Why Are FZ Wafers Ideal for Optical Components?


June 28, 2024

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The field of semiconductors is by no means basic. Besides using various materials, different techniques for processing silicone can give you different types of silicon wafers with unique and distinctive properties. That’s the case when you manufacture a float zone or FZ wafer.

The Czochralski process, commonly used for commercial wafers, is more cost-effective but offers some severe disadvantages for niche-market optical components. In this article, we’ll explain why while showcasing how FZ wafers are the best type for certain applications.

Let’s go!

What Are FZ Wafers?

Float Zone Silicon (FZ) is an extremely pure silicon obtained by vertical zone melting. The procedure was created at Bell Laboratories in 1955 by modifying an earlier technique created for germanium.

Due to the vertical configuration, molten silicon has enough surface tension to stop the charge from separating. Moreover, the crucibleless growth prevents contamination of the silicon from the vessel itself.  

This leads to a silicon wafer with very low concentrations of light impurities, like carbon and oxygen. During growth, nitrogen is often intentionally added to control micro defects and improve the wafer’s mechanical strength.

For this reason, FZ silicon wafers are used in applications where a much higher degree of purity is needed, which Czochralski (CZ) grown silicon cannot provide. They’re also called undoped or intrinsic. Some of these applications are:

  • Power devices that need high resistivity.
  • Detectors.  
  • Optical components such as lenses and windows for terahertz applications.
  • Solar cells, due to their higher conversion efficiency.  
  • Solar arrays of satellites.
  • Space-grade electronics, including microprocessors, memory chips, and sensors for spacecraft.
  • Radiofrequency (RF) chips.

But why is that?

What Differentiates CZ and FZ Wafers?  

Researchers who need higher-purity silicon use FZ silicon substrates instead of Czochralski-grown silicon. Simply put, Float Zone Silicon is most commonly used for low-volume applications that require high efficiency, while CZ Silicon is used for high-volume, less expensive applications.  

The Czochralski process leads to wafers with a large amount of oxygen in the silicon. When used for solar cells, these impurities reduce the minority carrier lifetime, thus reducing the voltage, current, and efficiency.  

Oxygen and its complexes with other elements may also become active at higher temperatures, making the wafers sensitive to high-temperature processing. Float Zone (FZ) wafers may be used to overcome these problems.

With less oxygen (O) and carbon (C), these wafers are less likely to become active and damage themselves at higher temperatures. Unlike CZ silicon, FZ silicon has a very high resistivity.

Are Float Zone Silicon Wafers Transparent?

Because of terahertz radiation, float zone silicon wafers are highly transparent. The electromagnetic waves with wavelengths between 3 mm and 30 mm, or 100 GHz and 10 THz, are called the Terahertz range.  

Many materials that block the visible and infrared spectrum appear to be transparent in the terahertz range, which is the primary advantage of terahertz waves. Imaging depth is limited because terahertz waves are typically more strongly attenuated by media than microwaves are.  

This increases the effectiveness of FZ wafers in the fabrication of optical components, such as windows and lenses, for terahertz applications.

Why Is a FZ Wafer Better por Optical Components

What is the IR Transmission of FZ Wafers?  

Only in the wavelength range of 1.2-6.5 µm, the undoped FZ crystallized double-side-polished silicon wafer exhibits >52% Infrared (IR) transmission.  

There is no absorption, but surface reflections restrict transmission.  

Transmission significantly decreases, and lattice absorption becomes significant at longer wavelengths (8-14) µm.

How Are FZ Wafers Grown?

FZ Float Zone crucible-free zone melting method. To successfully “grow” a flat zone (FZ) silicon wafer, the main condition is creating and maintaining a stable zone. Multi-turn inductors with turns positioned in the same plane are utilized for this purpose.

Additionally, the entire procedure takes place in an inert gas purge, also known as an evacuated chamber. On it, FZ crystal ingots are grown from a localized molten zone created by passing a polycrystalline rod made of ultrapure electronic-grade silicon through an RF heating coil.  

At one end, a seed crystal starts the growth. After the molten phase ends, impurities usually stay in the molten region rather than being absorbed into the solidified region, leaving a pure single-crystal region.  

Repeated movement of the zone is used to increase the degree of purification. When using the FZ method, the rate of crystal growth is twice as high as when using the Czochralski method.

That way, you end up with a high-purity silicon wafer.

What Are the Properties of FZ Wafers?

The Float Zone (FZ) technique emphasizes all important parameters for process control. Because of that, they can be used to develop high-efficiency solar cells. Some unique properties of FZ wafers are:

  • High bulk lifetime of minority carriers.
  • Minimal oxygen concentration.
  • Low resistivity variation.  

They can be classified into P-type boron and N-type phosphorous formulations.

Surface tensions during growth force Float zone wafers tend to be generally no greater than 200 mm. Modern technology allows single crystals to grow up to 125 mm in diameter by using an inductor whose diameter is smaller than the diameter of the welded rod.

For larger diameter crystals, production is only possibly using a single-turn coil of the "needle eye" type. However, this is much more complex and has slower production times.  

That, combined with the high-quality requirements during their growth, makes FZ ingots considerably more expensive than CZ wafers. Because of this, FZ wafers are less common in commercial production and are usually reserved for use in laboratory settings.

Unique Properties of a FZ Wafer

Find a Reliable FZ Wafer Provider  

In summary, due to their high purity, FZ wafers play a crucial role in various technological applications, including satellite technology. Their unique properties make them valuable for space exploration and other advanced systems across the photovoltaic industry.

Most people don't know that there are different types of silicon wafers, and even fewer know about the properties of float zone ones. That’s why at Wafer Word, we’re passionate about informing others about the great advancements in our industry.

If you want to learn more, contact us!

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