In the previous two articles in this semiconductor-focused series, we presented ICP-QQQ performance data for aqueous and organic process chemicals and reagents. In this article, we shift our focus to other critical analyses conducted throughout the IC fabrication process, with an emphasis on key materials such as substrates and photoresist.
Silicon wafers
Trace element contamination control begins with wafer substrates. Electronic-grade silicon typically requires a purity level of 9 to 11 nines (N) (99.999999 to 99.999999999%) since metal impurities of just a few picograms (pg) on a 200 mm wafer surface can lead to defective devices.1 As chip fabrication technologies advance, maintaining strict quality control of the substrate is needed to support higher-density architectures. ICP-MS or ICP-QQQ can be used for both bulk silicon (Si) analysis and surface element extraction using vapor phase decomposition (VPD).
In this study of high Si matrix samples, the Agilent 8900 ICP-QQQ was used to analyze two Si samples prepared at matrix levels typically analyzed in the semiconductor industry. Excellent precision was achieved in measuring 50 ppt spikes in Si matrix samples over a 1-hour analytical run, demonstrating the method's effectiveness, robustness, and sensitivity.2
In a separate study, working with experts at IAS Inc., the 8900 ICP-QQQ was used as part of an automated system to monitor contaminants collected from Si wafer substrates and associated layers and coatings via VPD (Figure 1). The system integrates into a fab's computer integrated manufacturing system, enabling continuous, unattended operation and contamination control of silicon wafers.
In the VPD technique, the surface layer (bare Si, or naturally or thermally oxidized SiO2) is dissolved using hydrofluoric acid (HF) vapor. Any residual metal-ion contamination is collected in a droplet of an aqueous solution of HF and H2O2, which is scanned over the surface of the wafer. This solution is then analyzed using ICP-MS or ICP-QQQ. H2SiF6 present on the surface will also be collected in the scanning droplet, so the solution will contain a few ppm of Si. This makes trace analysis of some elements difficult due to the formation of Si-based polyatomic ions, e.g., 30SiH on 31P and 28Si16O19F on 63Cu.
Using the automated system, detection limits below 3.0 E+07 atoms/cm2 (<1 pg/mL) were achieved for all elements, and the spike recovery data demonstrated the method’s accuracy in determining ultratrace contaminants in Si wafers.3
Wide-bandgap (WBG) substrates
In addition to conventional Si wafers, modern power semiconductor fabrication increasingly relies on wide-bandgap (WBG) substrates—such as silicon carbide (SiC) and gallium nitride (GaN)—to support advanced electrical and thermal performance. These materials are gaining popularity in power electronics due to their higher breakdown voltages and superior thermal tolerance. As with Si-based semiconductors, the electrical performance of WBG devices is highly sensitive to substrate purity. To meet stringent performance and quality requirements, contaminant levels in SiC and GaN wafer substrates must be closely monitored using appropriate analytical techniques. As discussed in another collaborative study with the team at IAS Inc., VPD is unsuitable for non-Si wafers because these materials cannot be decomposed by the HF vapor.4 So, a newly developed laser ablation-based ICP-MS system was used.
Other materials used in chip manufacturing
Other materials used in chip manufacturing that are suitable for analysis by ICP-MS or ICP-QQQ, include metal organic compounds such as trimethyl gallium (TMG), trimethyl aluminum (TMA), dimethyl zinc (DMZ), tetraethoxysilane (TEOS), and trichlorosilane (TCS). Such compounds are precursors used to grow thin metal films or epitaxial crystal layers in metalorganic chemical vapor deposition (MOCVD) and atomic layer deposition.
Pure metals such as Al, Cu, Ti, Co, Ni, Ta, W, and Hf are used as sputtering targets for physical vapor deposition (PVD) to deposit thin metal films on wafer surfaces. High-k dielectric materials include chlorides and alkoxides of Zr, Hf, Sr, Ta, and the rare earth elements (REEs). Each of these materials has a limit for acceptable levels of contaminants, requiring analysis using ICP-MS or ICP-QQQ.
Photoresist
Photoresist (PR) is one of the key materials used in the lithography step of IC fabrication, where intricate patterns are etched into the wafer's surface to build the IC circuit. Photolithography uses a UV image, projected onto the wafer surface, to reproduce the specific patterns required for each layer of the circuit, and PR is a crucial material for the lithography process (Figure 3). Different classifications of PR are used depending on whether the FAB is manufacturing printed circuit boards (PCBs), liquid crystals (LCs) and liquid crystal displays (LCDs), or ICs.
In this comprehensive study, an 8900 ICP-QQQ was used to quantify 20 critical trace elements in three IC-grade photoresist samples. The PR samples and calibration standards were prepared in Propylene Glycol Methyl Ether Acetate (PGMEA) solvent.
Single- or sub-ppt BECs were achieved for all the elements in a single acquisition (Table 1), and excellent precision and recoveries were achieved for repeated measurement of a 0.1 ppb spike in PR over a 1-hour run. The three different IC PR samples were measured after being diluted 10 times in PGMEA. The concentrations of the analytes present in the PR samples were determined against the external calibrations in PGMEA, and the dilution-corrected results (in µg/L in the original PR samples) are shown in Table 1.
Low levels of contamination were found in all three PR samples. A few elements were present at raised levels, for example, Fe, Zn, and Sn in PR Sample 1 and Ca in Sample 2. However, none of the analytes were above the current maximum 1–10 ppb range for contaminant elements in IC PR. Sample 3 did not contain any of the monitored elements at levels above 0.2 ppb, meeting future requirements for <1 ppb contaminant levels in PR. The results demonstrate the sensitivity, stability, and robustness of the 8900 ICP-QQQ method for the routine quality assurance testing of complex materials.5
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