All sample preparation and analysis were conducted in a Class 100 laminar flow hood in a Class 10000 cleanroom.

98% sulfuric acid is used in semiconductor manufacturing, and when analyzed by ICP-MS, ultrapure water is usually used to dilute it 10 times. In this study, high-purity concentrated sulfuric acid (98 wt%) from BASF (Taiwan, China Factory, China) was used, and ultrapure water (Millipore,>18.2 Ω M · cm) was used for 10 times dilution by weight to obtain a final concentration of 9.8%.

Quantitative analysis was conducted using the standard addition method (MSA). By continuously diluting the 1000 ppm single element stock solution (TruQ) listed in the "Consumables Used" table^?ms， PerkinElmer prepared a 10 ppb intermediate concentration multi-element reserve standard solution using 2% nitric acid. At 9.8% H₂SO₄Add the final standard solution to obtain concentrations of 1, 5, 10, and 20 ppt.

All experiments were performed using NexION 5000 ICP-MS (PerkinElmer)The instrument components and working conditions are shown in Table 1.

The main challenge in sulfuric acid analysis is its high viscosity, which may lead to sample deposition at the interface and potentially affect the long-term stability of the measurement. To address this issue, it is crucial to maintain analytical sensitivity while minimizing the injection volume as much as possible. This can be achieved by generating finer, drier, and more stable aerosols.

In this study, a Peltier cooling (PC3X) injection system set at 5 ℃ was used, combined with a swirl chamber and an all matrix injection system (AMS) gas inlet port. Before introducing the plasma, the PC3X system cools the aerosol, effectively filtering out larger droplets and allowing only finer aerosols to reach the plasma. AMS gas further dilutes aerosols, helping to form a finer and drier stable aerosol flow.³

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)Easily triggered by multi atomic and isotopic ions generated by plasma gas and/or matrix componentsSpectral interference influenceCommon plasma source interferences include Ar⁺、ArH⁺And ArO⁺Affects Ca separately⁺、K⁺And Fe⁺Analysis of. In reactive mode, the use of proprietary universal pooling technology (UCT) can effectively address these challenges.⁴Instruments equipped with multiple quadrupole technology, such as NexION 5000 ICP-MS, can more effectively eliminate interference.

When running in MS/MS mode, the first quadrupole analyzer (Q1) selectively transmits the target mass number while removing interfering ions, thereby purifying the ion beam and reducing background noise. Subsequently, the ions enter the quadrupole universal cell (Q2) and eliminate spectral interference through controlled gas-phase reactions. Finally, the second quadrupole analyzer (Q3) will filter out all residual interferences to ensure that only the target analyte reaches the detector. Ammonia (NH)₃）It has a low ionization energy (9.7 eV) and a pair of electrons, making it a highly reactive gas. In this study, ammonia gas was used as the reaction gas for the analysis of all target elements. As shown in Table 3, mass transfer mode was used for Zn, while MS/MS in-situ mode was used for the remaining elements.

In Ag and Pb analysis, spectral interference is not the main issue when using ammonia gas. On the contrary, using ammonia can utilize its collision focusing effect to improve sensitivity. For elements analyzed in situ mode, cold plasma conditions were also used to achieve lower background equivalent concentrations (BEC).

Table 2. Elements affected by S interference

(Click to view large image)

Among the affected elements, zinc (Zn) suffers the most severe interference and poses the greatest challenge for trace analysis. Zinc lacks the main isotopes, with its three most abundant isotopes（⁶⁴Zn、⁶⁶Zn and⁶⁸Zn) is severely interfered by sulfur based atoms. Among them, the most abundant isotope⁶⁴Zn will also be affected⁶⁴Ni's allosteric interference. In semiconductor grade process chemicals, the content of metal impurities is usually extremely low. due to⁶⁴The natural abundance of Ni is relatively low (0.926%), which has an impact on⁶⁴The interference of Zn is usually minimal. However, if the Ni level is high, it may cause significant interference.

The first ionization energy of Zn is higher than that of other target elementsMoreover, due to the lack of major isotopes, its sensitivity is inherently low. In addition, signal suppression with 9.8% sulfuric acid will further reduce sensitivity. For ultra trace analysis, maintaining sufficient sensitivity is crucial as it directly determines the detection limit and data accuracy.

The most effective method to reduce interference with Zn in sulfuric acid analysis is to use the mass transfer mode under ammonia gas mode.This method enables Zn to form clusters in the reaction pool, thereby avoiding interferenceThe universal cell equipped with Axial Field Technology (AFT) plays a crucial role in this process, optimizing reaction kinetics and efficiency by precisely controlling the residence time of ions in the reaction cell.

The AFT voltage can be adjusted between 0 and 475 V, and the reaction (DRC) mode is usually set to 250 V. However, for reactions that form high valence clusters and require longer residence times, lower voltages are usually preferred.

Zn can react with ammonia gas, and multiple Zn clusters will form in the reaction pool, which can be observed using sub ion scanning mass spectrometry. As shown in Figure 1, when the mass number of Q1 is set to 66, the highest peak is observed at the mass number of Q3 at 117, corresponding to Zn (NH)₃)₃The cluster with the highest abundance. Compared to lower levels of cluster formation,The higher degree of advantage is greater, as it can reduce the possibility of forming high priced interfering clusters with ammonia gasThis ensures that there is less background noise. When the mass number of Q1 is set to 64, a similar cluster formation pattern is also observed.

Figure 1. Display of Zn and NH₃Cluster formed sub ion scanning mass spectrometry image (click to view larger image)

AFT on Zn (NH)₃)₃The optimization of cluster formation is shown in Figure 2. As shown in the figure, the sensitivity of Zn cluster ions (66/117 and 64/115) varies with the AFT voltage, and the highest sensitivity is observed at 20 V, which is much lower than the commonly used voltage of the reaction cell. In other words, a significant improvement in sensitivity has been achieved through specific AFT optimization.

Figure 3. Calibration curves of all target elements (using 9.8% H of standard range 1-20 ppt)₂SO₄Solution (click to view large image)

Table 3. Method Settings (Click to View Large Image)

BEC depends on the impurity content of the sample itself, as well as the sample processing and pollution control in the laboratory environmentExcept for Fe, Ni, and AI, the BEC of most elements is less than 1 ppt. Sensitivity is also an important factor in obtaining accurate results at the ultra trace level of one trillionth (ppt). For H₂SO₄The highly challenging element Zn in the matrix achieved a sensitivity of 20 cps/ppt using 66/117 and a sensitivity of 35 cps/ppt using 64/115, which is currently the highest sensitivity reported in sulfuric acid analysis.

Towards 9.8% H₂SO₄Add 10 ppb of intermediate concentration multi-element standard to the solution to achieve a level of 10 ppt, and conduct stability tests using the obtained solution. The robustness of the instrument was evaluated by continuous injection and analysis of standard solutions without rinsing between samples. Using the variation of element recovery rate over time as a stability indicator.

As shown in Figure 4, during the 4-hour operation period, the recovery rates of all target elements remained within the range of 85% -115%,Indicating excellent stability of analytical methods and instrumentsIt is worth noting that this experiment is a pressure test aimed at evaluating the extreme robustness of the system. In routine analysis, it is recommended to use 1% HNO between samples₃Washing with dilute acid solution can not only remove residues from previous sample operation, but also clean up sulfuric acid deposits, thereby further improving the long-term stability of the instrument.⁵

Figure 4. 9.8% H spiked at a concentration of 10 ppt through continuous suction and analysis₂SO₄Stability test of solution conducted for 4 hours (click to view large image)