A. H. Reader

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.

The formation of an amorphous silicide by thermal reaction of sputter‐deposited Ti and Si layers

The initial reaction in amorphous Si‐Ti‐amorphous Si trilayers was investigated with Auger electron spectroscopy, transmission electron microscopy, and x‐ray diffraction. It was clearly demonstrated that at temperatures not exceeding 450 °C an amorphous Ti‐Si alloy is formed. At temperatures of 500 °C and higher, crystalline TiSi2 with the ZrSi2 (C49) structure was found. The growth kinetics of the amorphous silicide could not be described by a simple diffusion controlled process. It was shown that at 400 °C the thickness of the amorphous silicide is limited to approximately 18 nm. Since it was found that the growth of the amorphous phase is accompanied by excessive Kirkendall void formation, it is proposed that these voids eventually suppress the growth of amorphous silicide. The composition of the amorphous phase was determined to be between TiSi0.9 and TiSi1.2, i.e., close to the composition of the monosilicide.

Nucleation and growth of titanium silicide studied by in situ annealing in a transmission electron microscope

We have studied the crystallization of sputter‐deposited amorphous Ti‐Si alloy thin films of different compositions. A versatile means of discriminating between phenomena occurring in the nucleation and subsequent growth stages was offered by in situ annealing in a transmission electron microscope. Nucleation of TiSi2 with the ZrSi2 structure was observed for all alloys studied (Ti:Si=1:2 to Ti:Si=1:3). The activation energy for nucleation was independent of composition. TiSi2 crystallites appeared to possess a rather high density of stacking faults, predominantly concentrated in the center of the crystallites. The character of these stacking faults was analyzed. The growth of TiSi2 was anisotropic in amorphous alloys up to a composition of Ti:Si=1:2.5; more silicon‐rich alloys yielded isotropic growth. The anisotropy in growth rate, and the resulting morphology of the crystallites were related to the crystal structure. The crystallization of the amorphous alloy with the composition Ti:Si=1:2 proceeded vi...

MICROSTRUCTURE OF OXIDIZED LAYERS FORMED BY THE LOW-TEMPERATURE ULTRAVIOLET-ASSISTED DRY OXIDATION OF STRAINED SI0.8GE0.2 LAYERS ON SI

Ultraviolet‐assisted low‐temperature (550 °C) dry oxidation of Si0.8Ge0.2 strained layers on (100)Si has been studied. The oxidation rate of this material was found to be a factor of 2 greater than that of pure Si oxidation under identical irradiation conditions. Initially, the structure of the oxidized material consists of a SiO2 layer on top of a strained Si1−xGex layer with a Ge concentration significantly higher (x≳0.2) than the initial value. Increasing the oxidation time produces more SiO2 and a Si1−xGex layer further enriched with Ge. However, the oxidation rate is reduced and some of the Ge becomes trapped inside the growing SiO2 layer. For a prolonged irradiation time (≳5 h) SiGe oxidation still continues, unlike the case for pure Si, while the Ge trapped inside the SiO2 forms isolated microcrystalline regions.

Low temperature synthesis of Ge nanocrystals in SiO2

A novel and simple technique for the synthesis of Ge nanocrystals embedded in SiO2 is reported. The method is fully compatible with silicon microelectronic technology and relies solely upon low temperature (only 550 °C) ultraviolet oxidation of Si0.8Ge0.2 strained layers. This temperature is significantly lower than that usually used for the formation of Ge nanocrystals from SiGe oxides by H2 reduction.

Thin‐film reaction between Ti and Si3N4

The thermal reaction of Ti with Si3N4 was studied over the temperature range from 500 to 800 °C with Auger electron spectroscopy, Rutherford backscattering spectrometry, and transmission electron microscopy. The initial reaction consumes part of the silicon nitride and yields a two‐layer morphology of Ti(N) on top of Ti5Si3. As the reaction proceeds, the Ti5Si3 layer is converted to TiSi2. At 800 °C, a multilayer morphology is observed containing primarily TiN and TiSi2.

Crystallization of amorphous Ti‐Si alloy thin films: Microstructure and resistivity

Cosputtered amorphous thin films consisting of Ti and Si were crystallized to the C49 TiSi2 phase at temperatures of about 390 °C. Prolonged annealing of the crystalline C49 TiSi2 phase at higher temperatures resulted in a gradual decrease in resistivity. We investigated the changes in microstructure which were associated with this slow decrease of the resistivity of the C49 phase. In situ resistivity and Hall‐effect measurements, electron microscopy, and x‐ray diffraction were used. It was demonstrated that the slow decrease in resistivity of the crystalline C49 phase was associated with a decreasing stacking fault density (typically 2×106 cm−1) and a decreasing density of point defects. The presence of point defects was found to be related to stoichiometry deviations. Polymorphous crystallization of a slightly Si‐rich sample was found to result in a supersaturated solid solution of Si in C49 TiSi2. It was proposed that the phase field of the C49 phase is significantly wider than that of the equilibrium ...

Influence of microstructure on the resistivity of MoSi2 thin films

A study of electrical transport properties in MoSi2 thin films revealed a large resistivity difference of 57 vs 157 μΩ cm at room temperature between films formed from a codeposited Mo/Si structure and layers formed by reaction of deposited Mo with Si. The resistivity difference was found to be temperature independent. The Hall effect in the films formed from deposited Mo was a factor of four larger than in films formed from a codeposited alloy. The temperature dependencies of the Hall effect were also found to be different. Analyses of the films by Rutherford backscattering and transmission electron microscopy revealed no significant differences in thickness or grain size of the layers. The only microstructural difference is the stacking fault density, which is very high in the high‐ohmic films. The mechanism by which the stacking faults influence the electrical properties of MoSi2 and other refractory metal silicides is discussed, and relations are established between the method of deposition, the micro...

Characterization of amorphous silicon

S‐parameter positron beam measurements have been done on several kinds of a‐Si: Kr‐sputtered a‐Si, PECVD a‐Si, MeV ion beam amorphized Si and a‐Si grown in an MBE‐system at a low deposition temperature. Kr sputtered a‐Si becomes denser for higher Kr concentration. PECVD a‐Si:H contains micro‐cavities with a size depending on growth temperature. MeV ion beam amorphized Si contains 1.2 at. % small vacancies, which decreases upon annealing (relaxation) to 0.4 at. %. This effect can be mimicked by H‐implantation and subsequent annealing, showing that at least some of the dangling bonds in a‐Si are located at these vacancy‐type defects. Finally positron measurements show that MBE‐system grown a‐Si contains large open‐volume defects. The positron annihilation data are supplemented by data from some other techniques.

A Solid State Amorphisation Reaction in Ti-Si Diffusion Couples: The Phase Field

The reactions in sputter deposited Si-Ti-Si diffusion couples were investigated with X-ray diffraction, Auger electron spectroscopy and cross-section transmission electron microscopy. Anneals at a temperature of 400°C resulted in the growth of an amorphous phase at the Si-Ti interfaces. Crystalline silicides were only found after an anneal at temperatures of 500°C or higher. It was demonstrated that an amorphous layer of approximately 8 nm thickness sustained a concentration gradient from about 73% Si at the Si side to about 28% Si at the Ti side of the diffusion couple. The measured width of the phase field agreed with the width predicted from a calculated free energy versus composition diagram. Actually the observed phase field was found to be so wide, that it contains the stoichiometry of all equilibrium silicides. The consequences of our results for the explanation of silicide first phase nucleation were discussed.