Charge state contamination during ion implantation of triply charge phosphorus

The need for higher energy implants in advanced semiconductor processing has led to the use of multiply charged species in order to overcome the high voltage limitations of conventional ion implanters. It is normally not recommended that such implants be performed in equipment without some form of filtration for the removal of low energy contaminants. However, multiply charged implants have been performed with success using implanters with no such filtration. In these cases, special precautions must be taken to minimize the implantation of low energy contaminants. An important specific case of such contamination during the implantation of a multiply charged atomic species Xⁿ⁺ is the extraction of a singly charged molecular species X_n ⁺, which dissociates prior to magnetic analysis, yielding (among other things) the singly charged atomic X⁺. This latter species has a magnetic rigidity identical to that of the extracted, desired species Xⁿ⁺, and is therefore analyzed out to become a potential low energy contaminant. This paper presents a method that has been used to 1) identify such contamination as a cause of semiconductor device failure where the desired implanted species is P ⁺⁺⁺ and the contaminant is P⁺, and 2) define an operating space that minimizes the production of P⁺ in an implanter that uses no energy filtration to protect against such contamination. The method involves the monitoring of P₂⁺ , the diatomic, singly charged byproduct of the disintegration of P₃⁺