Disilane-based cyclic deposition/etch of Si, Si:P and Si1−yCy:P layers: I. The elementary process steps

Abstract

We have benchmarked the 550 °C, 20 Torr growth of Si:P and Si1−yCy:P using SiH4 and Si2H6. P segregation has prevented us from reaching P+ ion concentrations in Si higher than a few 1019 cm−3 using SiH4; the resulting surface ‘poisoning’ led to a severe growth rate reduction. Meanwhile, [P+] increased linearly with the phosphine flow when using Si2H6 as the Si precursor; values as high as 1.7 × 1020 cm−3 were obtained. The Si:P growth rate using Si2H6 was initially stable then increased as the PH3 flow increased. Mono-methylsilane flows 6.5–10 times higher were needed with Si2H6 than with SiH4 to reach the same substitutional C concentrations in intrinsic Si1−yCy layers ([C]subst. up to 1.9%). Growth rates were approximately six times higher with Si2H6 than with SiH4, however. 30 nm thick Si1−yCy layers became rough as [C]subst. exceeded 1.6% (formation of increasing numbers of islands). We have also studied the structural and electrical properties of ‘low’ and ‘high’ C content Si1−yCy:P layers (∼ 1.5 and 1.8%, respectively) grown with Si2H6. Adding significant amounts of PH3 led to a reduction of the tensile strain in the films. This was due to the incorporation of P atoms (at the expense of C atoms) in the substitutional sites of the Si matrix. Si1−yCy:P layers otherwise became rough as the PH3 flow increased. Resistivities lower than 1 mΩ cm were nevertheless associated with those Si1−yCy:P layers, with P atomic concentrations at most 3.9 × 1020 cm−3. Finally, we have quantified the beneficial impact of adding GeH4 to HCl for the low-temperature etching of Si. Etch rates 12–36 times higher with HCl + GeH4 than with pure HCl were achieved at 20 Torr. Workable etch rates close to 1 nm min−1 were obtained at 600 °C (versus 750 °C for pure HCl), enabling low-temperature cyclic deposition/etch strategies for the selective epitaxial growth of Si, Si:P and Si1−yCy:P layers on patterned wafers.