Oxidation and diffusion

What problem occurs when mobile ions such as sodium are present in the gate oxide?

  1. AThe oxide emits light
  2. BThe ions move under electric fields and temperature, shifting the threshold voltage
  3. CThe mechanical strength of the oxide increases
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Answer: B. The ions move under electric fields and temperature, shifting the threshold voltage

Alkali metal ions such as sodium move easily in oxide, and when electric fields or temperature drive them toward the interface, they shift the threshold voltage. This was a major problem in early MOS devices, addressed by contamination control in cleanrooms, chlorine oxidation and trapping with phosphosilicate glass.

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More Oxidation and diffusion questions

  1. Which n-type dopant diffuses relatively slowly in silicon during heat treatment, making shallow junctions easier to form?
  2. Which correctly describes pyrogenic oxidation (hydrogen combustion oxidation)?
  3. What is the approximate breakdown field of high-quality thermal oxide (SiO2)?
  4. Which method reacts hydrogen and oxygen near the wafer surface to form a high-quality thin oxide in a short time?
  5. What is the treatment called that introduces plasma-generated nitrogen into the surface of a thin gate oxide?