Agilent has been working closely with leading semiconductor manufacturers and chemical suppliers since the early 1990s—a relationship that continues to thrive today. The needs of the industry have driven the development of many of Agilent’s key ICP-MS innovations, from off-axis ion lenses and the ShieldTorch System with cool plasma to the unique, high-sensitivity of the Agilent 8900 ICP-QQQ with MS/MS operation. In the first of a series of articles, this overview focuses on the multi-element determination of contaminants at ultratrace concentrations in aqueous process chemicals using the 8900 Semiconductor configuration ICP-QQQ.

Monitoring trace metals during all stages of integrated circuit fabrication

Semiconductor device fabrication requires strict control of sources of contamination to avoid the production of unstable microchips and yield losses. Metallic contaminants are of concern because they can affect the electrical properties of the finished device, for example by reducing dielectric breakdown voltage. Major sources of inorganic contaminants include wafer substrates and the chemicals and reagents used during the manufacturing process of integrated circuits (ICs).

Multi-element analysis of aqueous process chemicals

During IC fabrication, wafers undergo many processing steps, as illustrated in Figure 1.

Among the most critical process chemicals in terms of controlling contamination are ultrapure water (UPW) and the RCA Standard Clean (SC) solutions SC-1 and SC-2. The RCA cleaning procedure removes chemical contaminants and particulate impurities from the wafer surface without damaging the chip. SC-1 (NH₄OH and H₂O₂ in deionized water (DIW)), removes organic residues, films, and particles from the wafer surface. SC-2 (HCl and H₂O₂ in DIW) then removes ionic contaminants.

Organic materials, such as photoresist polymer patterns, must be thoroughly removed from the surface of the silicon wafer following ion implantation. This cleaning step is performed using a sulfuric peroxide mix (SPM) ‘piranha’ solution: a mixture of H₂SO₄ and H₂O₂. Ensuring a low level of metal impurities in these chemicals is vital to ensure that contamination of the wafer surface is avoided at this stage in the manufacturing process.

HNO₃ also plays an important role in the fabrication of semiconductor devices. For example, a mix of HNO₃ and HF is used to etch single-crystal silicon and polycrystalline silicon. HNO₃ is combined with phosphoric acid and acetic acid for wet etching of aluminum.

Proof-of-performance studies

Many of the leading semiconductor manufacturers use multiple Agilent ICP-MS systems in their facilities and laboratories. Since its launch in 2016, the 8900 ICP-QQQ has been used extensively to analyze many of the aforementioned chemicals and reagents in line with SEMI guidelines, as summarized in the following examples. The instrument typically comprises a Microflow nebulizer (flow rate: 200 uL/min), a quartz sample introduction system (with optional gas port), platinum interface cones, an s-lens, and ORS⁴ with axial acceleration.

Meeting single- and sub-ppt guideline levels for ASTM/SEMI elements in ultrapure water

This study demonstrated the suitability of the 8900 ICP-QQQ with optional m-lens for the measurement of ultratrace-level contaminants in UPW. The m-lens ensured that background signals for the easily ionized elements (EIEs)—K, Na, Ba, and Li—were minimized, allowing all 26 SEMI-critical elements to be measured at ppt levels using hot plasma conditions (CeO/Ce ratio < 2%). All potential spectral interferences were resolved by operating the 8900 in MS/MS mode using a single multitune method with no gas and two reaction gas modes.

As shown in Figure 2, the BECs and DLs for all elements were well below the recommendations set by ASTM and SEMI for the quality of UPW related to semiconductor-industry manufacturing at < 0.045 m linewidths. All values were below 0.5 ppt, with the exception of boron. However, as reported in an article published in a previous issue of The ICP-MS Journal, the BEC and DL for boron can be improved to 0.63 and 0.12 ppt, respectively, using the 8900 combined with the Organo Puric ω II system that includes a boron filter.

SEMI Grade 5 high-purity hydrogen peroxide

In this study, TAMAPURE-AA-10 hydrogen peroxide (35%) was used as the sample matrix. To stabilize the spiked elements, ultrapure HNO₃ (TAMAPUREAA-10) was added to the H₂O₂ samples at 1 part of 70% HNO₃ to 1000, giving a final acid concentration of 0.07%. A mixed multi-element standard solution (SPEX CertiPrep, NJ, US) was prepared and spiked into the blank H₂O₂ matrix at 10, 20, 30, 40, and 50 ppt to create the standard addition calibration solutions. Solutions were prepared just before the analysis.

Quantitative results and detection limits for the SEMI elements measured in H₂O₂ are shown in Table 1, including sulfur (S) and phosphorus (P). For the results of elements that are not specified in the SEMI standard, see the application note. Each detection limit was calculated as 3-sigma of 10 replicate measurements of a blank H₂O₂ sample.

Per SEMI C30-1110 Grade 5, the maximum concentration of the listed elements is 10 ppt. Analyte elements were therefore spiked at 10 ppt except sulfur, which was spiked at 100 ppt. Reproducibility between 1.0 and 8.1% RSD was obtained at the 10 ppt level (100 ppt for S) for the spiked analytes, for a high purity 35% H₂O₂ sample analysis sequence lasting 3 hours 40 minutes.

