Metallic contamination in semiconductors adversely affects device performance. As linewidths decrease the allowable levels of metal contamination get smaller and smaller. The most commonly occuring forms of metal contamination are either in the middle of the periodic table, called heavy metals, or are in the alkaline metal column, at the left of the periodic table.
Heavy metals diffuse into the bulk silicon. This type of contamination is discussed in the contamination section of this website. Alkaline metal contamination, such as sodium, usually occurs during processing and does not diffuse into the bulk silicon. Alkaline metal contamination affects oxides. The charge of ionized alkaline metals in oxides affects the oxide charge, measured as flat band voltage or equivalent.
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Often, it is desirable to separate charge due to mobile ions from charge due to other things in the oxide. Sodium is the most commonly occurring alkaline metal contaminant, and its small diameter, ionized atoms migrate through the oxide in response to bias voltage. This variation in embedded charge in an oxide changes device performance, and the specs become unstable. Thus, it is desirable to limit the mobile charge contamination in an oxide.. Mobile charge can be detected via bias temperature stress tests.
The WT-2000 monitors oxides via a measurement technology we call VQ. It is a benchtop system. The WT-3000 offers similar capability via a dual FOUP system for 300mm wafers. The WT-2000 can also make the bias temperature stress test, if equipped with that optional measurement capability.
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