(Invited) Metallic Contamination Issues in Advanced Semiconductor Processing
Authors: Saga Koichiro 
Published: February 2020

As semiconductor devices continue to shrink, metallic contamination on silicon surfaces become to have a detrimental impact on VLSI device performance and yield. Surface metal impurities degrade gate oxide integrity [1] and cause the whisker growth in film deposition [2], while metal impurities dissolved in silicon cause recombination centers, junction leakage [3] and dark current in image sensors [4]. In this article, we have extensively reviewed metallic contamination issues including the analysis, prevention, removal, and gettering in advanced semiconductor processing.
Trace metallic contamination on the silicon surfaces can be detected with VPD- or LPD-TXRF, AAS, and ICP-MS and these methods have been widely used. These analyses provide average metal concentration on the whole wafer surface with high detection sensitivity, while they do not give in-wafer distribution of metal concentration. However, metal impurities accidentally deposited in wafer processing are often localized. Direct TXRF provides in-wafer distribution of metal concentration, while its sensitivity is insufficient. Therefore, the methods to detect localized metal atoms with high sensitivity are required. TXRF following vapor phase treatment (VPT) has been developed to enhance the detection sensitivity without droplet collection [5] and the influence of VPT-induced trace particles on the detection sensitivity has been studied [6]. When III-V compound semiconductor materials are introduced for high mobility channels, the detection sensitivity on the III-V semiconductor surface by TXRF [7] and the metal collection efficiency [8] becomes critical issues.
In order to control metal impurities in silicon, understanding of their penetration and diffusion behaviors is valuable. The amount of metals penetrating the silicon substrate depends on metal species, underlying films, their thickness, and annealing temperature. Cr and W penetrate the silicon substrate through the Si3N4 film more easily than through the thermally-grown SiO2 [9]. These metals on Si3N4 films must be strictly controlled before ion implantation. In-wafer distribution of recombination centers should be detected with high sensitivity and visualized with high detection sensitivity for advanced contamination control [10]. The deep energy levels, their depth profiles, and dark currents induced by metal impurities are varied with metal species [10-12]. The sensitivity of the deep level measurements should be improved.
Gettering is effective to mitigate the influence of metal impurities on device characteristics. When extrinsic gettering such as ion implantation defects is used, the gettering design based on physical properties of metal species is required [13]. One of the intrinsic gettering techniques is gettering using oxide precipitates in silicon. For effective gettering by the oxide precipitates, the sufficient total surface area of the oxide precipitates should be close to the DZ [14].
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