Table 1. Quantification of trace elements and stability test results for SEMI specification elements in high purity 35% H₂O₂

In a separate study the 8900 ICP-QQQ was used to overcome problematic spectral interferences on non-metallic impurities, P, S, Si and Cl in UPW and P, S and Si in H₂O₂. The results in Table 2 highlight the advanced performance of the 8900 ICP-QQQ for the analysis of these challenging elements, by achieving extremely low BECs for the elements, especially in UPW.

Table 2. BEC and DL of P, S, Si, and Cl in UPW and P, S, and Si in the highest grade H₂O₂.

Trace metal impurities in high purity hydrochloric acid

Semiconductor grade HCl is 37–38%, compared to 20% or 36% for commercial grades (as used in this study). In all grades of HCl, the very high chloride matrix leads to the formation of several polyatomic ions, which cause significant spectral interferences on some key elements. For example, H₂³⁷Cl⁺ on ³⁹K⁺, ³⁵Cl¹⁶O⁺ on ⁵¹V⁺, ³⁵Cl¹⁶OH⁺ on ⁵²Cr⁺, ³⁷Cl¹⁶O⁺ on ⁵³Cr⁺, ³⁵Cl³⁷Cl⁺ on ⁷²Ge⁺, ³⁷Cl₂⁺ on ⁷⁴Ge⁺, and ⁴⁰Ar³⁵Cl⁺ on ⁷⁵As⁺. In total, in this study, 50 elements, including all SEMI standard C27-0708 Tier-C specification analytes, were measured using the 8900 ICP-QQQ operating in multiple tune modes using the method of standard addition (MSA). Single figure ppt or sub-ppt DLs and BECs were achieved for all 50 elements in 20% HCl. Quantitative data acquired in each tune mode was combined automatically into a single report for each sample. Table 3 shows quantitative data for all SEMI specification elements determined by MSA in high purity 20% HCl. Allowing for the difference in the % concentration of the acid, the results show that the 8900 ICP-QQQ can measure contaminants at levels far lower than the requirements for high-purity semiconductor grade HCl specified in SEMI C27-0708.

Table 3. DLs and quantitative results for SEMI specification elements in high purity HCl. Units: ng/L

Note: The arsenic DL was measured in a different high-purity grade HCl (34% high-purity grade diluted to 20% with DIW), due to suspected contamination for this element in the original sample. The As contamination was confirmed from the product ion spectrum measured at m/z 91 and 93. See this Agilent publication for details.

Determination of trace elements in ultrapure semiconductor-grade sulfuric acid

In this study, 42 elements were measured using the 8900 ICP-QQQ operating in multiple tune modes using MSA calibration. High purity 98% H₂SO₄ (TAMA Chemicals Co. Ltd. Japan) was diluted ten-fold with UPW. To run ten-fold diluted sulfuric acid routinely, it is recommended that the large (18 mm) insert Pt cone is fitted. Long-term corrosion of internal ICP-MS components can be minimized by fitting the dry pump option and ball-type interface valve kit. Multi-element standard solutions were prepared from XSTC-331, XSTC-7, XSTC-8 (SPEX CertiPrep, USA) and a Si single-element standard (Kanto Chemical Co., Inc., Japan).

Sub-ppt DLs were achieved for all elements apart from Si (44 ppt), P (3 ppt), and Zn (1.5 ppt). The DLs were determined from 10x replicate measurements of the blank 9.8% H₂SO₄. The quantitative results (BECs) of the analysis of 9.8% H₂SO₄ are shown in Table 4a and b. Recoveries and RSDs were determined from 10 replicate measurements of a 20 ng/L spiked solution of 9.8% H₂SO₄. Excellent performance was achieved for all elements, including Ti, V and Zn, indicating the effective suppression of S-based matrix interferences using the 8900 method.

Table 4a. Quantitative results for 22 of 42 elements in 9.8% H₂SO₄. *2 µg/L spike for Si.

Table 4b. Quantitative results for the remaining 20 of 42 elements in 9.8% H₂SO₄.

Direct analysis of trace metal impurities in high purity nitric acid

The 8900 ICP-QQQ was used for the direct analysis of undiluted commercial grade (61–68%) HNO₃. Direct analysis simplifies sample preparation and avoids the potential introduction of contaminants during dilution. In total, in this study, 49 elements were measured using the 8900 operating in multiple tune modes, switched automatically during a single visit to each sample vial. Good linearity and sub-ppt DLs were obtained for all SEMI target elements. Representative standard addition calibration curves are shown for Na, K, Ca, and Fe (Figure 3). All 49 elements were determined at significantly lower levels than the <1 μg/L (ppb) maximum limit specified for 69–70% HNO₃ in SEMI standard C35-0708 Tier-B.

What about non-aqueous matrices?

The Agilent 8900 ICP-QQQ operating in MS/MS mode provides the high sensitivity, low backgrounds, and unmatched control of interferences required for the analysis of ultratrace elements in aqueous process chemicals. In Part 2, we look at how the 8900 ICP-QQQ handles high-purity organic reagents.

 

Read more here:

• Applications compendium: Measuring Inorganic Impurities in Semiconductor Manufacturing
  
   

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