Plasma processing of the silicon surface: A novel method to reduce transient enhanced diffusion of boron

Abstract

We investigate in detail the effect of plasma processing on the transient enhanced diffusion of implanted boron in silicon. Thermally oxidized silicon wafers were first processed with CHF3/CF4 plasma and subsequently implanted with boron, with energies ranging from 3 to 20 keV and a dose of 1×1013/cm2. Chemical profiles were measured by secondary ion mass spectrometry while lattice extended defects induced in silicon by plasma processing were characterized by transmission electron microscopy. Secondary ion mass spectrometry measurements reveal that the transient enhanced diffusion of boron after rapid thermal annealing is strongly reduced in plasma processed samples with respect to unprocessed samples. Defects induced by plasma processing are responsible for the reduction by acting as very efficient traps for the interstitial atoms generated during the implant. We note that the trapping efficiency is critically dependent on the projected range of the boron implant, being extremely evident at low energies and less marked as the energy is increased (i.e., when the interstitials generated by the B implant are far away from the trapping sites). By varying the plasma conditions (an argon plasma is used instead of a CHF3/CF4 plasma), we are able to establish a general correlation between trapping defect centers and transient enhanced diffusion reduction. Finally, spreading resistance measurements reveal that the amount of electrically active boron in plasma processed pure epitaxial Si is almost equal to that obtained in samples not exposed to plasma bombardment, thus demonstrating that the plasma processing has no detrimental effect on the boron electrical activation.