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Why does Versum Materials prefer digestive metal analysis methods for Organometallic Materials (OM)?

As semiconductor dimensions continue to shrink, customers are requiring very tight control limits on trace elemental impurities in the precursors used for deposition. It is important for the supplier to ensure that the reported levels of trace metals in the Certificate of Analysis is an actual representation of the metallic impurities in the product. Inductively Coupled Plasma Mass Spectrometry (ICPMS) is the technique of choice for determining trace metals concentration. ICPMS technique is ideally suited for determining metals in an aqueous solution. It is not possible to analyze the deposition precursors directly using ICPMS. One needs to quantitatively transfer the metallic impurities from the precursor sample to an aqueous solution.

For process chemicals and most organosilicon precursors the metallic impurities present in the precursors are not volatile and an evaporation technique is used. The sample is evaporated and the nonvolatile residue (which will contain the metallic impurities) is dissolved in acid and analyzed using ICPMS. However, for most OM precursors, this technique will not work since the metallic impurities in these precursors are themselves volatile. In fact, sublimation is often used for purification of these precursors. Consequently, the product will be certified as having very low metallic impurities. The following is an example of this issue:

The following is an example of this issue: Comparison of ICPMS Analysis of Hafnium Tertbutoxide (All quantities in ppbw)

 Batch 1 Batch 2
ElementEvaporationDigestionEvaporationDigestion
Al0.5431.128
As0.1350.179
Bi1.9981.5109
Cd0.2100.334
Crnd30.113
Cond60.125
Fe1.8370.549
Pbnd140.114
Mg0.3210.150
Mnnd4nd7
Mo0.1240.118
Nind100.128
Ag0.2950.1123
Ti7.613800.42283
Zr253257667161288667

In order to accurately quantify the level of metallic impurities in our OM precursors, Versum Materials has developed microwave digestion procedures to capture the metallic impurities in an aqueous matrix, which can then be easily analyzed using ICPMS instrumentation. The acid mixtures used for the digestion are carefully selected to ensure that the OM precursor and the metallic impurities are fully dissolved in the resultant digested solution. The OM matrix in the digest presents challenges due to suppression of sensitivity and interference with metallic impurities of interest. These matrix issues are alleviated by using high sensitivity, triple quadrupole based ICPMS instrumentation. Reaction and collision gases are also carefully chosen to remove the effect of interference on the metallic impurity of interest.

The Certificate of Analysis provided with the OM precursors from Versum Materials accurately represent the actual level of metallic impurities in the OM precursor.

Author

Suhas Ketkar has been with the company for 26 years and is the director of advanced analytical technology. Suhas graduated from the University of Texas at Austin with a Ph.D. in Physics and from Wharton/University of Pennsylvania with a master’s degree in management of technology.

Suhas.Ketkar@nullversummaterials.com