Low Temperature Oxidation of PTSI on As-Doped Si
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LOW TEMPERATURE OXIDATION OF PtSi ON As-DOPED Si J.P. GAMBINO, B. CUNNINGHAM, F.E. TURENE, IBM East Fishkill, Hopewell Junction, NY
J.F. SHEPARD 12533
ABSTRACT PtSi on As-doped polysilicon oxidizes rapidly at temperatures as low 0 as 500 C. The resulting SiO2 -PtSi and PtSi-polysilicon interfaces are very rough. Silicide inclusions are present in the oxide, probably due to differences in oxidation rate between different PtSi grains. The presence of some inclusions near the Si02 surface suggests that PtSi dissociates during oxidation. Rapid oxidation does not occur for PtSi on B or P-doped 20 3 polysilicon, or for As concentrations of lx10 cm- or less.
INTRODUCTION It is well known that heavily doped Si [1] and some silicides [2] oxidize more rapidly than undoped or lightly doped Si. However, relatively little is known about the effect of doping on the oxidation of silicides. Liu et al. [31 found that for TaSi 2 , B enhances the oxidation rate whereas As retards the oxidation rate. In contrast, Lechaton [4] observed enhanced oxidation for PtSi on As-doped Si at 550°C in air. The oxidation of PtSi on As-doped Si is of both technological and scientific interest. In an earlier study, it was observed that PtSi penetration of As-doped polysilicon contacts to shallow junctions is enhanced by annealing in oxygen ambients, resulting in high leakage The system is of scientific interest because the combination currents [5]. of a silicide and a dopant leads to faster Si0 2 formation than either the silicide alone or the dopant alone. In the present study, the oxidation of PtSi on As-doped polysilicon was measured as a function of temperature, time, and As concentration, in an effort to understand the oxidation mechanism.
EXPERIMENT A 35 nm field oxide was grown on the starting substrates, then stripped off one half of each wafer, resulting in wafers half covered with Si02 . Polysilicon (180 nm) was deposited, implanted with As, and annealed (880°C, nitrogen). In most cases, the concentration in the polysilicon was 21 3 Ixl0 cm . However, some samples were20 prepared 19 20 21 with - different As 1 concentrations (0, 5x0 , 1x1O , 5x1O , 2x10 cm ), and some were 2 3 prepared using B or P (ixl0 ' cm- ) rather than As. Pt (45 nm) was deposited on the polysilicon and sintered at 4000C in nitrogen for 30 min, forming -90 nm of PtSi and consuming -60 nm of polysilicon. Wafers were then oxidized in dry oxygen at 1 atm between 400 and 6500C. Oxide thicknesses were measured using Auger Electron Spectroscopy on the half of the wafer with polysilicon on the field oxide. The Si0 2 sputter rate was calibrated using the underlying 35 nm oxide layer, comparing the area under the oxygen signal of the field oxide to that of the PtSi oxide. The chemical identity of the PtSi oxide was determined Mat. Res. Soc. Symp. Proc. Vol. 106.
1988 Materials Research Society
182
using X-ray Photoelectron Spectroscopy (XPS) and the As distribution was measured by Secondary Ion Mass Spectroscopy (SIMS). The microstructure of the oxide and of the underly
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