Issues involved in the use of an As++ punch-through implant for a PMOS transistor

Use of an arsenic (As) punch-through implant in 0.5μm PMOS transistors is shown to have the benefits of better short channel characteristics for an equivalent junction breakdown voltage when compared to a phosphorous implant. Implementation of this double charged implant in a production flow however gave rise to problems in the Vt control of the devices. An energy contamination issue has been identified that depends on the type of source that is used in the implanter. This was confirmed by SIMS analysis. Further study points to the breakup of the As₂ dimer as the cause of the energy contamination. The As⁺⁺/As₂ ratio was found to depend on the type of source used. An optimization of source conditions is presented which lowers the risk of this contamination and gives stable transistor Vt´s. It is also shown that proper dose matching of this implant is crucial in Vt control due to the strong influence of the tail of this implant on the transistor Vt.