Secondary ion mass spectrometry helps semiconductor manufacturing

Semiconductor devices are ubiquitous in today’s electronics. At their core, they are simple: a transistor containing semiconductor materials with dopant. Rapid thermal anneal, the process of heating a wafer to high temperatures often over 900°C, activates the dopant and enforces a specific distribution of the dopant in a transistor.

Because the process is highly sensitive to temperature differences, sheet resistance (Rs) measurements are commonly used to monitor RTA, but the method may introduce unexpected errors. Instead, Jiang et al. adapted secondary ion mass spectrometry (SIMS) for RTA monitoring.

“Rs measurements have been reportedly subjected to errors caused by the wafer aging effect,” said author Zhixiong Jiang. “Also, variation in test wafers caused by ion implant can add uncertainty, and the Rs measurement is not sensitive enough to catch the process variation when the anneal temperature is very high.”

They pivoted to SIMS and used a non-annealed wafer in a batch implant as a reference for the remaining wafers in the same batch. To identify the ideal conditions for SIMS monitoring, they analyzed key parameters of SIMS depth profiles in wafers annealed at various temperatures and continued to refine their data process to achieve the best sensitivity and repeatability.

This new process boasts numerous benefits over the previous Rs technique.

“SIMS monitoring eliminated the impact from aging effect and test wafer variations as seen in some Rs measurements. It has a sensitivity to detect 1°C change in an anneal temperature higher than 1110°C,” said Jiang. “The long-term uncertainty in the SIMS monitoring data is ≤±0.5%, compared to the long-term uncertainty in normal carefully controlled SIMS analyses which is ±1.5%.”

Source: “Monitoring of rapid thermal anneal with secondary ion mass spectrometry,” by Z. X. Jiang, A. Ravi, T. Breeden, K. Khmelnitskiy, A. Duncan, D. Huynh, S. Butler, B. Granados, D. Acker, J. Luebbe, D. Sieloff, S. Bolton, and G. Prieto, Journal of Vacuum Science and Technology B (2024). The article can be accessed at https://doi.org/10.1116/6.0003599 .