Charge Collection Measurements for Energetic Ions in Silicon

Measurements of charge collection efficiency have been performed using 4He and proton beams from a Van de Graaff accelerator incident on test structures fabricated in MOS and diffused junction versions on nominal 1, 4 and 100 ohm-cm silicon. The purpose of these measurements is to determine the importance of the funnel effect in charge collection under a variety of conditions. The relevant dimensions of the three rectangular structural elements used in this study are approximately 2, 10, and 100 μm in width by 100 μm in length. The diameter of the incident ion beam was reduced by passing it through a 2.5 μm diameter pinhole aperture. Data on the 2 μm by 100μm diffused junction element on 4 and 100 ohm-cm silicon show evidence of charge funneling in the diffused junction version. No evidence for funneling was observed in 1 and 100 ohm-cm silicon MOS structures with a 1000 Å thick gate oxide, but funneling may occur in 4 ohm-cm silicon MOS structures with a 500 Å thick gate oxide, an important result for VHSIC-type devices.