Optimization of irradiation and measurement parameters for analyzing depth profiles of trace heavy elements in silicon using RBS technique

Abstract

The thesis work presents a new concept that integrates heavy ion Rutherford Backscattering Spectrometry with microbeam technology combined with high detection solid angle for the measurement of the depth profile and areal distribution of heavy elements in silicon using a Si microbeam. The concept revolves around using a probing beam equal or heavier than the major substrate's elements,aiming not only to increase the ackscattering cross-section, similar to well established heavy element Rutherford Backscattering Spectrometry, but also to eliminate pile-up due to backscattering from substrate, which is essential for sensitive analysis. This allows a significant increase in microbeam currents without increasing the count rate in large area detectors. The choice and characteristics of the probing beam are important for ensuring precise and reliable measurements. Various aspects of the probing beam, such as brightness,spatial resolution, and beam halo, were thoroughly investigated to enhance result accuracy and precision. A detection system with a total detection solid angle of approximately 1 sr was utilized in an unconventional geometry,where backscattering ions enter the detector at different angles resulting in longer distances in the passivation layer of the detectors.An examination into the effect of entrance angle on detector resolution was conducted,and a spectral analysis model was developed to address kinematic spread issues resulting from large detection solid angles based on Au reference thin films. Focussed 2.4 MeV Si2+ ions were scanned over the frontal or lateral surface of silicon wafers to measure both the areal concentrations and depth profiles of all elements heavier than silicon. Tests were done on samples with Platinum concentration range of 4 × 10 12–2 × 1014 at/cm3 that was diffused via the Platinum Silicide surface layer process from one lateral surface into the 400 µm thick Si wafers. An appropriate microbeam collimator was used to shield the detectors from events caused by forward scattering and scattering of the Si ion beam halo from different components in the reaction chamber. Lateral scans of the silicon wafer were performed by focussed Si ions, with ~10 µm spot size and of the nA range beam currents.The Rutherford Backscattering Spectrometry intensity maps containing the platinum depth profile were obtained with a sensitivity of 6.7 × 109 at/cm2