Abstract
The formation and thermal stability of titanium silicides in blank and patterned (001)Si have been studied by sheet resistance measurement, secondary ion mass spectroscopy (SIMS), transmission and scanning electron microscopy (TEM, SEM), glancing incidence x-ray diffractometry (GIXRD), high-resolution transmission electron microscopy (HRTEM) in conjunction with auto-correlation function (ACF) analysis, Auger electron spectroscopy (AES), and energy dispersion analysis of x-ray (EDAX).For Ti on patterned Si samples, the phase transformation of C49- to C54-TiSi2 becomes increasingly difficult as the size of oxide openings decreases. The results can be attributed to the low density of nucleation sites for the C54-TiSi2 phase. The miniature size oxide opening patterns of 0.1 - 10 mm in size were defined by standard optical lithography processes.For Ti on patterned (001)Si samples, the samples were first annealed at 600 ℃ for 60 s followed by annealing at 800 or 900 ℃ for 60 s. From TEM observations, after second step annealing at 800 ℃, C54-TiSi2 was the only silicide phase formed inside larger than 3 mm contact holes. Inside 1.4 to 2 mm contact holes, both C49-TiSi2 and C54-TiSi2 were observed to form. C49-TiSi2 was the only silicide phase formed inside 0.22 - 1.2 mm contact holes. As the second step annealing temperature was increased to 900 ℃, C54-TiSi2 was the only silicide phase formed inside 1.4 mm or larger contact holes. However, severe agglomeration of TiSi2 was observed. On the other hand, only C49-TiSi2 phase was detected inside 0.35 - 0.7 mm contact holes. Similar trends were found for silicide formation inside 0.9 - 4 mm linear openings. In addition, for Ti on 30 keV, 2 x 1015 /cm2 BF2+ implanted patterned (001)Si samples, C49-TiSi2 was the only silicide phase detected to form inside 0.3-0.6 and 0.18 - 0.7 mm contact holes after 800 and 900 ℃ annealing, respectively. End-of-range (EOR) defects as well as fluorine bubbles are evident.In order to reduce the phase transformation temperature of C49- to C54-TiSi2, an ultrathin interposing Mo layer (~ 0.5 nm) was used as a seed layer. For Ti on blank and patterned (001)Si samples with a thin interposing Mo layer, complete formation of C54-TiSi2 at 650 ℃ was achieved. The island formation of C54-TiSi2 and formation of pinholes were found for 900 ℃ annealed samples. Based on the sheet resistance, TEM, GIXRD, EDAX and SIMS data, it is conjectured that the redistribution of Mo atoms, in the form of a ternary Ti-Mo-Si phase, leads to the enhancement of the formation of C54-TiSi2 by providing more heterogeneous nucleation sites needed for the transformation from C49- to C54-TiSi2 phase. In addition, for Ti/Mo bilayer on patterned (001)Si samples after first step annealing at 650 ℃ for 30 s, C54-TiSi2 phase was the only silicide phase formed inside 0.16 - 0.36 mm oxide openings. The results clearly demostrate that the complete formation of low-resistivity C54-TiSi2 can be achieved by a thin interposing Mo layer in deep submicron devices.For ultrashallow junction formation, nitrogen ion implantation was found to suppress the B and As diffusion in silicon and the effect increases with the nitrogen dose. For N+ doses at a level of 5 x 1015 /cm2 or higher, simultaneous activation of both B and As was impaired.For Ti on nitrogen ion implanted ultrashallow junctions with a thin interposing Mo layer, a continuous C54-TiSi2 layer was found to form in all implanted samples annealed at 650-900 ℃. From the analysis of SIMS and ESCA, the presence of nitrogen atoms in TiSi2 is thought to lower the silicide/silicon interface energy and/or the silicide surface energy to maintain the integrity of the C54-TiSi2 layer without strong bonding with Ti during annealing at high temperatures. For Ti/Mo bilayer on nitrogen ion implanted patterned Si samples after annealing at 900 ℃, stable and continuous C54-TiSi2 layers were found to form. As a result, with appropriate control, an interposing Mo layer combined with N2+-implantation can be successfully implemented in forming low-resistivity C54-TiSi2 contacts and enhancing the thermal stability of TiSi2 layer on shallow junctions in deep submicron devices.Effects of stress on the formation of titanium silicides have been investigated. The C54-TiSi2 phase transformation temperature in tensily stressed samples was found to lower by about 100 ℃ than that in compressively stressed samples. For Ti on stressed (001)Si after rapid thermal annealing, the thickness of TiSi2 films was found to increase and decrease with the tensile and compressive stress level, respectively. In addition, the thickness of amorphous interlayers (a-interlayers) between Ti films and Si substrates was found to be thicker and thinner in the tensily and compressively stressed samples, respectively. From auto-correlation function (ACF) analysis, the thicker a-interlayer was found to consist of a higher density of crystallites. The crystallites provide nucleation sites for C49-TiSi2 and facilitate the formation of C49-TiSi2 of small size. The small grain size of C49-TiSi2 in turn enhances the formation of C54-TiSi2.