Correlation of threshold voltage (Vt) on long channel transistors with Vt implant dose determined by high precision SIMS

A large variation in the threshold voltage (Vt) of long channel PMOS transistors was observed over a three month period. The Vt varied for 15 lots by 9.2% (min-to-max). Therma Wave measurements were made directly on the wafers of each lot after the Vt implant. Therma Wave readings varied for the 15 lots by only 1.2% (min-to-max). No correlation was found between Vt and TW. SIMS monitor wafers were implanted with each lot. High precision SIMS depth profiles were measured on each monitor wafer. SIMS dose readings varied for the 15 lots by 9.3% (min-to-max). An excellent correlation was found between Vt and SIMS dose. The squared correlation coefficient (r²) was 0.92. This correlation verified that the root cause of the Vt variation was the dose delivered by the implanter. Details of the high precision SIMS technique are discussed, as well as a comparison between measurements by 2 SIMS labs