Silicon wafers

When a doped crystal is pulled by the CZ method, the dopant concentration tends to be higher toward the bottom of the crystal (later in the pull). Why?

  1. ABecause gravity makes the impurities sink
  2. BBecause many dopants have segregation coefficients below 1, so dopants that are not readily incorporated become concentrated in the melt
  3. CBecause the temperature is lower later in the pull
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Answer: B. Because many dopants have segregation coefficients below 1, so dopants that are not readily incorporated become concentrated in the melt

As the crystal solidifies, many dopants are not readily incorporated (segregation coefficient below 1) and remain in the melt. As pulling proceeds, the dopant concentration in the melt rises, so later parts of the crystal have higher dopant concentrations and lower resistivity. Resistivity therefore varies with position along the ingot.

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More Silicon wafers questions

  1. Which correctly describes 450 mm wafers, developed as the next diameter after 300 mm?
  2. What is the purpose of using bulk microdefects (BMDs), formed by oxygen precipitation inside CZ wafers?
  3. Heat treatment of a wafer diffuses oxygen out from near the surface, creating a low-defect layer at the surface where devices are made. What is this layer called?
  4. What is the purpose of the laser mark (ID mark) inscribed on the front or back of a wafer?
  5. Which polishing method is common for 300 mm wafers?