Evaluation of Metal Contamination Behavior on Silicon Wafer Surfaces Rinsed with Deionized Water Containing pg/L-Level Impurities
Authors: Tsutano Kyohei , Mawaki Takezo , Shirai Yasuyuki , Kuroda Rihito 
Published: December 2024

Contamination control is a crucial aspect of semiconductor device manufacturing, as it has a significant impact on device performance and yield. Metallic impurities on the silicon surface are known to diffuse into the bulk silicon during annealing and ultimately degrade device performance [1,2]. Deionized water (DIW) plays a critical role in the wet process, and a control value of metal concentration is defined by IRDS [3]. Recently, a metal analysis method for pg/L levels in DIW using a solid phase extraction method followed by inductively coupled plasma mass spectrometry (SPE-ICP-MS) has been developed to analyze at lower concentration [4]. However, the behavior of metal contamination on wafer surfaces rinsed with DIW containing trace metal impurities is unclear. Therefore, this paper aims to investigate the metal contamination behavior on bare silicon wafer surfaces rinsed with DIW containing pg/L metals and to obtain the relationship of the metal concentration between DIW and the wafer surface. In this study, we discovered that Cu was easily adsorbed compared to other metals, which is consistent with previous studies at the μg/L level.

Experiments

The boron-doped (100) CZ silicon wafers (diameter: 200 mm; thickness: 725±15 μm; resistivity: 7-12 Ω・cm) after removal of a native oxide film was employed for this test. The rinsing test were carried out using a single-wafer machine at a flow rate of 1 L/min, utilizing DIW intentionally contaminated with a metal solution at the pg/L level. The results obtained from the detection methods of SPE-ICP-MS and vapor phase decomposition-ICP-MS (VPD-ICP-MS) were compared.

Result and discussion

The data presented in Figure 1(a) indicates that the silicon wafer surface contamination levels were highest for Cu at 1E10 atoms/cm2, followed by Al at 1E9 atoms/cm2, Ti and Pb at 2E8 atoms/cm2, and Co, Mn, and Sr at 1E7 atoms/cm2 after an 8-hour rinsing time with approximately 20 pg/L. Cu was easily adsorbed compared to other metals, which is consistent with previous studies at the μg/L [5]. In addition, the linear relationship of metal concentration between DIW and the wafer surface for these metals was observed as concentration was increased from 20 to 100 pg/L with the same rinsing time. Figure 1(b) demonstrates that Cu exhibited a linear relationship between rinsing time and surface concentration when rinsing time was increased from 4 to 16 hours with 50 pg/L. Furthermore, surface concentration of Mn, Sr, and Pb were not significantly impacted by the rinsing time. On the other hand, that of Al, Ti, and Co increased by 5 to 10 times when the rinsing time was increased from 16 hours compared to 4 and 8 hours. It is likely that Al, Ti, and Co are easily contaminated with increased rinsing time.

Conclusion

Metal impurity control in DIW is one of the key factors in semiconductor manufacturing processes due to its direct impact on the performance and yield of devices. However, there is limited understanding of the effects of ultra-trace impurities in DIW on the silicon wafer surface metal contamination. In this study, we investigated the metal contamination behavior on bare silicon wafer surfaces using a single-wafer machine with DIW containing pg/L metals. We found that Cu was easily adsorbed compared to other metals, which is consistent with previous studies at the μg/L level. These findings provide novel insights into the effect of metal contamination on a silicon wafer surface by pg/L levels impurities in DIW and are expected to contribute to enhancing semiconductor manufacturing processes.
[1] A. Shimazaki et al., Solid State Phenom. 145, 115–121 (2009)
[2] K. Saga, ECS Trans. 86, 113(2018)
[3] International Roadmap for Devices and Systems, 2022 Edition. YIELD ENHANCEMENT (2022).
[4] K. Tsutano et al., Solid State Phenom. 346, 170-175 (2023)
[5] T. Ohmi et al., 1992 J. Electrochem. Soc. 139,3317-3335 (1992)

Figure 1