Junichi Takahashi
Agilent Technologies, Japan

Keywords

semiconductor, process chemicals, sulfuric acid, H₂SO₄, titanium, vanadium, chromium, ammonia mass-shift, oxygen mass-shift

Introduction

High purity H₂SO₄ is frequently used in the manufacturing of semiconductor devices, in processes such as the removal of organic substances from the surface of silicon wafers. The required metallic impurity level is lower than 100 ppt in the concentrated (usually 98%) acid. ICP-MS is the technique of choice for the measurement of trace metal impurities in semiconductor process chemicals. There are, however, some limitations for the measurement of elements such as Ti, V and Cr in H₂SO₄. Because of its high viscosity of 27 cP, it is not possible to introduce H₂SO₄ directly into the ICP without dilution. A 10 times dilution in UPW is normally applied, thus the BEC of the calibration curve must be lower than 10 ppt in the 9.8% H₂SO₄ solution measured. In addition, spectral interferences from SO⁺, S₂⁺ and ArS⁺ originating from H₂SO₄ make it difficult to determine elements such as Ti and Cr at low concentration even by quadrupole ICP-MS (ICP-QMS) equipped with collision/reaction cell (CRC). As outlined in this report, the Agilent 8800 ICP-QQQ with MS/MS mode allows the successful determination of the most problematic elements including Ti, V and Cr in H₂SO₄.

Experimental

Instrumentation: Agilent 8800 #200. Operating parameters are given in Table 1.

Reagents and sample preparation: Highly purified H₂SO₄, TAMAPURE-AA-100 (98% H₂SO₄) was purchased from Tama Chemicals Co., Ltd. (Kanagawa, Japan). 5g of H₂SO₄ was diluted by a factor of 10 in a chilled PFA bottle.

Results and discussion

Of the potential polyatomic interferences formed from the H₂SO₄ matrix, the SO⁺ ion is very stable and difficult to eliminate because its dissociation energy is as high as 5.44 eV. In addition, its ionization potential is 10.3 eV, which is almost the same as that of S, 10.36 eV. The spectral interferences caused by SO⁺ and
SOH⁺ overlap with ⁴⁸Ti (³²S¹⁶O),
⁵¹V (³³S¹⁸O, ³⁴S¹⁶OH and ³²S¹⁸OH) and
⁵²Cr (³⁴S¹⁸O). Quadrupole ICP-MS operating in He collision mode provides BECs of 60 ppt for ⁴⁷Ti (the BEC for the preferred isotope ⁴⁸Ti is much higher), 3 ppt for V and 8 ppt for Cr in 9.8% H₂SO₄. The BEC of Ti, in particular, is not acceptable for producers and users of semiconductor grade H₂SO₄.

Appropriate reaction gases to remove SO⁺ successfully in ICP-QMS are difficult to find. NH₃ can reduce SO⁺ by two orders of magnitude but the background signal remains too high for this application. Additionally, cluster ions of NH₃ such as N_mH_n produced by the reaction between Ar⁺ and the NH₃ cell gas lead to new reaction product ion interferences that increase the background at m/z 51, for example.

The 8800 ICP-QQQ operating in MS/MS mass-shift mode with NH₃ or O₂ reaction gas provides reliable and consistent measurement of Ti as ⁴⁸Ti¹⁴NH(¹⁴NH₃)₃ (Figure 1) and Cr as ⁵²Cr¹⁶O in H₂SO₄. Furthermore, in MS/MS mode, the Ar⁺ ion is removed by Q1, preventing it from reacting with NH₃ to form new product ion interferences in the cell. This reduces the background at m/z 51 improving the BEC for V, as shown in Figure 2. The final BECs obtained by ICP-QQQ in 9.8% high purity H₂SO₄ are summarized in Table 2.

Conclusions

ICP-QQQ operating in MS/MS mode provides a reliable means for manufacturers of high purity H₂SO₄ to guarantee all metallic impurity concentrations at less than 100 ppt in the concentrated acid.

More Information

Determination of challenging elements in ultrapure semiconductor grade sulfuric acid by Triple Quadrupole ICP-MS, Agilent application note, 5991-2819EN.