Application of ultra-high energy boron implantation for superjunction power (CoolMOS™) devices

Superjunction devices (SJDs) are a novel class of power devices which break the physical limit of silicon with respect to the area-specific turn-on-resistance. SJDs consist of a modified vertical MOSFET structure which is characterized by additional deep pillar-like p-type regions formed inside the n(−) epi-layer below the transistor gate. In the present investigation ultra-high energy boron ion implantation of 2-25 MeV was applied for forming the deep p-type regions laterally structured using Si stencil masks. For energies above 12 MeV the incident ions exceed the Coulomb barrier for Si which leads (i) to a significant gamma and neutron emission during implantation and, (ii) an activation of the wafer and the mask material. However, the most relevant reaction for activation (11B + 28Si → 39K(nα) → 34mCl(β, EC) → 34S) has a half-life time of only 32 min, hence the radiation level of the wafers drops below the critical limit within the processing time. Based on the described technology Infineon Technologies successfully prepared a set of prototype wafers with fully functional high-voltage transistors. Typical blocking capability was approx. 560 V with an area-specific turn-on-state resistance of about 3.85 Ω mm2.