demonstrates the potential of the ion beam synthesis technique.

Observation and analysis of epitaxial growth of CoSi2 on (100) Si

CoSi2 layers formed by the thermal reaction of vapor‐deposited Co films on Si(100) substrates have been studied by transmission electron microscopy, and x‐ray diffraction. It is shown that first a layer of CoSi is formed between Co and Si. Only thereafter is the formation of CoSi2 initiated at the Si/CoSi interface. In view of the similarity of the crystal structure and the small lattice mismatch between the Si and the CoSi2, epitaxy of aligned (100) CoSi2 is expected to occur. However, in addition to an aligned (100) orientation, CoSi2 occurs in a number of orientations, including a (110) preferential orientation. Many individual grains are composed of subgrains, slightly rotated with respect to each other and connected by small‐angle boundaries. It is shown that the observations can be largely attributed to the geometrical lattice match between CoSi2 and Si. A computer program has been developed that searches systematically for a large number of possible geometrical matches. It allows us to calculate ep...

Ion beam synthesis of heteroepitaxial Si/CoSi2/Si structures

The formation of buried single crystalline CoSi2 layers within a monocrystalline Si substrate by high‐dose ion implantation of Co has been studied. Comparison of measured Co distributions with profiles obtained from Monte Carlo calculations has revealed the two basic phenomena that are responsible for the formation of buried layers. The enhanced stopping due to the incorporation of high concentrations of Co into Si has been identified as the dominant effect in the ion beam synthesis of buried layers. The high stopping near the top of the implanted distribution causes accumulation of Co at this point, which promotes buried layer formation. Sputtering brings the entire Co profile closer to the surface. After implantation at a temperature of 450 °C, Co is present in the form of coherent CoSi2 precipitates. Precipitates occur both in a twinned and an aligned orientation and are highly strained due to the lattice mismatch with Si. For high doses a buried monocrystalline and aligned CoSi2 layer forms within the...

Amorphous and crystalline oxide precipitates in oxygen implanted silicon

We studied precipitation of oxygen in the region below the buried oxide of a silicon‐on‐insulator structure formed by high‐dose implantation of oxygen. Underneath the oxide layer there is first a region containing amorphous precipitates, spherical in shape. At greater depth, platelike precipitates of the monoclinic silica phase coesite are observed on {113} silicon planes. The lower interface of the buried oxide is very rough compared to the upper interface. The morphology of the implanted structure is attributed to intrinsic point defects. In particular it is proposed that a high concentration of self‐interstitials occurs below the oxide as soon as it becomes a continuous layer. This leads to a large reduction of the oxidation rate in this region. Oxidation then only occurs above the buried oxide, reducing the thickness of the superficial silicon film.

Examination of models for Zn diffusion in GaAs

Concentration profiles of Zn in GaAs have been calculated for two models: the Frank–Turnbull model involving Schottky defects and the ‘‘kick‐out’’ model involving gallium Frenkel defects. Comparison with experimental data shows that the latter model gives a better description. This indicates that Frenkel defects play a predominant role in this diffusion process.

New trends in SIMOX

Abstract Recent developments of the SIMOX technology, in which a silicon-on-insulator structure is synthesized by high-dose implantation of oxygen into Si, have been reviewed. A problem with these structures is that the Si layer generally contains a high density of dislocations (10 8 –10 9 dislocations/cm 2 ). However, recently three different methods have been reported in which the density of these defects was reduced to less than 10 5 dislocations/cm 2 by optimization of the implantation conditions. These methods have been discussed and relations have been established between the processing conditions, the steady-state point defect concentrations and the microstructure of the material. The relations may be used as guidelines for further improvement of these structures. The success of the SIMOX technology has also stimulated the research on the synthesis of other compounds in Si. In general the structures formed by the ion beam synthesis technique are not only of good quality, but they frequently exhibit unique features, illustrating the potential of the SIMOX and other ion beam synthesis techniques.

Properties of CoSi2 formed on (001) Si

Properties of CoSi2 films formed on (001) Si have been investigated for silicide films formed by solid‐state reaction of either evaporated or sputtered Co with (001) Si substrates. It turns out that CoSi2 films formed from evaporated Co have a systematically higher resistivity than films formed from sputtered Co, which can be attributed to contaminants that were present in the evaporated metal film. Silicide films formed from evaporated Co develop a strong (110) texture, which becomes more pronounced with increasing annealing temperature. Microstructural analyses revealed that this is due to epitaxial growth of (110) CoSi2 upon (001) Si. This is a surprising phenomenon as the epitaxy of (001) CoSi2 on (001) Si would be expected in view of the similarity of the crystal structure and the small lattice mismatch of CoSi2 and Si. The expected epitaxy of (001) CoSi2 on (001) Si is also present but for a smaller fraction of the silicide grains. The film stress has been analyzed in terms of thermal and intrinsic ...

Ordering of oxide precipitates in oxygen implanted silicon

An order network of oxide precipitates has been found in the monocrystalline superficial silicon layer of a silicon‐on‐insulator structure obtained by high‐dose oxygen implantation. The network consists of oxide precipitates 2 nm in size spaced about 5 nm apart. Electron diffraction patterns indicate that ordering occurs both parallel and perpendicular to the surface along the 〈100〉 directions of the silicon lattice. The precipitate network has a cubic symmetry.