Characterization of Shallow Junctions Fabricated by Gas Immersion Laser Doping (Gild)

Abstract

ABSTRACTThe viability of Gas Immersion Laser Doping (GILD) for VLSI processing of ultra shallow junctions is assessed using chemical, electrical and structural characterization of boron doped diodes. Diodes with good ideality factors (1.1) overarange of junction depths (50nm Xj 200 nm) have been fabricated by GILD. This process uses a pulsed XeCI excimer laserincident on a silicon surface saturated with B2H6.Dopant profiles as a function of laser energy and number of pulses aredetermined using Secondary Ion Mass Spectrometry(SIMS). For low energy or a large number of pulses, comparison with computer modelling suggests the junction is determined by melt depth. For higher laser energy and few pulses, liquid phase diffusion limits the depth of dopant incorporation.Leakage current measurements as a function of diode perimeter to area (P/A) ratio and Deep Level Transient Spectroscopy (DLTS) suggest that leakage occurs along the diode perimeter, and is dueto point defects generated from thermal stresses during melt regrowth. Diodes show good I-V characteristics after GILD alone, yet subsequent rapid thermal annealingisfound to further reduce leakage currents, probably due to relief of thermal stresses. Sheet carrier densities from Halleffect measurements show that 5 - 10% of the boron is activated, with doping levels exceeding 1020 cm−3 in some samples. Transmission Electron Microscopy (TEM) demonstrates that reasonable crystalline quality is maintained for moderate GILD conditions with a defect density at the surface of approximately 108 cm−2 .For higher laser energy with boron incorporation exceeding solid solubility, TEM shows stacking faults along <110>directions. Electron diffraction on highly doped samples shows extra spots indicating a high degree of strain in the doped layer.