CZ vs FZ Silicon: Czochralski and Float-Zone Crystal Growth

Almost all silicon wafers start as a single crystal grown by one of two methods. In the Czochralski (CZ) method, a seed crystal is dipped into molten silicon in a quartz crucible and slowly pulled up. In the float-zone (FZ) method, a polysilicon rod is melted in a narrow zone by a coil and the zone is moved along the rod, so no crucible touches the silicon. CZ is cheaper and makes large wafers; FZ gives purer silicon.

AspectCZ methodFZ method
How the crystal is grownA seed is dipped into silicon melted in a quartz crucible and pulled up while rotatingA radio-frequency coil melts a narrow zone of a polysilicon rod, and the zone is moved along it
Contact with a crucibleYes, so oxygen from the quartz dissolves into the meltNo, so there is very little oxygen
Oxygen contentHighVery low
Purity and resistivityLower purity; resistivity limited (the MCZ variant with a magnetic field helps)Very high purity and very high resistivity
DopingDopant is added to the melt; concentration varies along the ingotGas doping, or neutron transmutation doping (NTD) for uniform phosphorus
Wafer diameterUp to 300 mm in productionTypically up to about 200 mm; larger sizes are difficult
CostLower, and large ingots give good throughputHigher
Mechanical strength and getteringOxygen precipitates add strength and trap metal impurities (intrinsic gettering)Lacks these benefits, so wafers can be more fragile
Typical usesThe large majority of wafers: logic, memory and solar cellsHigh-voltage power devices (such as IGBTs), detectors, high-resistivity RF devices

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

CZ
The default for mainstream chips. It scales to the largest wafer sizes and is the most economical.
FZ
When high purity and very high resistivity matter more than cost or wafer size, mainly for power devices and sensors.

FAQ

Why is CZ the most common method?

It is cheaper, produces large ingots (up to 300 mm wafers in production) and gives wafers with oxygen precipitates that add strength and trap impurities.

Why does FZ silicon have higher resistivity?

No crucible touches the silicon, so it picks up very little oxygen and other impurities, which allows very high purity and resistivity.

What is neutron transmutation doping?

Irradiating FZ silicon with neutrons converts some silicon atoms into phosphorus, giving very uniform n-type doping.

What are FZ wafers used for?

High-voltage power devices such as IGBTs, detectors and high-resistivity RF devices.

Check your understanding

Which is a characteristic of single-crystal silicon made by the FZ (floating zone) method?

  1. AIt is the easiest to scale to large diameters and is the mainstream method for 300 mm wafers
  2. BBecause no quartz crucible is used, it contains little oxygen and other impurities, making high-resistivity crystals easy to obtain
  3. CIt turns polycrystalline silicon into a single crystal without melting it
Show answer and explanation

Answer: B. Because no quartz crucible is used, it contains little oxygen and other impurities, making high-resistivity crystals easy to obtain

The float-zone (FZ) method melts a narrow band of a polysilicon rod with RF induction heating and moves the molten zone along the rod to convert it to a single crystal. With no crucible, little oxygen enters, and impurities are swept away in the molten zone, yielding high-purity crystals with resistivities above 1000 Ω·cm. FZ silicon serves high-voltage power devices such as IGBTs and radiation detectors, but CZ scales more easily, with FZ limited to about 200 mm.

Which impurity is incorporated in relatively large amounts from the crucible into silicon crystals grown by the CZ method?

  1. AGold
  2. BOxygen
  3. CHelium
Show answer and explanation

Answer: B. Oxygen

CZ silicon is melted in a quartz (SiO2) crucible, and oxygen dissolving from it enters the crystal at about 10¹⁸ cm⁻³. Oxygen pins dislocations, strengthening the wafer, and its precipitates act as gettering sites, but excess oxygen causes defects and thermal donors. Concentration is therefore controlled, for example with magnetic CZ (MCZ) that suppresses melt convection, and specified per application.

In the CZ method, what is the main purpose of controlling the ratio of pull rate to temperature gradient in the crystal (V/G)?

  1. ATo control the type and amount of point defects (vacancies and interstitial silicon) incorporated, reducing crystal defects
  2. BTo change the color of the crystal
  3. CTo extend the life of the crucible
Show answer and explanation

Answer: A. To control the type and amount of point defects (vacancies and interstitial silicon) incorporated, reducing crystal defects

The ratio of pulling speed V to the temperature gradient G near the interface determines whether excess point defects incorporated into the crystal are vacancies or self-interstitials. High V/G leaves surplus vacancies that cluster into voids (COPs); low V/G leaves interstitials that form dislocation loops. Growing within the narrow window where they balance yields nearly defect-free ("perfect") silicon.

  1. Which impurity is typically added to the melt to make p-type silicon crystals by the CZ method?
  2. Which method irradiates FZ silicon with neutrons to convert some silicon into phosphorus, making it uniformly n-type?
  3. Which method applies a magnetic field to the crucible in the CZ method to suppress melt convection and control oxygen concentration and other properties?

Practice Silicon wafers questions

Key terms explained

Czochralski (CZ) method
A method of making single-crystal silicon by dipping a seed crystal into molten silicon in a crucible and slowly pulling it up while rotating.
FZ method (float zone)
A method that melts a zone of a polysilicon rod and moves that molten zone along to form a single crystal. With no crucible, oxygen content is low, suiting high-resistivity wafers.
MCZ method (magnetic-field-applied CZ)
A CZ variant that applies a magnetic field to the crucible to suppress melt convection. It makes the oxygen concentration in the crystal easier to control and is used for large-diameter crystals.

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