Study Shows Wafer Cleaning May Improve Film Thickness Metrology Results
Rudolph Technologies, Inc., a provider of film thickness metrology systems, has released the results of a one-year study that investigated the film thickness stability of the thin SiO2 films on silicon that are used to calibrate and cross calibrate film thickness metrology systems. Film thickness metrology tool stability is a crucial issue in wafer processing, and it is important to use a reproducible reference standard to facilitate accurate tool matching. Tests were performed using a Rudolph Model 436 research grade ellipsometer. Results suggest that cleaning the reference wafer before using will maintain a thin oxide film as a reliable thickness standard.
In the study, the thickness of a thin oxide and the dynamics of what appeared to be a contaminant layer that forms on the oxide surface were monitored. As expected, the data indicate that an oxide film grows rapidly on a bare silicon surface. Thereafter, the measured film thickness continues to increase at about 0.2 angstroms per month. This work indicates that the slow thickness increase is not solely due to diffusion limited oxide growth, but is primarily due to what is believed to be airborne molecular contaminants adsorbing onto the oxide surface. This is a particular problem when comparing data from different locations.
At the beginning of the study, a silicon wafer was etched to bare silicon. Immediately after etching, the wafer was measured and 12 angstroms SiO2 (oxide) film was detected. Six months later, the measured film thickness had increased to approximately 17 angstroms and continued to grow at a rate of about 0.2 angstroms per month. H2O2 cleanings were performed every two to three months thereafter, and after each cleaning the oxide film repeatedly measured 14 angstroms. This indicates that the underlying oxide film was stable and it remained stable over the one-year duration of the study. The measured film thickness (which resulted from the underlying oxide plus the adsorbed contaminants) did, however, demonstrate a rapid thickness increase after each cleaning, suggesting that cleaning may be the best method to achieve repeatable film thickness measurements of reference wafers. Results were confirmed by measuring a similar phenomenon on two different multi-domain production ellipsometers in a tool matching study.