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Routine determination of ultra trace elements in semiconductor-grade nitric acid

Thermo Fisher has developed a system for ultra-trace elemental analytics in applications such as geoscience, which require high elemental sensitivity

Thermo Scientific iCAP Qs ICP-MS has been specifically developed for ultra-trace elemental analysis in applications such as semiconductor, nuclear and geoscience that require the highest elemental sensitivity. The iCAP Qs ICP-MS is equipped with an inert sample introduction system and a high transmission interface. Through the combination of a lightning fast, solid-state, swing frequency RF generator, proprietary 90-degree ion optics and effective interference removal using the Thermo Scientific QCell, the iCAP Qs ICP-MS provides the high elemental sensitivity and low backgrounds required for ultra-trace elemental analyses.

In semiconductor manufacturing support applications (for example, incoming supplier or process control) target concentrations are generally below 10 ng/L. In this experiment, a single ICP-MS measurement using three instrumental modes (hot plasma, cold plasma and hot plasma with KED) was used for the analysis of semiconductor-grade nitric acid.

For many elements, the use of single-mode He KED with hot plasma (HP) is sufficient for the suppression of background and sample matrix induced spectral interferences to allow for reliable measurement at these concentration levels.

For some elements however, in particular first and second group metals as well as some transition metals, analysis using cold plasma is preferable to hot plasma since contamination from the sample introduction system is reduced, leading to lower backgrounds and improved detection power.

Sample and calibration solution preparation

Precleaned PFA bottles were used for the preparation of all blanks, standards and samples. The bottles were rinsed with ultra-pure water (18.2 MΩ) and left to dry in a laminar flow clean hood before use. Multi-element standards at concentrations of 50, 100, 500 and 1000ng/L were prepared by gravimetrically adding the appropriate quantity of a multi-elemental stock solution (SPEX Certiprep, Metuchen, USA) directly to the one per cent HNO3 samples. In order to assess recovery in the one per cent HNO3, a spike recovery test at 10 ng/L was performed. Semiconductor grade HNO3 (Fisher Scientific OPTIMA) was used for the rinse and blank solutions.

Instrument configuration

The iCAP Qs ICP-MS was configured for routine ultra-trace elemental analyses in semiconductor applications. The instrument configuration and operation parameters are shown in Table 1. The iCAP Qs ICP-MS used in this study was not installed in a cleanroom.
The iCAP Qs ICP-MS was equipped with a self-aspirating PFA 100 MicroFlow nebuliser (ESI, Omaha, USA), a peltier cooled quartz spray chamber (operating at 3oC), a 2.0mm ID sapphire injector and a demountable quartz torch.


Background equivalent concentrations (BEC) and detection limits (LoD), based on three times the standard deviation of ten replicate measurements of the calibration blank, were determined for 62 elements in one per cent HNO3. Each sample was analysed in a single mixed mode acquisition with automated switching between hot and cold plasma using the same instrument configuration. Results from this measurement as well as percentage recoveries from a 10 ng/L spike recovery test are presented in Table 2.

The results in Table 2 show the suitability of the iCAP Qs ICP-MS for ultra-trace multi elemental measurement at sub ng/L concentration levels in common semiconductor process control matrices.

Improved cold plasma performance

With some earlier RF generator designs cold plasma wasn’t suitable for the routine analysis of high acid concentration samples and further dilution was required, risking contamination from additional sample handling. With the introduction of the swing frequency RF generator on the iCAP Q ICP-MS, matrix stability in cold plasma was improved allowing for higher acid concentrations to be routinely analysed.

In order to test this improved cold plasma performance, calibrations at 1, 2, 3, 4 and 5 ng/L were achieved for 12 common cold plasma elements in a seven per cent HNO3 matrix using the iCAP Qs. A 1 ng/L spike recovery test was also performed.

It was shown that cold plasma on the iCAP Q ICP-MS effectively suppresses argon based interferences and provides the high sensitivity required for pg/L (ppq) level LoD and BEC values required in semiconductor applications.

Spike recoveries from 91 per cent to 108 per cent for 12 elements at 1 ng/L in seven per cent HNO3 further support the good performance of the iCAP Qs ICP-MS in this application.


The Thermo Scientific iCAP Qs ICP-MS has been shown to provide the high sensitivity and freedom from interferences required for the measurement of ultra-trace (sub ppt) concentration levels in semiconductor grade acid samples.

Fast, automated, in-measurement switching between hot and cold plasma is made possible with the swing frequency RF generator leading to improved reliability in mixed mode applications.

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