Pd Measurement on GaAs Wafer

Problem statement

GaAs wafers containing palladium present two major analytical problems:

Palladium Kα overlaps with the arsenic sum peak. Arsenic is abundant everywhere in the wafer making it impossible to measure palladium Kα alone.

Palladium Lα overlaps with argon Kα. Argon is the third-most abundant gas in the atmosphere; therefore, present in every XRF spectrum.

Unique solution

Elemental analysis and detection of low energies is performed with micro spots using the unique Onyx 3000 detector array, and by purging the air between the X-ray beam and sample using helium, which excludes argon from the measurement area. A controlled helium flow removes the argon from the area above the sample, resulting in a clear palladium Lα energy peak and allowing a precise measurement of the palladium thickness/concentration.

Using helium to purge argon from measurement area does not require a load lock chamber for vacuum, hence allowing measurement of large samples and improving throughput.

Figure 1: ONYX 3000 helium purge hemisphere demonstration

Figure 2: Background with and without the use of helium. The Ar peak disappears when using helium. 

Figure 3: Spectrum zoom in at the Pd Kα/As sum peak energy line. On- and off-site Pd Kα ROI intensity is very similar due to the presence of As in the wafer

Figure 4: Typical spectra measured on and off a Pd-containing feature with helium purge. The off-site spectrum shows no Pd or Ar peaks.

Conclusion

This report shows the utilization of the micro-spot ONYX 3000 capabilities for measuring palladium on a GaAs sample using a unique detector array and helium purge to efficiently remove air (argon) from the X-ray path.

Compared to WDXRF technology, ONYX 3000 EDXRF uses micro spots down to 10 µm (depending on the sample material) versus 1-20 mm in most WDXRF tools. In addition, a load lock chamber for vacuum is not needed.

This method was tested and found to be highly effective for measuring palladium thickness or concentration on GaAs samples.