There is a bit of a battle in the analytical arena when it comes to the best method for measuring trace contaminants on the surface of a silicon wafer. We described the analysis of trace metals on silicon wafers by Vapor Phase Decomposition – Inductively Coupled Plasma Mass Spectroscopy (VPD-ICPMS) in a previous blog post. TXRF or Total Reflection X-Ray Fluorescence is another technique that is often used in semiconductor manufacturing to monitor contamination. Where VPD-ICPMS collects the contaminants in a droplet that is then analyzed by mass spectroscopy, the TXRF instrument uses an x-ray beam to excite the wafer surface. Elements fluoresce after being excited, the fluorescence is then measured to determine what elements are present and in what amounts. There is considerable overlap between the two techniques, but each has its own unique features.
VPD-ICPMS on one hand is a very well-defined process and provides analysis of the top surface of the wafer, about 20 Angstroms. Whereas TXRF is more versatile and can be used on different wafer surfaces. Below is a comparison of the two techniques.
|Contaminants from the entire wafer surface are collected and analyzed.||Only the area excited by the X-Ray beam (~2cm spot) is analyzed. To do the entire wafer multiple spots are required e.g., a 300 mm wafer will need 350 spot analyses to cover the full surface of the wafer.|
|Results are cumulative for the wafer surface, no spatial information is available.||TXRF can produce maps showing the impurity distribution on the wafer surface.|
|10-100X greater sensitivity.|
|Detects light elements like Li, Na and Mg where TXRF cannot.||Detects non-metals like Cl, and Ar where VPD-ICPMS cannot.|
|Considered destructive analysis||Non-destructive analysis.|
|Wafer can only be analyzed 1x.||Wafer can be reanalyzed as needed.|
|Analyzes ~20 Angstroms or thickness of oxide film.||X-Ray beam penetrates ~ 500 Angstroms.|
|Can only analyze bare Si or Si with SiO2 films.||Will analyze oxide and any amorphous or crystalline film beneath it.|
VPD-ICPMS and TXRF use very different methods to analyze for contamination on wafer surfaces. These different mechanisms however provide certain benefits for each. In fact, chemists and engineers realized the advantages and now many semiconductor fabrication facilities use an integrated VPD-TXRF system to monitor contamination.